NASA Conference Publication 10180

Space Photovoltaic

Research and

Technology

1995

Proceedings of the fourteenth Space Photovoltaic Research and

Technology Conference (SPRAT XIV)

NASA Lewis Research Center

Cleveland, Ohio

Proceedings of a conference held at

Lewis Research Center

Cleveland, Ohio

October 24-26, 1995

NASA

Scientific and Technical

Information Program

1996

https://ntrs.nasa.gov/search.jsp?R=20000033702 2020-03-30T14:05:18+00:00Z
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Space Photovoltaic Research and Technology 1995

Table of Contents

Contents

Foreword

The Irving Weinberg Award

Session 1: Invited papers

C.P. Bankston, R.B. Bennett and P.M. Stella

G. Strobl et al.

il:

i

I

Planetary and Deep-Space Requirements for Photovoltaic Solar Arrays 2

Development of Advanced Si and GaAs Solar Cells for Interplanetary Missions 10

G.P. Summers

Naval Research Laboratory's Programs in Advanced Indium Phosphide Solar

Cell Development 21

C. Goodbody and R. Kimber

The Defense Research Agency Photovoltaic Programme 24

T. Saga, Y Kiyota, T. Matsutani, A. Suzuki, O. Kawasaki, T. Hisamatsu and S. Matsuda

Japanese Photovoltaic Power Generation for Space Application 31

Session 2: Multijunction Cells

E.M. Gaddy

Cost Trade Between Multi-junction, GaAs and Silicon Solar Cells 40

P.K Chiang, D.D. Krut and B.T. Cavicci

The Progress of Large Area GaInP2/GaAs/Ge Triple Junction Cell Development at

Spectrolab 47

YC.M. Yeh and C.L. Chu

Status of Multijunction Solar Cells 57

M.E V'dela et al.

InP tunnel Junctions for InGaAs/InP Tandem Solar Cells 65

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Session 3: Radiation Damage

G.R Summers, R.J. Walters, S.R. Messenger and E.A. Burke

The Role of Radiation Hard Solar Cells in Minimizing the Costs of Global Satellite

Communications Systems 71

G. Rybicki etaL

High and Low Energy Proton Radiation Damage in p/n InP Solar Cells 80

R. Waiters, G.P. Summers, S.R. Messenger and E.A. Burke

Correlation of Electron and Proton Irradiation-induced Damage in InP Cells 89

S. Wojtczuk etal.

Diffusion Lengths in Irradiated n/p InP on Si Solar Cells 95

S. Messenger, H.L. Cotal, R.J. Waiters and G.P. Summers

Electron and Proton Damage on InGaAs Solar Cells Having an InP Window Layer

100

A. Meulenberg

Photo-Recovery of Electron-Irradiated GaAs Solar Cells 106

Session 4: Cell Technology

L.C. DiNetta et al.

Initial Results for the Silicon Monohthically Interconnected Solar Cell Product

114

L. DiNetta and M.H. Hannon

Monohthically Interconnected GaAs Solar Ceils: A New Intercormection

Technology for High-Voltage Solar Cell Output 122

B. Chatterjee, W.C. Davis, S.A. Ringel and R. Hoffman, Jr.

Hydrogen Passivation of n+p and p+n Heteroepitaxial InP Solar Cells 127

R.E. Welser and L.J. Guido

The Growth of Low Bandgap InAs on (i 11)B GaAs Substrates 137

J. Moulot etal.

Three-layer Antireflection Coating for High Efficiency InP Solar Cells Using a

Chemical Oxide as First Layer 142

M.H. Hannon, M.W. DashieI1, L.C. DiNetta, and A.M. Barnett

Lightweight, Light-trapped Thin GaAs Solar Cell for Spacecraft Applications:

Progress and Results Update 150

Session 5: Measurements

J.R. Woodyard

Laboratory Instrumentation and Techniques for Characterizing Multi-junction Solar

Cells for Space Applications 158

D.R. Burger and R.L. Mueller

Solar Cell Angle of_incidence Corrections __ _ 168

D.A. Scheiman, P. Jenkins, D.J. Brinker and J. Appelbaum = :

Low Intensity Low Temperature (LILT) Measurements and Coefficients on New

Photovoltaic Structures 178

D.J. Brinker et al.

A Summary of the International Workshops on Space Solar Cell Calibration and

Measurement Techniques 186

..,

-Ill-
Session 6: TPV and advanced concepts

D. Chubb et al.

Review of Recent Thermophotovoltaic Research at Lewis Research Center 191

L.M. Fraas, J.E. Samaras, J.E. Avery and R. Ewell

Thermophotovoltaic Generators using Selective Metallic Emitters 208

Z.A. Shellenbarger, M.G. Mauk, L.C. DiNetta, and G.W. Charache

InGaAsSb/GaSb Thermophotovoltaic Ceils 215

S. Wojtczuk

Multijunction InGaAs Thermophotovoltaic Power Converter 223

P.E. Simms, L.C. DiNetta, K.D. Cavanagh and M.A. Goetz

Gallium Phosphide Energy Converters 231

Session 7: Assembly and Panels

M.R. Brown et al.

Characterization Testing of MEASAT GaAs/Ge Solar Cell Assemblies 237

E.L. Ralph and E.B. Linder

Advanced Solar Panel Designs 247

A. Meulenberg

Space Qualification of U'V and IR Reflecting Coverslides for GaAs Solar Cells

257

Session 8: Flight Experiments and Space Plasma

H. Curtis and D. Marvin

One Year of Flight Data From the PASP-Plus Experiment

V.A. Davis, B.M. Gardner, D.A. Guidice and P.S. Severance

Parasitic Collection by PASP-Plus Solar Arrays

D.A. Guidice

High Voltage Space Plasma Interactions on the PASP-Plus Test Arrays

W.R. Mackey

Preliminary Chaotic Model of Snapover on High Voltage Solar Cells

P.A. Jones and D.M. Murphy

A Linear Refractive Photovoltaic Concentrator Solar Array Flight Experiment 304

G.A. Landis and P. Jenkins

Development of a Solar Cell Dust Opacity Measurement Instrument for Mars

Pathfinder 313

Workshops

1. Multijunction Cells

2. Radiation Damage

3. Characterization and Testing of New Solar Cell Types

4. Solar Power Satellite

268

274

286

296

318

320

323

325

List of Attendees 328

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FOREWORD

Navid S. Fatemi

Essential Research, Inc.

NASA Lewis Research Center

The Fourteenth Space Photovoltaic Research and Technology (SPRAT XIV) Conference was

held October 24-26, 1995 at the NASA Lewis Research Center, bringing together representatives of the

space photovoltaic communityfrom the U.S., Europe, and Japan. In attendance were about 100

scientists, engineers, program managers, and others representing 21 commercial corporations, 7

govemment agencies, and 9 universities.

The latest results of research and development activities,commercialization plans, and flight data

on photovoltaic power generation for space were presented at the meeting. During the course of the

meeting, it became apparent that PV power generation needs of the near future will rest more with the

commercial sector in general and the communication satellite industry inparticular, than withthe

traditionally govemment-funded missions. This was indeed good news for the space PV community.

In addition, it was clear that advanced solar cell technologiessuch as GaAs on Ge, InP on Si or Ge,

and multibandgap cells are rapidly gaining acceptance and utilization as viable cost-effective alternatives to

the conventional Si technology. This fact gave mere urgency to the discussionsheld by the attendees at

the conference workshops dedicated to multibandgap cells, radiation resistance issues, characterization

and testing of new cell types, and solar power satellites.

As with its predecessors, the invitedpapers, the contributed papers, and the summary of the

workshops presented in this volume make it one of the most up-to-date compendia of space solar cell and

array literature available anywhere.

In conclusion, I would like to thank all those who helped organize and run the conference. They

are: publicationschair,Geoffrey Landis;logisticschair,George Rybicld;social events chair, Karen Wester;

secretarial and registration, Jenise Veris, Brunilda QuiSones, and Pat Wielinski; and finally Dennis Flood

for his mentorship.

-i-
The Irving WeinbergAward

Thefourteenth Space Photovoltaic Research and Technology executive committee has established the

Irving Weinberg Award, in memory of Dr. Irving Weinberg, a leading contributorto the field of space

photovoltaic research and development for most of his professional career. This award is to be given at

every SPRAT meeting to persons who have made significant contributionsto the field of space

photovoltaics. This award is meant to be inclusiveof all aspects of space photovoltaic research and

technology, from fundamental investigations of semiconductor materials, to device improvements, and

finally to innovations in hardware for actual missionapplications. The recipient of the first Irving Weinberg

award is Professor Chandra Goradia.

Professor Chandra Goradia

Professor Goradia received his M.Sc. degree in physics from the University of Bombay, India, in 1962. He

then received his M.E.E. and Ph.D. degrees in electrical engineering from the University of Oklahoma,

Norman, Oklahoma, in 1964 and 1967 respectively.Since 1967, he has been a faculty member in the

Electrical Engineering Department of the Cleveland State University, where he has been a full professor

since 1981.

Professor Goradia has conducted solar cell research since 1974. For the firstt'weyears, his research was

concentrated on theoretical modeling, experimental fabrication and performance evaluation of the Vertical

Multijunction silicon solar cells. Subsequently, inspiredby and in collaborationwith Dr. Irving Weinberg, he

worked on the theoretical optimal design and performance prediction of a variety of space and terrestrial

solar cells of different materials and geometries. These included the single- and double-connected silicon

Tandem Junction Cell (TJC), high base resistivityconventional silicon nip cells, space GaAs concentrator

cells, CulnSe 2 and CdTe thin film terrestrial cells, and InP space solar cells.

In 1988, professor Chandra Goradia established, jointly with his research colleague and wife, professor

Manju Ghalla-Goradia, the NASA Lewis-funded Space Photovoltaic Research Center (SPRC) within the

Electrical Engineering Department at Cleveland State University. They have been the co-directors of the

SPRC since its inception.

Professor Gomdia has published quite widely in the semiconductors and solar cells area and is the

principal author or joint author of over seventy-Fn/epublications injoumals and conference proceedings.

Professor Goradia is also a Senior Member of the Instituteof Electricaland Electronics Engineers (IEEE).

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PlanetaryandDeepSpaceRequirementsfor Photovoitaic Solar Arrays1

C. P. Bankston, R. B. Bennett, and P. M. Stella

Jet Propulsion Laboratory

California Instituteof Technology

Pasadena, CA 91109

INTRODUCTION

In the past 25 years, the majority of interplanetary spacecraft have been powered by nuclear

sources. However, as the emphasis on smaller, low cost missions gains momentum, more deep

space missions now being planned have baselined photovoltaic solar arrays due to the low power

requirements (usually significantly less than 100 W) needed for engineering and science

payloads. This will present challenges to the solar array builders, inasmuch as planetary

requirements usually differ from earth orbital requirements. In addition, these requirements often

differ greatly, depending on the specific mission; for example, inner planets vs. outer planets,

orbiters vs. flybys, spacecraft vs. landers, and so on. Also, the likelihood of electric propulsion

missions will influence the requirements placed on solar array developers.

This paper will discuss representative requirements for a range of planetary and deep space

science missions now in the planning stages. We have divided the requirements into three

categories: Inner planets and the sun; outer planets (greater than 3 AU); and Mars, cometary,

and asteroid landers and probes. Requirements for Mercury and Ganymede hinders will be

covered in the Inner and Outer Planets sections with their respective orbiters. We will also discuss

special requirements associated with solar electric propulsion (SEP). New technology

developments will be needed to meet the demanding environments presented by these future

applications as many of the technologies envisioned have not yet been demonstrated. In

addition, new technologies that will be needed reside not only inthe photovoltaic solar array, but

also in other spacecraft systems that are key to operating the spacecraft reliably with the

photovoltaics.

GENERAL CONSIDERATIONS

Planetary and deep space scientific exploration is faced withthe same programmatic requirements

common to virtually all space programs inthe 1990's, and beyond. These programs will face

tightly constrained budgets that are often near $100 millionfor the design, fabrication and

integration of the spacecraft. Launch services costs must be kept to a minimum. "13ghtfunding

requires the payload system developer to produce mission hardware that is mass and volume

efficient. The more capability which can be packaged in a small, lightweight system the smaller

and less expensive the launch vehicle that is required to boost it. Spacecraft and payload

development cycles will frequently be only 2-3 years. This will demand that proven or accepted

designs and technologies must be ready for implementation once project start is approved, since

critical design reviews may be schedrJled withinthree months of project start.

Accordingly, overlaying requirements associated with low mass, low cost systems will always be in

place as part of the system trade-offs. This means that compact packaging and low mass

structures conforming to launch vehicle restraints will be needed. In general, the highest

efficiency cell technologies consistent withthe cost constraints will be sought as planetary and

deep space missionsare usually power limited. Finally, short cycle times also require that

The work described in this paper was performed by the let Propulsion Laboratory, California Institute of

Technology, under contract with the National Aeronautics and Space Administration.

-2-
constituent components are readily available for rapid assembly and that vendor source

agreements are in place.

Finally, beyond direct power system considerations, there are a host of ancilliary spacecraft

technology considerations for the described mission concepts. Such considerations play a key

role in determining power system feasibility. Some of these considerations include fault tolerance

and reliability requirements, on-board autonomous operations capability, power-down cruise

capability, low-temperature tolerances and thermal control constraints, and power storage

requirements and capabilities for off-sun events and pointing anomalies. While beyond the scope

of the photovoltaic solar array requirements addressed in this paper, such considerations and the

technologies needed to address them are enabling for application of photovoltaics in many of the

applications described here, especially for those missions beyond the orbit of Mars.

INNER PLANETS AND THE SUN

Several inner body explorations are under study at the Jet Propulsion Laboratory.

Missions being planned for exploration inside 1 AU include a Mercury lander and orbiter

combination, and a "solar probe"to investigate solar regions within one-half of a solar radius. A

mission to Venus is also under study, but has not identified exceptional technology demands that

are beyond the need for rapid, low cost implementation with the best performance, lowest mass,

and most compact packaging.

A missionto Mercury is now being planned for launch around 2010. It would consist of an orbiter

carrying a separate lander package. The orbiter design life is one year following orbit insertion,

while the lander will descend to the surface and would function for one Mercurian day (176 Earth

days). Orbiter and lander power needs are 210 W and 25 W respectively. For both orbiter and

lander, the thermal environmentsare extraordinary, where the solar flux at 0.4 AU is about 104

W/m2. On the surface the subsolartemperature is 700K, with a black body emission from the

surface of 530K. At nightthe lander temperature would drop to 90K. The high temperatures will

require welded solar army construction along with many new array materials. These must also be

capable of withstanding wide temperature swings in eclipse or the day/night cycle. Also, off-sun

pointing performance of the baselined GaAs solar arrays will be required and characterization of

the performance of the system at highly oblique angles (10"-25") will be necessary. Solar radiation

levels will require the use of heavy shielding. Note that a solar electric propulsion option is being

considered for this mission; the advantages of such an approach are described in the SEP

discussion below.

The solar probe mission(Figure 1) has the goal of making insitu measurements as far into the

solar atmosphere (solar corona) as possible. Its perihelion is 4 solar radii where the spacecraft

would be exposed to 3000 suns solar flux at 0.3 AU. For a solar powered mission,two solar arrays

may be required. A large, low mass array would be required from launch outward to 5.2 AU,

inasmuch as the spacecraf_requires a gravity assist at Jupiter to place it on the desired trajectory.

This array would also be utilized on the cruise inward toabout 0.7 AU. The area of the large solar

array will be determined by the requif'ementto provide 60 W at 5.2 AU for spacecraft survival. This

array must perform under low intensity, low temperature conditions, and will be gimbaled to

provide pointing capability on and off sun as the spacecraft moves away and then retums. Low

mass designs will be required, with flexible fold out panels. An inflatable structure is presently the

baselined concept. This array would be jettisoned at about 0.7 AU (or at 400K) in favor of a

second smaller panel for the region 0.7-0.3 AU. This would be a rigid panel with welded cells for

survivability. This array will also require pointing off-sun as it approaches the sun. It is possible that

the second array may not be necessary if the primary array can be feathered sufficiently and

controllably to keep temperatures within a safe range. The characteristics of cell performance at

highly oblique, yet high intensity, incident solar fluxes must be determined for the cells selected.

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OUTERPLANETS

Very low-power missions to the outer solar system that have been studied include a Galilean

orbiter and Ganymede lander combination, and a flight to the Kuiper belt. Also, a much higher

powered mission to search for extra-solar planets is being considered that would perform its

investigations from up to 5 AU. While onboard power requirements would be moderate, the low

solar intensitywill require very large array areas. For example, the solar intensityat 5 AU would be

only 54 W/m =and even if as much as 25 percent can be converted (a highvalue) to electrical

power, this would yield only 13.5 W/rr_ of array area. As a result,such missionswill demand

extremely compact packaging and low mass structuresto conformto launch vehicle constraints.

In tum, the large area, low mass designs will impactallowable spacecraft loads. Inflatable array

structures, with and withoutconcentration, and multi-band gap cells will be considered if available.

In general, the highest efficiency cell technologies operable under low intensity, low temperature

conditions will be needed.

For the Galilean orbiter, the spacecraft power required is 50-100W at Jupiter, equivalent to at least

2500 W at 1 AU. Ifradiation damage, either from solar flares, or trapped radiationin the Jovian

belts is included, the 1 AU power level equivalent may be on the order of 4 kW. These arrays will

require both early cruise off-sun pointing and reliable performance under low intensity, low light

(LILT) conditions. The Ganymede lander willrequire only a few watts, but must operate in the

average temperature range 150-170 K under the extreme low lightand high radiation (100 kRad

qualification level) conditions near Jupiter. The low light conditions would require stowage of an

exceptionally large solar array incomparison to the lander. Furthermore, the difficultyin providing

energy storage and thermal control under such conditions may make a radioisotope source

necessary for the lander.

A =Kuiper Express" (Figure 2) is being studied to conduct reconnaissance of the primitive objects

of the Kuiper Belt. Launch would take place after 2000 with a ten year cruise phase. Mission

options include fly bys of Mars, an asteroid, Uranus, and Neptune in route. Solar electric

propulsion via 6-2.5kW ion engines would be used out to 3 AU, with solar power continuing to

provide at least 10 W out to 50 AU. An inflatable, "mini-dome" fresnei concentrator array has been

baselined to meet the challenging power requirement in the low light conditions, while meeting

packaging and mass constraints. Thus, significanttechnology challenges remain before these

mission requirements can be met.

Finally, the search for extra solar planets with a spacecraft (Figure 3) at 5 AU has the goal of

identifying the presence of planets within 10 parsecs. The spacecraft, a large aperture

interferometric telescope, would require about 600W generated from varying sun angles under

LILT conditions, Launch mass constraintsdictate a solar array pefformace target of >200W/kg at 1

AU. The plan is for 2 single axis, articulated arrays for this specialapplication. Inflatable array

technology is presently baselined.

MARS, COMETARY, AND ASTEROID LANDERS AND PROBES

Most studies of other landers and probes usually focus on the Mars environment, since that

planet is is the one for which a series of landers and probes are planned well into the next century.

Energy management on the Martian surface will be affected by the weather, latitude, and

temperature swings during the day/night cycle. On the other hand, conditions on a comet or

asteroid may provide different challenges. The surface density and composition of a comet are

unknown to the extent that a probe could encounter rock, "sand", or ice.

................. ._ = .... _ ................ =._, . ...... . .......

The environment encountered by Mars landers depends significantly upon the latitude of the

landing and weather conditions, particularly the likelihood of dust storms. Power levels vary from

as low as miiliwatts in a quiescent mode to near I wattfor brief periods for a so-called

4-
microlander/penetrator, up +to100 watts for more _complexrovers_orother science packages.

Surface temperatures will likely range between 150K (nighttime=at70" south latitude) to 300K in

the day time. Low mass, highly compact packaging will be essential for these systems. In

addition, the probe may experience landing conditions ranging from a soft landing to shock

Ioadings up to 10,000 g's for the case of a penetrator afterbody. These conditions might also be

common to asteroid landings.

Surface weather conditions on Mars are of special concern due to the possibilityof blowing dust.

This will affect solar array performance either through obscuration of the sun or deposition of dust

on the surface of the array. Optical depths of 3.6 have been measured by the Viking I lander and

upto 6 are possible in low to middle latitudes,thus reducingthe solarflux by as much as 85

percent. The Mars Pathfinder/Rover missionwill carry a small experiment, supplied by NASA's

Lewis Research Center, to determine the effects of dust on solar cells in the Martian environment

at Ares Vallis(19"N, 32"W).

The use of solar arrays for a cometary lander (Figure 4) would present additional problems. A

proposed lander for the Rosetta missionto the comet 'NVirtanen"would require 1-10 W of power

at about 3.25 AU. The surface temperatures are expected to be 130-150K with power

requirements being sized to recharge batteries during a 15 hour rotation cycle. A comet lander

system powered by a solar array may require that that rendezvous occurfar enough from the sun

such that the comet is relatively inactive to minimize the effects of dust or other debris

contamination. Due to these issues, along with mass and volume constraints, no solar array

system has yet been baselined for a comet lander misssion.

SOLAR ELECTRIC PROPULSION

One of the more exciting applications for interplanetary photovoltaics is solar electric propulsion

(SEP). Combining high power lightweight solar arrays with efficient ion propulsion will provide a

multitude of benefits for future missions. For many missionsthe use of SEP will dramatically

reduce flight times to distant targets. For others,the use of SEP will allow for increases in the

spacecraft science mass. These benef'dsarise from the advantage of SEP in achieving high

spacecraft velocities with a minimum of mass. These advantages are well understood by mission

designers and the first New Millennium (NM) mission is planned to space qualify an SEP stage

suitable for a wide range of small spacecraft missions.

Unlike conventional Earth orbiting spacecraft, where high powered array systems are combined

with heavy battery storage systems, SEP stages will relyonly on PV for propulsion,with no

allocation for thrusting during non-illuminatedperiods. As a result, the solar array can be a

significantfraction of the SEP mass and low mass array technology is directly applicable. SEP

arrays may also serve a dual purpose for outbound missions,for as the array power drops with

increasing solar distance, eventually being insufficientfor propulsion stage operation, it is in most

cases mere than sufficientto power a small spacecraft. In this manner a well designed mission

would utilize a single array for both propulsion and the spacecraft. This is the approach that will be

followed for the first New Millennium flight. For the NM flighttest, it will be criticalto verify that the

solar array is not degraded to any extent by metallic erosion products of the SEP thruster. Since

the thruster operates best at high voltages, the NM array will be operated at approximately 100 V

with down conversion used for the spacecraft (28 V) and up conversion used for the ion thruster

(1000 V). Due to inefficiencies in the conversion process and the high power level required for

the thruster, future efforts will attempt to design direct drive solar arrays that can operate at 1000 V

and directly feed the propulsion stage. At present, these voltage levels have not been

demonstrated with solar arrays and will be a formidable challenge. Concentrator solar arrays that

require fewer cells are the most likely choices for such systems.

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EXAMPLESEPBENEFITS

Anexample of the advantages attributable to the use of SEP is inthe Mercury Orbiter/Lander

mission described previously. Optimum SEP trajectories, based on the performance of xenon ion

=NSTAR" thrusters, with flight times between 600 and 880 days were developed. The spiral

trajectories have flight times equal to or less than those required for multiple Venus and Mercury

gravity assists. Launch opportunities exist approximately every four months. Various SEP system

mass reductions were examined includingan "advanced SEP" design which incorporates _AL"

(anode layer) thrusters.

Unlike the all chemical propulsion design, the maximum landed mass for an SEP-delivered system

occurs for lander deployment from low circular orbits. The reason is that the AV required to

circularize the orbitat Mercury is less than 10% of the total AV required for the mission (for the

chemical mission option, the maximum orbiter and lander masses are delivered for Mercury orbits

which are highly eccentric). In this case, the orbiter propellant savings associated with usinga

high eccentricity orbit do not compensate for the higher lander propellant loads.

Assuming major improvements in high-I,_, high thrust, lightweight chemical propulsion, the

Mercury orbiter/lander mission is extremely challenging and requires the use of low mass

spacecraft systems. Advanced SEP offers the promise of significantly higher delivery masses and

shorter flight times compared with the all chemical option.

CURRENT SPACECRAFT DEVELOPMENTS

Several photovoltaic powered planetary missions are in development or will begin development in

fiscal year 1996. These include the Mars Pathfinderlander and rover mission,the Mars Global

Surveyor orbiting mission, the Mars 1998 orbiter and lander mission,the Near Earth Asteroid

Rendezvous (NEAR) mission, and the first New Millennium flight now planned for an asteroid and

cometary rendezvous. These missions utilize advances in solar array technology that have

emerged in recent years, especially the emergence of GaAs/Ge solar cells. The Applied Physics

Laboratory's NEAR will be the first NASA planetary missionto launch with GaAs/Ge. The NEAR

array will provide 1800 W at I AU and 400 W at 2.2 AU on its trajectoryto the asteroid Eros. In

addition, all of the planned Mars missionswill utilize GaAs/Ge solar cells for the orbiter or lander, or

both, in order to achieve the desired energy balance, especially when packaging has limited

available surface area. This marks a clear transitionto the acceptance of GaAs/Ge solar arrays for

planetary missions.

The Mars Global Surveyor orbiter,to be launched in November of 1996,employs 2-GaAs/Ge and

2-Si panels to provide 667 W of power. It will collect a variety of data on Mars surface and

atmospheric characteristicsover a two year mission. Of particular note is the fact that the arrays will

be utilized for aerobraking to circularize the orbitfollowing insertion. This has resulted in special

design considerations relating to thermal issues. The Mars Pathfinder Mission to be launched in

December of 1996 employs GaAs/Ge for cruise, lander and a 6-wheeled rover vehicles. This

mission is to be the first of a series of missionsto place weather, seismological, and other

monitoring and scientific instruments on the Martian surface. The lander array will provide about

200 W on clear days and the roverarray 16 W. Forthe Mars 1998 mission, now in the early phase

of development, the baseline now includes GaAs/Ge on an orbiter (1440 W), GaAs/Ge on a lander

cruise array (470 W), and Si on a lander array (1440 W, all powers are BOL).

The first New Millennium flight plans to employ a linear concentrator array, supplied by the Ballistic

Missile Defense Organization in partnership with NASA Lewis Research Center, with

GalnP/GaAs/Ge, 2-junction, muiti-bandgapsolar cells. This missionwill demonstrate solar electric

propulsion for the first time in a planetary exploration environment. The array will provide 2.6 kW at

1 AU (BOL) to an NSTAR xenon ion thruster system. Approximately 200 W will be used by the
NewMillenniumspacecraftorengineeringandsciencefunctions_. The combined demonstration

of concentrator array, multi-bandgapcells, and solar electric propulsionwill open a new era in

scientific exploration of deep space.

SUMMARY

We have provided a repesentative sampling of studies that provide a wide range of requirements

for the future use of photovoltaic solar arrays in planetary and deep space scientific missions.

These requirements represent major challenges for solar array technology developers. Inner

planet missionsface Specialthermal and possibly high radiation environments. Missions that

would travel to more than 3 AU or beyond must perform under low intensity, low temperature

conditions, also with the possibility of high radiation environments. A completely different set of

requirements await solar arrays that must be landed on the surfaces of terrestrial planets or small

planetary bodies, where dust and atmospheres, in addition to possible wide temperature cycles

must be taken into account. Finally, requirements for low mess and compact packaging will require

innovative structures and deployment techniques, such as inflatable systems, in order to meet

launch vehicle constraints. Such concepts may also include innovative architectures like a

combined power and telecommunications system using a dployable concentrator/antenna. If

proven feasible, a "power antenna" system might enable a low power photovoltaic power source

to be used at greater distances from the sun while meeting high science telemetry data rates. The

realization of such innovative architectures will, of course, depend heavily on parallel advances in

other spacecraft technologies. Spacecraft pointing, attitude control, thermal control, and fault

tolerance requirements will be key drivers in the power system evolution to come.

EaCh

Balledes

Figure 1. Solar Probe Spacecraft Concept

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2.3m IICA\

ACS/TCM TIII_USTI,;I_ (x t,I )

Figure 2. Kuiper Express Concept

Figure 3. Extra Solar Planetary Search Spacecraft Concept

-8-
e5

lllllltl

lm

Figure 4. Comet Lander Concept

-9-
DEVELOPMENT OF ADVANCED SI AND GA AS SOLAR CELLS FOR INTERPLANETARY MISSIONS

G.Strobl _,P.Uebele _,R.Kem 1,K.Roy _,C.Flores2, R.Campesato 2,C.SignorinP, K.Bogus 3, P.Coz 4

_ASE, Heilbronn Germany; 2CISE spa, Milano Italy; 3ESA/ESTEC, Noordwijk The Netherlands; 4CRSC,

Roma Italy.

Abstract

The deep space and planetary exploration project have been acquiring more and more importance and

some of them are now well established both in ESA and NASA programs.

This paper presents the possibility to utilise both silicon and gallium arsenide solar cells as spacecraft

primary power source for missions far from the Sun, in order to overcome the drawbacks i'elated to the

utilisation of radioisotope thermoelectric generators - such as cost, safety and social acceptance.

The development of solar cells for low illumination intensity and low temperature (LILT) applications is

carded out in Europe by ASE (Germany) and CISE (Italy) in the frame of an ESA programme, aimed to

provide the photovoltaic generators for ROSETI'A: the cometary material investigation mission scheduled

for launch in 2003. The LILT cells development and testing objectives are therefore focused on the

following requirements: insolation intensity as low as 0.03 Solar Constant, low temperature down to -150

C and solar flare proton environment.

At this stage of development, after the completion of the technology verification tests, it has been

demonstrated that suitable technologies are available for the qualification of both silicon and gallium

arsenide cells and both candidates have shown conversion efflciencies over 25 % at an illumination of

0.03 SC and a temperature of -150 C. In particular, when measured at those LILT conditions, the newly

developed "HI-ETA/NR-LILT" silicon solar cells have reached a conversion efficiency of 26.3 %, that is the

highest value ever measured on a single junction solar cell.

A large quantity of both "HI-ETA/NR-LILT " silicon and "GaAs/Ge-LILT" solar cells are presently under

fabrication and they will be submitted to a qualificationtest plan, including radiation exposure, in order to

verify their applicability with respect to the mission requirements. The availability of two valid options will

minimise the risk for the very ambitious scientificproject.

The paper describes how the technical achievements have been possible with Si and GaAs LILT solar

cells (including a comparison between measured and modelled I-V characteristics) and it presents the

technology verificationtests results.

1. Introduction

In the last few years, both in Europe and in U.S.A., considerable resources have been dedicated to the

development of solar cells capable of good performances under low light intansity and low temperature

conditions. The effort is motivated by the increasing scientific interest in space missions far from the sun.

As a matter of fact, interplanetary explorations and cometary investigation projects are well established

both in ESA and NASA scientific programmes.

Although the spacecraft primary power source in deep space could be provided by the Radioisotope

Thermoelectric Generators (R.T.G.), the use of photovoltaic generators would be preferred because of a

number of factors, like cost, safety, social acceptance and, as far as European Missions are concerned,

non availability of R.T.G. technology in Europe.

The R.& D. programme "Solar cells for Low Intensity / Low Temperature (LILT) applications" was initiated

by ESA in 1991 with the general aim of supporting future interplanetary projects and with an initial target

for 0.1 Solar Constant (S.C.) and -100"C. LILT conditions. More recently, the LILT R.& D. programme has

been finalised to a specific mission: ROSETI'A, the ESA project for cometary investigation, due for launch

in 2003. The ROSEI-rA baseline mission foresees 10 years lifetime and two options power requirements

at 5,8 A.U. 260W and 680W respectively. The LILT cell development programme objectives have been

consequently adapted to the ROSEI-I'A LILT conditions, i.e. low intensity equivalent to 0.03 S.C. and

temperature down to -150 C.

The ESA LILT cell R&D. programme consists of the parallel developments of two different technologies,

silicon and gallium arsenide, and it is carded out by the co-operation of two European Companies: ASE,

taking over the leadership of the programme, the silicon cell development and the whole characterisation

work; CISE, executing the gallium arsenide development. Both the two technology developments have

been based on existing space proven cell structures, initially characterised at LILT conditions and then

modified in order to reduce the LILT degradation effects [2,3,4]. Subsequently, a technology verification

test programme has been accomplished on the newly developed Si and Ga As LILT cells and, finally, 150

samples are presently under manufacturing.

-10-
2. Silicon solar cell developmsnt

2.1 HI-ETA/NR-LILT silicon solar cell

The interest in using silicon solar cells to power spacecrafts for interplanetary missions began over three

decades ago. Anyhow, severe fill factor degradation effects under LILT conditions, occurring on a

statistical basis and which could not be assessed from room temperature measurements, prevented

silicon solar cells to be used under these particular conditions so far [1].

With the newly developed HI-E'I'AINR-LILT silicon solar cell, it has been demonstrated for the first time

that the detrimental effects of fill factor deterioration are suppressed, even more that the fill factor is

increasing at low temperature as expected from Shockley's diode theory.

The HI-ETAINR-LILT silicon solar cell structure, its LILT performance, fundamental LILT characterisation

and first results obtained during technology verification testing have already been presented in previous

papers [2,3,4]. Here a review of the main achievements will be given, recent results of electron irradiation

test with HI-ETA/NR-LILT solar cells will be presented and another inherent design feature of HI-ETA/NRLILT solar cells, namely an integrated Zener diode (IZD) for by-pass shunting will be discussed.

The HI-ETA/NR-LILT silicon solar cell has all the characteristic features of the standard HI-ETA

technology, such as 10 Qcm/CZ base material, n*lpp ÷ structure, front and rear side oxide passivation, fine

grid line pattern defined by photolithography, double layer antireflection coating, AI rear side reflector and

space proven TiPdAg contact system. In addition, the HI-ETA/NR-LILT silicon solar cell has a nonreflective front surface realised by inverted pyramids for a high current output, it is equipped both with a

planar structure including a p*- guard-ring channelstopper for eliminating edge channel currents and with

heavy n*÷ - diffusion under the front contact grid for reducing diode loss currents caused by filamentary

paths under the front contact metal. By these measures a high efficiency is achieved and the diode loss

currents under LILT conditions are reduced resulting in no fill factor degradation.

Although the structure of the HI-ETA/NR-LILT silicon solar cell is more complicated than standard HI-ETA

cells, it can be fabricated in a standard HI-ETA silicon solar cell production line.

The illuminated I-V characteristics of bare HI-ETA/NR-LILT silicon solar cells as measured under an

insolation intensity of 0.11 SC and 0.037 SC in the temperature range of +25°C to - 150°C are displayed

in Figure 1. The measurements have been performed in a vacuum chamber with quartz window, where

the samples could be cooled by liquid nitrogen and the insolation intensity of the solar simulator could be

adjusted by a grey filter and the lamp current.

As can be realised from Figure 1, the current is decreasing at low temperature for both intensities due to

an increase of energy gap F-.o and a reduction in minority carrier lifetime • [2]. The voltage strongly

increases at low temperature due to a reduction of the diode saturation current Jo which is varying with the

square of the intrinsic carder density n, [5]. The most remarkable result is the fill factor behaviour at

extreme LILT conditions. Even for an insolation intensity of only 0.037 SC and temperatures as low as -

150°C with its very low photogenerated current there exists no fill factor degradation, since the diode loss

currents are reduced to very low levels. Instead the fill factor is increasing at low temperature following

Shockley's diode theory due to a reduction of the thermal voltage Ur=kT in the exponential function of the

diode equation [5].

The dark forward J-V characteristics of HI-ETA/NR-LILT and standard HI-ETA solar cells are displayed in

Figure 2 and 3 as measured for varying temperatures between +25°C and - 175°C and fitted with the

well-known two diode model [2]. The two-diode model is consisting of two diodes in parallel with one

following ideal Shockley's behaviour and the other taking into account recombination/generation currents

within the space charge depletion region. From Figure 2 and 3 it can be realised that in case of HIETA/NR-LILT silicon solar cells the measured curves are very close to the theoretical ones, whereas for

standard HI-ETA solar cells there are strong deviations especially in the low current, low temperature

region. These deviations of measured dark J-V characteristics from modelled ones are responsible of the

fill factor deterioration of standard HI-ETA solar cells under LILT conditions and have already been

observed in previous work [6] ("flat spot" effect).

2.2 Intearated Zener Diode (IZD)

From standard silicon space solar cells it is known that in reverse bias conditions, e.g. in partially

shadowed arrays, the solar cell may fail due to a non-reversible localised break-through of the pnojunction

for voltages higher than 20V. This effect is well-known as the hot-spot phenomenon. Highly doped

junctions, however, such as the n'/p ÷ (emitter/channelstopper) junction, exhibit a relatively large leakage

current under low reverse bias voltages: this results in the reversible Zener break-through of HI-ETA/NRLILT solar cells, displayed in Rgure 4 as measured in dark and under AM0 illumination for temperatures

varying from +25°C to -150°C. From the temperature behaviour of the Zener voltage Uz, which is in the

range of 6 - 8 V, it can be deduced that avalanche break through is the responsible mechanism. Thus the

-1 1-
behaviourof HI-ETA/NR-LILTsiliconsolarcellsin reversebiasis characterisedbyanintegratedZener

diode(IZD)forby-passshunting.TheequivalentcircuitcanbeinspectedinFigure5.

3. Ga As so!ar cell development

3.1. LILT GaAs solar cell _tructure

The GaAs solar cells were grown utilizing a MOCVD system working at a very low pressure (20 mbar) in

order to improve the gas velocity and therefore the layer uniformity and to reduce the gas consumption.

The sources for Ga, As and AI are trimethylgallium, arsine and trimethylalluminium. The dopants are

silane and dimetylzinc. The basic structure of a GaAs LILT solar cell is illustrated in Figure 6.

The epitaxial GaAs layers can be grown on GaAs or on Ge substrates.

A GaAs LILT solar cell differs from a standard GaAs solar cell for these characteristics:

- very low doping level in the cell base

- high doping level in the cap layer

- antireflection coating deposition

- introductionof a i-layer into the junction

The impact of these items on the LILT performances will be explained in the following.

Low .dopin.q level in the base. The short circuit current of a solar cell decreases at low temperature

because of the energy gap increasing. The temperature coefficient of the short circuit current is affected

by the temperature behaviour of the hole diffusion length in the base region. As demonstrated [7], the

temperature behaviour of the diffusion length depends on the doping level, in particular at low doping level

the hole diffusion length increases as the temperature decreases. Therefore a doping level of 2x1018cm3

was chosen for the base of LILT solar cells.

High dopin.q level of the cap. The dominant mechanism for the ohmic conduction between GaAs and

metal depends on the doping level of the semiconductor. In general there are three mechanisms of

conduction: a) thermoionic emission (TE), b) thermoionic field emission (TFE), c) field emission (FE). The

first two mechanisms are temperature dependent and at low temperature present a high contact

resistance value. The FE mechanism is tunneling dominated and then temperature independent. In order

to obtain a good contact for a LILT solar cell it is recommended that the FE mechanism dominates, then a

high doping level of the cap layer is requested. For LILT solar cells a doping of 2x1019cm3 was selected

for the cap layer.

ARC deposition. The antireflection coating deposition technique affects the dark currents. The electron

beam evaporation forces the tunneling and the generation-recombination currents to increase and lower

open circuit voltage and FF were measured at low temperature and low intensity conditions. For these

reasons, thermal evaporated coating (ZnS/MgF2) was used for preliminary samples. This double

antireflection coating is optimal for a bare GaAs solar cell operating at low temperature because of its low

reflection in the UV part of the spectrum (in fact at low temperature the energy gap increases and shifts

the spectral response towards higher energies).

i-laver. The dark currents of a GaAs solar cell at low temperature are affected by the tunneling of carders

through impurity states in the space charge region [2]. In order to limit the tunneling current, an undoped

i-layer was introduced into the junction. The spacer thickness was varied from 12 nm to 35 nm. The effect

of the thinner spacer was negligible because of the Zn diffusion during p-type layer growth. GaAs LILT

solar cells were characterized in low temperature, low intensity conditions. Table I reports the

performances in AM0, 0.11 - 0.03 suns and 25#150 C for different GaAs samples with and without spacer

in the p-n junction. The cells with spacer have higher FF at low temperature and low intensity. Thus this

cell type is the most suitable one for LILT applications.

From Table 1, it is possible to notice that GaAs/GaAs cells with 35 nm spacer exhibit very good LILT

performances, an efficiency of 25.4% was reached at -150 C, 0.11 suns and an efficiency of 24.7% was

reached at -150 C, 0.03 suns.

In order to understand the FF temperature behaviour and to estrapolate GaAs solar cells behaviour at

LILT conditions, dark measurements at low temperature are necessary.

The experimental dark I-V curves of different GaAs samples were recorded and fitted using a triple diode

model developed by CISE [4 ]; the introduction of the i-layer lowers the tunneling saturation current of one

order of magnitude.

-12-
3.2 (_gmparison between GaAs/GaAs and GaAs/Ge LILT cells

Using the MOCVD technique it is possible to grow the GaAs epitaxial layers on GaAs or Ge substrates.

Basically there are no differences in the performances of a LILT GaAs cell grown on GaAs or Ge

substrate even if we expect for GaAslGe cells a conversion efficiency a little bit smaller than for the pure

GaAs cell. The main advantages of growing on Ge are:

- lower cell cost;

- higher mechanical strength;

- lower thickness with the possibility to decrease it down to 100 urn;

- lightweight solar cells, as a consequence of low thickness.

The main difficulty to transfer the LILT structure on a Ge substrate is to obtain a passive GaAs/Ge solar

cell. The passivity means that the Ge substrate does present neither a heterojunction with GaAs nor a

diffused junction into Ge due to the different diffusion coefficient and solubility of Ga and As in the Ge.

In fact, as demostrated in [8], an active GaAs/Ge solar cell is not suitable for LILT application because of

the difference between temperature coefficients of Isc for the GaAs and the Ge junctions. Moreover an

active GaAs/Ge cell shows a knee in the I-V curve under sunlight because of the imperfect matching of

current between the two junctions.

From LILT characterization it is possible to determine if a GaAs/Ge cell is active or passive by means of

the open circuit voltage temperature coefficient. In fact while for passive solar cells the Voc temperature

coefficient is equal to that of GaAslGaAs cell (1.9-2.0 mV/C), for active GaAs/Ge solar cell the Voc

temperature coefficient exceeds 2.2 mV/C. The Ge contribution to the open circuit temperature coefficient,

in an active cell, is quite variable and depends on the dark mechanism that dominates the dark I-V curve

of the Ge junction.

The analysis of the voltage temperature coefficient is not a very precise method to decide if a cell is active

or passive. In fact, according to our experience, if the Ge extravoltage is in the range of 10-30 mV, the

temperature coefficient could be similar to that of a pure GaAs cell. For this category of cells the best way

to decide the existence of a Ge P/N junction is to measure the cell voltage under a solar simulator, at the

same current, using both a neutral filter and a short-wavelength pass filter.

Some high efficiency GaAs/Ge LILT solar cells were produced and measured. Table II reports the low

temperature, low intensity characteristics of a GaAs/Ge solar cells. The efficiency of this cell in standard

conditions was 20.9 %, AM0, the cell area is 2x4 cm2. At low temperature (-150 C), 0.11 suns, the

efficiency of this cell was 24.5 %. The passivity of the cell was verified by means of the open circuit

voltage temperature coefficient that was similar to that of GaAs/GaAs solar cells, and by means of filters.

Some dark I-V curves of GaAs/Ge solar cells have been recorded. Rgure 7 shows a comparison between

dark I-V curves at room temperature and low temperature for GaAs/GaAs and GaAs/Ge solar cells. At

low voltages, a strong contribution of the dark current is shown in GaAs/Ge solar cells while it is not

noticeable in GaAslGaAs solar cells. This contribution seems to be related to shunt paths across the

junction. The shunt paths can be induced by dislocations originating at the cell edge during the dicing saw

cutting process, while the GaAs cells are usually cleaved after scribing. A mesa etch before cell cutting

could be the best solution to this problem.

3.3 Correlation between room and low temperature measurements

rl_e electrical characterization of illuminated parameters of GaAs solar cells in low temperature, low

intensity conditions, needs quite sophisticated apparatus to be made and it is time-consuming.

Then it is advisable to completely characterize only few samples under LILT conditions and identify a

correlation method between room temperature and LILT performances.

In fact it is possible to predict the behaviour at low temperature and low intensity of short circuit current

and open circuit voltage of a given sample, when the illuminated I-V performances and the dark curve at

room condition are known.

In fact Isc is linearly depending on the solar intensity and the temperature coefficient is well established.

From the dark curve it is possible to estimate the Voc at low intensity, and then applying the temperature

coefficient for the open circuit voltage it is possible to predict the low temperature behaviour.

The estimation of FF and efficiency of the sample is much more complicated because the temperature

coefficient of the FF differs from one sample to another. In order to have a preliminary evaluation of the

LILT performances of a GaAs solar cell, the room temperature performances and the dark I-V curve at

300 K and 123 K are needed.

A computer code was developed that extrapolates the illuminated performances from the dark curves

measurements.

-13-
Table III provides with a comparison between the LILT performances (directly measured under the solar

simulator) and the values extrapolated from the dark curves for a GaAs LILT sample.

4 Technolpav verification testinq

In parallel with the cell technology development work a wide range of testing activities has been

performed with both 40 HI-ETA/NR-LILT and 40 GaAs-LILT solar cells and solar cell assemblies (SCA's)

including front interconnector adherence, rear interconnector adherence, extended storage simulation,

antireflection coating adherence, BOL performance and EOL performance measurements[4]. 1 MeV

electron irradiations with fluences ranging from 1013e/cm2 to 1015eI have been performed at the Interfaculty Reactor Institute of the Technical University in Delft/The Netherlands

In this paper the EOL performance data of HI-ETA/NR-LILT solar cell assemblies are presented in detail.

The front and rear side silver interconnectors have been welded by using the parallel gap welding process

and for glassing 100pm thick CMX coverglasses have been used.. Before electron irradiation, the HIETA/NR-LILT SCAs have been measured under low intensity illumination at room temperature,

subsequently the 1 MeV electron irradiation has been carded out at room temperature and after electron

irradiation the cells have been measured again under low intensity illumination at both room temperature

and low temperature. The degradation data for the different electron irradiation dosages have been

determined from different HI-ETA/NR-LILT SCAs of the same structure and the absolute low temperature

EOL data of one SCA have been related with the absolute low temperature BOL data of another SCA in

order to calculate the relative degradation behaviour.

In F_gures 8 to 11 the relative degradation data of HI-ETNNR-LILT SCAs measured during this electron

irradiation experiment are displayed including absolute BOL values for open-circuit-voltage (Voc), shortcircuit-current density (Jsc), maximum power density (P_=), fill factor (FF) as measured under an

insolation intensity of 0.11 AM0 and temperatures ranging from +25°C to -175°C.

Five LILT Ga As solar cells (three GaAs/GaAs and two GaAs/Ge) were dressed with silver plated

molybdenum interconnectors and 150 um thick coverglasses and submitted to electron irradiation at Delft

University. The cells were measured at LILT conditions before and after irradiation.

The electrical tests, carried out after irradiation, showed some incongruities, probably because of difficult

measurement accuracy at very low intensities.

A new batch of Ga As samples has been submitted to electron irradiation and sent to Spasolab laboratory

(Madrid, Spain) for electrical performance testing: the discussion on Ga As solar cell radiation hardness at

LILT conditions shall be based on those measurement results, expected in November '95.

5. Conclusions and future work

The ESA R. & D. programme "Solar cells for Low Intensity / Low Temperature applications" is coming to a

conclusion: suitable technologies are available for the qualification of both silicon and gallium arsenide (on

germanium) LILT solar cells, accordingto the ROSETTA mission requirements.

The availability of two technologies will minimize the riskfor the project.

With the newly developed HI-ETA/NR-LILT silicon solar cells it has been demonstrated for the first time

that the fill factor deterioration which prevented silicon solar cells to be used under these conditions so far

can be suppressed. In future work emphasis will be paid on raising the current especially at low

temperature operation.

The LILT GaAs solar cell structures were optimized both on GaAs and Ge substrates using a small scale

MOCVD Aixtron reactor.According to :the experimental results, the highest efficiency can be obtained at

LILT conditions with GaAs solar cells grown on GaAs substrates. The GaAs/Ge structure seems to be

slightly affected by the lattice mismatch between the substrate and the grown layers.A test procedure was

proposed to predict the LILT behaviour of GaAs solar cells avoiding time-consuming and expensive low

temperature measurements.

100 HI-ETA/NR-LILT silicon solar cells and 50 LILT GaAs solar cells are being produced and will be ready

by December 1995; a dedicated test plan has been defined, including: LILT electron and proton

irradiation, I-V measurements at high intensities, LILT electrical performance measurements on large

number of samples for statistical assessment.

Low temperature electron irradiation expedments have been set up, including in-situ electrical

measurements without any warming-up annealing and, on this purpose, dedicated test facilities have been

developed at the University of Paris.

The final part of the development and the additional work needed to achieve the qualification are expected

to be carried out in the frame of the ROSET-I'A project.

-14-
TABLE h LILT performances of bare GaAs/GaAs solar cells

Isample: moc88b

without spacer

AM0 performances Isc = 260.5 mA FF = .81

1 sun, 25 C Voc = 1037 mV Eft. = 19.8 %

s.c.= 0.1092 s.c.= 0.0299

25 9261 28.45! 0.736, 16.40 847, 7.81 0.740 15.11

0 983' 28' 0.779' 18.15 9131 7.691 0.7841 17.01

-50 1084! 27.6 _ 0.805 ;, 20.37 1017] 7.6 ! 0.815 19.46

-100 27.3 I, 0.841 _ 22.95 1132! 7.5' 0.830 21.771

-130

-150

1182 !

i

1237' 26.9_ 0.839 i 23.63

1272 i 26.65, 0.842 24.14

1193 _ 7.38! 0.825 22.44!

I

1218; 7.271' 0.8301 22.71

d19a IAM0 performances Isc = 264.7 mA FF = .81

spacer 30 sec I1 sun, 25 C Voc = 1039 mV Eft. =20.2 %

s.c.= 0.1124 | _ s.c.= 0.0327

25 9511 29.7 i 0.7551 17.59 8671 8 64 0.7741 16.74

0 10001 29.25' 0.788 r 19.02 9421 8.581 0.7901 18.43

-50 11101 28.85! 0.829i 21.91 10421 8.5 p 0.815! 20.83

-100 12111 28.31 0.847 _, 23.93 11471 8.43_ 0.834 23.29

-130 12691 28.1_ 0.8481 24.93 1210 8.28; 0.831, 24.04

-150 1300', 27.8 0.851! 25.37 1251 8.13 0.833 24.46

sample: d20A2 IAM0 performances lsc = 263 mA FF = .81

spacer 30 sec 11sun, 25 C Voc = 1036 mV Eft. =20.1%

2O

-5(

-100

-130

-150

l s.c.= 0.1114

9621 29.3' 0.764' 17.92

10011 28 8 !. 0.793 ! 19.03

1111', 28.61 0.8131 21.51

1216 28.51- 0 8471 24.44

1256 28, 0.842 24.65

1286 27.5' 0.852 25.08

s.c.= 0.0319

868! 8.4, 0.759_ 16.03

926 6.26! 0.7891 1753I

1038! 8.17 0.828i 20 331

1149! 8.08; 0 8551 22.99 !

1216! 8.04 _, 0.861 24.33_

1254 '_ 7.941 0.856 24.701

-15-
TABLE I1: GaAs/Ge solar cell LILT characteristics

Sample: GaAs/Ge C85A2 with ARC

Performances AM0, 1 SC, 25 C • I_ = 264 mA V_= 1079 mV FF=.79 Eft.= 20.9

iii:.i::i_ii ilii__ii!iliiiiiiiiiiil iiii! i i ii_._i.. _.i_ INSO_TION := :0_:11 SC; i i!:ii! iii_ili _iii_i ii!i!_ii!i!i:_ !_ iiii:iii.liii i_::i;iil ;ii_i_ii iii_, iii_iii_iil_: :,iii!ii_:i ii:_:__:_:

T

[c]

25

Voc

[mV]

975

Isc

[rnA]

28.8

Vm

[mV]

826

Im

[mA]

26.5

FF

0.78

dl/dT

[uA/C]

0 1023 28.5 865 26.3 0.78 12.0

-50 1126 28.0 976 26.1 0.81 10.0

-100 1223 27.7 1064 26.0 0.82 6.0

-130 1278 26.9 1128 25.0 0.82 26.7

-150 1309 26.9 1141 25.3 0.82

Eft dV/dT

[%] [mV/C]

18.6

19.3 -1.92

21.6 -2.06

23.4 -1.94

23.8 -1.83

24.5 -1.72 0.0

i_iJ_!!_`i_`ii__i_:_i_i_:_i_i_ii_ii_i_i_i_ii_;iiiii_ii!!_iiiii_!!i!!!_ii! INSOLATION

Voc

[mV]

9O5

953

ISC

[mA]

9.3

9.1

Vm

[mY]

745

803

Im

[mA]

8.5

8.3

= 0.03 SC

FF

0.75

Eff

E%]

16.7

17.6

dV/dT

[mY/C]

-2.18

dl/dT

[uA/C]

9.1

22

0.77

-50 1051 9.0 901 8.2 0.78 19.6 -1.98 2.0

-100 1145 8.9 982 8.2 0.79 21.3 -1.88 2.0

-130 0.80

0.80

1042

1065

1201 22.3

22.5

8,8

8.7

-1.87

-1.74

8.1

8.0

3.3

-150 1232 5.0

TABLE III " Comparison of the predicted and measured values of a GaAs LILT solar cell

T(K)

295

223

123

295

223

123

i ! ! ! !!i iii i iiii i ! iiiiii ii iiiiiiii!ii! i i ii!iiii i!iii iiiiiii iiiiiiiii ii iiiiiiiiiiii!i iiiii 

v=(mv)

951

1110

1300

l=(mA)

29.7

28.8

27.8

FF

.76

.83

.85

Eft(%)

17.6

21.9

25.4

V=(mV)

954

1107

1306

I=(mA)

29.7

28.8

27.8

FF

.76

.82

.85

Eft(%)

17.8

21.6

25.4

.77 16.7

.82 20.8

874

1042

1255

8.64

8.5

8.1

.76

.82

.83

16.2

20.5

24.1

867 8.64

1042 8.5

1251 8.1 .83 24.1

-1 6-
6

IHI-ETA/NR-ULTSIIIc0n SolarCell

S

E 4

v) 3

_2

_2_c \-_o.c",,:loo-c"_-lso,c

0.11 +25 583 5.42 78.2 15_9J 1

0.11 -150 934 4.70 92.1 27J_ _t

0.037-150 920 127 915 26.3] i

i

',' ii .i I .

1oo :_o _oo 4o0 soo 6oo 7oo aoo 9oo _ooo

Voltage rnV

Fiqure 1 Illuminated l-V characteristics of bare HI-ETA/NR-LILT silicon solar cells (3.78cm*6.19cm) as

measured under an insolation intensity of 0.11SC and 0.037SC in the temperature range of +25°C and -

150°C.

loo 10-1

10-2

_ io-3

"_ 10 "4

1o°

1o, -

(_ +25°C /" 2" / ,

," " modelled ....

'°"o 0.2 04 o.s 0.'8 _ .2

Voltage V

Ficlure 2: Measured dark J-V characteristics of HI-ETA/NR-LILT silicon solar cell (3.78cm*6.19cm) as a

function of temperature with theoretical fit curves of double diode modelling.

10 0

10-I

10-2

10._

_10-4

10 -s

10-6

10 7

10 "e

0 0.2 0.4 0.6 0.8

Voltage V

measured --

modelled ....

.2

Ficlure 3: Measured dark J-V characteristics of standard HI-ETA silicon solar cell (3.78cm.6.19cm) as a

function of temperature with theoretical fit curves of double diode modelling.

-1 7-
0.0

-0.2

-0.4

< -0.6

-1.0 -0.8

-1.2

-1.6 HI_rr/VNR-LILT SILICONSOLARCELLS -7 ) -1 -2 -3 -4 -5 -6

Voltage V

Fiaure 4: Dark reverse I-V characteristics of HI-ETA/NR-LILT silicon solar cells as measured in dark and

under AM0 illumination for temperatures varying between +25°C and -150°C.

1 gen Z

Rs

Nn+ emitter

p base

Rp

n+

emitter

p+ cha_nr e_stopper

I

Fiqure 5: Equivalent circuit for I-V characteristics of HI-ETA/NR-LILT silicon solar

Zener diode (IZD) for by-pass shunting.

cell with integrated

l_we G=._ =-

buJ_er Ge_ n+

subeU_te GaAs or Ge ss+

Fiqure 6: Ga As LILT solar cell structure.

-1 8-
Fiqure 7:

+ ou( ...._

, ooE,..c3

• ooE.¢:

• O0E..o.

• ooE,-0o

1

00_

000_

00_O_

_00_tll

."_C I0¢ _00 Y¢_ "00C ,;_¢ .4¢O

•

,,Itq)e tmv_

6..' _//P F .... ,_>

• j_' c. .... .,-....

• ,MRII t.,_

Dark I-V curves for GaAs/Ge and GaAs/GaAs solar cells.

" " _. ¢

F t'-*-..,c _+,,v I

_ t_ _C B_OT_OmVI

o.7t- l--._--._p,,= _o.m_v !

11 _ I-'+---'_'c _x_0'.Vl

081 _._t . . . _ .... , . . J .... z . .

0 1101n $ 10 's S fO _ S 10 Is

I MeV_irr_doeeetr_omt_oe'eu_e le/c_l

Fiqure 8: Degradation of open-circuit-voltage Voc of HI-ETA-NR/LILT SCAs (3.78cm.6.19cm) with 100pm

thick CMX coverglasses under LILT conditions after 1MeV electron irradiation at room temperature•

C;

I 0,11

15

0.11

0,7

0_0 ._...,_....+: i - . I .... I , . . I, ,,,l . . . i .... z . . . i...

10 rl S _O tl S 1074 S I0 +1 S 10 _ll

1 MeV ek)c_l InlldialJ<:_ _ It room tempe_l_Jre Iok:m'_

Fiqure 9: Degradation of short-circuit-current density Jsc of HI-ETA-NR/LIL SCAs (3.78cm.6.19cm) with

100pm thick CMX coverglasses under LILT conditions after 1MeV electron irradiation at room temperature

-19-
Fiqure 10: Degradation of maximum power density Pm_xof HI-ETA-NR/LILT SCAs (3.78cm.6.19cm) with

100/Jm thick CMX coverglasses under LILT conditions after 1MeV electron irradiation at room temperature

1.0

|-

u

I0_ I I0_I I 10W I 1¢1a= 6 1_

I MeV eleelnm IrmdlI_n d_e ,,_morn timp_,lum le-/em_

Fi.qure11 Degradation of fillfactor FF of HI-ETA-NR/LILT SCAs (3.78cm.6.19cm) with 100pm thick CMX

coverglasses under LILT conditions after 1MeV electron irradiation at room temperature

References

[1] P.M.Stella, F.S.Pool, M.A.Nicolet, P.A.Iles, ,PV technology for low intensity, low temperature (LILT)

applications", 1= WCPEC - World conference on photovoltaic energy conversion (1994)

[2] G.Strobl, C.Flores, P.Uebele, R.Kern, R.Campesato, K.Roy, M.Marteila, K.Bogus, Silicon and gallium

arsenide solar cells for deep space missions", Proceedings of ESPC-93 (ESA-WPP-054, European Space

Power Conference, p.603-610)

[3] G.Strobl, P.Uebele, R.Kem, K.Roy, C.Rores, R.Campesato, C.Signorini, K.Bogus, Silicon and

gallium arsenide solar cells for low intensity, low temperature operation", Proceedings of 1= WCPEC -

World conference on photovoltaic energy conversion Vo1.2,(1994), p.2124.

[4] G.Strobl, P.Uebele, R.Kem, tC.Roy, R.Campesato, C.Rores, C.Signorini, K.Bogus, High efficiency Si

and GaAs solar cells for LILT applications", Proceedings of ESPC - European Space Power Conference,

(1995), pp.471-477.

[5] H.J.Hovel, Semiconductors and Semimetals Vol.ll, ed. by Willardson and Beer Solar Cells, Academic

Press 1975

[6] V.G.Weizer, J.D.Broder, On the cause of the flat-spot phenomenon observed in silicon solar cells at

low temperature and low intensities", J.Appl. Phys.,Vol.53,No.8, (1982), p.5926

[7] R.Campesato, C.Rores, "Ga As solar ceils for low temperature, low intensity applications",

Proceedings of ESPC- European Space Power Conference, (1991), p.759.

[8] R.Campesato, C.Fiores, "Effects of low temperature and intensity on Ga As and Ga As / Ge solar

cells", IEEE Transaction on Electron Devices, Vol.38, n.6, (June 1991).

-20-
NAVAL RESEARCH LABORATORY'S PROGRAMS IN ADVANCED INDIUM PHOSPHIDE

SOLAR CELL DEVELOPMENT

Geoffrey P. Summers

Naval Research Laboratory, Code 6615, Washington, DC 20375, and

Department of Physics, University of Maryland Baltimore County, Baltimore, MD 21228

INTRODUCTION

The Naval Research Laboratory (NRL) has been involved in the development of solar

cells for space applications since the 1960s. It quickly became apparent in this work that

radiation damage caused to solar cells by electrons and protons trapped by the earth's magnetic

field would seriously degrade the power output of photovoltaic arrays in extended missions.

Techniques were therefore developed to harden the cells by shielding them with coverglass, etc.

Ultimately, however, there is a limit to such approaches, which is determined by the radiation

response of the semiconductor material employed. A desire for high efficiency and radiation

resistance led to the development of alternative cell technologies such as GaAs, which has since

become the technology of choice for many applications. InP cells are currently the most

radiation resistant, high efficiency, planar cells known.

NRL first sponsored InP solar cell technology in 1986, when Arizona State University

was contracted to grow p/n cells by liquid phase epitaxy. NRL's interest in InP cells was

generated by the results presented by Yamaguchi and his co-workers in the early 1980s on the

remarkable radiation resistance of cells grown by diffusion of S into Zn doped p-type InP

substrates. These cells also had beginning of life (BOL) efficiencies ~16%(AM0). Related to

the radiation resistance of the cells was the fact that radiation-induced damage could be optically

annealed by sunlight. Relatively large quantities of lx2 cm2 diffused junction cells were made

and were used on the MUSES-A and the EXOS-D satellites. These cells were also available in the

U.S. through NIMCO, and were studied at NRL and elsewhere. Workers at NASA Lewis became

involved in research in InP cells about the same time as NRL.

SINGLE JUNCTION AND TANDEM CELLS GROWN ON InP SUBSTRATES

In 1987 a high level Navy sponsor became interested in InP cells, which led to a one

year contract from NRL to Spire Corporation in 1988-89. The goals of this program were to

produce large area (2x2 cm2) cells with BOL efficiencies >16%. The cells were to have space

qualified contacts and antireflective (AR) coatings and to show a radiation resistance better than

both single crystal GaAs and Si cells. The shallow homojunction technology which was developed

in this program enabled cells to be made with AMO efficiencies >19%. More than 300 cells

were eventually produced, many of which have been flown on space experiments such as PASP

PLUS on the APEX satellite. Thes_ tests have confirmed the high radiation resistance of InP

cells. NRL has published widely on the radiation response of these cells and also on radiationinduced defect levels detected by DLTS and other techniques.

During the late 1980s NRL began sending small amounts of funding to SERI (now NREL)

for the development of tandem junction cells made by epitaxially growing an InP top cell

(Eg~1.32 eV) on a lattice matched GalnAs bottom cell (Eg~0.73 eV). The cell was grown in both

3- and 2-terminal versions, with the latter requiring a connecting tunnel junction. This early

work led in 1991 to a three year contract with NREL with the goals of producing large area

(2x2 cm2), 2-terminal ceils with high BOL efficiencies and with EOI_ efficiencies greater than

-21-
epitaxiallygrown, singlejunctionInP cells. Duringthe courseof this program,severalcells

withAM0efficiencies>22%wereproduced,but the funding ran out before the grid design and

the AR coatings could be fully optimized. It was thought that with such optimization efficiencies

close to 25% could have been readily achieved. In a 2-terminal tandem junction cell the

photocurrent of both subcells has to be matched. Although this can be achieved relatively easily

at BOL, superior radiation response for the cell requires that the current matching be

maintained as both subcells degrade individually. Research at NRL showed that current

matching in the suboells could be maintained to higher radiation levels by reducing the base

doping of the GalnAs cell below the initial levels used (~5x1017 cm3). Data were taken in these

studies which will enable InP-GalnAs tandem cells to be grown in the future that can maintain

current matching under AM0 light up to 1 MeV electron fluences >101s cm-2.

Five 2x2 cm2 tandem cells were provided to the U.K. DRA for the STRV l b solar cell

experiment and two of these cells were incorporated on the satellite. These were the only 2-

terminal, tandem cells on the STRV experiment.

DEVELOPMENT OF CELLS GROWN ON Si and Ge SUBSTRATES

Both the single junction and the tandem junction cell development were very successful

as research programs. However, it was realised that the high cost and relative brittleness of

InP wafers meant that if inP cell technology were to become a viable space power source, the

superior radiation resistance of InP would have to be combined with a cheaper and more robust

substrate. The main technical challenge to this approach was to overcome the effect of the

dislocations produced by the lattice mismatch at the interface of the two materials. Building on

the success of the initial program, NRL secured funding from ONR to support a Phase 1 and 2

effort at Spire to produce InP on Si cells. The goals of this program were to produce cells as

large as 2x4 cm2 with high BOL efficiencies and with EOL efficiencies comparable to InP

homoepitaxial cells. Both cell polarities were investigated and several schemes for alleviating

the ~8% lattice mismatch between inP and Si have been tried, including the use of GalnAs and

GalnP grading layers. The best efficiencies to date (~13% on a 2x4 cm2 cell) were achieved

with n/p cells. It is hope to continue this cell development into a Phase 3, with the goals of

increasing the BOL efficiency and fabricating more than 400 2x4 cm2 cells for assembly into

two small power panels for the STRV 1 c/d satellite for launch in 1998.

As of the Phase 2 program at Spire, ONR/NRL sponsored a study with an independent

contractor to evaluate the potential commercial market for InP/Si cells into the next decade.

This study was completed in July of this year. The main conclusions of this study, which was

made by Booz Allen & Hamilton, were that a niche market exists for InP/Si technology for

missions that operate in high radiation environments. These would include several of the

proposed global satellite communication systems such as Ellipso and Odyssey. Based on current

and projected systems in these orbits, the addressable market for inP/Si technology by the year

2007 was estimated to be 15 kW per year. Outside the niche market the study found that InP/Si

technology would have comparable performance and cost to GaAs/Ge.

In June of this year, NRL negotiated a contract with Applied Solar Energy Corporation as

prime contractor, and with RTI and NREL as subcontractors, to develop a 2-terminal, InPGalnAs tandem junction cell on a robust substrate. The goals of this program are essentially to

produce a cell that substantially exceeds the efficiency of the InP/Si cell at all particle fluences.

It has ben decided that a Ge substrate will be used initially, although the possibility of

eventually employing a Si substrate is included in the program. Because of the problem of autodoping, the p/n polarity is preferable from an epitaxy viewpoint and one of the early successes

in this program is that by carefully controlling the diffusion of Zn in a relatively thick p-type

-22-
emitter,RTI have producedsinglejunctionp/n InPcells with efficiencies>16%. It is thought

that p/n cells can be grown eventually with efficiencies close to the best achieved in nip cells.

The proton response of InP and GalnAs single junction cells is the same for both pin and n/p

polarities, so there is much flexibility for the best ultimate cell geometry. It is hoped that this

development program will last several years.

REMAINING CHALLENGES

Although NRL's InP cell development has been very successful, there are several

technical challenges still remaining. The most elusive of these has been that the ready optical

annealing of radiation-induced damage observed in diffused junction InP cells has never been

fully reproduced in cells grown by epitaxy, even In cell structures that appear to be identical.

Secondly, efforts to minimize the effects of dislocations produced in the heteroepitaxy of InP on

Si and GalnAs on Ge cells have still not yet been fully explored. This problem needs to be tackled

both by reducing the number of dislocations produced and by reducing the electrical effect of

those remaining. The task of SCaling up the InP/Si cell to much larger dimensions also still

remains to be attempted, but there seems no reason in principle why cells as large as 8x8 cm2

could not be grown if required. Finally, of course, cell structures have to be developed that can

be produced in large quantities at a competitive cost.

-23-
The Defence Research Agency Photovoitaic Programme

C. Goodbody and R. Kimber

Defence Research Agency

Space Department

Farnborough

Hants, GU14 6TD

UK.

INTRODUCTION

The current DRA photovoltaic programme is funded mainly by the UK Ministry of Defence. The

programme is aimed at research and development into the performance of new solar cells and array

concepts. The core of the programme consists of studies to determine the potential benefits and

disadvantages of using the different solar cell/array combinations on future MoD spacecraft. The main

areas of interest are cost, mass, volume, lifetime, radiation hardness, area, reliability and when the

relevant technologies are likely to reach maturity. The programme addresses two timeframes, 5-10

years and 10-15 years. This is backed up by an extensive programme of cell characterisation and

environmental testing to provide data for the studies. When the opportunity arises flight experiments

are conducted to verify the results from the ground testing and to demonstrate the performance in the

real environment with all its synergistic effects.

SOLAR ARRAYS

The initial trade-off studies have looked at three typical missions, 3 and 6 kW GEO communications

satellites and an 8.7 kW LEO remote sensing spacecraft, all powers being end of life. They have

considered the impact of a range of cell types on conventional rigid arrays eg. the Fokker ARA and

Aerospatiale GSR3 and on the conventional flexible array, eg. the TRW EOS and Spar Olympus. The

6 kW study has been reported in reference 1.

The LEO study involved the sizing for a single wing remote sensing spacecraft which would

generate 8.7kW after 5 years. Six cell types were involved in the study: Si BSR, Si BSFR, Si HiETA,

GaAs/Ge and GalnP/GaAs/Ge tandem cell. These cells were assumed to be laydown onto

conventional flexible and conventional rigid substrates and the salient parameters for each array

design were calculated (mass, area of wing, cost of protoflight array and recurring cost). In the case

of the tandem cell, educated e-_timates were made of the likely performance and cost once the cell

is fully qualified. Figures 1 and 2 illustrate the cost and mass for the rigid and flexible respectively.

(The estimated costs are relative for comparative purposes only).

The 3kW GEO conventional flexible and rigid array study considered a number of advanced cell

technologies in two timeframes:

In the Medium term (5-10 years), the cells used in the study were the Silicon high efficiency and IIIV high efficiency cells. The study _howed that at the power level of 3kW the mass of a rigid array

using the III-V cells were substantially lower than for Silicon (55kg, GalnP/GaAs against l O0kg, Si

HiETA). The flexible array showed no advantages, being heavier and more expensive in most cases.

In the Long term (10-15 years), the cells used in this study were the advanced thin Silicon,

ultrathin Ill-V, multijunctions and thin film low efficiency CIS, CdTe and c_-Si. Due to the long

timeframe the cell cost and operating performance could be only roughly estimated. However the

study showed that in terms of cost, the thin film cells could effect substantial savings at array level

in comparison with the higher efficiency cells.

-24- o
SOLAR CELLS

New solar cells, as they become available from the manufacturers, are being acquired and subjected

to qualification and environmental tests to determine likely in-orbit losses (2). This is to verify

manufacturers claims and provide specific performance data for use in the solar array trade off

studies. The environmental testing is primarily electron and proton irradiations performed at AEA

Technology (UK Atomic Energy Authority). A dedicated facility has been developed for the proton

irradiation of solar cells and is being used on a collaborative programme with Phillips Laboratory that

has just commenced.

PROTON IRRADIATION FACILITY

The facility is based on the AEA Technology, 7 MV Tandem Van de Graaff accelerator, and consists

of a well characterised source of protons with a dose rate range of 106 to 109 protons cm "2 s-1,

providing an energy range of 2-10 MeV on the target plane. The proton beam is scattered using a

thin foil to provide irradiation over a large area. Reproducibility of exposure dose is better than 5%,

and the energy of the protons to better than 1%, with an energy distribution at the target plane of

typically 150 keY. The proton intensity distribution across the target plate is currently uniform to

_-5.10%. This can be made better by moving further away from the scattering foil but with a

reduction in dose rate.

A new target chamber has been developed to allow the irradiation of cells under a wide range of

conditions: unidirectional or isotropic irradiation, at different temperatures -170 °C to + 150 °C, with

or without illumination, with or without the cells electrically active. Provision has also been made for

a solar simulator to allow the in-situ measurement of the cells, see figures 3 and 4. The target plate

was designed to accommodate a number of single cells or strings, in any arrangement, to a

maximum size of 1 lOmm square. Gas nitrogen and or liquid nitrogen is passed through the plate to

control the temperature. Typical temperature stabilities observed to date are + 100 + 1°C; -100

+ 2°C. The target plate is driven by a computer- controlled stepper motor and may be set at any

angle in 360 degrees, in steps of 0.9 degrees, to the beam, or oscillated with a selected angular

velocity profile to simulate isotropic conditions.

Secondary energy reducing foils, of increasing thickness, can be placed between the scattered

proton flux and the cell to reduce the incident proton energy from 10 MeV to 2 MeV in IMeV steps.

With increasing foil thickness the proton energy distribution increases, to the extent that it is

possible to simulate the proton spectra between 1 and 10 MeV for different orbits rather than

treating it as discrete energies. With the inclusion of even thicker foils and/or dropping the initial

proton energy it will be possible to subject cells to spectra of keY protons, which can be

characterised, but not discrete energies.

FLIGHT EXPERIMENTS

UoSAT-5

UoSAT-5 satellite was launched intp a 770 km Sun Synchronous orbit on the 16th July 1991 on an

Ariane 4 launcher as a secondary payload to ERS-1. One of the payloads is the DRA solar cell

experiment. The experiment consists of the I-V measurement of various types of silicon, gallium

arsenide and indium phosphide solar cells from the UK, Europe and the USA. Results have been

presented before, references 3,4. The experiment continues to work with data being collected every 3

months. Figure 5 shows a comparison of the short circuit current of 2 GaAs cells on the experiment,

one with a conventional adhesive bonded 2001_m CMG coverglass and the other with the same

coverglass but teflon bonded to the celt by Pilkington Space Technology. The degradation in current is

less than 1% and there is no significant difference between the 2 cells.

-25-
STRV- 1A/B

STRV-1A/B were launched on 17 June 1994 into a geostationary transfer orbit, GTO, (200x36000

km), figure 6 shows STRV-1A. Unfortunately the solar cell technology experiment (Ref 5) on board

suffered a major failure with its sun overhead detectors during launch. Attempts to overcome this

problem have failed resulting in no meaningful I-V data being obtained. The only positive result from

the experiment is the temperature difference between two GaAs cells. One cell has a conventional

AR coated CMG coverglass and the other has an early version of the Pilkington Infra-Red rejection

coating for Silicon on the coverglass. Even though this coating is for a Silicon cell and not fully

optimised nor designed for operation on a spin stabilised satellite such as STRV, this cell is running

6 °C cooler than the cell with the conventional AR coating.

On STRV-IB we are able to monitor the current of the solar panels at the operating point of 28V.

The data from the panels has been corrected for temperature, Earth-Sun distance and solar aspect

angle by Dr Dean Marvin of the Aerospace Corporation. Figure 7 shows the degradation of the

current at 28V for the 4 panels. For the -Y, -X and +Y panels, all GaAs/Ge, the actual degradation

was approximately agreeing with the prediction up to 300 days and then deviates. After 450 days

the panels have degraded by approximately 8% compared to the predicted 10%. However, for the

+X panel, which is GaAs/GaAs, the actual degradation is slightly worse than predicted up to 300

days and then converges with the predicted degradation curve. The GaAs/Ge panels were typically

18.5% efficient at beginning of life compared to 19.8% for the GaAs/GaAs panel. Radiation

monitors on A and B are indicating that the radiation environment is less severe than predicted by

the AE8 and AP8 environment models and so the GaAs/Ge results are not unexpected. The

GaAs/GaAs result is probably due to the cell structure, giving higher initial performance but softer

radiation characteristics.

In this year's work programme the existing Surrey Satellite Technology design of the measurement

electronics package is being reviewed to identify all the short comings and potential modes of

failure. A new design is being developed to overcome these. This will then followed by the

manufacture of a breadboard model that can be fully tested to' ensure correct, reliable operation.

This is to give confidence for any future flight of a solar cell experiment.

LABORATORY

TO support the Ceii/panel characterisation and environmental testing programme the DRA maintains

a comprehensive test laboratory. This is subject to a continual prOgramme of equipment upgrades to

improve the quality of the measurements and meet new testing requirern'ents. We _re'cUrrently in

the process of developing a new close Spectral match simulator for the testing of multi-junction

solar cells. Figure 8 shows the spectrum o7 the basic s|mu|ator _desi,qn, giving a very good :class A

spectrum. The simulator uses a compact Xenon arc lamp and an incandescent lamp with simple

filtering. Work is now ongoing to modify this spectrum to produce the close match design. In

addition to measuring multi-junction cells it is intended that the Close Match simulator will be used

to generate primary standard reference cells.

- FUTURE WORK

Solar Arrays

The above solar array trade off studies will be repeated as and when more cell data becomes

available, to verify the assumptions made or to correct them as necessary. New data Will be derived

from in-house testing, DRA flight experiments, from manufacturers and also from the exchange of

information through conferences and collaborations. The studies will also be extended to include

new solar arrays designs, eg. Able's Ultraflex and the Linear Concentrator array and new concept

arrays.

-26-
Solar Cells

The solar cell testing programme will continue as new and/or better cells become available from

manufacturers. We also intend to shortly commence a long term UV irradiation programme on a

range of cell types.

Flight Exoeriments

The flight of STRV-1C/D is currently in the planning stage, proposed launch end of 1998 into GTO.

We are currently looking into the possibility of the re-flight of the solar cell experiment, determining

costs and looking for collaborators on the experiment. The opportunity also exists for the flight of

small panels of advanced high power cells, with the current being monitored at the operating point

as on STRV-1B. It is hoped that the flight, if it occurs, will be backed up by a comprehensive ground

based radiation test programme to allow accurate e_timates of the degradation to be made for

comparison with the actual in-flight degradation. The possibility of flying strings of advanced cells

as an experiment on the proposed Skynet-4F Communications satellite is also being investigated.

Laboratory

In addition to the introduction of the close spectral match simulator, work will be undertaken to

automate the measurement of cell I-V and spectral responses. The intention is to have the cells on

most test programmes to be hard wired onto a substrate (removing any possibility of handling and

probe damage), with connection of the cells to the electronic load through a 20 channel multiplexer.

All of the measurement conditions, light intensity, measurement block temperature and cell selection

is to be controlled via computer. The required sequence of measurements will then be programmed

in removing the need for operator intervention and hence reduce testing costs.

CONCLUSIONS

The DRA photovoltaic programme is wide ranging in its activities, providing an independent

assessment of new solar cell and array technologies in development around the World, for our MoO

customer. Further activities are planned in these areas to meet the future requirements of MoO

spacecraft.

REFERENCES

1.

2.

3.

4.

5.

R. Kimber, C. Goodbody, "Cell types for Geostationary Array Applications", 4th European

Space Power Conference, SP369, Poitiers, France, Sept 1995.

C. Goodbody, R. Kimber, "Achieving the optimum solar array size for spacecraft", 29th

Intersociety Energy Conversion Engineering Conference, Monterey, CA, USA, Aug 1994.

C. Goodbody, "The UoSAT-5 Solar cell experiment - First year in orbit", Proc. Xll Space

Photovoltaic Research and Technology Conference, Cleveland, Ohio, USA, Oct 1992.

C. Goodbody, " The UoSAT-5 solar cell experiment", 3rd European Space Power Conference,

WPP-054, Graz, Austria, Aug 1993.

C. Goodbody, N.Monekosso, "The STRV-1 A & B Solar Cell Experiment", Proc. 23rd IEEE

Photovoltaic Specialists Conference, Louisville, KY, USA, May 1993.

© British Crown Copyright, 1995/DRA.

Published with permission of the Controller of Her Britannic Majesty's Stationary Office.

-27-
Mass and Cost for 8.7kW Flexible Array (Single Wlng)

II r -- OC_I

Si 10 OhnH:m _ 10 Ohm-era 51HiETA

BSR BSFR

Figure 1.

m

|

r0

GaAMGe GalnP/GaAs/Ge

Pm

Comparison of Array Mass & Costs for LEO Flexible Array

Mass and Cost for 8.7kW Rigid Array (Single Wing)

I 3OO

Si f0 Ohm-era SI 10 Ohm-era Si H/ETA GgAS/Ge GIInP/C-g&.S/GI

BSR BSFR Pas

Figure 2. Comparison of Array Mass & Costs for LEO Flexible Array

f- GN2 exhaust /-"- Vacuum gauge heads

/ Port for sotar simulatio

E_-_-- Vacuum shaft seal & bearing - top and bottom ! during irradiation

Access door__._ e _"- P°n :mc_ibraut_atl:[_ si:ulat°r _0 I/

Spec mou g p at

Hi-vat valve-

_<_ Turb°-m°tecular Pump--T-----_ I

LN2in "eat" !b_tePP" m°1°r I : "L

x LN2 Sprays

Figure 3. Schematic of Proton Irradiation Chamber.

-28-
.For in-situ measurement

_- Adjustable plame mirror

_- VL_uum I

window

For illumination during irradiation

Figure 4. Schematic of Proton Irradiation Chamber.

]Ol

,oo

o

,..,..__'• I

• _bl •

• ...._..: :

99

0 0,5

'

•_¢ ; _:-_, "='

g

1.5 2.5

Diys 5imc¢ LiunCh

iT=tl_

__J

| •

: :".

Figure 5. Comparison of the Isc of GaAs cells with Adhesive and Teflon

bonded coverglasses: Flying on the UoSAT-5 solar cell experiment

Figure 6 : STRV-IA.

-29-
102

lO0

i 98.

%

94

90.

SI'RV-I BSOLAR PANELS

88

86

50 1(30 150 200 250 300

C_a aven_m e,,eq S ct_s Days Sinee Laumdt

!

350 400 450 500

Figure 7. STRV-1B Solar Panels : Degradation of the Current generated at 28V

with time in orbit, Normalised to the Average Day 3 value.

A Simulator

'ii

] i .

_a

3oo

_k

I P ,

600 900 1200 1.500 1_00

Wavelength(sam)

2100 2400

Figure 8. Spectrum of AMO Class A simulator in development

-30-
T°

JAPANESE PHOTOVOLTAIC POWER GENERATION

FOR SPACE APPLICATION

Saga, Y. Kiyota_ T liatsutani, A. Suzuki, 0. Kawasaki*, T. Hisamatsu* and S. iatsuda*

SHARP Corporation

282-1 Hajikami, Shinjo-cho, Kitakatsuragi-gum Nara 639-21, Japan

* National Space Development Agency of Japan(NASDA)

2-I-I Sengen, Tsukuha-shi, lharaki-ken 305, Japan

ABSTRACT

This paper describes Japanese activities on mainly silicon solar cell research.

development and applications. The high efficiency thin silicon solar cells and the same

kinds of solar cells with integrated bypass function (IBF cells) were developed and

qualified for space applications. The most efficient cells (NRS/LBSF cells) showed average

18% at kll0 and 28°C conditions. After electron irradiation, I_S/BSF cells showed higher

efficiency than NRS/LtK_ cells. The IBF cells do not suffered high reverse voltage and can

survive from shadowing_ The designs and characteristics of these solar cell are presented.

In the last sectiom our future plan for the solar cell calibration is presented.

INTRODUCTION

Silicon solar cells have been used as the most popular electrical power source for

spacecrafts over thirty years. They have been in the advantages of high reliability,

low mass and low cost co_pared to other solar cells for many years. The performance of

silicon solar cells had increased from 10% efficiency in the early 1960's to 15% in the

end of 1980' s. NASI)A and SHARP have continued to develop Si solar cells with higher

efficiency, higher radiation resistance and lower mass. We developed the ultrathin 50_ BSFR

cells in 1980' s. The ultrathin solar cells have been used for JERS-1 and ETS-VI and the

solar cell _onitor (SCI) on ETS-V showed their excellent radiation performance (Ref. 1).

However the spacecraft have m_de a de,and of higher power from year to year. GaAs solar

cells have been expected to supplant Si solar cells. Although the GaAs solar cells have high

efficieny and high radiation resistance, they have also the disadvantages of high mass, high

fragility and high material cost resulted from the source Iimitswhich will become very

important factor considering the future large scale space application, for example, space

stations or space platforms. We thought the high efficiecy $i solar cells compared

favourably to the GaAs solar cells could be realized and the development was started in

1990. We have finished the develoments of several types of high efficiency Si solar cells in

1995. They are 100_ thick NRS/LBSF (Non _Reflective S_urface/_Localized _Back _Surface Field)

cells with about 18% efficiency, NRS/BSF (_Non _Reflective S_urface/_Back _Surface Field) cells

with about 17% and g_S/LBSF & NRS/BSF cells with IBF. First, the solar cell applications

for NASDA satellites are introduce& then the designs and characteristics of these advanced

-31-
Si cells are presented and discussed. Our activities for the universal calibration system of

space solar cells are also introduced in the last section of this paper.

SOLAR CELL APPLICATIONS

The historical solar cell applications for NASDA satellites are shown in Table 1.

This table shows the application and trends in development of several kinds of solar cells

for space. The solar cells except high efficiency Si solar cells shown in Table 1 were

introduced at the SPRAT XR (Ref. 2). The 10tic= NI1S/BSF cells have been brought into mass

production for ADEOS-II program and the 2_c= NliS/BSP cells with IBF are studying to be

adoped for OICETS progra_ The NRS/BSF cells will use the blue red reflective (BFdi)

coverglasses made by OCLI to reduce the cell solar absorptivities and the operational

temperature of solar array.

HIGH EFFICIENCY SILICON CELLS

Schematic drawings of the NI_S/LBSF cell and NI_S/BSF cell are shown in Figure 1 and

Figure 2 respectively. Their designs are the same except two differences. The N'RS/LBSF cell

has a locally diffused P+ back surface field layer and a planar PN junctiom On the other

hand, the NRS/BSF has a normal P+ back layer and a normal PN junctioIL Both solar cells have

20_m squared inverted pyramids on the ceil active area to reduce light reflection loss and

the SiOz passivation layers on the front and back surfaces to reduce surface recombination

loss of minority carriers. Direction of the inverted pyramids was declined by 45 degrees to

the cell edge to minimize a risk of open failure of a strin_ The NI_/LBSF cell was made

from only 2_cm Si substrates because the fill factor of the 10g2cm NI_S/LBSF cell was lower

than the 2g2cm Nl_S/LBSF cell (l_ef. 3). The NRS/BSF cells were made from 2g2cm and 10g_cm Si

substrates and both cells showed each advantage for different radiation conditions. The

typical electrical parameters and solar absorptivities of three types of high efficiency

cells are given in Table 2. The NRS/LBSF cells showed the highest efficiency (average

18. 0%) of three kinds of cells. The NILS/BSF cells showed lower efficiency (avrage 17. 0 to

17. 3%) than the NIkS/LBSF cell. We measured reverse saturation currents J0_ and J02 of these

cells using two diodes model and ND filters methocL The NI_S/LBSF cell gave lower J0, (6x

10-_A/cm _) by about one third and a little lower 10_ (7xl0-gA/cm 2) than the NRS/BSF cell

(Jo _ 1. 8x10- l _A/cm2 102 : 1. 0xl0-Sk/cm_). In generally the localized BSF is neccessary to get

high open circuit voltage (low 1o_) and the planar PN junction is neccessary to get large

fill factor (low J0_-). But the'planar PN junction did not improve the fill factor of the _S

/BSF cell and was not neccessary for them. We found that the planar PN junction decreased

J02 and was effective to increase the fiI1 factor of the NRS/UkSF cell with low I0L but not

effective for the NRS/BSF cell with relatively high J0 _. Solar absorptivities of these cells

were about 0. 85 and lower by about 0. 05 than those of the conventional textured cells. The

improvement was resulted from the introduction of the SiOz passivation back layer.

The results of 1WeV electron irradiation to these solar cells are shown in Figure 3.

The NRS/LBSF cells showed larger degradation than the N'RS/BSF cells and conventional BSl_

-32-
cells with the samethickness in the low fluence range (less than lxl0_4e/cm2).Onthe other

hand, the NRS/BSF cells showed smaller degradation rare than the conventional BSFR cells.

The 2_cm NRS/BSF cells showed higher power at lower fluences than lxlOL4e/cm 2 compared to

the 10_cm NRS/BSF cells. The 10g2cm NRS/BSF cells showed the highest power at higher

fluence range than lx10 '_ e/cm 2. These results suggest the radiation characteristics of NRS/

BSF ceil is dominated mainly by the bulk damage and the damage of Si/SiO2 interface is added

to the bulk damage in the NRS/LBSF cell (Ref 4). The back Si/SiO_ interface of the NRS/LBSF

cell is susceptible to the electron irradiation and increase 10, rapidly and decrease open

circuit voltages of the NRS/LBSF cells. The radiation characteristics of NRS/BSF cells were

similar to that of the conventional BSFR cell. The inverted pyramid surface made more

minority carriers near to the PN junction than the flat surface and even the normal pyramid

surface. They improved the cell radiation characteristics (Ref. 5). These high efficiency

cells showed about 20% higher power than 200_m BSR cells used for geostationary satellites

at their EOL (ll[eV electron fluence of lxl0 t _e/cm_).

The NRS/LBSF cells will be applicable to the satellites with low radiation at low

altitude or with short mission lives. The NRS/BSF cells will have wide space applications

due to their high radiation resistance and relatively low costs.

HIGH EFFICIENCY SILICON CELL WITH IBF

A Schematic drawing of the high efficiency Si cell with IBF is shown in Figure

The idea of IBD (integrated bypass diode) to protect the solar cell from the hot spot due to

cell shadowing was studied in 1970's (Ref. 6). However the IBD cell needed a small diode

integrated in a solar cell and a special interconnectiom It needed neccessarily high cost

and did not become popular. The IBF (integrated bypass function) is a simple idea to protect

the solar cell from the cell failure due to shadowin_ The IBF cell has many small dots of

P+ diffused layer in the front N÷ diffused layen The P+ dots are formed by the same boron

diffusion as the hack surface P+ diffusion and form high doped P+N÷ junctions in the N÷

diffused layer. Therefore the IBF cells need only a small amount of additional cost compared

to the usual cells. Because the high doped P+N÷ junctions can flow much more reverse current

compared to the cell PN junction and more current than the cell short circuit curt-rent at low

reverse voltage (about -3V), the IBF cell does not suffered a high reverse voltage and

can survive from shadowin_ ks the reverse current increases in proportion to the total

length of P÷N÷ junctions, we can easily control the reverse chracteristics of the cell

without a remarkable power loss (Ref. 5). Considering the temperature effects of the reverse

I-V chracteristics (the revers_current decreases with temperature increase), we think the

avalanche breakdowns of P+N+ junctions are being induced at low voltages in the IBF cells.

The typical reverse I-V characteristics of a 2×2cm 2 IBF cell is shown in Figure 5.

The NRS/LBSF and NRS/BSF cells with IBF were developed and qualified for space applicatiom

The electrical parameters of them are shown in Table 3. The efficiencies of the NRS/LBSF

cell and NRS/BSF cell with IBF were lower by 0. 8% and 0. 3% respectively than those of the

same kinds of usual cells. The specification of reverse current was determined to be more

than 0. 2 t per 2X2cm2 at the reverse voltage of -3 V using the electrical and thermal

-33-
simiulations of solar array with the IBF cells. The radiation characteristics of the IBF

cells were almost the same as those of the same kinds of usual cells. The reverse

characteristics of the IBF cells were hardIy changed by 1 leY electron irradiation to 3x10 _s

e/cm 2. The temperature coefficients of the electrical parameters and solar absorptivities of

them were almost the same as those of the same kinds of usual cells.

The reverse bias tests using NRS/BSF cells with IBF were successfully performed

using the test configuration shown in Figure 6. The test results showed the IBF cells did

not suffered hot spots or PN junction failures. We think the IBF cells are usefull to

realize the solar array with a simple design, high reliability and less cost which have cell

shadowing problems.

FUTURE PLAN FOR SOLAR CELL CALIBRATION

We have proposed the solar simulator calibration method of space solar cells to the

space agencies in the world. The solar simulator method is shown in Figure 7. The advantages

of the solar simulator method are as follows;

1. A large number of cells can be calibrated in a short period.

2. Cells can be calibrated regardless of weather or environmental contaminatiom

3. Cells can be calibrated at relatively low cost.

The solar simulator calibration value and the balloon flight calibration value of a

solar cell was compared- The difference between our calibration value and the balloon flight

calibration value Fas 1. 1% to 2. 2 % using spectral irradiance of tWO (World lfeteorogical

Organization). And we caluculated confidence intervals froE these results to analyze the

uncertainty of this meth0cL A very small value of total confidence interval (0. 25°A)

indicates that the solar simulator method is highly accurate systematically. The accuracy of

the solar simulator method is comparable with the balloon flight calibration methock We

belive that the solar simulator method can be applied to the primary calibration of Si solar

cells and are going to prepare the ISO draft standards on space solar cell calibration

CONCLUSIONS

New high efficiency silicon solar cells with efficiencies of 17% and 18% (AMO,

1 sun) were qualified for spac_ use and their characteristics were clarified- The NRS/LBSF

cell gave the highest efficiency at BOL condition But the NRS/BSF cells showed higher

radiation resistances than the NRS/LBSF cells. Both cells gave about 20% higher

efficiencies than 200_m thick BSR cells used for geostationary satellites at their EOL

condition (1MeV electron fluence of 1×10' Se/cm2).

The design, characteristics and test results of the IBF cells were presented. By

forming smaIt P+ dots in the N+ diffused layer, the reverse characteristics of the cells

could be controlled without remakable power losses. The IBF cells would contribute to

-34-
realize the solar array with a sidle design, high reliability and less cost.

The advantages of the solar simulator calibration method were presented. We are

going to prepare the ISO draft standards and expect this method will be accepted as primary

calibration of Si solar cells by ISO in future.

_EFERENCES

L O. Kawasaki et al, "NASDA's Space Solar Cell -Ground Test Data and Flight Data-', Proc.

1994 IEEE 1st World Conference on Photovoltaic Energy Conversion', 1994, P.2224

2. S. Matsuda et all,"NASDA Activities in Space Solar Power System Research. Development and

Applications', Proc. of the _ Space Photovoltaic Research and Technology Conference

(SPRSAT _I), 1992, P. 318

3. T, Hisamatsu et al, "Electrical Properties of Thin Silicon Space Solar Cells', Proc. 7th

International Photovoltaic Science and Engineering Conference, 1993, P. 573

4. T. Hisamatsu et al, "Advanced Thin Silicon Space Solar Cell', Pro. 4th European Space

Power Conference, 1995, P.371

5. T. Katsu et al, "Development of High Efficiency Silicon Space Solar Cells', Proc. 1994

IEEE Ist World Conference on Photovoltaic Energy Conversion', 1994, P.?-133

6. K S. Rauschenback "Solar Cell Array Design _mdbook', Van Norstrand Reinhold Company,

1980, P. 300

Table I Solar Cell Applications of NASDA Satellites

CS-3a

F_-v C_-_ ,._-_

Lamch 87.: 27 88.2.19 89.9.6

O=t_ _. 9.16

Satellite &,x WlindricalCylindrical

sl,we

Satellite 5501(C _ 323Ki

11ei_t

Satellite 1.5'/ 7y Sy

l_ife

Sate I I ite 84b'W 62W 265W

3_er _OL)

solar Si _ G_a $i

_ell T,_po

Solar Zmx4_ 2¢mx 2c= S._xZ. 2m"

ColI Size 2¢1x_¢="_

Solar Cell 280,m 2.80,au ZOQ,_m"_

THickness, _0 Mm'Z

Cover- 1 _ISO CII_150 OCLI

f

_ass I 150.um

*1 : min array

*Z : d'large array

BS-3=

ms_-Lb es-m3 JB_I ETS-_ SuS--_ AOEOS ..c_-rs , 01C_TS _eos-2

90.2.7 90.8.28 92.2.11 94 95 96 97 98 99

91._ 25 mm=m_, WINT_ NINT'E_ WINTER _ll_ WIHT'_.R

Box Box Box Box _I indrical Box Box Box Box

7_ 5_(¢ 13,1_Q¢ ZO001_ 3_g.¢ J 3500_ 2000K¢ 500K¢ 3500K¢

I

2y 7y 2y IOy $y 3,/ 3y ly 5y

540_ 148211 205311 410011 450011 5_OW 1320lll 5000W

$i BS_ Si BSR Si BSFR Si 8SFR Si _ $i _ G_ts [$i NR$/BSFSi NR$/BSF

-e 20_cm 10Ocm

2cm<4¢m 2c_ ×4_ 2c_x 4oa 2c_x _,c_ 2. 43crux 2cax 4c_ Zcmx 4ca ,_= x 6,'- 2cax4a_

6._

2._O_Jm 200_m 50urn SOUm 200urn lO0_m 200_zm lOOu= IO0/Jm

CII_IGO OCLI ASI_ _gSSO OCLI XSIO0 _I00 0C%.1 OCLI

IGOu.m,, !GOum _lOOum 8RRIO0u m

-35-
Non-Reflective Surface

N-contact 6ridline

SiO Passivation and ARC

N_Diffused Layer

P* Diffused

Fig. 1

SiO2 Layer

AL BSR and P-Contact

Schematic Orawin{ of NRS/LBSFCell

Non-Reflective-Surface SiO Passivation and ARC

I_Diffused Layer

SiOz Layer

P" Diffused Layer (BSF) AL BSR and P-Contact

Fi_.2 Schematic Drawing of NRS/BSFCell

-36-
Table 2

ii |

Cell Type

NRS/LBSF

NRS/BSF

NRS/BSF

Typical Electrical Parameters and Absorptivities

Resistivity

(Q ore)

2

Voc

(my)

Is¢ FF Pmx Eff.

(=A) (d) .... (_)

_S

637 194 0.79 97.6 18.0 0.84

2 630 191 0.78 93.6 17.3

10 625 191 O.77 92.0 17.0

O.85

O.85

Note ' /_0 , 135.3mW/_z , 28"C

Cell Size 2cmx2cm, lO0#m thick

1""1

E

X

LI=

I00

90

8O

7O

6O

50

4O

•lO0#m BSFR

o 10.<2.cm

0 1013 1014 |0 _5 1016

1MeV Electron Fluence [e/cmz]

Fi¢.3 Pmax Degradation of..Hi_h Efficiency Cells and Conventional Ce118

-37-
c

.=

L_

L_

_n-Refiective Surface SiOz Passivation and ARC

F

I_ Diffused Laver (BS_ AL _R and P-Contact

0.5

Fi¢4 Schematic Drawing of NRS/BSF Cell with IBF

0.4

0.1 /

/

/

/

/

O

0 1 2 3 4

Reverse Voltage 0/)

Fig. 5 Typical Reverse i-V Character i st i cs of 2 x 2cmz IBF Cell

Table 3 Typical Electrical Parameters and Absorptivities

Resistivity Voc

Cell Type /Gc_) (mY)

NRS/LBSFwith IBF 625

Isc FF P_x Eff.

2 193 0.77 92.9 17.2 0.84

NRS/BSFwith IBF 2 625 191 0.77 92.0 17.0 0.85

NRS/BSFwith IBF 10 620 O.76 go.3

Note " AMO, 135.3mW/cmz , 28°(3

Cell Size 2r..mx2cm, lO0.m thick

191 16.7 0.85

-38-
r

I

I

I

I

I

I

I

I

I

II

I

I

I

f

I

I

I

J

I

i

I

L.

Module $imzlator

Constant

Current

Source

!

Test Submod_le

IllJ J9!lJltl/JlliJllJ nil , ,_ _._-:

Shad6_

<

f

AMO Illumination SSo_'_lar_

1Q

Data

L._ Recorder

F

_Fig. 6 Reverse Bias, Test Configuration

" Solar 1

Spectral Simulatort

Resp. onse _ _ I_ Standard

_¢o_ _,---,;,-,--,., Solar Cell ,,_.._,.--.;.:._, : o. ...,..:,

Radiometer " _ _' _( ",, radiorn°leter

(NIST) _ C°mputer I

Fig. 7 Solar Simulator Calibration Method

-39-
COST TRADE BETWEEN MULTI-JUNCTION, GALLIUM

AND SILICON SOLAR CELLS

Edward M. Gaddy

Goddard Space Right Center

Greenbelt, MD 20771

ARSENIDE,

Abstract

Multi-junction (MJ), _gallium arsanide (GaAs), and silicon (Si) solar cells have respective test eff'K:ienciesof

approximately 24%, 18.5% and 14.8%. Multi-junctionand gallium arsenlde solar cells weigh more than silicon solar

cells and cost approximately five times as much per unit power at the cell level.2 A trade is performed for the TRMM

spacecraft to determine which of these cell types would have offered an overall performance and price advantage

to the spacecraft. A trade is also performed for the multi-junction cells under the assumption that they will cost over

ten times that of silicon cells at the cell level. The trade shows that the TRMM project, less the cost of the

instrument, ground systems and mission operations, would spend approximately $552,000 dollars per kilogram to

launch and suppo_ science inthe case of the spacecraft equipped withsilicon solar cells. If these cells are

changed out for gallium arsenide solar cells, an additional 31 kilograms of science can be launched and serviced at

a price of approximately $90 thousand per kilogram. The weight reductionis shown to derive from the smaller area

of the army and hence reductions in the weight of the army substrate and supporting structure.

If the silicon solar cells are changed outfor multi-junctionsolar cells, an additional 45 kilograms of science above

the silicon base line can be launched and supported at a price of approximately $58,000 per kilogram. The trade

shows that even if the multi-junctioncells are priced over ten times that of silicon cells, a price that is much higher

than projected, that the additional 45 kilograms of science are launched and serviced at $180,000 per kilogram.

This is still much less than the original $552,000 per kilogram to launchand service the science. Data and

qualitative factors are presented to show that these figures are subjectto a great deal of uncertainty. Nonetheless,

the benef'dof the higher efficiency solar cells for TRMM is far greater than the uncertainties in the analysis.

Tradeoff Between Multi-Junction, GaAs, and Silicon Solar Cells

The TRMM spacecraft has actually already chosen gallium arsenide cells. These were selected in 1991, before

multi-junctionceils were remotely available, and just as adequate technical and price information on gallium

arsenlde cells was attainable. Thus what follows only estimates the relative value of the different types of solar cells

to a spacecraft. The weight data used inthis exercise, which includes more than just the weight of the cells e.g.

the weight of the array's mechanical systems, is based on that which was originallyestimated for siliconand gallium

arsenide solar cells, with the estimates for the multi-junctioncells scaled from the estimate for the gallium arsenide

cells. Subsequently, the weightfor the selected gallium arsenide array has increased due to an increase inthe

weight of the array mechanical systems. At least an equal, and probablya greater increase would have occurred for

the silicon array. This means that the weight estimates for all the cell types should be higher than given inthis

paper and that the resultingtrade between the cells, computed below, predicts a too small advantage for the more

efficient cells.

The TRMM spacecraft is planned to be earth pointingand to fly at an initial altitude of 350 km and an inclinationof

350. For any of the solar cells,the spacecraft has two solar army wings and flies with the velocity vector

perpendicular to a line drawn between the wings. From here, the designs depart. The multi-junction or gallium

arsenide solar arrays can power the spacecraft withfour panels or two panels per wing. The wings for the more

efficient multi-junctioncells are of course smaller than the wings for the gallium arsenlde solar cells. The silicon

-40-
solararrayneedstwelvepanelsorsixperwing.Thismeansofcoursethattheweightofthesiliconpanelsis

signif'cantlygreater.AllofthisissummarizedinTableI.

• ]

Table I

TRMM Array Comparisons for a 2674 Watt at End of Life Array

Parameter Si GaAs MJ

Weight of Cell Stack, Wiring, Connectors & 48 kg

Misc.

47 kg 38 kg

Array Area 26.2 rn2 18 rn2 13.4 m2

Array Temperature 74 C 87 C 87 C

BOL Efficiency at Operating Temperature 11.3% 15.8% 21.4%

EOL Efficiency at Operating Temperature 9.4% 13.3% 18.0%

Number of Panels 12 4 4

Number of Panel Hinges 20 4 4

Number of Delay Actuators 2 0 0

Number of Potentiometers 12 2 2

Mechanical System Weight 144 kg 94 kg 81 kg

Total Weight 192 kg 141 kg 119 kg

The cost of these three arrays, includingtest costs, even though the only test specifically called out is a

deploymenttest, is summarized in Table II. With respect to the multi-junctionsolar cells some caution is in order.

The price estimates used assume that the multi-junctionsolar cells are mature and is derived by usinga ratio of .86

between gallium arsenide and multi-junctioncell arrays on a basis of power produced.4 This ratio will not holdfor

the firstseveral multi-junctionsolar cell arrays produced. As a result, the price for the multi-junctioncell array in

Table II is too low for the firstfew multi-junctionarrays. This underestimate is taken into account later.

Table II

TRMM Solar Array Cost Comparison

Component si Cost ($) GaAs Cost ($) "MJCost ($)

Cell Stack, Wiring, 2,470,000 4,200,000 3,620,000

Connectors & Misc.

Panel Substrate , 436,600 300,000 . 223,333

Panel Hinges & Boom 200,000 80,000 80,000

Delay Actuators 70,000 0 0

Potentiometers 3,000 500 500

Deployment Test 150,000 75,000 75,000

Total 3,329,600 4,655,500 3,998,833

i

-41-
TableIIdoesnotshowthepriceadvantageofthemulti-junctionandgalliumarsenidesolararraystothespacecraft.

Thisisbecausethesearraysoffersubstantial weightreductions,seeTableI,andsomaofthisreductioncan

increasetheamountofsciencethatthespacecraftcancarryhenceincreasingthecosteffectivenessofthe

spacecraft.Below,thisadvantageis estimated firstfor the gallium arsenide and then the multi-junctioncells.

Value of GaAs Array Weight Reduction on Spacecraft Science

From Table I, the gallium arsenide array is 51 kilograms less than the siliconarray. Not all of the 51 kilogramscan go

into science. Some of the weight must go into the various spacecraft systems that support the increase in science.

Such systems are increased in weight in rough proportion to the percentage weight increase in science. The

amount of increase is computed using equations (1) through (6). Some spacecraft systems, these whose size is

primarilydetermined by the spacecraft weight, remain unaffected by the increase in science. These systems are

attitude control, propulsion, structure and thermal.

The equations (1) through (6) and their solutions account for the effect of the spacecraft systems that increase in

weight. These equations predict an increase in the weight of the solar array as well as other spacecraft systems.

The solar array weight increases, as do the other subsystem weights, to support the increase in the science

capability. This means that the GaAs array will have to be more powerful than the siliconarray. The data inTables I

and II do not reflect this. To obtain final costs for the spacecraft array this change in array and other subsystem

costs is accounted for later in the paper. In equations (1) through (6), the variables INSTR, CDH, COMM, HGAS,

ELEC, PWR, and SA are respectively the weights of the scientificinstruments; command and data handling,

communications, high gain antenna, electrical, power exclusive of the solar array, and solar array systems on the

spacecraft with the siliconarray. The variables ACDH, ACOMM, t_HGAS, AELEC, APWR and t_SA are the increases

in the weights of the respective systems as a result of the additional capability they must have to serve the

additional science when the siliconarray is replaced by a gallium arsenide array. The variable CF is the fractionof

the command and data handling system that is used to support science. This fraction is obtained because about

13.85 idYsec are used for the spacecraft while about 169.80 kb/sec are used for the instruments. This same

fraction is used for the communications and high gain antenna. The variable PF is the fraction of power that is used

by the instruments in normal operation, in this case 382 watts out of the total spacecraft wattage of 938.5 watts. SF

is the fraction of weight by which the solar array increases when there is an increase in the array's power producing

capability. In this case it means that the solar array increases .863% for every 1.0% increase inthe array's power

producing capability. The six equations imply that the capability of the spacecraft systems is proportionalto weight.

Although this is a reasonable approximation, it is not necessarily the case. Forthe greatest accuracy, each

spacecraft subsystem would have to be redesigned for the increased capability and then its weight reestimated. In

the context of this paper, the resources to do this are notavailable and the approximation used is good enough.

(I) CF*CDH

(2) CF*COMM

(3) CF*HGAS

(4) PF*ELEC

(5) PF*PWR

(6) SF*PF*SA

(51-ACDH-ACOMM-APWR-_ELEC-&HGAS-_SA)/INSTR = ACDH,

(51-ACDH-_COMM-APWR-_ELEC-_HGAS-_SA)/INSTR = _COMM,

(51-_CDH-_COMM-APWR-_ELEC-AHGAS-_SA)/INSTR = _HGAS,

(S1-ACDH-_COMM-_PWR-_ELEC-_HGAS-ASA)/INSTR = _ELEC,

(51-_CDH-ACOMM-_PWR-_ELEC-_HGAS-_SA)/INSTR = _PWR,

(51-_CDH-_COMM-_PWR-_ELEC-_HGAS-_SA)/INSTR = _SA,

INSTR = 633 kg

CDH = 83.0 kg

COMM = 32.4 kg

HGAS = 81.8 kg

ELEC = 263. kg

PWR = 172. kg

SA = 141. kg

CF = .918

PF = .407

SF = .863

-42-
Solving these equations yields the following results:

ACDH = 3.74 kg,

ACOMM = 1.46 kg,

AHGAS = 3.68 kg,

AELEC = 5.25 kg,

APWR = 3.43 kg,

ASA = 2.43 kg,

Total = 20.0 kg.

The above results mean that the spacecraft can accommodate 51 - 20 = 31 kg of additional science. A value for

this increased science can be computed as follows. The launch costs for the TRMM spacecraft are approximately

$116.8 million and the spacecraft costs, excluding instrument, ground systems, and mission operations, are

approximately $232.7 million. This means that the total costs to get the science to orbit, and to support it with the

spacecraft are $349.5 million. This figure must be increased because the capability and complexity of some of the

subsystems has increased. Using the results of equations (1) through (6) as shown in Table III, leads to a price

increase of $2.8 million.Again, this assumption is a reasonable approximation, but not completely accurate.

The scientificinstruments weigh 633 kilograms (The TRMM spacecraft itself will weigh approximately 3,512

kilograms). This means that the launch costs for the original scientificpayload are $552,000 per kiiogram. On this

basis the launch and support of the additional 31 kilograms cost about $17.1 million.This far exceeds the price of

the more expensive gallium arsenide solar cells. Another measure of this is that the added 31 kilograms cost

$2.8 million or $90,000 per kilogram to launch and support, about a factor of 5 less than the first 633 kilograms on

a per weight basis. Another measure is that the silicon powered spacecraft launches 633 kilograms of science at a

cost of $552,000 per kilogram. The gallium arsenide spacecraft launches 664 kilograms of science at a cost of

$531,000 per kilogram.

This analysis underestimates, to a degree, the value of the gallium arsenide solar cells. For example in the power

and electrical systems, the predicted increase in weight is assumed to be directly proportionalto the increased

power requirement. This is notthe case; the power system weight will go up somewhat less than the linear

increase predicted. The analysis, in another way, overestimates the value of the gallium arsenide solar cells. This is

because the value of the first 633 kilograms of science is undoubtedly higher on a per weight basis than the value

of the next 31 kilograms. The analysis also overestimates the value of the gallium arsenide cells in that their use

resulted in an atypical reduction in the complexity of the solar array. This made the gallium arsenide cells look

'1:)etter"than they would be on average.

Most importantly,this analysis presumes a clean possibility to organize and plan the spacecraft to the optimum.

This is plainly not the case. For example, the satellite weight becomes a much more criticalparameter as the

spacecraft weight is about to be too heavy for the planned launch vehicle and to bump the spacecraft to the next

larger vehicle. As another example, the power system weight is heavily dependent on batteries whose weight and

size are not readily available inthe exact optimum size. In addition,the design of spacecraft is suchthat

subsystems are frequently not completely optimized for many reasons, one of which is just to get the heritage and

known price of a previously existing subsystem. Furthermore, most of the GSFC spacecraft fly instrumentsand

technology that have not been flown before or even done before. As a result, the spacecraft must carry rather

significant weight contingencies. In short, the business of fabricating a spacecraft such as TRMM involvesa great

deal of intuitive judgment and a high degree of uncertainty. It is messy. Nonetheless, averaged over a large

number of spacecraft, the predictions made here have merit in assigningvalue to the weight saved by a solar array

and serve as a guide as to whether more efficient solar cells have value.

The historyof TRMM illustrates the uncertainties of the predictions made above and the messiness of planning a

unique spacecraft. On TRMM the gallium arsenide solar cells were selected, butthe saved weight went into added

fuel to keep the spacecraft aloft for the required time ratherthan into the scientific payload. Subsequently, the

atmospheric models used to evaluate the decay of the spacecraft orbitchanged, so the additional fuel was no

longer necessary to meet mission requirements.

-43-
Table III

Cost of SpacecraftSystemsfor TRMMin Millions of Dollars

Item(Anasteriskindicatesthatsubsystemprice CostW'dhSi CostW'dh Cost With

changes with solar cell price) Army GaAs Array MJ Array

Test 4,7 4.7 4.7

Structure 12.6 12.6 12.6

Deployables 7.0 7,0 7.0

Optical Alignment .7 .7 .7

Gimbal 2.8 2.8 2.8

ACS 15.2 15.2 15.2

*Power (Exclusive of Array) 7.3 7.4 7.5

Array 3.3 4.7 4.1

Thermal Design 3.5 3.5 3.5

Thermal Contamination 1.6 1.6 1.6

Thermal Coatings .3 .3 .3

Reaction Control System 5.3 5.3 5.3

*Electrical includingI & T 21.4 21.8 22.0

*C & DH 11.0 11.5 11.7

Software 2.1 2.1 2.1

*Communications 6.0 6.3 6.4

Subtotal 104.6 107.6 107.4

Software Management 1,8 1.8 1.8

System Engineering 4,9 4.9 4.9

Project Support 4.1 4.1 4.1

Instruments ($73.2) N/A N/A N/A

P.A. 5.5 5.5 5.5

Ground Systems(S25.2) N/A N/A N/A

Mission Operations(S3.7) N/A N/A N/A

MPS (Center Tax-Overhead) 26.9 26.9 26.9

Subtotal 148.0 150.8 150.6

Contingency 9.0 9.0 9.0

Civil Servants 75.7 75.7 75.7

Total 232.7 235.5 235.3

-44-
Value of MJ Array Weight Reduction on Spacecraft Science

The methodology used in this section is the same as that used in the previous section. From Table I, the multijunction cell array is 73 kilograms less than the silicon array. Again, not all of the 73 kilograms can go into science.

The spacecraft system

the constants have the

CF*CDH

CF*COMM

C F* HGAS

P F* ELEC

PF* PWR

SF*PF*SA

weights will increase per the equations below. The variables have the same meaning and

same values as in equations (1) through (6) except for SA which is given its value below.

(73-ACDH-_COMM-_PWR-_ELEC-_HGAS-ASA)/INSTR = _CDH,

(73-ACDH-ACOMM-_PWR-AELEC-_HGAS-_SA)/INSTR = _COMM,

(73-ACDH-_COMM-APWR-_ELEC-AHGAS-ASA)/INSTR = _HGAS,

(73-ACDH-_COMM-APWR-_ELEC-AHGAS-ASA)/INSTR = AELEC,

{73-ACDH-_COMM-APWR-_ELEC-_HGAS-_SA)/INSTR = _PWR,

(73-ACDH-ACOMM-APWR-AELEC-_HGAS-_SA)/INSTR = _SA,

SA = 119 kg

Solving these equ_lons yields the following results:

_CDH = 5.39 kg,

ACOMM = 2.10 kg,

_GAS = 5.31 kg,

AELEC = 7.57 kg,

_PWR = 4.95 kg,

ASA = 2.95 kg,

Total = 28.3 kg.

This means that the spacecraft can accommodate 73 - 28 = 45 kg of additional science. The total costs to get the

science to orbit, support it, and send raw data back are computed as $235 million in Table III. This figure includes

the increased capability and complexity of some of the subsystems due to the increase in science. Again the

launch and support costs for the original scientific payload are $552,000 per kilogram. On this basis the additional

45 kilograms cost about $24.8 million to launch and support the spacecraft. Once again this far exceeds the price

of the more expensive solar cell array. The solar array makes the added 45 kilograms available at $2.6 million or

$57,000 per kilogram, an order of magnitude less than the first 633 kilograms on a per weight basis.

Sens#ivity Analysis

That the cost of the first several multi-junction solar arrays would be significantly greater than that quoted in Table II

and Table III was mentioned earlier. Making the assumption that this price is twice that given for gallium arsenide

cells in Table III will give a notion of how sensitive this analysis is to the price of the arrays. This assumption is quite

conservative even for the first few multi-junction arrays in that if the entire difference is due to cell cost, the multijunction cells should be about an order of magnitude more expensive than gallium arsenide cells. This price

difference cannot begin to be predicted. The material cost of the multi-junction cells should be quite close to that

of gallium arsenlde solar cells. The capital equipment used to produce them should be the same as for gallium

arsenide cells. The labor used to produce multi-junction ceils maySeslightly greater than gallium arsenlde cells in

that the multi-junctions must stay longer in the reactor that grows additional cell layers. The major difference may

well be that the multi-junction cells will havea lower yield than the gallium arsenide cells or that some sort of

protection against reverse bias will have to be added; but in short, the price estimate from reference 1 appears

reasonable. The first few arrays may be much more expensive because the cells may well give "teething problems"

of an as yet unknown nature. This means that the manufacturer will face possible losses that will cause a significant

protective increase in price. A factor of two is more than adequate to cover contingencies.

Making an assumption of a factor of two in array price will change the analysis as follows. If the multi-junction solar

array is priced at twice that of the gallium arsenide array as well as increased in power output to support additional

science, it will cost $9.6 million rather than the $4.1 million shown in Table Ill. This means that overall cost to the

project will rise $5.5 million to $8.1 million from the $2.6 million given in the preceding analysis. This is $180,000

-45-
per kilogram; stillsubstantially less than the $552,000 per kilogram costto launch and service the science with a

siliconarray.

Range of Validity of the Analysis

This results of this analysis depend on the higher efficiencysolar cells to reduce the mechanical weight of the solar

array substantially.This istrue with an array such as TRMM that has relatively heavy substrates,and mechanical

deployment and positioningSystems. The analysis may lose validity if the ceils can no longer leverage their

efficiency advantage to cut array mechanical systems weight.

There are two types of arrays where the analysis inthis paper may not held true. The first is a body mounted array

on spinningspacecraft. If such a spacecraft can obtain adequate power with the area available on its body withthe

use of silicon arrays, the use of higher efficiency cells would be counterproductive because the higher efficiency

cellsare heavier. The second type of array for which the analysis may be invalid is a deployable array such as the

Advanced Photovoltaic Solar Array (APSA) sor Solar Array Right Experiment (SAFE)6or the Hubble Space

Telescope Array7. For these arrays, the reduction of weight withthe increasing efficiency of cells is not necessarily

achievable because most of the mechanical weight of these arrays is in the blanket storage and deployment

mechanisms. Changing to a more efficient cell will have littleeffect on reducing this mechanical weight. This issue

is quantitatively discussed by Ralph_who uses only the launchcost per unit weight. In this case, the analysis still

favors the higher efficiency cells.

Conclusions

From this study, both gallium arsenide and multi-junctionsolar cells offer significantcost advantages to spacecraft

having deployable arrays. Gallium arsenide solar cells offer increases in the scientificpayload at about $90,000 per

kilogram and multi-junctionsolar cells offer increases in the scientific payload at about $ 58,000 per kilogram. This

compares to the payload that costs $533,000 to launch and support. The magnitude of this advantage is

dependent on the higher efficiency cells to reduce the mechanical and structural weights of the solar array

through a reduction inthe array's area.

P. K. Chiang, et al, =LargeArea GalnPz/GaAs Tandem Cell Development for Space Power Systems," TwentyThird Photovoltaic Specialists Conference, 1993, pp. 659-664.

=Gene Ralph, "High Efficiency Solar Cell Arrays System Tradeoffs," IEEE FTrstWorld Conference on Photovoltaic

Energy Conversion, 1994, Hawaii, Figure 1, p.1998.

3Throughout this paper the term "support"means to provide the scientif'¢ instrument with environmental

protectionfrom the space environment, to keep it at acceptable temperatures and to send the data it

produces to earth in readable form. In short, the services provided by the spacecraft.

4Gene Ralph, =High Efficiency Solar Cell Arrays System Tradeoffs," p. 1998.

s Paul Stella and Richard Kurland, =Thin Film GaAs for Space-Moving Out of the Laboratory,"Twenty-Third

Photovoltaic Specialists Conference, 1993, pp. 21-26.

6"Solar Array Right Experiment," Final Rel_od, LMSC-F087173, under Contract NAS-8-31352, April 1986.

7Lothar Gedach, "The Solar-Power Generator for the Hubble Space Telescope," ESA Journa/, vol. 14, no. 2,

1990, pp. 149-168.

6Gene Ralph, =HighEfficiency Solar Cell Arrays System Tradeoffs," Figure 7, p. 2000

_46-
THE PROGRESS OF LARGE AREA GalnP2/GaAs/Ge TRIPLE-JUNCTION CELL DEVELOPMENT AT

SPECTROLAB 1

P. K. Chiang, D. D. Krut and B. T. Cavicchi

Spectrolab Inc.,

Sylmar, California

ABSTRACT

This paper describes the successful demonstration of high efficiency, large area monolithic triplejunction, n on p, GalnP2/GaAs/Ge cells. The highest open circuit voltage and cell efficiency (cell size: 2 cmx 2

cm) measured to date are 2.573 V and 23.3%, respectively, under 1 sun, AM0 illumination. A very uniform

distribution of cell efficiency across a 3" diameter wafer is also achieved. The temperature coefficient and 1 MeV

electron irradiation results are obtained. We have incorporated a triple-junction cell in the 1995 JPL balloon

flight. The I-V result obtained from the balloon flight and the solar simulators are compared. Finally, we have

fabricated more than 50, 2 cm x 2 cm triple-junction cell-interconnect-cover (CIC) assemblies. The highest CIC

efficiency is 23.2%. No degradation in CIC performance after 100, -120 "C to +140 °C thermal cycles was

observed.

INTRODUCTION

The GalnP2/GaAs/Ge monolithicmulti-junctioncell is an attractivetechnologyfor space applicationsdue to

its high efficiency and radiationhardness. Ge is a low cost(compared to GaAs or InP) substrateavailable in 100 mm

wafer diameters sufficiently rugged for thin, large area cell fabrication. In past years, Spectrolab has demonstrated

dual-junctioncell efficienciesupto 24.2% (cell size: 0.5 cmx 0.5 an) under lx, AM0 illuminationon Ge substrates [1].

Concurrently, we have modeled a triplejunctionve_on of the cell utilizingthe Ge as a voltage booster. Model results

indicatedwith the inciusionof a thirdjunctioninthe Ge substrate,the cell efficiencywill have an additional2% to 2.5%

(absoluteefficiency)increase.

Under this U.S. Air Force development contract, the objectivehas been to demonstrate proof-of-concepttriple

junction devices and to deliver cells with a goal of 24% efficiency. The ultimate objectives are (1) to demonstrate a

26.5% cell efficiencyand (2) to extend the growthsto larger reactorscapable of supporting volume production. In this

work, we have utilizeda multiple wafer reactorto growthe triple-junctionGalnP2/GaAs/Ge cells. The highest efficiency

achieved to date is 23.3% for a cell measuring 2 cm x 2 cm. We have also achieved a very uniform distributionof cell

efficienciesacross 3" diameter wafers. Average cell efficiency of 22.8% across the 3" diameter wafer indicates

large area cells, up to 4 x 6 cm2 across 3"wafer are possible. Cells have been characterized by electron irradia1This Work is supported by the Departmentof the Air Force and Managed byPhillipsLaboratory,Space Power and

Thermal Management Division, underPhillipsLaboratory contractno. F33615-91-2146.

-47-
tion stability (1 MeV), and temperature coefficient measurements for both dual and tdple-junction cells. The

results indicate the difference in absolute efficiency between these two devices is 2% at an operating

temperature of 54 °C.

In addition, we have incorporated a triple-junction cell in the JPL balloon flight 1995. The same cell was

also measured by NREL and Spectrolab. The efficiency measured from the balloon flight is 0.7% and 1.9%

(relative efficiency) lower than what measured by NREL and Spectrolab, respectively.

Finally, we have fabricated more than 50 triple junction welded CICs. The highest CIC efficiency is

23.2%. The average efficiency for the 50 CICs (CIC size: 2 cmx 2 crn) was 22.0% and is very comparable to

that which was measured on the bare cells (average cell efficiency is 22.1%) before they were fabdcated into

CICs. Several CiCs were subjected to a 100 thermal cycle test with the temperature varying from -120 °C to

+140 °C in each cycle. No degradation was observed for this test.

GalnP=/GaAslGe TRIPLE-JUNCTION CELL RESULTS

GalnP=/GaAs/Ge Triple-Junction Performance Modeling

In thisworkwe fi_t performed modelingto calculatethe minimumachievable efrciency in the tripie-junction

cell in the high volume production environment. Our approach has been to combine what we have already

demonstratedon the dual-junctionwith conservatively modeled performancefor the Ge bottom cell. The triple-junction

cell structure shown in Fig. 1 consists of a dual-junctionGalnP2/GaAs interconnectedto the Ge bottom cell througha

secondGaAs tunneljunction. The resultantI-V curve for a triple-junctionGalnP2/GaAs/Ge cell is shown in F_g.2. The

expected minimum average efficiency at beginning-of-life(BOL) is 26.5%. Previouswork on the electron irradiationof

Ge cellsdemonstrated a low rate of current and voltage lossupto highfluence levels [2,3]. At an EOL fluence of 1 x

10is, 1 MeV electrons/cm2, the Ge cell will continue to function as a voltage booster and the triple-junctioncell will

have an efficiencygreaterthan 21%.

GalnPz/GaAs/Ge Triple-Junction Solar Cell Fabrication

The cell structure utilizedin thiswork has been shownin Fig. 1. The thicknessof the emitter and base layers

in GalnP2 cell were 0.1-0.15 um and 0.4-0.48 urn, respectively. The carrier concentrationin the emitter of both cells

was 1-3x1018cm"3.The base of the top cellwas dopedto a level of 1-3x1017crn"z,while a base dopingof the 3.6 x 10'8

cm"3was targeted in the GaAs cell to maximize EOL current collection. A high bandgap AlinP2 layer was used to

passivatethe front of the GalnP2 cell; the back surfacewas passivatedin this cell with an AIGalnP layer. A GalnP2

layer was used for the window on the GaAs cell. An AIGaAs (or GalnP2) was used for the GaAs cell back surface

passivation. The Ge bottomcell active junctionwas formed by As diffusioninto a p-type Ge substrate.

The GalnP2/GaAs/Ge triple-junction cells were grown on 3-inch diameter Ge substrates. The grown

wafers show mirror like shiny surface morphology. Cells were then processed into a 2 om x 2 cm cell size using

standard single junction GaAs/Ge producing procedures. After processing, cells were evaluated by light I-V

measurements. Since current in these cells is limited by the top two cells, the X-25 simulator intensity was set

with JPL balloon flight GalnP2 and GaAs (filtered by GalnP_) standard.

The highestefficiencymeasured to date is 23.3% (AM0, 28 °C) for a 2 cm x 2 cm cell. As shown in F'_. 3

the open circuit voltage (Voc), short circuit current (lsc), and fill factor (FF) are 2.573 V, 58.06 mA and 86.1%,

respectively. To our knowledgethis is the first, and the highestefficiency reportedfor, an rVp GalnP2/GaAs/Ge triple

junctioncell.

-48-
Figure 4 shows the best measured cell efficiency distribution for cells grown on a 3 inch diameter

substrate. The cell efficiencies, with an average of 22.8% (lx, AM0), were uniformly distributed across the

wafer. The average Voc, Isc and FF, were measured at 2.549 V, 58.1 mA and 84.7%, respectively, in this wafer.

Good cell uniformity across the 3 inch diameter wafer indicates large area, 4 cm x 6 cm cells could be fabricated.

The spectral response was measured using light bias at different wavelength. The extemal quantum efficiency

(Q.E.) of the three subcells are clear from the data in Fig. 5. After integration of the external Q.E. with AM0

spectrum we obtained a much higher current in the Ge cell and confirmed that the current triple-junction cell

performance is limited by the top two cells.

GalnPz/GaAs/Ge Triple-Junction Solar Cell Characterization

Electron Irradiation Test Results

In this test we irradiated a small quantity of bare cells. The electron energy and fluence used in this test

were 1.0 MeV and 1E15 e/cm 2, respectively. After the irradiation, cells were re-measured with X-25 simulator.

Due to a lack of balloon flight standard, for end-of-life (EOL), the X-25 simulator intensity was set with JPL

balloon flight GalnP2 and GaAs (filtered by GalnP2) standard cells for BOL.

The light I-V results for the irradiated cells are summadzed in Table 1. Vocl, Iscl, FF1 and Eft1 are the

open circuit voltage, short circuit current, fill factor and efficiency, respectively, for BOL. Voc2, Isc2, FF2 and

Eft2 are the results for EOL. Average Voc, Isc, FF and Eft ratios for cells measured at EOL to BOL are 92.4%,

81.1%, 99.2% and 74.4%, respectively. Since the ratio of Isc is very close to that measured in GaAs single

junction cells, we concluded that the EOL performance of triple-junction cell is limited by the degradation of

GaAs cell. In order to vedfy this, we performed spectral response measurements on these cells. As shown in

Fig. 6 the extemal Q.E. for both GalnP2 and GaAs cells degraded at EOL. The ratio of integrated current for

EOL to BOL are approximately 93% and 80%, respectively, for the GalnP2 and GaAs cells, which confirms that

the EOL performance of current triple-junction cell is limited by the degradation of the GaAs cell. Recently, we

have improved the GaAs single junction cell electron irradiation performance. We will incorporate this into the

triple junction cell growths and expect a better EOL performance in optimized triple junction cells.

GalnPz/GaAs/Ge Triple-Junction Cell Temperature Coefficient Measurement

Several triple-junction cells have been used for the temperature coefficient measurements. Light I-V

measurements were performed at four different temperatures (10, 28, 50 and 80 °C). The open circuit voltage

decreases with increasing temperature is shown in Fig. 7. The decrease in the Voc is due to the increasing dark

currents. The rate of decrease of Voc with temperature is 6.07 mV/°C. For comparison, the temperature

coefficient of 3.99 mV/°C for the dual-junction cell is also plotted in the same figure.

The short circuit current increases with increasing temperature is shown in Fig. 8. The improvement in

Isc with increasing temperature is due mostly to the shift in the abso_rP2!ionedge of three cells. The rate of

increase of short circuit current density with temperature is 0.0162 rnA/cm/'C.

The efficiency decreases with increasing temperature for both dual and triple-junction cells are shown in

Fig. 9. The rate of decrease of Eft with temperature are 0.040 and 0.053 %PC (absolute), respectively, for dual

and triple-junction cells. This figure also cleady shows the difference between these two devices is 2% at an

operating temperature of 54 °C, as we previously reported [1].

-49-
GalnPz/GaAs/Ge Triple-Junction Cell Balloon Flight Measurement Results

Triple junction device measurements are performed using Spectrolab's modified X-25 simulator. The

simulator contains an attachment with a movable set of filters that allow trimming simulator's spectral content to

achieve proper color balance between the GalnP2 top cell to GaAs middle cell with respect to the set of balloon

flight standards.

Device 2T110A4 was measured using this simulator with appropriate set of top and middle cell

standards. This cell was also measured at NREL using multi-source filtering attachment. In addition to

Spectrolab and NREL measurements, Cell 2Tl10A4 was mounted onto the standard JPL balloon package and

flown on the 1995 balloon flight. The results and temperature corrected results are shown in Table 2.

Measurement differences between Spectrolab ground and balloon measurements are less than 2%.

Most of the error is in the voltage measurement, rather than Isc and fill factor. This error may be related to the

difference in temperature coefficients of the measured cell and the values measured for other devices (presented

in this paper). Similarly, the difference between NREL and Spectrolab measurements is less than 2%. Good

agreements in measured currents between three measurements show that terrestrial measurements of

multijunctions within 1% are possible.

GainPz/GaAs/Ge TRIPLE-JUNCTION CIC RESULTS

In this work, we have successfully fabricated full CIC structures. In order to meet the delivery

requirement, fifty bare cells with an average efficiency of 22.1% were selected for the CIC fabrications. Silver

straps were used to weld to the top metal contact of the cells. Three mii thick, CMX glasses were cut to the size

of the cells and cemented to the surfaces of the cells. After fabrication, light I-V were re-measured. The highest

efficiency measured to date is 23.2% (AM0, 28 °C) for a 2 cmx 2 cm CIC. The Voc, Isc, FF are 2.564 V, 57.76 mA

and 86.3%, respectively. The average efficiency for these 50 CICs is 22.0%, which is very comparable to the

average bare cell efficiency of 22.1% before they were fabricated into the CICs.

Several CICs were subjected to a 100 thermal cycle test. In this test, the temperature varied from -120

°C to +140 °C at the rate of 15 °C/rain in each cycle. After the test, they were re-measured. The I-V results are

very close to what were measured before the thermal cycle test indicating no degradation occurred during this

thermal cycle test.

CONCLUSIONS

GalnP2/GaAs/Ge triple-junctioncells have been successfullydemonstrated. The best cell efficiency of 23.3%

AM0, 28 °(3)is the highestreportedefficiency forthis device. With optimization,these cells are capable of providinga

minimum average efficiency of 26.5% in volume Auction. The electron irradiation of first few cells yielded

average P/Po at 0.744. This performance is limited by the GaAs middle cell With optimization the P/Po in this

device is expected to improve to 0.80 at 1.0 MeV (1E15 e/cm2) electron irradiation. We have completed

temperature coefficient measurements shown 2% absolute efficiency difference between the dual and triplejunction cells at operating temperature of 54 "C. Finally, the triple junction CICs were successfully fabricated

indicating the "transpancy" of the product to standard Spectrolab's welded panel fabrication process. The best

CIC (2 cm x2 cm) efficiency is 23.2%. No degradation in CICs was observed after thermal cycle testing.

-50-
REFERENCES

[1] P.K. Chiang et. al., "Large area GalnP2/GaAs/Ge multijunctionsolarcellsfor space applications',Proc. 1stworld

conf. on PV energy conversion, 1994, pp. 2120-2123.

[2] R. Venkatasubramanian et. al., "Hightemperature performance and radiationresistanceof highefficiency Ge and

SiooTGeomsolar cellson lightweightGe substrates', Proc. 22nd IEEE PVSC, 1991, pp.85-89.

[3] D. D. Krut et. al., "The development of Ge bottomcell for monolithicand stacked multijunctionapplications',Proc.

22nd IEEE PVSC, 1991, pp. 90-92.

Table 1 Light I-V test results for BOL and EOL

Vocl Voc2 Ratio lscl Isc2 Ratio FF1 FF2 Ratio Eft1 Eft2 Ratio

(V) (V) (rnA) (mA) (%) (%) (%) (%)

2T101A-5 2.524 2.324 0.921 60.68 49.10 0.809 81.16 81.08 0.999 22.5 16.8 0.744

2T110A-5 2.526 2.342 0.927 60.66 49.35 0.814 83.67 82.42 0.985 23.2 17.3 0.743

Avg: 2.525 2.333 0.924 60.67 49.23 0.811 82.42 81.75 0.992 22.9 17.0 0.744

Table 2 Triple Junction Cell Testing Results

Isc

Voc

Eft

FF

Spectrolab Measurement

(ASTM AM0)

Measured at 28°C

JPL Balloon

Corrected to

28oc

Measured at 53.5°C

NREL

0/VRRL AM0)

Measured at

25oc

58.47 mA 59.98 mA 58.83 mA 58.97 mA

2.541 V 2.355 V 2.510 V 2.553 V

22.95% " 21.15% 22.51% 22.6%

83.94% 81.96% 83.89%

-51 -
n - AIInP

n - GalnP

p - GalnP

n - GalnP

n - G_s

,Top

Cell

Middle

Cell

2ridTunnd

F'_. 1. Cross-section of a triple-junction cell

30.0

25.0

10.0

Current Density (mA/cm =)

:',,.

Perfo rmunce

Jsc(mA/cm:) Voc(V) FF(%) Eft(%)

GalnPz/GsA8 15.96 2.39 86.7 24.2

Ge 24.60 0.24 61.0 2.6

GalnP_/GaAs/Ge 15.96 2.62 85.7 26.5

Voltage (V)

F'_g. 2. Modeled BOL performance of proposed n/p GainPz_3aAs/Ge monolithic Vipte-junction cell (AM0 28 °C)

-52-
r.

•0E

.05  ̧

.04  ̧

.03,

,02,

.01.

ZV PLOT 2T2228 I 04 I 08/04/95J 12:16

" _: :oseo6

V_ 2. 5728

_F\ .s. 09

Pma_ . t286

V|PmI_X 2.2943

ZOIOm_x .05605

_IBVlola .05255

I

. • , ,. . .

._ ; 1:s _ z:s 3

voltage (v)

Fig. 3. I-V curve for the highestefficiencyGalnP2/GaAs/Ge triplejunctioncell

I

SIJb: 3" Diameter I_Ge

(Cell Area: 4.078 cm=)

Fig. 4. Efficiency distribution of triple-junction cells across a 3" diameter substrate

-53-
Ext. & Int. QE (%) Reflectance (%

100 1- ,! _,"I , [ S/N: 2R1062-I#4

IO0

5o _ "! ....... ! ....... _---_:- ...... ! ..... 8o

• : -," _ II _,,l •

. . ,./ ,LT'"_: I :

I I "

,

40 -_--/-_. _,_._.E;1 2,! ..... 40

20_ 20

0 _ 0

350 550 750 950 1150 1350 1550 1750

Wavelength (nm)

Fig. 5. External quantum efficiency and reflectance of a triple-junction cell

100

8O

6O

4O

2O

0

35O

External O.E. (%)

S/N: 2T101A-sT: : _ !..,.,a_

-'- "°_J.i ........... ,...... ,.... /:.._..i ..... i..... i\...

• : : i i,

400 450 ._00 550 600 650 700 750 800 850 900

Wavelength (nm)

Fig. 6. External quantum efficiency of a GalnP2/GaAs/Ge triple junction ceil measured before and after electron

irradiation

-54-
2.7

2.6

2.5

2.4

2.3

2.2

2.1

2

0

Open Circuit Voltage (V)

iiiiii!!!!iill ....iii iiiiiiiiiiiiiiiiiiiii

iiiiii iliil

i i = , I , , , , I , , I I | n , r z I , , ; J I , , , , I , s L J I , z , T I , • , I

10 20 30 40 50 60 70 80 9{

Temperature (C)

Fig. 7. Open circuit voltage of dual and triple-junction cells as a function of temperature

6o

5o

4o

3o

2o

10

Short Circuit Current (mA)

-I (Cell Area: 4.076cm=) I ............................. _";'_ .....

......................................... ............

............................... ;.;,,,.: ..............................

,F j

.... _. " "" . .... 1 Temperature Coefficient: 0.0160 mA/cm=*C

0 [ I , I _ I _ I _ I , I , I , I ,

0 10 20 °30 40 50 60 70 80 90

Temperature (C)

Fig. 8. Short circuit current of triple junction cell as a function of temperature. (note: vertical bar in each

temperature represents the range of measurements in three cells)

-55-
Efficiency (%) (@ lx AM0)

25

24

23

22

21

20

16

0 1 0 20 30 40 50 60 70 80 90

Temperature (C)

Fig. 9. Efficiency of dual and triple-junction cells as a function of temperature

-56-
STATUS OF MULTI JUNCTION SOLAR CELLS

Y.C. M Yeh and C.L. Chu

Applied Solar Energy Corporation

15251 Don Julian Rd,

City of Industry,CA 91745-1002

INTRODUCTION

This paper describes AppliedSolar's present activity on Multijunctionspace cells. We have worked on a veriety

of MJ cells, both monolithic and mechanically stacked. In recent years, most effort has been directed to

GalnP2/GaAs monolithic cells, grown on Ge substrates,and the statusof thiscell design will be reviewed here.

Purposeof Work

MJ cells are in demand to provide satellite power because of the acceptance of the overwhelming importance of

high efficiency to reduce the area, weight and cost of space PV power systems. The need for high efficiencies

has already accelerated the productionof GaAs/Ge cells, with efficiencies 18.5-19%. When users realized that

MJ cells could provide higher efficiencies (from 22% to 26%) with only fractional increase in costs, the demand

for productionMJ cells increased rapidly.

The main purpose of the work described is to transfer the MOCVD growthtechnologyof MJ high efficiency cells

to a productionenvironment, providingall the space requirementsof users.

TECHNICALAPPROACH

The feasibility of the high effÉciencyMJ cells considered here, was demonstrated pdmadly by an NREL group

under Dr. Jerry Olsen. The challenge is to transferthis MJ cell to productionstatus. This required validationthat

large area MJ cells could be fabricated evaluation of the relative merits of P/N and N/P configurations, and

demonstration that Ge substratescould replace GaAs, to provide larger area, thin MJ cells.

The main technical challenges for MJ cells result from the larger numbers of cell design options (10-12 layers)

and from the controlled MODVD proceduresrequired for uniformmultilayer growthover substrates.

The approach used is to systematically vary the various layers usingdesign of experiment methods to optimize

the layers for high efficiency. These tests are made in a large capacity MOCVD reactor to validate that these

optimization procedures can be achieved at production levels. Production operation includes the need to

upgrade the toxic disposal (involving both P and As) and establishing of maintenance schedules to provide

round-the-clockoperation,to meet userdelivery schedules.

There is one otherimportant factor in control of the growthprocedures. The BOL cell efficiency depends on the

current match between the GalnP2 and GaAs cells, and is highest when the sub-cell currents are equal.

However, because radiation degrades the subcellsat different rates, (usuallythe GaAs cell degrades faster when

exposed to only the longer wavelengths transmitted by the the GalnP2 cell), in order to meet typical space

mission requirements, it is necessary to modify the growth proceduresto over-match the GaAs cell. Although

this reduces the BOL efficiency slightly,the EOL efficiency is improved. We have investigated this tailoring of

radiation performance, including steps to increase the radiation resistance of the GaAs sub cell, and have

validated a model which allows predictionand monitoringtests that ensure than large numbers of MJ cells have

consistentEOL performance, meeting specific flight conditions.

-57-
ForMJcells,thecontactrequirementsarenotexcessive.Thehigherinternalimpedanceofthecellallowsvery

lowseriesresistance(highCFF)tobeobtainedwithlowmetalshading(~3%).Therequirementsforbondability

contactadhesionandminimization of interactionbetweenthe contact metallization and the MJ cell structure can

be fulfilled using experience gained on GaAs/Ge cells. We are continuing to evaluate possible improved

metallization schemes. For MJ cells, there is only slight decrease in CFF resultingfrom the larger grid path

length on larger size substrates.

The AR coatings need modification,to ensure the correctmatch for 2J (and 3J cells).

To control and monitor MJ cell performance, we have developed a set of inline characterizationtests. A major

requirement is that the solar simulators used can replicatethe AM0 spectrum for all cells in the stack, and over

the whole cell area. We are usingtwo-light simulators,one an existing Hoffman simulator, the other a Xe light

simulator,with added optical filters. We are also modifying a large area pulsedsolar simulatorfor testing strings

and panels made from MJ cells. The simulator coefficient is checked by radiometers, and also using balloonflown subcelisor secondary standards to calibrate simulator.

The I-V data are analyzed in detail to extract the performance of the overall cell and of the subcells.

Other tests include:

- Visual check of surface morphologyof MGCVD grownlayers. (This follows procedures developed

successfullywith GaAs/Ge cells)

- Detailed analysisof illuminated I-V performance

Spectral analysis of individualcell output

C,-V Polaron profiling,to dedve the impurity concentration and thicknessof the main layers, usually

the emitter and base of the subcells.

X-ray diffractionto check the latticespacing and strain

Spectral reflectance to check solar absorptance andAR coatings

For tests of external cell features, (contact strength,temperature cycling, humidity etc.) the usual QA equpment

and procedures are used.

RESULTS ACHIEVED

The initial efficiency goal (21.5 - 22%) has been achieved for large area cells (over 4 cm2, up to 36 cm2), grown

in a large productionMOCVD reactor.

As mentioned above, the major factor in achieving high efficiency for large area MJ ceils is notto correct for the

slight loss in CFF resulting from longer grid length, but the need to optimize all the critical layer growth

parameters over larger areas.

Figures 1 and 2 show AM0 I-V curves for two cells of different area. The radiation performance has been

modeled. Figures 3 and 4 show the performance of dual-junction cells for a range of 1 MeV fluences, as a

function of the subcell current mismatch. Figure 5 showssome experimental radiation data for dual junction

cells, plottedwith the model predictions, o

Figures 6 and :Zshowssome in-line characterization results,the measurement of the spectralresponse of each

subcell, and the deviation of the bandgap of the GalnP2 cell.

These MJ cells have successfullypassed the space qualificationtests shown in Table1.

Present efforts continue to increase the yield for cells made underproductionconditions.

-58-
DEVELOPMENT WORK

In additionto the on-line yield improvements, several other areas of MJ cell development are proceeding at

Applied Solar, to meet future productionrequirements.

Enhanced Dual JunctionCells

Work is proceeding to develop dual-junctioncells with enhanced output. This involves improving the passivation

at several cell interfaces, and also some fine-tuning of the growth parameters. The design of the enhanced

outputcells (minimum average goal 24%) will also ensurethat the cells meet specifiedradiation exposures.

Triple-JunctionCells

The user-demands for GaAs/Ge cells led to the requirement that the GaAs/Ge interface should be

photovoltaically inactive, and the growthprocedureswere definedto tenderthis interface inactive.

For MJ cells, it is easier to provide matching currentin a Ge cell undera 2J cell, and work has proceeded to add

a third (Ge) cell underthe dual junctioncell. In the best case, this should further increase efficiency to 26%, with

no penalty in radiation resistance,and minimum increase in temperature coefficients.

Applied Solar has continued work begun (with RTI) on making dual junction monolithic cells from AIGaAs &or

GalnAsP) cells grown on Ge cells. We. are re-evaluating the options for forming Ge cells, which retain best

performance alter exposure to the full growthsequence of GaAs and GalnP2 cells.

On a NASA-Lewis SBIR, in Phase I we studied the options available to include Ge PN junctions. The methods

adjustment of the growth parameters to form the PN junction in-situ (the method used to make GaAs/Ge cells

with active interface), or preforming Ge PN cells, by epitaxial growth, or by ion implantation or diffusion. We

have established that high emitter dopingdensity (>5 x 10la cm_) is needed to ensure effective PN junctions in

P/N structures after As in-diffusionoccursduringgrowthof the GaAs cell.

We are also studying the many impurity interactionsthat take place across the GaAs/Ge interface during the

growthschedule.

We have established that the quality of present N-Ge substratescan provide matching currentwhen illuminated

under the dual-junction layers. The main emphasis is on maximizing the Voc and CFF of the Ge cell under the

same illumination conditions.

Other MJ Cells

We have begun work (with NREL and RTI) on an NRL contract, to investigate MJ cells (InP/InGaAs) grown on

Ge substrates. The goal of this contract is to provide high EOL performance. The main technical emphasis is to

optimize the cell performance by growingintermediate layers with increasinglatticespacing on the Ge substrates

to reduce the effects of the lattice mismatch between Ge and the othertwo cell materials.

We are also continiung to explore the methods developed with RTI, on mechanically stacked MJ cells with

improved cell-cell bonding methods. This option allows cells with suitable bandgaps but widely varying lattice

spacingsto be combined effectively.

SUMMARY

This paper described Applied Solar's currentactivity in MJ cells. The goals are to meet increased user demands

for higher efficiency large area space cells, at productionlevels. We have described the productionbuild up of

dual junction cells, primarily in demonstrating growth of all the DJ cell layers in a large throughput MOCVD

reactor. We have also established effective in-line characterization, analysis and testing methods. Spacequalified post-growthprocessmethods have been demonstrated.

-59-
Weoutlinedworkto enhanceDJcells, and to add a thirdjunction. We also described work on some associated

MJ cell designs.

Table I Space Qualification Tests Completed

PARTICULATE RADIATION

UV RADIATION

HUMIDITY

OPTICAL PROPERTIES

VVELDABlUTY

SOLDERABIUTY

ANGLE OF INCIDENCE

REVERSE BIASSOAK

REVERSE BIAS CYCLING

TEMPERATURE COEFFICIENTS

THERMAL CYCLING

PROTON ENERGY:

200 KeY

FLUENCES

1E + 11 TO 1E + 13 e/cm2

ELECTRON ENERGY:

1-MeV

FLUENCES

3E+13 TO 3E + 15 e/cm2

550 - SUN-HOURS AT 1-SUN AMO

95% AT 45°C FOR 30 DAYS

ABSORPTIVITY = 0.89, EMISSIVITY = 0.85 W1TH OCLI 0213

GLASS COVERSLIDE

FRONT/BACK CONTACT 45° PULL, 350 GRAMS

FRONT/BACK CONTACT 45" PULL, 600 GRAMS

0" TO 80° SUN ANGLE AT ROOM AMBIENT

30-MINS DARKREVERSE AT 120°C,1.67 X 1so

> 25,000 CYCLES AT 120°C, 1.67X 1so

28"C TO 120"C, 0 TO 3E +15, 1-MeV ELECTRONS

5,000 CYCLES FROM -110"C TO 120"C

-60-
i/I

I,.

¢)

(3.

E

<

E

I,.

I,.

-1

C.)

v

.14

.12

.I0

.08

.06

.04

.02

Isc= 121.7rnA

Voc= 2420.8mV

Cff = 82.6 %

Eff = 22.5 %

Pm= 243.4 mW

Vm = 2091.1mV

Im= 116.4mA

Pm

,,l,t,,I,

0 .5 1 1.5 2 2.5

(Volloge,Volts)

Figure t I-V Data (AM0, 28°C) for

Dual-Junction Cell, Area 8 cm2

L

0.

E

<

r-

"I

u

v

Isc= 532.3mA

Voc= 2394mV

Cff = 81.1%

Elf = 21.4

Pm= 1033.5mW

Vm = 2077.7mV

Im= 497.4mA

Pm

,,,,,,,,,l,,,,1,,,,l,,,,,,,,,l,,,,,,,,,l,,,,w,,

.5 I 1.5 2

i

(Volfoge,Volls)

llllll

2.5

Figure 2 I-V Data (AM0, 28°C) for

Dual Junction Cell, Area - 36 cm2

-61-
/

24.0

22.0

Q}

0

=n

¢:: 16.0 LU

C)

'[4.0

12_0

I0.0

/ 3EI4

_'-" 7- 7¢3Et5

f

J

08 09 , _1 1.a *-_

.TscRatio (GaA_/Ga.Co.P2)

Figure 3 Influence of Current Mismatch on AMO

Efficiency of a Dual Junction Cell Exposed

to Several I MeV Electron Fluences

C_

tc

(o

E

03

OE

n

_E

_.0 _

0.9"

0.8

0.7

0

r- 0.6

o

u_ 0._

I.

7 0.8

.L

7

./

/

/

/

L. -/

0.9 1.0 1.1 1.,?.

Js'c Ratio (GaAs/GalnP2)

" 3EI3

1EI4

3E14

-1EI5--

-3E15

1.3 1.4

Figure 4 Influence of Current Mismatch on Fraction

of Power Remaining, for a Dual-Junction Cell

Exposed to Several t MeV Electron Fluences

-62-
>..

C)

(.--

(D

e--

0

em

t.i._.

t.¢._

i,!

23

22

21

2O

19

18

17

IG

15

14

13

12

11

10

0

L

L

b

MISMATCH RATIO

V = 1.00

O = 1.05

171-- MEA_3_ED

X = L20

0

• .,x .... I .... t .... ! .... v.... 1.... I .... I .... I .... l .... v.... I .... I .... l .... 1.... 1

1013 1014 10 is 1016

1 MeV Electron Fluence (cm 2)

Figure 5 Radiation Performance of Dual-Junction

Cell [Modeled and Expedmental]

L

I

I

It¢.t

i

I

q

I

I

q!

I

I

to',e

Figure 6 Spectral Response of Dual-Junction Cell

-63-
0.72 •

0.70'

0.68

0.66

0.64'

0.62 '

0.60

0.58

0

y - 0.68602- 4.8S6Se-3z R^2 - 1.000

| "

o

D

a - i • i • _ "- _1

2 4 6 8 10

Q.E"° Z

Computed bandgap - 1.8o7ev

Figure 7 Bandgap of GalnPz Derived

From Spectral Response

-64-
InP tunnel junctions for InP/InGaAs tandem solar cells (11

M. F. VILELA =, N. MEDELCI b, A. BENSAOULA b, A. FREUNDLICH a, and P. RENAUD a

=Space Vacuum Epitaxy Center, University of Houston, Houston, TX 77204-5507

bAlso International Stellar Technologies, Inc., 9209 Hilldale St., Houston TX USA 77055

d . We report, for the _rst time, an epitaxially grown lnP p+/n ++ tunnel junction. A diode with peak current

ensizies up to 1600 A/cm'- and maximum specific resistivities (Vp/Ip - peak voltage to peak current ratio) in

the range of 10"4.Q.cm 2 is obtained. This peak current density is comparable to the highest results previously

reported for lattice matched In0.53Ga0.47As tunnel junctions. Both results were obtained using chemical beam

epitaxy (CBE). In this paper we discuss the electrical characteristics of these tunnel diodes and how the growth

conditions influence them.

1. Introduction

The drive for higher photovoltaic efficiency has led the technology away from single-junction cells and toward

multiple-bandgap (tandem) cells. These multi-bandgap devices Can be better matched to the solar spectrum. In

this approach, cells of different bandgaps are placed optically in series, either during the growth process

(monolithic tandems) or by being joined together after individual processing (mechanically stacked tandems).

With regards to production costs and compatibility with current panel fabrication standards, the monolithic

approach is superior.

InP solar cells have shown higher radiation resistance than the more traditional solar cells such as Si

and GaAs, and are ideally suited for space applications. Due to their potential high efficiency, InP/InGaAs

tandem solar cells seem to be the ideal doublet for both space and terrestrial applications.

Chemical Beam Epitaxy (CBE) has been shown to be a powerful technology for the growth of

phosphorus-based compounds. Moreover, CBE permits reproducible and precise control over the layer

composition and doping and has demonstrated growth of complex heterostructures with interfaces below one

monolayer fluctuation. Therefore, CBE appears to be the technique of choice for solar cells such as the twoterminal multi-junction InP/InGaAs.

The key to achieving very high conversion efficiencies for monolithically integrated tandem solar cells is

realizing stable and optically transparent tunnel junctions. We have recently demonstrated In0.53Ga0.47As

tunnel junctions with very high peak current densities on InP, GaAs, and Si substrates using CBE. Growing InP

layers on top of these tunnel junctions did not degrade them [1]. However, InGaAs tunnel diodes absorb a

significant portion of the low energy photons thus limiting the efficiency of the tandem device. In this work we

report the first realization of an InP tunnel junction with a peak current density of 1600 AJcm '_. The InP tunnel

junction is utilized to provide the crystallographically compatible and optically transparent ohmic interconnect

between the InP and InGaAs solar cells.

2. Experimental growth

Epitaxial runs were accomplished in a Riber CBE 32 system using Trimethyl-lndium (TMI), TriethyI-Gallium

(TEG), and pre-cracked Arsine (ASH3) and Phosphine (PH3) as growth precursors. Solid Beryllium and Silicon

were used as p and n doping sources respectively. 17 .3 2o -3

Be-doped p-lnGaAs with net hole concentrations varying frorq72 x._0 cm tq92 x .130 cm and Sidoped n-lnGaAs with net electron concentrations varying from 1 x 10 cm to 2 x 10 cm were achieved;

more details on these results can be found in reference [2].

(1) This work was supported by the following State of Texas Advanced Technology and Research Programs:

#93-03652-224, #93-03652-236, #93-03652-243, #93-03652-260. The work at I.S.T. Inc. was supported by

NASA SBIR Program #NASW-4093.

-65-
The InPlayersweregrownat temperaturesrangingfrom753K(480°C)to 803K (530°C) and growth

rates }b_.aryi_r_gfrom 0_5 _._/hr to 1 _m/hr. Be-doped p-lnP with net hole concentrations varying f.r_om

2 x 1019 cm.s to 1 x 10 cm and Si-doped n-lnP with net electron concentrations varying from 1 x 10 cm to

4x 10 cm were achieved.

The inP beryllium doping study shows that the p-type carrier concentration levels vary with Be source

temperature, the growth temperature and growth rate, Fig. 1. A similar behavior was observed for Be-doped

lnGaAs layers grown by CBE [3]. A drastic drop in the carrier concentration for higher Be source temperatures

was observed; the same behavior exists for Be-doped InP epilayers grown in our laboratory by molecular beam

epitaxy (MBE). The threshold for this drop seems to be related to other growth parameters. For example, at a

growth rate of l_tm/hr and a growth temperature of 783K (510°C), the maximum doping level reached was for a

Be source at 1203K (930°C). Using a growth rate of 0.75 p.m/hr and the same growth temperature of 783K

(510°C), the maximum doping level was reached for a Be source at 1153K (880°C).

Fig. 1 shows that the carrier concentration is more sensitive to growth temperature than to growth rate.

For instance, a change in the grq_th ._ate from 11t_o0.7_5 p.m/hr (2.78 to 2.08 A/s) only slightly increases the

hole concentration, from 4 x 10 cm to 5x 10 cm'3; this difference is within the experimental error.

However, if the growthTtempjerature isl_Jecr.e3ased by 45 K (from 828K (555°C) to 783K (510°C)) the doping level

increases from5xl0 cm tolxl0 cm .

The Be source activation energy was determined to be 3.0 eV.

o

tO

tQ)

fJ

tO

O

E}}

t-

.w

t_

0

"0

e_

m

fi2

e1E19

1E18

1E17

1E16

).80

4-

I ! I

,+

rate = 1 _Lm/h, Tg = 783 K

rate = 0.75 l.tm/h, Tg = 783 K

rate = 0.75 t.tm/h, Tg = 828 K

rate = 0.75 _Lm/h, T9 = 763 K

... ,,0.J85 0.90' 0.95 "',

Be effusion celt temperature (1000/T(K))

1.00

Fig. 1: Hole concentration in InP as a function of the Beryllium source temperature.

Fig. 2 shows the electron concentration as a function of the Si effusion cell temperature. High and low

electron concentrations were obtained. The electron concentration was found to be minimally sensitive to both

the growth temperature and the growth rate.

The silicon activation energy was found to be 7.57 eV.

-66-
E

E:

O

e.-

O

O

towQ..

O

.w

CO

¢-

1E19

1E18:.

1E17

v

I " I I ' ! I "

rate= 1 _En/h

rate = 0.75 p.mlh

1E16

0.68 0._0 " '0.72 0.74' ' 0.76 ' 0.'78 0.80

Si effusion cell Temperature (1000Fr(K))

Fig. 2: Electron concentration in InP as a function of the Silicon source temperature.

3. Photovoltaic results

The CBE technique is expected to allow the realization of tunnel junctions with superior properties. Those

devices require both degenerately doped semiconductor layers and low interdiffusion of doping species in the

narrow (~20 nm) space charge region of the junction. This is possible since high quality lnP and InGaAs layers

can be grown at much lower temperatures than those required in more conventional techniques, such as liquid

phase epitaxy (LPE) and metallorganic vapor phase epitaxy (MOVPE).

In previous reports [1 2] we have reported In 0 53Ga0 47As tunnel junctions with peak current densities of

up to 1015 A]cm 2 using CBE. In0 53Gao 47As tunnel jun_ions grown on GaAs and Si substrates which yield

peak current densities as high as 560 AJcm 2 were also demonstrated [1], see Table 1. We would like to point

out two major results of that work. First, the peak current densities reached by those diodes were the highest

ever reported for this kind of tunnel diode. Second, and most important, those characteristics were unchanged

even after subsequent growth of a thick (3 p.m) InP solar cell at high growth temperature (>833K (560°C)),

representing more than 2 hours of growth. We summarize our In0.53Ga0.47As tunnel diodes results in

Table 1.

Table 1

InGaAs Tunnel Diodes

substrate peak current resistivity annealing

A_cm 2 (xl 04).O.-cm 2

InP 1015 2.5 No

GaAs 452 4 No

Si 560 4.8 No

lnP* 580 3.14 No

InP** 860 2.9 2 hr (> 560°C)

* tunnel diode belongs to the same wafer used for **

** half wafer used in * was reloaded in the growth chamber and had an additional InP solar cell grown on it.

-67-
Our preliminary work on the growth of InP and InGaAs solar cells resulted in photovoltaic converters with

AM1.5 efficiencies equal to 18% for lnP and 10.2% for In0.53Ga0.47As. We have also fabricated InP/InGaAs

tandem devices which were grown using a 0.1 _m thick InGaAs tunnel diode as the interconnect. The tunnel

diodes are shown to provide a low resistivity ohmic contact, even at simulated high concentration sunlight. The

open circuit voltage of the tandem device is the sum of the InP and InGaAs individual cell voltages. The

spectral response of this tandem revealed minimal sensitivity for photons with energies between 1.4 eV and

0.75 eV. This is due to photon absorption in the InGaAs tunnel diode. The use of very thin tunnel diodes should

minimize this absorption. However, Wanlass et al [4] showed, by using MOVPE, that the use of ultra-thin tunnel

diodes precludes using this tandem technology for high concentration applications, because the degradation of

the tunnel diode electrical characteristics.

We summarize our photovoltaic results in Table 2.

Table 2

Photovoltaic Performances

Solar Cell Voc Jsc FF TI

(AM1.5 values) V mA/cm 2 % %

InP 0.81 28 80 18.1

InGaAs 0.25 60 62 10.2

InP/InGaAs tandem* 1.2 60 82 m

InP/lnGaAs tandem 1.0 8 76 6.3

* - under concentrated light; Voc - open circuit voltage; Jsc - short circuit current density; FF - fill factor;

- efficiency; AM1.5 - air mass 1.5.

4. InP tunnel diodes devices

The samples grown in this study are simple InP p+/n ++ junctions. In order to investigate the evolution of the

tunnel characteristics for tandem solar cell applications, all samples have an additional InP p-type layer grown,

which simulates the InP top solar cell in the actual tandem device (the thickness was determined by computer

modeling [5]). The growth is terminated with an In0.53Gao.47As p-type contact layer. The complete structure is

shown in Fig. 3. For all samples analyzed, the n-doped InP layer characteristics were kept constant, with the Si

doping at N D- N A = 1 x 1019cm "3, a thickness of 750 )_, a growth temperature (Tg) of 783K (510"C), and a

growth rate (rq) of 1 i_m/hr. Only the p+ layer growth parameters were varied. _=ig. 4 shows the I-V

characteristics bf a device, CBE 399, exhibiting a peak current density of 1,600 A/cm. This tunnel junction

imposes a voltage drop on the tandem device in the microvolt range, for a sunlight concentration of 100x (AM0

spectrum).

The Be-doped InP layer for this device was grown under the following conditions: T9 = 773K (500°C);

rg = 0.751_m/hr; and TBe = 1153K1_880.°_C). The doping level in this layer, as extracted from the Hall

measurements, is N A - N_ = 7x 10 cm . To our knowledge this is the first demonstration of an

epitaxially-grown InP tunne_diode.

The dependence of the tunneling characteristics on the Be-doped InP layer growth parameters was

investigated. The tunneling characteristics are completely lost if, taking the growth conditions of the sample

CBE 399 as reference (see Table 3), we,increase the growth temperature by 30K (from 773K (500°C) to 813K

(530°C)), or.if we increase the growth rate by 33% (from 0.75 to 1 i_m/hr (2.08 to 2.78 A/s)), or if the Be source

temperature rises beyond the threshold, as explained in the section 2. The results are summarized in Table 3.

The samples shown in the table were selected to stress the level of growth control required in order to achieve

tunnel diodes with superior properties. More studies are necessary to figure out the mechanisms controlling the

diffusion and/or the incorporation of Be in lnP.

The doping levels of both layers composing tunnel diode CBE-399, as deduced from our Hall

measurement calibration graphs, were: 7 x 101_cm 3 for the p layer (see Fig. 1) and 1 x 1019 cm -3 for the n

layer (see Fig. 2). Those doping concentrations give an effective doping level, N*= NAX ND/(NA + No), of

4 x 1018 cm 3. The theoretical expression developed by Kane [6] (see Vilela et al [2]), correlating the effective

doping level and peak current in tunnel diodes, predicts a peak current of less than 200 A/cm'. However, the

-68-
2

1,600 AJcm peak current density exhibited by CBE-399 demands an effective doping level of 8 x 1018 cm "3, at

least 2 x 1019cm s for each adjacent layer in the CBE 399 device. In other words, the theoretical calculatio_j

implies that one or both doping layer levels in that tunnel junction are underestimated. Also, the 1,600 A/cm

peak current value shows a minimal interdiffusion between both layers. Indeed, if Be or Si were fast diffusing

elements in CBE grown InP material, tunnel diodes heated to 8_3K (530°C) for 20 minutes, as CBE-399 was,

would show peak current densities much lower than 1,600 A/cm.

Secondary Ion Mass Spectrometry (SIMS) profiles will be performed in this device, in order to address

this discrepancy between theory and experiment.

Table 3

InP Tunnel Diodes Growth Parameters Effects of the Be doped InP layers.

SAMPLE Growth Temp. Growth Rate Be Temp. Peak Current

°C _tm/h °C A/cm 2

CBE 399 500 0.75 880 1,600

CBE 469 500 1 880 No (B)

CBE 449 530 0.75 880 No (Z)

CBE 407 500 0.75 920 No (S)

For all the samples the n++doped (silicon) InP layer characteristics were fixed at a ND- NA = 1 X 10

thickness of 750 ,_, a growth temperature (Tg) of 783K (510°C) and a growth rate (rg) of I i_m/hr.

No - means no tunnel characteristics presented.

(Z) means Zener, (B) means Backward or (S) means Simple diode.

cm ,

In 0 5_Ga0 47As:Be

'p_+=1x1020

0.1 I._m

lnP:Be

p=2x1018

0.25 _m

InP:Be

p+=5x1018-7x1018

0.075 _m

InP:Si

n++=1x1019

0.o75

InP:Si

n=2xl 018

0.05 _m

InP:S

n-type substrate

n=2xl018

=400 l.Lm

Fig. 3: Schematic representation of the lnP tunnel diode Studied in this work.

-69-
Fig. 4: I-V characteristic of an InP tunnel junction (mesa 200 l_m in diameter) fabricated on InP

substrateo This tunnel diode has a cap layer simulating the lnP solar cell growth during the tandem

fabrication ( 20 rain growth at 803K (530°C)).

5. Conclusion

In summary, using Chemical Beam Epitaxy, we have demonstrated the fabrication of InP tunnel diodes with

very high peak current densities (up to 1600 Ncm"). This peak current is one of the highest ever reported for

tunnel junctions fabricated with III-V compounds. This realization should allow the growth of InP/InGaAs

tandem solar cells with optically transparent and electrically compatible interconnects for concentrator

applications. This tunnel junction induces a voltage drop in the micrcvolt range for a concentration of 100x

AM0. We are currently working toward the fabrication of such monolithically integrated InGaAs/InP tandem

solar cells. We have also demonstrated the drastic influence of the growth rate and growth temperature on the

tunneling characteristics of these devices. Finally, the absence of any degradation in InP tunnel diodes

subjected to the growth of a top InP solar cell, combined with the efficiencies reached by our single solar cells

(InP and InGaAs in Table. 2), make CBE the ideal technology for the fabrication of high performance

InGaAs/InP tandem solar cells.

The authors would like to thank Alex Ignatiev for helpful discussions, David Moore for the legibility of this

text and Ping-chi (Pablo) Chang for his assistance during CBE growth.

6. References

[1] A. Freundlich, M. F. Vilela, A. Bensaoula and N. Medelci, Proc. 23rd IEEE-PV Conf. (1993) 644.

[2] M. F. Vilela, A. Freundlich, A. Bensaoula and N. Medelci, 5th International Conference on InP and Related

Materials, Paris, France, IEEE Catalog #93CH3276 (1993)227.

[3] R.A. Hamm, M. B. Panish, R. N. Nottenburg, Y. K. Chen, and Humphrey; Appl. Phys. Lett. 54 (25) (1989)

2586

[4] M.W. Wanlass, J. S. Ward, K. A. Emery and T. J. Coutts, Proc. 23rd IEEE-PV Conf. (1993) 621.

[5] P. Renaud, M. F. Vilela, A. Freundlich, N. Medelci and A. Bensaoula; Proc. of the Xll Space Photovoltaic

Research And Technology Conference (SPRAT Xtl), Cleveland (1994) 141.

[6] E. O. Kane; J. Appl. Phys. 32 (1961) 83.

-70-
THE ROLE OF RADIATION HARD SOLAR CELLS IN MINIMIZING THE COSTS OF GLOBAL SATELLITE

...... COMMUNICATION SYSTEMS .....

Geoffrey P. Summers

Code 6615, Naval Research Laboratory, Washington, DC 20375 and

Department of Physics

University of Maryland Baltimore County, Baltimore, MD 21228

Robert J. Waiters

Code 6615, Naval Research Laboratory, Washington, DC 20375

Scott R. Messenger and Edward A. Burke,

SFA, Inc., Landover, MD 20785

INTRODUCTION

An analysis embodied in a PC computer program is presented, which quantitatively

demonstrates how the availability of radiation hard solar cells can help minimize the cost of a

global satellite communications system. An important distinction between the currently

proposed systems, such as Iridium, Odyssey and Ellipsat, is the number of satellites employed

and their operating altitudes. Analysis of the major costs associated with implementing these

systems shows that operation at orbital altitudes within the earth's radiation belts (103 to

104km) can reduce the total cost of a system by several hundred percent [1,2], so long as

radiation hard components including solar cells can be used. A detailed evaluation of the

predicted performance of photovoltaic arrays using several different planar solar cell

technologies is given, including commercially available Si and GaAs/Ge, and InP/Si which is

currently under development.

Several examples of applying the program are given, which show that the end of life

(EOL) power density of different technologies can vary by a factor of ten for certain missions.

Therefore, although a relatively radiation-soft technology can usually provide the required EOL

power by simply increasing the size of the array, the impact upon the total system budget could

be unacceptable, due to increased launch and hardware costs. In aggregate, these factors can

account for more than a 10% increase in the total system cost. Since the estimated total costs of

proposed global-coverage systems range from $1B to $9B, the availability of radiation-hard

solar cells could make a decisive difference in the selection of a particular constellation

architecture.

EMERGING SATELLITE COMMUNICATION SYSTEMS

The first satellite communication systems deployed provided mainly point-to-point

communications, although connectio_ to mobile handsets was possible where cellular systems

were available. The new systems are designed to provide global communication services

directly to maritime and land-mobile terminals in addition to fixed communication centers. To

provide global coverage, some of these systems plan to orbit as many as 840 satellites

(Teledesic). The orbital altitudes selected range from low earth orbit (LEO, 1000 km or less)

to geostationary earth orbit (GEO, 35,793 km). Medium earth orbits (MEO) are used to

designate altitudes between these extremes and would include highly elliptical orbits (HEO).

The geostationary orbit, initiated with INTELSAT I in 1965, has been widely used for global

communications. However, the propagation delay (0.6 seconds for a typical international

-71-
connection), high latitude shadowing effects, the power required for the up-link and the

possibility of catastrophic system failure due to the loss of a single satellite, have led to the

development of alternative LEO and MEO systems.

THE OPTIMUM ALTITUDE

The total cost of deploying a global satellite communications system is a strong function

of altitude. The requirement for global coverage defines the minimum number of satellites

required as a function of altitude. The number decreases by a factor of about 25 in moving from

LEO to GEO. On the other hand launch costs increase markedly as the altitude increases. When

communication systems costs are calculated based upon satellite cost per kg and launch cost per

kg, a shallow minimum in the cost curve is found at altitudes between 2,000 and 10,000 km.

Fig. 1 shows typical estimates for launch costs and the total mass of satellites in orbit plotted

versus altitude. The total mass is the product of the number of satellites and their individual

mass. Launch costs are taken from the TRW Space data Book and are an average for several

launch vehicles, including various configurations of the Titan IV, and the shuttle.

The exact location of the minimum in the cost curve is sensitive to relatively small

changes in the detailed parameters of a given system especially when details about satellite

capability are included, e.g. the number of communication channels, the type of data

transmitted, etc. A general estimate of system costs based on Fig, 1 are compared with actual

cost estimates for individual systems in Fig. 2. As can be seen all of the data indicate that below

2,000 km the costs of these systems increase precipitously. A curve similar to Fig. 2 has also

been discussed by Logston [1].

There are a number of other reasons why a MEO system is a favored configuration. These

include lower life-cycle costs than LEO because of fewer satellites (and ground stations for some

systems), less potential for launch failures than LEO, shorter transmission delay than GEO,

higher elevation angles than LEO or GEO, and less frequent handoffs than LEO [3].

The major obstacle to establishing cost-effective MEO systems is the fact that the earth's

radiation belts reach a maximum intensity in this region. Of electronic satellite components,

solar cells are particularly vulnerable to radiation damage because of the necessity to maximize

their exposure to the sun. The availability of radiation hard solar cells are therefore critical to

achieving he potential benefits of MEO operation.

SELECTING AN OPTIMUM SOLAR ARRAY FOR A MEO COMMUNICATIONS SYSTEM

In order to compare different array technologies and to optimize the implementation of a

given technology, many different factors must be accounted for [4]. The computer program

developed for this purpose calculates the EOL power density of solar arrays taking into account

the cell geometry, coverglass thickness, support frame, electrical interconnects, electrical

harness, adhesive, and packing density. The EOL power density can be determined for any

altitude from LEO to GEO, and for e_uatorial to polar planes of inclination. The mission duration

can be varied over the entire range planned for the proposed satellite systems. An algorithm is

included in the program for determining the degradation of cell efficiency due to proton and

electron irradiation for different solar cell technologies [5] using the data given in the Solar

Cell Radiation Handbook [6]. Solar flare effects can also be included.

The program can be used to determine the optimum configuration for any cell technology

as a function of a particular orbit and a specified mission life. Consequently, it is possible to

compare the expected performance of all kinds of cells including those that are currently

available and those that are expected to be available in the near future. The critical factor from

-72-
the cost point of view is the EOL power density in watts per kilogram. This can be translated

directly into cost. Factors such as beginning of life (BOL) efficiency and even the cost per cell

are found to have a much weaker impact on the final cost of the system.

RESULTS OF THE ANALYSIS

An example of the results obtained with the program is shown in Fig. 3 for three

different solar cell technologies: Si, GaAs/Ge, and InP/Si. It will be shown below that if the

array support material is kept the same, the single most important variable is the coverglass

thickness. In Fig. 3, the optimum coverglass thickness for each technology for orbits in the

radiation belts has been used. The values for the remaining parameters such as the cell

thickness, the support frame and the mission life were made identical. It can be seen that the

InP/Si technology surpasses all others in EOL power density even though the BOL efficiency was

equal to or less than the other two technologies. The difference was especially noticeable in

orbits in the radiation belts where the InP/Si EOL power density exceeds those of the other

technologies by 50 to 300%.

In Fig.4 we show that there is an optimum coverglass thickness for each technology if the

EOL power density is to be maximized for a particular mission. For operation in the radiation

belts, the optimum thickness for InP/Si is -12 mils, whereas for GaAs/Ge it is -30 mils and

for Si -20 mils. Slightly different curves would be obtained for different missions.

The sensitivity of the cell types to factors such as BOL efficiency and substrate thickness

has also been examined. These factors are not as readily changed as coverglass thickness but can

be expected to improve as cell development continues. Results appear in Figs. 5 and 6. In Fig. 5

the EOL power density is given versus the BOL efficiency. These curves were derived assuming

that as-grown and radiation-induced defects have the same effect on cell efficiency. For silicon,

the BOL efficiency has no effect on EOL power density over the full range studied for this

particular mission. Some improvement is obtained for InP/Si and GaAs/Ge up to about 15%,

but little enhancement in EOL power density occurs beyond that point. InP/Si is markedly

superior to the others for BOL values ranging from 10 to 25%. Fig. 6 shows that the EOL power

density is relatively insensitive to cell thickness. As was found for BOL efficiency, the relative

ranking of the different technologies is not changed as cell thickness varies from 2 to 16 mils.

Some insight into the contribution of the various factors to the EOL power density can be

obtained by examining their relative weights for typical input values. Results for the three

technologies are given in Table I for the particular mission used for Figs 3 - 6.

Table I

Percentage Contributions of Major Elements to the Total Array Weight

InP/Si _ Si

12 mil glass ° 30 mil glass 20 mil glass

12 mil cell 8 mil cell 4 rail cell

14% BOL 18% BOL 14% BOL

Frame 59.7 43.4 60.2

Glass 19.4 33.7 31.6

Cells 19.1 21.1 2.9

Elect. Ham. 1.8 1.7 1.8

-73-
As can be seen in Table 1, the largest single contributor to the total weight of all the

arrays is the honeycomb frame holding the cells. It should be noted that in addition to structural

support, the frame plays an important role in shielding the cells from the underside,

contributing the equivalent of a 30 mil coverglass. Unlike efficiency and cell thickness, the

array material can be changed relatively easily. Results of substituting a flex array with 1/10

the density of a honeycomb are shown in Fig. 7.

Fig. 7 shows that marked improvement occurs in the EOL power density of the InP/Si

technology as a result of using the flex array• The influence upon the other two cell types is

marginal. The reason for the difference is the radiation hardness of the InP/Si cell. Note that

the shielding effect of the substrate has been taken into account for InP/Si and GaAs/Ge in Fig. 7,

but not for the -4 mil thick Si cells, because it has such a small effect.

SUMMARY

We have examined the various factors which influence the EOL power density for three

different planar solar cell technologies. The performance of the InP/Si technology in the middle

of the radiation belts was found to exceed those of alternative technologies by a substantial

margin. Factors such as BOL efficiency and cell thickness did not strongly influence the results.

However, the density of the array support frame has a marked effect on the result.

To achieve the same total power output as the InP/Si technology, the mass of the more

vulnerable solar arrays would have to be increased by as much as 450%. The additional

hardware and the associated launch costs add significantly to the cost of the total system.

Technical problems introduced by the larger arrays can further magnify the costs. The

availability of radiation-hard solar cells, therefore, could make a decisive difference in the

selection of a particular constellation architecture when total system costs are measured in

billions of dollars.

ACKNOWLEDGEMENTS

This work was supported in part by the Office of Naval Research. The authors would like

to acknowledge the assistance of Harvey B. Serreze of Spire Corporation with the computer

programming.

REFERENCES

•

2.

.

T. Logston, "Mobile Communication satellites", McGraw-Hill, 1995.

W.W. Wu et al., "Mobile Satellite Communications", Proc. IEEE, 82, 1431-1448,

1994.

o

N.D. Hulkower,"An Update on*The Mobile Communications Scene", 1EEE Communications

Chapter Boston, MA, 14 Sept. 1995.

° S. Bailey and G. Landis, "Use of Advanced Solar Cells for Commercial Communication

Satellites", IEEE Proc. 1st World Conference on Photovoltaic Energy Conversion,

Hawaii, Dec. 1994, pp 2002-2005.

. G.P. Summers, R.J. Waiters, M.A. Xapsos, E.A. Burke, S.R. Messenger, P. Shapiro and

R.L. Statler, "A New Approach to Damage Prediction for Solar Cells Exposed to Different

-74-
°

Radiations", IEEE Proc. 1st World Conference on Photovoltaic Energy Conversion,

Hawaii, Dec. 1994, pp:2068-2073.

H.Y. Tada, J.R. Carter, B.E. Anspaugh and R.G. Downing, in "The Solar Cell radiation

Handbook", JPL Publication 82-69 , 1982 and B.E. Anspaugh, "Solar Cell Radiation

Handbook - Addendum 1" 1982 - 1988", JPL Publication 82-69, Addendum 1, 1989.

-75-
GLOBAL MOBILE SATELLITE COMMUNICATION SYSTEMS

Cost Determining Factors as a Function of Altitude

Fig.1

100 -_ i ..j ....

so......................... I i

O ........... i

.......! iiiltt -=,o..........\ /

:: / CO

• _ .-

I 0 .....

100 1,000 10,000

Orbital Altitude (km)

100

8O

6O

4O

2O

0

100,ooo

r--

c

o

:y

(3

The total mass of a global mobile communication satellite system and the launch cost per

kilogram, both as a function of orbital altitude.

GLOBAL MOBILE SATELLITE COMMUNICATION SYSTEMS

Cost vs Orbital Altitude

Fig.2

4.0

3.5

3.0

2.5

O

•-- 2.0

_o 1.5

O

1.0

0.5-

0.0

100

= IRIDIUM

.i, []

ELUPSO

MSAT

t I

1,000 10,000 100.000

Orbital Altitude (kin)

The estimated total cost of a global mobile satellite communication system as a function

of orbital altitude, based on inputs such as the total mass of the system and launch costs.

Actual estimated costs by proposers of several commercial systems are also shown.

-76-
EOL SOLAR ARRAY POWER DENSITY vs ALTITUDE

10 Year Mission - Circular Orbit -60 ° Inclination

Fig. 3

6O

5O

o'l

C

¢ 30-

Q

o 20-

o.

,.J

0

UJ

10-

_ 12 mil coverglass

_'," _'"- InP/St__ 30 rail coverglass ",,,.___.7 .....-.................................

_ : ." 20 rail coverglass

'z,, .._:.-"

BOL Efficiency

InP/SI 14%

GaAs/Ge 18%

Si 14%

I I I

16 18 20

0 I 1 I I I I ,, I

0 2 4 6 8 10 12 14 22

Altitude ( 103 km)

End of life (EOL) power density as a function of orbital altitude for arrays of InP/Si,

GaAs/Ge and Si solar cells for a 10 year mission in a circular orbit with a 60o angle of

inclination. A standard AI honeycomb array material has been used in each case.

EOL POWER DENSITY vs COVERGLASS THICKNESS

5 Year Mission - 7400 km Circular Orbit - 60" Inclination

Fig. 4

50

8 mil thick cells

"_ 40 - InP/Si

30 - GaAs/Ge .....

• .,"

0 .-" ._/ ....................

...., ;i:>.........................

,--I // _ j

O " BOL Efficiency

uJ 10 - / SI 14%

0 I

/

I I I I t t I

GaAs/Ge

InP/SI

I I

14%

18%

I 1

0 5 10 15 20 25 30 35 40 45 50 55 60

Coverglass Thickness (mils)

End of life (EOL) power density vs coverglass thickness for arrays of InP/Si, GaAs/Ge

and Si solar cells, for a 5 year mission in a 7,400 km circular orbit with a 60o angle of

inclination. This is an orbit in the heart of the earth's radiation belts.

-77-
60

EOL POWER DENSITY vs BOL EFFICIENCY

5 Year Mission - 7400 km Circular Orbit - 60 ° Inclination

Fig. 5

5O

O}

,,'5

40

c 30 ID

Q

o 2O

p

12 mil coverglass

30 mil coverglass

_ GaAs/Ge _ .......................

. _. ........... 20 mil coverglass

..J

0

U,I

10- 8 rail thick cells

2 kg/m 2AI-Honeycomb

t

0

10 15 20 25

BOL Efficiency (%)

End of life (EOL) power density vs beginning of life (BOL) efficiency for arrays of

InP/Si, GaAs/Ge and Si solar cells, for a 5 year mission in a 7,400 km circular orbit

with a 60o angle of inclination. Note that the optimum coverglass thickness has been

used for each cell technology.

EOL POWER DENSITY vs SOLAR CELL THICKNESS

5 Year Mission - 7400 km Circular Orbit - 60 ° Inclination

Fig. 6

6O

50

40

3o

o

no 20

,_1

O

I,LI

10

InPtSt /

2 kg/m 2N-Honeycomb

..............................

...................... ,;7 • .........................

.. BOL Efficiency

Si 14%

- InP/Si 14%

GaAs/Ge 18%

0 | ] | I ,. I l ,

2 4 6 8 10 12 14 16

Solar Cell Thickness (mils)

End of life (EOL) power density vs cell thickness for arrays of InP/Si, GaAs/Ge and Si

solar cells, for a 5 year mission in a 7,400 km circular orbit with a 600 angle of

inclination.

-78-
EOL SOLAR ARRAY POWER DENSITY vs ALTITUDE

10 Year Mission - Circular Orbit ° 60° Inclination

Rex Array - 0.2 kg/m 2

Fig. 7

80 _._ "_

qt°

,q

W

e-

¢=

6O

_ 40-

-,I

8

20-

_ GaAs/Ge_..."

20 mil coverglass

| I I I I f

4 6 8 10 12 14

Altitude ( 103 km )

0 !

0 2

12 rail coverglass

30 rail coverglass ..---

.°-'' •

i

J

/

J

s

BOL Efficiency

InP/Si 14%

GaAs/Ge 18%

Si 14%

1 1 I I

16 18 20 22

End of life (EOL) power density as a function of orbital altitude for arrays of InP/Si,

GaAs/Ge and Si solar cells, for a 10 year mission in a circular orbit with a 60o angle of

inclination. A flex array material has been used in each case. Compare this figure with

Fig.3 to see the effect of the lighter array material.

-79-
High and Low EnergyProton Radiation Damage

in pin InP MOCVDSolar Cells

GeorgeRybicki and Irv Weinberg

Photovoltaic Branch

NASA Lewis Research Center, Cleveland, Ohio.

Dave Scheiman

NYMA Inc. Brookpark, Ohio

Carlos Vargas-Aburto and Roberto Uribe

Kent State University, Kent, Ohio

Abstract

InP p+/rYn+ solar cells, fabricated by metal organicchemical vapor deposition, (MOCVD) were

irradiated with 0.2 MeV and 10 MeV protonsto a fluence of 1013/cm2.The power output degradation,

IV behavior, carrier concentration and defect concentration were observed at intermediate points

throughout the irradiations. The 0.2 MeV proton-irradiated solar cells suffered much greater and more

rapid degradation in power output than those irradiated with 10 MeV protons. The efficiency losses were

accompanied by larger increases inthe recombinationcurrents in the 0.2 MeV proton-irradiated solar cells.

The low energy proton irradiationsalso had a larger impact on the series resistance of the solar cells.

Despite the radiation induced damage, the carrier concentration in the base of the solar cells showed no

reduction after 10 MeV or 0.2 MeV proton irradiationsand even increased during irradiation with 0.2 MeV

protons. In a deep level transient spectroscopy (DLTS) studyof the irradiated samples, the minority carder

defects H4 and H5 at Ev + 0.33 and Ev + 0.52 eV and the majority carder defects E7 and E10 at Ec- 0.39

and Ec-0.74 eV, were observed. The defect introduction rates for the 0.2 MeV proton irradiations were

about 20 times higher than for the 10 MeV proton irradiations. The defect E10, observed here after

irradiation, has been shown to act as a donor in irradiatedn-type InP and may be responsible for obscuring

carder removal. The results of this study are consistentwith the much greater damage produced by low

energy protons whose limited range causes them to stop in the active region of the solar cell.

Introduction

A near recordAMO efficiency InP solar cell of 19.1% was achieved through MOCVD growth of a

homoepitaxial InP p+/n/n+ structure.(1) This type of structure has the potential to simplifythe growth of

heteroepitaxial InP cells on Si by eliminatingthe problem of counter doping of the base region inthe InP

on Si n on p cells. Electron irradiationtesting of MOCVD InP p/n cells has also shown that they have

superior radiation resistance to MOCVD InP n/p cells. (2)

4

Several phenomena are observed in irradiationstudies of n-type InP that may contribute to its improved

radiation resistance, it has been observed that carder concentration in n-type InP, rather than decreasing,

increases after electron and proton irradiation.(3,4) This phenomenon, which we shall call carrier

enhancement, has been observed only in pn diodes in which the p region was zinc doped. (5)

Furthermore, this effect is correlated witha specificdefect through the use of DLTS. (3) In contrast, it has

been shown that the carder concentration in p-type InP decreases after either proton or electron

irradiation.(6,7) In the present study we observe the effects of high and low energy proton irradiations on

the defect structure and carrier concentration in the n-type region of the diode. In particular we focus our

attention on the radiation effects which involve low energy protons whose range is such that they stop

within the active n-type region of the pn diode. We concern ourselves mainly with the defect structures,

-80-
possible carrier enhancements and the differences in the effects of low and high energy particles on the

series resistance and performance of InP p+/n/n* solar cells. It is of particular interestto observe the

effects of carrier enhancement on the performance of the solar cells.

The diodes and solar cells used inthis study were fabricated by MOCVD at the Spire Corporation under

contract with NASA Lewis. The cell configuration,dopants, and concentrations are shown in Figure 1.

Cell performance was measured in the calibration laboratoryof the PhotovoltaicBranch at NASA Lewis,

using a Spectrolab X-25 xenon arc solar simulatorand a flight-calibrated InP standard cell. The low energy

proton irradiationswere performed at the Universityof Michigan's ion Beam Laboratory and the high

energy proton irradiationswere performed inthe tandem Van de Graaff accelerator of Western Michigan

University. Spectral response and IV measurements were made priorto the irradiationsand at each step

throughout. Carrier concentrations were measured usingthe capacitance-voltage (CV) technique. The

concentration and energy levels of the radiation-induced defects were measured by DLTS. The

concentration of defects have been corrected for incomplete trap filling (8), and the energy levels have

been corrected for the FrenkeI-Poole effect. (9)

Results and Discussion

The pre-irradiationperformance parameters of the p on n cellsare given in Table 1.

Jsc (ma/crn2) Voc (mV) FillFactor % Efficiency %

0.2 MeV 23.6 851.4 84.7 12.41

10 MeV 22.5 854.9 85.2 11.95

Table 1. Pre-irradiation cell performance parameters

The performance of these cells was clearly notstate of the art. As indicated before, cells of up to 19.1%

have been fabricated in this configuration,(1)The cells used inthis study were produced as part of a

development program; therefore, growth and cell design parameters had not yet been optimized. The

post irradiationcell performance parameters are given as a function of fluence in Table 2. The normalized

efficiency as a function of fluence in presented in Figure 2.

IEnergy (MeV)

Ruence cm-2

0

1011

lOre

1013

0.2

efficiency %

12.41

8.37

4.85

2.75

0.2

(ma)

23.6

19.8

14.3

10.8

0.2

Voc (mV)

851

772

649

545

10

efficiency %

11.95

11.81

10.45

7.90

10

 c(ma)

22.5

22.4

21.4

19.0

10

vcc(ma)

855

852

828

755

Table 2. Pre and Post Irradiation cell performance parameters

The dramatic difference in damage rates is immediately evident. "]"helow energy protons produce far

more degradation than the 10 MeV protons. The reason is that the range and damage production rate of

the two particles differ dramatically.The 10 MeV proton has a projected range of 497 um and will pass

-81 -
completelythroughtheactiveregionofthecell.The0.2MeVprotonhoweverhasa projected range of

1.56 um and will stop inside the active region of the cell. (10) An incident proton produces maximum

damage at the end of its track and, since the 0.2 MeV proton stops in the active region of the solar cell it

produces a greater number of defects inthis region of the cell. The low energy proton thus produces a

much greater amount of degradation. The degradation is dominated by a reduction in short circuit current

in the 0.2 MeV cells but is comparable inopen circuitvoltage and short circuitcurrent in the 10 MeV

irradiated cells. This is also related to the differences in damage productionof the two particles. Although

the 0.2 and 10 MeV irradiationsboth increased the dark currents, the 0.2 MeV proton irradiations had a

greater effect by more strongly reducing the diffusion length and thus reducing the current collected. This

effect is clearly visible inthe exlemal quantum efficiency curves given in Figure 3.

Figure 3 shows the extemal quantum efficiency of the unirradiatedand irradiatedcells. The 0.2 Mev

proton irradiations had a more pronounced effect on the quantum efficiency. The 10 MeV irradiations

produced a relatively uniform degradation in output as a function of wavelength. The 0.2 MeV irradiations,

however, produced a larger degradation in the red end of the spectrum. This reflects the strong effects of

the 0.2 MeV protons in reducing the diffusion lengths inthe cells. The quantum efficiency in the red

portion of the spectrum, where the light is absorbed more weakly, and thus penetrates to a deeper depth,

is more strongly affected by the low energy protons which have a greater effect in reducing the diffusion

length of photogenerated carders.

The 10 MeV proton radiation resistance of the p/n/n+ MOCVD solar cells versus n/p MOCVD solar

cells (2) and diffused junction n/p solar ceils(11), is shown in Figure 4. The radiation resistance of the

p/n/n+ cells inthis study is better than that of the n/p MOCVD configuration but is still not as good as that

of the diffused junction cells. The starting efficiencies of all three cells however, were not the same and so

although the results suggest that the p/n MOCVD configuration is better than the n/p MOCVD

configuration, the data must be viewed cautiously. The variations in radiation resistance between the

diffused junction and MOCVD n/p configurations has been the subject of some study. (12) The

differences in performance between MOCVD and diffused junction cells have to do with the annealing of

defects in the diffused cells. In the diffused junctioncells the radiation induced defects anneal easily

during operation and the power output recovers, while in MOCVD cells, the defects, as identified by

DLTS, anneal out, cell power output recovery is notobserved, or at least notto the same extent. (12) The

variation in radiationresistance between cells based on n and p type InP materials may also have to do with

the introduction rate of the defects. It has been demonstrated that the introduction rate of defects is lower

in n type than in ptype materials. (13,14) An absence of radiation induced carrier removal may also help

improve the radiation resistance in n type InP.(3)

Carder concentrationdata for the irradiated samples are presented in Figure 5. It can be seen that not

only is carder removal not observed after proton irradiationbutthat the cartier concentration increases

slightlyin the case of 10 MeV irradiations,and stronglyinthe 0.2 MeV irradiations.The lack of carder

removal has been previouslyobserved in n-type InP and has been correlated with a radiation induced

defect. (3) As the 0.2 MeV protons have a higher defect productionrate than the 10 MeV protons, they

produce a larger increase in carder concentration.

IV curves were also taken throughout'irradiationsand cell parameters were measured. The data is

presented in Table 3. The series resistance as a function of fluence is plotted in Figure 6.

From the data it can be seen that the 0.2 MeV proton irradiationsproduce a larger increase in the

recombination currents inthe solar cells. The differences produced in series resistance as shown in

Figure 6, became more pronounced at higherfluences. The recombination current is related to the defect

concentration in the depletion region and the 0.2 MeV protons produce more defects in this region and

thus a greater increase in recombination currents. In the case of the low energy protons the degradation in

the diffusioncurrent is slightly higher than the degradation in the recombination current. This indicates

significant damage in the depletion region, emitter and base. In the case of the 10 MeV irradiationsthe

degradation in the recombination current is very limited, but much greater inthe diffusion

-82-
IEnergy (MeV)

!Ruence (cm-2)

0 3.2 x 10-17

0.2 0.2 0.2 10 10 10

J01(Ncm2) Jo2(Ncm2) Rs (ohm) Jol (Ncrn2) J02(A/cm2) Rs (ohm)

5.3x10-14 0.49 5.85x10-18 1.17x10-12 0.51

1.08xl 0-9 5.91xl 0-18 2.22xl 0-12 0.52

5.5 x10-8 0.56 1.04x10 -17 7.84x10 -12 0.63

1011 1.85xl 0-11

1012 1.5 xlO -11

10_3 1.2 x10-9 1.75x10-8 1.7 9.09x10-12 7.11x10-11 0.61

Table 3. Pre and Post Irradiation IV Data

current. This suggests that little damage is produced in the emitter and the depletion region by the

10 MeV protons and that most of the damage occurs in the base. This again is a result of the high damage

production rate of the low energy protons which stop inthe active region of the solar cell as opposed to

the high energy protons which pass through the cell or produce damage deeper inthe base of the cell.

The series resistance of the solar cells was also more stronglyaffected by the lowenergy protons but

the effect was pronounced only at the highest fluence. This effect occurs at the same point where the

carder concentration increase was observed inthe low energy proton irradiated samples. It is apparent

that the carrier concentration increase, which has been related to the presence and concentration of a

radiation induced defect (3), does not reduce the series resistance or benefit cell performance. The

increase in carrier concentration has not been observed inthe 10 MeV proton irradiated cells due to the

lower defect introduction rates of the 10 MeV protons.

DLTS was used to study the defect spectra and to measure the defect introductionrates in the

samples. The DLTS spectra are shown in Figures 7 through 11. A preirradiationanalysis showed that no

deep levels were present in the samples. The defect concentrations were measured throughout the

experiments and the introductionrates calculated. The data appears in Tables 4 and 5.

Fluence,_ (cm-2) 1 x 1011 5x 1011 I x 10_ I x 1013

NT (cm°3) E7 1.08x 1014_+0.1 3.65x1014+0.16 5.87x 1014_+0.20 18.67x 1015_0.41

NT (cm"3) H4

NT(cm -3) H5

NT (cm-3) E10/I I

NT/(I)(cm-3) E7

-- 1.55 x 1014:1:0.1 2.96 x 1014_+0.08 --

1.06x 1014 3.08 x 1014+ 0.28

4.88 x 1015_0.41

1080 73O 587 865 (873)

NT/¢ (cm-1) H4 -- 310 296 -- (296)

NT/$ (cm-l) H5 -- 212 308 -- (308)

NT/(_(cm-1) E10/11 -- -- -- 488 (488)

Table 4. Defect Concentrations and Introduction rates in 0.2 MeV Proton Irradiated InP

(Numbers in parenthesis are a least squares fit of all Nt vs. _ data.)

-83-
Fluence,_) (cm-2) I x1011 1x1012 1x1013

NT (cm-3)E7 m 6.48x 10134_-_0.53 5.24x 1014+0.21

NT (cm-3) H4 m 2.92x 1013__.0.28 1.82 x 1014+ 0.08

r ,,

NT (cm"3) H5 _ 1.56x1013+-0.41 1.22x1014+0.13

,=,

NT (cm"3) E10/11 m _ 7.79x1013_+0.40

NT/_ (cm-1) E7 64.8 52.4 (51.8)

N-r/_) (cm-1) H4 _ 29.2 182 (17.7)

N1V(1)(crn-1) H5 m 15.6 12.2 (12.1)

_ -- 7.8 (7.8)

NT/_ (cm -1)E10/11

Table 5. Defect Concentration and Introduction Rates in 10 MeV Proton Irradiated InP

(Numbers in parenthesis are a least squares fit of all Nt vs. (_data.)

The post irradiation DLTS analysis revealed the presence of two deep levels in the minority carrier

spectra and three deep levels in the majoritycarrier spectra. Only two of the majority levels could be

resolved in both the high and low energy proton irradiations, the first, Ec-0.37 eV is referred to in the

literature as E7.(5) The second level was found considerably deeper in the gap at Ec-0.74 eV, and is

known as E10 or E11, (4,5). An additional level, E9 at Ec-0.64 eV was observed after the 10 MeV proton

irradiations but it could notbe clearly resolved in the 0.2 MeV spectra and no systematic study of this

defect was performed. The defect E7 is associated with an antisite vacancy pair, (Vin-Pln)-(17) The defect

E10/11 is correlated with a displacement inthe P sublattice of InP and thought to be a complex between

this displacement induced defect and an intrinsicdefect or impurity. (3)

The relative concentrations of the two defects inthe majority carrier DLTS spectrum in the case of the

0.2 MeV irradiations is not in agreement with some of the results in the literature, ie. E7 not E10111 is the

defect with the largest introductionrate.(5) This however is a result of the metastable nature of the defect

E7. The defect E7 is the major defect in the spectrum known as the B configuration, which results from

cooling the sample under reverse bias.(17)

The minority carrier spectra inboth highand low energy proton irradiationsshowed two deep levels at

Ev+0.29 and Ev+0.52 eV, referred to in the literature as H4 and H5.(15). The defect H4 is thought to be a

displacement on the P sublattice of InP, possibly Pin- H5 is thoughtto be a complex of a defect and a

dopant impurity, Vin-Zn or PIn-Zn, as evjdenced by the strong dependence of its introduction rate on the

dopant concentration. (18) The minoritycarrier spectra shown here in which H4 is the defect with the

highest introduction rate, is in agreement with the spectra published inthe literature.(15)

The defect introduction rates were calculated from a least squares fit of the defect concentration

versus particle fluence. The data are shown graphically in Figures 11 and 12. The data presented in the

graphs corresponds to a least squares fit of all data up to the highestfluence at which reliable data could

be extracted, the final value calculated in this manner is presented in parenthesis in the highest fluence

column. This was a particularproblem inthe case of minoritycarrieranalysis after 0.2 MeV irradiationsat the

highest fluences. A high concentration of defects can affect the qualityof a DLTS analysis by introducing

leakage currents, increasing series resistance or by producingvery high defect concentrations. (16) In the

-84-
caseofthelowestfluence10MeV irradiations the defect concentrations were below the detection limitof

our instrument. The variation in defect introductionrates calculated witheach additional data point is

illustrativeof the error which might be introduced by estimating introductionrates from a single data point,

a practice which is common in the literature.This estimationhowever was necessary in this case for the

calculation of the defect E10/11 which required scans up to 400K due to its large activation energy.

Intermediate measurements were not made in this case to avoid annealing out the radiationdamage.

The defect introductionrates measured here are very much higherin the case of the 0.2 MeV protons

than for the 10 MeV protons and both are considerably higherthan those measured for 1 MeV electrons.

(6,13) An attempt was made to correlate the defect introductionrates with the energy loss rates of the

particles in the material. The great majorityof the energy lossof a high energy particle incident on a

material is by ionizationof the latticeatoms, buta smallfraction is lostproducinglattice atomic

displacements. The energy lost by atomic displacement can be estimated usingTRIM. (10) The software

predicted that the 0.2 MeV protons would produce 25 times as much damage as the 10 MeV protons. In

comparing the introduction rates observed here, the ratio of defect introduction rates for 0.2 MeV to

10 MeV for the defects H4 and H7 is roughly 16, but for the defect H5 it is exactly the 25 predicted. Thus

there seems to be a reasonable correlation between the predicted energy loss and defect introduction

rates.

The exception to this observation is the relationshipin the introductionrates of E10/11 in the two cases

above, the prediction is a ratio of 25 but the observed ratio is63. The reason behind this discrepancy may

be related to the nature of the defect E10/11. This defect has only been found in p/n junction diodes

where the p dopant is zinc, it has not been found in irradiated Schottky barrier diodes. (5,13) Thus, it is

theorized that the formation of E10/11 requires the participationof Zn in the n-type material from the

diffusion front tail produced during the formation of the pn junction. (5) The introduction rate of E10/11

was also found to depend strongly on electron energy, leading one to believe that Zn atoms ejected into

or implanted intothe n region of the diode by proton irradiation may enhance the formation rate of E10/11.

(5) In this case the low energy proton irradiationsmay have caused a large redistributionof the Zn due to

the large number of displacements they cause in the junction region, as compared to the 10 MeV protons

and 1 MeV electrons. The 0.2 MeV protons thus produced an anomalously high defect introduction rate

for E10/11 by implanting more Zn into the n-type region of the diodes.

The superior radiation resistance of the p/n-type InP solar cellsmay be related to the defect

introductionrates. In this studythe introductionrate of the dominant majority carrier defect El0 in n-type

InP was about 8 per 10 MeV proton. The defect introductionrate for the dominant majority carder defect

H4 in p-type InP was found to be approximately 90 per 10 MeV proton. (12) The lower defect introduction

rate in n-type lnP may contributeto enhanced radiationresistance in p/n InP solar cells. This findingis an

agreement with previous authors who found the same relationship in electron irradiated INP.(13,14)

Conclusiorl_

The 0.2 and 10 MeV proton irradiationsproduced dramatically differentrates of degradation in the

p/n/n+ InP solar cells studied. The relative rates of damage were correlated withthe different ranges and

damage production rates of the two particles. The low energy protons stopped in the active region of the

cell and produced a dramatic decrease in efficiency,dominated by a decrease inthe diffusion length of

photogenerated carders. The 10 MeV protons produced degradation dominated by an increase in dark

currents in the cell. The MOCVD p/n/n+ InP solar cell configurationstudied here showed better radiation

resistance than the MOCVD n/p configuration in the literature. Studies of the radiation damage showed

that the lack of carrier removal in n-type InP was notthe reason forthe enhanced radiation resistance, as

increases in series resistance and no cell performance recovery as observed to attend this phenomenon.

A DLTS study of the defects showed no significantdifferences in the defect spectra generated by low

and high energy proton irradiation, or differences withthe 1 MeV electron irradiationspectra in the

literature, but that the low energy protons produced 15 to 60 times as many defects as the high energy

protons. The defect introductionrate for the dominant majority carrier defect in n-type InP was found to be

-85-
lowerthan that in p-type InP, in agreement with the findingson electron irradiatedmaterials. This fact may

contribute to the superior radiation resistance in n-type InP materials.

Acknowledgements

The authors would liketo acknowledge the help of Dr. Stephen Ferguson, from Westem Michigan

University and Dr. Victor Rotberg of the Michigan Ion Beam Laboratoryof the University of Michigan for

their help with the proton irradiationsreported in thiswork. One of us (C.V.-A.) would also liketo

acknowledge partial support for this work from Wilberforce University, under subgrant number NCC3-281,

and from NASA LeRC, under grant numbers: NAG3-946 and NAG3-1726.

References

1. C. Keavney, V. Haven and S. Vernon, Proc. 21St. IEEE PhotovoltaicsSpecialist Conference,

p.141 (1990).

2. C. Keavney, R. Waiters and P. Drevinsky, J. Appl. Phys., 73, 60 (1993)

3. A. Sibile, Appl. Phys. Lett., 48, 593 (1986).

4. I. Weinberg, G. Rybicki, C.Vargas-Aburto, R. Jain and D. Scheiman, Proc. Of the XIII Space

Photovoltaics Research and Technology Conference, p. 147 (1994).

5. J. Benton, M. Levinson, A. Macrander, H. Temkin and L. Kimerling, Appl. Phys. Lett, 45, 567 (1984).

6. I. Weinberg, C. Swartz and P. Drevinsky, J. Appl. Phys., 72, 5509 (1992).

7. G. Rybicki and C° Zorman, J. Appl. Phys., 75, 3187 (1994).

8. Y. Zohta and M. Watanabe, J. Appl. Phys., 53, 1809 (1982).

9. G. Vincent, A.Chantre and D. Bois, J. Appl. Phys., 50, 5484 (1979).

10. J. Ziegler, J. Biersack and U. Littmark, "l'he Stopping and Range of Ions in Solids",

(Pergamon Press, New York, NY,1985).

11. I. Weinberg, C. Swartz, R. Hart and M. Yamaguchi, Proc. Of the 5th. European Symposium

on PhotovoltaicGenerators in Space", ESA SP267, p. 415 (1986).

12. R. Waiters, S. Messenger, G. Summers, E. Burke and C. Keavney, Proc. 22nd. IEEE Photovoltaics

Specialist Conference, p. 1560 (1991).

13. M. Levinson, J. Benton, H. Temkin and L. Kimrneding, Appl. Phys. Lett., 40, 990 (1983).

14. M. Levinson, H. Temkin and W. Bonner, J. Elec. Matls., 12, 423 (1983).

15. A. Sibile and J. Bourgoin, Appl. Phys. Lett., 41,956 (1982).

16. D. Schroder, Semiconductor Material and Device Characterization,

(Wiley Interscience,NY, NY, 1990), p. 336.

17. J. Wager and J. Van Vechten, Phys. Rev .B., 32, 5251 (1985).

18. M. Yamaguchi and K. Ando, J. Appl. Phys., 60, 935 (1986).

-86-
p+ 2 x 101 8 (Zn)

n 3.2x1016 (Si)

n+Sx 1018 (Si)

n 5 x 1018 (S)

Figure 1. Details of InP cell configuration

0.25 um

1.5 um

0.5 um

o

re

,.=

o

E

I/4

e

E

o

Z

1

0.8

0.6

0A

02

Normalized Efficiency as a

Function of Proton Ruence

0

10 lo

_...................,,........................'""_:_'"ff$'9"......

• • • =t_.l . . • aHHI • • • • i|=

10" 1012 10 I_

Fluence (cm"=)

F3gure2. NormaJizedef_ency as a

functJ_of protonfluence.

1

o

¢=

e

u 0.8

E

E 0.6

.=.

c

g

= 0.4

0

"' 02

i

0

0.3

''''1""' I'''" I''''1'''' I''''1'''" I ''='

E : ! .=

== 1 uilm_N

= ,

= ,

°°....._

: .. . , : _- :

0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

Wavolongth um

F'cjure3. External quantum efficiency of cells,

unirradiated and irrad_ted tol 012/cfrl2

1=3

1

0.95

0.9

0.85

0.8

0.75

0.7

0.65

:::"i....::...:..:..::::

% i =

.......................",,.,.,%...............%..........

i _ := ......

..................... :::::::::::::::::::::

•-'4-K._ey_,_tocvo • ........_::

0.6 .................................................... :..,,:.,"...;.;;._.

I0 Io 10'I 1012 101_

F]uence (cm")

F'_ure4. Normalizedefficiencyof various

cellsafter 10 MeV irradiations.

_', 5.0 1016

e

4.5 1016

,iqll

el

Ii

4.0 1016 ,

e,

o

3.5 1016

s_

II

_ 3.0 10 TM

101°

10 MeV

10 t] "0 x2 1013

ProtonFluence_m "2)

F-cjure 5. Carrier concentration as a

function of proton fluence.

'2

E

J¢

O

U

efO

4=1

w

1.8

1.6

1.4

1.2

I

0.8

0.6

0.4

101°

• 0.2 V.

I0 MeV . _ ........ ,r ..... :" ........ i

10 I1 1012 10 I_

Fluence (cm"=

Rgure 6. Series resistance as a function

of protonfluence.

-87-
-6O

"E -50

3

_£ -40

• _ ._

"6

J

a -10

0

0

2OO

Ec-OJI7 lO_=/cm= 1

_ t

250 300 350 400

L Kel_n

Rgum 7. MajoritycarrierDLTS spectrumof

02 MeV protonrradated InP.

0

__ -s

e=

= -10

-15

C

o

m -30

g

-4O

-' " " " I ' " ' " I _'" " " i ' '' ' "

I0 n/cm =

• • l • I l a i • I | i i z I • • • •

150 200 250 300

Kelvin

F_jure 8. Minoritycarder DLTS spectrumof

02 MeV protonirrad'BtedInP.

-35

_-3o

¢.

=1

J_

_-2o

Q

-15

'1

"6 -10

J

o -5

• " I ' " " " I ' ' ' ' I ' ' ' = I ' " " "'I

_ -10

i

-15

I , , = I

,',. i o-20

200 250 300 350 400 100

Kelvin

' ' = " I " " " ' I • • " ' I " ' " '

, • = • | • | • i | • • • • | • • • •

150 200 250 300

Kelvin

Rgure 9. MajoritycarrierDLTS spectrumof

10 MeV protonirradBtedInP.

Rgurel0. MinoritycarrierDLTS spectrumof

10 MeV protonirradBtedInP.

9.0 1015 .--. 6.0 10z4

' 8.0 1015 _ 5.0 1014

"" 7.0 1015 = _ 4.0 I0 z'

 .o5.0 10" lO Is ==_ 3.0 1014

4,0 1015 _ 2.0 1014

o= 3.0 10 _

r_ 2.0 I015 _ 1.0 1014

1,0 101'

210 t2 41012 61012 8101_ 11013 11013 21012 41012 61012 81012 11013 11013

Proton Fluence (era") Proton Fluence (cm "2 )

Rgure 11. Defect introduction rates in0.2 MeV

pinionradated InP.

F'_jure12. Defect introductionrates in 10 MeV

protoniradatedInP

-88-
Correlation of Electron and Proton Irradiation-Induced

Damage in InP Solar Cells

Robert J. Walters

Naval Research Laboratory, Code 6615, Washington, DC 20375

Geoffrey P. Summers

Naval Research Laboratory, Code 6615, Washington, DC 20375, and

Department of Physics, University of Maryland Baltimore County, Baltimore, MD 21228

Scott R. Messenger and Edward A. Burke

SFA, Inc., Landover, MD 20785

ABSTRACT

The measured degradation of epitaxial shallow homojunction n+p In_P solar cells under 1 MeV electron

irradiation is correlated with that measured under 3 MeV proton irradiation based on "displacement

damage dose". The measured data is an_yzed as a function of displacement damage dose from which an

electron to proton dose equivalency ratio is determined which enables the electron and proton

degradation data to be described by a single degradation curve. It is discussed how this single curve can

be used to predict the cell degradation under irradiation by any particle energy. The degradation curve is

used to compare the radiation response of InP and GaAs/Ge cells on an absolute damage energy scale.

The comparison shows InP to be inherently more resistant to displacement damage deposition than the

GaAs/Ge.

INTRODUCTION

When determining the best solar cell technology for a particular space flight mission, accurate

prediction of solar cell performance in a space radiation environment is essential. The current

methodology used to make such predictions requires extensive experimental data measured under both

electron and proton irradiation. Due to the rising cost of accelerators and irradiation facilities, such data

sets are expensive to obtain. Moreover, with the rapid development of novel cell designs, the necessary

data is often not readily available. Therefore, a method for predicting cell degradation based on limited

data would be most useful. Such a method has been developed at the Naval Research Laboratory based

on damage correlation using "displacement damage dose" which is the product of the non-ionizing energy

loss (NIEL)and the particle fluence. 1 Displacement damage dose is a direct analog of the ionization

dose used to correlate the effects of ionizing radiations. In this method, the performance of a solar cell in

a complex radiation environment can be predicted from data on a single proton energy and two electron

energies, or one proton energy, one electron energy, and Co 60 _ammas. Summers et al. 2 have used this

method to accurately predict the data measured by Anspaugh -_ on CoaAs/Ge solar cells under a wide

range of electron and proton energies. In the present paper, the method is applied to InP solar cells using

data measured under 1 MeV electron and 3 MeV proton irradiations, and the calculations are shown to

agree well with the measured data.

-89-
In addition to providing accurate damage predictions, this method also provides a basis for

quantitative comparisons of the performance of different cell technologies. The performance of the

present InP cells is compared to that published for CmAS/Ge cells. The results show InP to be inherently

more resistant to displacement energy deposition than CmAs/Ge.

EXPERIMENTAL NOTES

The 1 MeV electron irradiations were performed using the Van De Graft accelerator at the

National Institute of Standards and Technology (NIST). The 3 MeV proton irradiations were performed

using the Pelletron accelerator at the Naval Surface Warfare Center (NSWC) in White Oak, MD. In both

cases the beam current was kept low enough so that no sample heating occurred during irradiation. The

irradiations were done at room temperature, in the dark, and at open circuit. For the electron

irradiations, dosimetry was achieved with a Faraday cup and current integrator. The proton fluences

were determined by collecting all the charge striking the sample holder through a current integrator.

An Oriel 1000W Xe arc lamp solar simulator with AM0 filtering was used for PV measurements.

The lamp intensity was adjusted to 1 sun, AM0 using an InP reference cell calibrated by Keith Emery at

the National Renewable Energy Laboratory (NREL). Current-voltage (IV) curves were measured using

two Keithley 617 electrometers and a Kepco 50-2M bipolar amplifier.

The solar cells studied here are n+p shallow homojunctions grown epitaxially by metalorganic

chemical vapor deposition (MOCVD) on p-type InP wafers doped with Zn. The cells were grown by

Spire Corporation under contract to the Naval Research Laboratory. The cell base was 3p.m thick with

Zn as the dopant atom. The emitter was 300 A thick with Si or Se as the dopant atom. The cells were

square, and 0.5 cm on a side. The total area of 0.25 cm 2 was used in all calculations.

CORRELATING 1 MeV ELECTRON AND 3 MeV PROTON DAMAGE

For the present study, the degradation of some InP solar cells under 1 MeV electron and others

under 3 MeV proton irradiation has been measured. The irradiations were done incrementally with the

cells being characterized after each fluence increment. The measured degradation of the cell maximum

power (Pmax) under both irradiations is shown in figure 1. Since 3 MeV protons are more damaging, i.e.

have a larger NIEL, than 1 MeV electrons, the data sets are separated along the fluence axis. The goal is

to correlate these data so that the degradation can be described by a single curve. In the present method,

the first step is to convert the fluence values to displacement damage dose by multiplying by the

appropriate NIEL value. The calculated NIEL values for electrons and protons over a wide energy range

incident on Si, GaAs, and InP are tabulated in reference 2. From these tables, the NIEL for 1 MeV

electrons incident on InP is 3.348x10 "5 0VteV cm2/g) and that for 3 MeV protons is 2.03 lxl0 -2 0VleV

cm2/g). The resultant plot of the degradation ofPma x vs displacement damage dose is shown in figure 2.

As can be seen in figure 2, analyzing the degradation data as a function of displacement damage

dose greatly reduces the separation of the electron and proton data sets. The next step in correlating the

data is to determine an electron to proton dose equivalency ratio (R e ) Rep is defined as the ratio of the • P "

dose along the 1 MeV electron degradatmn curve to the dose along the 3 MeV proton degradation curve

-90-
which cause equal cell degradation. For the present data, Pep = 4_12. By dividing the dose values &the

1 MeV electron data by this ratio, the electron and proton degradation data are correlated (figure 3). The

reason that the electron and proton data do not initially correlate when plotted as a function of

displacement damage dose (i.e why Pep ¢ 1) is that electron damage coefficients in p-type InP do not

vary linearly with NIEL as will be discussed below.

Now that the degradation data has been properly correlated, a characteristic degradation curve for

these InP solar cells can be determined. This curve is determined by fitting the correlated data to the

following expression:

Dd

P_ (Dd)= A- C log(1 +-=-)

D,

D d -ffidisplacement damage dose

A,C,D x - constants

(1)

which is essentially the degradation equation given in the Solar Cell Radiation Handbook 4 except that,

here, the parameters are functions of displacement damage dose instead of particle fluence. For the

present data sets, these constants were determined: A = 24.3 (mW/cm2), C = 4.87 (mW/cm2), and D x =

1.79x109 (MeV/g). The fit is shown as a solid line in figure 3.

CALCULATING THE CELL RESPONSE TO OTHER IRRADIATIONS

Given the characteristic degradation equation, the response of these InP cells to any other

irradiation can be calculated. This is possible because radiation damage in semiconductors can be related

to NIEL. In the case of proton irradiation &p-type InP, the damage coefficients have been found to vary

lineraly with NIEL 5-7. Therefore, the degradation of these InP cells under any proton irradiation can be

calculated by simply dividing the dose range &equation (1) by the appropriate NIEL.

The case of electron irradiation of p-type InP is more complicated. The fact that the measured

electron and proton degradation data do not directly correlate when plotted as a function of displacement

damage dose (figure 2) indicates that the electron damage coefficients in p-type InP do not vary linearly

with NIEL. It has been found in p-type Si and C.mAS that the electron damage coefficients vary with the

square of the NTEL1, 5. Preliminary results indicate the same to be true for p-type ImPg, but more data is

needed to confirm this. Once the dependence on NIEL is established, the degradation under any electron

and proton irradiation can be calculated from the characteristic equation.

COMPARISON OF THE PERFORMANCE OF inP AND GaAs/Ge SOLAR CELLS

By applying the displacement damage dose concept, the response of a particular solar cell to

irradiation by a spectrum of particle energies can be expressed in a single curve. Therefore, this method

of analysis provides a basis for comparing the radiation response of different solar cell technologies on an

absolute scale. As an example, the response of CraAs/Ge solar cells can be compared with the response

of the InP cells studied here. In reference 2, a characteristic degradation curve was determined for

GaAs/Ge solar cells based on measurements made by Anspaugh 3. This curve is reproduced in figure 4

along with the characteristic degradation curve for ImP solar cells derived here. The InP curve lies above

-91-
the CraA_Ge curve. This indicates that InP solar cells are intrinsically more radiation resistant than

GaAs/Ge solar cells to displacement damage energy.

SUMMARY

The measured degradation of epitaxial shallow homojunction n+p InP solar cells under 1 MeV

electron and 3 MeV proton irradiation has been correlated based on displacement damage dose. From

the correlated data a characteristic degradation curve for these InP cells has been derived which can be

used to calculate the cell response under any particle irradiation. Therefore, through this damage

correlation method, the cell response to irradiation by any spectrum of particles can be calculated from a

minimum of experimental data. The characteristic equation has also been used to compare the radiation

response of these InP cells with that published for GaAs/Ge solar cells. The comparison shows these InP

cells to be inherently more resistant to displacement energy deposition than GaAs/Ge cells.

REFERENCES

1 G.P. Summers, E.A. Burke, and M.A. Xapsos, "Displacement Damage Analogs to Ionizing

Radiation Effects", Radiation Measurements, 24, No,. 1, 1-8 (1995).

2 G.P. Summers, R.J. Waiters, M.A. Xapsos, E.A. Burke, S.R. Messenger, P. Shapiro, and ILL.

Statler, "A New Approach to Damage Prediction for Solar Cells Exposed to Different Radiations", IEEE

Proc. 1st World Conference on Photovoltaic Energy Conversion, Waikoloa, Hawaii, Dec 5-9, 1994, p.

2068-2073.

3 B.E. Anspaugh, "Proton and Electron Damage Coefficients for GaAs/Ge Solar Cells", IEEE Proc.

22nd Photovoltaic Spec. Conf., Las Vegas, NV, Oct., p1593 (1991)

4 H.Y. Tada, J.R. Carter, B.E. Anspaugh, and R.G. Downing, "The Solar Cell Radiation

Handbook", IPL Publication 82-69, 1982.

5 G.P. Summers, E.A. Burke, P. Shapiro, S.R. Messenger, and R.J. Waiters, "Damage Correlation

in Semiconductors Exposed to Gamma, Electron, and Proton Radiations", IEEE Trans. Nuc. Sci., 40,

1300-1306, Dec (1993)

6 T. Takamoto, H. Okazaki, H. Takamura, M. Ura, M. Ohmori, M. Yamaguchi, S. Ikegami, H.

Arai, T. Hayashi, A. Ushirokawa, K. Takahashi, M. Koubata, and A Ohnishi, "InP Solar Cells for Space

Applications", Technical Digest of the International PVSEC-5, Kyoto, lapan, p547, (1990)

7 M. Yamaguchi, T. Hayashi, A. Ushirokawa, K. Takahashi, T. Takamoto, H. Okazaki, M.

Ohmori, S. Ikegami, and H. Arai, "Analysis of Space Flight Data of InP Solar Cells in EXOS-D Orbit",

IEEE Proc. 22rid Photovoltaic Spec. Conf., Las Vegas, NV, Oct., (1991)

8 S.R. Messenger, "Electron and Gamma - Induced Displacement Damage Effects in InP

Semiconductors Devices", Ph.D. Dissertation, Univ. of Maryland Baltimore County, 1995
25

20

=_

M

W

M 15

_Jl JJJJH_JJJJnJnqJoaJJ,tJI tJ tJJJJHt JJ tJJJHJ IJolnaJ,I IJ iaJJJJn I JJJtJJJJ

• 1 sun

m

10

10 9 1010

• AlVI0

• 298K

I

n

m

I

I IIIIIII[

l •

•

II I IIIIIII

3

1 MeV MeV

II IIIIIIII

electrons protons

II II1111_II IIIIIIII

epitaxial

II I IIIIId

n÷p

II

InP/lnP

I IIIIII1II 111

,1

1011 1012 1013 1014 1015 1016 1017

Particle Fluenee (cm "2)

FIGURE 1: Measured degradation of the maximum power of epitaxial shallow homojunction n+p InP

solar cells under 1 MeV electron and 3 MeV proton irradiation. Since 3 MeV protons have a larger

NIEL value than 1 MeV electrons, the data is separated along the fluence axis. The goal of the present

analysis is to correlate these data.

25

20

15

I I lllllq

I"

10

107

I l aJll. I I I llllll I

I MeV electrons

3 MeV protons

I I llllll[ l I IIIIII

Istm

AMO

29gK

I 1 IIIIIII I I IIIIIll I I IIIIIll I I IIIIII1 I II11111

l0 s - 109 lOIO l0 n lO12

Displacement Damage Dose (MeV/g)

FIGURE 2: Measured degradation of the maximum power of epitaxial shallow homojunction n+p InP

solar cells under 1 MeV electron and 3 MeV proton irradiation plotted as a function of displacement

damage dose. These are the same data as in figure 1 except that the fluence has been converted to

displacement damage dose by multiplying by the calculated NIEL value for each particle energy.

Analyzing the degradation data as a function of displacement damage dose brings the degradation curves

much closer together.

-93-
25L i tll.ll_lll.I I J lJlJllt I I JJll.q I I IIIIII

lsunepitaxial n 'p InP/InP _ 298K __

2O

_MM" _ . lMeVdectrons ¢orrc,at_d t °'K _

 ls- t • .. I \ -

-I Characteristic degradation CtaW¢I _-

B

lO I IIIII[II I l llillll I l lililll I l lllllll I IIIIIII

10v 108 109 lO1o lOll 1012

Displacement Damage Dose (MeV/g)

FIGURE 3: Correlation of the measured 1 MeV electron and 3 MeV proton irradiation damage. By

applying the Pep value of 4.12 to the data of figure 2, the degradation data is correlated. The

degradation under electron and proton irradiation can now be described by a single degradation curve

given by equation (1). The solid line is a fit of the correlated data to equation (1).

25

20

i 15

I.I

el

E

..... I I I i illl[ I i i l IIii[ i I i _

1 sun -

i ........." oo _

"'"""'-,..,..,. ,, ,.

'"'-% _

%.

10 I] ..............

5 I 1

"°.,..

-- Characteristic InP degradation curve ] ........ "q

ChEacteristic GaAs/Ge degradation curve

(from ref. 2) r

i Ililll ] J J Illill I i i lil

10s l(P 101° 10 u

Displacement Damage Dose (MeV/g)

FIG_ 4" Comparison of the radiation response of CraAs/Ge and InP solar cells. Since the

displ-acement-d-am_age dos_d-arn_ige correlation method allows-dae so|_-ceil degradat_onunderi_irradiatipn

by electrons and protons of any energy to be described by a single curve, it provides a basis for the

comparison of the performance of different cell technologies on an absolute scale. These data show that

InP is inherently more resistant tO displacemefit damage deposition than GaAs/Ge.

-94-
DIFFUSION LENGTHS IN IRRADIATED N/P InP-on-Si SOLAR CELLS

Steven Wojtczuk and Claudia Colerico

Spire Corporation

Bedford, MA 01730-2396

Geoffrey P. Summers and Robert J. Waiters

Naval Research Laboratory

Washington, DC 20375

Edward A. Burke

Independent Consultant

Woburn, MA 01801

OVERVIEW

Indium phosphide (InP) solar cells were made on silicon (Si) wafers (InP/Si) by to take advantage

of both the radiation-hardness propertiesof the InP solar celland the lightweight and low cost of Si wafers.

The InP/Si cell application is for long duration and/or high radiation orbit space missions. Spire has made

N/P InP/Si cells' of sizes up to 2 cm by 4 cm with beginning-of-life (BOL) AM0 efficiencies over 13% (onesun, 28C). These InP/Si cells have higher absoluteefficiency and power density after a high radiation dose

than gallium arsenide (GaAs) or silicon (Si) solar cells after a fluence of about 2e15 1 MeV electrons/cm2.

In this work, we investigate the minority carrier (electron) base diffusion lengths in the N/P InP/Si cells. A

quantum efficiency model was constructed for a 12% BOL AM0 N/P InP/Si cell which agreed well with the

absolutely measured quantum efficiency and the sun-simulator measuredAM0 photocurrent (30.1 mA/cm2).

This model was then used to generate a table of AM0 photocurrents for a range of base diffusion lengths.

AM0 photocurrents were then measured for irradiations up to 7.7e16 1 MeV electrons/cm2 (the 12% BOL

cell was 8% after the final irradiation). By comparing the measured photocurrents with the predicted

photocurrents, base diffusionlengthswere assigned at each fluence level. A damage coefficient K of 4e-8

and a starting (unirradiated) base electron diffusion length of 0.8 pm fits the data well. The quantum

efficiency was measured again at the end of the experiment to verify that the photocurrent predicted by the

model (25.5 mA/crn2) agreed with the simulator-measured photocurrent after irradiation (25.7 mA/cm2).

QUANTUM EFFICIENCY MODEL

The quantum efficiencymodel used, described well by HoveF, breaks upthe total quantum efficiency

into three components. The first is from the cell emitter. In an N/P InP cell, the emitter is very thin (300A)

to limitlightabsorbed in the emitter which is subjectedto a high front surface recombinationloss (le7 cm/s).

The emitter (hole) diffusion length is in almost all conceivable cases larger than the emitter thickness.

Therefore, the model results are virtually independent of emitter diffusion length. Emitter diffusion lengths

of 10, 1,0.1, or 0.05 pm give the same result as far as the quantum efficiency and the AM0 photocurrent are

concerned since all of these lengths are in excess of the 300A emitter thickness. This non-sensitivity of the

photocurrent to emitter diffusion length makes the study of the base diffusion length much easier.

-95-
Thesecondcomponent thatcontributestothe quantum efficiency is the NP junctiondepletionspace

charge region (SCR). In the NP InP cell the emitter is very heavily doped (> lelg/cm3) so that a one-sided

step junction approximation is used to calculate the zero-bias SCR width in the base (doping 3e17/cm3),

which is about 630A. To first order, the model assumes that any carriers photogenerated in the SCR are

immediately collected and this component does not depend on either base or emitter diffusionlengths.

The third model component is from the base region of the solar cell, and the model is essentially

similar to that of the emitter (butof opposite polarity and minority carrier types). The surface recombination

velocity at the back of the 31Jmthick cell was taken to be le4 cm/s, but the results are very insensitiveto this

value, since the base diffusionlengths are all less than 1pm and very little is collected from 3pm away from

the junction.

Figure 1 shows (black dots) the measured quantum efficiency of a l crn2 12% InP/Si cell before

irradiation. The AM0 photocurrentfrom the measured quantum efficiency and IoVmeasurements at one-sun

on a sun simulator(set witha NASA-calibrated InP reference cell) agreed (30.1 mA). The quantum efficiency

data were non-linear least squares fitted to the model usingthe Marquandt-Levenberg algori_m. The model

fit is shown as the uppermost solid line, and when integrated against the AM0 power specVum, gives a

photocurrentof 30.3 mA, in close agreement with the measured data. The base (electron) diffusion length

extracted at this pointwas 0.8pm. The cell had not yet been irradiated; the diffusion length is lower than in

homoepitaxial InP due to dislocation defects from the 8% lattice-mismatch inthe heteroepitaxial InP/Si cell.

:>,

O

riw

O.m

IJJ

E

tO

m

ri_

4)

,k,*

r"

m

1.0

0.9

measured QE of 12% BOL InPISi

(30.1 mA sun-simulator Jsc)

QE model (30.3 mA)

•" . "Lb:0.8pm . ° cell 5854-3533

" SCR (10.1 mA .... .

base (16.6 mA)

........ , -" le7 cm/s

• " Sb: le4 ¢m/s

. Le > We (0.03pro

500 600 700 800 900 1000

Wavelength (nm)

Figure I Measured (black dots) and model (solid line) quantum efficiencybefore irradiationfor a 12%

N/P InP/Si cell. Solid line (equivalent to 30.3 rnA AMO) is the sum of the three dotted lines,

representing contributions from the base (16. 6 mA), depletion space-charge region (SCR)

(10.1 mA), and the emitter (3.6 rnA). An electron base diffusion length of 0.81Jmfit the data.

-96-
DIFFUSIONLENGTHDETERMINATION

Usingtheabovequantumefficiency(QE)model,thepredictedphotocurrent wasobtainedversus

thebasediffusionlength(Figure2). Thiscurveis then used to estimate the diffusion length from the

measured photocurrent. The key to our experiment is the assumption the emitter diffusion length, when

irradiated, is always larger than the 300A emitter thickness, and that the space charge region is to first-order

constant under irradiation. With these assumptions, the base component of the quantum efficiency curve

dominates the photocurrent degradation with irradiation (the other components stay relatively constant).

_"E"35

E _ _ ....,,..._

: 30 ""

//

/

2o

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Base(Electron)DiffusionLength0un)

Figure 2 Mode/AMO photocurrent for an N/P InP/Si cell vs. base electron diffusionlength.

The AM0 I-Vs for the 12% AM0 BOL N/P InP/Si cell were measured from no irradiation to an

equivalent fluence of 7.7e16 1 MeV electrons/crn2, where the efficiency was 8%. The cells were irradiated

by alpha particles from a 1 mCi Am-241 source. This alpha source is small, self contained, and delivered

an equivalent fluence of 7.7e16 1 MeV electrons/cm2 in only 333 hours. Damage in InP from alphas is

accurately converted into 1 MeV electron equivalent fluence using the non-ionizing energy loss (NIEL)

method_'4. Forthe source-cell distance used, the 3.9 MeV alpha fluxwas calculated as 1.03e6 odcm2Js. The

calculated equivalent I MeV electronflux was 6.45e10 electrons/cm2/s. Table I showsthe equivalent 1 MeV

fluence, the measured photocurrent, and the base diffusion lengths obtained from Figure 2.

Table I F/uence vs. Measured AMO Photocurrent and Estimated Base Diffusion Lengths

Eqv. 1 MeV Electron Fluence

#/cm2

AM0 Photocurrent

mA/cm2

Base (Electron) Diffusion Length

(pm)

0 30.1 0.8

1.2 E 14 29.9 0.8

1.4 E 15 29.5

1.7 E 16 26.8

3.8 E 16 25.8

7.7 E 16 25.7

0.7

0.4

0.3

0.3

-97-
A standard empirical model for diffusion length versus fluence 5 was fit using Table I data (Fig.3).

The model parameters are the unirradiated electron diffusion length Lo (0.8pro, Fig. 1) and K, the damage

coefficient (4e-8). The fit is good except at the highest fluence; this may be due to radiation damage carder

removal effects changing the width of the space charge region; this effect was not included in our modeling.

A

E

=L v 1.0

.J

--=--0.9

e,

=, o.gq

c

o 0.7

W

=1

== 0.6

¢=

_" 0.5

2

_ o.4

lip

v 0.3

o

m 0.2

m

. 0.1

e

-- 0.0 t,1

[] extract

=(K_. ln.o2)-o.5 ",ol --o.- L

with K = 0.4e-7 -I

....... and Lo.-,0.8 .pm................. .._1

1012 10TM 10TM 1015 1015 1017

Eqv. 1 MeV Electron Fluence $ (#/cm2)

Figure 3 Estimated base electron diffusion length vs. fluence for an N/P inP/Si ceil

We could not measure the quantum efficiency after every irradiation due to some scheduling issues.

However, to confirm the QE model used in calculating Figures 1 and 2 and _e diffusion lengths in Table I

was still accurate after heavy irradiation, we measured the cell after the irradiations were all completed. The

data is shown in Figure 4. The fit still agrees reasonably well with the measured sun-simulator photocurrent.

1.0

J) " m_asure_l QE of'12;/o B_L inP/Si "

0.9

o_0.8

==

"_ 0.7

0.6.

E0.5

0 0.4

0.3

_ o.2

0.1

0.0

now 8%(25.7 mA sun-simulator Jsc)

after 7.7e16 1 MeWele/cm2

del (25.

Le > We (0.03_m)

:._ base (1"1.9m,) u!

:'.. SC'R (10.1 mAi'" .... . "

," ..._-.... ,q

' emitter (3.6 mA) ......

' ' ' ! I °] ° " I" " °|"

500 600 700 800 90(

nA

Wavelength (nm)

1000

Figure 4 Measured (black dots) and model (solid line) quantum efficiency after 7.7e16 1 MeV

electrons (12% BOL NIP lnP/Si cell is now 8%). Solid line (equivalent to 25.5 mA AMO) is

the sum of the three dotted lines, representing contributions from the base (11.9 mA),

depletion space-charge region (SCR) (10.1 mA), and the emitter (3.6 mA). An electron base

diffusion length of O.31Jm fit the data.

-98-
CONCLUSIONS

The minority carrier (electron) diffusion length in the P-type base of a 1 cm2 N/P InP/Si cell starts

at about 0.8pm (measured 12% AM0 efficiency,30.1 mA photocurrent) before any irradiation, and drops to

about 0.3pro after an extremely high7.7e16 1 MeV electron fluence (measured 8% AM0 efficiency,25.7 rnA

photocurrent). Except at possibly the highest fluence tested, it seems that the emitter and space-charge

region contributions to the quantum efficiency and photocurrent may vary only a little in these thin emitter

(300A) cells, with most of the degradation caused by decreased photocollectionfrom the base of the cell.

REFERENCES

I,

.

3.

o

o

S.J. Wojtczuk, N.H. Karam, P. Gouker, P. Colter, S.M. Vernon, G.P. Summers, R.L. Waiters

and R. Statler, "Development of InP Solar Cells on Inexpensive Si Wafers," Proc. of 1st

WCPEC (24th /EEE PVSC), 1994, pp. 1705-i708.

H.J. Hovel, Solar Cells, Vol.11 Semiconductors and Semimetais, Academic, 1975, pp. 17-20.

G.P. Summers, E.A. Burke, P. Shapiro, S.R. Messenger, and R.J. Walter, "Damage Correlations

in Semiconductors Exposed to Gamma, Electron and Proton Radiations',/EEE Trans. Nuclear

Science, 40, 1993, pp. 1372-1379.

G.P. Summers, R.J. Waiters, M.A. Xapsos, E.A. Burke, S.R. Messenger, P. Shapiro, and R.L.

Statler, "A New Approach to Damage Predictionfor Solar Cells Exposed to Different Radiations',

Proc. of 1st WCPEC (24th IEEE PVSC), 1994, pp. 2068-2073.

H.Y. Tada, J.R. Carter, B.E. Anspaugh, and R.G. Downing, Solar Cell Radiation Handbook, 3rd

Ed., JPL Publication 82-69, p. 3-19.

-99-
ELECTRON AND PROTON DAMAGE IN Ino.s3Gao.47As SOLAR CELLS

HAVING AN InP WINDOW LAYER

Scott R. Messenger

SFA, Inc.

Landover, MD 20785

Hector L. Coral and Robert J. Waiters

Naval Research Laboratory

Code 6815, Washington, DC 20375

Geoffrey P. Summers

Naval Research Laboratory

Code 661 5, Washington, DC 20375

and

Department of Physics

University of Maryland Baltimore County

Baltimore, MD 21228

ABSTRACT

As part of a continuing NRL program to optimize the space radiation resistance of InP/Ino.s3Gao.47As

tandem solar cells, Ino.53Gao.47As (referred to as InGaAs below) solar cells were irradiated with 1 MeV electrons

and with 3 MeV protons. The cells were grown with a 3/lm n-lnP window layer to mimic the top cell in the

tandem cell configuration for both AM0 solar absorption and radiation effects. The results have been plotted

against "displacement damage dose" which is the product of the nonionizing energy loss (NIEL) and the particle

fluence. A characteristic radiation damage curve is then obtained for predicting the effect of all particles and

energies.

Air mass zero (AMO), one sun solar illumination IV measurements were performed on the irradiated

InGaAs solar cells and a characteristic radiation degradation curve was obtained using the solar cell conversion

efficiency as the model parameter. Also presented are data comparing the radiation response of both n/p and

p/n InGaAs solar cells. For the solar cell efficiency, the radiation degradation was found to be independent

of the sample polarity.

INTRODUCTION

The lnP/Ino.s3Gao.47As tandem solar=cell appears to be a promising solar cell technology for use in severe

space radiation environments, primarily due to the known superior radiation resistance of the InP top cell.

Several reports on both the tandem cell design and radiation response have been presented previously. One

sun, AM0, conversion efficiencies as high as 21.1 % and 22.2% at 25°C were measured for monolithic, twoterminal tandems with total areas of -4 cm 2 and -1 cm 2, respectively, for unoptimized cells(ref.1). The

efficiency could be increased to -26% when fully optimized(ref, t ). Several radiation studies have shown that

the InP/inGaAs tandem cell displays superior radiation resistance(refs.2,3). A new program is now underway

involving NRL, ASEC, RTI and NREL to grow the InP/InGaAs tandem cell on Ge substrates, thereby greatly

increasing cell durability and decreasing cost. Several cell designs are being considered, including both n/p and

p/n polarities. Since Ge is an n-dopant in both InP and InGaAs, the n/p configuration will have to include an

additional tunnel junction between the substrate and the bottom cell of the tandem.
To optimize the radiation degradation of InP/InGaAs, the radiation response of each component cell,

as well as combined in the tandem configuration, must be investigated. Furthermore, current-matching

between the subcells under irradiation is necessary while still maintaining the open circuit voltages. Otherwise,

current-limiting conditions set in and the tandem cell current would then be controlled by the less radiation

resistant subcell current. The optimization of each subcell can be achieved using such quantities as doping

concentrations and layer thicknesses(refs.2,3).

The radiation degradation of InP cells due to electrons and protons of several different energies for both

the n/p and p/n polarities have been reported(refs.4-6). Also, the radiation degradation of n/p and p/n InGaAs

solar cells to 1 MeV electron irradiation has been reported(refs.7,8). The radiation response of the twoterminal InP/InGaAs tandem cell (on InP substrates) in the n/p configuration under both 1 MeV electron and

3 MeV proton irradiations have been reported (BOL efficiency -20% on a 4 cm 2 cell)(refs.2,3). The results

showed that the InGaAs cell was the limiting subcell. Therefore, increasing the radiation response of the

InGaAs subcell is imperative for increasing the radiation response of the tandem cell.

This paper reports experimental results of both 1 MeV electron and 3 MeV proton irradiations on

InGaAs solar cells. Cells of both the n/p and p/n polarities were irradiated, with the p/n cells having only 1

MeV electron irradiation exposure. The results obtained from the 1 MeV electron and 3 MeV proton

irradiations were compared using "displacement damage dose", thus giving another example of how useful this

technique is in predicting the damage in any radiation environment(refs. 10,11 ). A single radiation degradation

curve will be presented which fully describes displacement damage effects in InGaAs solar cells. Several base

carrier concentrations were also considered in an attempt to optimize the cell radiation response to 1 MeV

electrons.

EXPERIMENTAL DETAILS

The irradiations were performed on two different InGaAs cell structures both grown by MOCVD.

InGaAs solar cells of the n/p polarity (A = 13.6 mm 2) were fabricated by the Research Triangle Institute (RTI)

while the p/n cells (A =25 mm 2) were grown by the National Energy Renewable Laboratory (NREL). Both cell

types had a base doping concentration of 2x101_ cm 3. The complete cell structures are given in references

7 and 8. An InP window layer was grown on both cell polarities to mimic the tandem cell.

The 1 MeV electron irradiations were performed using a Van de Graaff accelerator either at NASA

Goddard in Greenbelt, MD or at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD,

while the 3 MeV proton irradiations were performed using a Pelletron either at NRL or the Naval Surface

Warfare Center (NSWC) in White Oak, MD. In all cases, the beam currents were kept low enough to avoid

significant sample heating during irradiation. This is important, especially in InP, since both thermal and

injection annealing during irradiation can occur.

Current-voltage (IV) measurements were performed both in the dark and under one sun, AM0 solar

illumination using a Spectrolab X25 solar simulator at 298K. A silicon reference cell was measured on a Lear

jet and then extrapolated to AM0 courtesy of NASA Lewis. The simulator intensity was then set for a

measurement on InGaAs. The IV measurements were performed using HP34401A multimeters and a Kepco

36-1.6M bipolar amplifier, with full experiment automation achieved through IEEE-488 GPIB operation with a

QuickBasic program. The measurement errors are expected to be less than 1%.

4

CORRELATION OF 1 MeV ELECTRON AND 3 MeV PROTON DAMAGE

Figure 1 shows the radiation results for 1 MeV electrons and 3 MeV protons on n/p InGaAs solar cells.

The normalized solar cell efficiency is plotted as a function of the particle fluence. The data will be analyzed

in terms of displacement damage dose. To calculate the displacement damage dose, the fluences are

multiplied by the respective nonionizing energy loss (NIEL) values for the particle and energy in question. The

NIEL values for 1 MeV electrons and 3 MeV protons in InGaAs are 2.88x10 s and 2.33x10 2 MeVcm2/g,

respectively. The method of calculating NIEL values has been outlined in several prior publications(refs. 11,12).

Figure 2 shows the data from Figure 1 plotted as a function of displacement damage dose. It can be seen that

the curves are much closer together when presented in this manner. The next step in the correlation is to

-101-
determine the electron to proton dose equivalency ratio (R_p) as described in reference 9. Rep is defined as the

ratio of the dosealong the 1 MeV electron degradation curve to the dose along the 3 MeV proton degradation

curve which cause equal cell degradation. For the present data, R,p is found to be 2. By dividing the dose

values of the 1 MeV electron data by this ratio, the electron and proton degradation data are correlated as is

also shown in Figure 2.

Now we have a characteristic degradation curve for radiation damage in n/p InGaAs solar cells. To

describe this radiation damage quantitatively, an equation very similar to the one given in the Solar Cell

Radiation Handbook is shown (for the solar cell conversion efficiency (r/) as an example):

(1)

where A, C, and D, are the fitting parameters and D_ is the displacement damage dose. The degradation as

a function of displacement damage dose is the difference between this equation and the one given in the Solar

Cell Radiation Handbook. Upon a numerical data fit, the following values for the constants were determined:

A =4.76 (%), C= 1.353 (%), and Dx= 1.433x10" (MeV/g). The solid line in Figure 2 shows the characteristic

degradation curve for n/p InGaAs given by Eq. (1).

CELL POLARITY EFFECT ON EFFICIENCY DEGRADATION WITH 1 MeV ELECTRONS

inGaAs solar cells of the p/n polarity fabricated by NREL were also irradiated with 1 MeV electrons.

The radiation results for the solar cell conversion efficiency are shown in Figure 3, where the efficiency

degradation of cells having different polarities for the same base dopant concentration (2xl 01_cm 3) are plotted

as a function of 1 MeV electron fluence. A complete analysis, using the other solar cell parameters as well

as dark IV data, is currently underway. It can be seen from Fig. 3 that the cell efficiency degradation is

independent of cell polarity. This behavior is different than that reported for InP and Si(ref. 1 3).

CONCLUSION

Having a characteristic degradation curve as a function of displacement damage dose is greatly

beneficial in the prediction of cell response in different radiation environments. It also makes comparison

straightforward with other cell technologies. Using this curve, one can predict the response of the cell in any

radiation environment by simply calculating the NIEL of the cell material for the particle and energy of interest.

A spectrum of particle energies, such as that produced by a Co 6° source, can also be handled quite simply(ref.

13). This characteristic curve also aids in the absolute comparison of results from different cell technologies.

As an example, the comparison of the degradation of n/p InP homojunction and GaAs/Ge solar cells is given

in the paper by Waiters et al. in these proceedings. It is shown that InP degrades less than GaAs/Ge for any

displacement damage dose.

The polarity independence of solar cell efficiency degradation of InGaAs is an important result in the

design of the InP/InGaAs tandem cell. The'polarity independence gives us considerable flexibility in optimizing

the final design for the InP/InGaAs tandem cell.

-101-
REFERENCES

1. M.W.Wanlass,J.S.Ward,K.A.Emery,A.Duda,andT.J.Courts,"ImprovedLarge-Area,Two-Terminal

InP/Ino.s3Gao.47As TandemSolarCells",Proceedingsof the 1" IEEEWorldConferenceonPhotovoltaicEnergy

Conversion,Waikoloa,HI,1994,pp.1717-19.

2. H.L. Coral, R.J. Waiters, S.R. Messenger, and G.P. Summers, "Performance of Proton- and ElectronIrradiated, Two-Terminal, Monolithic InP/Ino.s3Gao.47As Tandem Solar Cells", Proceedings of the 1't IEEE World

Conference on Photovoltaic Energy Conversion, Waikoloa, HI, 1994, pp. 2088-95.

3. H.L. Cotal, R.J. Waiters, G.P..Summers, and S.R. Messenger, "Electron Irradiation of Two-Terminal,

Monolithic InP/Ino.53Gao.4_As Tandem Solar Cells", J. Appl. Phys. 77, 2173 (1995).

4. C.J. Keavney, R.J. Waiters, and P.J. Drevinsky, "Optimizing the Radiation Resistance of InP Solar Cells:

Effect of Dopant Density and Cell Thickness", J. Appl. Phys. 73, 60 (1993).

5. I. Weinberg, G.C. Rybicki, C. Vargas-Aburto, D. Scheiman, "Performance, Defect Behavior, and Carrier

Enhancement in Low Energy, Proton Irradiated p÷nn ÷ InP Solar Cells", Proceedings of the 13 'h Space

Photovoltaic Research and Technology Conference, NASA Lewis Research Center, Cleveland, OH, 1989, pp.

149-58.

6. S.R. Messenger, R.J. Waiters, M.J. Panunto, and G.P. Summers, "Radiation Effects on p+n Junctions

Grown by MOCVD", Proceedings of the 13 thSpace Photovoltaic Research and Technology Conference, NASA

Lewis Research Center, Cleveland, OH, 1994, pp. 167-80.

7. R.J. Waiters, G.J. Shaw, G.P. Summers, and S.R. Messenger, "The Effect of Base Dopant Level and

Thickness on the Radiation Response of Gao.471no.s3AsSolar Cells", Proceedings of the 5" IEEE International

Conference on Indium Phosphide and Related Materials, Paris, France, 1993, pp. 600-4.

8. S.R. Messenger, R.J. Waiters, H.L Coral, and G.P. Summers, "Radiation Degradation in Ino.53Gao.47As

Solar Cells", Proceedings of the 6 t" IEEE International Conference on Indium Phosphide and Related Materials,

Santa Barbara, CA, 1994, pp. 504-7.

9. G.P. Summers, R.J. Waiters, M.A. Xapsos, E.A. Burke, S.R. Messenger, P. Shapiro, and R.L. Statler,

"A New Approach to Damage Prediction for Solar Cells Exposed to Different Radiations", Proceedings of the

1=' IEEE World Conference on Photovoltaic Energy Conversion, Waikoloa, HI, 1994, pp. 2068-75.

10. G.P. Summers, E.A. Burke, P. Shapiro, R. Statler, S.R. Messenger, and R.J. Waiters, "The Use of

Displacement Damage Dose to Correlate Degradation in Solar Cells Exposed to Different Radiations",

Proceedings of the 13 th Space Photovoltaic Research and Technology Conference, NASA Lewis Research

Center, Cleveland, OH, 1994, pp. 197-201.

11. G.P. Summers, E.A. Burke, R.J. W'alters, G.J. Shaw, P. Shapiro, R. Statler, and S.R. Messenger, "A

General Meth(_d for Predicting Radiation Damage to Solar Cells in the Space Environment", Proceedings of the

23 r_ IEEE Photovoltaic Specialists Conference, Louisville, KY, 1991, pp.1426-31.

12. G.P. Summers, E.A. Burke, P. Shapiro, S.R. Messenger, and R.J. Waiters, "Damage Correlations in

Semiconductors Exposed to Gamma, Electron and Proton Radiations", IEEETrans. Nucl. Sci. 40, 1372 (1993).

13. S.R. Messenger, "Electron and Gamma-Induced Displacement Damage Effects in InP Semiconductor

Devices", Ph.D. Dissertation, University of Maryland Baltimore County, 1995.

-103-
-103-
3 MeV Protons

RTI n/p InGaAs

1 Sun, AM0

98K

1 MeVElectrons_

0109 10 l° 10 11 10 12 10 13 10 14 10 is 10 16 10 17

Particle Fluence (particle/cm 2)

Figure 1 Degradation in nip InGaAs solar cell efficiency (1 sun, AM0) due to 3 MeV proton and 1 MeV electron

irradiation as a function of particle fluence.

A

O

tG)

,m

o

UJ

O

4

2

10

1 Sun, AM0

"_, 298K

z_\& ^ & characteristic

"_- _ radiation curve

RTI nip InGaAs "_ •

' 3M ev Pr°t°ns

• 1 UeVElectrons _ •

_(raw data) -X_-

A 1 MeV Electrons

(i°rlelated' Rep=2 )' "_

2 3 4 109 2 3 4 1010 2 3 4 1011 2 3 4 1012

Displacement Damage Dose (MeV/g)

Figure 2 Data from Fig. 1 plotted as a function of displacement damage dose. The 1 MeV electron data is

correlated with the 3 MeV proton data using the factor P_, and a characteristic radiation curve is obtained.

-104-
o_

V

¢.)

o_

U

E

Ill

(I)

L)

_I_ Ino.53Gao.47As Solar Cells

4_ (N=2x1017cm'3)

_'__ 1 Sun, AM0

3 _ 298K

2 • pin (NREL) "_

1 • nip (RTI)

01013...............................1014 1015 '" ................10 is

1 MeV Electron Fluence (e'/cm 2)

Figure 3 Efficiency degradation of n/p and p/n InGaAs solar cells.

independent.

The degradation is shown to be polarity

-105-
Photo-Recovery of Electron-Irradiated GaAs Solar Cells I

Andrew Meulenberg (301-926-3609)

Consultant

Gaithersburg, MD 20855

ABSTRACT

The first long-term (3000 hours) UV testing of unirradiated and 1 MeV electron-irradiated GaAs/Ge solar

cells, with multilayer-coated coverslidesto reduce operating temperature, has produced some unexpected results.

The cells used for this series of tests displayed a much higher radiation degradation than that predicted based on

JPL Solar Cell Radiation Handbook data. Covered cells degraded more than did bare cells and use of multilayercoated coverslides further increased the radiation degradation in short-circuitcurrent (Isc).

Electron radiation damage to these GaAs solar cells anneals at 40°C when exposed to -1 sun AM0 UV

light sources. The effect appears to be linear with time (-1% of Isc per 1000 UVSH), has not yet saturated (at

3000 hours), and may not saturate until recovery of electron damage is complete. If electron, and perhaps proton

damage, to GaAs solar cells recovers totally with extended exposure to sunlight, the financial implications to the

satellite community are immense. This effect must be confirmed by further laboratory and flightdata.

NOTATION

Coverslide Coatings:

ARR = antireflecting

IRR = infrared (IR) reflecting

UVR = ultraviolet (UV) reflecting

BRR = blue-red reflecting (UVR on front; IRR on back)

DSR = double-sided coated coverslides (coating contains both UVR and IRR in the

same narrow-band-pass multilayer coating)

SSR = single-sided coated coverslides (coating contains both UVR and IRR in the

same narrow-band-pass multilayer coating)

INTRODUCTION

Five types of coverslide coatings (see notation above), designed for GaAs solar cells, have been tested as

part of a NASA-sponsored, space-flight qualification for Blue-Red-Reflecting (BRR), multi-layer-coated,

coverslides. Covered cells have been tested for degradation from the humidity, thermal, solar UV, and radiation

environments representative of near-earth orbits (1). Coverslides and solar cells were characterized at each step

of the test to identify the variation within the components and the reasons for changes observed with each step.

As a control on the UV test, several covered INTELSAT-6 silicon solar cells (designated as I-6) were included.

Solar cells change their spectralresponse with particulate irradiation. In the case of GaAs cells, the

response degradation is highestat the UV and IR ends of the spectrum. This is just where the BRR filters have

their greatest impact; so, it was anticipated that GaAs cells with BRR filters might degrade less under irradiation

than would the same cells without filters. Therefore, a set of solar cells from this test was irradiated to explore this

hypothesis and to provide data for space-radiation-damage predictions. Since extended UV exposure of

preirradiated GaAs cells has not been reported, we used available silicon solar cell data as the only potential

guide.

1 This paper is based on work performed at COMSAT Laboratories under contract from Goddard Space

Flight Center. The final analysis and paper presented here was funded by Hi'Consultants.

-106-
Siliconsolarcellshavedemonstratedaneffectcalled"Photo-Redegradation." This effect shows up in

cells that have been electron irradiated and then exposed to lightfor an extended period: the cells degrade

beyond the point that was measured rightafter irradiation. When first discovered and studied in depth (in the late

'70s), the effect was determined to saturate rather quicklyand to appear primarily in float-zone refined material.

The procedure developed at the time was to anneal and stabilize the cells after irradiationwith an overnight bake

at 60"(3 under flood-lamp illumination.

In recent years, photo-redegradation has also been observed in crucible-grown silicon material, when

solar cells have undergone UV testing after 1 MeV-electron irradiation and stabilization (2,3). With this

background, the decision was made to run a comparison of irradiated and unirradiated GaAs solar cells in the

same UV test to determine if a similar effect existed in this material as well. This paper describes the electronirradiation and subsequent UV-exposure results.

ELECTRON IRRADIATION

One (1) MeV electron irradiations of the GaAs solar cells were carded out at the National Institutes of

Standards and Technology. Cells were irradiated, in various combinations, at 2, 4, and 4x10" e-/cm 2 to provide

data points at 2, 4, 6, 8, and 10x10" e-/cm= levels. Selected cells were removed from the test at different points to

provide W-irradiation test samples at the radiation levels more likely to be encountered in common missions (4-

6x10" e-/cm2). The primary purpose of the electron irradiationwas to provide irradiated samples for the UV test.

However, useful data for the particular GaAs/Ge cells provided for the test was anticipated. The total number of

cells was limited; therefore, no coverslide group had many cells included. In addition to the normally covered cells

in the test, some bare cells and cells with the coverslides on backwards were also included in an attempt to isolate

damage effects and mechanisms in the electron- and UV-irradiated cells.

O

O

W

co

0.95

0.9

0.85

0.8

0.75

0.7

0.65

\,

._O

-0- "_= ,,

O

X

×

-t

+

0

I I ! I f I I I

10

1 MeV Electron Fluence (xle14/cm2)

e ARR avg

(6,3)

A BRR avg

(5,2)

:= DSR avg

(5,2)

+ IRR avg (4,2)

x SSR avg

(4,3)

o bare avg (5)

.... bare1980's

cells, JPL

Handbook

Figure 1 Relative Degradation of Short Circuit Current for I MeV

Electron Irradiated GaAs Cells

-107-
Figure 1 indicates the effects of coverslide type on the electron irradiated cells. This plot compares the

average degradation in short circuit current (Isc) relative to the unirradiated cell current (Isco). The numbers

beside each coverslide type indicate the total number of cells irradiated in each group and the number irradiated

beyond 4 x 10" e-/cm 2 respectively (e.g., 5,2 indicates 2 out of 5 cells were irradiated past 4 x 10" e-/cm2). The

data plotted at "4.5" and "8.5" x 10" e-/cm2 are values remeasured after a -24 hour, 60°C, thermal anneal

following the irradiations to 4 and 8 x 10" e-/cm 2 respectively.

The dashed curve in Figure 1 is taken from the JPL Solar Cell Radiation Handbook, Addendum 1: 1982-

1988 and represents bare GaAs/Ge solar cells from this period. The curve is quite different from the present

covered-cell data, both in slope and degradation level, and thus indicates the danger of depending upon previous

data for projections in a developing technology.

A basis for some of these differences might reside in the damage profile of covered cells versus that of

bare cells. Back scattering of electrons from the surface layer (<5 pm thick) of silicon can reduce the damage in

this surface layer by more than 20% compared to that in the subsurface material or in the surface layer with

coverslide applied. In silicon solar cells, this has little effect, since bulk damage to the cell dominates and the

damage in most of this region (which is 100-300pm thick) is independent of coverslide. In GaAs solar cells, with a

total active volume of less than 6 microns thickness and a higher average atomic number, the backscattering

effect and the effect on cells are both greater. If a 30% effect is assumed for the backscattering in GaAs cells, the

ARR covered cell data at 2E14 is in better agreement with the JPL data for bare cells. However, the cnrrect[on is

inadequate at the higher fluences (the difference in slopes also eliminates a simple dosimetry error). Furthermore,

bare cell results from this batch, irradiated at 4E14, also were significantlylower than the JPL data.

Another reason, suggested for the higher than predicted electron-induced degradation, is the possibility

that the Ge substrate is not inactive, as designed, and current may be collected from this region. With irradiation,

loss of this Ge contribution to the Isc would be greater than the loss from the GaAs. The problem with this

explanation is that cells with IR reflectors would be less sensitive to this effect. As seen below, this is contrary to

the experimentally observed results. Therefore, the full high-degradation slope of the present cells, cannot be

attributed to dosimetry, to damage profile problems, or to the Ge substrate alone.

The second important feature of the data in Figure 1 is the difference between the IRR-coated coverslides

and the others. The IRR data, which consists of one cell irradiated to 2 and 6 x 10" e-/cm 2,two cells at 4 x 10" e-

/cm 2, and one cell at 4 and 8 xl0 _4e-/cm 2, is internally consistent and clearly different from all but one individual

cell in the 4 other groups of cells. Values of Isc for the three cells, irradiated to 4 x 10" e-/cm 2, are within _+0.03.

Cell variation is therefore not enough to explain the difference with the other coverslide types. The data does not

depend upon coverslide odentation. No explanation for the high electron degradation of the IRR coverslides

relative to the other covered cells is offered at this time, beyond the possibility that the coatings used are

susceptible to electron damage.

ULTRA-VIOLET DEGRADATION

The UV degradation results illustrated in Figure 2 are represented by the relative short-circuit current (UV

test data divided by the initial data, I / Io, where Io is the data point taken at the beginning of the UV test)

normalized against the relative Isc of the .control cells. The normalized relative currents (I/Io / Ic/Ico) thus

compensate for any changes in solar-simulator output intensity and spectrum experienced during a scan of the

test and control cells. (The initial data point, Io, is artificiallyplaced at 1.2 UVSH to provide a starting point on the

log scale and to indicate that some UV exposure takes place during all of the initial measurements pdor to

beginning the UV test exposure.)

Most of the cells in this second of two UV tests had their coverslides reversed (and indicated in the

figures by -r) to determine the effects of filtering the UV light through the coverslide (the DSR double-sided

coverslides did not change with inversion), therefore relative values, not absolute values of change are to be

considered in the analysis.

-108-
The most unexpected feature of Figure 2 is the recovery in lsc experienced by the pre-irradiated cells.

Since the unirradiated cells (no-rad) follow the expected UV degradation profiles, recovery of the irradiated

GaAs/Ge cells is neither experimental error nor is it associated with the coverslides. The DSR and SSR cell

results in the figure clearly show a reproducible recovery of the electron-irradiated cells relative to the nonirradiated cells during extended UV exposure. The data indicate that the extended UV exposure anneals the

electron damage at about 1% per 1000 hours. Recovery curves therefore are generated by adding the UVSH

times 10-5 to the unirradiated cell values at a given time (e.g., at 1000 UVSH, add 0.01). In figure 3, curves are

provided to indicate the predicted level of recovery in Isc for the pre-irradiated cells.

1.01

o

(,9

m

f-

°_

tm

ttG)

t_

n,,

0.99

0.98

0.97

0.96

0.95

• O • •

• _ •0 0 • •

0

• & 0

OA

• • •

• 56B2-r (SSR, 4E14)

o 56A2-r (SSR, no tad)

• 58B-r (DSR at 4E14)

A 58A-r (DSR, no rad) Z_

10 100 1000 10000

UV Solar Hours

Figure 2. UV Degradation and Electron-Irradiation Recovery for SSR and DSR

Covered GaAs Solar Cells in Test 2.

The UV degradation results in Figure 3 are those of Figure 2 with the addition of recovery curves and BRR

cells from the same test. This time, the unirradiated cell data are represented by fitted curves, rather than by data

points,to remove some clutter. UV degradation control cells were provided by inclusion of INTELSAT-6 cells that

have been reproducible and well-characterized inpast UVtests.

It is clear that qualitatively the pre-in_adiated DSR and SSR covered cell data in Figure 3 follow the "DSR +

recovery" and "SSR + recovery" curves quite well. However, the irradiated BRR cells fit the "GaAs + recovery"

curve, rather than the expected "BRR + recovery" curve. The "GaAs + recovery" curve assumes only recovery

from electron irradiation and no UV degradation (the predicted curve for BRR covered cells would be close to that

of the irradiated "SSR + recovery" curve, since the BRR and SSR UV degradation curves are so close). The high

peak values (-1.025) are unexpected for a recovering irradiated-BRR cell since more than 2% UV degradation

has been observed in both Test I and Test 2 for unirradiated-BRR cells. These cells therefore appear to have an

unusually high recovery from electron irradiation (4-5% at 3000 UVSH, ratherthan 3%).

-109-
1.03

1.02

>_ 1.01

:_ 1.00

t-

•_ 0.99

0.98

6

0.97

0.96

I/

/

,/

• - i

.... _- _ "_-._ • "_',. A-"'

- _ _ %.. =_m)_ /

•,,

\ .

........ I ........ 1 ........ 1 . .... ' '1

10 100 1000 10000

UV Solar Hours

1

__ GaAs recovery I

• BRR-r 4E14

---- -- BRR-r no tad

i SSR-r4E14

•--- = SSR-r no tad

_ - _ SSR-r + recov

• DSR-r 4E14

•--- = DSR-r no racl ,

.... DSR-r + recov

= = -I-6norad :

Figure 3. UV Degradation and Electron-Irradiation Recovery for BRR, SSR, and DSR

Covered GaAs Solar Cells in Test 2.

1.04

O

¢tJ

¢.-

o_

Q_

¢-

m

¢..-

¢O

O

.>_.

.._m

n1.03

1.02

1.01

0.99

• 56B3-r(ARRat4E14)

O 39B (ARR at6E14) l

..... ARR norad + recovery I

ARR norad j

-Q

B

• w w_

B w"

.wo o°

.... =• ........ .m...... n

£3 13

o o

t

Q _

I I I I I I i I I I I i I I i t

100 1000

UV Solar Hours

10000

Figure 4. UV Degradation and Electron-Irradiation Recovery for ARR

Covered GaAs Solar Cells in Test 2.

-110-
Figure 4 displays the ARR covered cells in the same format as used in Figure 3. The unirradiated ARR

covered cell data (from Test 1) shows little UV degradation (Reference I data suggests 0.2% loss for a set of ARR

cells). The ARR covered cell (with reversed coverslide) irradiated to 4E14 1 MeV electrons shows a recovery

characteristic of those in Figure 2. The fit is even better if the "ARR + recovery" curve is raised by -0.005. This

offset could be from statistical variation in the first measurement; or, it could be real and a result of a rapid UV

induced change in refractive index of the optically mismatched system of adhesive and coverslide AR coating

(matched to air, not adhesive).

While the "ARR at 4E14" data in Figure 4 are not fitted as well to the "ARR + recovery" curve as were the

DRR and SSR data, the pattern is consistent with the estimated recovery formula. The "ARR at 6E14" cell cannot

be forced to fit the recovery model. This cell starts the expected recovery in Isc beyond 100 UVSH; however, the

"ARR + 6E16" data deviates dramatically from the recovery curve beyond 1000 UVSH hours 2. This is clearly not

the behavior identified in the earlier cells. The fact that the two ARR cells in Figure 4 had their coverslides applied

oppositely would not account for the observed difference in recovery. The 50% higher electron fluence of the one

cell is not expected to be significant; yet, the same effect is seen in Figure 5 for the IRR cells.

1.01

o

¢/)

¢.-

°_

C_

¢-

¢U

fO

re'

1 •

0.99

0.98

0.97

o

•0

0 0

• ¢

o o

o

0

O 49A-r (IRR, no rad)

• 31B (IRR at 6¢ 14)

'_O

O

' ' ..... I ........ I ...... _...... I ....... '!

10 100 1000 10000

UV Solar Hours

Figure 5. UV Degradation and Electron-Irradiation Recovery for IRR

Covered GaAs Solar Cells in Test 2.

The irradiated IRR covered cell in Figure 5, with correctly odented coverslide at 3000 UVSH, shows only a

0 - 1.5% increase in Isc relative to the unirradiated IRR-covered cells in Test 11. In that test, the unirradiated IRR

data vaded between 0 and 1% below the initial value. The 9RR at 6E14" cell mimics the "ARR + 6E14" data of

Figure 4 in that it displays an initialrecovery followed by a drop in Isc recovery beyond 1000 UVSH. Thus the 50%

increase in prior electron-irradiation fluence appears to be significant. This fluence dependence is unexplained at

this point; unless the higher electron irradiation of the GaAs begins formation of a different defect type that

predisposes the material to subsequent UV degradation.

The unirradiated IRR cell in Figure 5, with inverted coverslide, shows a significantlyhigher UV degradation

than do the cells with correctly odented IRR coverslides in Reference 1 (2% vs. 0 - 1% at 3000 UVSH). If this

result is a consequence of the unfiltered UV exposure to the multilayer coating, then the implication is that these

layers are more sensitive to energetic radiation (UV with %< 0.35 pm or I MeV electrons) than to the lower energy

2 The deviation begins earlier, if the 1000 UVSH data is lowered by 0.5%, or later, if the data cluster at 1500

UVSH is raised by 0.5%. Such a potential offset in the data points is seen in all of the figures and would be a

consequence of the normalization.

-111-
UVthatpassesthroughthe CMG coverslide. This sensitivityto energetic radiationcould explain the higher loss in

Isc of the IRR-covered cells from the electron irradiationthan that observed for the other cells in the same test.

The plotted data in Figures 2-5 include a correction, mentioned for Test 2, in the appendix of Reference

1, that consists of reducing the initial control cell data by 1%. Without thiscorrection,the normalized values shown

(other than the initial values) would be 1% lower. Figure 6 displays the unilluminated control cells on a linear plot

to emphasize the later portion of the test where a deviation is noted between the different cell types. Since the cell

Isc values are self normalized, the -1% offset in the initial point raises all of the other data points toward 1.01.

The 100% line is an average of the GaAs control cell data beyond the initialpoint.

+ ARR 4E14

• SSR 6E14

DSR 6E14

o

x 1-6

100% line

,,,I .... I .... I .... I .... I,,'i .... I

500 1000 1500 2000 2500 3000 3500

UV Solar Hours

Figure 6. Self-Normalized Control-Cell Data for Test 2 of the Experiment

The GaAs control cells have been electron irradiated, but not exposed to UV. An important question

concerns the long-term recovery of these cells when exposed to 40°C and vacuum but no intense light beyond the

short periods of I-V test measurements. Examination of the data in Figure 6 indicates that the I-6 cell has

degraded relative to the GaAs control cells. (For this reason, it was removed from the control cell average.)

However, the =ARR at 4E14" cell has degraded less than the =DSR and SSR at 6E14" control cells. Initially,the I-6

degradation had been attributed to scattered UV lightaffecting the sensitive silicon cell opticalstack. However, the

ARR cell is the least UV sensitive of the GaAs cells. Therefore, scattered UV could not be a valid explanation.

Recovery of electron damage is greater forJhe 4E14 cells than for the 6E14 cells after 1000 UVSH. Data at the

lower UV exposure points is too confused by other effects to allow comment. There is no other data on thermalvacuum recovery of these cells, so this must be high on the list of possibilitiesto be considered.

If thermal vacuum recovery is real for the electron-irradiated GaAs cells, then, the normalization procedure

should be based on the I-6 cell rather than the irradiated GaAs cells in Figure 6. the consequences of such a

renormalization would be an increase in Isc of all the data by -0.5% at 2-3000 UVSH. Such a change would

generally improve the fit to the recovery model, reduce the apparent UV degradation of unirradiated cells, and

result in a saturation, rather than a tumover, in the 6E14 cell data beyond 1000 UVSH. It would also indicate that

the recovery of electron damage to GaAs cells will occur with or without light; but, it will be accelerated by the

presence of photo-generated minoritycarders.

-112-
DISCUSSION

While variations exist in the individualcells and coverslides of this program and the sample size is small,

detailed analysis of the components and combined structures has allowed an understanding of the loss

mechanisms to be expected from the space environmental effects. Nevertheless, there are two things to consider

in this postulation of photo-recovery for electron irradiated GaAs solar cells

1. The measured electron-induced damage in the GaAs/Ge solar cells involved in this test was higher than

expected.

• The degradation in Isc of the solar cells in this test was significantly higher than that seen in prior data (>14%

vs. -11% at 4 x 10" 1 MeV electrons/cm2and >22% vs. 16% at 8 x 10'4 lcm2).

= The covered cells degraded further than did the bare cells of this test (2 - 8% more in Isc / Isco at 8 x

10'4/cm2).

2. Since the test from which the photo-recovery data has been extracted was not designed with this study in

mind, the data is limited and statisticsare poor. Nevertheless:

• A significant photo-recovery effect has been seen in 5 out of 7 electron-irradiated cells that were exposed to

extended UV illumination. (The other two cells, that displayed less recovery, had been exposed to a 50%

higher radiation fluence.)

• None of the many unirradiated cells in the two tests showed any recovery beyond their initial values with

extended UV exposure.

• Three out of three irradiated GaAs control cells, that were exposed to the same handling, measurements, and

thermal-vacuum environment (but not to the UV source), showed much less recovery. Light is therefore

critical to the rate and perhaps to the magnitude of the effect, but not necessarily the only source of the effect.

In summary:

• The initial electron degradation for Isc of these recovering cells was -15%.

• No cells from this batch showed >1% recovery after irradiation when exposed to a floodlamp and

60°C for -24 hours.

The long-term photo-recovery is a significant portion of the total electron damage (-3 out of 15%). Nevertheless,

it has so far only broughtthe cells back to the level observed in the JPL irradiated GaAs solar cells.

A question remains: are we seeing a recovery of the basic GaAs radiation damage or only a recovery of the

excess damage? Longer-term testing and analysis will be required to properly address this question and others

on the generality of the effecL

REFERENCES

1. A. Meulenberg, "Space Qualification of UV and IR Reflecting Coverslides for GaAs Solar Cells," Proc. of

the 14th S0ace Photovoltaic Research and Technology Conference. NASA / LeRC, Cleveland, OH, October 24-

26, 1995

,o

2. A. Meulenberg, "UV Testing of INTELSAT VII, Vlla, and VIII Solar Cells." Proceedinc_ of the 13th Space

Photovoltaic Research and Technology (_onference. NASA/LeRC, Cleveland, OH, June 14-16, 1994

3. A. Meulenberg, "UV Testing of Solar Cells; Effects of Anti-Reflective Coating, Prior Irradiation, and UV

Source," Proceedings of the 12th Space Photovoltaic Research and Techno!ogv__Conference, NASA / LeRC,

Cleveland, OH, 1992

-113-
INITIALRESULTSFORTHESILICONMONOLITHICALLYINTERCONNECTEDSOLARCELLPRODUCTt

L.C.DiNetta,K.P.Shreve,J.E.Cotter,S.Sun,and A. M. Barnett

AstroPower, Inc.

Newark, DE 19716-2000

ABSTRACT

This proprietary technology is based on AstroPower's electrostatic bonding and innovative silicon solar

cell processing techniques. Electrostatic bonding allows silicon wafers to be permanently attached to a thermally

matched glass superstrate and then thinned to final thicknesses less than 25 IJm. These devices are based on the

features of a thin, light-trapping silicon solar cell: high voltage, high current, light weight (high specific power) and

high radiation resistance. Monolithic interconnection allows the fabrication costs on a per Watt basis to be roughly

independent of the array size, power or voltage, therefore, the cost effectiveness to manufacture solar cell arrays

with output powers ranging from milliwatts up to four watts and output voltages ranging from 5 to 500 volts will be

similar. This compares favorably to conventionally manufactured, commercial solar cell arrays, where handling of

small parts is very labor intensive and costly. In this way, a wide variety of product specifications can be met using

the same fabrication techniques. Prototype solar cells have demonstrated efficiencies greater than 11%. An

open-circuit voltage of 5.4 volts, fill factor of 65%, and short-circuit current density of 28mA/cm 2at AM1.5

illumination are typical. Future efforts are being directed to optimization of the solar cell operating characteristics

as well as production processing. The monolithic approach has a number of inherent advantages, including

reduced cost per interconnect and increased reliability of array connections. These features make this proprietary

technology an excellent candidate for a large number of consumer products.

INTRODUCTION

The monolithically interconnected solar cell (MISC) array benefits from the advantages afforded by thin

solar cell design technology. AstroPower has been developing thin, electrostatically bonded silicon solar cells for

a variety of applications. Specifically, the advantages and benefits of any thin solar cell technology can be summarized as below.

ADVANTAGES

X Minority carder diffusion length required to be two times the layer thickness permitting efficient

operation from poor quality or degraded lifetime material.

X The thin device design opportunity allows high open-circuit voltage.

X Light trapping leads to good light absorption.

,X Good carrier collection leads to high short-circuit current.

RESULTING IN

_( High Specific Power

,/ High Efficiency

/ High Degree of Radiation Tolerance

In addition to the previously stated advantages, the integration of the monolithic interconnection technique

allows for a number of inherent benefits. These are enhanced reliability of interconnections, lower 12Rlosses, and

minimization of the impact of shorts and opens in adeployed array.

t This research is supported by BMDO and managed by USASSDC through a current SBIR program, contract

#DASG60-95-C-0007.

-114-
HIGHPERFORMANCE DESIGN

This thin electrostatically bonded solar cell structure incorporates a number of features that will enhance

its efficiency. They include: a thin base layer, high base doping, lighttrapping and front and back surface

passivation. The thin base layer can be highly doped thereby increasing the output voltage without paying a

penalty in collectionefficiency; i.e. current is maintained while the voltage is increased. This is due to the reduced

dependence of collectionefficiency on minority carrier lifetime for thin silicon solar cells. This same effect reduces

the solar cell array's dependence on lifetime reducing damage caused by radiation. The degradation

characteristics of thin silicon solar cells are expected to be comparable to GaAs and lnP solar cells.

The back of the thinned silicon solar cell is micro-machined using an orientation-dependent etch to

produce random pyramids in the surface. A back surface reflector is then deposited, so that the long wavelength

light that enters the silicon is totally internally reflected by the textured back surface and planar front surface. It is

possible to obtain optical thicknesses in the thin silicon that approach twenty times the actual thickness. Light

trapping has been demonstrated in thin silicon structures with textured back surfaces [1, 2]. Light-trapping is

incorporated into the array structure by the use of randomly--oriented pyramids etched into the back surface. The

pyramids will cause light incident on the back surface to be reflected oblique to the plane parallel with the front.

This will trap weakly absorbed light by total internal reflection at the top surface. Light-trapping allows the effective

thickness to be much thicker than the actual thickness of the device. A reflective metal such as gold or silver

evaporated onto the back surface will make the back surface nearly 100% reflective.

Another contribution to a high open circuit voltage is from the reduced recombination volume of the thin

solar cell. This will require front and back surface passivation, which can be achieved with a thermal oxide on the

front of the device, and a PECVD silicon nitride or silicon dioxide layer on the back. Fill factor improvements can

be achieved because low resistivity silicon base layers can be employed in this solar cell design. This reduces the

series resistance of the base layer in comparison to the high resistivity base layers currently in use for silicon

space solar cells.

Surface passivation is achieved by forming a thermally grown silicon dioxide layer on the front surface

before the electrostatic bonding step. Also, a thermal oxide is grown on the back side, since the glass/silicon

laminate can withstand high temperatures. Passivating silicon oxides can also be deposited onto the back by

plasma-enhanced chemical vapor deposition (PECVD), although thermally grown oxides are preferred.

Electrical isolation of the array elements is achieved by an orientation selective chemical etch process

called '_/-groove" etching. The groove is etched completely through the silicon layer after it has been bonded to

the glass and thinned to a nominal thickness of 30 microns. The glass superstrate provides both mechanical

support and electrical isolation.

Re-interconnection is provided by forming ohmic contacts on the side-walls of the V-grooves, which have

been formed so that the base-layer of one array element is exposed on one side of the V-groove and the emitter of

the adjacent array element is exposed on the other side of the V-groove. Thus, by depositing a metal layer in the

groove, series interconnection is achieved. The wrap-around emitter is formed by solid-state diffusion after the

bonding, thinning and V-groove processes. Utilizing a high-temperature process compatible glass superstrate is

critical to forming the wrap-around emitter.

Parasitic losses are minimized by the device design. All contacts are formed on the back side of the solar

cell array, therefore losses due to shading are minimized and can approach zero. Shunt conductance losses,

which have been characteristic of previous monolithic array designs, will also be minimized by careful selection of

contact metals and by complete element-to-element isolation provided by the V-groove and glass superstrate.

Series resistance losses will be minimized by,proper selection of the device geometry or by the incorporation of

grid lines on the emitter and/or base. Modeling predicted losses of less than 5% due to shading and series

resistance losses for optimized array geometries. (Shading and series resistance losses of production solar cells

are typically greater than 10%.)

-115-
HIGH RADIATION TOLERANCE

Radiation damage is the primary

degradation mechanism of silicon solar

cells deployed in space and results in

reduced performance with time and

depends on the pal:titular orbit and

exposure level. The gradual degradation

in solar cell performance is due to the

reduction in the minority-carrier lifetime

that results from cumulative damage to the

crystal lattice. One approach to increasing

silicon solar cell radiation tolerance has

been to reduce the silicon base thickness

as much as possible. Although silicon

solar cells formed on free standing,

thinned wafers are presently available,

their minimum thickness is limited to 65

m due to handling and yield concerns.

Improvements in the radiation tolerance of

silicon solar cells can occur if the active

layers are thinner (20-35 ilm).

Modeling and experimental data

showing the efficiency degradation of

o

.E

>,

Iii

:>

G)

n,"

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 ,

1.00E+12

i_ _.---x- - -x .... _. x

?_" _ "" ,x

...... 20 um Si, _ "

theoretical "i_ "_C

%

-- -- -- 100 um silicon, % ,&

theoretical

55-76 um silicon, X \ &

\

experimental

X GaAs/Ge,

experimental

X InP, experimental

I I ",.''1 ........ i , , . ,,,=,1 • , i i,|.,i

i i = m

1.00E+13 1.00E+14 1.00E+15 1.00E+16

1 MeV Electron Fluence

Figure 1: Radiation resistance of space solar cells [3, 4, 5].

candidate space solar cells as a function of 1 MeV electron fluence is shown in Figure 1. Thin, lighttrapped,

silicon solar cells have a theoretical radiation resistance similar to InP solar cells and better radiation resistance

than GaAs/Ge solar cel]s. Experimental and theoretical values for a 4-rail thick silicon solar cell are shown to

verify the model.

Because the absorber (base) layer is very thin, the solar cells will be extremely insensitive to changes in

minority-carrierlifetime caused by irradiation. The efficiency of the array is roughly independent of the minoritycarTier diffusionlength untilit is less than the thickness of the absorber layer. For silicon base layers on the order

of 25 I_mor less, this is equivalent to a minority-carrier lifetime of 250 nanoseconds (as-grown, non-irradiated

silicontypically has a lifetime greater than 10 microseconds). In contrast, present high-performance siliconsolar

cells require minority-carrier

lifetimes on the order of 1

millisecond. This thinned silicon

array design reduces the minoritycarrier lifetime requirement by

more than a factor of onethousand. This design will

substantially increase radiation

tolerance and significantly extend

the useful life of silicon solar cells

deployed in space.

RELIABILITY

Due to the higher voltage

attainable by the monolithically

interconnected solar cell the I'R

array losses will be minimized for

any bus voltage when compared

to conventional silicon solar cells.

Utilizinga higher voltage also

results in a weight savings at the

(/)

3_

Q.

Figure 2:

40 ................................................

35 T ........... -- ...........................

30 :--................................................. ii

20_25:- ....................................................... ' .....

5 _- ...............................................

01 "

12 24 36 48 60

BUS Voltage

(volts)

Conventional (0.5 volts) ---,=--- MISC (12 volts)

Solar ceil voltage effect onpower loss.

-116-
array level from the smaller quantity of high current carrying wire necessary for solar cell intei'connection and the

reduced dependence on stringing of low voltage devices to achieve the needed bus voltage.

This monolithic interconnection technique offers several additional advantages compared to conventional

array interconnection technologies for space applications. The reliability of the array is enhanced by reducing the

complexity of the interconnections [6], and monolithically interconnected solar cell arrays offers potentially higher

yield compared to conventional array manufacturing processes such as welding [7] or soldering. Typical yield

reductions in standard array interconnection are the result of breakage due to either mechanical or thermal stress.

Another reliability benefit from the high voltage device is the resultant lower power loss from shorts,

opens, and impact damage at the array level. If the monolithically interconnected solar cells are operated either at

the bus voltage or some factor of the bus voltage, loss of a single solar cell within a string will have minimal impact

on the array power generation when compared to typical silicon solar cells. This is shown in Figure 2 using, for

instance, a nominal 0.5 volt silicon solar cell and a 12 volt monolithically interconnected silicon solar cell. As is

shown the power loss is quite dramatic due to the loss of a string of devices as compared to a single or a few

devices when the string of devices becomes disabled.

EXPERIMENTAL RESULTS

The long term goals of this program are to develop

and manufacture lightweight,radiation hard, high

performance, highvoltage solar cells for advanced space

power systems and the terrestrial market. Results from initial

prototype solar cell testing are discussed in the following

sections.

I-V CHARACTERISTICS

Device #56 is a 27-micron thick device that is

electrostatically bonded to an akali free high temperature

Coming glass. This device consists of twelve monolithically

series interconnected segments. The I-V curve is shown in

Figure 3.

Fill factor is limited to 61.5% by the high series

resistance and low shunt resistance. Shunt resistance is

approximately 20 ohm/cm 2and has limited both Jmpand Vmpof

this device. High series resistance is the other limiting factor

of this device. Figure 4 shows the gray I-V response. It is

=

+ + I I/

+ , 1

.... _......... r r .... _" .... _' .... 'I ........

i : I , , ,

l

I I I f * i

I t i i

+ I I , I ,

, • i i + i

I

i

Figure 3. Light I-V curve #56.

evident from this curve that the device is largely

limited by the low shunt resistance. The total

series resistance of#56 can be approximated

from the slope of the light I-V curve in the

forward bias region near Vo_. It is estimated at

10

1 i •

150 ohms total for this device and is the major

contributor to the low fill-factor. This resistance

is made up of an emitter component, a bulk or

base component and a metallization

component. The modeling of this structure °

indicates that both the emitter and bulk

components contribute significantly to the high

overall R,e__,value. Lower resistivity bulk

material is needed, in addition to a lower Rme+t

emitter value. The device operation will be

optimal when bulk material resistivity is

approximately 0.3 ohm-cm, and emitter Rm_,t

value is approximately 45 - 50 ohms/square.

A second device, #28, is 28-microns

thick and is processed similarly to the previous

0.1

o. 0.01

E

o_

o 0.001

..,}

0.0001

0.00001

0.000001

Figure 4.

n=2• ,+ t

1.00E-05-/- ----4

Ik + i n=i = , -_

'J' ! ! 1.00E-11 _ , ,

0 0.12 0.24 0.36 0.48 0.6 0.72

Voc (Volts)

Gray I-V curve (#56).

-117-
iil--- I

" t

Voc 5.5 volts

Isc 9.8 mA

"" FF 64.5% i :..... "....

2 I , Area 3.12cm , . , ,

-- Eft 11.1%AM1.5G ,- .... ," "',..... r---

, , , ', ; ', ,,

..... T .... r .... ,t .... _ ........ "1..... i .... 1..... t" ---

z i _ r , r

P J _ , I I ,

, I l , i t i

i l , , i i , ,

r

Figure 5. Light I-V response (28)

solar cell. Refer to Figure 5 for the light I-V

characteristics and Figure 6 for the gray I-V

characteristics. Fill factor has improved to

64.5% due to reduced 12Rlosses in the base.

1E+01

1E+00

1E-01

1E-02

1E-03

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

Figure 6.

: I I l i

i j , 7"--_

mr .--_-GreyI-V ! ._," -"

-'--i _210 kohm Rshunt i 1-._'_

- ' i i .I.=_._..'-_>_n:2 -.

f i .'"i"" I Ie-12

• " :'" le-ll i

i i _'' "' i

0.000 0.120 0.240 0.360 0.480 0.600 0.720

Voc (volts)

Gray I-V curve (FZ28).

Performance of #28 shows improvements over #56 which can be attributed to the use of more heavily doped bulk

material. The bulk resistivity is approximately optimal at 0.3 ohm-cm, but the emitter Rsheet iSnearly the same as

#56 at 130 ohms/square, which is nearly three times the ideal value. The gray I-V curve, Figure 6, indicates good

shunt resistivity(23k ohms total) and reasonable J01and J02values of 5 x 1012 and 1 x 10"7,respectively, for a

silicon diode. The series resistance has improved to 110 ohms. Total series resistance is still limitingthe fill-factor

of thisdevice, and improvements will be gained with the use of more heavily doped emitters.

SPECTRAL RESPONSE

Quantum efficiency measurements, Figure 7 and 8, indicate reasonable material quality after the

non-optimized high temperature post bond processing. The silicon-glass laminate is subjected to a high

temperature processing step and stress related degradation of the material qualitywould be expected to degrade

the blue response of the solar cell. Previous work with electrostatically bonded silicon solar cells has shown that

excellent blue response can be attained and that there are no detrimental effects from the bonding step itself or at

post bond processing temperatures below 900°C. Experiments are being conducted to reduce the temperature of

this processingstep and to improve the front surface passivation which should result in significantly improved blue

response.

At greater than 500 nm, both of these devices compare favorably to control samples fabricated on the

same quality starting material. Furthermore, the effective diffusionlength, which is derived from the long

wavelength response is 195 microns and 93 microns. This demonstrates a lighttrapping coefficient of greater

than 7 and 3 respectively. Inconsistencies inthe effective diffusionlength can be remedied by optimizingthe solar

cell processing and carefully controllingthe starting material specificationsas related to specific process

parameters.

-118-
Figure 7.

100 100

i i

90 - - 90

8o - 8o

= 7o 70

_ 60 60_

w

_0 50

=_ 40 4o

30 / 30

/ - , -

20 I ...... _, ...... -'...... '......-'...... _........... 20 10 -__ - I0

0 : E : 0

350 450 550 650 750 850 950 1050

Wavelength (nanometers)

i--, --External QE -,------Internal QE --Reflectance

l ,

Spectral response curves of MISC device #56

Figure 8.

100 "r 100

' , i _ p ' = |

9080 ..... r= ...... .................................. _"+_i ; -_,;- - -I 9080

e

o ....... 60 o =, +o .............. !

E _o ---I/...... !....... :....... :....... _-..._\ ....:---+_o_.

= i ' Solar Cell Thickness = 28 _m " '; -tl

"

m

:=

= 40 .... .... Es°lar ' Diffusion Length = 93 _m ___:_x_ _____ ,o

O 30 30

20 20

10 10

0 + ; , 0

350 450 550 650 750 850 950 1050

Wavelength (nm)

I__ --External QE --Internal QE --Reflectance !

L

Spectral response curves of MISC device #28

-119-
HIGH TEMPERATURE SURVIVABILITY

High temperature survivable glass/silicon laminates are necessary to enable post bond processing and

long term operational stability. Upper temperature survivability limits have been evaluated for electrostatJcally

bonded silicon to Coming's 1737 glass structures. Interracial degradation has been observed at approximately

70°C below the specified softening point of the specified glass. The upper limit of the 1737 glass bonded to silicon

is approximately 900°C and is related to the softening point of the glass. High temperature survivability of Pyrex

glass bonded to silicon was found to be approximately 750°C. Both of these upper limits were approximately 70°C

below the accepted softening point values for the glasses. In order to increase the temperature survivability of the

glass bonded silicon, a higher softening point glass was formulated specifically for this work and evaluated. This

glass formulation was used to electrostatically bond to silicon and has shown good survivability to 900°C. The

projected upper temperature survivability of this bonded glass is approximately 950°C.

High temperature survivability and optical transmission tests were conducted using various commercially

available space quality celia doped glasses and other glass compounds including a custom formulated ceria

doped glass. Celia doping is important in the space environment to prevent darkening of the solar cell coverglass

due to ionizing radiation over the projected lifespan of the satellite. The celia, although imparting some initial tint

to these glasses, makes them more stable and inhibits the formation of color centers.

A high temperature survivable, custom celia doped glass was formulated successfully by Sere-Corn for

our experiments. This glass has been successfully electrostatically bonded to silicon and has survived processing

to 900°C without optical degradation. It has a softening point of 1050 ° C which is 75 ° C higher than the Coming

1737 glass we used to successfully fabricate the 10.3% and 11.0% devices: The TCE (thermal expansion

coefficient) of this glass is more closely matched to silicon at 33.5 x 10"7/oc than the 1737 glass which has a TCE

of 37.8 x 10"7/°C. This should provide for reduced electrostatic bond induced stress at the glass-silicon interface.

Projected maximum processing temperature of this glass (electrostatically bonded to silicon) is approximately

975°C. Figure 9 shows optical transmission data test results of various glasses including the custom compound.

10o

0//_( / / "-0-- Sere-Corn Ceria doped glass 20.5

'-_ 8ollY ¢ --o--Sem-Com _il Ceria doped " glass - 6.5 mil

g

70

60

50

400 500

= i , , ...... z ......

600 700

wavelength (nm)

Sem-Com Ceria doped glass 50.5 mil

--,x---Pilkington CMZ glass 8 miI

-_--Coming 1737 glass 26.5 mil

--o-- CMZ 80 glass 3 mil

_Ce doped Coming 213 glass 5 rnil

Coming 7740 glass 73 mil

Coming 7059 glass 20.5 mil

800 900

Figure 9. Optical transmission properties of various glasses

The change in output power of MISC device #CZ55 over 100 cycles from -80°C to +130°C was measured.

Duration of one complete cycle was approximately seven minutes. I-V measurements were taken at 0 cycles, 25

cycles, 57 cycles, 76 cycles, and 100 cycles. No significant change in output power was found over the duration.

-120-
SUMMARY

A wide variety of military, space, and commercial markets can be served by this technology. In order to

penetrate these markets with this new class of solar cell it is necessary to demonstrate the feasibility of solar cell

array products that will be cost-effective, lightweight,high power, high voltage, and radiation resistant. The

specific technical objective of the Phase II program is to fabricate a prototypeof a lightweight, high-efficiency,

monolithicallyinterconnected solar cell array with the previously defined operating parameters.

The advantages of utilizingthis technology are:

X Higher Performance

J" High efficiency thin-device structure and lightweight resultin high specific power

t/" Higher output power and better stabilitythan amorphous or polycrystallinesolar arrays

)(' Higher Reliability

t/" High reliability interconnections

j" Radiation hard

)(' Higher Design Flexibility

_/" Any size or voltage array can be fabricated cost effectively

_" High power, large area, cost-effective, array technology

j" Current and voltage configurable

_/ Cost effective for high voltage in a small area

q/" Arrays are practical for charging battery banks, includingapplications of voltages of 1 to 500 volts

_" Per-watt cost is independent of the number of interconnectionsand the array size

_" No labor/material cost as associated with cut and tabbed cells less than 100cm2

FUTURE PLANS

A preliminary process outline has been designed for productionof large quantities of MISC devices for the

terrestrial applications market. Various size wafers can be utilizedand these cut to size in the final fabrication step

to obtain the desired current/voltage output. This process has been cost analyzed and the initialresults are quite

favorable. AstroPower will direct market the space power applications. The use of high voltage solar cell arrays

has been identifiedas a critical need for electric propulsion. This solar cell technology can enable electric

propulsionfor orbit raising missionswith a considerable savings in launch cost. Secondly, the use of MISC

technology for applications in concentrator solar cell arrays is very interestingdue to the lower I=R losses

associated with the high voltage output of the solar cells as compared to high current. Presently AstroPower is

preparing to deliver prototype MISC terrestrial and concentrator solar cells for integration into array products by

other manufacturers.

REFERENCES

1. Rand, R.B. Hall, and A.M. Barnett, "Light Trapping in Thin Crystalline SiliconSolar Cells", 21st IEEE

PhotovoltaicSpecialist Conference, May 1990 page 263.

2. Rand and P.A. Basore, "Light Trapping SiliconSolar Cells, Experimental Results and Analysis", 22nd IEEE

PhotovoltaicSpecialists Conference, October 1991.

3. Tada, J.R. Carter, B.E. Anspaugh, and _G Downing, "Solar Cell Radiation Handbook", JPL Publication 82-69,

1989.

4. Weinberg, D.J. Brinker, C.K. Schwartz, and R.E. Hart, "Progress in Indium Phosphide Solar Cell Research,"

SPIE Indium Phosphide and Related Materials for Advanced Electronic and Optical Devices Conference,

I989, page 434.

5. Anspaugh, "Solar Cell Radiation Handbook, Addendum 1", 1989.

6. J.R Woodyard, "Thin Film Cell Development Workshop Report", Proceedings of the Space Photovoltaic

Research and Technology Conference, May 1991, page 48-1.

7. Smith and B.T. Cavicchi, "GaAs Solar Cell Circuit Assembly", 19th PhotovoltaicsSpecialists Conference, New

Orleans, May, 1987, page 448.

-121-
MONOLITHICALLYINTERCONNECTEDGAASSOLARCELLS:

A NEWINTERCONNECTIONTECHNOLOGYFORHIGHVOLTAGESOLARCELLOUTPUT

LC. DiNetta and M.H. Hannon

AstroPower, Inc.

Newark, DE 19716-2000

ABSTRACT

Photovoltaic linear concentrator arrays can benefit from high performance solar cell technologies being

developed at AstroPower. Specifically, these are the integration of thinGaAs solar cell and epitaxial lateral

overgrowth technologies with the application of monolithically interconnected solar cell (MISC) techniques. This

MISC array has several advantages which make it ideal for space concentrator systems. These are high system

voltage, reliable low cost monolithically formed interconnections, design flexibility, costs that are independent of

array voltage, and low power loss from shorts, opens, and impact damage. This concentrator solar cell will

incorporate the benefits of light trapping by growing the device active layers over a low-cost, simple, PECVD

deposited silicon/silicon dioxide Bragg reflector. The high voltage-low current output results in minimal 12Rlosses,

while properly designing the device allows for minimal shading and resistance losses. It is possible to obtain open

circuit voltages as high as 67 volts/cm of solar cell length with existing technology. The projected power density

for the high performance device is 5 kW/m 2for an AM0 efficiency of 26% at 15X. Concentrator solar cell arrays

are necessary to meet the power requirements of specific mission platforms and can supply high voltage power for

electric propulsion systems.

It is anticipated that the high efficiency, GaAs monolithically interconnected linear concentrator solar cell

array will enjoy widespread application for space based so!ar power needs. Additional applications include remote

man-portable or ultra-light unmanned air vehicle (UAV) power supplies where high power per area, high radiation

hardness and a high bus voltage or low bus current are important. The monolithic approach has a number of

inherent advantages, including reduced cost per interconnect and increased reliability of array connections. There

is also a high potential for a large number of consumer products. Dual-use applications can include battery

chargers and remote power supplies for consumer electronics products such as portable telephones/beepers,

portable radios, CD players, dashboard radar detectors, remote walkway lighting, etc.

INTRODUCTION

This monolithic interconnection process is capable of achieving high bus voltages while significantly

reducing the number of ultrasonic welded, tabbed and soldered, or wirebonded array interconnections commonly

used for space solar cell array fabrication. The low system current results in minimal 12Rlosses, while properly

designing the device allows for minimal shading and resistance losses.

A schematic of the device is shown in Figure 1. This device is fabricated on a semi-insulating or surrogate

substrate. The epitaxial layers are grown on°a Bragg reflector which results in light trapping and also offers

additional electrical isolation from the substrate. The areas in between the solar cell elements are seperated by

either rendering them non-conductive or physically dividing the epitaxial layer into segments. Following this the

base layer and emitter of adjoining segments are connected in a series configuration by etching and patterning

appropriately and applying a suitable metallization.

In addition to the reduced interconnect costs and increased reliability, another key benefit of this

technology is the wide range of specifications that Solar cell arrays can be designed to meet. Monolithic

interconnection allows the construction costs to be roughly independent of the array size, power or voltage.

Therefore, the cost to manufacture solar cell arrays with output powers ranging from tens of milliwatts up to tens of

watts and output voltages ranging from 5 to 500 volts will be roughly the same. This compares favorably to

conventionally manufactured, commercial concentrator solar cell arrays, where handling of small parts is very

-122-
interconnect metal

(base, emitter contact)

p-emitter

growth front

F

semi-insulating region via

GaAs or surrogate PECVD deposited Bragg reflector

substrate

Figure 1. GaAs monolithic array for linear concentrators.

labor intensive and costly. The array can also be configured to provide power at a specified current or voltage by

simply changing the metallization mask set. in this way, a wide variety of product specifications can be met using

the same fabrication techniques.

Due to the higher voltage attainable by the monolithically interconnected solar cell, the 12Rarray losses will

be minimized for any bus voltage when compared to conventional GaAs solar cells. Utilizing a higher voltage also

results in a weight savings at the array level due to the smaller quantity of high current carrying wire necessary for

solar cell interconnection and the reduced dependence on stringing of low voltage devices to achieve the needed

bus voltage. This monolithic interconnection technique offers several additional advantages compared to

conventional array interconnection technologies for space applications. The reliability of the array is enhanced by

reducing the complexity of the interconnections [1], and monolithically interconnected solar cell arrays offer

potentially higher yields compared to conventional array manufacturing processes such as welding [2] or

soldering. Typical yield reductions in standard array interconnection are the result of breakage due to either

mechanical or thermal stress.

m

m

3_

w

== m

=.

=o T

I

35 2

15 - _ -_-Conventional 11 volts}

1 J " '-_-- MISC (12 volts,

10÷

0 ==-_= _' : = = , ,

24 36 48 60 72 B4 96 t08 120

Bus Volzage

(volts)

Figure 2. Solar cell voltage effect on power loss.

-123-
Anothereliabilitybenefitfromthe high voltage device is the low power loss from shorts, opens, and

impact damage at the array level. If the monolithically interconnected solar cells are operated either at the bus

voltage or some factor of the bus voltage, loss of a single solar cell within a string will have minimal impact on the

array power generation when compared to typical GaAs solar cells. This is shown in Figure 2 using, for instance,

a nominal 1.0 volt GaAs solar cell and a 12 volt monolithically interconnected GaAs solar cell. As is shown, the

power loss is quite dramatic due to the loss of a string of devices as compared to a single or a few devices when

the string of devices becomes disabled.

A monolithic interconnection technique offers several advantages compared to conventional array

interconnection technologies. These include:

• The cost of fabricating the array interconnections is reduced and the process can be

automated.

• 12Rlosses are minimized

• The complexity of the interconnections is reduced thereby enhancing the reliability of the

array.

• The interconnection cost is independent of the array size.

• Yields are higher compared to conventional array manufacturing processes such as

welding or soldering.

This monolithic interconnection technique offers the potential to significantly reduce the high current levels

of concentrator systems without degrading the total system power. The monolithic array will supply power at a

lower current and higher voltage because it is configured as a series connected array of small area devices. This

type of interconnection cannot be done by any other method simply because of the number and size of the

components involved. By reducing the area of array elements, the current is reduced. Reducing the system

current has considerable benefit in reducing the power lost to series resistance. For example, an 8 cm2GaAs

solar cell at 15-suns delivers on the order of 3.6 A and 1 volt for a power of 3.6 Watts. A similar cell configured as

a monolithic array (80 elements at 0.05 x 2 cm2) would deliver the same power at 0.045 A and 80 volts. This is a

considerable reduction of the

600.00 _-

,L

500.00 i

 oo.ooT

_E 300.00

200.00 -

O L

n

1oo.oo

L

0.00

Figure 3.

zero grid width

; (prismatic cover)

' t// '

ij

_/ 50 micron grid width

/ , .

J

i

'E 'E

J

0 0.05 0.1

Active Area Width [cm]

o

Predicted monolithic array performance. The 0% loss line

is an ideal case which includes no shading or series

resistance losses (AMO, 15X).

system current.

The monolithically

interconnected array in its simplest

form consists of a lateral array of

devices connected end to end.

Figure 3 shows the projected power

as a function of array element width

and bus bar width. Using a properly

designed prismatic cover, the grid

shading can effectively be reduced

to zero. Therefore, using coarse,

50 micron grid widths, a power

output of over 500 mWlcm 2is

feasible with the prismatic cover.

With an array element or active area

width of 0.01 cm, it is possible to

achieve 67 volts/cm. An active area

width of 0.05 cm enables

18 volts/cm, and an active area

width of 0.1 cm results in

10 volts/cm. Higher voltages are

feasible using photolithography to

reduce the grid width.

-124-
Theother cell parameters for this model are: base doping of 3x1017/cm3, base thickness = 1 micron, and emitter

sheet resistivity of 18 ohm/r-I.

The monolithic,ally interconnected concentrator solar cell incorporates the light trapping benefits of an

ultra-thin GaAs solar cell by laterally overgrowing a PECVD deposited Bragg reflector. This reflector consists of

alternating layers of silicon and silicon dioxide deposited by the PECVD process. The silicon/silicon dioxide

system offers significant advantages over the use of Bragg reflectors grown by MOCVD or MBE, foremost of

which is the low cost. For the concentrator solar cell technology, a non-conducting reflector is desirable for

electrical isolation of the substrate and solar cell active layers. The use of lateral overgrowth means the

advantages of light trapping are achieved while maintaining a rugged structure supported by a GaAs or surrogate

substrate. There is the potential for using low cost substrates such as silicon with a low cost, proprietary, GaAs

interlayer. This further enhances the low cost benefits of this technology.

The reflectivity spectrum of a Bragg reflector can be tuned for high reflection depending on the thickness

and number of layers. Refiectivity data of a typical silicon/silicon dioxide Bragg reflector is Shown in Figure 4.

100-

==

o=

90  ̧

8O

7O

6O

50

4O

3O 1-

20_

10_

!

0

35O

i

450 550 650 75O

Wavelength (nm)

850 950 1050 1150

Figure 4. Reflectivity of a two pair siliconsilicon dioxide (55/120/55/120 nm) Bragg reflector on silicon.

To obtain lateral overgrowth, the substrate is masked and patterned using standard photolithography

techniques. Openings in the mask (vias) are defined by selective etching to expose the substrate surface. The

areas of exposed substrate serve as sites of preferential nucleation; nucleation is limited to the vias, with virtually

no deposition on the masking layer. As growth proceeds, crystals nucleated at the vias overgrow the masking

layer.

For the.monoiithicaily interconnected'concentrator device, the interdevice semi-insulating regions can be

situated directly over both the growth front and the via openings (see Figure 1). Therefore, the material quality in

these regions is not critical. The highest quality material will be in the areas of film laterally overgrown on the

Bragg reflector.

AstroPower has extensive experience with the lateral overgrowth of GaAs and AIGaAs over both metals

and dielectrics. Figure 5 shows a cross sectional photomicrograph of consecutive AIGaAs layers laterally

overgrowing a tungsten mask. The vias were 10 microns wide on 400 micron centers. The overgrowth layer was

close to impinging (30 microns between growth fronts) and there was no solvent entrapment or voids. The

consecutive layers were uniform and planar. By adjusting the mask spacing and via dimensions, impinging growth

layers are feasible and have been demonstrated.

-125-
AI soGa.4oAs--_

overgrowth layer

AIGaAs DH

LED structure

Figure 5. Cross-section of AIGaAs layers laterally overgrowing a tungsten mask ( I OOX).

SUMMARY

This unique device uses monolithically interconnected elements for linear concentrator arrays. The low

system current results in minimal 12R losses and device modeling enables minimal shading and resistance losses.

The technology takes advantage of the benefits of light trapping by incorporating epitaxial lateral overgrowth of a

PECVD deposited Bragg reflector. This process is capable of achieving high bus voltages while significantly

reducing ultrasonic welded, tabbed and soldered, or wirebonded array interconnections commonly used for space

solar cell array fabrication.

REFERENCES

1 J.R. WOODYARD, "Thin Film Cell Development Workshop Report", Proceedings of the Space Photovoltaic

Research and Technology Conference, May 1991, page 48-1.

2 B.S. SMITH AND B.T. CAVICCHI, "GAAs SOLAR CELL CIRCUITASSEMBLY", 19TH PHOTOVOLTAICS SPECIALISTS

CONFERENCE, NEW ORLEANS, MAY 1987, PAGE 448.

-126-
HYDROGEN PASSIVATION OF N*P AND P*N HETEROEPITAXIAL

InP SOLAR CELL STRUCTURES 1

B. Chatterjee, W.C. Davis and SJ_. Ringel

Department of Electrical Engineering

The Ohio State University

Columbus, OH 43210

R. Hoffman, Jr.

Essential Research, Inc.

Cleveland, OH 44135

ABSTRACT

Dislocations and related point defect complexes caused by lattice mismatch currently limitthe

performance of heteroepitaxial InP cells by introducingshunting paths across the active junction and by

the formation of deep traps within the base region. We have previously demonstrated that plasma

hydrogenation is an effective and stable means to passivate the electrical activity of such defects in

specially designed heteroepitaxial InP test structures to probe hydrogen passivation at typical base

depths within a cell structure. In this work, we present our results on the hydrogen passivation of actual

heteroepitaxial n*p and p÷nInP cell structures grown on GaAs substrates by metalorganic chemical

vapor deposition (MOCVD). We have found that a 2 hour exposure to a 13.56 MHz hydrogen plasma at

275 C reduces the deep level concentration in the base regions of bath n*p and p*n heteroepitaxial InP

cell structures from as-grown values of 5-7xl 0 TMCm "3, down to 3-5xl 0 2 crn-3, All dopants were

successfully reactivated by a 400 C, 5 minute anneal with no detectable activation of deep levels. I-V

analysis indicated a subsequent ~ 100 fold decrease in reverse leakage current at -1 volt reverse bias,

and an improved built in voltage for the p÷nstructures. In additionto being passivated, dislocations are

also shown to participate in secondary interactionsduring hydrogenation. We find that the presence of

dislocations enhances hydrogen diffusion into the cell structure, and lowers the apparent dissociation

energy of Zn-H complexes from 1.19 eV for homoepitaxial Zn-doped InP to 1.12 eV for heteroepitaxial

Zn-doped InP. This is explained by additional hydrogen trapping at dislocations subsequent to the

reactivation of Zn dopants after hydrogenation.

INTRODUCTION

Electrically active dislocations within InP layers grown on lattice mismatched substrates such as

GaAs, Si and Ge currently limit the efficiency of heteroepitaxial InP solar cells (ref. 1). The ~ 8%

mismatch in lattice constant for InP/Si, and 4% for both InP/Ge and InPIGaAs, typically resultin

threading dislocation densities in the range 1-10x108 cm2 within the InP layers, much higher than the

theoretically predicted value of 105cm2 necessary to achieve heteroepitaxial cell performance

comparable to homoepitaxial InP (refs. 1,2). This fact has prompted the application of many approaches

to reduce this high dislocation density and/or the electrical activity of dislocations, including

compositionally graded buffer layers, thermally cycled growth and hydrogen passivation (refs. 3,4).

Our earlier work on hydrogen passivation of heteroepitaxial InP test structures grown on GaAs

and Ge substrates demonstrated the effectiveness of a post-growth plasma hydrogenation treatment

Work supported by NASA grant no. NAG3-1461.

-127-
which reduced the dislocation related deep level concentration from ~ 6x1014 crn"ato ~ 3x10 t2 cm"ain the

InP layer (refs. 2,5,6). This passivation was stable up to ~ 550 °C,;Nhich combined with dopant

reactivation occurring at *.400 °C, opens a 150 °(3 post-passivation processing window for cell

completion. The observed deep level passivation was consistent with a 2-3 order of magnitude reduction

in reverse leakage current for diodes fabricated within the heteroepitaxial InP test structures. Further

detailed analysis showed that hydrogen incorporation fundamentally changes the trapping mechanism of

dislocations from the expected extended state behavior, to point-defect-like behavior, significantly

reducing the role of dislocations as the dominant feature in the deep level spectra. In fact, the dominant

level responsible for reverse biased generation-recombination current switched from a deep dislocation

level to a shallow center within the InP bandgap after H-passivation. In this paper we present the first

report of deep level passivation by plasma hydrogenation in actual heteroepitaxial InP cell structures

grown by low pressure metalorganic chemical vapor deposition (MOCVD). We investigate passivation

efficiency in both p+n and n*p cell structures. Results of a comparative study of homoepitaxial InP/InP

and heteroepitaxial InP/GaAs cell structures with regard to hydrogen diffusion and dopant reactivation

kinetics after hydrogenation are discussed to reveal the impact of dislocations on the passivation

process.

EXPERIMENTAL

The heteroepitaxial InP on GaAs structures were grown by low pressure MOCVD in a reactor of

our own design and construction as previously described [ref. 7]. Precursor species of phosphine,

trimethylindium (TMIn), silane and diethylzinc were injected into the hydrogen carder gas. Reactor

chamber pressure was maintained at 150 ton"throughout the growth process. A typical InP on GaAs

heteroepitaxial growth included a five minute, 620"C anneal in 1.5 tort partial pressure phosphine priorto

nucleation of a thin layer of InP at 550"C. The susceptor temperature was then raised to 620"C for the

remainder of the growth of the 3-4 pm thick heteroepltaxial layem. A TMIn flow rate of 14.5 pmol min"1

resulted in a growth rate of 0.61 nm sec"1. Doping of the InP layers was achieved by injection of sllane

and diethylzinc for n-type and p-type respectively. Figure 1 shows the measured doping profiles of the

structures investigated here.

Hydrogenation was performed in a Technics Planar Etch II parallel plate, 13.56 Mhz plasma

reactor, using a 30 sccm flow rate of semiconductor grade !"12,chamber pressure of 750 mTorr, and

power density of 0.08 W/cm 2. Substrate temperatures were varied from 250 ° C 275° C. Hydrogen

exposure time was varied from 1.5 - 2 hours. Immediately priorto hydrogen exposure, all samples were

capped with a thin, hydrogen-permeable, cap layer of SiN=to prevent InP surface degradation by

preferential loss of phosphorous dudng hydrogenation. Optimum cap thickness was found to be 20 nm

for a 2 hour exposure, based on SEM studies of the underlying InP surface as a function of nitride cap

thickness. The nitride layer was etched off in dilute HF before metaliization. DLTS measurements were

performed using a Biorad DL4600 DLTS spectrometer and I-V measurements were done using an HP

4145 semiconductor parameter analyzer. Post hydrogenation dopant reactivation annealing was

performed in flowing N2 at 400 0(3for 5 minutes. Metallization was performed in an e-beam evaporator

usingAuGeNi for n-type ohmic contacts and Ag/Zn/Ag for p-type contacts (ref. 8). Test devices were

isolated by mesa etching to an active junction area of 0.785 mm'.

RESULTS AND DISCUSSION

3.1 Defect passivation in n'p and p*n heteroepitaxial InP cell structures

Both n*p and p*n cell structures have been used to achieve high efficiency InP homoepitaxial cells to

date. For heteroepitaxial cells grown on group IV substrates such as Ge and Si, the p*n design is

preferred due to potential autodoping and the development of back to back diodes for n*p structures. In

view of hydrogen passivation however, both the direction of the built in field of p+n cells and the strong

Zn-H bonding typically observed in p-lnP might be expected to impede the in-diffusion of hydrogen into

the base region of the heteroepitaxial cell. Hence, hydrogen passivation was performed on cell

-128-
structures of both types to compare passivation effectiveness. Figure 2 showsdark I-V plots for the two

cell types pdor to hydrogen processing (as-grown) and alter a 1.5 hour hydrogen exposure plus several

post-hydrogenation annealing temperatures. It is seen that the built in voltage improves after

hydrogenation for p*n but not n*p structures, indicating that Vocfor the former should be improved.

Furthermore, in both cases, hydrogen is seen to significantly improve the reverse leakage current

characteristics, shown in table I. The 400 C anneal is required to reactivate the Zn dopants in both cell

structures (confirmed by C-V measurements).

Table I. Reverse bias leaka_le currents for vadous hydrogenation anneal conditions.

Device areas are 0.785 mm'.

I,e,@ -1V n+pdevices.......... -p*rtdevices

As grown 1.39 mA 1.11mA

hydrogen + 400 UCanneal 10.74 I_A 5.21 I_

hydrogen + 512 Uc anneal 198 i_ 991 i_,

hydrogen + 602 Uc anneal 1.25 mA 1.79 mA

From this table, we see that leakage currents are reduced by more than 2 orders of magnitude for both

cell types after the 400 C dopant reactivation anneal. Only alter annealing in excess of 500 C does the

leakage current increase toward its original, non-passivated value. What is clear from these results is

that neither the direction of the built-in electric field nor preferential trapping of hydrogen in the emitter

significantly impedes the desired in-diffusion of hydrogen. However, table I indicates that p°n structures

exhibited a greater rate of thermal degradation. While the masons for this are unclear, one possibility

might be that Zn is being gettered by threading dislocations as interstitialswhich are shunting the

junction. We have reported in an eadier work using photoluminescence that interstitial Zn is Indeed

gettered by dislocations in heteroepitaxial InP and we are currently performing SIMS and polaron

profiling studies to investigate this possibilityin these samples (ref. 9). This suggests that altemaUve

dopants to Zn may be appropriate for heteroepitaxial p*n cells.

Figure 3 showsthe corresponding DLTS measurements for both the p*n and n*p structures which

confirm that in both cases, hydrogen is passivating deep levels by Iowedng their concentration by more

than 2 orders of magnitude. To our knowledge, this is the first report of successful hydrogen passivation

in heteroepitaxial InP cell structures. For the n*p structure, the hole trap at 0.69 eV is very close to the

peak T1 (comprised of sub-peaks T1A and Tli3) that we have reported previously for heteroepitaxial pInP test structures as related to extended defects resulting from lattice-mismatch (either dislocation cores

or point defect clusters) (ref. 2). The dislocation-related hole trap previously labeled T2 in the eadier test

structures is not observed in the cell structure until~ 0.5 um is removed from the surface. This depthdependent concentration is reasonable if this level is related to threading dislocations that have

nucleated from the InPIGaAs interface. The DLTS spectra of the p*n structure indicates passivaUon of

two electron traps in the n-type base having activation energies of 850 meV and 509 meV for the high

and low temperature peaks, respectively. This is the first report of deep levels in n-type heteroepitaxial

InP and we are currently analyzing the_OLTS spectra in detail to determine the relationship of these

states with the presence of dislocations and associated defects, and with our results on hetemepitaxial ptype InP.

One concem of the hydrogen passivation process is its stability with respect to moderate

temperature exposure and to light exposure. While we are presently investigating the latter, figure 4

demonstrates excellent passivation stability against exposure to a temperature of 80°C. As can be seen,

no detectable deep level reactivation was observed for the 24 hour pedod studied here. Light soaking

experiments will be performed on actual cells (i.e. with a gdd pattern) that are currently being fabricated.

-129-
3.2 Hydrogen diffusion in InP/GaAs and InP/lnP cell structures

One of the key issues for hydrogen passivation is the depth of hydrogen diffusion into the cell

structures. Our earlier work on specially designed test structureswhich incorporated a pn junction buffed

2 um below the InP surface demonstrated that hydrogen easily diffuses through the typical base

thickness of a heteroepitaxial cell. Here we investigate the impact of dislocations on the hydrogen

diffusion process within the n'p cell structures by monitoring the Zn dopant deactivation using

electrochemical C-V profiling for homoepitaxial and hateroepitaxial structures. It has been reported by a

number of authors that the dislocations provide 'expressways' for hydrogen to diffuse into the bulk of the

samples, with hydrogen also showing a strong preference for these extended defects (ref. 10). We

hydrogenated both homoepitaxial and a heteroepitaxial cell structure for 2 hours at 275°C. Figures 5

presents the results of C-V depth profiling of Zn acoeptors, before and after hydrogen exposure for the

two structures. Both samples show deep passivation depths. However the heteroepitaxial sample shows

a deeper and much greater degree of acceptor passlvation than its homoepltaxial counterpart. This result

confirms that the threading dislocations running from the InP/GaAs interface up to the surface provide a

pathway for fast hydrogen diffusion into the base region from the top surface compared with the case

where these dislocations are absent. Similar studies were attempted for the n-base regions in the p÷-n.

However, donor passivation was not observed, consistent with the results of Pearton et al. who have

reported that hydrogen is only weakly bonded to Si donors in InP compared to Zn acceptors (ref. 11).

3.3 Acceptor reactivation in n*p structures

One of the factors which affect hydrogen diffusion is the chemical affinity between the

passivating hydrogen species and ionized dopant atoms. In InP0Zn acceptors are known to be strongly

passivated by positively charged hydrogen ions, where hydrogen is thought to bond with the neighboring

P atom in a bend centered position (ref. 12). It is reasonable to expect that the presence of dislocations

in heteroepitaxial material may significantly complicate this process,for at least two reasons. First,

dislocations appear to be a major sink for hydrogen. Second, the presence of dislocation strain fields

have been shown to getter Zn interstitials, and thereby reduce the substitional Zn concentration (mE 9).

Zn-H reactivation kinetics for homoepitaxial and hetemepitaxial p-type InP are compared below.

In the absence of an applied electric field, hydrogen-zinc complexes dissociate at temperatures

close to 400°C. Such high temperatures are necessary to (1) separate H from Zn, and (2) provide

enough energy so that the H species can diffuse away from the Zn ion after reactivation. Zundel et al.

(ref. 13) however, have shown that in the presence of a sufficiently strong electric field the dissociation of

these zinc-hydrogen complexes follow first-order kinetics since the applied electric field sweeps out the

reactivated hydrogen which prevents retrapping inside the depletion region. Following this analysis, we

conducted a sedes of experiments to elucidate this information in heteroepitaxial and homoepitaxial Zndoped InP using the n'p cell structure. C-V measurements were made on hydrogenated n+p structures

after systematically annealing under a reverse bias of-8 volts as a function of temparature (70 - 120 °C)

and time (5-30 min ). Following the analysis of Zundel et al., the inactive carder concentration, N_,which

is the concentration of carders removed by passivation, is plotted in figure 6 according to

Ni = Ni - NHexp (- vat)

where N, is the initial carder concentration of the as grown sample, NH is carder concentration at the

chosen depth after hydrogenation, v_ is the thermally activated dissociation frequency and t is the time

over which the reverse biased annealing (RBA) takes place. The dissociation frequency has a

temperature dependence, which is given by

Vd = VOexp ( -ED / K'I")

where Eo is the dissociation energy of the zinc-hydrogen complex. First, the annealing experiments were

done for different time periods at a fb(ed annealing temperature. Then these experiments were repeated

-130-
for other temperatures generating figure 7. The slopes of the lines in figure 7 when plotted against the

annealing temperatures gives the dissociation energy ED which is shown in figure 8. Analysis of the

homoepitaxial data gives a value of 1.19 eV for Eo., consistentwith previous reports by other

investigators for zinc acceptors in homoepitaxial InP (ref. 12). However for heteroepitaxial Zn-doped InP,

we calculate ED to be 1.12 eV. To the best of our knowledge this is the first reported values for

heteroepitaxial InP and we attribute this lowering to interactions with dislocations in the depletion region.

The local electdc and strain fields surrounding the dislocations aid the removal process of hydrogen

from the acceptors once hydrogen is thermally liberated, thereby Iowedng the apparent dissociation

energy. DLTS measurements shown in figure 9 support this notion, where additional reduction, or

passivation, of the T1 (dislocation-related) level is observed as a function of the RBA process. Since no

additional hydrogen is being provided to the InP structure from the ambient, this passivation can only

occur by trapping hydrogen that has been liberated from Zn acceptors by the low (70-120 °C)

temperature RBA.

4. Conclusions

Hydrogen passivation of n*-p and p*-n InP heteroepitaxial cell structures grown on GaAs

substrates has been demonstrated and investigated. The passivation resulted in a - 2 order of

magnitude reduction in reverse leakage current at -1 volt, which corresponds to a > 2 order of magnitude

reduction in DLTS trap concentration within the base region of each cell structure. No reactivation of

deep levels was detected for at least 24 hoursdue to heating at 80°C in the dark. Fundamental studies

of hydrogen diffusion, and Zn reactivation kinetics with regard to the participation of dislocations were

performed. It was found that dislocations aid the hydrogen passivation process by providing fast

diffusion paths and by lowering the apparent dissociation energy of Zn-H complexes. We are currently

processing hydrogen-passivated cells using the procedures presented here, and this will be reported the

future.

5. References

1. R.K. Jain and D.J. Flood, "Influence of the Dislocation Density on the Performance of Heteroepitaxial

Indium Phosphide Solar Cells," IEEE Trans. Electron. Dev., vol. 40, pp. 128-1934, 1993.

2. B.Chatterjee and S.A. Ringel, "Hydrogen Passivatlon and its Effects on CarTier Trapping by

Dislocations in InP/GaAs Heterostructures, J. Appl. Phys. vol. 77, pp. 3885-3898, 1995.

3. C.J. Keavney, S.M. Vernon, V.E. Haven, S.J. Wojtczuk and M.M. AI-Jassim, "Fabdcation of n*lp InP

Solar Cells on Si Substrates," Appl. Phys. Lett., voi. 54, pp. 1139-1141, 1989.

4. I. Weinberg, C.K. Swartz and D.J. Bdnker, "Hetemepitaxial InP Solar Cells on Si and GaAs

Substrates," Tech. Digest Int. PVSEC-5, pp. 81-84, 1990.

5. B. Chatterjee, S.A. Ringei, R. Sieg, R. Hoffman and I. Weinberg, "Hydrogen Passivation of

Dislocations in InP on GaAs Heterostructums," Appl. Phys. Lett., vol. 65, pp. 1-3, 1994.

6. S.A. Ringel, B. Chatterjee, R.M. Sieg= E.V. Schnetzer and R. Hoffman, Jr., "Matedal Quality and

Dislocation Trapping in Hydrogen Passivated Heteroepitaxial InP/GaAs and InPIGe Solar Cells," Proc.

1st IEEE WCPEC, pp. 2204-2207, 1994.

7. "High Efficiency InP Solar Cells from Low Toxicity Tertiarybutylphosphine and Tdmethylindium by

OMVPE", R.W. Hoffman Jr., N.S. Fatemi, D.M. Wilt, P. Jenkins, D.J Bdnker and D.A. Scheiman, Proo.

1st IEEE WCPEC p. 1882, 1994.

-131-
8. N.S.Fatemi,V.G.Weizer,D.MWiltandR.W.Hoffman,"ASimple,LowResistanceContactSystem

for Shallow Junction p'nn° InP Solar Cells That Preserves Emitter Integrity During Sintering, Proc. IEEE

WCPEC, pp. 2129-2132, 1994.

9. R.M. Sieg, B. Chatterjee and S.A. Ringel,'Evidence for Enhanced Zinc Interstitial Concentration in

Strain-Relaxed Heteroepitaxial Indium Phosphide, "Appl. Phys. Left. vol. 66, pp. 3108-3110, 1995.

10. J.M. Zavada, S.J. Pearton, R.G. Wilson, C.S. Wsu, M. Stavola, F. Ren, J. Lopata, W.C. DautremontSmith and S.W. Novak, "Electrical Effects of Atomic Hydrogen Incorporation in GaAs on SiT J. Appl.

Phys. vol. 65, pp. 347-353, 1989.

11. W.C. Dautrernont-Smith, J. Lopata, S.J. Pearton, LA. Koszi, M. Stavola and V. Swaminathan,

"Hydrogen Passivation of Acceptors in p-lnP," J. Appl. Phys. vol. 66, pp. 1993-1996, 1989.

12. S.J. Pearton, W.S. Hobson and C.R. Abernathy, "Dissociation Energies of Acceptor-Hydrogen

Complexed in InP," .Appl.Phys. Lett. vol. 61,pp. 1588-1590, 1992.

13. T. Zundel and J. Weber, "Dissociation Energies of Shallow-Acceptor-Hydrogen Pairs in Silicon,"

Phys. Rev. B. vol. 39, pp. 549-552, 1989.

_1E+19

E

._.

1E+18 8

o

1E÷17

4"

n

p - base

÷

p -BSF

" hole I

• electronI

I I I I l !

0 0.5 1 1.5 2 2.5 3 3.5

Depth [ urn]

lal

__IE+19

E

o.

c

o

g IE+18

u

o

"E

1E+17

n-base

IIW • Q • • • • • • 0(I

n%BSF n+.-contact

layer

D• O

I -I I

0 0.5 1 1.5 2 2.5 3

00000•000 B•O•O••0••O•O0 • ••0

'_ hole 1

• electront

I

i

Depth [urn]

Ib]

Figure 1. Electrochemical C-V profiles of carder concentration for (a) n'/p/p" and (b) p"/n/n°/n . cell

structures grown on p°-GaAs substrates used in this study.

-132-
_F (aA)

=o. oo n'h

IE.OOO _v[

b

-,o.o,

J. 8O0 VTr

I

I

I

_11000/_$v

I I

(v)

ZF (mu_)

AO.OC

,.oo_ t /

/cl_v

,/

_, /. I I ]

- . o l.uoo :L.600 $ 600 VF .mOOO/cl_v (V)

Figure 2. Dark I-V characteristics of (a) n'p and (b) p'n structures. For each case, a-d denote as-grown,

1.5 hr. H-passiv_ion + 400 *C anneal, 1.5 hr. H-passivation + 512 'C anneal, and 1.5 hr. H passivation +

602 °C anneal conditions. NI anneals were for 5 minutes.

E

8.01E+14

7.01E+14

6.01E+14

5.01E+14

E

._.4.01E+14

z-3.01E+14

2.01E+14

1.01E+14

1.00E+12

100 150 200 250 300 350 400

Tempemture[K]

4.50E+14

4.00E+14

3.50E+14

300E+14

2.50E+14

2.00E+14

1.50E+14

100E+14

5.00E+13

I

100 400

.,oro 7

/ \

•_.j_j hydrogenated

150 200 250 300 350

Temperature [K]

Figure 3. DLTS spectra of for (a) n'/p and (b) p'/n InP/GaAs device structures in the as-grown state and

after a 2 hr. H-passivation exposure plus dopant reactivation anneal.

-133-
1.00E*13

E

z1.00E+12

J_hyclrogen,no RBA

0 5

I I I I

10 15 20 25

AnnealingTime [hours]

Figure4. Stabilityof passivateddeep levelsagainstlongtermannealinginthe darkat 80oC as obtained

fromDLTS peak heightsoftheannealedsamples.

,_, 1.00E+ 18

1.00E+17 ,e,,,,

C

8

C

o 1.00E+16 (.,1

"E

¢0

1.00E+15

0

_-- n+ emitter

?

I

0.5

hydrogenated n*lp cell

_oe_ p]

' i

; _ hom_pi

I I I I I I

1 1.5 2 2.5 3 3.5

Depth [urn]

Figure5. ElectrochemicalC-V Zn-acceptorprofilescomparingpassivationdepthin heteroepffaxialand

hornoepitaxialn'/p structuresaftera 2 hourhydrogenplasmaexposure.

7.00E'17'

5.00E÷17

E

=_ 1 80E÷16

ne/p =ell

e=

U 8.00E.15

!

;_ Asgrown

I _:4_'- 2 hr H 2, no RBA

J H-* 5 mm RBA

]

I_ H2'* 10rain RBA

i i

0.2 0.3

Depth[urn]

vr=-_v

T = I00°C

I I I

0 0.1 04 0.5

Figure 6. C-V doping profiles of heteroepitaxial n'/p InP/GaAs structures, for as grown and

hydrogenated ( 2 h, 275°C) samples annealed at 100 ° C under -SV reverse bias for various time periods.

The plots were obtained from C-V data.

-134-
m

Z

co

la.

>=

c

0.1

0.01

0

_m u

......... °wm_o_mi_Db._ =-2===== _m_ m

%

%

'%

%

(a) ,,

Homoepi, n+lp " ".,

%

I I I _,, I

5 10 15 20

--4--70C

--7- _C

--'*--_C

-"_--1_C

-_- 120C

Annealing Time [min]

I

25 30

Z

¢.-

.Q

u.

U

.>_

0.1

0.01 :

i

0.001

0

• 70C I ".. "-

o 80C i ".

• 9oc ! Co) ",.

• 100 C i ",., heteroepi, /p

%

I I I "=,1. I I

5 10 15 20 25 30

Annealing Time [rain]

Figure 7. Inactive acceptor fraction versus bias application time at different temperatures in the high field

region of hydrogenaed n'p diodes for (a) homoepitaxial and (b) heteroepitaxial structures.

-135-
0.1

0.01

u. 0.001

8

_ 0.0001,

E =1,12 eV

d ,,=_

E =1.19 eV "- .,-.. d

• Heteroepi

• Homoepi

I 0.00001 i I q

2.6 2.7 2.8 2.9 3

1000/'r(K)

Figure 8. Arrhenius plots of the dissociation frequencies of Zn-H oompiexes in homoepitaxial and

heteroepitaxial n°/p devices. The dissociation energies are indicated inthe figure.

o.

w

z

7.00E+12

6.00E+12

5.00E+12

4.00E+12

3.00E+12

2.00E+12

1.00E+12

0.00E+00

100

hydrogen, no RBA //_

hydrogen+ 10 minRBA " _//_

I I. I I I

150 200 250 300 350

I

400

Temperature [K]

Figure 9. Effect of RBA at 80 °C on the DLTS spectra, showing further passivation of the deep levels after

RBA experiments, for heteroepitaxial n'/p structures.

-136-
THE GROWTH OF LOW BAND-GAP InAs ON (111)B GaAs SUBSTRATES ]

R. E. Welser and L. J. Guido

Yale University

New Haven, Connecticut 06520-8284

Summary

Growth on the (111)B orientation exhibits a number of advantageous properties as compared to the

(100) during the early stages of strained-layer epita,xy. In accordance with a developing model of

nucleation and growth, we have deposited thin (60 A - 2500 A), fully relaxed InAs films on (111)B GaAs

substrates. Although thicker InAs films are subject to the formation of twin defects common to epitaxy on

the (111)B orientation, appropriate control of the growth parameters can greatly minimize their density.

Using this knowledge base, InAs films up to 2 I_m in thickness with improved morphology and structural

quality have been grown on (111)B GaAs substrates.

Introduction

The use of low band-gap materials is of interest for a number of photovoltaic and optoelectronic

applications, such as bottom cells of optimized multijunction solar cell designs [1], long wavelength light

sources, detectors, and therrnophotovoltaics. However, low band-gap materials are generally mismatched

with respect to lattice constant, thermal expansion coefficient, and chemical bonding to the most

appropriate commercially available substrates (Si, Ge, and GaAs). For the specific case of III-V

semiconductor heteroepitaxy, one must contend with the strain induced by both lattice constant

mismatch at the growth temperature and. differences in the rates of mechanical deformation during the

cool-down cycle. Several experimental techniques have been developed to minimize the impact of these

phenomena (i.e., compositional grading, strained-layer super!attices, and high-temperature annealing).

However, in highly strained systems such as InAs-on-GaAs, three-dimensional (3-D) island formation and

large defect densities (> 108 cm"2) tend to limit their applicability. In these particular cases, the surface

morphology and defect density must be controlled during the initial stages of nucleation and growth.

! This work has been supported by the NASA Graduate Student Researchers Program (NGT-50832) and

by the National Science Foundation via the Presidential Faculty Fellowship Program (ECS-9253760).

-137-
Thin Film Evolution

At the last SPRAT conference, we reported on a study of the evolution of InAs islands on (100) and

(111)B GaAs substrates [2]. Highly strained semiconductor systems like InAs-on-GaAs, with a 7.1% lattice

mismatch, grow in the Stranski-Krastanov growth mode in which the deposition of a two-dimensional (2-D)

wetting layer is followed by the formation of discrete 3-D islands. The density of the discrete islands is a

function of growth parameters, which, for the metalorganic chemical vapor deposition (MOCVD) system

used in these investigations, are substrate temperature (Ts), TMIn flow (fTMIn), and AsH3 flow (fAsH3).

Moreover, the density, geometry, and defect structure of the 3-D islands differ between the two

orientations.

As the film thickens, the islands coalesce to form a continuous film. The thickness at which this

happens is a function of the density (Ni) and the width-to-height ratio (w/h) of the initially discrete 3-D

islands (Figure-I). Specifically, a 2-D film forms more quickly w'rth higher Ni and the larger w/h.

10 4

_1000

_m

J

@

Z

100

° ''°"'1 ° ' °''"1 'I ,o,,.,i'" I

I • (100) - continuous

._ • [ [] (I00) - islands

[ • (111)B - continuous

_ Z_ (Ill)B-islands

",, • • -

/x /x "- []

/x

10 i i ,,,HJ l , ,,,,,J , , Ill,,,| , , ,i,,,J , , t,,,,,| ' I II'"

105 IO k 107 10 s 109 101° 1011

Island Density (cm "z)

Figure-1 The transition in thin film morphology from discrete 3D islands to a continuous layer as a

function of nominal film thickness, island density, and width-to-height ratio. The critical thickness (tc) at

which this transition takes place can be expressed as tc = Ni'1/2(w/h) 1 ' The solid line assumes a w/h ~ 7

for the (100) orientation, while the dashed line takes w/h - 100, which is more appropriate for the (111)B

substrate.

-138-
Featureless 250 A films have been deposited on both substrate orientations by adjusting the growth

parameters in such a way as to increase the island density. However, these films differ remarkably in terms

of surface roughness and defect density. The improved properties of films on (111)B substrates can be

attributed to the lower density, flatter geometry, and fewer threading defects of the initial islands on that

orientation. In particular, (111)B films are atomically terraced with at least an order of magnitude reduction

in threading dislocation density as compared to films on (100) substrates. While these thin 250 A films

on the (111)B approach the idealized condition for lattice-mismatched films of efficient strain relaxation

between the epilayer and the substrate with a minimal number of threading defects, thicker InAs films on

the (111 )B orientation are subject to the formation of hillocks which quickly degrade the film morphology.

Growth on (111)B Substrates

Even in the simple case of homoepitaxy, hillock formation is a common problem which has been

attributed to the formation of twin defects [3,4]. While tilted substrates have typically been employed in

the past to reduce hillock density, recent molecular beam epitaxy (MBE) results suggest that nearly

featureless films can be deposited on (111)B substrates under a narrow range of growth conditions, even

if substrates are on-orientation [5,6]. We have reproduced these results in our MOCVD system (Table I).

Table I : Hillock density as a function of growth parameters for 2500 A homoepitaxial GaAs films on

(111)B substrates.

II

Ts (°C) fTEGa (sccm) fAsH3 (sccm) Hillock Density (cm -2)

525 23 100 5 x 106

600 23 100 5 x 106

650 23 100 5 x 103

700 23 100 50

600 140 100 l x 108

600 140 500 1 x 107

600 23 100 5 x 106

600 23 500 1 x 106

llr i ITI II

Although hillock density is particularly sensitive to substrate temperature, it is also found to be a

function of all the growth parameters. At a fixed temperature, slower growth rates and a higher As

overpressure both tend to reduce the number of twins. By appropriately altering growth conditions,

hillock density can be reduced by over six orders of magnitude.

-139-
2-Step Approach

With this in mind, we have implemented a 2-step procedure for the growth of thicker InAs films on

(111)B GaAs substrates. In the first step, optimized conditions for the deposition of a specular, strainrelaxed 250 .&film of InAs are employed. Next the growth is interrupted, and the conditions altered to

approximate thoseyielding specular, lattice-matched GaAs films, in this way, the hillock density on latticemismatched 2500 A InAs films has been reduced by over an order of magnitude.

InAs films grown on (111 )B GaAs substrates employingthis 2-step procedure still exhibit similar

improvements in structural properties compared to InAs films on (100) GaAs. Figure-2 shows the x-ray

diffractionpeaks from 2500 A InAs films on (100) and (111)B GaAs substrates. In both cases, the

separation of the film and substrate peaks suggests that both films are nearly fully relaxed. However, the

full width at half maximum (FWHM) of the film peak on the (100) orientation is roughlytwice that of the

substrate, indicatinga substantial degradation instructural quality. On the other hand, the film peak

FWHM on the (111)B is nearly the same as that of the substrate, suggesting an improved structural quality.

Indeed, cross-sectionalTEM of these films indicates at least an order of magnitude lowerthreading

dislocation density in the (111)B film.

Conclusions

We have produced atomically terraced, fully relaxed lnAs films on (111)B GaAs substrates with misfit

dislocations largely confined to the epilayer/substrate interface. The advantages of the (111)B

orientation stem from the shape of the initial InAs islands, their density (which can be manipulated by

adjusting growth conditions), and the nature of their defect structure. Thicker InAs layers on the (111)B

are susceptible to growth defects similar to those observed for GaAs homoepitaxy on (111)B substrates.

However, by implementing a 2-step growth approach, thicker layers of InAs films with reduced growth

defect density have been grown on the (111)B orientation. Thus, the use of (111)B substrates may have

the potential to yield high quality, strain-relaxed material for device applications.

References

1. R.E. Welser and L. J. Guido, Solar Energy Materials and Solar Cells 36, 349 (1995).

2. R. E. Welser and L. J. Guido, Proc. o_ SPRAT XIII, Cleveland, OH, June 14-16, 1994, p. 409.

3. A.Y. Cho, J. Appl. Phys. 41, 2780 (1970).

4. K.C. Rajkumar, P. Chen, and A. Madhukar, J. Appl. Phys. 69, 2219 (1991).

5. L. Vina and W. I. Wang, Appl. Phys. Lett. 48, 36 (1986).

6. P. Chen, K. C. Rajkumar, and A. Madhukar, Appl. Phys. Lett. 58, 1771 (1991).

-140-
d

.I-4

0.8

0.6

0.4

0.2

// "..XI -- InAs Film

(200) // '. '_] ......... OaAs Substrate

/ '.. -

| •

f •

_ -

i i

o lel b

I •

.R •

;-l-l-t-r-,-1-,. L..B_j.l_L.pwq_l*_'rWl ! I ! !

fl , , z i i i i .... , , , !

|

q,J

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2

A(2®) (degrees)

(a)

.w,,4

1'===4

1 7"" .... ' .... " 7"_'."1 InAsmim

08 ...... G ssbs, ,e 0.6- / ',,. i

0.4 - !': ''. !

0.2 i -'!

0 ", i , , I , , , , l , , , , I .... I , , , , I , ,., , I , , , , I'; .......

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2

A(2®) (degrees)

(b)

Figure-2 X-ray diffraction data from 2500 A films grown on (a) (100) and (b) (111)B GaAs substrates.

-141-
DESIGN OF A THREE-LAYER ANTIREFLECTION COATING FOR

EFFICIENCY INDIUM PHOSPHIDE SOLAR CELLS

USING A CHEMICAL OXIDE AS FIRST LAYER

Jacques Moulot

Cleveland State University,

Cleveland, Ohio 44115

Chandra Goradia

Cleveland State University,

Cleveland, Ohio 44115

Mircea Faur

Cleveland State University,

Cleveland, Ohio 44115

Manju Goradia

Cleveland State University,

Cleveland, Ohio 44115

Maria Faur

NASA Lewis Research Center

Cleveland, Ohio 44135

Sheila Bailey

NASA Lewis Research Center

Cleveland, Ohio 44135

HIGH

ABSTRACT

It is well known that the behavior of III-V compound based solar cells is largely controlled

by their surface, since the majority of light generated carders (63% for GaAs and 79% for InP)

are created within 0.2 pm of the illuminated surface of the cell. Consequently, the always

observed high surface recombination velocity (SRV) on these cells is a serious limiting factor for

their high efficiency performance, especially for those with the p-n junction made by either

thermal diffusion or ion implantation. A good surface passivation layer, ideally, a grown oxide as

opposed to a deposited one, will cause a significant reduction in the SRV without adding

interface problems, thus improving the performance of III-V compound based solar cells. Another

significant benefit to the overall performance of the solar cells can be achieved by a substantial

reduction of their large surface optical reflection by the use of a well designed antirefiection (AR)

coating.

In this paper, we demonstrate the effectiveness of using a chemically grown, thermally

and chemically stable oxide, not only for surface passivation but also as an integral part of a 3-

layer AR coating for thermally diffused p+n InP solar cells. A phosphorus-rich interfacial oxide,

In(PO3) 3, is grown at the surface of the p+ emitter using an etchant based on HNO 3, o-H3PO 4

and H20 2. This oxide has the unique propertiesof passivating the surface as well as serving as

a fairly efficient antireflective layer yielding a measured record high AM0, 25Oc, open-circuit

voltage of 890.3 mV on a thermally diffused lnP(Cd,S) solar cell. Unlike conventional single

layer AR coatings such as ZnS, Sb203, SiO or double layer AR coatings such as ZnS/MgF 2

deposited by e-beam or resistive evaporation, this oxide preserves the stochoimetry of the InP

surface. We show that it is possible to design a three-layer AR coating for a thermally diffused

InP solar cell using the In(PO3) 3 grown oxide as the first layer and AI20 3 , MgF2 or ZnS, MgF 2

as the second and third layers respectively, so as to yield an overall theoretical reflectance of

less than 2%.

Since chemical oxides are _eadily grownon III-V semiconductor materials, the technique

of using the grown oxide layer to both passivate the surface as well as serve as the first of a

multilayer AR coating, should work well for essentially all III-V compound-based solar cells.

INTRODUCTION

Calculations show that the majority of light generated carriers in direct bandgap III-V

compound semiconductor based solar cells, are created within 0.2 pm of the illuminated surface

of the cell(l). For such materials, surface recombination velocity (SRV) and pin junction depth

-142-
considerationsareof criticalimportanceinthedesignof highefficiencysolarcells.Eventhough

publishedvaluesof SRVonInPandGaAsvaryoverawiderange,dependingonthesurface

preparationandthetechniquesusedformeasurement (2), it is estimated that SRV values can be

as high as 1.8x107 cm/s (3,4), even for epitaxially grown cells. Therefore, thin emitters and

reduced SRV appear to be necessary conditions for the fabrication of high efficiency III-V

compound based solar cells. A good surface passivation layer, ideally a grown oxide, will cause

a significant reduction of the SRV without adding semiconductor/oxide interface traps and oxide

bulk defects which are inherent to deposited oxides, thus improving the overall performance of

the IlI-V compound based solar cell. Yet another obvious but not often propedy addressed

limitation to high performance of these cells, is their high surface reflection, particularly in the

blue region of the solar spectrum.

In this work, we address these two problems with a new optimized three-layer

AR coating for the p+n InP(Cd,S) thermally diffused solar cell. A chemical oxide grown on the

p+-doped emitter serves as the first layer of a three-layer AR coating composed of

in(PO3)3/AI203/MgF2. This In(PO3) 3 oxide, which is described in detail elsewhere(3), is

primarily designed as a surface passivating layer, and has yielded a record high AM0, 25°C

open circuit voltage Voc of 890.3 mV, measured on a thermally diffused p+n InP(Cd,S) solar

cell(S). We also investigate a three-layer AR coating composed of In(PO3)3/ZnS/MgF2. Even

though, in this paper, we demonstrate the effectiveness of the above described AR coatings on

p+n InP solar cells, the concept of using a native oxide passivating layer as the first layer of a

multilayer AR coating should benefit any III-V compound semiconductor based solar cells, since

chemical oxides are readily grown on most III-V semiconductor materials.

EXPERIMENTAL

The 0.8 cm x 0.6 cm p+n InP solar cells were fabricated usin._the closed ampoule

technique to diffuse Cd into n-lnP:S (ND-N A = 3.5 xl016 to 3.1 xl017 cm- ) with Cd3P 2 as the

diffusion source. Diffusion temperatures were from 560o to 660 °C. The substrates were

Czochralski (LEC) grown with EPD of about 5-7 x 104 cm"2. Diffusions were performed through

chemically grown P-rich oxide cap layers.

Au-Ge was used for the back contact. The Au-Zn (0.18 to 0.3 pm thick) front contact grid

was deposited by evaporation and defined using existing photolithography masks, designed for

the nip cell configuration. Because Au based contacts are known to penetrate into InP during

sintering at 430 °C, up to depths which are over three times the initial thickness of the

evaporated Au-Zn-Au layer, the thickness of the emitter was kept at quite a high value (>3.5 IJm)

while keeping the thickness of the evaporated contacts below 0.3 pm. After sintering, the thick

emitter was thinned down over the uncontacted area using a chemical etch which we call the

PNP etch, based on HNO3, o-H3PO4, and H20 2, especially developed for this purpose(6),

The thermal diffusion process just described is inherently a substantially lower cost

process as compared to the epitaxial process of junction formation and can potentially be used

for large scale batch production of,solar cells. To minimize the surface damage which ordinarily

always occurs during thermal diffusion, we used a 3 to 5 nm thick In(PO3)3-rich oxide cap layer

in our diffusion process. In addition, we further improved the quality of our diffused surface by

doing a chemical treatment with the PNP etchant after sintering the front contacts of the cell.

Simple chemical treatments of InP_surfaces using HNO3 and HF based etchants were found to

decrease the SRV to below 5 x 10b cm/s, e.g. 1.7 x 105 for n+-InP and 4.7 x 105 cm/s for p+-

InP, after rinsing the substrates in a HNO3 (15%) solution(4).

Using the PNP etch, from low frequency EG-V measurements, we recorded a surface

state density minimum (hiss) at the Cd-diffused p+-InP/passivating layer interface as low as 2

-143-
x1010crn-2eV-1(7).About40 nm was removed from the surface of a the p+n InP structure

diffused at 660 °C (surface acceptor concentration:- 4 x 1018 cm'3; junction depth: ~ 3.5 Urn).

The first layer of the designed three-layer AR coating is chemically grown in the process

of thinning the emitter using the PNP etchant. A more detailed description of the composition of

this oxide is published elsewhere(z). Here, we will simply state that the residual chemical oxide

is a two-layered oxide composed of a thick In-rich outer layer and a P-rich layer at the interface

with the emitter. This interfacial oxide is rich in In(PO3) 3 and has a bandgap of 6.8 _'-0.2eV(8).

Because the dissolution rate is quite reproducible, a controlled thickness of oxide can be grown

to serve as the first layer of the designed three-layer AR coating.

The SiO, Sb203, MgF 2 and ZnS layers are all deposited by evaporation techniques.

DESIGN OF THE THREE-LAYER AR COATING

The theoretical design of the three-layer AR coating was conducted using standard

optical theory based on the matrix description of each layer of a multilayer system. The optical

impedance of each layer is assumed known, in order to reduce the number of unknown

parameters in the minimization of the reflectance function, and to maintain the problem within

the realistic boundaries of commonly used optical materials. The MATLAB algorithm developed

to calculate and optimize the parameters of the AR coating is based on work done by Nagendra,

and Thutupalli (1988)(9).

For solar cell applications, the design of a good AR coating, contrary to common

practice, should not be based merely on reducing the light reflectance but rather maximizing the

extemal quantum efficiency or minimizing the loss of short circuit current due to reflection. This

last criterion, expressed in terms of the integrated current loss, defined as the ratio of the

integrated short circuit current of the cell to the ideal zero-reflectance integrated short-circuit

current, allows one to take into account the incident light spectrum, and eventually the presence

of a protective transparent cover glass.

Our experiments have shown that the commonly used double-layer AR coating,

ZnS/MgF 2, deposited on p+n InP solar cell surface leads to a net decrease of the open circuit

voltage (Voc) of the cell by as much as 50 mV due to an increase in SRV(7). For that reason, we

recommend a three-layer AR coating with In(PO3) 3 passivating oxide as the first layer, followed

by either ZnSIMgF2or AI203/MgF2 . The In(PO3)3/AI203/MgF 2 is a new optical system we are

proposing for the p_'n lnP solar cell. AI203 is known for its stability, its high dielectric constant,

and its radiation tolerance (higherthan that of commonly used $iO2). Also, AI20 3 has a closed

packed hexagonal crystal structure, closer to that of In(PO3) 3. We expect AI203 to be superior

to ZnS. MgF2 is a generally preferred outer layer which refractive index of 1.37 gives a good

optical transition to many protective coverglass/adhesive systems(i.e, fused silica/adhesive

1.46/1.43, microsheet/adhesive 1.53/1.43).

RESULTS AND DISCUSSION

Table 1o and Table 2. give optimum designs of three-layer AR coatings using

In(PO3)3/ZnS/MgF2 and In(PO3)3/A!203/MgF 2 for different thicknesses of the first layer

In(PO3) 3 and a AMO spectrum ranging from 300 nm to 950 nm. The symbols ICL and IRL

represent the Integrated Current Loss, and the Integrated Reflection Loss respectively defined as

-144-
ICL= ] _' and IRL=

The results suggest that the In(PO3)3/AI203/MgF2 coating gives a lower ICL than

In(PO3)3/ZnS/MgF 2 mainly because of a lower blue reflectance, as shown in Fig. 1.

Considering the first layer of the AR coating, Fig.2 shows that the two-layered oxide

(-750 A) reduces the surface reflectance of an InP solar cell from an average 40% (curve a) on

a bare surface with no oxide, to about 20% (curve c). In this particular case, after removing the

In-rich outer oxide layer, the reflectance of the remaining thin In(PO3) 3 oxide (300A) is about

25% (curve b). In Fig. 3, it can be seen that a thick two-layered oxide

In203(-900A)/In(PO3)3(-300A ) gives a lower overall reflectance than a single layer of SiO

(800A thick). Its blue reflectance is lower than either SiO, Sb20 3 (750A thick), or a double layer

of ZnS/MgF 2 (not shown here). However, Sb203 has lower mid-spectrum reflectance. Our

experiments show that single layers of SiO, Sb20 3 as well as a double layer of ZnS/MgF 2, even

though they reduce the surface reflection, also tend to increase the surface recombination

velocity on the emitter of a p+n InP solar cell, leading to a decrease of the open circuit voltage

by as much as 50 mV(7). Although the reflectance of the double-layer chemical oxide is lower

than that of SiO, it is still too high for use as the only layer of an efficient AR coating.

Furthermore, the In-rich outer oxide layer is unstable and quite electrically conductive, causing a

noticeable drop in the shunt resistance Rsh and Voc of the cells. It therefore cannot be used, but

should rather be removed so that only the P-rich oxide remains for use as the first of a threelayer AR coating.

Curve d) of Fig. 2 shows the calculated reflectance of a three-layer antireflective coating

composed of In(PO3) 3 (400A), AI20 3 (549A), and MgF2 (712A). It can be seen that the

reflectance is reduced to an average of less than 2% when the three-layer coating is used. As,

shown in table 1, the actual loss of current density due to reflection loss, will be less than 1%,

when one takes into account the spectral response of the InP solar cell.

In Table 3 are presented measured and predicted performance parameters of p+n

(Cd,S) InP solar cells. Rows 1,3 and 4 of the data refer to thick In-rich chemical oxides (~900A to

1500A) over an interfacial P-rich oxide (~300A to 400A). The parameters in rows 2 and 5 were

measured after removing the In-rich oxide from the surface. The increase in Voc and FF are

expected as current leakage occurs through the In-rich oxide outside the mesa etched active

area. The increase in FF is a consequence of an increase in Rsh when the In-rich oxide is

removed. After depositing 850A of SiO over the 400A thick P-rich oxide, the increase in Jsc is

consistent with a decrease in reflectivity. The last row of table 3. gives an experiment-based

projection of the performance of the cell in row 6, using the optimum three-layer AR coating

designed, the grid coverage of a newly acquired photolithography mask (5%), and an

experimentally achievable improved series resistance of about 0.8 Q-cm2. The projected Jsc

and Voc values of 37 mAlcm" -and 894 mV respectively can be obtained based on the

quantum efficiency curve of this cell. A fill factor, and efficiency values of 80% and 19.4%

respectively can be achieved on such a cell. It should be pointed out that these values are

below the experiment-based projections for our state-of-the-art p+n lnP solar cells where in

addition to using the designed 3-layer AR coating, reducin_ the emitter junction depth to less

than 0.3 pm, the specific contact resistance to less than 10-° _-cm, should make a solar cell of

efficiency, -q=21.3% achievable (s),

-145-
CONCLUSIONS

The dual problemof surfacepassivationand antireflectioncoatingdesignof liI-V

compoundbasedsolarcellshavebeenaddressed.We havedesigneda three-layeroptimized

ARcoatingforp+nInPsolarcell,whichbringstheaveragereflectancedownfromabout40%to

lessthan2%whileatthesametimesignificantlyimprovingtheJscandVocbypassivatingthe

topsurfaceof theemitter.Thishasbeenaccomplishedbyusinga chemicallygrownIn(PO3)3-

richpassivatingoxidelayerasthefirstlayerof a 3-layerAR coating, with AI20 3 and MgF2 as

the second and third layers. We believe that the significantfront surface passivation is to a large

extent responsible for our achieving the record high efficiency Voc of over 890 mV. This concept

of using a passivating chemically grown oxide as the first layer of a multilayer AR coating can be

beneficial to other III-V compound solar cells as well.

(1).

2).

(3).

4,)°

(5).

(6).

7),

(8).

(9).

REFERENCES

M.B. Spitzer, B. Dingle, J. Dingle, and R. Mordson, "A review of Indium Phosphide

Space Solar Cell Fabrication Technology*, Proceedings of the 21st IEEE Photovoltaic

Specialists Conference, 1990, pp. 196-206.

J.F. Wager and C.W. Wilmsen, Physics and Chemistry of IIl-V compound

Semiconductor Interfaces, edited by Carl W. Wilmsen, Plenum Press, 1985, pp. 165-

211.

M.Faur, M.Faur, PoJenkins, M.Goradia, S.Bailey, D.Jayne, I.Weinberg and C.Goradia,

"Study of Surface Passivation of InP," Surface and Interface Analysis, 15, 745 (1990)

P.Jenkins, M.Goradia, M.Faur, S.Bailey and MFaur, "Measurement of Surface

Recombination Velocity on Heavily Doped Indium Phosphide," Proc. 21st IEEE PVSC,

1990, p.399.

M. Faur, M. Faur, D.J. Flood, C. Goradia, D.J. Brinker, M. Goradia, J. Moulot, S. Bailey,

N.S. Fatemi, P.P. Jenkins, and D. M. Wilt, "Status of Diffused Junction p+n InP Solar

Cells for Space Power Applications", First World Conference on Photovoltaic Energy

Conversion, December 5-9, 1994, Hawai.

M.Faur, M.Faur, D.T.Jayne, S.Bailey and M.Goradia, "Etchant for Chemical Thinning of

InP and itsApplications in the Fabrication of InP Diffused Junction Solar Cells," Surface

and Interface Analysis, 21, 110 (1994)

M.Faur, M.Faur, D.J.Flood, O.J.Brinker, C.Goradia, S.Bailey, I.Weinberg, M.Goradia,

D.T.Jayne, J.Moulot and N.Fatemi, "Effective First Layer Antireflecting Coating on InP

Solar Cells Grown by Chemical Oxidation," Proc. 6th Int'l IPRM Conf., 1994, p. 492.

J.Joseph, Y.Robach, G.Hollinger, P.Ferret and M.Pitval, "The Passivation of InP by

In(PO3) 3 for MISFET Applications,* Proc. 1st Int'l Conference on Indium Phosphide and

Related Materials IPRM, 1989, p.268.

C.L. Nagendra and G.K.M. Thutupalli, "Design of Three-layer Antireflection Coatings: A

Generalized Approach", Applied Optics, vol. 27, no.11, 1 June 1988, pp. 2320-2332

-146-
Table 1: Summary of 3-layer ARC design with In(PO3)3/AI203/MgF 2

dln(PO3)3 dAl203 dMgF2 Jsc (AMO) ICL IRL

(A) (A) (A) mA/cm 2 (%) (%)

50 647 490 43.99 1.62 2.12

100 602 510 44.00 1.60 2.09

150 534 558 44.03 1.54 2.01

200 460 616 44.06 1.46 1.85

250 420 641 44.11 1.37 1.73

300 422 635 44.17 1.23 1.54

350 425 624 44.24 1.06 1.38

400 549 712 44.43 0.64 1.03

450 425 Ir'6'561" 44.34 0.83 1.81

500 500 465 44.06 1.48 2.79

Table 2: Summary of 3-layer ARC design with In(PO3)3/ZnS/MgF 2

dln(PO3)3

(A)

50

dZnS

(A)

466

415

dMgF2

(A)

899

ICL

(%)

0.95

Jsc (AMO) IRL

mA/cm 2

44.29

44.28

44.27

43.08

43.75

44.3

43,68

43.78

43.69

43.64

(%)

1.30

100 898 0.97 1.36

150 363 894 0.99 1.40

200 417 767 3.66 6.13

250 197 825 2.15 2.59

300 239 918 0.93 1.79

350 291 773 1.88 1.89

400 240 760 2.10 2.06

450 178 753 2.29 2.22

500 114 752 2.41 2.33

-147-
Table 3: AM0, 25Oc performances of selected p+n InP diffused solar cells measured at NASA

LeRC.

AR Coat;.g Approx. Emitter Rs Jsc Vo¢ RF TI

Thickness

(_) (Q-cm2) (mNcm2) (mv) (%) (%)

In203 (.1500 A)/In(PO3) 3 (-400A) 0.45 3.52 26.3 887.6 69 11.98

In(PO3) 3 (_400A) 0.45 3.24 27.5 884.6 73.7 12.95

In203 (-900A)/In(PO3) 3 (-300A) 0.4 3.35 28.2 881.7 72.6 13.2

In203 (-1100 A)/In(PO3) 3 (.430A) 29.4 877.2 61.7 11.63

In(PO3) 3 (.400A) 0.3 4.38 27.6 886.6 62.8 11_5

SiO(-850A)/In (PO3)3(-400A) 30.95 887.5 61.5 12.35

Optimized In(PO_) 3 (400A)I 0.3 0.8 37.1 8.94.2 80.1 19.43

AI203(549A)/MgP:_ (712A)

Ceil area: 0.48 cm"; front coverage: 9.55%; distance between the 0.31Jm thick fingers: 620pro * E_ _edment-based projected

parameters for the cell in row 6.

Three-layer AR Coating Design

with In(PO3)3 oxide as first layer

20 "t " " " " i , , • , ! • , , . | , , . . i • • • , i • • • • i

16

l: In(PO3)3-ZnS-MgF')

"_4) _ 8 I,__ ab. ._ . . . . b: InlPO313--AI203--MgF2

4

or. .

300 400 " 500 600 700 800 900

Wavelength (nm)

Fig. 1. Reflectance on optimized Three-layer AR coating using: a)In(PO3)3/ZnS/MgF 2 b)

In(PO3) 3/AI203/Mg F2

-148-
¢:

60

48

36

m

tJ

(I) 24

1ira

Q

n12

Reflectance on InP Surface

......... • . • • i i •

C

b

d

0

300 400 500 600 700 800 900

Wavelength (nm)

Fig. 2: Surface reflectance vs. wavelength of a) a bare p+n InP surface (no oxide), no grid

fingers; p+n InP solar cells with: b) 10.5% grid coverage, 400 A thick surface oxide layer; c)

10.5% grid coverage and 750 A surface oxide; d) a 3-layer AR coating, In(PO3)3/AI203/MgF2.

Surface Reflectance of InP Solar Cell

w_th one layer ARC

::'........4

0 28

14

0

300 400 500 600 700 800 900

Wavelength (nm)

Fig. 3: Surface reflectance vs. wavelength of p+n InP solar cells usingsingle layer AR coating of

the passivating chemical oxide (In203/In(PO3)3 -1100/400A); SiO (800A); and Sb20 3 (750A).

-149-
LIGHTWEIGHT, LIGHT-TRAPPED, THIN GaAs SOLAR CELL FOR SPACECRAFT APPLICATIONS:

PROGRESS AND RESULTS UPDATE I

M.H. Hannon, MW. Dashiell, L.C. DiNetta, and A.M. Barnett

AstroPower, Inc.

Newark, DE 19716-2000

ABSTRACT

Progress is reported with respect to the development of ultra-lightweight, high performance, thin, light

trapped GaAs solar cells for advanced space power systems. Conversion efficiencies of 17.9% (AM0, lX) have

been demonstrated for a 3 I_mthick, 1 cm2solar cell. This results in a specific power of over 1020 W/kg (with a 3-

mil cover glass) and a power density of 240 W/m 2. Device parameters were 1.015 volts open circuit voltage, 80%

fill factor, and a short-circuit current density of 29.54 mA/cm 2. In addition to silicone bonding, the use of

electrostatic bonding to attach the cover glass support to the front surface enables an ultra-thin, all back contact

design that survives processing temperatures greater than 750°C. This also results in a 10% reduction of the cell

weight for a potential specific power of 1270 W/kg. All back contact, ultra-thin, electrostatically bonded GaAs solar

cell prototypes have been fabricated demonstrating an open circuit voltage of 1 volt for a cell base thickness of 1

_m with a 0.5 p.m emitter. This technology will result in a revolutionary improvement in survivability, performance,

and manufacturability of lightweight GaAs solar cell products for future Earth-orbiting science and space

exploration missions. The thin, electrostatically bonded, all back contact GaAs device technology has multiple

uses for specialty high performance solar cells and other optoelectronic devices.

INTRODUCTION

A schematic cross-sectional representation of the silicone bonded AstroPower prototype thin GaAs solar

cell design is shown in Figure 1. This device is supported by a 3-mil cover glass which has been attached to the

front surface with a 1-mil silicone adhesive.

cover glass

F_um 1.

p-type contact

-- ,W_ ... bonding agent

4

GaAs cap layer -I-_. I I I 1_, dielectric

AIGaAs window layers _'J"_. n re'L---:-, GaAs emitter, 0.5p.m

_'. GaAs base, 1.0

= = H • m---

_. back surface

diele_dc •reflectodn-_pe contact

Ultra-thin GaAs solar cell with light trapping.

1This research was supported in part by the Department of the Air Force and managed by Phillips Laboratory,

Space Power and Thermal Management Division under SBIR contract #F29601-93-C-0188.

-150-
The improved device design utilizes electrostatic bonding to attach the cover glass support to the front

surface, enabling an ultra-thin, all back contact design that eliminates grid shading. The electrostatically bonded,

ultra-thin structure survives process temperatures exceeding 750°C. The conceptual design of this unique solar

cell is shown in Figure 2. The p-type region is diffused from the back of the device to the emitter.

diffusionfront _

(<1% of tota.area)_--_,_ Glass superstrate ! _

junction t, L._ _ _ _'__ _ _ _G_s ..... k_-_!

r - _: 1" \ I

n-typecontactlines or dots n-busbar/reflector p-busbar/reflector

Figure 2. Electrostatically bonded, all back contact, ultra-thin GaAs solar cell.

The benefits of this device technology include the following:

• specific power improvements over state of the art GaAs/Ge devices

• high radiation resistance and lower on-orbit operating temperature

• all back contact design which simplifies electrostatic bonding and eliminates grid shading

• array tabbing does not require wraparound interconnections

• enables cost-effective manufacturing, eliminates adhesive degradation, and provides high

structural integrity

• transferable to any epitaxial growth technology and various solar cell materials and designs

including tandem solar cells and high voltage concentrator cells

• applicable to integrated logic components, LEDs, LED displays, flat screen display drivers,

waveguides, and microwave devices

High Performance Benefits

The ultra-thin, lightweight, light-trapped GaAs solar cell design offers a high specific power in comparison

to silicon and GaAs/Ge devices, which is important for space applications (ref. 1). Light trapping increases the

effective optical path length with the use of a reflector. The benefits of light trapping in GaAs can be realized by

increased optical absorption, collection efficiency and photon recycling (ref. 2). These features lead to increased

open circuit voltages and short circuit currents (ref. 3).

Radiation damage is the primary degradation mechanism for GaAs solar cells deployed in space. The

ultra-thin, light-trapped GaAs solar cell will have significantly increased EOL efficiencies compared with

conventional solar cell structures because of the thin device layers associated with the structure. This design will

be less sensitive to changes in bulk diffusion length due to the increased optical path length and decreased

recombination volume.

Thermal stability and tolerance to UV degradation are inherent to the thin device structure and

electrostatically bonded 3-mil glass superstrate. There is neither a darkening effect such as that which occurs with

adhesives after extended exposure to UV light, nor degradation of the bond interface. The maximum power to

-151-
weight ratios can be attained since no additional material is used to form the bond and the electrostatic bond will

not suffer from degradation upon exposure to high temperatures.

The all back contact technology enables tabbing to the p-type and n-type regions of the device to be easily

accomplished from the back of the structure. Placement of the grid pattern for both the n- and p-type contacts on

the back of the solar cell eliminates grid shading losses for light entering the front of the device. In contrast to

other coplanar contact designs, this technology eliminates the need for micro-machining the solar cell

The high performance benefits of AstroPower's ultra-lightweight, thin, light trapped GaAs solar cells

enable the devices to meet the technology demands for solar cells with increased performance, as required for the

space cell industry (ref. 4).

RESULTS AND DISCUSSION

The highest efficiency obtained for an ultra-thin, adhesive bonded, LPE grown device achieved to date at

AstroPower is 17.9%. The results of the current-voltage and quantum efficiency measurements are shown in

Figures 3 and 4. As can be seen, the open circuit voltage and fill factor are quite high. The quantum efficiency

measurement indicates some losses in blue response which can be improved with optimization of the emitter and

window layer.

Table I lists the weight contribution of the major material components for this solar cell. Reducing the

device thickness to 2 microns, with a 2 micron-thick GaAs contact layer, would reduce the GaAs contribution to

5% of the total cell weight. Also, with the electrostatically bonded, all back contact device, the weight of the

adhesive, which is approximately 10% of the total cell weight, would be eliminated. This weight reduction will lead

to the highest possible power densities (greater than 1270 W/kg) for these ultra-thin solar cells.

Figure 3.

Voc 1.015V

Jsc 29.54 mNcm 2

Fill Factor 80.3%

Area 1 cm2

Thickness 3 _m

Specific Power 1020 W/kg

AM0, lX efficiency 17.9%

........................................ i ..........

.............. _.... _.............. .:..............

I

i

, • : . .y

-.... ':................ i

i Horz: 0.2 v/div vert: 5.0 mA/div

Current voltage measurement for ultra-thin device G13901A.

-152-
Figure 4.

Table I.

Z

u.

I,I.

UJ

3

z

o

x

W

100 -

90-

80

70

60

50

4O

30

20

10

0

350 450 550 650 750 850 950

WAVELENGTH (rim)

External quantum efficiency measurement for ultra-thin device G13901A.

Weight contribution of the major solar ceil components for the u/tra-thin device (G13901A).

Material

Pilkington CMG glass (3-mil)

Sylgard Silicone Adhesive (1-mil)

Gallium Arsenide

Density

2.554g/cm 3

0.gg/cm3

5.32g/cm 3

Total Weight/cm 2

19.46mg

2.29mg

1.85mg

Percentage of Total Cell

Weight

82%

10%

8%

Typical values for the dark diode reverse saturation current densities for the GaAs solar cells are

3x1019Ncm 2and 5x10lIA/cm 2for the diffusion and depletion region recombination components respectively.

These current densities provide an indication of the junction quality, minority carder lifetime, and surface

passivation for the device. The dark diode current values obtained at AstroPower are among the best reported by

a number of researchers for high efficiency GaAs solar cells (refs. 5, 6, and 7), further demonstrating the value of

near equilibrium growth processes.

Light-trapping has been demonstrate_l on the ultra-thin devices. The external quantum efficiency curve

illustrated in Figure 5 shows an increase in long wavelength response (between 650 and 870 nm) of the thinned

solar cell with a back surface reflector, compared to the same device before the thinning procedure (on the GaAs

substrate). The external quantum efficiency of this device was increased by 5.2% at 850 nm with the incorporation

of a back surface reflector. The gain in short circuit current density for this solar cell is approximately 0.7 mA/cm 2.

This gain is expected to increase as the active device thickness is decreased to less than 2 microns. The blue

response of this device was low due to a non-optimized AIGaAs front surface passivation layer.

-153-
9O

>-

o

z

iiJ

tL

1,

W

Iz

g

o

z

lg

w

I,-

85

80

75

7o

65

"'%°

%

,°

...... on GaAs substrate

---thinned, w_th reflector

60 ! , i

650 700 750 800

WAVELENGTH (nm)

85O

F_um 5. Extemal quantum efficiency of sample G13405B.

Photographs of the ultra-thin, lighttrapped device are shown in Figure 6. The front surface is shown in

Figure 6a and the back surface including the n-type contacts and silver reflector is shown in Figure 6b.

4

(a) (b)

Figure6. Photographofthefront(a)and back (b)surfaceoftheultra-thin, lightrappedGaAs solarcell.

Fabricationoflargearea (8crn2)devicesisunderway. A photographofan 8 cm 2GaAs solarcell

fabricatedon theGaAs substrateisshown inFigure7. Similardevicesarebeingprocessedas ultra-thin, light

trappedsolarcells.The resultsofsixI cm 2devicesprocessedfromone 2x4 crn2 LPE growthareshown in

TableII.The performanceofthesedevicesdemonstratesthecapabilityofthematerialtosupportlargearea

devices.

-154-
_ :_ /Z__:i __:ZI) -,-_--___ _r__! .....

Figure 7.

Table II.

Photograph of a large area (8cm 2)GaAs solar cell.

Current-voltage characteristics of six 1 cm2devices fabricated from one large area (8cm 2) LPE

growth.

Cell

Number

G13901A

G13901B

G13901C

G13901D

G13901E

G13901F

Median

Voc

(v)

I 1.02o

1.015

1.007

1.008

1.011

0.987

.01o

Jsc

(mA/cm 2)

29.0

30.0

29.1

30.2

31.0

30.8

30.1

Fill Factor

(%)

78.9

79.3

• 78.3

73.8

78.4

72.5

76.4

Efficiency (AM0, lX, 25°C)

on GaAs substrate (%)

17.29

17.89

16.98

16.64

18.20

16.33

17.14

Efficiency (AM0, lX, 250C)

of Ultra-thin Device (%)

17.98

17.56

16.98

13.94

17.30

15.17

17.14

For the all back contact, electrostatically bonded, ultra-thin GaAs solar cells, the p-type emitter is extended

to the back of the solar cell by a selective diffusion. The surface area of the diffusion front is less than 1% of the

total area of the Liltra-thin solar cell. Zinc diffusion profiles were determined by electrochemical CV profiles at

BioRad Semiconductor in Mountain View, California. The electrochemical CV profiles for two zinc diffusions into

n-type GaAs substrates (Si: 0.89-3.92x10_S/cm 3)are shown in Figures 8. Figure 8a shows the results of a 2 hour

zinc diffuison at 700°C. The p-region extends at least 1.5 microns into the GaAs substrate and has a high

conductivity. The results of a 2 hour zinc diffusion at 750°C are shown in Figure 8b. the p-region extends at least

3 microns into the GaAs substrate and has a high conductivity (50(_-cm)'_). These measurements indicate that

the resistance of the zinc diffused regioos is minimaland the Width of the back contact fingers can be reduced to

less than 25 microns without hindering the performance of the solar cell.

-155-
Ip2

20_ ° p'

ll ,_ +_-,u._.._ _

17

i

0 0,5 1 +0 .w15 ;o 21s 3'.o 31s 41o

20 n • _

18

15

,.to , _ , + , _ , _ . + , J.

F_u_ 8. Electrochemical CV profiles for zinc diffusion into a GaAs substrate a) 2 hour zinc diffusion at

700_C and b) 2 hour zinc diffusion at 750°C.

In order to achieve a high efficiency, ultra-thin, all back contact solar cell, a high temperature glass

formulation that is CTE matched to GaAs and has a high softening point has been developed. This glass has a

softening point of 890°C, and a CTE of 6.0xl0SlK. The annealing point of this glass is approximately 650°C.

Void-free, 6 cm 2 bonds to LPE GaAs layers on GaAs substrates have been obtained with this high temperature

glass.

GaAs solar cell structures electrostatically bonded to this glass survive the substrate removal procedure

and subsequent processing steps. Ultra-thin (less than 5 microns) GaAs/glass laminates have been heat cycled

to 750°C for two hours and cooled in liquid nitrogen with no degradation of the bond interface. Electrostatic

bonding to this high temperature glass formulation enables high temperature device processing to occur after

coverslide bonding. Future plans include space qualifying this glass with the appropriate testing laboratory and

continuing to work with the glass manufacturer to ensure space survivability of the glass superstrate.

Prototype all back contact devices are presently being processed. To date, open circuit voltages of 1 volt

have been demonstrated for a cell base thickness of 1.0 micron with a 0.5 micron emitter. In addition to

completing 16 and 25 cm 2 all back contact solar cells on LPE material, this technology will be demonstrated on

MOCVD material over the next few months.

CONCLUSIONS

High performance, lightweight, thin, light trapped GaAs solar cells have been demonstrated. Conversion

efficiencies of over 17.9% (AM0, lX) have been demonstrated resulting in a specific power of 1020 W/kg (with a

3-mil cover glass) and a power density of 240 W/m 2. The incorporation of light trapping has increased the external

quantum efficiency of these solar cells in theJong wavelength range. Large area, electrostatically bonded, ultrathin GaAs solar cell structures have demonstrated survivability to 750°C, with no degradation of the bond

interface. Prototype all back contact devices with open circuit voltages of 1 volt have been fabricated. Future

plans include completing 4 cm 2 all back contact, electrostatically bonded, thin, light trapped GaAs solar cells on

both LPE and MOCVD material for a potential specific power of 1270 W/kg

The success of this program can lead to the deployment of high performance, thin GaAs solar cells in the

space environment. AstroPower's solar cell design can have a significant impact on the longevity and power

generation capabilities of space power supplies. The fabrication technology has multiple uses for specialty high

performance solar cells and other optoelectronic devices.

-156-
REFERENCES

1. Kelly Gaffney_ "Air Force Activities in Space Photovoltaic Power System Technology", Proc. NASA Space

PhotovoltaicRe_ee_ch and Technology Conference. Cleveland. Ohio (1994).

2. G. Lush and M Lundstrom, =Thin film approaches for high efficiency III-V cells",Solar Cells. 30 (1991).

3. C.B. Honsberg and A.M. Barnett, "LightTrapping in Thin Film GaAs Solar Cells",

Photov_oJtaicSpeCialists Conference, Las Vegas, Nevada, (1991).

4. P.A. lies and F. Ho, =Technology Challenges for Space Solar Cells", proc. 24th IEEE Photovoltaic Specialists

Conference, (1994).

5. L.C. Olsen, G. Dunham, F.W. Addis, D. Huber, and D. Daling, =Electro-optical Characterization of GaAs Solar

Cells, Proc. 19th IEEE Photovol_aicSpecialists Conference. (1987).

6. P.D. DeMoulin, C.S. Kyuno, M.S. Lundstrom, and M.R. Melloch, =Dark IV Characterization of GaAs pin

Heteroface Cells, Proc. 19th IEEE Photovoltaic Specialists Conference. (1987).

7. S.A. Ringel, A. Rohatgi, and S.P, Tobin, "An Approach Toward 25% Efficient GaAs Heteroface Solar Cells,

IEEE Transactions on Electron Devices. vol. 36, No. 7, July 1989.

-157-
LABORATORY INSTRUMENTATION AND TECHNIQUES FOR CHARACTERIZING

st

MULT-JUNCTION SOLAR CELLS FOR SPACE APPUCATIONS

James R. Woodyard

Wayne State University

Detroit, Michigan48202

ABSTRACT

An integratedsystemisdescribedwhichconsistsof a spectral radiometer and dual-source solar simulator,and

personal computer based current-voltage and quantum efficiency equipment. The spectral radiometer is calibrated

usinga tungsten-halogenstandard lamp witha calibration based on NIST scales. The quantum emciency apparatus

includes a photodiode calibrated using NiST scales and a monochromatic probe beam. The apparatus is used to

measure the dependence of the absolute external quantum efficiencyof solar cells at various forward-bias voltages

includingthe ma)dmum-power pointunderAM0 light bias. Quantum effciendes of multi-junctionceils were measured

with both spectral-light bias and AM0 light bias. Measured speckal irradiances of the dual-source simulator were

convolutedwithcellquantumefficienciesto calculatecell currents as func_on of voltage. The calculated currents agree

with measured currents at the 1% level.

INTRODUCTION

Mul_junclJonsolar cellsare atlx-addvefor space applicationsbecause they can be designed to convert a larger

fractionof AM0 into electrical powerthan single-junction cells. The performance of mulS-junctionceils is much more

sensitive to the spectral irradiance of the illuminating source than single-junc'don cells. The design of high efficiency

multi-junction cells for space applications requires matching the optoelectronicproperties of the jun_ons to AM0

spectralirradiance. Unlike single-jun_on cells, it is not possibleto determine the quantum effciency of multi-junction

cells usingonly a monochromatic probe beam. It is necessary to use a lightbias because of the series nature of the

cell structure. Burdick and Glaffelter (1) reported a measurement technique for mulU-junctJon cells which employs a

spectrallight-biastechnique. While the method is usefulfor understandingand improving mul'd-junclionsolar cells, it

does notyield the absolutequantumelTciencyunderAM0 light-bias. Because of the non-linear nature of multi-junction

cells, current-voltage characteristicsunder AM0 conditionscannot be calculated from measurements under non-AM0

conditionsusing spectral-correc'donmethods.

The goal of the research is to develop a solar simulator which appro]dmatesAMO spectral inradiance,and

laboraton]insbumentationand techniques,for use in measuring the quantum effciency and I-V characteristiceof multijunction solar cells under AM0 power-generating conditions. An integrated system is described which consists of a

spectral radiometer and dual-source solar simulator, and personal computer based current-voltage and quantum

efficiency equipment. The spectral radiometer is calibrated using a tungsten-halogen standard lamp which has a

calibrationtraceable to NationalInstituteof Science and Technology (NIST) scales. The calibrated spectral radiometer

is used to measure the spectral irradiance of the light-biasbeam and obtainan integral fit of it in two spectral regions

to the World RadiationLaboratory(WRL) AM0 spectralirradiancedata. The solar simulator produces a light-biasbeam

which is used for current-voltage and external absolute quantum effdency measurements. The quantum efficiency

apparatus includes a photodiodecalibrated usingNIST scales and a monochromatic probe beam, The apparatus is

usedto measure the absoluteextemal quantum efficiencyof triple-junctionsolar cells at vadous forward-bias voltages

under both spectral-light and AM0 light-biasconditions.

SOLAR SIMULATOR DEVELOPMENT

The spectral irradianceof the solar simulator plays an importantrole inthe characterizal_onof multi-junction

solar cells. Characterization techniques such as lightI-V and light-biasedquantum efficiency measurements require

-158-
matching the solar simulator spectral irradiance as close as possible to the AM0 spectrum in order to predict the

performance of cells in a space environment with AM0 illuminance.

The spectral irradiance of the solar simulator used in this work was measured with a spectral radiometer

constructed and calibrated in our laboratory. The spectral radiometer employs an integrating sphere, order-sorting

filters, single-stage monochromatorand detector;the detectoris a silicon photodiodeand temperature stabilized. The

instrument is computer interfaced for control purposes, and data acquisition,display and analyses. The spectral

radiometerwas calibrated with a type FEL 1000 W quartz tungsten-halogen standard lamp traceable to NIST scales;

calibrations were carded out using the procedure specified in NBS Special Publication 250-20. The accuracy of

measurementsinthe 350 to 900 nm range is believedto be better than 4%; the precision of consecutivespectral scans

is better than 1%.

A modelSS1000 solarsimulatormanufacturedby Optical Radiation Corporation (ORC) was used in this work.

The lightsource inthe simulatoris a xenon high-pressuredischarge lamp. The speckal irradiance of the solar simulator, as delivered by ORC, is shown in

Figure 1 by the filled circles. The

spectral irradiance of the WRL AM0

solar spectrum is shown for comparison purposesin the figure by the solid

line. The spectral irradiance of the

simulator differsfrom AM0 in a major

way in the 300 to 370 nm and 650 to

1000 nm wavelength ranges. Above

800 nm the spectral irradiance of the

solar simulatoris dominated by xenon

lines. The differences in spectral

irradiancesofthe ORC solar simulator

when compared to AM0, while generally not important in characterizing

single-junction solar cells, can introduce major errors in light I-V and

quantum efficiencymeasurements of

multi-junctioncells.

The solarsimulatorwas modified to produce a spectral irradiance

in closer agreement with the spectral

irradianceof the AM0 solar spectrum

500 -- •

450

,_ 400 --

350

300-

Spectral Irradlance of Single-Source Solar

Simulator Col AM0

WRL _M0

• Single -Sour,:e

E 250

I

0 " =

300 400 B00 600 T00 000 900 t000 1100

Wavelength (nm)

Figure 1: Measured spectral irradiance of single-source solar simulator

compared withWorld Radiation LaboratoryAM0 spectral irradiance.

using a design reported by Bennett

and Podlesny(2). In order to obtain a better match withthe AM0 spectral irradiance at wavelengths above 700 nm, a

600 W tungsten-halogen lamp and cold mirrorwere added to the solar simulator. A diagram illustratingthe optics of

the modifiedsimulatoris show in Figure

2. The design includes two elliptical

mirrors and a fiat cold mirror to focus

illuminationfrom the tungsten-halogen

and xenonlamps on an opticalintegratoe the design produces a horizontal

lightbeam which is compatible with the

horizontalopticsofthe spectral radiometer and quantumefficiency apparatus.

The cold mirror was custom fabricated

to Vansrn_ wavelengths greater than

750 nm and reflect shorter wavelengths. The originalfolding mirror in

the simulator was replaced with the

cold mirror;it is mounted at forty-five

degrees withrespect to both the xenon

and tungsten-halogenlamps. The cold

mirror serves two purposes. It transCold Mirror

" J

Tungsten- Halogen enon Lamp

Lamp

Figure 2: Opticsof Dual-Source Simulator

-159-
milsthexenonspectrumatwavelengthsgreaterthan750nmandreflects wavelengths less than 750 nm. The effect

ofthe mirroron the speclralirradBnceof the xenon lamp isto attenuate the xenon lines shown in Figure 1. The mirror

transmitsthe spectralirradiance of the tungsten-halogen lamp withwavelengths greater than 750 nm while reflecting

wavelengths less than 750 nm.

The oplicsof the solar simulator,as deliveredbyORC, incorporateda second folding mirror,Vicor beam splitter

and Pyrex collimating lens. The Vicor beam splitterwas used to produce an opticalsignal for the feed-back circuit

whichstabilizesthe xenon lamp current. The folding mirror and collimalJnglens produce a ve_cal beam focussed on

a horizontalwork surface. The folding mirror and lens were removed to produce a hortzontal light beam. The Vicor

beam splitterwas replaced with a high-qualityquartz beam splitter. The modificationin the optics also increased the

UVthroughput ofthe simulator.AddiUonalair cooling capacitywas added inorder to dissipatethe power produced by

the tungsten-halogenlamp. The electronics supplied by ORC withthe simulator were used to power and control the

xenon lamp. A D.C. power supply regulated at the 0.01% level was added to the system for powering the tungstenhalogen lamp. The stabilityof the dual-source lightbeam intensityis atthe 0.1% level.

The spectralirradianceofthe dual-sourcesolarsimulatorwas fit to the WRL AM0 spectrum by integratingand

cempadng the two spectral irradiances in two regions. The simulator and WRL spectral irradianceswere integrated

intwo regionsand compared. One region employed wavelength limitsof 350 and 750 nm; the other region had limits

of 750 and 900 nm. The limitsof 350 and 900 nm were selected because the quantum efficiencyof the triple-junction

solarcells investigatedinthis workis negligibleout_e this wavelengthrange. The wavelength limitof 750 was selected

because it is the bandpass of the cold mirror. Seleddon of the two regions inthis manner made it possible to obtain

integrated spectral irradiance fits in each region almost independently of each other by adjusting the current inthe

corresponding lamp; most of the spectral irradiance in the 350-750 nm range is due to the xenon lamp while the

tungsten-halogen lamp produces most of the spectral irradiance inthe 750-900 nm range.

The fitofthe solarsimulator spectral irradianceto the WRL AM0 spectrum was carried out using a procedure

whichincludedcalibratingthe spectral radiometer; adjustingthe currentsin the xenon and tungsten lamps; measuring

the simulator spectral irradiance; integratingthe measured and WRL AM0

spectral irradiances inthe 350 to 750

nm and750 to 900 nm ranges; calculalJngthe percentage error in the integrated measured and WRL AM0

spectral irradiances in the two wavelength regions; and repeating the

process untilthe error in each of regionswasless than 1%. The spectral

irradiance of the dual-source simulator compared to the WRL AM0 spectrum is shown in Figure 3 by the filled

circles. Comparison of the spectral

irradiances in Figures 1 and 3 shows

the dual-source solar simulator producasa speclralirradiance which is in

significantJybetter agreement withthe

WRLAM0 speclrum. The percentage

difference between the integrated

spec_'alirradianceof dual-source simulatorcompared to WRL AM0 in F'_-

ure 3 is +0.06% inthe 350 to 750 nm

wavelength range and -0.9% in the

750 to 900 nm range.

30O

250

160

100

so

0

Spectral Irradlance of Dual-Source Solar

81mulator Compared to WRL AMO

_RL A 140

: 0 n,m *0.06'

_00 400 EO0 SO0 700 800 900 1000 1100

Wavelength (nm)

Figure 3: Spectral Irradiance of dual-source solar simulator compared

withWorld Radiation Laboratory AM0 spectral irradiance.

QUANTUM EFFICIENCY SYSTEM DESCRIPTION

The quantumefficiency(QE) systemwas designed and constructedto carry out measurements under

three cond_ons: dark, spectral-light bias and AM0 lightbias. A second personal computer is used to control the QE

system,as well as data acquisition,displayand analyses. The design permits quantum efficiency measurements with

an accuracy better than 2% over the 400 to 1000 nm wavelength range. The probe beam intensityand calibrated

-160-
detectorresponseresultinredudionsinaccuracyouts_ethiswaveleng_range.Themajorcomponentsofthe system

includetwo cornputerqnterfacedScientificMeasurement System, Inc. MonoSpec 27 monochromators; two computerinterfacedmotorizedfilter wheelswithfilters for order sorlJngand spectral-light bias;UV-grade fused silica lenses and

beam splitter; two magnesium fluoride coated AI mirrors; computer interfaced Stanford Research Systems model

SR830 DSP lock-inamplifier and chopper; 60 watt quartz-tungsten-halogen (QTH) lamp; and a computer-interfaced

Hewlett-Packard model 6038A power supplyfor the QTH lamp.

The two monochromators were mounted in tandem and used with the QTH lamp to produce a

monochromaticprobebeam for QE measurements of solar cells. Two monochromators are employed to reduce the

shay-lightlevel in the probe beam. Each monochromator has a ruled 1200 grooves/mm grating blazed at 500. The

subtractk'emode is usedfor the physical configurationof the monochromators along with 4 mm slitwidths to optimize

lightthroughput. The measured resolulJonof the two monochromators is 20 nm. The monochromators are capable

of higher resolution at the expense of probe beam intensity. The monochromator containing the entrance slit and

locatednextto the input opticsis referredto as monochromator#1 inthe following discussion;the other monochromator

containsthe exit slitand is referred to as monochromator #2.

OplJcalcomponents located in front of the entrance silt of monochromator #1 serve to focus an A.C.

lightbeam onthe entrance slit and provide order-so_ng capability.The optics include a QTH lamp, collimating lens,

beam chopper and filter wheel. The beam chopper and filter wheel are mounted nextto the entrance slit of the first

monochromator.The beam chopperis used in conjunctionwiththe lock-inamplifier to detect the response of the test

solar cellto the monochromatic A.C. probe beam. The filter wheel containsfour long pass filters. The filters provide

order-soRtingofthe light-beams passingthrough the monochromators. The filters serve to reject the nth order beams

of wavelength _/n inthe QE probe; the higher order beams can introducelarge errors in QE measurements.

The optical components at the e_t slit of monochromator #2 produce two light beams from the A.C.

monochromaticprobebeam which passesthrough the exitslit. The beams are used to measure the absolute extemal

QE of a test solar cell. The configura-

_on of the opticalcomponents is shown

inFigure 4. The components include a

beam splitter, two mirrors, lens, calibrated silicon photodiode, test-cell

holder and filter wheel, and miscellaneous optical rails and holders. The

components are enclosed in a black

light-tight aluminum box. The monochromatic A.C. probe beam emanating

from the e_ slitof monochromator #2

issplitintotwo separate lightbeams by

the beam splitter. The lightbeam reflected from the beam splitter is focussed on the calibrated detector by

the lens; the detector is a calibrated

silicon photodiode and serves as the

referencedetectorfor the QE measurements. The photodiode calibration is

traceable to NIST scales and permits

determining the absolute number of

photons incident on the test cell. The

second lightbeam transmits the beam

splitter and isincidenton the fiat mirror;

it is reflected onto the concave mirror

which focusses the beam on the test

cell. The opfics do not permit meaLight-TightBox

Rail

/ Cooling Air Exhaust Port

Filter Wheel

Test-Cell Holder

D.C. Ught Beam\

Detector

Calibrated

Flat

Mirro

Lens

ExitSlit

Beam Splitter

Cooling Air

Inlet Port A.C. Monochromatic

Probe Beam

Concave Mirror

Monochromator #2

Solar Simulator LightBeam

Figure 4: Top view of opticalcomponents located at the eMitslitof

monochromator #2.

surement of the reflected lightfrom the test cell. The QE values measured by the system and reported in thispaper

are absolute external quantum efficiencies.

A D.C. light beam originatingfrom the solar simulator and passingthr0ugh thefilter wheel shown in Figure 4

is incident on the test cell; it is coincidentwith the optical axis defined by the test cell and filter wheel. The spectral

content of the lightbeam is determined by the filterwheel. Three positions on the filter wheel are used to hold filters

which pass speclrafor measuring QE of triple-junctioncells under spectral-light bias conditions(1); each filter passes

-161-
a porlJonof the simulator spectrum for "turning-on"two of the three junctions of a test cell. One positionon the filter

wheel is open and usedforAM0 light-biasmeasurements; another position(:0ntains a black aluminum disk and is used

for measuring QE under dark conditions.

Temperature control of solar cells during light-bias measurements is accomplished by directing room

temperatureair on the back of the cell mounted inthe test-cell holder. Figure 4 showsthe cooling air inlet portwhich

ismountedoppositethe test-cellholder. Airis exhaustedthroughthe cooling air exhaust port which is baffled to prevent

ambient light from entering the light-tightbox. The air flow is set to insurethe active junction of the test solar cell is

maintained at room temperature; the junctiontemperature is monitored by measuring the open circuitvoltage of the

test cell. Temperature probes are includedin the system to monitorthe ambient airtemperature inthe light-tightbox

and the temperature of the test-cell holder.

The systemwas designedto minimize electrical noise. Electrical connection of the test and reference cells to

a mul_plexerand lock-in-amplifier are made withBNC cables and connectors. Ground loops are minimized through

the use of a singlecommon groundforallthe electricalcomponents; all electrical components are also electrostatically

shielded. The background currentlevelsare lessthan 1E-12 A and the A.C. monochromaticprobe currents of the order

of 1E-6 A. The wavelengthis scanned bystepping both of the monochromators in 20 nm intervals, and measuring and

loggingten valuesof the currentat each wavelength. The standard deviation of the ten current values is typicallyof the

order of 1E-3 of the average value of the measured current. The calibrated detector and test cell currents are

measured sequentially at each wavelength usinga multiplexingcircuitand lock-inamplifier.

Calibrationofthe QE system is accomplished followingalignment of the optical components and carryingout

two scans. One scan iscarriedoutwiththe calibrated detector inthe position shown in Figure 4; a second scan is done

with the calibrated detector positioned in place of the test-ceU holder. Computer software is used to calculate a

calibration vector which is used insubsequent scans to determine the absolute external QE of solar cells mounted in

thetest-cell holder. The calibration vectors are saved on the hard drive and used to determine the long-term stability

of the system. Following a wavelength scan, absolute external QE values are displayed on the computer monitorin

graphical form and in tabular form on the printer. The tabular data are also saved on the computer hard drive for

archivalpurposesandsubsequentanalyses. Computercontrolof the filterwheels, monochromators, QTH lamp power

supply, and lock-in amplifier makes it possibleto measure QE over a selected wavelength range in about twenty

minutes. The stability of the system for successivescans is at the 0.1% level.

SINGLE-JUNCTION QE RESULTS

The insbumenta_n was used to measure the absolute external QE of a single-junction a-Si:H alloy solar cell

with a superstrate structure (3). The

results of the measurements for the

cell without light bias at 24 °C under

short-circuit conditions are shown in

Fcjure5 bythe open-square symbols.

The maximum value in QE is 0.81 and

occurs at 590 nm. QE decreases

below 400 nm because of absorption

in the glass superstrate and top pdoped layer. The reduction in QE

above 700 nm is due to the band gap

of the intrinsiclayer. The details of QE

inthe 400 to 700 nm range reflect the

roles of the doped layers, intrinsic

layer thickness and carder transport.

QE was measured at 24 °C usingthe

AM0 light bias shown in Figure 3; the

results are shown in Figure 5 by the

openbJangularsymbols. The effect of

AM0 lightbiason QE is not discernible

on the graph. The measurements

show that changes in the occupancy

of the sub-band-gap states resulting

from highcarrier injectionlevels do not

1.00

_:_ 0.80

0.60

UJ

0.40

O' 0.20

0.00

300

81ngle-JuncUon QE va Wavelength

r, 2 | C

Dark,

v D'-.'_.., ! ¢ C

A Light E las, 24 C

400 600 I00

\

700 8O0 gO0

Wavelength (nm)

Figure 5: Quantum Efficiency of a single junction a-Si:H alloy solar cell

under dark and light-biasconditionsat 24 °C, and under dark conditionsat

55 °C.

-162-
have a major effect on carder transport and collec'donin single-junc'dona-Si:H alloy solar cells. The light-tightbox

showninFigure4 was heatedto 55 =Cand QE measured under dark condi_ons. The results of the measurements are

shown bythe invertedtliangularsymbols in F'_ure 5. QE differences of the order of a few percent are discernible and

may be attn'butedto the temperaturecoelTcientsof both the test cell and calibrated detector. The 55 °C measurements

show the techniquesemployedare notcriticallysensitiveto room temperature vada_ons of the order of a few degrees.

The effect ofronsard-biasvoltageon QE fora single-junctiona-Si:H alloysolar cell was investigated under lightbias conditions at 24 °C. The voltage was stepped in increments of 0.1 V from 0.0 to 0.8 V. The results of the

measurements are shown in F'_ure 6.

The measurementsshow QE is insensitiveto forward biasfrom 0.0 to 0.5 V.

The peak value in QE is 0.8 and occurs at 560 nm. At a forward bias of

0.6 V, approximately the maximumpower point, QE begins to decrease

and the peak value shifts to lower

wavelengths. Significant decreases

occur in QE as the forward bias is

increased and approaches the opencircuitvoltage. At 0.8 V, the peak in

QE is 520 nm and the value is 0.08.

An interpretation of the wavelength

shift inthe peak of QE shown in Figure

6 is as the forward bias voltage is

increased, the trapped charge in the

Intrinsiclayer plays an increasingimportant role in skewing the electric

field distribul_ontowards the front of

the cell; this in turn shiftsthe peak in

QE to a lower wavelength, and influences carder transport and decreases

carder coileddon.

Single-Junction QE Dependence on Voltage Blaa

0.8

W O.6

°°.,

0.2_

0.0

0.2

Wavelength (nm)

Figure 6: Three dimensional view of the quantum efficiency dependence

on forward-bias voltage for a single-junction a-Si:H alloy solar cell.

The instrumentation and measurement methods were evaluated by comparing the short-circuit current

measured under AM0 light bias, Isc (meas), with a calculated short-circuitcurrent, lsc (calc). The value of the

calculatedshort-circuitcurrentwas obtained by convol_ng the measured quantum emciencyvalues, QE (X_), shown

inF'_ure 5 wi_ the measured spectral irradiancevalues, SI( Ai ), shown in Figure 3. The convolutionwas carded out

overthe 350 to 900 nm wavelength range. The calculated short-circuitcurrent was obtained usingthe expression:

Equation 1

_ A Coulombs

Isc(calc) _ QE( hi ) x SI( X_) x X_x _ [. ]

1239 "Joulesxnrn"

where 8, isthe wavelength, _ the monochromator step@ and A the cell area. The results of the convol_on are

shown in Table I. The agreement of isc (meas) with Isc(calc) is better than 1%. Isc (meas) was measured with a

Keithley model 236 source measurement unit recently

calibrated withNIST scales referenced to NIST publications

#252194 and #251357; isc (calc) was caTculated using Table I

measured QE( X=)and SI( X_)values traceable to NIST

scales, as indicated in the preceding section. The agree- Cell isc(meas) Isc(calc) &lsc/Isc

merit between Isc (calc) and Isc (meas) is believed to (ma) (rnA) (%)

result from using good measurement techniques and X302 6.315 6.353 0.6%

instrumental_oncalibrated with scales traceable to NIST.

The measurements characterize the behavior of a singlejun_on a-Si:H cell under forward and lightbias. They also demonstrate the validityof the measurement techniques

for characterizing single-jun_on solar cells. These observations alos prove useful in understanding the behavior of

triple-junction solar cells undervarious biasing cond_ons.

-163-
SPECTRAL-B_S TRIPLE-JUNCTION QUANTUM EFFICIENCY RESULTS

QE measurements of _ple-junct]on a-Si:H cells under short-circuit condi_ons were carded out using the

spectral light-bias technique (1). The sbucture of the triple-juncUon cells has been previously discussed (3). The lightbias technique is based on selectively injecting carders in cell jun_ons, and "tuming-off" and "turning-on" juncUons.

A junction is referred to as "turned-off" when there is relatk'ely little opt_,al inje_on of carders in the junction; it is

qumed-on" when there is a relatively large opUcal inje_on of carders. When one junction is _tumed-off', and the other

two jun_ons are "turned-on', the "tumed-off" junction limits the current in the cell. QE of a junddon limBng the cell

current can be determined using an A.C. monochromatic light beam, referred to as the probe beam. The carders

injected by the probe beam in the "turned-off" junction increase the photoconduddvity and produce an A.C. current which

characterizes QE of the junc'don under the conditions of the measurement. The injection of carders in the juncUons is

determined by the spectral irradiance of

the D.C. light beam and the optoelectronic prope_es of each of the three

junc'dons. The spectral irradiance of

the D.C. light beam used in the spectral

light-bias technique is varied to selectively "turn-on" two of the three junctions in the triple-junction solar cell.

The filter wheel contains three filters;

each filter has a spectral transmission

which filters the AM0 solar simulator

beam to produce a spectral irradiance

which "tums-on" two of the three junctions. The first filter qums-on" the middle and bottom junctions, the second

filter "turns-on" the top and bottom

filters, and the third filter "turns-on" the

top and middle jun_ons. Thus the first

filter makes it possible to measure QE

of the top juncUon, the second filter the

QE of the middle junction, and the third

filter the QE of the third junction.

The quantum efficiencies measured with the spectral-light bias technique for the top, middle and bottom

junctions of an a-Si:H alloy triple-junction solar cell are shown Figure 7; the

values were measured at 24 °C. the

top junction is represented by the curve

with the closed circles. The peak in QE

of the top junddon is about 0.54 at 440

nm. QE of the middle junction is shown

by the closed squares; it peaks at 600

nm with a value of about 0.53. QE of

the third jun_on is represented by the

closed triangles and peaks at 720 nm

with a value of about 0.53.

The relative intensity of the A.C.

probe and D.C. spectral light-bias was

invesUgated and the results are shown

in Figure 8. The intensity of the probe

beam is important in obtaining reliable

QE values. The probe beam intensity

must be much less the intensives of the

three D.C. spectral light-bias beams.

Spectral-Light Bias Triple-Junction Solar Cell QE

1.00

_ 0.80

_e

o

E o.00

UJ

E

= 0.40

¢=

el

¢1

O.20

• Top Ju

• Middle1

" Bottom

4104141

41 •

41 I!1. .... ..•

400 600

+ctlon

Junction

Junctior

w:

•e

e

0.00

80O

d&

e

k•A

! •

_.1,. .... ±

800 900

Wavelength (nm)

Figure 7: Spectral light-bias quantum efficiencies of the top, middle and

bottom junctions for an a-Si:H triple-junc'don solar cell.

1.00

o 0.80

e

_0.60

. E 0.40

48 0

O 0.20 o

B

[]

0.00

Effect of Spectral-Light Bias Intensity on

o8

Trl )Is-Junction Solar Cell QE

FIIle | Symbol s.100% hJton411ty

Open 8ymb© L8-l;0% Intensity

G

+B o n A AAA

G *

DO O

0 A

0 f]

D 0000

lq|l_ . _.A A

400 600 000 700

A

A

800 000

Wavelength (nm)

Figure 8: Effect of spectra-light bias intensity on QE for an a-Si:H alloy

_ple-junction solar cell.

-164-
The roleof the relativebeam intensitieswas evaluatedby measuringQE withan attenuated probe beam and comparing

it withQE values obtained with an unattenuated probe beam. The results of measurements with a 50% probe beam

attenuationare shown inFigure 8. The symbolsusedto represent the various jun_ons are the same as Figure 7. Filled symbols represent QE measurements with the unattenuated probe beam; the QE values measured with the

unattenuatedbeam are referred to as 100% intensityin F'_ure 8. The open symbols inthe figure are plotted overthe

dosed symbolsand correspondto QE values measured withthe probe beam attenuated 50%. Figure 8 shows QE of

the middle and bottom junc'donsis not influenced by the probe beam intensity;the topjunction shows a reductionin

measured QE values of the order of a few percent when the probe beam intensityis attenuated by 50%. Hence it may

be concludedthe QE values obtained_th the spectral light-biastechnique are notsignificantlyinfluenced by the probe

beam intensitiesused in these measurements.

The measurements onthe _ple--juncSonsolar cells show the topjunction is effective in absorbing the shorter

wavelengths of an AM0 spectrum, while the bottom cell absorbs the longer wavelengths, and the middle cell the

intermediatewavelengths. Sincethe junctionsare inseries, the photocurrentin each junction is the same under powergenemSngconditions. The design of a triple-junctioncell requires optimal junctions to convertthe largest fraction of

the AM0 spectrum into elecffical energy. I! the cell design is not optimal, then one of the junctions may limitthe

photocurrent,and carder recombinationinthe othertwo junction willresultin lower cell efficiency. The structureof the

quantum efficiencyof a _ple-junciion cell measured under AM0 D.C. light biasis useful in evaluating the design of the

cell. If the measured QE resembles one of the curvesin F'_ure 7, the resultswill suggest cell performance is limited

by the junddonwhich corresponds to the curve.

AM0 UGHT-BIAS TRIPLE-JUNCTION QUANTUM EFFICIENCY RESULTS

The quantum elficieny of

triple-juunc'dona-Si:H alloy solar cells

were measured using a D.C. light-bias

beam produced by the solar simulator

adjustedto produce the spectral irradiance shown inF'_ure 3. The measuremerits were carded out with the cells

under short-circuit current conditions.

The results of measurements are

shown in Figure 9. The peak value of

QE is at 460 nm with a value of about

0.40. The curvein F_ure 9 has approximatelythe same shape as the curve in

F_gure7 forthe top-junction of the cell.

Figure 7 showsthe peak value is 0.54

at 440 nm for the top jun_on of the

cell measured. The results suggest

the performance of the triple-junddon

cell under AM0 light bias is limited by

the top junddon. QE of three _plejunction cells were measured; the

results of the measurements were

convoluted with the solar simulator

0.50

Triple-Junction QE vs Wavelength

Under AM0 L!_lht Bias

0.30 ij

E0.20

_0.10

.

0.00 " "• ....

400 500 tl00 700 BOO 901

Wavelen nl____n___ th

Figure 9: Quantum efficiency of a-Si:H alloy triple-juctionsolar cell under

AM0 light bias.

spectral irradiance using Equation 1 in +

orderto obtaina calculated value forthe short-circuitcurrent.

The results of the calculations are shown in Table II.

Isc(calc) is within about 1% of Isc(meas) for the three cells.

The calculaSons confirm the validity of the D.C. light-bias

technique for determining QE of triple-jun_on calls.

The role of forward bias on QE of a triple-junc'don

solar cells underAM0 D.C. light biaswas investigated. A cell

was maintained at 24 °C and QE measured with forward

biases ranging between 0 and 1.82 V. The voltage at the

Table II

Cell Isc(meas) Isc(calc) Alsc/Isc

(mA) (ma) (%)

ST05 2.338 2.361 1.0%

ST17 2.386 2.395 0.4%

ST38 22.53 2.279 1.1%

-165-
ma_dmum-power point was about 1.7

V and the open-circuit voltage was

about 2.2 V. Forward biases of 0,

0.62, 1.02, 1.42, 1.62 and 1.82 V

were used. The results of the mea°

surements are shown in F_gure 10.

Both the wavelength corresponding

to the QE peak value and the peak

value of QE are strongly influenced

by the forward-bias voltage. The

peak value of QE decreases from

0.40 to 0.08 as the forward bias increases from 0 to 1.82 V; for the

same increases in the forward bias,

the wavelength for the peak QE

value increases from 460 to 600 nm.

The measurements show QE under

short-circuit cond_ons is considerably different than QE measured

near the maximum-power point.

While the short-circuit measurements suggest the performance

of the cell is limited by the top junction, the measurements near the

maximum-power point suggest both

the top and middle junddons are

limiting the operation of the cell.

The response of the triplejun_on cell was further tested by

qualitatively varying the spectral irradiance of the solar simulator by

changing the current in the tungstenhalogen lamp. The lamp current

was varied from 5.0 to 2.9 A; 5.0 A is

the current which produced the fit to

WRLAM0 shown in FKiure 3. Asthe

lamp current was decreased, the

spectral irradiance beyond 750 nm

decreased. Figure 11 shows the

behavior of QE. The quantum efficiencywith a lamp current of 5.0 A is

the same as the values plotted in

I=_gure 9. The QE curve with a lamp

current of 2.9 peaks at 720 nm and

has a value of 0.47; it is similar to the

curve produced by the bottom junction shown in Figure 7. Thus as exTriple-Junction QE Dependence on Voltage Blaa

Under AM0 Light Bias

,_ 0.4 ___--_ _

O.S _ _ soo

_e/_j 2.0 _ Wavelength (nm)

Figure 10: The dependence of the quantum efficiency of a triple-jun_on

a-Si:H alloy solar cell on forward-bias voltage for AM0 light bias.

Triple-Junction QE Dependence on Solar

Sire ulator Spectral Irradlanca

:::

0.2

0ol

0

" qi s_ ole_ Q

Figure 11: Dependence of triple-jun_on quantum efficiency on spectral

irradiance of solar simulator. The spectral irradiance of the solar

simulator was varied by changing the filament current in the tungstenhalide lamp.

pected from the changes in the speclxal irradiance produced by reducing the current in the tungsten-halogen lamp, the

cell current is limited by the top junction when the lamp current is

5.0 A; the current in the bottom junction limits the cell current when the lamp current is 2.9 A.

The investigations of QE measured under AM0 light bias conditions reported in this seddon are preliminary,

however, the results show agreement at the 1% level between measured short-circuit currents and currents calculated

from measured cell quantum efficiencies and solar simulator spectral irradiances. Studies need to be done to

determine the specifications of both the spectral irradiance and radiance of AM0 solar simulators to be used in the

characterization of mul_junddon cells. The accuracies will depend, among other things, on the number of junctions

in the multi-junction cells and the optoelectronic properlJes of each of the junctions. The work to determine the

-166-
s_tJons for AMOsolar simulators can be carded out using small area cells, i.e., cells of the order of 1.0 cm2in

area. While device simulal|on studies are usefulfor designingcell structuresand defining the optoelecb'onicof each

junc_on,laboratorycharacterizationof cellsshould be carded out both to facilitate solar cell development and oplJmize

the limited resources available for space tesUng.

SUMMARY

An integratedsystemwas described which consistsof a spectral radiometer and dual-source solar simulator,

and personal computer based current-voltage and quantum efficiency equipment. The spectral radiometer was

calibrated witha tungsten-halogenstandardlamp. The quantumefficiencyapparatus employed a calibrated reference

detectorwhichwas usedin measuringthe absolute extemal quantum efficiencyof triple-junc'dona-Si:H ally solar cells.

The calibrationsof the lamp and photodiode, as well as the source measurement unit used to measure cell currents

were basedon NIST scales. Quantum ef6ciencieswere measured usingbeth the spectral-light bias and AM0 light-bias

techniques. Quantum efficiencies measured with the AM0 light-biastechnique were shown to be dependent on

forward-bias voltage and the specb'al irradiance of the AM0 light-bias beam. Measured spectral irradiances and

quantumeffidendes were convolutedto calculate cell short-circuit cell currents. Calculated currents compared with

measured short-circuitcurrents at the 1% level,

REFERENCES

* AppredatJonis expressedto KennethR. Lord II,Michael R. Waiters and Fazal UrRahman Syed for their contributions

to this work.

1. Joseph Burdick and Troy Glatfelter, Solar Cells 18 (1986) page 301.

2. M. Bennett and R. Podlesny Proceedings of_e 21st IEEE PhotovoltaicSpecialists Conference (1990) page 1438.

3. Kenneth R. Lord II, Michael R. Waiters and James R. Woodyard, Proceedings of the 23st IEEE Photovoltaic

SpecialistsConference (1993) page 1448.

-167-
SOLAR CELL ANGLE OF INCIDENCE CORRECTIONS 1

by

Dale R. Burger and Robert L. Mueller

Jet Propulsion Laboratory, California Institute of Technology

Pasadena, California 91109

The Mars Pathfinder mission has three different solar arrays each of which sees changes in

incidence angle during normal operation. When solar array angle of incidence effects was researched

little published data was found. The small amount of published data created a need to obtain and

evaluate such data. The donation of the needed data, which was taken in the fall of 1994, was a

major factor in the preparation of this paper.

BACKGROUND

The Mars Pathfinder cruise phase solar array is body-mounted and will be providing primary

spacecraft power at angles from normal to at least 60 degrees off-normal. The lander solar array is

also fixed and, due to its location on the surface of Mars, will see a constantly changing incidence

angle during a day. The Microrover is a mobile experiment on Mars with a solar array as its top surface

and thus will also see constant changes in incidence angle both from sun position and vehicle

orientation.

A search of the literature for angle of incidence data turned up only a few references (1-4) and

no tabular data. Reference 1 lists the following possible causes of off-normal array power loss other

than the cosine correction:

a. "Optical effects relating to the first-surface reflectance of the cover slide;

b. Optical effects relating to apparent changes in the optical thickness of coatings and filters,

thereby causing apparent changes in the spectral transmittance and reflectance values;

c. Edge effects relating to refraction, scattering and additional light collection by solar cells and

covers, especially by thicker covers; and

d. Shadowing of solar cells by solar cell cover edges, solar cell interconnectors, wires, and other

relatively small array components."

Reference 2 mentions as possible causes items a. and b. above while Reference 3 mentions

items a., b., and expands upon c. by varying cell-to-cell spacing and cover slide thickness. Reference

3 also adds the following possible power loss cause:

e. Accelerated UV degradation of adhesive due to edge channelling of UV light.

Reference 4 mentions item a. "and adds the following possible causes (which are primarily

terrestrial concerns):

f. Shadowing by particulate soiling; and

g. Effect of white or black background.

Due to limitations in space and our data we will be discussing only items a., b., and c. in this

paper.

IThe work described in this paper was carried out by the Jet

propulsion Laboratory, California Institute of technology, under a

contract with the National Aeronautics and Space Administration.

-168-
DATACOLLECTION

TheLargeAreaPulsedSolarSimulator(LAPSS)is a researchtoolat JPLandwasusedfor

creatingandcollectingallof thedatainthispaper.TheLAPSShasbeendescribedindetailelsewhere

(5)andthuswill notbedescribedhere.ThedatausedinthispaperwascontributedbyAppliedSolar

EnergyResearch,Cityof Industry,Californiaandisgratefullyacknowledged.

CELLS

The28 cellsusedinthisstudywerefromsevendifferentlotsmadewithstandardprocesses

butwereconsideredto beresearchcellsandthusnoeffortwasmadeto compareanycellperformance

valuesexceptthe responseof theirshortcircuitcurrent(Isc)to changes in angle of incidence.

Angle of incidence measurements were made on the cells by attaching them to a divider head

which was carefully aligned normal to the light beam at 0 angle of incidence.

The Applied Solar Energy Corporation (ASEC) cells were all 4 cm by 4 cm GaAs/Ge with the

usual two layer anti-reflection coating on the top surface. Each cell had a CMX coverglass having a

cerium oxide ultra-violet absorption capability. The Pilkington specification for CMX coverglass shows

a quarter-wavelength anti-reflection coating of MgF2 on the front surface to maximize transmission at

600 nm.

THEORY

Before data reduction and analysis was attempted, a number of potential angle of incidence

dependent effects were examined.

Cosine - The cosine correction is due to the change in effective array collection area due to the

projection of the off-normal array surface onto a plane normal to incident sunlight.

Fresnel Reflectivity - Fresnel reflection is due to the interface between two optical media having

different indices of refraction. The reflected portion of the incident light is given by:

I[ tan2 (_i- _2) sin2 (_i- #2)

/?= +

tan2 (_i + _2) sin2 (_z + _2)

(z)

where ¢1 is the incident beam angle and the refracted beam angle,_,, is:

_2=arcsi_ sin 'I)

(2)

COATINGS AND FILTERS

Coverglass AR Coating - The MgF= antireflective coating that is normally used on the front

surface of a coverglass is a single layer and thus an increase in the angle of incidence will show little

effect from this coating (6). There could be a small shift of the center frequency toward shorter

wavelengths which could cause a small decrease in output.

Filters - There were no low absorption filters such as blue-red reflection or infra-red reflection,

on the coverglass or the cell front surface therefore optical effects from these filters was not a

concern.

-169-
EXTREMEANGLEEFFECTS

Thereisa possibility of trapping light or shadowing a cell when very large incidence angles are

encountered. These effects are difficult to quantify but have been reported [3].

LOW INTENSITY EFFECTS

Temperature - A decrease in temperature is normal when the angle of incidence increases for

a solar array exposed to constant sunlight. The pulsed light source of the LAPSS (about 3 msec every

5-10 min.) should not create any temperature effects so this factor could be ignored.

Voltage - Lower intensities due to angle of incidence do result in lower cell voltages. This

study however only focused upon the short circuit current and thus should not have to correct for this

effect.

DATA REDUCTION

The raw data was reduced by use of a JPL BASIC program called REPLOT. This program takes

header file information such as temperature, standard cell Isc, and analog-to-digital counts and uses

this information to convert and correct the digital count IV data for each separate angle of incidence.

This data is shown on Figure 1 as a solid line.

The normal (0 degree angle of incidence) short circuit current (Isc) value was first corrected

for the small reflection loss at normal incidence (i.e. 1.7% for index of refraction of 1.3) by dividing

the Isc value at normal incidence by one minus the reflection loss. This corrected Isc value was then

used to create two other sets of values: a predicted Isc versus angle of incidence using the cosine

correction only (shown on Figure 1 as long dashed lines); and a predicted Isc versus angle of incidence

using both cosine and Fresnel corrections (shown on Figure 1 as short dashed lines).

While it was known that the front surface was coated with magnesium fluoride (MgF2), the

exact value of the coverglass front surface index of refraction was unknown and coverglass from the

same lot was not available for measurement. A published material value of 1.38 was first used in the

Fresnel equation (1) but later a better fit between actual and predicted values was found by using an

index of refraction of 1.30.

A prediction error was used to normalize the results as well as to determine the validity of using

the cosine and Fresnel reflectivity approach. The prediction error was calculated by subtracting the

measured data from the predicted value and then dividing by the measured data. The results for 27

of the experimental cells is given in Table I below and shown in Figure 2. One cell was dropped from

the study due to loss of one measured value.

Table I- Prediction Errors

Cell Angle oflncidence

No. 10 _0 30 40 65

1 -0.00063 +0.00121 -0.00064 -0.00290 -0.00756

2 -0.00166 -0.00495 -0.00222 -0.00375 -0.00742

3 -0.00027 -0.00020 -0.00033 -0.00243 -0.00878

4 -0.00299 -0.00402 -0.00363 -0.00565 -0.01030

5 -0.00080 +0.00027 -0.00045 -0.00010 -0.00796

6 -0.00049 +0.00171 +0.00084 +0.00031 -0.00704

7 -0.00363 -0.00468 -0.00722 -0.00927 -0.01534

8 -0.00051 -0.00323 -0.00471 -0.00541 -0.01001

9 +0.00018 -0.00202 -0.00168 -0.00586 -0.01372

10 -0.00055 -0.00167 -0.00262 -0.00151 -0.00281

-170-
11 -0.00086 -0.00517 -0.00421 -0.00637 -0.01178

12 -0.00382 -0.00624 -0.00921 -0.01081 -0.01434

13 -0.00141 -0.00230 -0.00513 -0.00631 -0.01056

14 -0.00143 -0.00411 -0.00345 -0.00474 -0.00813

15 -0.00135 -0.00225 -0.00267 -0.00497 -0.00608

16 -0.00261 -0.00404 -0.00604 -0.00930 -0.01696

17 -0.00108 -0.00043 -0.00284 -0.00417 -0.01113

18 -0.00045 -0.00032 +0.00059 -0.00408 -0.01258

19 -0.00151 -0.00187 -0.00317 -0.00440 -0.01308

20 -0.00175 -0.00376 -0.00490 -0.00856 -0.01828

21 -0.00094 -0.00326 -0.00582 -0.00717 -0.01259

22 -0.00456 -0.00740 -0.00758 -0.00742 -0.00995

23 -0.00209 -0.00287 -0.00224 -0.00388 -0.00935

24 -0.00102 -0.00305 -0.00206 -0.00486 -0.00919

25 -0.00217 -0.00027 -0.00538 -0.00782 -0.01326

26 -0.00376 -0.00661 -0.00737 -0.00851 -0.01402

27 -0.00322 -0.00697 -0.00717 -0.00598 -0.01447

Note: the expected

0.003.

random error for Isc values taken with the LAPSS is no greater than +/-

STATISTICAL FIT

For small angles of incidence the cells had cosine times Fresnel predicted values which closely

matched the measured values. However, it was noted that the error between predicted values and

measured values increased with angle of incidence. This trend is seen in Figure 2 and Tables I and I1.

It can also be noted in Figure 2 that the spread increases with angle of incidence which is expected

from the effects of the systematic errors which are discussed below.

Table II

Ang. of 10 20 30 40 65

Incidence

Mean of -0.0017 -0.0029 -0.0038 -0.0054 -0.0110

Error

Std. Dev. 0.0013 0.0025 0.0027 0.0027 0.0035

Max. Error 0.0002 0.0017 0.0008 0.0003 -0.0028

Min. Error -0.0046 -0.0074 -0.0092 -0.0108 -0.0183

Also of interest in Figure 2 is that there seemed to be a normal distribution of the errors around

the means. This hypothesis was checked using a chi-squared test with 3 degrees of freedom. The

results for 10, 20, 30, 40, and 65 degrees were 8.68, 5.475, 1.967, 1.47, and 1.493 respectively.

These chi-squared values were found to be acceptable at the 0.05 level when compared with the 0.95

percentile chi-squared value of 7.815 except the measurements at 10 degrees where there were 15

values that fell between the mean and + 1 Sigma and only 4 between the mean and -1 sigma. The

only reason that could be found for the skewed values was the possibility that running all of the tests

by rotating the dividing head clockwise as viewed from the top could have introduced some systematic

error. After considering the size of the values involved compared to the LAPSS measurement accuracy

of +/-0.003 and the sensitivity of the mean to outlying values it was decided not to pursue this

-171-
finding.

TREND ANALYSIS

Since the plot in Figure 2 shows a definite trend this trend was plotted as Figure 3 by using

the mean values at each angle as a data point. A fourth order regression curve was selected as the

best fit and this curve gave a good fit to the plotted data with an R value of 0.99988. No conclusions

were drawn from this finding.

Possible explanations for the trend of larger measured data values than predicted values include

the following:

a.

b.

Onset of the expected extreme angle improvement;

Improved cell performance due to the MgF z AR coating center frequency shift toward

shorter wavelengths;

c. Improved cell performance due to longer optical path length near the cell junction.

d. Error in the assumed effective index of refraction.

There was no practical way to check on possibilities a., b., and c. Possibility d. however was

checked by selecting a "typical" experimental cell (i. e., one which had prediction errors similar to the

mean of all of the cells). Cell #13 prediction errors were a reasonable fit to the mean values. Values

for an effective index of refraction of nz were assumed as 1.25, 1.30, and 1.35 with results as shown

in Table III.

Table III

Index Angle of Incidence

n2 10 20 30 40 65

1.25 -0.00175 -0.00260 -0.00527 -0.00593 -0.00223

1.30 -0.00141 -0.00230 -0.00513 -0.00631 -0.01056

1.35 -0.00159 -0.00250 -0.00546 -0.00713 -0.01788

Table III shows that the 1.30 effective index of refraction value minimizes the prediction error

at the small angles where the error should be small and does have a trend of larger negative errors at

larger angles of incidence. The 1.25 effective index of refraction value minimizes the total prediction

error but with no trend to the error except a possible positive prediction error at angles above 65

degrees. The 1.35 effective index of refraction value produces prediction errors which are in all cases

larger than those of the 1.30 value. Considering the previously reported performance increase at

extreme angles of incidence it has been decided to remain with the assumed 1.30 effective index of

refraction value. What would have been very useful in this study is measured data beyond 65 degrees

which might allow a better resolution of this question.

While an increasing angle of incidence effect was probably seen, extreme angle effects were

not conclusively noted since the measurements were only taken to an angle of incidence of 65

degrees. A more detailed search of the region between 80 and 90 degrees is required in order to fully

answer this question. Possible additional factors of interest for extreme angle effects are noted in

Reference 3 as: cover slide optical properties, cover slide thickness, and inter-cell spacing.

SYSTEMATIC ERRORS

There are three different systematic error causes: large incidence angles; reduced reading size,

and assumed index of refraction.

Large Incidence Angles - The cosine function is changing very rapidly at large incidence angles

so small angle errors become a problem. Alignment of the cell samples to the light beam was done

with care but the source is 36 feet away from the sample and there is no hard-mounted goniometer

-172-
to use as a reference. An additional problem would be stray room light and wall reflection which

become larger percentage effects as the incidence angle increases. This effect was minimized by

turning off the room lights.

Reduced Reading Size - As the incidence angle increases the Isc decreases so small

measurement errors in Isc become magnified. Of particular interest is the rounding error since all

readings large or small are presented with only two decimal digit accuracy.

Assumed Index of Refraction - As mentioned above, the coverglass front surface index of

refraction used for all predicted values was set at 1.30 to obtain the best data fit. This was done early

in the analysis by using data from only seven of the ASEC cells. Predicted cosine times Fresnel curves

for all cells used this value. A better controlled experiment would measure this value using coverglass

from the same lot.

There is some beam decoliimation but this effect was measured in Reference 3 and found to

be less than 0.5 degrees for a setup similar to the JPL LAPSS. The apparent size of the JPL LAPSS

light source is calculated to be 0.56 degrees on the diagonal of the rectangular source area. This

compares favorably with the sun's apparent size from Earth which is 0.53 degrees.

RANDOM ERRORS

LAPSS - The best estimate of the LAPSS random measurement error is a maximum of +/-

0.3%.

AID Count - The data acquisition system used on the LAPSS is typical in that it uses a digitized

representation of the analog data which consist of whole counts. Rounding off to the nearest count

is then a random error equal to, at most, 1 part in 4096 or roughly 0.024%.

Temperature - Temperature measurements are displayed on the meter to the nearest tenth of

a degree so the rounding of this value is a random error equal, at most, to 5 parts in about 2800.

Fortunately the Isc sensitivity to this error for gallium arsenide cells is about 0.0175%/degC and

0.0145%/degC for silicon cells so the overall temperature error effect of a 0.05 degree rounding error

is small.

Angle - The divider head has 0.1 degree gradations and a backlash of 1.5 gradations. Care was

taken to always move in the same direction to minimize backlash so the setting error would be about

0.05 degree or about +/-1.0% in the worst case at 85 degrees angle of incidence. This study only

took readings in a counter-clockwise direction as viewed from above.

TOTAL RANDOM ERROR

The LAPSS error can not be included in the total random error since it already contains the

other two errors. With that in mind and the fact that the count and temperature errors are

independent, the total random error is calculated as the square root of the sum of the squares:

0.000255 or 0.0255%. This total random error is not large enough to invalidate the conclusions made

below.

OTHER ERRORS

At least one other error source must be mentioned in this discussion. The tests were run only

on single cells which would not necessarily give the same results as tests run on full arrays. Edge

effects, reflections, and shadowing effects are all possible confounding errors if the conclusions drawn

below are applied to arrays which was the original intent.

CONCLUSIONS

-173-
UsingthecombinedcosineandFresnelcorrectionsgaveverygoodpredictionsofthemeasured

data. ForatypicalexampleseeFigure1.

Anempiricallyderivedvalueof thecoverglassfrontsurfaceindexofrefractionshouldbeused

if possibleratherthana publishedmaterialvalue.

Thereis probablyaneffectfromincreasingangleof incidencewhichcouldnotbeexplained

indetail.

REFERENCES

1. SolarCellArrayDesignHandbook,Vol. 1, JPL Document #SP 43-38, Vol. 1, Jet Propulsion

Laboratory, Pasadena, CA, Oct. 1976, pp 4.2-1 &-2 and 4.8-1 &-2.

2. Ross, R. G., Jr., et. al., "Measured Performance of Silicon Solar Cell Assemblies Designed for Use

at High Solar Intensities," JPL Publication #TM 33-473 (N71-23780), March 15, 1971, pp 113-115.

3. Opjorden, R. W., "Solar Cell Optical Design Considerations," Conf. Rec. of 9th IEEE PVSC, Silver

Springs, MD, May 2-4, 1972, pp 164-173.

4. Wilson, A. and Ross, R. G., Jr., "Angle-of-Incidence Effects on Module Power and Energy

Performance," Progress Report 21, April 1982 to January 1983, and Proc. of 21st PIM, JPL Doc.

#5101-222, Jan. 1983, (NTIS, PC A22/MF A01; 1 ), pp 423-426.

5. Mueller, R. L., "The Large Area Pulsed Solar Simulator (LAPSS)," JPL Publication #93-22, Rev.

A, July 15, 1994.

6. Heavens, O. S, "Optical Properties of Thin Solid Films," Butterworths Scientific Publications,

London, 1955, pp. 208-215.

-174-
0

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-175-
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-176-
0

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-177-
LOW INTENSITY LOW TEMPERATURE (LILT) MEASUREMENTS AND COEFFICIENTS

ON NEW PHOTOVOLTAIC STRUCTURES

David A. Scheiman and Phlllip P. Jenkins

NYMA Inc.

Brook Park, Ohio, 44142

David J. Brinker

NASA LeRC

Cleveland, Ohio 44135

Joseph Appelbaum

TeI-Aviv University

TeI-Aviv, Israel, 69978

ABSTRACT

Past NASA missions to Mars, Jupiter and the outer planets were powered by radioisotope thermal generators

(RTGs). Although these devices proved to be reliable, their high cost and highly toxic radioactive heat source has

made them far less desirable for future planetary missions. This has resulted in a renewed search for altemate

energy sources, some of them being photovoltaios (PV) and thermophotovoltaics (TPV). Both of these altemate

energy sources convert light/thermal energy directly into electdcity. In order to create a viable PV data base for

planetary mission planners and cell designers, we have compiled low intensitylow temperature (LILT) I-V data on

singlejunction and multi-junction high efficiency solar cells. The cells tested here represent the latest photovoltaic

technology. Using this LILT data to calculate Short Circuit Current (IJ, Open Circuit Voltage (VJ, and Fill Factor

(FF) as a function of temperature and intensity, an accurate prediction of cell performance under the AM0 spectrum

can be determined. When combined with QUantum efficiency at Low Temperature (QULT) data, one can further

enhance the data by adding spectral vadations to the measurements. This paper presents an overview of LILT

measurements and is only intended to be used as a guideline for material selection and performance predictions.

As single junction and multi-junction cell technologies emerge, new test data must be collected. Cell materials

included are Si, GaAs/Ge, GalnP/GaAs/GaAs, InP, InGaAs/InP, InP/InGaAs/InP, and GalnP. Temperatures range

down to as low as -180°C and intensities range from 1 sun down to .02 suns. The coefficients presented in this

paper represent expedmental results and are intended to provide the user with approximate numbers.

BACKGROUND

With increasing concems over the safety and cost of RTGs, altemate power sources are being sougH. NASA's

current stand on this issue is to avoid using nuclear power sources unless there is no feasible alternative. One

such alternate source of power is photovoltaics, which are widely used today in both space and terrestrial power

systems. Most solar cells are designed to operate at 1 sun intensity (AM0, 136.7 mW/crn2) and moderate

temperatures (20° to 80°C). As space exploratory missionsextend beyond earth's orbit, temperature and intensity

become a concern. Missions are being proposed for Mars, Jupiter,the outer planets, and beyond the solar system.

At these distances, both intensity and array operating temperature drop. Intensitychanges inversely as the square

of the distance. Temperature calculations _re based on intensity and emissivity. The array temperature can be

as low as -140°C at 6 astronomical units (A.U.), i.e. Jupiter intensity is 5 mW/crnz and -130°C at 5.2 A.U. (1). A

plot of Intensity vs distance is shown on the following page, this plot also includes relative array temperatures at

various planetary distances.

With early LILT measurements dating back 15-25 years, most of the available data is outdated. Solar cells have

become more efficient and more reliable over a range of environmental conditions. Eady LILT data was also

performed using older techniques with limited temperature and intensity regulation, and less sensitive measuring

equipment. Flight hardware costs continue to increase, which decreases their allowable design margins. Updating

these measurements is crucial for the recent resurgence in PV for interplanetary missions.

Most temperature effects on solar cell output are understood. As cell temperature drops open circuit voltage Vo_

will increase linearly, and short circuitcurrent I,_will decrease due to a shift in bandgap (the absorption coefficient

-178-
alsodecreaseswithtemperature).FillFactor

willtendto increaseproportionallywithvoltage

but thereare manyothermechanismsthat

contributeto itstemperaturedependence(2).

The most importanteffect is that the dark

currentIodecreases as temperature decreases.

The temperature effects on voltage and current

can be seen in the following equations (3,4):

ykTII, + 1/ (1)

I(V)= !,=-/o[e (7"_r)- 11

!-

II

(2) -=

=

-E,. (3) /o a "r'e-_- =

where T is temperature, y is the ideality factor,

typically between 1 and 2, k is Boltzman's

constant, F.=ois the bandgap, and q is the

charge on an electron. As temperature

decreases, the bandgap of the semiconductor

material increases. This decreases the

spectrum which can be absorbed and reduces

the photocurrent.

.1

.01

.001

I:0A:U .......... '

ArmyOper_ng Temperature:

Mars - -9.5 °C

SJupil.52A.U. Jupiter- -130 °C

Saturn- -168 °C

Uranus- -lgg °C

ter 52 A.U. Neptune=-214 °C

uto=-221 °C

_e=_.e _o.oe.A.u: , .

10 20 30 40

DistanceFromtheSun(A.U.)

Figure 1" Solar Intensity vs. Distance From the Sun

Other LILT effects are not well known. Tandem cells in series must be current matched. As the band gap shifts

with temperature, the current matching may be lost. AS cells drop intemperature and intensity,these changes can

be nonlinear. Cells may become shunted and/or carders and dislocations may be "frozen out'. Three common

LILT phenomenon that lead to performance degradation include cell shunting, formation of a rear contact Schottky

barrier, and the "broken knee" or "flat spot" curve shape (5,6).

CELL TYPES

The cells used for this experiment represent a broad range of new cell materials. Only one of the cells tested

was obtained from a production run; all other cells were grown in research labs. These materials were grown on

substrates which include Si, GaAs, Ge, and InP. The cells are:

• GalnP/GaAs two-terminal monolithic tandem grown on GaAS.

- GainP cell on GaAs (inactive)

- GaAs cell with a GalnP window layer.

• InP/InGaAs two-terminal monolithic tandem grown on InP.

- InP cell "

- InGaAs cell with a InP window and grown latticed matched on InP

• GaAs/Ge (passive Ge), GaAs grown on Ge.

• Si 2 .Qcm with BSF. This a production cell.

• InP MOCVD

• .72 eV InGaAs (InP window, InP substrate)

• GaSb (bottom cell of GaAs on GaSb tandem stack)

TEST DESCRIPTION

The test consisted of measuring IV curves of solar cells at varying light intensities and temperature. The

-179-
temperaturesrangedfrom 25°C to -185°C. The

intensities ranged from 1 sun down to .03 suns, or

equivalent distances of 1 to 6 au. I-V curves were run

every 25°(3 at 2.8, 4.7, 11.5, 46, and 136.7 mW/cn_

intensities. The information included in this paper is

only a summary of the data analysis. Figure 2 shows

a diagram of the test setup.

The tests were all conducted at NASA Lewis in the

Solar Cell Evaluation Lab. A Spectrolab X-25 solar

simulator was used to measure the cells. This

simulator provides a close match to the AM0 spectrum

but it is not exact. A monitor cell was placed outside

the low temperature plate to correctfor flicker in the arc

lamp lightsource. All the cells were mounted to a test

plate and placed in a closed environment with a quartz

window and constant nitrogen purge. Temperature of

the test plate was maintained by cooling with liquid

nitrogen and heating with resistive heaters. Up to eight

cells can be tested simultaneously with this setup. All

of the cell measurements and temperatures are

computer controlled. Cells were measured with

standard 4-wire techniques and contacted using Kelvin

probes; no epoxies or solders were used to contact the

cell.

:: QuartzWindow

= /

ells

?--X-25

Heaters /

" r-AMO

--- ess "_

Test I late iMontor

1 Screen Cell

Figure 2 LILT Test Setup

A singlethermocouple embedded in the test plate is used for temperature control. Additionally,four witr]esscells

of similar material and thickness as the test cells were mounted to the test plate and used as a temperature

reference for the cells. A temperature measurement was made at the beginning and end of each IV curve so that

accurate V= vs T and I,,=vs T correlations could be made. Typically, a temperature drift of less then 2° was

observed during an IV curve. Each IV curve was performed from V= to I==.

Light intensity was set up for 1 sun by adjusting the lamp intensity to match I==on a calibrated GaAs/Ge cell at

the plane of the test cells. Intensitywas decreased by using metal screens, which lowerthe amount of lighton the

cells without changing the spectrum. The cells were placed far enough behind the screens to avoid 'hot spots' on

the individual cells.

LILT DATA

All the test data was used to calculate temperature coefficients for V=, I,=, and FF. The data analysis is

presented by cell type. Any anomalies in the cells are shown in the plots of the data or mentioned in the text. All

of the data are normalized to the value at 25°C so that they can be used independently of cell size. Temperature

coefficients are presented in Tables I and II on the following pages. All of these cells were optimized for 1 sun

or greater intensities.

GalnP/GaAs

The GalnP/GaAs cell is a monolithic tandem cell consisting of series connected current matched cells. The cells

are series connected using a tunnel junction. This cell had nearly linear temperature/intensity dependence to about

-90°C, with peak efficiency at around -50°C. Below -90°C, the cellvoltage flattened and then dropped to near room

temperature values. A plot of this data at 1 sun is shown in Figure 3. This loss of output below -90°C can be

attributed to the eventual current mismatch of the two cells, parasitic losses in the tunnel junction, and additional

voltage loss from changes in dark current.

A GalnP cell and a GaAs cell with a GalnP window layer were measured separately. Data on these two

individual cells show that the drop in current is due to limiting by the bottom cell. Both of these cells continue to

operate well below -90°C and indicate that the probable loss in tandem performance could be in the tunnel junction.

-180-
InP/InGaAs

The InP/InGaAs cell is a monolithic tandem cell consistingof series-connected current matched cells. This cell

also had typical temperature/intensity dependence to about -90°C. This cell had a peak efficiency at near -90°C.

Below -90°C, the cell voltage becomes nonlinear. A plot of this data is shown in F_ure 4. The voltage change

does not coincide with the current drop.

Plots of an InP cell and an InGaAs cell with an InP window layer measured separately show typical

temperature/intensity dependence over the entire range of measurements. The voltage slope of both cells tends

to lessen below -90°C. The current of the InGaAs cell changes very littlewith temperature. This is due to the shift

at beth ends of the spectrum. The InP window layer is shiftingalong with the band edge of the InGaAs cell which,

when integrated over an AM0 spectrum, shows little net change in current. This is clearly demonstrated in the

QULT measurements (7).

1.3

1.2

G"

1.1

>!

J 1.0

I=

wl-

.9

.... , .... . .... . . . . ....

I =----.v= .......

/t ,\

5O 0 -5O -100

TemperaIure (_3)

Figure 3 GalnP/GaAs at 1 Sun

%. •

-,.,=,,

-150 -20O

1.75

e_" 1.50

&

@

1.00

.75

5O

0

/

+'- --_ FF i ". ./

......... 1 , ,, , • i . | | ......

0 -50 -100 -150 -200

Tempera_re (%')

Figure 4 InP/InGaAs at 1 Sun

S_LCells

The Si cell is a 2_<:m cell with a BSF. The 1 sun temperature data is shown in Rgure 5. Below -100°C the

voltage slope is much lower. This cell had typical temperature/intensity dependence over the entire range of

measurements. Si efficiency increased by 70% from 25° down to -180°C, where it peaks. This cell tends to

operate the best at low temperature due to its shift in bandgap. The bandgap shifts from 1.21 eV up to 1.45 eV,

which is the optimum bandgap single-junction cells under AM0.

InP Cell

This InP cell had typical temperature/intensity dependence over the entire range of its measurements. The

voltage slope did change at temperatures below -75°C, but the change was not as much as seen on the previous

cells. The efficiency on this ceil continued to rise over the entire temperature range, increasingby 30% from room

temperature down to -180°C.

InGaAs Cell

The InGaAs cell is grown lattice matched'(.72 eV) to InP with an inP window layer: The voltage also exhibits a

prominent slope change below -100°C. The two InGaAs cells measured here had slightly different coefficients,

which may be a function of their design (two different research labs).

GaAs/Ge and GaAs Cell

The GaAs/Ge cell was cut down from a large area cell and shows severe shunting at low intensities due to the

cutting. Full area cells had no shunting problems. This cell also had a slope change in voltage below -75°C. The

cell had a Schottky barrier at temperatures below -125°C, seen as a bend in the IV curve near Vo¢.

Low Intensity measurements were conducted on all cells at every temperature recorded above. The behavior

of I==and Vo¢followed predicted performance within the ranges of the temperature coefficients presented above.

-181-
The short circuit current varied linearly with intensity

and the open circuit voltage vad&:l with the linearly

logarithm of I=. The Fill Factor tended to follow Vo=.

The GalnP/GaAs cell at room temperature and -90°C

data follow typical temperature trends. The changes in

voltage slope at lower temperatures reflect possible

changes in dark current Io as voltage is defined in

equation 1.

TEMPERATURE CORRECTION

The basis for this paper is to attempt to create a data

base for temperature coefficients for a wide variety of

current cell structures. Use of these coefficients can be

derived from the following equation:

1 _P=,= . l__d/_. 1 W==+ 1_tiFF (4)

P,., o7" /,,,dr V=, dr FFdr

From the above equation, which is based on the

maximum power point, temperature correction can be

applied directly. Simpler techniques apply correction

1.7

'1.5

®

1

e_ 1.1

.9

5O

v- ----v Isc f

0- .... O VO C .///

... _... __J"

0 -50 -100 -150 -200

Temperature (°C)

Figure 5 Si at 1 Sun

to Vo=,I,=,and P,= (or FF), then use curve fdting to generate the IV curve. This correction works well with normal

IV curves, but does not accurately represent larger cells or arrays which contain steps or inconsistencies in the IV

curve. The following two equations can be applied on a point by point basis to generate an approximate

temperature corrected IV curve.

-a--r+ p.=_ < rjj Is)

and

ldl=

,_..,_, /]

tc: jj

(6)

The Fill Factor correction is applied to the voltage equation, but it could be used in the current equation if preferred.

Second order equations can be substituted directly for the single coefficients. In all cases, voltage goes up and

current goes down as temperature decreases. For use in arrays, series and parallel multipliers must also be used

(series cells add in voltage, parallel cells add in current).

CONCLUSION

The data presented inthis paper presents a brief overview of the temperature and intensitycharacteristics of new

cell technologies. The temperature coefficients will help create a database for mission planners. This work is a

continuation of the QULT and LILT measurements published previously (7,8). A comparison of the results of this

paper with those obtained by QULT shows'that I,= obtained with temperature-dependent spectral response is in

good agreement with I,_ dependence measured with an AM0 simulator. It should be noted that temperature

coefficients tend to vary among similar cells, and the spectrum of the X-25 simulator does not exactly match the

AM0 spectrum (it contains more infrared and less ultraviolet).

The coefficients are indicated for the typical characteristics of cells showing common trends. These common

trends are; higher bandgap cells have lower coefficients; voltage increases and current decreases with lowering

temperature; Vo=is proportionalto the log of intensity, current is directly proportionalto intensity, and fillfactor tends

to drift up to a peak and drop down.

Although multi-junctioncells offer higher efficiency than single cells,they do present problems if used over a wide

range of temperatures. Monolithic tandem cells must be designed to match current over a wide range of

-182-
temperatures, wherechangesintemperaturecausea

shift in bandgap. In both tandem cells presented here,

the bottom cell current remained relatively flat, this is

due to the bandgap shift of both cells, the spectral

window to the bottom cell remained constant. Tandem

cells measured here worked well together to -90°C and

then started to drift nonlinearly.

Most of the cells measured exhibited two slope

curves for Vo=vs temperature. This characteristic is

indicative of a change in the Ioas well as I,. Different

recombination mechanisms affect different voltage

ranges and temperatures, i.e., Hall Schottky Read,

tunneling recombination, junction recombination, and

surface recombination. The voltage slope at lower

temperatures tended to be less then near room

temperature. Within the range of temperatures

measured for most cells, a peak in fillfactor peak could

be observed; this required a second order equation for

curve fitting.

The plots shown in Figure 6 indicate that the voltage

- .015

.01

_.- O-- "---" .........

./ ..i°I"

1.1 u---e InGaAs Bottom

/-""_ =" "'= GalnPIGaAs

¢=.i" ,P---4, InP

O-----O Si i ..... =

.1

RelativeIntensity(I=AMO)

Flgure6 Voc Coefficientvs Log(Intensity) on Cells

coefficients tend to increase linearly as a function of the

log of intensity and that their slope also increase with decreasing bandgap. This trend can be mathematically

demonstrated. It can be used to extrapolate temperature coefficients for a wide range of intensities.

The authors would like to graciously thank National Renewable Energy Labs, Applied Solar Energy Corporation,

Spire Corporation, Boeing Corporation, and JX Crystals for providing cells which were used for these

measurements. The authors intend to continue to add to this data as new requirements and cells become

available.

REFERENCES

1) Ho, J.C., Barrels, F. T. C., and Kirkpatrick, A. R., 1970, "Solar Cell Low Temperature, Low Solar Intensity

Operation'. Conference record of the 8th IEEE Photovoltaic Specialists Conference, pp. 150-154.

2) Hovel, Harold J.,'Semioonductors and Semimetals, Volume 11, Solar Cells', Academic Press, 1975, pp 166-180

3) Hu, Chenming, and White, Richard M., 1983, "Solar Cells, From Basic to Advanced Systems', McGraw-Hill, pp

57-63

4) Yang, Edward S., "Fundamentals of Semiconductor Devices', McGraw Hill Book Company, 1978, pp 103-108.

5) Stella, Paul M., Pool, Frederick S., Nicolet, Marc A., and lies, Peter A., 1994, "PV Technology for Low Intensity,

Low Temperature, (LILT) Applications', First World Conference on Photovoltaic Energy Conversion, pp. 2082-

2085

6) Weizer, V. G., and Broder, J. D., 1982, "On the Cause of the Flat-Spot Phenomenon Observed in Silicon Solar

Cells at Low Temperature and Low Intensities', J. Appl. Phys., Vol. 53, No. 8, August 1982, pp. 5926-5930.

7) Jenkins, PhUlip,Scheiman, David, and Blinker, David, 1994, "Prediction Short Circuit Current of Single Junction

and Multi-Junction Solar Cells at low temperature for Planetary Missions', First World Conference on Photovoltaic

Energy Conversion, pp. 2012-2015. "

8) Scheiman, David A., Jenkins, Phillip P., Brinker, David J.,1995, "Low Intensity Low Temperature (LILT)

Measurements on New Photovoltaic Structures', 30th IECEC, 95-353, Vol. I, pp.327-332

-183-
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-185-
A SUMMARY OF THE INTERNATIONAL WORKSHOPS ON SPACE SOLAR CELL

CALIBRATION AND MEASUREMENT TECHNIQUES

D.J. Brinker, H.B. Curtis and D.J. Rood

NASA Lewis Research Center

Cleveland, Ohio

P. Jenkins and D.A. Scheiman

NYMA Setar, Inc.

Brook Park, Ohio

ABSTRACT

The first two of a planned series of internationalworkshops concerning space solar cell calibration and

measurement techniques have been held within the past year. The need for these workshops arose from the

increasing complexity of space solar cells coupled with the growing international nature of the market for space

cells and arrays. The workshops, jointly sponsored by NASDA, ESA and NASA, have the objective of

obtaining international agreement on standardized values for the AM0 spectrum and constant,

recommendations for laboratory measurement practices and the establishment of a set of protocols for making

interlaboratory comparison measurements. The results of the first two workshops, held in Waikiki, Hawaii, USA

in 1994 and Madrid, Spain in 1995, are presented.

INTRODUCTION

The market for both space solar cells and arrays has become more intemational in recent years. At the

same time, space cell design is becoming increasingly complex with a corresponding increase in the difficulty

of providing accurate on-orbit performance predictions. Thus the need for the universality of calibration and

laboratory measurements was recognized and a series of workshops concerning these issues has been

initiated. The workshops, of which two have been held and a third is planned, have been jointly sponsored by

the European Space Agency (ESA), the National Space Development Agency of Japan (NASDA) and the

National Aeronautics and Space Administration (NASA). The objectives of the workshops include agreement

on standardized values of the Air Mass Zero (AMO) solar constant and spectral intensity distribution,

recommendations for laboratory measurement practices and establishment of a set of protocols for making

interlaboratory comparison measurements. The international intercomparison will include both primary

reference standards and laboratory techniques. The workshops were to be held at approximately one year

intervals in conjunction with major space photovoltaic conferences in order to maximize attendance.

1st WORKSHOP

The First International Workshop on Space Solar Cell Calibration and Measurement Techniques was

organized by the NASA Lewis Research Center. It was held on December 12-13, 1994 in Waikiki, Hawaii,

directly following the 1st World Conference on Photovoltaic Energy Conversion. Thirty-four participants from

England, France, Germany, Japan, Spain and the United States attended the workshop. Their affiliations are

listed in Table I. During an opening plenary session, each of the sponsoring agencies presented their

objectives for the workshop. Furthermore, it was decided that smaller working groups would be optimum in

light of the diverse topics to be addressed and the short time (two days) allotted for the workshop. The three

-186-
Table I - Affiliationsof 1st Workshop Participants

Adv. Eng Services Co.

Hi-Reliability Comp.

ISAS

JQA

NASDA

Opto Research Corp.

Sharp Corp.

Wacom R&D Corp.

M. Wantanabe & Co.

United States

Hughes Space & Com.

NASA LeRC

NREL

Spectrolab

USAF/Phillips Lab

Deutsche Aerospace

DRA

EEV Ltd.

ESA-ESTEC

ESTI

Fraunhofer Institute

INTA/Spasolab

working groups would discuss primary standard solar cells, multijunction devices and laboratory practices.

Reports from each of the three groups would be presented at a closing plenary session, with written reports to

be prepared and distributed after the close of the workshop.

Primary Standard Solar Cells

The need for accurate laboratory measurement of space cells is increasing as the cells become more

complex and mission planners demand ever more precise guarantees of on-orbit performance as the margins

in power systems shrink. These laboratory measurements are possible only with the use of primary (or

reference) standard cells. Historically, primary standards have been made through the use of high altitude

balloons (JPL and CNES), Shuttle experiments (NASA and ESA) and high altitude aircraft (LeRC). The two

balloons and the aircraft are the only methods currently utilized. Concems of primary standard cell users were

voiced in the working group and include: the seasonal nature and limited space available on calibration flights,

the cost of an independent calibration program prohibits users from generating their own standards, and the

aging of standard cells, creating the need for regular re-flight. An indoor (laboratory) method of primary cell

calibration was proposed. It is based on the the measurement of the spectral response of the cell and the

spectral irradiance of the solar simulator lamp. A number of disadvantages and concerns with this method

were identified and a comparison with traditional methods was recommended before any further action could

be taken on the proposal. The conclusions and recommendations of the working group were: 1) increased

opportunities to create space-based primary standards are needed, 2) a research effort to measure the AM0

spectrum should be advocated, 3) an international standard for the AM0 spectrum and constant needs be

chosen and, 4) an international intercomparison of primary standards should be established.

Multijunction Devices

Multijunctionsolar cells present new problems because of the requirement for current matching of the

two or three cells of the devices currently under development. Not only are primary standards of complete

devices necessary, but calibrated subcells will most likely be required. Because the different subcells in the

multijunction device generally have different radiation-hardness characteristics, care must be taken in

differentiating between Beginning of Life (BOL) and End of Life (EOL) performance, with each condition

requiring a set of primary standards. Accurate determination of temperature coefficients is also required, with

the different subcells having different temperature coefficients. Most single source solar simulators, unless

carefully filtered, are inadequate for determination of these coefficients. Precision current-voltage

characterization of subcells and full MJ devices will require spectrally tunable solar simulators and subcell

standards. The working group presented the following issues and recommendations to the full workshop: 1)

determination of how well reference cells must be spectrally matched to test cells to avoid spectral mismatch

corrections, 2) optimization of spectral adjustment of simulator for production testing, 3) perform an

-187-
uncertaintyanalysis to allow extrapolation of laboratory measurements to actual space performance and, 4)

round robin cross-checking.

Laboratory Practices

It was recommended in that the World Meteorological Organization's (WMO) solar spectrum and

constant (136.7 mW/cm2) be adopted as a standard. However, each organization will continue to use its own

standards until formal agreement is reached. For intercomparison, a cell temperature of 25 °C will used. Full

area illumination for spectral response/quantum efficiency measurements and total area for efficiency

calculations was also recommended. A round robin intercomparison was proposed for current-voltage and

spectral response measurements. Each of the three sponsoring agencies will provide cells as follows:

Celltvpe Irradiated Japan Europe United States..

Silicon No X X X

Silicon Yes X X X

GaAs No X X

GaAs Yes X X

Hi Eft. Si No X

Hi Eft. Si Yes X

A solar cell holder compatible with both the JPL and CNES balloons was designed and will be used to

mounting the cells designated for the intercomparison.

2nd WORKSHOP

The final action of the 1st Workshop was agreement as to the necessity of a second workshop, to be

held within the next year. This workshop, organized by the European Space Agency and hosted by the

Spanish National Institute of Aerospace Technology (INTA), was held in Madrid on September 12-13, 1995,

directly following the Fourth European Space Power Conference. Twenty-five participants from China,

England, France, Germany, Japan and the United States attended; the affiliation of these attendees is given in

Table II. The three working groups from the 1st Workshop reported their findings at an opening plenary

session. The primary objectives of the Workshop were to conclude the discussions begun in the 1st

Workshop and to finalize and implement plans for the round robin comparisons. As in the case of the 1st

Workshop, smaller working groups were formed. These groups were primary standards, AMO solar spectrum

and constant; laboratory measurement practice; multijunction devices and round robin measurements.

Primary Standards, AM0 Solar Spectrum and Constant

The tentative agreement on the WMO solar spectrum and constant from the 1st Workshop was

reaffirmed. Participants in this worksh_o agreed to survey the "radiometry" communities in their respective

countries for recommendations on AM0 spectrum and intensity. A temperature of 25 °C was decided upon for

all laboratory current-voltage and spectral response measurements. The calibration of primary standards was

discussed, with continued reservations about the indoor methods expressed. It was decided that calibration

methods be designated as either "synthetic" (laboratory-based, spectrally corrected) or "space-based" (high

altitude balloon or aircraft, Shuttle or similar flight experiment).

Laboratory Measurement Practice

A general agreement was reached on 25 °C as a reference temperature for all round robin measure-

-188-
TableII- Affiliationsof 2nd Workshop Participants

United States Europ__

Adv. Eng Services Co. NASA LeRC CNES

Hi-Reliability Comp. Deutsche Aerospace

ISAS EEV Ltd.

JQA China ESA-ESTEC

NASDA China Acad. Space Tech. Fraunhofer Institute

Opto Research Corp. INTA/Spasolab

Sharp Corp.

ments. After a discussion Of laboratory practices at the various institutions represented in the working group, it

was decided that two areas that would best benefit from a comparison of results would be the determination of

temperature coefficients and spectral response characteristics. The cell complement would be the same as

that designated during the 1st Workshop, except the cells would not be mounted. The temperature range for

temperature coefficient determination would be +20 to +80 °C, all results would be blind until full completion of

the testing. ESA, NASDA and NASA LeRC would participate. The same cell set will be used for the spectral

response measurements, with test conditions left up to the individual agencies but fully documented. An

exchange of test procedures for common laboratory measurements was agreed to by ESA, NASDA and

NASA.

Multijunction Devices

Various measurement techniques and equipment are in use for characterizing multijunction solar cells.

The strengths and weaknesses of the techniques, as well as any previously unforeseen anomalies, need to

be identified. This can best be accomplished through a MJ solar cell measurement investigation. It will not be

a comparison of results, but a practical opportunity to assess measurement techniques for real space cells.

NASA LeRC will obtain and mount about 10 GalnP/GaAs two junction solar cell. LeRC willtest the cells on the

Lear aircraft (both Isc and IV measurements) and distributethem to the following possible collaborators:

United States Europe Japan

Agencies: LeRC ESTEC NASDA

JPL INTA ISAS

CNES

DRA

ISE (Freiburg)

After testing at the various agencies, the cells will be returned to LeRC for reflight on the Lear aircraft. All

resultswill be distributed to all participants.

.=

Round Robin Measurements

A working group devoted to round robin measurements was created at the 2nd Workshop. However,

its results mirrored some of those from the working groups previously discussed. They suggested blind

round robin of current-voltage and spectral response measurements be conducted and agreed with the MJ

working group that a around robin for multijunction cells is premature. A round robin of primary calibration

standards would be conducted between the agencies currently calibrating cells, i.e., CNES, JPL and NASA

LeRC. Each participant will provide two cells, one silicon and one gallium arsenide, six cell in all. The

intercomparison will be conducted fully blind, with all data to be distributed only after the calibrations are

complete.

-189-
The 2nd Workshop ended with a plenary session at which time the four working groups reported their

results. Preliminary written reports from two of the groups were distributed, with a final, complete Workshop

Report to be distributed by year's end. A third workshop was decided upon, as a forum for reporting and

discussionof the results of the several measurement activities begun at this workshop. The 3rd Workshop will

be held in Japan in November, 1996, following the 9th Intemational Photovoltaic Science and Engineering

Conference. The workshop will be hosted by NASDA.

CONCLUSION

The first two of a planned series of Intemational Workshops on Space Solar Cell Calibration and

Measurement Techniques have been held during the past year with some forty participants from six countries.

The need for these workshops arose from the increasing complexity of space solar cells coupled with the

growing international nature of the market for space cellsand arrays. The objectives of the workshops include

agreement on standardized values of the (AM0) solar constant and spectral intensity distribution,

recommendations for laboratory measurement practices and establishment of a set of protocols for making

interlaboratory comparison measurements. Toward meeting these goals, three round robin measurement

activities have been started: 1) primary calibration standards, 2) laboratory current-voltage and spectral

response characterization and 3) laboratory temperature coefficient determination. A multijunction cell

measurement investigation will also be conducted. The results of these activities will be reported and

discussed at a third workshop, to be held in Japan during November of 1996.

-190-
RevlQw of Recent Thermonhotovoltalc fTPV) Research at Lewis Research Center

Donald L Chubb, Brian S. Good and David M. Wilt

NASA Lewis Research Center

Cleveland, OH 44135

and

Roland A. Lowe, Navid S. Faternt and Richard H. Hoffman

Essential Research, Inc.

Cleveland, OH

and

David Scheiman

NYMA, Inc.

Brookpark, OH

ThermophotovoP,aic (TPV) research at NASA Lewis Research Center that began in the late 1980's is reviewed.

This work has been concentrated on low bandgap indium gaBurn amenide (InGaAs) PV cells and rare earth -

yttrbm aluminum garnet (YAG) thin film selective emitters, as well as, TPV system studies. An emittarme theory

has been developed for the thin film emitters. Expedmar_al spectral emittance results for erbium Er-YAG and

holmium Ho-YAG show excellent emittance (p...7)within the emission bands. The .75 eV InGaAs PV ceils

fabricated at Lewis have excellent quantum efficiency. An eff'¢lency of 13% has been measured for this cell

coupled to an Er-YAG selective emitter and a shodpass IR filter.

Thermophotovoltalc (TPV) energy conversion is not a new concept. It has its origine at MIT in the late

1960's. Early TPV work was confined to the use of siliconSi photovoltalo (PV) cells, which require high

temperature (p..2000K)emitters to make aneffident system. In recent years, however, two significant

advances have occurred that make poss_oleefficient TPV energy conversion at moderate temperatures

(<2000K). The first of these developments is efficient low bandgap photovo¢:_lo(PV) cells such as gallium

antimonide (GaSb) and indium gallium arsenide (inGaAs). Efficient selective emittem that have single strong

emission bands at photon energies that match the PV cell bandgap energy is the second important

development.

Before beginning the review of the TPV research at Lewis a definition of a "I'PV system is in order.

Figure 1 is a schematic of a general TPV system. Any thermal energy source, such as the sun, combustion or a

nuclear reaction can be the input energy for the system. However, the thermal energy must be converted to

radiant energy that can be efficiently converted to electdcaJenergy by the photovoltaic (PV) cells. Thus the

radiant energy must have a photon energy equal to or greater than the PV cell bandgap energy. There are two

methods for obtaining the bandgap-matched radiation. E_har a selective emitter, which emits most of its

-191-
energy in a single bendgap-matched emission bend, or a bandpass filter - grey body emitter combination,

which results in a single bendgap-matched emission bend can be used to obtain efficient TPV conversion.

Therefore, there are two basic TPV systems; selective emitter or filter. In this paper we review the research on

selective emitters, low bandgap InGaAs PV cells, as well as, TPV systems studies.

11th{thermalellidency) x TIEf (emitler-filerefidmcy) x qW (PVelldency) = 11T(systemellidemy)

sun

combuslon

gN

radioisolq:)eUoc_

--E

--E

_-_ er

FigureI Thennophoto_llalc('TPV)EnergyCormemlonConcept

SelQctlve Emitters

The ideal selective emitter would have a single emission band with an emittance approaching one

within that band and negligible emittance outside the emission band. For the photon energy or wavelength

region of interest in TPV (1-31un), an electronic transitionof an atom or molecule is required to produce the

desired radiation. However, when atoms are compressed to solid state densities the emission is not

characterized by narrow line emission as with an isolated atom, but by a continuous emission spectrum.

Most solid state materials behave like a grey body emitter, namely they have nearly constant spectral

emittance. The only way to obtain a selective emitter isto find a materialthat behaves like an Isolated atom. At

lowdensities such as with a gas or plasma the emissionis likethat of an iso_ed atom. Therefore, the first

selective emitter we looked at was a cesium plasma (1). And infact the emitter efficiencywas large (>.7).

However, a plasma emitter has two problems. Rmt of all the operation temperature is too high and secondly

the low plasma density means the power density in the emission band is low.

Fortunately,there are a group of atoms that at solid state densities behave nearly I_e isolated atoms.

These are the rare earth atoms. For doubly and triplycha_ed ions of these elements in cn/stals the orbits of

the valence 4f electrons, which accountfor'emission and absorption, lie inside the 5s and 5p electron orbits.

The 5s and 5p electrons "shield"the 4f valence electronsfrom the surrounding ions in the crystal. As a result,

the rare earth ions inthe solid state emit in narrow bands ratherthan in a continuous grey body manner. For

temperatures of interestthe rare earths have one strong near-infrared band associated with electron

transitionsfrom the lowest energy level to the ground state The spectra of these rare earth ions in crystals

have been extensively studied. Most of this work is summarized inthe text of Dieke (2).

The first selective emitter investigation (3) of the rare earths in oxide form showed the strong infrared

emission bends. However, emittance outside the emission band was also large so that the emitter efficiency

was low. In the late 80's Nelson and Parent (4, 5) reported a large improvement in rare earth oxide emitters.

Their emitters are constructed of bundles of small diameter (5-10 lun) rare earth oxide fibers similarto the

construction of the Welsbach mantle used in gas lanterns. The very small characteristic dimension of these

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• . . - ,

emitters results in low emittance outside the emission band and thus greatly increased efficiency. At the

Auburn Space Power Institute fibrous rare earth oxide emitters are being fabricated by a paper making process

(6). The fibrous selective emitters are well suited to a combustion driven system were the fibrous mantle

surrounds the flame. However, for coupling to other thermal energy sources likely to be used for a space

power system, such as nuclear or solar, the fibrous emitter is not so well suited. As just stated, itwas the small

characteristic dimension that made the fibrous emitters efficient. Another geometry for achieving a small

charactedstiodimension and also easily coupling to any thermal source is a thin film. A thin film containing a rare

earth on a low emittance substrate can be easily attached to any thermal source. In addition, a thin film is mere

durable than a fibrous geometry. Therefore, we began theoretically and experimentally investigating rare earth

containing thin film selective emitters (7, 8). Until now, the most successfulthin film emitter is yttrium aluminum

garnet (YAG, Y3AisO12)doped with rare earths (9). The rare earth doped YAG, where the rare earth ion

replaces the yttrium ion, is grown as a single crystal and then cut and polished to the desired thickness. A low

emittance, opaque substrate must be placed between the emitter and thermal source to block the grey body

emission from the thermal source.

Although YAG can be doped with any of the rare earths most of our research has been confined to Er -

YAG and Ho -YAG (9, 10, 11, 12). W'dhemission bands at Zb=1.55 pm and ;Lb=1.95 wn, respectively, they are

of most interestfor TPV application. The theoretical analysis of the thin film emitter is based on one

dimensional radiative transfer theory (8, 13, 14) that includes scattering, as well as, emission and absorption, ff

scattering is includedthe theory is quite complicated. However, neglecting scattering allows an analytic

solution for the spectral emittance, e_.(13).

Wheresxs is the substrate emittance, ffxois the reflectance at the emitter surface, rk is emitter material

index of refraction, KXd = otxdis the emitter optical depth where a_.is the extinction coefficient (= absorption

coefficient for no scattering) and d is the emitter thickness. Other quantities appearing in eq. (1) are the

following.

2 K

h+= I-4pxo(I-e_,)p..E3(K_)E3( ''2"_) (2)

It..

2 K_

h_ = E3(K _) -p.E3(--)

IJ'm

(3)

D).= 1-4pxo(l - e_,)E 32 (Ku) (4)

2

It,.: 1-1/n_. (5)

I

E3(x ) = lue-,/=du = 3"I_-3/2_ c _l:)o-nentiai Integral of order 3 (6)

O

As eq. (1) indicates the parameters that determine the spectral emittance are the optical depth, which

depends on the extinction coefficient and film thickness, the substrate emittance, emitter material index of

refraction, and reflectance at the emitter surface. Refraction at the emitter surface is accounted for by the

quantity Pm---cos Om,where Omis the maximum angle of incidence at the emitter surface for radiation to escape

the film (sin em= l/r_ from Snell's Law for refraction).

The dependence of ¢_.on Kxd is approximately 1 - exp (-3Kxd/2) since _ (x) -- 1/2 exp (-3x/2).

Equation (1) also shows the importance of substrate emittance. For regions outside the emission band where

ctxand thus KXdis small (E3 (0) = 1/2) the emittance will be dominated by the substrate emittance, eXs. Thus it

is important to have low substrate emittance to minimize the emission outside the emission band and thus

maximize the emitter eff'¢iency. Low exsalso impliesthe reflectance (1 - z-_ ) at the substrate approaches 1

which means that a signit'c,ant contribution (1 - ¢x_term in eq. (1)) to the emission band results from reflection at

the substrate. Therefore, low substrate emittance minimizes emission outside the emission band where a_.is

-193-
smallandincreasesemissionwithintheemissionbandwhereax is large.

Knowing the spectral emittanoe, _, the emitter efficiency, qE, defined as follows, can be determined.

I_E'-

power emittedinemissionband x= (7)

0

toudpower emitted

Where e_b is the black body emissive power and Zu is the short wavelength limit of the emission band and _ is

the longwavelength limit of the emission band.

(s)

f"°°l J-,]

And h is Planck's constant, co is the vacuum speed of light, k is Boltzmann's constant and TE is the

emitter temperature. We have found (11,12) that the spectral emittanoe can be splitinto 4 bands. For the

region 0 _;_.< _ the spectral emittance is nearly a constant, _. Within the emission band (_ =;_.=;24)the

emittance shows some variation but can also be approximated as a constant, _. For Xi< _.< X¢ the

emittance is also a constant, _, where Zc= 5 izmfor YAG. For Z¢ < _.<0o the extinctioncoefficient for YAG is

large so that the emittance, _ -"1.0. Therefore, if ¢_.is approximated by the four band model just described

equation (7) becomes the following.

11s= [ I+ _, F(ux,u=) G,(u=) + ¢= H(u=) + e= L(u=) 1-' ¢, 8, F(u,,u=) Iz,F(u,.u.)

Where,

F (upu=) = dx u I- , u= "_

u 3

o.(.°I=

- X 3

H(u.)=f-:--:, d=

°,¢ -1

f¢4 . X3 hc °

L(u,)= -_- -uj" _dx u=- X=kTE

¢ ,,.

(9)

(lO)

(11)

(12)

(13)

As equation (9) shows the emitter efficiency depends on the ratio of the emittances outside the

emission band (_, _, _ ) to the emittance for the emission band, _, as well as, the emitter temperature, TE,

and the emission band location (_ & ;_) and the cutoff wavelength, ;%. For given values of the emittance

ratios and emission band location there is an optimum temperature for maximum efficiency (7). Lowe (11, 12),

et al. have experimentally found that the optimum "rEfor Ho-YAG with an emission band centered at _.= 1.95

pm is approximately 1650K. For Er-YAG, which has an emission band at Xb= 1.55 _ the optimum TEwill be

higher. Theoretically (7, 8), it has been found that forthe case where ¢1= % = _, maximum qE occurs when

Eb/ k TE = 4, where Eb = hCo/ Zb= (hCo/ 2) (1/Zu + 1/7_ ) is the photon energy at the center of the emission

band.

-194-
As mentioned earlier, the rare earth oxide fibrous emitter is effident because of the small dimension of

the fibers. In the care of the thin film emitter there will be an optimum thickness for maximum efficiency. This

can be seen by considering equation (g). For a given TE and emission band, the efficiencywill be a maximum

when the ernittance ratios _ / £b, _/_) and _c/ _bare a minirnurrLAs already mentioned in discussingthe

spectral emittance (eq. (1)), the ernittance has nearly an exponential dependence 1 - exp (-3K_/2) on optical

depth, K_ (=o_d). Therefore, for d - 0, _- ¢x=and therefore ¢1- ¢.u- _ - _. As d increases _ will increase

much faster than _, £uand £c since c_ >> cr.i,_ and ¢c- constant. As a resultthe emittance ratioswill decrease

and qE will increase to a maximum for some optimum d. For d greater than the optimum value the emittance

ratios will begin to increase since_ will be nearly constant at its asymptoticvalue for KM --= while q and _WIB

be still increasing. Therefore, TIEwill start to decrease. Lowe (12), et al. expedmentally found the optimum

thickness for Er-YAG to be .7 to1.0 rnm. Since this is rather a large dimensionfor a thin film a more appropriate

title should be "thick"film selective emitter rather than thin film selective emitter.

Now consider experimental results for Ho-YAG and Er-YAG selective emitters. The first parameter of

interest in evaluating a selective emitter is the extinctioncoefficient c_.since the spectral emittance depends

primarilyon c_.(eq. (1)). The extinction coefrclent at room temperature was calculated using measured values

of the spectral transmittance and reflectance (9). Rgure 2 shows the extinctioncoefficients of Ho (25%) -YAG

and Er (40%) -YAG (the percentages refer to the percentage of Ho and Erions that replace the yttdum ions in

the YAG clystal). As can be seen the extinction coefficients within the emission bands are large (> 10 cm-l)

compared to regions outside the emission bands where c¢_,< 1.0 crn-1, Therefore, we would expect the same

result when the spectral emittance within the emission bands are compared to the spectral emittances outside

the emission bands.

25 .................. 50

i,0 j,0

-- 10 20

5 i 10

¢/J

0 0

1700 1800 1900 2000 2100 2200 1300

Wavelength,;Lnm

1400 1500 1600

Wavelength,;Lnm

1700

Rgure2(a)Ho(25%)- YAG Rgure2('o)Ep(40%)- YAG

Rgure 2 Extinction Coefficient from Measured Transmission and Reflectance

(a) Ho(25%)-YAG (b) Er(40%)-YAG.

-195-
1 " " : i " ' - ' i' - • - i - - - 1

O.8

i

0.6 i/ '11".." / " 0.6

= ..y _v, "... _ 0.4

0 ................. Z

1400 1600 1800 2000 2200 2400 0

Wavelength; ). nm

Figure 3 (a) Ho (25%)-YAG

l_abr, I_'perlment o al

""/ No scalterlng

I:k-O

EqualscaNerlnoand

rpUon coefllcents

i _ -O.S

.. .

1200 1400 1600 1800 2000 2200 2400

Wavelength,_.nm

Figure 3(b) F:r (40%)-YAG

Figure 3 Theoretical and expedmental spectral emittance _.with platinumfoil substrate. Theoretical

calculations assumed constant substrate emittance (_ =.2) and index of refraction for YAG (nf =1.9).

(a) Ho(25%)-YAG,.65mm thick; front surface temp. = 1380K, back surface temp. =1616K, average

temp. = 1500K. (b) Er(40%)-YAG, 1.04ram thick, front surface temp. =1398K, back surface temp. =

1606K, average temp. = 1500K.

In Figure 3 the measured spectral emittances at an emitter temperature of 1500K with platinum

substrates are compared to the theoretical emittances. The emitter temperature is the average of the front

(emitting surface) and back (substrate surface) surface temperatures. The theoretical calculations (8, 14),

which include scattering, assumed a constant substrate emittance ( _ = .2) for platinum and a constant index of

refraction (nf = 1.9) for the emitter. Also, the theory used the measured, room temperature extinction

coefficient and assumed a constant emitter temperature. Since these calculations were performed the theory

has been modified to allow a linear temperature variation across the emitter (14). In comparing the theoretical

and experimental _. results, several features should be noted. First,the emittance in the emission bands is

large. Second, resultswith small scattering (£Z_.< .5) agree mere closelywith the measurements. Third, the

measured emittance outside the emission band is considerably higherthan the theoretical values. Part of this

discrepancy results from experimental error. In wavelength regions of low emittance, background radiation

becomes signif'cant so that the measured emittances are larger than the actual values (9, 10). Finally, the

theoretical emittance outside the emission band is smallerthan the assumed platinum substrate emittance

(zs = .2). This result occurs because part of the substrate emission is totally reflected at the emitter surface due

to the refractive limit(14m2term in h_term of eq. (1).

To summarize the selective emitter discussion the following points should be emphasized. Rrst of all,

the thin film Ho-YAG and Er-YAG selective emitters have large emittances (> .6) within their emission bands.

However, the emittance outside the emission bands is also larger than predicted. For _.> 51zrnYAG has a large

extinction coefficient therefore the emittance for _,> 5wn will be large. For _.< 5 run the source for most of the

emission outside the emission bands is the substrate. Therefore, it is essential that the substrate emittance be

small in order to obtain good emitter efficiency. As pointed out earlier, low substrate emittance (high substrate

reflectance) Increases the emittance withinthe emission band as well. Currently, we are investigating other

host materials for the rare earth ions. Also, other low emittance substrate such as rhodium are being

investigated.

-196-
INDIUM GALLIUM ARSENIDE PHOTOVOLTAIC CELLS

The TPV need for a low bandgap energy photovoltaic (PV) cell is the main reason the Lewis research

program (15,16) on indiumgallkJmarsenide (InxGal-x As) PV cells was initiated. Inx Gal-x As is a direct bandgap

semiconductor material that has a bandgap ranging from .35eV to 1.42eV depending on the IrVGa ratio. It is of

interestfor tandem solar cells (17), as well as forTPV. Besides Lewis, InxGal.x As research is being carded out at

several laboratories including NREL (17, 18, 19), Spire Corporation (20), Research Triangle Institute (21) and

Rensselaer Polytechnic Institute (22).

Inx Gal-x As devices were grown on InP substrates by Organo Metallic Vapor Phase Epitaxy (OMVPE) in a

horizontal, low pressure reactor designed and constructed at Lewis. Details of the growth method are given in

references 15 and 16. Three different bandgap devices have been grown. The firstcell grown was In.s3Ga.47 As

with a bandgap energy, Eg = .75eV, and a lattice constant matched to InP. Also, cells with Eg = .66eV and .60eV,

which are not lattice matched to InP, were grown by incorporatingstep graded buffer layers between the InP

substrate and the cell structure. These layers are intended to minimize the density of dislocations in the active

device. The device structures are shown in Rgure 4.

I

LT,J eV InGIAI ¢li _

Au/GI Fro_ MeUdnza_bn

T.2o5 cou o H , "-

1.,smn n÷ InpWindow

021un n InG_ks Erdl_

2J wn p InGa_ Bue

Front Metal_at/on

s

0.1 ILmn÷ tnPWlndow

0.27 I_n n InGe/w (0.6eV/0.6GeV}

2.25 _ p InGa._ (0.6 eV/0.SGeV)

0.17 _ p InGaAs(0._ eV.4).89eV)

0.15 Fm p InGa/_ (030 eV/0.72 eV)

0.3 wn p InGaAs(O.TSeV_-75 eV)

0.,?.lunp_InP Buffor

Rgure4 Lat cematched(0.75eV)andmismatched(0.6010.66eV)InGaAsdevice res.

In TPV apprcations the photon generated current densities will be large (1-10a/cm2). Therefore it is critical

to reduce series resistance losses as much as possible. For this reason highly doped InP window layers are used

to reduce the resistance between the current collecting grid fingers. A balance between the absorption losses of

the window layer and the reduction in series resistance means there will be an optimum window thickness. Due to

the lattice mismatch only thin window layers can be used on the .66 and .60eV cells. AnUreflecting (AR) coatings

of tantalum oxide (Ta20s) were applied to.reduce reflection in the wavelength regions near the cell bandgaps.

Detailed performance results for the three InGaAs cells are given in references 15 and 16. In this paporwe

will consider the spectral response and the unilluminated current-voltage relations (dark diode data). In F_jure 5

the external quantum efficiency for the cells is shown. As can be seen the latticed matched, .75eV cell attainsthe

largest quantum efficiency (> 90'/0). The .66 and .60eV cells have lower quantum eff¢iency and also show a "roll

off' for the long wavelengths that the .75eV cell goes not show. Because of the maw dislocations in lattice

mismatched cells the minoritycarder I'detimeswill be short. This fact coupled with longer absorption lengthsfor

long wavelength photons accounts for the "rolloff" at the long wavelengths. Optimization of base thicknesses,

doping levels and the lattice grading structure should improve the long wavelength response.

-197-
1

0.8

_0.6

_ 0.4

 0.2

w

0

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Z

Wavelength (prn)

F_jure 5 External Quantum Efficiency measurements of InGaAs

phototvoltaic devices with anti-reflective coatings.

Bandgap (eV) A J01

(Ncm^2)

Rs( ) Rsh (_)

0.75 1.01 3.6e-8 0.453 3.4e3

0.66 0.99 6.5e-6 0.431 2.5e3

22e-5 0.387 8.0e2

III

0.96

Table 1 InGaAs photovoltaic device diode characteristics for three different bandgaps.

The second column ('A') is the diode ideality factor, J01 is the reverse saturation

current (dark current), Rs is the series resistance and Rsh is the shunt resistance.

In Table Ithe dark diode data for the three coils is shown. As can be seen the dark saturation current, j Ol

increases considerably in going from the .75eV lattice matched cell to the .66eV and .60eV lattice mismatched

cells. There is also a significantdecrease in the shunt resistance, Rsh,in going from .75eV to .60eV. Also note

that the diode ideality factor A-1 for each qf the cells so that recombinationcurrent in the depletion region is

negligible. Finally, the series resistance, R=, shows a small increase in going from the .60eV cell to the .75eV coll.

The large dark saturation currents and "roll off" in quantum eff'ctency for longwavelengths for the lattice

mismatched cells result mainly because of dislocations. Currently, buffer layer design and dislocation passivation

techniques are under development to alleviate these problems. In addition a new cell structure that addresses two

important TPV issues is being fabricated. First of all, photons reaching the PV cell with energy below the bandgap

energy are a lossto the system unless they can be reflected back to the emitter where they are absorbed.

Therefore, placing a highly reflecting surface such as gold on the backside of the cell will reflect these low energy

photons back to the emitter as long as the substrate does not absorb them. The second issue is the high current

density and low voltage associated with low bandgap energy cells nocossanj for TPV. By using an army of series

connected InGaAs cells the total current can be reduced and output voltage increased thus reducing resistive

-198-
losses. The cell array structurethat uses sedes connection and also a backside reflector is the monolithicaily

integrated module (MIM) device shown in F'cjure6. In this case semi-insulating InP is used as a substrate. The

semi-insulating InP is transparent to low energy photons, which are reflected by the gold layer on the cell backside.

Reflection data for a 350pro thick semi-insulating InP wafer with polished surfaces and gold on one side is shown in

Figure 7Also shown is the reflectance of a MIM cell without an AR coating. As can be seen

• 80% reflection is achieved for the long wavelength (low energy) photons.

___zmect

#Front Contact _ .< .

...._-_.................................. _//z_ ---_=--_.... r.Ga_c_.

_"__ p LiP Conductor Layer

' - [ " " lit..... " it i

Imulat/o r DielectridGold Reflector

Figure 6 Monolithically Integrated Module (MIM) InGaAs device design.

10O

_, 80

"_ 60

2O

0 Ilia

tooo

F'_ure 7

i I I I I I I I I I I I I I I I I I I I I I I I I I I I I

i

,"_.._...r--p,,-- ,,, ^. ....... '---_''=.........

"_aA - - -t =.

sM]_ Stmcmre

tl,,l|i,,,i in lll,,i Iil,iI

1500 2000 2500

Wavelength (rim)

Spectral reflectances of polished 350 Iml thick InP wafer with

gold bac_ide arid Eg = .75 eV InGaAs MIM PV cell

To summarize the InGaAs research the following points can be made. Excellent quantum efficiency has

been measured for the three InGaAs cells investigated. However, dislocations in the lattice mismatched cells

causes a roll-off in the long wavelength quantum efficiency, as well as, reduced minoritycarder lifetimes and

therefore increased dark saturation currents. The new MIM structure, which is advantageous for TPV applications

as discussed above, is now being fabricated.

-199-
TPV SYSTEMS STUDIES RESULTR

A computer model for a general TPV system has been developed at Lewis to determine overall TPV

system performance (23). The model has been applied specificallyto a solar driven thermophotovoltaic (STPV)

system (24) in additionto the general case where the thermal source is not specified (23). Details of the model can

be found in references 23 and 24. In this paper we present results comparing filter and selective emitter TPV

systems obtained using the systems model.

An important assumption of the model is that the wavelength range (0___.< 00) is divided into four bands.

W'dhineach band the optical properties of each of the components in the system are assumed constant. The

boundaries of each of these bands is determined by the emitter properties or the filter properties depending on

which system is being considered. As already discussed the rare earth-YAG selective emitters are well

characterized by such a four band model. Since we are comparing filter and selective emitter systems the model

used for the PV cell is the same for both systems. We assume an ideal PV cell model. That is the cell quantum

efficiency is 1.0 and the ideal diode equation applies for the current-voltage relation. We also assume the PV cell

bandgap energy, Eg, is matched to the emitter emission band energy or the filter bandpass energy. In other words

hc o hc o

_,1_ __.==m (14)

El E s

Where as discussed earlier, ;q, is the longwavelength cutoff for the emitter emission band or the filter bandpass.

We also assume a constant dimensionless bandwidth A Eb/ Eb forthe emitter emission band or the filter bandpass.

= E,)/" E,)

Eb (15)

Where Eb is the photon energy at the center of the emitter emission band or the filter bandpass and F-.uis the high

energy cutoff for the emission band or filter bandpass.

The overall efficiency TITfor a general TPV system is defined as follows.

'q'r = 11o,1"1a"l'lpv (16)

Where the thermal eff'¢iency,Thh,emitter-filter eff'¢iency, 11Ef,and PV efficiency,TIPv, are defined as follows.

power input to emitter

Tl,a- power input to thermal source (I_

input power to PV ceils for _, < _'s

Tla- power input to emitter (18)

electrical power output from PV cells

TIFV--. .

input power to PV cells for _. <_.=

(19)

For cornparing selective emitter and filter TPV systems the thermal efficiency,_th, need not be considered

assuming it is the same for both systems. Therefore, the systems model is used to calculate TIEf and TIPV for a

given emitter temperature, T E, with bandgap energy Eg being the independent variable.

The firstthingto note about a TPV system is that for a given emittertemperature there is an optimum PV

cell bandgap energy for maximum efficiency and output power density. This is illustrated in Figure 8 where results

for a selective emitter system based on the rare earth-YAG emitter at TE= 1500K are shown. For this case the

-200-
assumed emittances for the emitter are _ = .75, q = .2, _ ,, .75, _ = .2 and a dimensionless bandwidth,

&Eb/ Eb= .15. These values are representative of the experimental values obtained for Er-YAG and Ho-YAG

(9-12). Reflectance for the PV cell was assumed to be zero for all wavelengths. As can be seen from Figure 8 an

optimum bandgap energy occurs because IrlEf decreases while TiPV,increases with increasing Eg. Also, Eg for

maximum power output is slightly lowerthan Eg for maximum eff'¢iency. Although not shown in the figure, as TE

increases the optimum values for Eg move to larger values.

O.8

U

O.4

0.=

O2

0.t

0

O.2 0.4 O-6 0.8 I 1.2

pV C.*I B,ndQm r=rUqly,*V

OUtletI_,

1.4

12

1

O.8

O.8

0.4

0.4 M U 1 1.2 1.4

Pv Cd B_mpp E,-,,q_. ,V

(a) EmitterEfficiency,_IEf,PVEfficiency,qpv,& ProductTIEf Irlpv ('o)OutputPower

Figure 8 Performance of Rare Earth-YAG selectiveemitter TPV system at TE = 1500K.

Propertiesbased on experimentaldata (q= _j = 2, _ = _ = .75, ,_Eb/Eb = .15).

uealPVcen(q.Brmmefr=:_r_= 1.).

Now consider the comparison between the rare earth-YAG selective emitter system and two filter systems.

The two filter systems are the resonant-array filter developed by Edtek Corporation (25) and a combination

interference-plasma filter being developed by Knolls Atomic Power Lab (26)° The resonant array filter (RAF)

consists of a reflective metal film (gold) containing precisely aligned (lnm precision) small cross shaped holes. The

outstanding feature of this f_er is the large reflectance outside the bandpass region (rpru=.95, r==.97). However,

it also has relatively low transmission (_ =.5) in the bandpass region (),u ._;L<ZI). The dimensionless bandwidth

for the RAF isAEb / Eb= .4. With further development it is hoped that a transmission of .8 can be obtained (25).

The combination plasma-interference filter (PIF) consists of alternating layers of high and low index of refraction

materials with a coating (such as indium tin_xide) on the back sidethat functions as a cutofffilter(26) (large

reflectance for 2,• ;L=). The combination filter has good transmission (_b =.7) in the bandpass region and large

reflectances (ru=rc=.8, rr=.9)outside that region. The dimensionless bandwidth for the PIF is &Eb/ Eb = .6. Forthe

RAF and PIF systems we assumed a grey body emitterwith and emittance q = Zb = q = _ = .9. Also, we assumed

the filters were lossless (no absorption) so that r + _ = 1, where r is reflectance and _ is transmission. This is a rather

large assumption since the filter receives the entire radiative outputof the grey body emitter. Thus a small (<.1)

abserptance is a significantlossto the system (7).

As well as low absorptance, a filter TPV system must have large reflectance outside the bandpass region

(ru,rl,rc ) in order to have high efficiency. Similarly, in a selective emitter system the emlttance outside the emission

band (_, q, _.) must be low compared to the emittance within the errdssionband, _b. One method for

-201-
compensating for the significant ernittance (q = Zu= .2, zc = .75) outsidethe emission band is to use a PV cell that

has large reflectance outside the emission band and thus reflect this out-of-the emission band radiation back to the

emitter. The MIM InGa As cell discussedeadler is just such a PV cell. Therefore, for comparing the selective

emitter system and the filter systems we assumed a reflecting PV cell for the out-of-the emission band radiation in

the selective emitter system (SER). Since the filter systems have large reflectances outside the bandpass region a

reflecting PV cell is not required in those systems. For the PV cell we assumed the following reflectances;

ru=rb=.03, r_ rc=.8, which should be representative of the MIM InGaAs structure.

In Figure 9, the product of eff'¢lencies,qEf qPV, for the two filter systems (RAF AND PIF) and the rare earthYAG selective emitter system (SER) are compared at TE = 1500K. As can be seen the RAF system yields the

largest eff'miencyfollowed closely by the PIF system. Also note that the optimum bandgap energy is largest for the

RAF system. There are two reasons why the filter systems have larger effioiencythan the selective emitter system.

First of all, as discussed earlier, the filters have been assumed to have no absorptance so the calculated efficiency

is larger than for a real system. Second, the large reflectances for regions outside the bandpass region inthe case

of the filters is the main reason for their good efficiency.

A comparison of the output power for the three systems is shown in Figure 10. In this case the PIF system

produces significantly more output power than either the RAF or SER systems. The SER system produces slightly

more power than the RAF. For a TPV system the output power, PEL,is proportionalto the following quantities.

Pie- '¢bEbAEb (20)

Therefore, it is the large dimensionless bandwidth (_=b / Eb = .6) and large transmJssion-emittance product in the

bandpass region (¢b_ = .63) that accounts for the PIF system having the largest output power. Even though the

SER system has a much smaller dimensionless bandwidth (&Eb / Eb= .15) than the RAF system (AEb/ Eb = .4), the

low bandpass transmission-emittance product (_boo = .45) for the RAF system results in the RAF system having

the lowest output power. Also, note from Figures 9 and 10 that the optimum Eg for maximum power output is

smallerthan the optimum Eg for maximum efficiency. This result was pointed out earlier in discussing Figure 8.

O.4

O_

O.2

O.1

0

O-2

• • • I • • • ! " " " : • " " i • • • I " " -

Pia_W_dmnce

FbrpF)

P,monm/way

r-t=pa_

IN Col bac__n_cs mfl_or (_R)

| | . . , ! . . • ! . . = i . . . t . , •

0.4 0-8 0-8 1 1.2 1.4

PV Cell Bandgap _neegy, eV

Fkjure9 Efficiencycomparisonof icssless filterandRare Earth-YAGselectiveemitterTPVsystemsat "rE=1500K.Filter

properties;PIF(rH-_-_,,.9, rt_9_ru_r_8_rb_3_1_r_&Eb/Eb_6)_RAF(r_-t_b_t_u'-r-_-_9_rI_ru_95_rc'_97_rb_.5_1-r_

&Eb/Eb-.4).Selective emitterproperties;SER(r=_-r=u-.2, _-r=c-.7S, ru-rb-.03,rl-rc,,.8,&Eb/Eb -.15)Ideal PV cellmodel

usedforall cases.

-202-
2

1.5

0.5

0

O.2

I I ' I " " " I " " " IJ ,,° ? "

_ Pbsnm/Irderbmnce

__ RareF_.,_h-YAGSelectiveemitterwith

or(SE )

I ! I ! i

0.4 0.6 0.8 I I,,2 1.4

PV Cell BandgapEnergy,oV

Figure 10 Power output comparison of Iossless filter & Ram Earth-YAG selective

emitter TPV Systems at TE = 1500K. Same conditions as Figure 9.

Based on the results of Figures 9 and 10 it would appear that the filter systems will perform better than the

rare earth-YAG selective emitter system. However, as already mentioned if filter absorptance were includedthe

calculated filter system efficiencies will be reduced. Also, Improvements to the selective emitter efficiency can be

made by reducing the emittances and increasing the PV cell reflectances in the regions outside the emission

band. Improving the filter systems efficiency will be more difficultsince they already have large reflectance inthe

regions outside the bandpass. However, the power output of the RAF system can be improved by increasing the

bandpass transmission from its present low value (_ =.5). Power output of the SER system can also be increased

by increasing the bandwidth (AEb) by doping the YAG emitter with two rare earths with contiguous emission bands

such as erbium Er and thulium Tm (10). At the present time there is no clear winner inchoosing between filter and

selective emitter TPV systems. Two important issues not yet considered, reliability and cost, will probably be the

deciding issues in determining whether any TPV system becomes a viable energy converter for space or

commercial application.

EXPERIMENTAL InGaAs PV CELL PERFORMANCE COUPLED TO Er-YAG SELECTIVE

EMITTER

To complete the review of the Lewis TPV research we will discuss recent experimental results obtained by

coupling the rare earth-YAG thin film emitter and InxGal.xAs PV cells (27). Several combinations of rare earth-YAG

and InxGal_xAsPV cells were investigated (27). However, in this paper only the Er-YAG and In.53Ga.47As(Eg =

.75eV (1.65 run) ) results will be discussed. This is a well matched system since the emission band of Er (Figure

3b) ranges from ;Lu- 1.4Wn to ;_ - 1.71zmand the bandgap energy for In.s3Ga 47As is ;Lg= 1.65wn.

A schematic of the experimental apparatus is shown in Figure 11. The Er-YAG emitter is heated on the

inside of the door of an atmospheric fumace. Radiation from the emitter is concentrated by a highly reflecting

platinum tube on to either a therrnopile detector to measure the total radiant power incident on the PV cell or on to

the InGaAs PV cell. The measured PV cell efficiency is therefore given by the following expression.

-203-
• measured maximum output pow_ Ps.

TI_= measured total radiant input power = P":-

(21)

The maximum output power, PEL, was determined from measured current-voltage curves. It should be pointed out

that l]'pv given by equation (21) is not the same as T1PVgiven by eq. (19). The radiation inthe denominator of eq.

(19) includes only photons with energy greater than the bandgap energy (;L<)_1) whereas Pinin equation (21)

includes all photon energies. As a result, (_lPV>rl'Pv).

Figure 11 Schematic diagram of test apparatus used to measure PV efficiency.

Properties of the Er-YAG emitter and In.53Ga.47AsPV cell used inthe experiment have already been

described. For these experiments the Er-YAG emitter was .4ram thick with a platinum foil substrate backed with

silicon carbide (SIC). SiC has large emittance (= absorptance) so that there is good thermal coupling between the

furnace and the emitter. This good thermal coupling reduces the temperature gradient across the emitter. The

.75eV InGaAs cell used inthe experiment had a thinner (.05-.llUn) InP window layer than the cell shown in F'Ngure4

and an area of lcm 2.

Two systems were experimentally considered. The first system consisted of the Er-YAG emitter and the

In.53Ga.47AsPV cell. The second system consisted of the same emitter and cell but added an interference

shortpass IR filter between the emitter and PV cell. The filter had a cutoff at ;L- 1.6Wn. For _.>1.6pro the filter has

greater than .9 reflectance. For ;L< 1.6 the filtertransmission- .7. Therefore, most all radiationwith _.> 1.Pawnwill

be reflected back to the emitter when the filteris used. (In that case _l"PV - _PV ).

The I-V curves and cell performar_ parameters obtained for the two systems operating with TE = 1373K

are shown in Rgure 12. Obviously, the addition of the filter greatly improves 11"Pv(going from 2.3% to 13.2%).

However, the output power (PEL= JscVocFF) is reduced by a factor of two with the filter because of the filter

transmission loss. Use of the MIM cell structurew'_ the_back surface gold reflector will give the eff'¢iency

improvement possible with the filter but not suffer the reduction in power resultingfrom the transmission loss.

-204-
250 , ,,, , ....

200

150

5O

0

0

Jsc = 990 mA/cn_

Voc = 387 mV

FF = 0.74

Eft. = 2.2%

Area = 1.0 cm2

Ta20s AR (2400A)

z • • _ z 1

100 200 300 400

V (mV)

(a) W'_houtFilter

150

120

.-. 9O

-- 6O

30

0

,, o

Jsc = 115 mAlcm 2

Voc = 376 mV

FF = 0.74

Eft. = 13.2%

Area -- 1.0 cm2

Ta2Os AR (2400A)

I I i I | " I !' |

loo 2oo 300 400

V (mV)

(b) W'dhF_er

F'_um 12 I-V curves for the 0.75 eV InGaAs PV AR coated cell coupled to the Er-YAG

selectiveemitterat TE= 1100° C (a) withoutfilter, (b) with_er.

£.0_H_g.I,ILSID 

Considerable TPV research has been carded out at Lewis beginning inthe late 1980's. The work has

been concentrated on thin film rare earth-YAG selective emitters and InxGal.xASPV cells, as well as, theoretical and

experimental systems studies. Rare earth-YAG selective emitters with excellent emission band emittance (> .7)

have been produced. However, further reseamh is required to reduce the radiation outside the emission band.

This research is directed at lookingat new host materials for the rare earths and lower emittance substrates.

The lattice matched .75eV InGaAs PV cells fabricated at Lewis have yielded excellent quantum effk;iency.

A coil efficiency of 13.2% was measured using an Er-YAG emitter and short pass filter. However, for the lattice

mismatched InGaAs cells, dislocationsresulting from the lattice mismatch cause reduced quantum efficiency and

increased dark saturation current. A new monolithicaily integrated module (MIM) structure is being fabricated that is

better suited for TPV applications.

A TPV systems model developed at Lewis is able to model any possible TPV system. The model has

-205-
been used to compare t'dterand selective emitter TPV systems. However, since filter absoq_tance losses were not

included in the model it is not poss_le to say which system will yield the larger efrK_ency. W'dh Iossless f_ers the

model predicts that the f_er systems wig yield the largest eff'ciency.

lo

2.

3.

4.

.

,

o

.

9.

10.

11.

12.

13.

14.

17.

REFERENCES

R. A. I.owe, C. Goradia, M. Goradia and D. L Chubb, J. Appl. Physics, 68, 5033 (1990).

G. H. Dieke, "Spectra and EnergyLevels of Rare Earth Ions in Crystals" (interscience, New York, 1968).

G. F_ Guazzoni, Appl.Spectra. 26, 60 (1972).

R. E. Nelson, in Proceedings of the 32rid International Power Sources Symposium (Electrochemical Society,

Pennington, NJ, 1986) pp 95-101.

C. R. Parent and R. E. Nelson, in Proceedings of the 21st IntersocietyEnergy Conversion Engineering

Conference (American Chemical Society, Washington, D. C., 1986), Voi. 2, pp 1314-1317.

P. L Adair and M. F. Rose, in Proceedings of The First NREL Conference on Thermophotovoltaic Generation

of Bectricity, (American Institute of Physics Conference Proceedings 321, 1994), pp 245-262.

D. L Chubb, in Proceedings of the 21st Photovoitaic Specialists Conference (IEEE, Now York, 1990)

pp 1326-1342; also NASA TM - 103290, 1990.

D. L Chubb and R. A. Lowe, J. Appl. Phys., 74, 5687 (1993).

R. A. Lowe, D. L Chubb, S. C. Farmer, and B. S. Good, AppL Phys. Lett. 64, 3551 (1994)

R. A. Lowe, D. L Chubb, and B. S. Good, in Proceedings of The First NREL Conference on

Thermophotovoitaic Genera_n of Electricity, (American Instituteof Physics Conference Proceedings 321,

1994), pp 291-297.

R. A. Lowe, D. L. Chubb, and B. $. Good, in Proceedings of the First World Conference on Photovolta;c

Energy Conversion, (IEEE, 1995).

R. A. Lowe, B. S. Good and D. L Chubb,in Proceedings of 30th Intersociety Energy Conversion Engineering

Conference Engineering Conference (IECEC), (Amsdcan Society of Mechanical Engineers, 1995), vol. 2,

511-515.

D. L Chubb, R. A. I.owe and B. S. Good, in Proceedings of The First NREL Conference on

ThermophotovoltaJc Generation of Electricity,(American Instituteof Physics Conference Proceedings 321,

1994), pp 229-244. --

B. S. Good, D. L Chubb and R. A. Lowe, in Proceedings of The First NREL Conference on

Therrnophotovoltalo Generation of Electriofly, (American Instituteof Physics Conference Proceedings 321,

1994), pp 263-275.

D. M. Wilt, N. S. Faterrd, R. W. Hoffman, Jr. et al., Appl. Phys. Lett. 64, 2415 (1994).

D. M. Wilt, N. S. Fatemi, R. W. Hoffman, Jr. et aL, in Proceedings of The Fi_'t NREL Conference on

Therrnophotovoltaic Generation of Electricity (American Instituteof Physics Conference Proceedings 321,

1994). pp 210-220.

M. W. Wardass, J. S. Ward, K. A. Emery and T. J. Coutts, in Conference Record of the 23rd IEEE Photovoltaic

Specialists Conference, 1993, pp 621-627. -206-
18.

19.

20.

21.

22.

3.

4,

2_°

6.

27.

J. S. Ward, M. W. Wanlass, X. Wu and T. J. Courts, in Proceea"mgsof The Firsl NREL Conference on

Thermophotovoltaic Generation of Electricity(American Instituteof Physics Conference Proceedings 321,

1994), pp 404411.

R. K. Ahrenkiel, T. Wangansteen, M. M. AI-Jasaim, M. Wanlass and 1".J. Courts, in Proceedings of The Rrst

NREL Conference on Thermophotovoltaio Generation of Electricity (American Instituteof Physics

Conference Proceedings 321, 1994), pp 412-424.

S. Wojtczuk, E Gagnon, L Geoffroy and T. Parodos, in Proceedings of The Rrst NREL Conference on

Thermophotovoltaic Generation of Electricity (American Institute of Physics Conference Proceedings 321,

1994), pp 177-187.

P. R. Sharps, M. I_ Timmons, R. Venkatasubramantan, J. S. Hills, B. O'Quinn, J. A. Hutchby, P. A. Illes and

C. L. Chu, in Proceedings of The First NREL Conference on Thermophotovoltaic Generation of Bectricity

(American Institute of Physics Conference Proceedings 321, 1994), pp 194-201.

H. Ehsani, I. Bhat, D. Marcy, G. Nichols, J. Borrego, J. Parrington and R. Gutmann, in Proceedings of The Rrst

NREL Conference on Thermophotovoltaic Generation of Electricity(American Institute of Physics

Conference Proceedings 321, 1994), pp 188-193.

B. S. Good, D. L Chubb and R. A. Lowe, in Proceedings of The Second NREL Conference on

Thermophotovoltaic Generation of Electricity (American Institute of Physics Conference Proceedings 1995).

K. W. Stone, E F. Lelngang, R. E Drubka, D. L Chubb, B. S. Good and D. M. Wilt, in Proceedings of 30th

IntersocietyEnergy Conversion Engineering Conference (IECEC), (American Society of Mechanical

Engineers, 1995), vol. 1, pp 713-718.

A. Schock and V. Kurnar, in Proceedings of The Rust NREL Conference on Thermophotovoltaic Generation

of l_lectricity(American Instituteof Physics Conference Proceedings 321, 1994), pp 139-152.

P. F. Baldasaro, E. J. Brown, D. M. Depoy, B. C. Campbell, and J. R. Parrington, in Proceedings of The Rrst

NREL Conference on Thermophotovoltaic Generation of Electricity(American Instituteof Physics

Conference Proceedings 321, 1994), pp 2943.

N. S. Fatemi, R. H. Hoffman, D. M. Wilt, R. A. Lowe, L M. Gawerick and D. Scheirnan, in Proceedings of The

Second NREL Conference on Thermophotovoltaio Generation of Electricity(American Institute of Physics

Conference Proceedings, 1995).

-207-
Thermophotovoltaic Generators Using Selective Metallic Emitters

Lewis M. Fraas, John EoSamaras, James E. Avery

JX Crystals Inc.

Issaquah, WA

and

Richard Ewell

Jet Propulsion Labs

Introduction

In the literature to date on thermophotovoltaic ('rPv) generators, two types of infrared

emitters have been emphasized : gray body emitters and rare earth oxide selective emitters.

The gray body emitter is defined as an emitter with a spectral emissivity independent of

wavelength whereas the rare earth oxide selective emitter is idealized as a delta function emitter

with a high emissivity at a select wavelength and a near zero emissivity at all other wavelengths.

Silicon carbide is an example of a gray body emitter (Ref. 1) and Er-YAG is an example of a

selective emitter (Ref. 2). The Welsbach mantle in a common lantem is another example of an

oxide selective emitter. Herein, we describe an alternative type of selective emitter, a selective

metallic emitter. These metallic emitters are characterized by a spectral emissivity curve

wherein the emissivity monotonically increases with shorter infrared wavelengths as is shown in

Figure 1. The metal of curve "A', tungsten, typifies this class of selective metallic emitters.

In a thermophotovoltaic generator, a photovoltaic cell typically converts infrared

radiation to electricity out to some cut-off wavelength. For example, Gallium Antimonide (GaSh)

TPV cells respond out to 1.7 microns (Ref. 3). The problem with gray body emitters is that they

emit at all wavelengths. Therefore, a large fraction of the energy emitted will be outside of the

response band of the TPV cell. The argument for the selective emitter is that, ideally, all the

emitted energy can be in the cells response band. Unfortunately, rare earth oxide emitters are

not ideal. In order to suppress the emissivity toward zero away from the select wavelength, the

use of thin fibers is necessary. This leads to a fragile emitter typical of a lantern mantle. Even

given a thin Er-¥AG emitter, the measured emissivity at the select wavelength of 1.5 microns

has been reported to be 0.6 while the off wavelength background emissivity falls to only 0.2 at 5

microns. This gives a selectivity ratio of only 3. Another problem with a delta function selective

emitter is its low power density at practical temperatures because of its narrow emission

bandwidth. The concept of sele_ivity can be generalized by noting that we simply wish to

maximize the ratio of in-cell-band power to out-of-cell-band power. Using this generalized

selectivity concept and assuming a GaSb cell covered by a simple dielectric filter, we note that

the emissivity selectivity ratio for tungsten is 0.3 (at 1.5 microns) /0.07 (at 5 microns) = 4.3.

In the following sections, we note that the selective metallic emitters can be valuable in

both radioisotope TPV generators in space and in hydrocarbon fired TPV generators here on

earth.

-208-
Radioisotope TPV Generators

The Boeing Company first proposed to the Jet Propulsion Labs (JPL) the use of GaSb

infrared cells for RTPV generators arguing that these RTPV generators could outperform

currently used RTGs. Subsequently, JX Crystals licensed the GaSb cell technology from

Boeing. JPL advocated the use of a tungsten emitter (Ref. 4) while NASA Lewis advocated the

use of a Er-YAG selective emitter for this RTPV application. JX Crystals then obtained a small

contract to do a RTPV design trade study with personnel at JPL and NASA Lewis as

subcontractors. In the following, we describe the results of this trade study.

At the beginning of our design trade study, JPL had already designed the small RTPV

generator shown in Figure 2. It used a single General Purpose Heat Source supplying 250 Watts

of thermal power. Our contract goal was to evaluate altemative emitters, infrared filters, and low

bandgap cells for use in the baseline design. Our most important finding was the fact that the

tungsten emlssivity curve is in fact selective.

For safety reasons, the emitter temperature for the radioisotopesource is limited to 1375

K. If a gray body operating at 1300 K were to be used, its peak energy wavelength would fall at

2.2 microns which is well beyond the GaSb cell band edge wavelength at 1.7 microns. A gray

body emitter model then suggests either the use of cells with longer wavelength response or the

need for an ideal filter or, altematively, the need for delta function selective emitters. However,

when the black body emission spectrum is multiplied by the Figure 1 curve "A" emissivity

function, the peak power wavelength shiftsto 1.7 microns as is shown in the spectrum in Figure

3. Given a simple dielectric filter with the transmission spectrum shown in Figure 4, it becomes

apparent that a practical RTPV generator can be fabricated with readily available components.

The key components then become GaSb cells, simple dielectric filters, and a tungsten emitter.

Undeveloped temary or quaternary cells are unnecessary. Fragile rare earth oxide emitters are

unnecessary. And finally, more exotic IR filters are not needed. The performance projections

for this simple RTPV generator are given in the following table.

Table I

GPHS Emitter GaSb Cell GPHS Gross Net Electric Overall

Temp Temp Temp Supply Electric Power System

(Kelvin) (Kelvin) (Kelvin) Power Power (Watts) Efficiency

(Watts) (Watts)

1324 1298 273 250 34o4 32.7 13.1%

1325 1299 293 250 31.9 30.3 12.1%

Hydrocarbon Fired TPy Generators using Catalytic Metallic Emitters

JX Crystals has been independently developing hydrocarbon fired TPV generators for

applications here on earth. After the above space contract had terminated, personnel at JX

Crystals discovered that the short wavelength bias of the tungsten emitter could be generalized

to other metals. The Figure 1 curve "B" metal contains a common oxidizing metal catalyst. We

demonstrated the usefulness of this curve "B" metal by fabricating a hydrocarbon fired TPV

generator using GaSh cells and a Bunsen burner. Our 2 Watt demonstration unit is shown in

Figure 5 operating an AM / FM / tape "boom box". This demonstration unit consists of a propane

bottle connected to a Bunsen burner with a coil of the curve "B" metal catalyst immersed in the

flame serving as the IR emitter. The 20 GaSb cell circuit shown in Figure 6 is then wrapped

-209-
around the IR emitter converting the infrared radiant energy into electric power. An illuminated

current vs voltage curve for a GaSb cell circuit in a Bunsen burner unit is shown in Figure 7.

The actual operation of Bunsen bumer TPV electric generator will be demonstrated at the

conference.

Figure 8 shows the measured spectrum from our Bunsen bumer over the wavelength

range of I to 5 microns. Note that the spectra from the blue flame from a Bunsen bumer without

an emitter inserted shows peaks at 4.5 microns and 2.7 microns. These emissions are

associated with CO 2 and H20 vibrations in the gas phase and fall outside of the cells response

range. Then with a catalytic metal emitter immersed in the flame we see in Figure 8 the

appearance of a substantial amount of radiated power with a peak falling within the response

range for GaSb cells. The emitter temperature in this case was measured to be 1520 C

(approximately 1800 K). Figure 8 also shows that the spectrum for the catalytic emitter is indeed

suppressed beyond 1.5 microns relative to a gray body emitter with an emissivity of 0.3. This

experimental result is consistent with our expectations given the Figure 1 Curve "B" emissivity

data. If combustion occurs in the gas phase, the energy must then be coupled to the solid

emitter which could be an inefficient process. The advantage for catalytic emitters is that the

combustion occurs on the emitter surface and the energy is automatically coupled efficiently to

the emitter.

Conclusions

We note that certain metals act as selective IR emitters serving to enhance the power

emitted at shorter wavelengths and to suppress the out-of-band power lost at longer

wavelengths. This is advantageous for GaSb TPV cells responding out to 1.7 microns. For

radioisotope TPV generators for use in space, a tungsten emitter operating at 1300 K shifts the

peak power wavelength from 2.2 microns for a gray body emitter to 1.7 microns. For a catalytic

emitter operating in a hydrocarbon flame at 1800 K, the peak power wavelength shifts to 1.3

microns.

References

o

,

Pemisz, U.C., Saha,C.K.."Silicon Carbide Emitter and Burner Elements for a TPV

Converter," The First NREL Conference on Thermophotovoltaic Generation of

Electricity, American Institute of Physics Conference Proceedings 321, pp. 99-105, 1994.

Lowe, R.A., Chubb,D.L., Good,B.S.," Radiative Performance of Rare Earth Garnet Thin

Film Selective Emitters," The First NREL Conference on Thermophotovoltaic Generation

of Electricity, American Instituteof Physics Conference Proceedings 321, pp. 291-297,

1994.

,

°

Fraas,L.M., Ballantyne, R.J., Samaras, J.E., SeaI,M.R. "Electric Power Production

Using New GaSb Photovoltaic Cells with Extended Infrared Response," The First NREL

Conference on Thermophotovoltaic Generation of Electricity, American Institute of

Physics Conference Proceddings 321, pp. 44-53, 1994.

Ewell,R., Chimielewski,A., Nesbith,W. and SerceI,J. "Thermophotovoltaic Power System

Final Report," JPL D-10814, May 1993.

-210-
0.4

.Z'

o_

03

03

°_

E

LU

0.3

0.2

0.1

0

0

\

B ) catalytic metal

A)

i

tungsten

2 4 6

Wavelength (microns)

8 10

Figure l. Emissivity of Metal Emitters

Figure 2. AeroSpace RTPV Generator Design

-211-
0.8

'_0.6

0.4

0.2

0

0

/_,-.,,,_ Temperature = 1300 K _ metal emitter

I _,_ 1.74micronpeak / graybody emi_er

., _" emissivity=.

_ . ?_24 micron peak

_ I I I i

I

2 4 6 8 10

Wavelength (micrometers)

Figure 3. Spectral Emissive Power Shitt with Metal Emitter

oo 0.8

_0.6

= 0.4

P" 0.2

0

2 4 6 8 10

Wavelength ( microns )

Figure 4. Near Term Dielectric Bandpass Filter

-21 2-
Figure 5. 2 Watt TPV Demonstration

Figure 6. 20 Cell GaSb TPV Receiver Circuit

-21 3-
0.4

0.3

0.2

0.1

.._

0 2 4 6 8

Volts

Figure 7. GaSb TPV Circuit with Gas Flame / Metal Mantle Light Source

Emitter Tempertrl;ure - 1520 C

'f

i 1920 C graybody

g ,

i.

|,

3 [- / only _ "-,.,

2 / ""-.

at .m -,r__ _

1 / r,_nn.,...._

•/¢

1 2 3 4 5

WavelenOth (mtcromote4"s)

Figure 8. Emissive Power of Proprietary Emitter

-214-
InGaAsSb/GaSb THERMOPHOTOVOLTAIC CELLS

Z.A. SHELLENBARGER, M.G. MAUK, and LC. DINETTA

AstroPower, Inc.

Solar Park, Newark, DE 19716-2000

G.W. CHARACHE

Lockheed/Martin Corp.

P.O. Box 1072, Schenectady, NY 12301-1072

SUMMARY

AstroPower is developing InGaAsSb thermophotovoltaic (TPV) devices. This photovoltaic cell is a

two-layer epitaxial InGaAsSb structure formed by liquid-phase epitaxy on a GaSb substrate. The (direct) bandgap

of the In_.xGaxAs_.ySbyalloy is 0.50 to 0.55 eV, depending on its exact alloy composition (x,y); and is closely

lattice-matched to the GaSb substrate. The use of the quaternary alloy, as opposed to a ternary alloy -- such as,

for example, InGaAs/InP -- permits low bandgap devices optimized for 1000 to 1500 °C thermal sources with, at

the same time, near-exact lattice matching to the GaSb substrate. Lattice-matching is important since even a

smalldegree of lattice mismatch degrades device performance and reliability and increases processing

complexity.

Internalquantum efficienciesas high as 95% have been measured at a wavelength of 2 microns. At

1 micron wavelengths, internal quantum efficiencies of 55% have been observed. The open-circuit voltage at

currents of 0.3 Ncm 2 is 0.220 volts and 0.260 V for current densities of 2 Ncm 2. Fill factors of 56% have been

measured at 60 mA/cm2. However, as current density increases there is some decrease in fill factor. Our results

to date show that the GaSb-based quaternary compounds provide a viable and high performance energy

conversion solution for thermophotovoltaic systems operating with 1000 to 1500 °C source temperatures.

1. INTRODUCTION

We report our latestresults on InGaAsSb thermophotovoltaic (TPV) cells. TPVs are p-n junction

semiconductordevices that convert photons emitted by a heated source directly into electrical power. For TPV

systems utilizing thermal radiation from an emitter heated at 1000 to 1500 °C, there is a need for Iow-bandgap

cells with a high spectralresponse in the range of 1500 to 2500 nm wavelength. This implies a TPV cell with a

bandgap of ~0.5 eV. One important potential application is the radioisotope General Purpose Heat Source

(GPHS) where 1100 °C blackbody radiation _n be used for thermophotovoltaic energy conversion. In this paper

we describe high-efficiency TPV devices based on lattice-matched Ino.osGao.92Aso.o7Sbo.93 (EG = 0.53 eV) epitaxia]

layers on GaSb substrates. To our knowledge, this is the first report of the InGaAsSb quaternary alloy applied to

TPV devices.

Several theoretical studies have indicated that photovoltaic cells based on the InGaAsSb quaternary alloy

are good candidates for TPV applications that require high spectral response in the 1500 to 2500 nm wavelength

range. Depending on its alloy composition (x,y), the direct bandgap of the In_.xGaxAsl.ySbyalloy varies from

0.18 eV (InSb) to 1.43 eV (GaAs). The quaternary alloy can be closely lattice-matched to the GaSb substrate

provided the composition is restrained to values such that y = 0.1 + 0.9 x. With this lattice matching condition, the

-215-
bandgapofthequaternaryalloy ranges from approximately0.3 to 0.7 eV. However, there is a further limitation

due to a wide solid-phase miscibility gap in this quaternary at typical growth temperatures. The miscibility gap

evidently precludes bandgaps in the range of 0.35 to 0.5 eV. Therefore, for the spectral range of interest, we

assume the lowest attainable bandgap is 0.50 to 0.52 eV. This bandgap range corresponds to an optical

absorptionedge of 2380 to 2480 nanometers.

It is worth emphasizing that the use of the quatemary alloy, as opposed to a ternary alloy--such as, for

example, InGaAs--provides the needed bandgap with, at the same time, near-exact lattice matching to the GaSb

substrate. Lattice-matching is important since even a small degree of lattice mismatch degrades device

performance and reliability. Although there are epitaxy techniques to partially ameliorate effects associated with

lattice mismatch of ternary alloy layers on binary substrates (e.g. defect-filtering superlattices, interrupted growth

regimens, etc.), we believe the use of the quaternary alloy to avoid lattice mismatch altogether is a simpler and

more effective approach.

The TPV device we are making is a two-layer epitaxial InGaAsSb structure formed by liquid-phase epitaxy

on a GaSb substrate at a growth temperature of 515 °C. Liquid-Phase Epitaxy (LPE) is a well-established

technology for III-V compoundsemiconductor devices. A major advantage of LPE for this application is the high

material quality, and more specifically,the long minority carder diffusion lengths, that can be achieved. This

results in devices which are equal or superior in performance to those made by other epitaxy processes such as

molecular beam epitaxy (MBE) or metal organic chemicalvapor deposition (MOCVD). Another major advantage

is that LPE is a simple, inexpensive, and safe method for semiconductordevice fabrication. Significantly, the LPE

process does not require or produce any highly toxic or dangerous substances-Jn contrast to MOCVD. Also, the

epitaxial growth rate with InGaAsSb LPE is ~2 microns]minute which is ten to hundred times faster than MOCVD

or MBE. We have successfully scaled up the LPE process for epitaxial growth in a semi-continuousmode on

3-inch diameter wafers. This, combined with the high growth rates, will dramatically improve the manufacturing

throughput compared to traditional and more costly epitaxy processes. Our objective is to develop an epitaxial

growth technology to produce low-cost, large-area, high efficiency TPV devices.

2. EPITAXIAL GROWTH AND FABRICATION OF InGaAsSb TPV CELLS

InGaAsSb photodiodes, light-emitting diodes, and double heterostructure injection lasers made by liquidphase epitaxy have been previously reported. We have adapted this technology for the production of lnGaAsSb

TPV cells.

We use a standard horizontal slideboat technique for the liquid-phase epitaxial growth of the InGaAsSb.

The graphite slideboat is situated in a sealed quartz tube placed in a microprocessor-controlled, programmable,

three-zone tube furnace. The growth ambient is palladium-diffused hydrogen at atmospheric pressure with a flow

rate of 300 ml/min.

The substratesare 500-micron thick, chemically polished (100) oriented, n-type GaSb wafers obtained

from MCP Wafer Technology, Ltd. (Milton Keynes, UK) or Firebird Semiconductor, Ltd. (Trail, BC, Canada).

17 3

Substrates are doped to 3-5 x 10 ore with tellurium. The substrateresistivity is 9 x 10_ .Q.cm, and the average

etch-pit density is approximately 1000 cm2. °

The growthsolutionsare indium (x_,=0.59), gallium (xGa=0.19),antimony (Xsb=0.21),and arsenic

(xA,=0.01). The melts are formulated with 3- to 5-mm shot of high purity (99.9999%) indium, gallium, and antimony

metals and arsenic added as undoped InAs polycrystalline material. The total weight of the melt is about 10 g.

Prior to growth, the melts are baked out at 700 °C for fi_een hours under flowing hydrogen to de-oxidize the

metallic melt components and outgas residual impurities After bake-out, appropriate dopant impurities are added

to each melt. The first melt for the growth of the n-type InGaAsSb base layer contains tin or tellurium. The small

amount of Te needed to dope the layer (atomic fraction in the melt = 10s) is problematic. For reproducible

doping, a weighable amount of Te is added as 100 to 200 mg of Te-doped GaSb (Cre=1019cm3). Tin is added to

-216-
themelt as 10 to 200 mg of high purity shot. Our preliminaryresults (Section 3) suggestthat highn-type doping

concentrations can be achieved more readily with tin than with tellurium. However, the relatively high liquid-phase

concentration of tin alters the melt composition needed to grow the lattice-matched InGaAsSb quaternary with the

desired bandgap, For higher tin doping levels, we will need to re-optimize the melt compositions to include the

effects of dilution with additional tin. This will require a phase equilibria analysis and model of a 5-component

system (In-Ga-As-Sb-Sn). The second melt for the growth of the p-type emitter contains 5 to 100 mg germanium.

Presently, we are beginning a more detailed and systematic characterization of impurity segregation and dopingin

the In-Ga-As-Sb quaternary system with the aim of achieving better control and a greater range of doping

concentrations.

The melts are equilibratedfor 1 hour at 530 °C and then cooledat a rate of 0.7 °C/min. At 515 °C, the

substrateis contacted with the first melt for two minutes to grow a 5-micron thick n-type InGaAsSb base layer.

Next, the substrate is moved to the second melt for 5 seconds to grow a 0.3-micron thick p-type InGaAsSb emitter

layer.

Front and back ohmic contacts are formed on the epitaxial InGaAsSb/GaSb structure by standard

processing techniques. The back of the substrate is metallized by plating with an 200-rim thick electron-beam

evaporated Au:Ge:Au:Ni layer and alloyed at 300 °C The front contact is a grid of 10-micron wide metallization

lines with 100-micron spacing and a single 1-mm wide center busbar. The grid is formed by a photolithography liftoff process with a 200-nm thick electron-beam evaporated Au:Zn:Au metallization. The front grid is thickened to

5 microns by gold electroptating. The front contact is not sintered. The substrate is masked and patterned to

define a 1 cmx 1 cm device and isolation etched with a potassium iodide - iodine "gold" etch. Most of our TPV

cells are 1 cm x 1 cm in area; although larger cells (2 cmx 2 cm) with comparable performance have also been

made. In order to simplify the spectralresponse analysis, we elected not to apply any anti-reflection coatings to

the cells. FIGURE 1 is a top-view photograph ofa 1 cm x 1 cm InGaAsSb TPV cell.

3. TPV DESIGN AND OPTIMIZATION

FIGURE 2 shows the TPV device design in cross-section. The fabricated cells have a 0.3 to 0.5 micron

thick p-type emitter with a Ge concentration of approximately 1019cm3, as indicated by Secondary Ion Mass

Spectroscopy (SIMS). A thicker, more heavily doped p-layer will reduce the sheet resistance of the emitter and

therefore improve the fill-factor, but will tend to reduce spectralresponse due to higher free-carrier absorption and

increased sensitivity to front surface minority carder recombination.

The base thickness in our cells ranges from 3 to 5 microns with a Te or Sn concentration of about 10is to

10TMcm_, as determined from capacitance-voltage measurements and SIMS. FIGURE 3 shows the SIMS depth

profile indicating the abruptness of the p-n junction and the depth uniformity of the doping concentrations. There is

apparently very little smearing of the doping profile due to diffusion or segregation of dopants. Discrepancies

between the Te dopant concentration measured by SIMS (total impurity concentration) and that implied by

capacitance-voltage measurements (net donor concentration) indicate that much of the Te is either not ionized or

else is compensated. This is a common problem in Te doping of III-V semiconductors, especially in GaSb-based

materials, and is probably due to the formation of electrically inactive telludde complexes or compoundsin the

material. Increasing the Te concentration in the melt showed a "saturation effect" in that the Te doping level did

not increase in proportion to the Te concentration in the liquid phase. Our most recent devices incorporate tin as

the n-type base dopant and have base dopings targeted around 101_cm3. Modeling indicates that base dopings

in this range will yield the optimum open-circuit voltages and short-wavelength quantum efficiencies.

-21 7-
4. TPV DEVICE EVALUATION

We present external and internalspectralresponseand current-voltagecharacteristicsfor 1 cm x 1 cm

p-lno.osGao.92Aso.0_Sbo.93:Ge / n-lno.osGao._Aso.oTSbo._: Te (or Sn) epitaxialcells on an n-GaSb:Te substrate

producedas described above. The externa/spectral response of a typical InGaAsSbTPV cell is shown in

FIGURE 4. FIGURE 5 shows the correspondinginternalspectralresponse. The lower external spectralresponse

is due to gridshading and reflectionof incidentlightfrom the uncoated InGaAsSb emittersurface. The grid

shading is 18.2%. The absorptionedge impliedby the spectralresponse measurementsof a number of samples

ranged from approximately2200 to 2250 nm. At a wavelengthof 2000 nm, internalquantumefficienciesas high

as 95% have been measured, and at a wavelength of 1 micron, internal quantum efficienciesof almost 55% have

been observed. The internal quantum efficiencyaveraged over the spectralregion from 1 to 2 microns

wavelengthis 60%. (It should be noted that for the intendedTPV applications,the responseof the cell for

wavelengthsless than 1.5 micronsis notimportant.)

The 1 cm x 1 cm InGaAsSb TPV cellswere tested under simulatedinfrared light usinga ZnSe-filtered

tungstensource (Carley Lamps, Inc., Torrance, CA) with a spectral emission in the 800 to 3000 nm wavelength

range. Under an illuminationintensitycorrespondingto a short-circuitcurrentdensityof 2 Ncm 2, open-circuit

voltages as high as 0.260 volts have been measured. FIGURE 6 shows the current-voltagecharacteristicsof a

1 cm x I cm InGaAsSb "T'PVcell under an infraredilluminationintensitythat yields a short-circuitcurrent densityof

62.4 mA/cmz and a open-circuitvoltage of 0.178 V. The fill-factor is 0.57. To date, the bestfill-factors observed

are less than 0.6. We believe that one cause of the somewhat lowfill-factors is series resistance,which is

discussedfurther inthe next section. FIGURE 7 showsopen-circuitvoltage vs. short-circuitcurrentfor varying

light intensity. The open-circuitvoltage increases logarithmicallywithilluminationintensityand an open-circuit

voltage of ~0.250 V is reached for current densitiesof 1 Ncm 2. The diode idealityfactor inthe voltage range of

0.1 to 0.25 V is close to 2, implyingthat highinjectionis dominant inthis voltage range.

5. CONCLUSION AND DISCUSSION

Our resultsto date have demonstratedthe potentialof InGaAsSb TPV devicesmade by liquid-phase

epitaxy. We believe there is stillroom for substantialefficiencyenhancements inthese devicesby optimizationof

the dopinglevelsand layer thicknesses. Furtherimprovementsmight includewide bandgap lattice-matched

AIGaAsSb windowlayers for front surface passivation,and AIGaAsSb back-surfacefield claddinglayersto reduce

the reverse saturationcurrent and thereby increase the open-circuitvoltage. Highlydopedcontact layerswill

providelower series resistance, as will substratethinning. Lower sedes resistancewill lead to higherfill factors.

Thinningthe substratewill also improve heat sinkingof the device.

The requiredperformance of a TPV device is dependent on its system application. Spectral controlof

thermalemitters,the use of selective filters and reflectors,heat transfer,and photonrecyclingeffects need to be

includedin the device designand systemoptimization. These considerations are not usuallyrelevantfor

conventional photovoltaicdevices and therefore the designand optimizationrulesfor TPVs will be significantly

differentthan those for solar cells. For example, gridobscurationand reflectionare notnecessarilylosses in TPV

systemsif photonsreflected from the front surfaceare re-absorbed by the emitter. Our next generationof

InGaAsSb TPV devices will incorporatedesignfeatures to fully exploitphotonrecyclingeffects.

REFERENCES

A. ANDASPAEVA,A.N. BARANOV,A. GUSEINOV,A.N. IMENKOV,L.M. LITVAK,G.M. FILARETOVA, and Y.P. YAKOVLEV,

"Highly EfficientGalnAsSb Light-Emitting Diodes (_.= 2.2 m, 1"1=4%,T= 300K)_ Soviet Technica/Physics Letters

14, 5 (1988) 377-378.

-21 8-
M. ASTLES, H. HILL, A.J. WlLL#kMS,P.J. WRIGHT, and M.L. YOUNG, "Studies of the Gal.xln_,sl.ySby Quaternary

Alloy System I. Liquid-Phase Epitaxial Growth and Assessment J. Electronic Matefia/s 15, 1 (1986) 41-49.

A.N. BARANOV, A.M LrFVAK,K.D. MOISEEV, N.A. CHARYKOV, and V.V. SHERTSNEV, "Melt-Solid Phase Equilibria in

the In-Ga-As-Sb and In-As-P-Sb Systems" Russian J. Physical Chemistry 64, 6 (1990) 884-886.

A.N. BARANOV,V.V. KUZNETSOV, E.R. RUBTSOV, Y.P. YAKOVLEV,and A.A. GUSEINOV, "Kinetics of Crystallization of

the Gaxlnl.xASySb_.y Solid Solutions from the Liquid" Russian J. Physic, a Chemistry 65, 12 (1991) 1713-1716.

C. CANEAU, A.K. SRIVASTAVA, A.G. DENTAl, J.L. ZYSKIND, and M.IL POLLACK, "Room-Temperature

GalnAsSb/AIGaAsSb DH Injection Lasers at 2.2 pm" E/ectronics Letters 21, 18 (1985) 815-817.

J.C. DEWlNTER, M.A. POLLACK, A.K. SRIVASTAVA, and J.L ZYSKIND, "Liquid Phase Epitaxial Gal.,inxAsySb__y

Lattice-Matched to (100) GaSb over the 1.17 to 2.33 _m Wavelength Range" J. Electronic Matedals 14, 6 (1985)

729-747.

A.E. DRAKIN, P.G. ELISEEV, B.N. SVERDLOV, A.E. BOCHKAREV,L.M. DOLGINOV,and L.V. DRUZH1NINA, "lnGaSbAs

Injection Lasers" /EEE J. Quantum E/ectronics QE-23, 6 (1987) 1089-1094.

A. JOULLII_,F. JIA HUA, F. KAROUTA,and H. MANI, =LPE Growth of GalnAsSb/GaSb System: The Importance of

the Sign of the Lattice Mismatch" J. Crysta/Growth 75 (1986) 309-318.

H. KANO, S. MIYAZAWA, and K. SUGIYAMA, "Liquid-Phase Epitaxy of Ga_.ylnyAs_Sb_.x Quaternary Alloys on GaSb"

Japanese J. Applied Physics 18, 11 (1979)2183-2184.

F. KAROUTA,A MARBEUF, A. JOLLIE,and J.H. FAN, =Low Temperature Phase Diagram of the Gal..lnxAsySbl.y

System" J. Crysta/Growth 79 (1986) 445-450.

N. KOBAYASHI, Y. HORIKOSHI, and C. UEMURA, "Liquid-Phase Epitaxial Growth of InGaAsSb/GaSb and

InGaAsSb/AIGaAsSb DH Wafers" Japanese J. App/ied Physics 18, 11 (1979) 2169-2170.

J.B MCNEELY, M.G. MAUK, and L.C. DINErrA, =An InGaAsSb/GaSb Photovoltaic Cell by Liquid-Phase Epitaxy for

Thermophotovoltaic (TPV) Application" Proc. 1st NREL Conf on 7"herrnophotovoltaic Generation of E/ectrfcity,

1994 T.J. Cou'rrs and J.P. BENNER,eds., (New York: American Institute Physics, 1995) 221-225.

K. NAKAJIMA,K. OSAMURA, K. YASUDA,and Y. MURAKAMI, "The Pseudoquatemary Phase Diagram of the

Ga-ln-As-Sb System" J. Crysta/Growth 41 (1977) 87-92.

N. KOBAYASHIand Y. HORIKOSHI, "Pseudoquatemary Phase Diagram Calculation of In_.xGaxAs_.ySbyQuaternary

System" Japanese J. Applied Physics 21, 1 (1982) 201-202.

K. NAKAJIMA,K. OSAMURA, K. YASUDA, and Y. MURAKAMI, "The Pseudoquatemary Phase Diagram of the Ga-lnAs-Sb System J. Crysta/Growth 41 (1977) 87-92.

,i

E. TOURNI_:,F. PITARD, A. JOULLIE,and R. FOURCADE,=High Temperature Uquid Phase Epitaxy of (100) Oriented

GalnAsSb Near the Miscibility Gap Boundary" J. Ctysta/Growth 104 (1990) 683-694.

M.M. WANLASS, J.S. WARD, K.A. EMERY, and T.J. COLrrTS, "Gaxlnl.xAS Thermophotovoltaic Conveners" IEEE

1st World Conference on Photovoltaic Energy Conversion (New York: IEEE Press, 1994) 1685-1691.

S. WOJTCZUK, E. GAGNON, L. GEOFFROY, and T. PARODOS, "lnxGa_._As Thermophotovoltaic Cell Performance vs.

Bandgap" Proc. 1st NREL Conf. on Thermophotovoltaic Generation of E]ectricity, 1994 T.J. Courrs and J.P.

BENNER, eds., (New York: American Institute Physics, 1995) 177-187.

-21 9-
FIGURE 1: Top-view Photograph of a 1 cm x 1 cm InGaAsSb TPV Cell.

Au/-/_nlAup-typeContact

_1 _ 0.3-0.5pmThickp-typeInGaAsSbEmitter

__-__:_-_ 7.... - _ _-__--_.__z_ ( 3-5_ Thickn-typeInGaAsSbBase

GaSbSubstrate

rX"------ Au/Ge/Au/Nin-typeContact

FIGURE 2: InGaAsSb/GaSb p-n Junction Thermophotovoltaic Cell Design.

Alll

| | .... ! • = =. = I . ! . . .

£ =' 3 4

FIGURE 3: SIMS Depth Profile of Doping•

LO|

1o=

Lo=

lo=

1.01

%

p

I

Z

IZ

N

M

).

X0

I

,(

-220-
IOO

8O

,,, 6O

O

n,

'" 40

20

_____ii __

t i i I l

, , , I !

i I i t •

i i I I z

=

: = ! i I

t i t I :

i i i i_ i

i

' ' ! ! L-4-'

0

1000 1200 1400 1600 1800 2000 2200 2400

WAVT=J.ENGTH

FIGURE 4: External Spectral Response of InGaAsSbtGaSb p-n Junction Thermophotovoltaic Cell.

1oo

II0

,,, SO O

Z

iii

I-

_z 4,0

2O

I

i; i :

i i i i w I i

l.._e'- I i ;

i .Jr ! ; :i

: i I 'i i'

•-=._i J. ! I ! ,

:-o! l, ,! ,t i ._

.... i ' i i _I ' ' i i i " i

k

800 lOGO 1200 1400 '_600 1800 2000 22C0 24,0C

WAVELENGTH

FIGURE 5: Internal Spectral Response of InGaAsSb/GaSb p-n Junction Thermophotovoltaic Cell.

-221-
.25

20

.15

.10

.05

i °

0

-.10

-.15

-.25

.... . .... .; ......... _ ....

......... _ ......... _..

i

i

- - -- .... P . -r .... '

I

i

, [

• - d ......... ,...

I

R

• ' " ' I ' • |

• i

' T

.... r .... x .... _ .... _ i ...............

o

, J

-1.0 -0.8 -0.6 -0.4 -0.2 0 02 0.4 0.6 O.B 1.0

Voltage (V)

FIGURE 6: Current-Voltage Characteristic of 1 cm x I cm InGaAsSb TPV Cell.

m

m

g

I

'-t

1.00E+O0

1.00E-01

1.00E-02

I ,

1.0OE-05

0 0.05 0.10 0.15 0.20 0.25

v. N1

FIGURE 7: Open-Circuit Voltage vs. Short-Circuit Current for InGaAsSb TPV Cell under Varying Illumination

Intensity.

-222-
MULTIJUNCTION InGaAs THERMOPHOTOVOLTAIC POWER CONVERTER

Steven Wojtczuk

Spire Corporation

Bedford, MA 01730-2396

OVERVIEW

The experimental performance of a multijunction monolithic lattice-matched 0.74 eV InGaAs

thermophotovoltaic (TPV) power converter under 980C blackbody irradiation is reported. Eight InGaAs PN

junctions grown epitaxiaily on a semi-insulating wafer were monolithically integrated in series to boost the

- 0.4 V photovoitage per typical InGaAs junction to over 3 volts for the 1 cm=chip. This chip was originally

designed and characterized for free-space 1.3 pm laser power beaming_. The power efficiency of this TPV

device is 16% for that part of the blackbody spectrum above the material bandgap. The device is shown to

deliver about 1 watt of output power when driven with enough light. This is the first report of such a

multijunctionTPV device. This is not a traditional tandem cell in which the junctions are stacked vertically.

Eight lmm long by 1 cm wide junctions are laterally connected across the device area. This multijunction

design has the potential for lower t2R power losssince the smaller PN jun_on area limitsthe current to oneeighth that of the equivalent surface area. In essence, the current is traded for voltage to avoid the FR loss,

analogous to the way power uttities avoid I=R loss in high-tension power lines, by transforming the high

current, low voltage generated at a power plant into a high voltage at a low current before transmitting the

power over great distances. Figure 1 shows the idea behind this device. The present multijunction TPV

does not yet work at the 100W/cm2 level; the figure is only meant to illustrate the concept.

- IOOAI e._q:_forMASS_PR_

BrokenIntolO _/_"9""""_"

Series-Connected

Junctions

I

,islil,/g

__r_al ou_ 50W

-0.SVx 10 =5V jl2Rloss 100Xless

- IOA | forthesameres/s_ance

Figure I Illustrationof how ohmic power losscan be greatly reduced with a multt]unction TPV device

-223-
DEVICE DESCRIPTION

Theepilayer structure and fabrication details were described in the paper! discussing 1.3 pm laser

power conversion with this device, and are summarized here. Much greater detail, including photographs

of the monolithic interconnections between junctions on the chip can be found in the earlier paper. Metal

organic chemical vapor deposition was used to epitaxially grow the converter on a semi-insulating InP wafer.

The non-conducting wafer is necessary to interconnect the junctions rnonolithically. This lattice-matched

0.74 eV InGaAs converter used a P-on-N structure to insure that the N InGaAs buried layer (the first layer

grown), which does not have the advantage of a metal contact gridto lower its sedes resistance, is of the

lower resistivityN type material. Both N and P InGaAs can be doped to a high level (> le19/cm3), but the

mobility of the N InGaAs is -25X higher. This layer is made about 3pm thick to lower its sheet resistance.

Next a thin, heavily-doped N-InP back surface field/etch stop is grown. This acts as a minoritycarder

mirror for carriers photogenerated in the device base layer, and aids in the device fabrication (back contact

connection) to stop a selective wet etch which etches InGaAs but =stops"at InP. The etch stop property is

important since it is possible to overetch the via for the back contact too far into the insulating wafer. The

back surface field is much more crucial for TPV than for standard solar cells. In a solar cell, much of the light

is at short wavelengths and absorbed in the emitter or space charge region;in a TPV cell most of the light

is at long wavelengths and is absorbed in the base region where a low BSF recombinationvelocity increases

the photocurrent noticeably. This base layer, also N-type inGaAs, is more lightlydoped than the buried layer

(by two orders of magnitude) to increase the diffusionlength. Low resistivityis not crucial since the current

path is vertical (only 3prn long) unlike the buried N-InGaAs where current flows laterally along 1000prn.

Finally, a heavily doped (1e19/cm3), thin (0.3pm) P-InGaAs emitter layer is grown. A front metal

contact grid optimized for the emitter sheet resistance is later evaporated, which compensates for the 250X

higher emitter sheet resistance (10X thinner, 25X lower mobility)compared to the buried N-InGaAs. A silicon

nitride layer was used to isolate the junction sidewall and prevent interconnect metal from shunting the

junction. A finished, packaged device is shown in Figure 2.

Figure 2 Packaged eight-junction 0.74eV InGaAs multijunctionTPV device. Contact grid too fine too

see; thin lines para/lel to busbars are the interconnects between the junctions.

-224-
IVCHARACTERISTICS

Figure3 showsa darkandilluminatedI-VcurveforthemultijunctionTPVunderaweak(i.e.small

area)980CInfraredAssociatesblackbodyoriginallymadefor long wavelength infrared photodiode tests.

The 980C temperature was used since this was the highest our calibrated blackbody source would reach.

For this 0.74eV bandgap converter, a higher temperature source would have been more desirable. The

multJjun_on short-circuit photocurrentwas 6mA (each junction is 0.1cm2 so the current density per junction

was 60mA/cm2). The open-circuit voltage at this 6 mA level was 2.3V. The I-V fill-factor is -65%.

Figure 3 Dark and illuminated I-V curves for a 0.74eV mu/tijunctionTPV converter under weak 980C

blackbody irradiation. Vertical scale is 2mA/div and horizontal is 500mV/div.

In order to calculate efficiency, we need to estimate the usable power falling on the device. The

blackbody source aperture had a one-half-inch radius ('RI") situated two-inches (=h') from the TPV device.

In order to use an analytical view factor formula for two coaxial circular disks, the rectangular TPV device

(lcm by 0.8 cm photoarea) is approximated as a 0.2-inch radius ('R2") circular area (equal to 0.8cm2, same

as the original photoarea). The view factor "F" for this arrangement isz.

F = R2 (X_ - 1 ) where X = h2 + R12 + R2 2

R] 2R] R2

or F is 0.009 (i.e. the TPV receives -1% of the lightemitted by the blackbody). The amount of power from

the 980C blackbody above the 0.74eV bandgap(1.65pm cutoff wavelength) is 1.16 W/cm2, and the oneinch diameter blackbody aperture has an area of 5.1 cm2 so that (5.1x1.16) 5.9 W was emitted at

wavelengths the InGaAs multijunctionconverter could convert. The amount of power incident on the device

was then (0.009x5.9) 55mW. The power the device developed was (2.3Vx0.006AxO.65) 9mW, for a power

efficiency of 16%.

-225-
Figure 4 shows several I-Vs of the device when driven by a solar simulator to various current levels.

A current level of 6mA (corresponding to Figure 3) is shown, along with the IV with illumination driving the

device to 50mA (best fill-factor, 70%) and up to the highest photocurrent we could obtain (0.425 amps, fill

factor series-resistance limited at 51%). The device delivered (0.425Ax3Vx0.51) 0.64 watts at the highest

illumination tested.

400

300

IZ

IAi

IZ

0

0

I0

Z

IL

2OO

100

\

\

o

II I , , ,

0.5 1.0 1.5 2.0 2.5 3.0

PHOTOVOLTAGE (V)

Figure 4 IVs obtained with the multijunction TPV cell at various illuminations witha solar simulator

-226-
OHMIC POWER LOSS CALCULATIONS

Figure 4 above shows that this first prototype multijunctionTPV technology can deliver about 1 watt,

and begins at this level to become limited by ohmic losses. The device can be further improved to lower the

ohmic loss. This section outlines the numerical calculations needed to simultaneously minimize:

IZR loss in buried N-type InGaAs lateral conduction layer

I=R loss in top P-type InGaAs emitter layer (loss lowered by use of metal contact grid)

shadow loss due to metallic contact grid

shadow loss due to interconnect

An incident optical power of 100 W/cm = was used in the calculations. Figure 5 illustrates one

junction of the eight-junction TPV and shows the dimensions and parameters used in the calculation:

Figure 5 Definition of terms used in ca/cu/ating /oss in one-junction of eight in the TPV.

Loss in buried IV"InGaAs - The TPV's are on a semi-insulatingInP substrate. The N+ buried layer

must conduct the photocurrentto the cell edge to be collected by the bottom contact. The total photocurrent

I(X) in the layer increases linearly from zero at the edge furthest from the bottom contact, to its maximum

value as the end contact at is approached:

/(x) - w x

hc

Maximum photocurrentfrom the incident optical power _ (100 W/cmZ), 7.6 A, is at X = L - IC, where L is the

junction length (1 mrn), and "IC" is the interconnect width (100 pm). Here rl is the external quantum

efficiency (0.8) at an assumed narrow selective emitter peak wavelength ,_(1.315 pm), q (1.602xl 019C), h

(6.626x 10_ J-s) and c (3x 10em/s) are constants, and W is the junctionwidth (1 cm) of the multijunctionTPV.

-227-
TheIZRpowerlossinthelowerN*InGaAslayeristhengivenby:

L-/C L-IC

PLOSSIV = f I2(X) dR =/ I2(X) pN dx - q2q2*2_'2WpN (L-IC)3

W t_ 3h2c2tlv

0 0

where dR is the incremental resistance, p, is the resistivity(10" Q-cm) of Spire's N-type InGaAs (N - 3x10'_

cm"3, mobility - 2000 cm=/V-s), and t, (4 pm) is the thickness of the N InGaAs buried layer. _ss, is

0.45W/junction. The available incident optical power on a single junction is:

Pava =_ WL

PAvAis 10W, SOthat FN , the ratio of I=FIpower loss in the IT buried InGaAs layer to the available

optical power is 4.5% and is given by:

Pzoss # q2q2_Z2PN (L-IC)3

PAVA 3h2c2tl¢ L

Loss in upper I_ InGaAs - The upper IP InGaAs emitter is doped (- 3xl 0 TM cm3) similar to the N+

InGaAs buried layer, but has a ~ 25X lower hole mobility (- 80 cm_V-s), which would dominate all losses

if a top contact grid was not used. Instead of photocurrent flow across the junction width L - IC (900pm),

with a contact grid the current now only flows across half of the gridline-to-gridline spacing S (100 pm), or

50 pro. This 18X shorter distance than the flow inthe N InGaAs buried layer compensates for the 25X lower

hole mobility. The photocurrent I(Y) increases from zero at the center of the gap between gridlines to a

maximum at a gridline:

I(Y) = qvl_. L Y

hc

The maximum photocurrent at Y = S/2 is 0.042 A. The I=FIpower loss, PLOSSP, is:

W s/2

f /2(y) dR W sj2 - = 1 i2(y) Pe dy Wq2Tl 2 _2Lpp

4: S 2

Pwss P S -s 1 2 S -5 1 2 L tp 12h2c2tp

where W/S is the number of gridlines per junction, ppis the resistivity(0.003 Q-cm) of Spire's P-type InGaAs

(- 3x10 TM cm"3doping, mobility - 80 crn_/V-s), and tp is the thickness of the InGaAs emitter (0.3 pm). The

emitter cannot be made much thicker than 0.3 pm without the external quantum efficiency q dropping below

80°/0. PLOSSPis then 0.52 W. Fp, the ratio of IZR power loss in the emitter sheet resistance to the available

-228-
opticalpoweris5.2% and is given by:

FLOSS P _ q2r124)_.2Pp S 2

PAVA 12h2c2te

Loss in top contact grid metal- The top contact grid also has I=R loss. Gold has a resistivity PM

of - 3x10_ Q-cm. The thickness (height) tMof the evaporated gold is 3 pm. The width "d"of a gold gridline

is 5 pm. The power loss PaF_O,0.34 W per junction is given by:

L-/C

W f 12(X ) dR - W L-/C I2(X) OM _

P GPJO='_ o S o dt_

Wq2112 2 .2SpM (L-IC) 3

3h2c2dtM

FG_o , the raUoof 121:1loss in the grid to available optical power is 3.5% and is given by:

PGRID q2TI2_.2pM S(L-]C) 3

F_ D -

P_vA 3h2c2tH dL

Shadow Loss from Grid and Interconnect- We are now planning to use an interconnectdistance

"IC" of 100 pm out of a total junction width of "L"of 1000 pro. This means 10% of the total area is "dead"

area (i.e. IC/L). In addition, we are now using a top contact grid with gridline width "d"of 5 pm on 100 IJm

spacings "S", for an additional shadow loss of 5% (d/S). Total shadow loss (the largest loss mechanism)

is 1.5W/junction or 15% and is given by:

P se,,,_w d IC

FSHADOW - _ + __

PAVA S L

- SUMMARY

We have demonstrated a monolithiceight-junction 0.74eV InGaAs thermophotovoltaic converterwith

an output power capability of at least 0.64 watts, with voltages up to 3V and currents to 0.425 amps. At this

level, the device was series-resistance limited (51% fill factor). At a lower photocurrent level, the best fillfactor was 70%. The device was tested with a sun-simulator and also under a weak, small-area 980C

calibrated blackbody source. Under the blackbody, this TPV device exhibited an efficiency of about 16% to

the blackbody emission wavelengths above the material bandgap (1.65 pm cutoff wavelength). We

discussed the ohmic power losses in the device. We calculated that an upper limitfor this device technology

may be an efficiency of 27% at power densities up to 100 W/cm2. Table I below shows the calculated

losses.

-229"
j. . . , ,

Table I Losses in P-on-N mulb_unctionTPV design under 100 W/crn_of i.3 pm light.

Power Loss ( W/cm=) Power Available ( W/cm =)

Incident Optical Power 100

I=R Loss in N÷Buried Layer 4.5 95.5

121:1Loss in P+Emitter 5.2

I=R Loss in Metal Contact Grid 3.4

Shadow Loss due to Grid 5.0

i

Shadow Loss due to Interconnect 10.0

Intrinsic38% Cell Efficiency

(loss due to Eg 0.74eV while

Voc-0.5, QE~0.8, imperfect FF

(-0.7) even with no series R)

44.6

90.3

86.9

81.9

71.9

27.3

.

.

REFERENCES

S. Wojtczuk, T. Parodos, and G. Walker, "P/N In(AI)GaAs MullJjunctionLaser Power

Converters," Proc. of XIII SPRAT NASA Conf. Pub. 3278, 1994, pp. 363-371.

M.F Modest, Radiative Heat Transfer, McGraw, 1993, pp. 166-173.

-230-
GALLIUM PHOSPHIDE ENERGY CONVERTERS

P. E. Sims, L. C. DiNetta, K. Dugan Cavanagh, and M. A. Goetz

AstroPower, Inc.

Newark, DE 19716-2000

SUMMARY

Betavoltaic power supplies based on gallium phosphide can supply long term low-level power with high

reliability. Results are presented for GaP devices powered by Ni-63 and tritiarated phosphors. Leakage currents

as low as 1.2x10"17Ncm 2 have been measured and the temperature dependence of the reverse saturation current

is found to have ideal behavior. A small demonstration system has been assembled that generates and stores

enough electricity to light up an LED.

INTRODUCTION

Betavoltaic power generation requires an efficient semiconductor device to convert the beta-generated

carriers into useful electric power. AstroPower has developed a gallium phosphide energy converter which is

evaluated for two methods of power generation: direct conversion and indirect conversion• Figure 1 shows the

indirect conversion concept, which is the most cost effective of the two methods. Figure 2 shows a current-voltage

curve for direct conversion using S3Nias a power source. Figure 3 shows data for an indirect conversion array.

For the direct conversion system, the short-circuit current density is 1.9xl 0"eNcm 2. The short-circuit current

density for the indirect conversion system is 2.4x10 .7A/cm 2, better than an order of magnitude improvement over

the 63Ni fueled device. Improvements in current generation are the key to building betavoltaic power supplies with

reasonable cost. Additionally, the utilization of tritium contributes to the safety of the device.

FIGURE 1.

ln,,cm._-nentation

I elooeoeool i

Tnllum Phosphc_ Power

Indirect betavoltaic conversion.

RESULTS

-o

Actual two-step tritium-fueled power generation has been accomplished since the last report [1] of a

prototype GaP semiconductor betavoltaic converter array. The initial light sources utilized were two custom-made

tritium light tubes fabricated by SRB Technologies (Winston-Salem, NC) for $150. The luminance of these highpressure tubes was 700_L. Each tube contained 15Ci of tritium and was half silvered to increase the light flux

impinging on the array. The short-circuit current of the GaP array illuminated by these two high-pressure tubes

was found to be 48nA without any optical containment techniques besides half-silvering the tubes. The effective

collection area of this array was 0.2 cm2, so the short-circuit current density with this light source was 240 nA/cm 2.

The average open-circuit voltage was 0.997 V/cell. Due to the Federal regulations involving tritium, these lights

are still at the factory in North Carolina while the array is in Delaware, so more detailed results other than those

presented in Figure 3 are not available.

-231-
A second demonstration unit was evaluated using three standard blue tritium markers (Figure 4) as a light

source in a reflective enclosure. The luminance of a standard commercial blue marker is approximately 5001_L

and there are no mirrors on the tubes. The best short-circuit current achieved with this configuration was 19nA,

which corresponds to 95 nNcm 2 current generation (Figure 5) and the single element open-circuit voltage was

0.912 volts. This power supply was found to be capable of charging a commonly available tantalum capacitor so

that an LED could be fired about once an hour. Manual triggering was required since the short-circuit current

provided by this betavoltaic generator was insufficient to sink into a firing circuit.

0

-O, I

-02

-03-

"_ -0.:- -

_E -0.5 -

-_ -u._ -

_ -o._ -

t

I

r

o -0.9' - •

1

_E -

w_ -1.2

_ -1 ..3

U --1.4

¢ -1.s

-I.6 -I

o 0:2 0:, o:,., 0:8

Cell V._ltoge C_

FIGURE 2. GaP betavoltaic device fueled by direct

conversion of =Ni beta-electrons [2].

0.2

'0.15

>._

z_

_ o._

0

12_1GmP rummy(2 s_ _ 8) J-V

of _ 70_JL lun',4m_,_ ledmv_t,_l

ell Itmmis 01 ¢m2

2 3 4

ARRAY VOLTAGE C_)

S E

FIGURE 3. GaP betavoltaic array fueled by two

custom-made T2-ZnS:Ag light sources.

FIGURE 4. Photograph of commercial tritium lamps (scale is in inches).

-232-
-4

4

"41

-t0

!.1=I

• -14 I

.16

-18

.2o

GoP _ i_J_._rocl by

0

i

I

I

I

J

i •

r

5

VOLTAGE {V)

FIGURE 5. GaP betavoltaic array fueled by three commercially available T2-ZnS:Ag light sources.

The cell characteristics for the 12-¢ell array are presented in Table I and the array geometry is detailed in

Figure 6. These cells have a lower short-circuit current compared to our more recent device results. Using the

following factors: 1) a current doubling due to better device characteristics, 2) a 20% AR gain, and 3) a 25% gain

due to enhanced optical coupling of the tritium lights to the cell elements, a realistic prediction of the possible

current generation density for this type of device is 720 nNcm 2 for the bright light configuration. The first factor is

confirmed by the last section of Table I, the second factor is the measured reflectance of GaP which can certainly

be reduced to near-zero with an AR scheme, and the third factor, while speculative, seems to be an achievable

goal.

A

B

1 2 3 4 5 6

FIGURE 6.

+

Array configuration.

Top View

Individual device area is 0.1 cm2

Table 2 displays some of the best leakage currents measured for devices fabricated during the past year.

The best measured leakage current is 1.2x10 -17 A/cm 2 for a device limited by space charge recombinationgeneration (n=2). Due to the low flux of either beta radiation or light impinging on the device, it is necessary for the

converter to possess a very small leakage current. In this respect, even though it is possible to tailor the light

spectrum for indirect conversion, currently available light sources can only support devices with leakage currents

on the order of those that can be obtained with GaP. As shown in Table II, very little current is required to

generate high voltages with these devices.

-233-
Table I.

Cell data for array elements illuminated by =_8.24 p,Wlcm 2 ZnS:Ag light (Ref. 1 )

Array Position Cell Voc (V) Isc (nA) Vmp (V) Imp (nA) I at 0.SV (hA) FF 11(%)

A1 B114-1405 0.99 51 0.8 45 45 0.72 4.37

A2 B114-10-05 1.03 54 0.8 48 48 0.69 4.66

A3 Bl14-10-07 0.95 52 0.8 48 48 0.78 4.66

A4 B114-12-05 1.00 51 0.9 44 49 0.77 4.81

A5 B114-10-04 1.03 51 0.8 50 50 0.76 4.86

A6 B114-14-01 1.04 53 0.8 50 50 0.73 4.86

B1 B114-12-04 1.06 52 0.9 46 51 0.75 5.03

B2 Bl14-12-02 1.01 52 0.9 47 52 0.80 5.14

B3 B114-12-01 1.12 53 0.9 53 52 0.76 5.46

B4 B114-14-02 1.14 54 1.0 50 53 0.81 6.07

B5 B114-10-01 1.08 55 0.9 52 54 0.79 5.68

B6 Bl14-10-03 1,16 55 1.0 51 55 0.80 6.19

Array 6.27 146" 5.25 116 121 _* 4.8 V 0.56 5.62

Improved

high-current Bl15-02-01 1.19 106 0.9 100 104 0.71 10.93

GaP device

Improved

high-current Bl15-05-02 1.11 111 0.9 108 10g 0.79 11.8

GaP device

1 cm2 GaAs cell 0.24 251 0.2 159 0.52 3.86

* Current boost is due to the elimination of probe shading after array fabrication.

Table II.

Leakage currents of GaP devices

Device Vo¢ I=¢ Jo2

(Volts) (hA) (Ncm 2)

B126-07-06

B123-15-17

B123-06-15

B126-04-19

B123-14-22

B123-05-16

B124--01-09

B126-05-02

B124-05-14

B123-03-23

1.302 252

1.288 234

1.272 300

1.281 373

1.276 344

1.266 343

1.230 273

1.212 259

t.153 207

1.154 320

1.2xl 0"lz

1.5x10 "17

2.6xl 0 "17

2.7x10 "17

2.8x10 "17

3.4xl 017

5,4xl 0"17

7,2x10 "17

1.8x10 °is

2.7x10 "is

-234-
Figures 7, 8, and 9 display measured values for the dark current, quantum efficiency, and shunt

resistance of a GaP device. These resultsare displayed as a function of temperature. We have found that GaP

behaves very ideally up to about 300°C and can perform for a short durationto about 400°C. The high values of

shunt resistance which have been achieved are particularlycritical for this application.

i¢-_=

i,, =

_,,.=

i

| ""

1E-_2

t¢_

IE,lJ

l .T..

I ,,-"*" o....... i

! ....t........I/ i

Figure 7. Temperature dependent leakage current of GaP pin junction diodes [3].

70

60

50

4O

2O

o

-'1' ..... '''t'

,4100, ' j ' ' _ f _ '5/'

.... '," , , , le+11 .... I .... I .... I .... I ....

300C _" le+10

× ,00c le+8 ; %.

\ 1e+7 le+6 _ P DEVICE

\ __ _p_ le+4 le+5 i \ u_

_. 1e+3 \ S'aDr_mlacE

\, _Z 1e+,1e+2_ \$1337-1010B

600 le+O F,,,, I, ,_ , I .... I .... I ,

WAVELENGTH (rim) 0 100 200 300 400 500

TEMPERATURE (C)

Figure 7. Temperature dependent external quantum Figure 10.

efficiency of a GaP photodiode (no AR) [3].

Estimated shunt resistance of a GaP

device at different temperatures compared

to a commercially available silicondevice

[3].

r 63 i The experimentally determined powe density for a direct conversion N"fueled system is

7 2

1.35xl 0.8W/cm 2, while the indirect conversion system power density was measured as 1.9xl 0 W/cm. The

significantly higher power available by the indirect conversion of beta radiation to electricity coupled with the safety

of the tritium light source as opposed to _Ni, is desirable for this application. Also of interest is that by using an

indirect conversion method, even with a low level beta emitter such as tdtium, more power can be generated than

with a high beta flux using direct conversion methods.

-235-
At the present time, work is ongoing in using tritiumlightsources to develop safe, high output lightsources

for a variety of applications. AstroPower is collaboratingwith Encapsulite, Inc. (Maplewood, NJ) to develop

prototype betavoltaic generators using GaP-based devices and microsphere illuminationtechnology. The

microsphere concept, which is the highpressure encapsulation of tritium and phosphors in small (<1ram dia.)

spheres, holds promise for the achievement of significantly brighterlightsources. Encapsulite Inc. believes that

luminance's of 1 to 10 FtL are possible using this approach (1FtL _.753 I_L). These light sources will enhance the

power generating abilityof the betavoltaic power conversion system by greatly increasingthe incident lightflux.

The output of the betavoltaic generator is expected to be very stable since there are no semiconductor

degradation mechanisms while the output of the generator assembly is expected to be predictable due to the well

known decay statisticsof the beta emitter. Useful generator lifetimes of 20-years are expected usingtritium as a

fuel.

CONCLUSIONS

Betavoltaic power generation using GaP devices and tritiumpowered lightsources is technically feasible.

A =realistic" power supply is proposed as having a Vmpof 5 V and an Iscof 20 Ida, which would require a converter

area of approximately 140 cm2 usinga projected current generation density of 720 nNcm 2. A much smaller

device area is possible if the microsphere concept is as bright as expected. This configuration would find use in

long-term space missionsas a housekeeping power supply. This device could both store power for periodic burst

transmissionsand command/control receptions. It is also possible that this type of device could switch on a more

powerful power source as the spacecraft reaches a vicinitywhere data acquisition activities are planned.

References

[1]

[2]

P.E. Sims, L.C. DiNetta, and A.M. Barnett, =High Efficiency GaP Power; Conversion for Betavoltaic

Applications', Proceedings of the XIII SPRAT Conference, June 1994.

G.H. Negely, =Gallium Phosphide Converters Optimized for use in Beta-Cells', Final Report for Contract #

09-2988, Sandia National Laboratories, Sept., 1990.

[3] P.E. Sims, L.C. DiNetta, M.A. Goetz, and J.B. McNeely, "Embedded High-Temperature Optical

Transducer for Real Time Combustion Control", Workshop on High Temperature Power Electronics for

Vehicles, Fort Monmouth, NJ, April 1995.

-236-
CHARACTERIZATION TESTING OF

MEASAT GaAs/Ge SOLAR CELL ASSEMBLIES

Mike R. Brown

Curtis A. Garcia

George S. Goodelle

Joseph S. Powe

Joel A. Schwartz

Hughes Space and Communications Company

Spacecraft Integration and Structures Business Unit

El Segundo, California 90245

Abstract

The first commercial communications satellite with gallium-arsenide on germanium (GaAs/Ge) solar

arrays is scheduled for launch in December 1995. The spacecraft, named MEASAT, was built by Hughes

Space and Communications Company. The solar cell assemblies consisted of large area GaAs/Ge cells

supplied by Spectrolab Inc. with infrared reflecting (IRR) coverglass supplied by Pilkington Space

Technology. A comprehensive characterization program was performed on the GaAs/Ge solar cell

assemblies used on the MEASAT array. This program served two functions; first to establish the database

needed to accurately predict on-orbit performance under a variety of conditions; and second, to

demonstrate the ability of the solar cell assemblies to withstand all mission environments while still

providing the required power at end-of-life. Characterization testing included measurement of electrical

performance parameters as a function of radiation exposure, temperature, and angle of incident light;

reverse bias stability; optical and thermal properties; mechanical strength tests, panel fabrication, humidity

and thermal cycling environmental tests. The results provided a complete database enabling the design of

the MEASAT solar array, and demonstrated that the GaAs/Ge cells meet the spacecraft requirements at

end-of-life.

Introduction

Fixed solar

j panel

:f!_ Deployable

solar panel

Figure 1. MEASAT HS-376 Spacecraft

For roughly thirty years since the advent

of satellite communications, spacecraft

power generation has been dominated by

the use of silicon solar cells. In recent years,

the spacecraft industry has experienced a

dramatic transition with the introduction of

higher efficiency devices, primarily galliumarsenide solar cells grown on germanium

substrates [1]. In the commercial satellite

industry, the use of GaAs/Ge (galliumarsenide on germanium) solar cells was

impeded by the high cost of these devices.

However, progress in GaAs/Ge

manufacturing technology, as well as the

demand for higher power spacecraft has now

made the use of GaAs/Ge solar cells on

commercial spacecraft both feasible and

desirable. The first commercial satellite to use

GaAs/Ge cells is the MEASAT spacecraft

(shown in Figure 1), built by Hughes Space

and Communications Company (HSC). The solar panels were built by Spectrolab Inc. and delivered to

HSC in October of 1995; the spacecraft is currently scheduled for launch in December of 1995.

-237-
The MEASAT spacecraft, illustrated in Figure 1, is a Hughes HS-376 model spin-stabilized spacecraft.

The HS-376 product line beganin 1979 and continues in production to this day. The MEASAT program

includes two spacecraft. The solar array consists of two cylindrical solar panels as shown in Figure 1. The

introduction of GaAs/Ge solar cells on this spacecraft significantly increased the power generation

capability of the solar panel. As a result, GaAs/Ge solar cell technology has made a major contribution to

the longevity of this product line.

A comprehensive test program was performed by HSC to characterize the on-orbit performance of the

solar cell assemblies used on the MEASAT solar panels. This program served two functions; first, to

establish the database needed to accurately predict on-orbit performance; and second, to demonstrate

the ability of the solar cell assemblies to withstand all mission environments while still meeting end-of-life

requirements. The test program and the resulting database are described herein. A summary of the

results is shown in Table I. The results provided a complete database enabling the design of the MEASAT

solar array, and showed that the GaAs/Ge solar cell assemblies meet the spacecraft requirements at endof-life.

Test Articles

The solar cell assemblies used on MEASAT consisted of GaAs/Ge solar cells with silver interconnects

and coverglass installed. These assemblies are also referred to as CICs (cell-interconnect-coverglass

assemblies). These assemblies were manufactured by Spectrolab, Inc.. The interconnects consisted of

25 _m silver mesh. Interconnects were installed by welding to the front ohmic contact of the solar cell.

Coverglass was bonded to the solar cells using Dow Coming 93-500 adhesive.

Several coverglass configurations were tested during the characterization test program. These

included CMG coverglass [2] supplied by Pilkington Space Technology and Coming 0213 coverglass [3]

supplied by Optical Coating Laboratories Inc.. The coverglass types used in the test program are listed in

Table II. For the MEASAT flight panels, 150 _m thick CMG/]RR was chosen. This coverglass has an antireflective (AR) front surface coating and an infrared reflective (IRR) back surface coating [4].

A total of 470 large area cells and 35 small area cells were used in the characterization test program.

Small area cells were used exclusively for ultraviolet radiation, proton radiation, spectral response and

temperature profile tests. All other tests were performed with large area cells and assemblies. The large

area cells included 100 bare cells (i.e. cells without covers) and 370 assemblies. Large area bare cells had

dimensions 3.447 cm x 6.888 cm; the assemblies consisted of bare cells with these same dimensions and

covers which were slightly oversized (3.459 cm x 6.904 cm). All large area cells had an average thickness

of 135 I_m.The small area cells included 20 bare cells and 15 assemblies. Cell dimensions were 2 cm x 2

cm; the small area cell thickness was 175 _m.

Table II. Coverglass Types

Type

CMG/AR

0213/AR

CMG/IRR

CMC.,-glRR

Description

CMG glass with anti-reflective front

surface coating

Coming 0213 glass with antireflective front surface coating

CMG glass with anti-reflective front

surface and infrared reflecting back

surface coatings

CMG glass with anti-reflective front

surface and infrared reflecting back

surface coatings

Thickness

80 I_m

80 i_m

80 _m

Manufacturer

PST

OCLI

PST

Quantity Of CICs

100

75

6O

150 I_m PST 150

-238-
Table i. MEASAT Solar Cell Assembly Characterization Tests

TEST RANG E/DURATION RESULT

Electrical Characterization

Electrical output 28 deg C; full I-V curve at AM0 Characterized performance

intensity.

...................................................................................... , ................................... ,,, ......................................................................................................................

Covering gain/loss Measure change in Isc when placing z_lsc/Isc = -1.1% with CMG/IRR

............................................................................. .c..o. ve.rs., onc.e!!.s.a_.28.d..eg..C ................................ c.over.g!a..s.s...............................................

Full I-V curves every 10° from 0 o to Charactenzed performance.

Angle of incidence 80° and at 85° EOL deviation from cosine

factor = 0.965 with CMG/IRR

............................................................................. coverglass

Temperature profile i_uii"i-5"'c'u__;";.i'":?_5:-?_'6T-_':'5':"_§:'" .......(,_ara'ci';'_'i';a'"_';_or_anc'e ......

50, 100 and 1S0 deg C ................................................................................ ,,,.,° ............................................................... ,°.,,,.°.o ........................... ,,,,,,,,°,,, .............................................

Spectral response Measure Isc at wavelengths from 300 Characterized response

to 1000 nm.

Reverse bias Full i-V curves before and after reverse Characterized performance; esbias at maximum expected voltage tablished screening procedure.

Radiation Tests

Electron radiation Irradiation with 1 MeV electrons at 4 Characterized performance

fluences from 0 to 1015 e/cm 2 (see Figure 6)

"'#'r'o:_o'n"r;_a'ia'ii_;'_ ......................................... irraai'aiion"_ii_"_"i_'i;9"'l;'r_;io'ns"_i .......................... E)ecjra_'_iion"i'e's;_"i'_';;,n"Jr"£ ........

3.33.1011 and 5 43o1011 p./cm2 model prediction

Reverse bias at maximum voltage after

Reverse bias after radiation 1 MeV electron irradiation at 2.5o10 _4 Passed

e/cm 2

Radiometric Properties

Determine from reflectance Average normal

Emissivity measurements at wavelengths emissivity = 0.84

between 4.8 and 26.2 _m

................................................................................ i'n:i_;'_'ra:t'e'_"_i;'so'_;ian ce'i'rom.......................... "_v_';'_"n_;r_ai"_'13so_i'a_'_e"'" .....

Solar absorptance measurements at wavelengths 0.77 with CMG/IRR coverglass

................................................................................ be_..e.e.n....0.-.2...5....Lo...2:s.._m.: ............................................................................................

Off-angle solar absorptance Measure solar absorptance at angles Cosine weighted average over

of incidence fro m 0 to 71 °. all angles = 0.72 with CMG/IRR

coverglass

Mechanical Tests

Humidity 30 days at 95% relative humidity and Passed

45 de_ C

.._on!.ac!..!n!..e..gr!_y.. ....................................... _.50.._ra.._..p.u!.!.s!reng.!h..re .qu.re_............................................... Passed..............................

Bending test Determine minimum bending radius; Passed at 18 cm

< 38 cm required

Thermal cycling Interconnected cells mounted on

(two 30 cm x 30 cm panels) curved panel; 1600 cycles from -150 Passed

............................................................................... !o 6.0.de_...C...............................................................................................................

Thermal cycling Interconnected cells mounted on

(76 cm x 76 cm panel) curved panel; 1200 cycles from -150 Passed

................................................................................ p....60.., de_...C................................................................................................................

Reverse bias after thermal Reverse bias at maximum voltage after Passed

cycling 1600 cycles from -150 to 60 deg C

-239-
Forthermalcyclingtests,solarcellassemblieswereweldedintostrings,andbondedontosolarpanel

coupons. The first coupon had dimensions 30 cm x 30 cm, and conta[Secr20 solar cell assemblies with 80

I_m thick CMG/IRR covers. The second coupon had dimensions 30 cm x 30 cm and contained a total of 12

solar cell assemblies; four cells had 150 I.tm thick CMG/IRR covers, and the remainder had 80 I.tm thick AR

coated covers (CMG and Coming 0213). Two of the cells on the second coupon with 80 p.m thick covers

were damaged during coupon assembly. These cells would normally be replaced on a flight panel; hence,

they were not included in subsequent testing. The final coupon had dimensions 76 cm x 76 cm and

contained 200 solar cell assemblies; of these, 124 had 150 _m thick CMGARR covers and the remainder

had 80 p.m thick AR coated covers (CMG and Coming 0213).

Electrical Characterization Tests

Electrical performance tests were performed at HSC using a Spectrolab X-25 solar simulator, set to

AM0 using appropriate balloon flown standard cells. Cell temperature was kept at 28 deg C using a water

cooled test fixture. The average electrical output at AM0 and 28 deg C is shown in Table III for each

configuration tested.

Table !11. Electrical Performance Results

Coverglass Dimensions

Type (cm)

None 3.447 x 6.888

CMCVIRR

AR coated

Voc (mY) Isc (mA)

1029 7O9

3.459 x 6.904 1018 720

3.459 x 6.904 1026 730

Prnax Efficiency

(mW/cm2 !

595 18.5%*

580 18.0%**

603 18.7%**

* Based on nominal bare cell dimensions (shown in Table) and a solar constant of 135.3 mW/cm 2.

** Based on nominal coverglass dimensions (shown in Table) and a solar constant of 135.3 mW/cm 2.

Covering gain tests were performed by measuring the change in short-circuit current (Isc) obtained

when placing a coverglass on a cell, and keeping the cell temperature at 28 deg C. To accomplish this,

amyl alcohol (with an index of refraction closely matching that of the adhesive) was used instead of

adhesive in between the coverglass and the cell. This method allowed a comparison of the covering gain

for different covers on the same solar cell. In addition, by testing the same coverglass on different cells,

the effects of variability in cell characteristics were established (they were much smaller than the

differences due to the type of coverglass coating).

The covering gain results are summarized in Table IV. As shown in the table, the covering gain for cells

with IRR covers was negative, indicating a loss. It should be noted, however, that this test only measures

changes in short-circuit current at a constant temperature. As shown below, the IRR covers also provide a

lower value of solar absorptance, which reduces the temperature and increases the cell voltage on orbit.

For the MEASAT spacecraft, shown in Figure 1, the lower solar absorptance also reduces the total heat

input to the spacecraft. The IRR covers were chosen for flight on the basis of all these considerations.

Table IV. Covering Gain Results

Coverglass Type

CMG/1RR

Coverglass

thickness

150 I_m

# covers

tested

21

80 p.m

# cells

tested

i i i

5

CMGARR 80 i.tm 6 5 -0.9%

CMG/AR 6 4 2.1%

Average covering

_ain

-1.1%

-240-
Measurements of electrical output versus angle of incident light were performed at JPL (with the

assistance of Bob Mueller) using a Spectrolab large area pulsed xenon solar simulator (LAPSS). I-V curves

were taken at angles of incidence ranging from normal incidence to 85°. To summarize the results in a

form which is applicable to spinning spacecraft, a loss factor was calculated which represents how the

output deviates from the simple cosine loss rule. This factor, known as the deviation from cosine factor, is

defined as follows:

F(Imp) =

.dO

Imp(0=0) J'(cose),de

where F(lmp) = deviation from cosine factor for Imp

Imp= current at maximum power measured at angle of incidence e

and the integral is taken over the range of angles from -90° to +90 °. For most silicon solar cells, the

deviation from cosine factor is roughly 0.97 (representing a 3% loss) [5]. The results of this testing

showed a deviation from cosine of 0.954 for unirradiated GaAs/Ge cells with IRR covers; for irradiated

GaAs/Ge cells with IRR covers, the deviation from cosine was 0.965 (cells were irradiated at 2.5.1014

e/cm 2 with 1 MeV electrons). GaAs/Ge cells with IRR covers are expected to have a slightly lower factor

because the infrared reflecting band moves toward the visible wavelengths at high angles ot incidence.

Temperature profile tests included measurements of full I-V curves with solar cells at temperatures

from -150 deg C to +150 deg C. Bare 2 cm x 2 cm cells were used for this test. Cells were irradiated with 1

MeV electrons at fluences of 0, 1014, 5,1014 and 1015 e/cm 2. Two cells of each fluence were tested. The

results are shown in Figures 2 through 4. As shown in the figure, these cells exhibit the characteristic

decrease in voltage and increase in current, with increasing temperature, over most of the temperature

range.

1.40 T......................................

1.3o ................................

1.20 __----__, -_-=-_ .........................

1.10 __"'''_'_",._-'"'=_- _< .................

1.00 _"........ __-_,_ - ............ o,o .... ......

> ...... 0.80

0.60 ......................................

0.50 ! I 1 !

-150 -50 50 150

Temperature (deg C)

_"_ 0 fluence

1E14 elcm2

_._ 5E14 e/cm2

1E15 etcm2

Figure 2. Open-Circuit Voltage Versus Temperature

-241 -
,1........................

...... ... ii _ I _ I _ D.._-- i i_I .--.

0.9 ............................................. UDirxed_d ....

II

0

z

0 , 8 IIIIIII I _ IIII II IIIl lllIIIIl: IIl I I llI I I lllIIIIlllll llIIIl Ill

0.7 I I I I I

-150 -100 -50 0 50 100

I

150

Temperature, deg C

Figure 3. Normalized Short-Circuit Current Versus Temperature

D.

E

13.

"O

q)

N

O

z

1.20 .............................................

1.10

1.05

1.00

0.85

0.80

0.75

0.70 I I I I I I

-150 -100 -50 0 50 100 150

Temperature (deg C)

4

Figure 4. Normalized Maximum Power Versus Temperature

Spectral response results are shown in Figure 5. Spectral response was also measured on 2 cm x 2

cm bare cells. These results show that, like silicon cells, the GaAs/Ge solar cell response is more

sensitive to radiation degradation at longer wavelengths. This change in spectral response is probably

responsible for the difference observed between the deviation from cosine factors for irradiated and

unirradiated cells. The irradiated cells are more dependent on light at blue wavelengths and less

dependent on light at red wavelengths. As a result, the irradiated cells are less affected by the

movement of the infrared reflecting filter to shorter wavelengths at high angles of incidence.

-242-
0.6 I .......................... ;'m;m_"_m _......... 0.5 -_

0.3 --- . ......

02 ....................... .......

........................... ......

o i ..........

300 5O0 700 900

Wavelength, nm

--,-- 0 Ftuence

_1E14

--°--5E14

----O----- 1E 15

Figure 5. Solar Cell Spectral Response

Reverse bias testing was performed on 370 solar cell assemblies. The procedure used was similar to

the screening procedure discussed in reference 6. However, the MEASAT requirements were slightly

less severe than those in the reference because the power system design restricted the maximum

voltage which a solar cell could reach in the reverse bias condition. The current at this voltage was

generally less than full Isc. Hence, each cell was subjected to reverse bias for one minute at the highest

reverse bias voltage possible on the MEASAT solar array (5.5 V). Electrical output was measured before

and after reverse bias using the X-25 solar simulator. Cells which exhibited a measurable decrease in

maximum power were considered failed. This established a procedure for screening of MEASAT flight

cells; only cells which passed this test were accepted for flight.

Further testing was performed to determine whether cells which passed the screening process once

would pass again after exposure to environmental testing. Environmental testing included exposure to

long duration reverse bias, multiple reverse bias exposures, radiation and thermal cycling. Long duration

exposure was performed on 25 cells up to 30 minutes. Multiple exposures were performed on 50 cells up

to five exposures. Radiation was performed on 7 cells with i MeV electrons at a fluence of 2.5-1014

e/cm 2. Thermal cycling was performed on coupon #2 and consisted of 1600 cycles on from -150 deg C to

60 deg C. The results showed that each cell which passed the screening test before exposure to the

environment also passed the screening test after exposure.

-!

Radiation Test Results

Radiation testing included electron, proton and ultraviolet exposure. Electron radiation was performed

at the JPL Dynamitron Laboratory. Large area bare solar cells were irradiated with 1 MeV electrons at

fluences of 0, 1014, 5-1014, and 1015 e/cm 2. At each fluence, 8 GaAs/Ge cells and 6 silicon control cells

were irradiated. The silicon controls served mainly to verify the radiation fluence. The results are

summarized in Figure 6. Regression plots for the normalized degradation of each operating parameter are

shown in the figure. For maximum power the normalized degradation factor at 1015 e/cm 2 was 0.765. This

value is slightly less severe than the value of 0.74 reported in the Solar Cell Radiation Handbook [7].

-243-
1.00 1

u 0.95 " .-. Imp Voc and Vmp

0.85

0.80

i._

O

z

0.75

1014 015

Fluence (e/ore2)

Figure 6. Radiation Degradation With 1 MeV Electrons

Proton radiation was performed at the Caltech Van de Graft accelerator. Small area bare solar cells

were irradiated with 9 MeV protons at fluences of 3.33.1011, and 5.43-1011 p/cm 2. Four GaAs/Ge cells

and two silicon controls were irradiated at each fluence. The predicted degradation factor for maximum

power was calculated using the proton relative damage coefficients [8], the equivalence factor for

conversion from 10 MeV protons to 1 MeV electrons [8] and the electron degradation curve shown in

Figure 6. The predicted degradation factors were 0.83 and 0.80 for the two fiuences tested; the

measured values were 0.86 and 0.81 for the two fluences respectively. In each case, the measured

degradation was slightly less severe than the predicted values. To complete the database for the

MEASAT program, these results were augmented by additional proton radiation data provided by

Spectrolab Inc..

Ultraviolet exposure was performed by James Chow et. al. at Hughes Aircraft Company Electro-Optical

Systems Segment. Electrical performance of solar cells during this test was measured using the LAPSS at

JPL with the assistance of Bob Mueller. Ultraviolet exposure and performance testing were both

performed with the cells under vacuum at 10 -5 torr. GaAs/Ge test samples included 4 cells with CMG/AR

covers, 4 cells with CMGARR covers and 4 cells with AR coated Coming 0213 covers. Control samples

included 4 silicon cells with fused silica covers and 4 silicon ceils with removable fused silica covers, and

no coatings or adhesive. Test methods for the UV exposure test are described in reference 9. All cells

were exposed to 3100 sun-hours of equivalent UV between the wavelengths of 200 and 400 nm.

Electrical output was measured after 0, 400, 1200, 2900 and 3100 hours. Based on the data, it was

concluded that UV effects saturated witflin 1200 hours of exposure. The results are summarized in Table

V. All GaAs/Ge solar cell assemblies showed a net degradation of less than I%.

Radiometric Properties

Normal emissivity was measured using Gier-Dunkle DB-100 infrared spectrophotometer. Solar

absorptance was measured using a Lambda 9 spectrophotometer with an integrating sphere. Solar

absorptance measurements were performed by James Chow and Lillian Hunter in Hughes Aircraft

Company Electro-Optical Systems Segment. The results for normal emissivity solar absorptance are

summarized in Table VI.

-244-
Table V. Ultraviolet Exposure Results (3100 UV hours)

GaAs/Ge cells with

CMG/AR covers

GaAs/Ge cells with

CMG/IRR covers

Average loss in lsc, 1.4% 1.6%

raw data

Measured system 0.9% 0.9% 0.9%

effects

Net loss in Isc T.T

(raw data less 0.5% 0.7% 0.6%

systems effects)

Table Vl. Normal Radiometric Properties

GaAs/Ge cells with GaAslGe cells with GaAs/Ge cells with

CMG/AR covers CMG/IRR covers AR coated Corning

0213 covers

Normal emissivity 0.84 0.84 0.84

GaAs/Ge cells with

AR coated Coming

0213 covers

1.5%

Normal solar 0.90 0.77 0.90

absorptance

Solar absorptance was also measured as a function of the angle of incidentlightfor GaAs/Ge cells with

CMG/IRR covers. Measurements at off-normal angles were made using the double bounce technique

developed by Andrew Meulenberg of Comsat Laboratories. This method uses a pair of solar cells placed

within the integrating sphere such that the incoming light is reflected from one cell to the next. Six

samples were measured at angles up to 71o. To determine the effective solar absorptance with CMC_MRR

covers on a spinning spacecraft, the cosine weighted average absorptance was calculated as follows:

(z(e) cos e de

Effective solar absorptance =

f cos e de

where ¢(= solar absorptance and 9 = angle of incidence. To estimate the error inherent in this calculation,

the data was fit with several different fitting functions. The final result was an effective solar absorptance of

0.72 + 0.02.

Mechanical Tests

Mechanical tests included humidity exposure, contact integrity tests, bending strength and thermal

cycling. Humidity exposure consisted of 30 days at 95% relative humidity and 45 deg C. A total of 4 bare

cells and 8 assemblies with assorted covers were tested. The maximum allowed degradation in Pmax was

2%. The bare cells passed with an average degradation of 1.0%; the assemblies passed with an average

degradation of 1.6%. No difference was observed in the performance of different coverglass types.

Contact integrity tests included front and back tape peel tests, interconnect pull strength, and back

contact pull strength. Each test was performed on both bare cells and assemblies. Tape peel testing

revealed no evidence of contact or coating delamination. The required pull strength was 150 grams

minimum. All cells and assemblies passed.

-245-
Bendingstrengthwasmeasuredbysubjectingassembliesto bendingaround a cylinder of known

radius [5]. Cylinders of decreasing radii were used until visual inspection showed breakage of the

coverglass or solar cell. A bending radius of 38 cm or less with no breakage was required. Four assemblies

with 80 _m thick covers and four cells with 150 _m thick covers were tested. Cracking was first observed at

a bending radius of 18 cm for assemblies with 150 I_mthick covers, and at 15 cm for assemblies with 80 p.m

thick covers.

Thermal cycling was performed on the 3 solar panel coupons described above. Coupons #1 and #2

underwent 1600 cycles from -150 deg C to +60 deg C. Coupon #3 underwent 1200 cycles from -150 deg

C to +60 deg C. Electrical output was measured before and after thermal cycling. Measurements were

taken at Spectrolab Inc. with the help of Robert Dally using a large area pulsed xenon solar simulator

(LAPSS). The maximum allowable degradation due to thermal cycling was 2% in Pmax. Coupon #1 passed

with an average measured degradation of 0.9%. Coupon #2 passed with a measured increase in average

power of 0.1% (consistent with no change). Finally, coupon #3 passed with nc) change (0.0%) measured

in average power output.

,_ummary

The solar panels used on the MEASAT spacecraft represent the first GaAs/Ge solar panels on a

commercial communications satellite. The solar cell assemblies installed on these panels have been fully

qualified and characterized by the test program outlined above. The database generated by this test

program can be used to accurately predict the electrical performance of the MEASAT solar array

throughout the mission lifetime. In addition, the test results have demonstrated the ability of the solar cell

assemblies to successfully withstand all mission environments.

References

[1] P.A. lies and F. Ho, Technology Challenges For Space Solar Cells, 1994 IEEE First World Conference

On Photovoltaic Energy Conversion Vol. II, IEEE, p. 1957, 1994.

[2] C.A. Kitchen, A.A. Dollery, K. Mullaney, K. Bogus, Teflon Bonding Of Solar Cell Assemblies Using

Pilkington CMZ & CMG Coverglasses - Now A Production Process, 1994 IEEE First World Conference

On Photovoltaic Energy Conversion Vol. II, IEEE, p. 2058, 1994.

[3] Qualification Test Results For Blue-Red Reflecting Solar Cell Covers And Other New Products For The

Solar Power Market, P. Beauchamp, B. McLean, M. Larro, 1994 IEEE First World Conference On

Photovoltaic Energy Conversion Vol. II, IEEE, p. 2064, 1994.

[4] Infra Red Reflective Coverglasses: The Next Generation, K. Mullaney, G.M. Jones, C.A. Kitchen, D.P.

Jones, IEEE 22nd Photovoltaic Specialists Conference, IEEE, 1991, p. 1593, 1991.

[5] J.S. Fodor, S.W. Gelb, L.J. Goldhammer, G.S. Goodelle, A.L. Judson, Characterization Testing Of

Large Area Solar Cell Assemblies, Proceeding Of The 19th IEEE Photovoltaics Specialists

Conference, IEEE, 1987

[6] L.A. Rosenberg and S.H. Gasher, Reverse-Bias Screening Of Large-Area GaAs/Ge Solar Cells At Low

And High Temperature, IEEE 22nd Photovoltaic Specialists Conference, 1993, p.1421.

[7] B.E. Anspaugh, Solar Cell Radiation Handbook Addendum 1: 1982-1988, JPL Publication 82-69

Addendum 1, Jet Propulsion Laboratory, p. 37, 1989.

[8] B.E. Anspaugh, Proton and ElectrOn Damage Coefficients For GaAs/Ge Solar Cells, IEEE 22nd

Photovoltaic Specialists Conference, 1993, p.1593.

[9] G.S. Goodelle, G.R. Brooks, J.R. Mosher, High Vacuum UV Test Of Improved Efficiency Solar Cells,

IEEE 1lth Photovoltaic Specialists Conference, IEEE, 1975, p.184-9.

-246-
ADVANCED SOLAR PANEL DESIGNS 1

E. L. Ralph and E. B. Under

Applied Solar Energy Corporation

City of Industry, CA

Solar panel designs that utilize new high-efficiency solar cells and lightweight rigid panel technologies are

described. The resulting designs increase the specific power (W/kg) achievable in the near-term and are well

suited to meet the demands of higher performance small satellites (smallsats).

Advanced solar panel designs have been developed and demonstrated on two NASA SBIR contracts at Applied

Solar. The first used 19% efficient, large area (5.5 cm x 6.5 cm) GaAs/Ge solar cells with a lightweight rigid

graphite epoxy isogrid substrate configuration. A 1,445 cm2 coupon was fabricated and tested to demonstrate 60

W/kg with a high potential of achieving 80 W/kg.

The second panel design used new 22% efficiency, dual-junction GalnP2/GaAs/Ge solar cells combined with a

lightweight aluminum core/graphite fiber mesh facesheet substrate. A 1,445 cm2 coupon was fabricated and

tested to demonstrate 105 W/kg with the potential of achieving 115 W/kg.

1.0 INTRODUCTION

This paper will address the construction details for the GaAs/isogrid and dual-junction GaAs/carbon mesh panel

configurations. These are ultimately sized to provide 75 Watts and 119 Watts respectively for smallsats or may

be used as modular building blocks for larger systems. A detailed weight estimate for each design is provided in

Table 1.0-1.

GaAs/isogrid and dual-junction GaAs/carbon mesh coupons have been fabricated and tested to successfully

demonstrate critical performance parameters and results are also provided here.

2.0 GaAs ISOGRID SOLAR PANEL ASSEMBLY

The ultimate technical objective is to develop and demonstrate standardized GaAs composite isogrid solar arrays

that provide -80 Watts per kilogram (W/kg) specific power and can be built for one-third current costs within a 3

month turnaround from order to delivery. The design will produce a 75 Watt array power output at Beginning of

"Life (BOL) using a modular configuration that is ideally suited for small satellites (smallsats) for remote sensing,

communication and experimental applications. This is significantly less time and money required to build solar

arrays using current practice and is an extremely attractive solution for smallsat developers seeking the best

performance value. Hardware and manufacturing processes developed during this program will be intended for

production use at a rate of two to three solar array shipsets per month, enabling a long term moderately sized

annual product line.

A preliminary design concept of a GaAs composite isogrid solar array was developed for potential future

production. This 75 Watt BOL, 0.94 kilogram solar array is ideally sized for smallsat developers and features

adaptable voltage output and mounting locations for interface flexibility. Figure 2.0-1 shows the baseline array

which includes large GaAs solar cells on a composite isogrid panel substrate, routing wiring and an attachment

hinge for spacecraft interface. The routing wiring may be integral to the panel facesheet which is used to isolate

The prima_ basis of the work discussed was two NASA MSFC SBIR Phase I contracts; NAS8-4057 advanced lightweight

high performance isogrid solar array, and NAS8-4056 multijunction GaInP/GaAs/Gc solar cells on carbon mesh substrate.

-247-
thesolarcoilsfromthesubstrate.Integralwiringreducesarrayassemblylaborforreducedcostandschedule

andimprovesre!lability.The=wires" are actually metal strips sandwiched in the polyimide facesheet structure

and are pre-printed by design. Array voltage is designed for either 35 or 70 Volts by selection of appropriate tap

points on the output connector. Four arrays can readily be used by a single spacecratt to provide 300 Watts total

power. The conveniently located hinge design will be both lightweight and reliable and may be detached for user

flexibility. The panel will be reinforced along the hinge mounting bar to support other attachment designs if

needed. The array will meet or exceed performance and manufacturing objectives because it uses:

0 large area (36 cm2) GaAs solar cells drawn from ongoing high volume production for ready

availability and low cost

0 lightweight composite isogrid technology that is rapidly fabricated with reusable tooling

0 a standardized, modular design that meets user requirements

0 a production ready configuration that has no non-recurring engineering cost

A proof-of-concept GaAs composite isogrid array coupon was successfully developed and demonstrated as an

initial project milestone and is shown in Figure 2.0-2. As shown, there are 30 solar cell assemblies on the panel

which are identical to current Applied Solar high-volume production cells. Measured results show 18.09% panel

efficiency at 28 °C, Air Mass Zero (AM0) which is near the 19% program goal. The best cell on the coupon

measures 18.99% efficiency. The panel substrate was manufactured by the Structures and Controls Division of

the Air Force Phillips Laboratory. The coupon successfully passed thermal cycling tests and was also examined

both visually and after X-ray without notable defects. Total coupon specific power is about 60 W/kg which is

approaching the 80 W/kg target. The total coupon manufacturing and test cycle time was under 35 calendar

days which met project objectives.

By successfully developing and demonstrating a working GaAs composite isogrid solar array coupon, this project

has shown that notable performance and producibility improvements can be gained and forwarded to the

smallsat developer who needs low-cost and high-performance solar arrays. Based on these benefits, it is

concluded that GaAs composite isogdd solar arrays, as demonstrated here, is an important new capability, and

should be qualified and brought into commercialization as soon as possible.

2.1 GaAs SOLAR CELLS

In recent years, GaAs solar cells have displaced silicon as the technology of choice for space systems. GaAs

solar cells fabricated on producible germanium substrates offer improved conversion efficiency (>18.5%) at a

competitive price. Applied Solar began developing GaAs technology in 1981 and initiated production in 1984.

Presently, Applied Solar is continuously producing GaAs solar cells from an existing facility with over 125 kW

annual capacity.

Applied Solar will deliver over 170,000 5.5 cm x 6.5 cm solar cells by 1997 for a single contract. The active

layers are grown on a 140 mm thick germanium substrate using Metal Organic Chemical Vapor Deposition

(MOCVD) and the total finished cell thickness is approximately 160 mm. Cells are normally covered with a

coverglass which provides protection and optical filtering. The baseline uses a 100 mm thick ceria doped

borosilicate glass. The cell structure is a P-on-N GaAs design on the N-doped germanium substrate which has

demonstrated good performance results with a producible manufacturing process. This is a two terminal device;

interconnect is made to the back surface metal contact and to a front metal contact which collects current from

gridlines that are distributed across the face of the coll.

To date, 50,000 large GaAs solar cells have been produced with 18.5% average conversion efficiency as shown

in Figure 2.1-1. 19% is the mode and efficiencies in excess of 19% are achievable by selecting high-end cells

from the population. Current large cell production rates are between 3,000 and 6,000 cells per month, and at

~0.9 Watts output per cell, this translates to about 60 kiloWatts per year for this single program.

-248-
2.2 " ISOGRID PANEL SUBSTRATES

isogrids are lightweight shell structures stiffened by triangular stiffening ribs integrally co-cured or bonded to the

shell. Carbon fiber/polymer composite isogdd structures are preferred to conventional metallic designs as they

offer relatively high strength and stiffness to weight ratios, flexibility in design, custom tailoring of desired

materials properties, and the ability to perform in diverse environments. Additionally, metal structures require

long manufacturing lead times compared to composites which can use advanced fabrication techniques including

automated processing.

Composite isogrid technology was originally developed in the 1970's and the Phillips Laboratory initiated

research in 1990. Several promising application areas were identified including aircraft and naval vessel

structures, terrestrial applications such as infrastructure (bridges) and automotive and space structures including

launch vehicles and spacecraft/solar panels. The composite isogrid solar panel design concept shown in Figure

2.2-1 has several performance advantages compared to the traditional aluminum honeycomb structure with

aluminum or graphite/epoxy skins. These include at least a 15% weight reduction and improved thermal

protection which reduces solar cell operating temperature by 20-30% and translates into a smaller array area

needed to meet power requirements. These size reductionscombine with improved manufacturing processes to

result in lower panel cost. Composite isogridspecific weight as a function of panel size is shown in Figure 2.2-2.

3.0 GalnP=/GaAs/Ge On CARBON MESH PANEL ASSEMBLY

The preliminary multijunction (MJ) carbon mesh panel for the smallsat mission is shown in Figure 3.0.1. In this

case, we utilize 22% efficient GalnP2/GaAs/Ge solar cells. Using the maximum power voltage of 2.093 V/cell for

the MJ, results in a 17 cell series string to meet 34 V bus voltage. Using a cell size of 5.0 cm x 4.9 cm, the

layout on the panel is 17 series x 10 parallel. The layout sketch shows two circuit segments -17S x 5P wired in

parallel which provides 119 W at 34 V (AM0 28°C). This represents a 67% increase in power output BOL (28°C)

for the same size panels, thus demonstrating the significant improvement available using the advanced

technology being developed.

The demonstration panel coupon is 38.1 cm x 38.1 cm in size with one circuit consisting of 30 series connected

GaAs/Ge solar cells (5.5 cm x 6.5 cm) and a second circuit consisting of 16 series connected MJ solar cells (2

cm x 4 cm). It was successfully fabricated and tested and is shown in Figure 3.0-2. CMX coverglass, 0.010 cm

(4 rail) thick with an A/R coating on the top surface, was selected for the MJ cells. The coverglass covers 100%

of the solar cell. The standard Applied Solar interconnect made from 25 i_n silver-plated Invar was selected.

The interconnect has an out-of-plane stress relief loop and there are two interconnects soldered to each MJ cell

and three interconnects on each large area GaAs/Ge cell.

The panel substrate is an advanced lightweightdesign fabricated by Ketema Corporation. It uses a one half inch

thick aluminum honeycomb core with high strength lightweight 0.005 inch thick "1"300/934graphite cloth/epoxy

resin facesheets. A 0.002 inch thick layer of Kapton is co-cured to one facesheet as a dielectric layer. The

weight of the 38.1 cm x 38.1 cm panel substrate is 138 grams for a specific weight of 0.9 kg/m" (0.19 Iblft')l

The 16 MJ cells plus the 30 large area GaAs/Ge cells represent a total of 128 cm2 and 1072.5 cm2. Average

output of the MJ cells was measured and this represents a 33.3 Watt coupon power output capability. Using the

actual measured weight of the coupon, one then gets a demonstrated specific power value of 105 W/kg (9.5

kg/kVV) and a value of 230 W/m" based on the total coupon area. The packing factor on this small

demonstration coupon was only 83% so this is really not representative of a larger panel which typically is 90-

95%.

3.1 Multijunction Solar Cells

This solar cell is a 22% efficient, two-junction device, built in a tandem configuration with only two contacts. The

active layers are deposited in series by MOCVD on a germanium wafer with a grid contact on top and a full cell

contact on the bottom. Therefore, from an external appearance standpoint, it looks identical to the conventional

GaAs/Ge or even Silicon solar cells. The two junctions are fabricated within two different semiconductor

materials with bandgaps selected to provide sunlight absorption over a wide wavelength range and optimize

-249-
collectionefficiencyfor eachbandgap.Electricaldataof a solarcellusedinthe panelcoupon are shown in

Figure 3.1-1. Germanium wafers are used as the substrate for the MOCVD deposition for cost reasons as in the

GaAs/Ge solar cell case. This cell is just completing qualification testing this year. It will become available in

production quantities in 1996. The early productionruns will start out with 2 cm x 4 cm by 200 idn thickness size

cells but with time will increase in size and become thinner, the same as GaAs/Ge type cells did for economic

reasons. For this project the 2 cm x 4 cm size cell was selected because of its early availability.

4.0 Benefits and Conclusions

Spacecraft designers often have different reasons for using an advanced solar cell array technology. Overall

system cost is usually a strong driving factor; however, there are those cases where the advanced technology is

enabling and the cost becomes secondary. In any case, the advanced technologies demonstrated here provide

another step function advancement in solar array performance. The higher efficiency and lighter weight panels

at beth 80 and 115 w/kg are significantly better than any existing technology presently being used on spacecraft.

Because of the high efficiency, the array area becomes smaller thus providingcost savings, since attitude control

fuel and drag recovery fuel is saved, and allow better stowage for launch. In addition the "standardized" isogrid

panel offers an attractive low cost short schedule solution to smallsat requirements.

Cost projections in the trade studies have shown that the mulUjunction solar cell cost be slightly higher (15-20%)

than the GaAs/Ge cells presently used; however, at the array level, costs will be about 5% less due to the fewer

number of cells needed on an array to achieve the same power level. At the overall system level, considenng

the value of the reduced weight, the cost benefits become very large and can represent cost savings equal to the

total cost of the array.

Before satellite designers utilize new advanced technology concepts such as those demonstrated in this program,

they need to be convinced that the performance properties are there and that the product is reliable. This usually

means that a large enough solar panel be built and environmentally tested to verify the design and provide

confidence. The design concepts presented here would be an excellent starting point. We have been in contact

with various potential users of this advanced users of there advanced technologies and there is genuine interest

in seeing them come into commercialization.

-250-
Table 1.0-1 Detailed weight estimates for each design approach

Performance summary Phase Ii Isog_rid Array

cell material

cell size

number of cells on panel

_anel size

panel power

array weight

array specific power

Solar Cell Assy Characteristics

cell material

cell size

cell thickness (l_'n)

coverglass thickness (l_n)

% efficiency (28 °C, AM0)

maximum voltage (V)

maximum current (A)

maximum power ON)

cell weight (gins)

Circuit Characteristics

cells per series string

stnngs in parallel

assembly loss (%)

diode voltage loss (V)

panel output voltage (V)

panel output current (A)

panel output power (W)

Panel Substrate Characteristics

structure material

structure thickness

skin (faceplate) material

skin thickness"

_eight density (bare)

)anel area

Panel Mass Analysis (all units in grams)

4dd-On Hardware:

solar cell

3MX coverglass

coverglass adhesive

circuit interconnects

cell adhesive

_nd terminations

wire, 26 AWG

terminal board

spot bonds

Insulated Panel Substrate

Hinge

Growth margin

gallium arsenide

5.5 cmx 6.5 cm

86

64 cm x 55 cm

75.1 W

.94 kg

80 W/kg

gallium arsenide

5.5 cm x 6.5 cm

139.7

100

19%

0.86

1.069

0.919

3.13

43

2

4%

0.36

35.14

2.14

75.13

graphite epoxy isogrid

0.863 cm

graphite epoxy plane

508

1.3 kg/m 2

64 crn x 55 crn

269.18

71.19

21.23

12.9

27.49

0.8

4.57

9.0

1.2

457.6

35.0

30.0

Total Estimated Mass 940.15

Phase II M J/Mesh Array

GalnP2/GaAs/Ge

5.0 cmx 4.9 cm

170

50.8 cJ'n x 88.9 cm

119W

1.o4_-g

115 W/kg

GalnP2/GaAs/Ge

5.0 cm x 4.9 cm

139.7

100

22%

2.03

0.359

0.729

2_15

17

10

4%

0.00

33.13

3.59

118.94

AI honeycomb

1.27 cm

graphite epoxy mesh

127 i_1

0.9 kg/m 2

50.8 cm x 88.9 cm

365.5

95.8

28.56

17.0

37.24

1.6

9.14

g.0

2.4

406.45

35.0

30.0

1,037.68

-251 -
Figure 2.0-1 The baseline Gaits composite isogrid solar array provides 75 Watts and weighs less than 1

kilogram.

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-252-
Figure 2.0-2 Proof-Of-Concept GaAs composite isogdd panel coupon measurees 38.1 crn on a side

-253-
Figure 2.1-1 19% conversion efficiel_cy solar co/Is are the most frequently produced by current production.

..:.:::.'...

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APPLIED SOLAR

1995 GaAs/Ge CELL PRODUCTION

(36 cm =)

DISTRIBUTION (%)

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30-_

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to

16.5 16.8 17.1 17.3 17.6 17.9 18.1 18.4" 18.7 18.9 t9.2 t9.5

CELL EFFICIENCY (%)

Figure 2.2-1 Composite isognd array technology uses a low-density triangular structure for high strength and

stiffness to weight ratios.

-254-
Figure2.2-2 Panels with edge lengths greater than 50 cm derive the most benefit from composite isogrid

technology

.-. 3

v

2

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0_5

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0 20 40 60 80 100 120 140 160 180 200

Square Panel Side Length (cm)

Figure 3.0-1 Mu/tijunct/on carbon mesh pane/provides 115 W/kg

889 mm (35")

O

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17S x 5P

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Cell:

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Multijunction Cell Panel

GaInP_/GaAs/Ge 2J 22%, 5.5 rail

5.0 cm x 4.9 cm, 6 rail CMG cover

17S x 10P, 170 cells/panel, 119 W at 34 V (AMO 28°C)

-255-
Figure3.0-2Proof-of-concept 2J.GaAs carbon mesh panel coupon measures 38.1 crn on a side

Figure 3.1-1 A 22.2% (Best) 2 cmx 4 cm 2J-GaAs solar cell used on the demonstration coupon

t_

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-256-
Space Qualification of UV and IR Reflecting Coverslides

for GaAs Solar Cells 1

Andrew Meulenberg (301-926-3609)

Consultant

Gaithersburg, MD 20855

ABSTRACT

As part of the space qualification effort for blue-red reflecting coverslides designed for use with GaAs

solar cells, the first long-term (3000 hours) UV testing of unirradiated and 1 MeV electron-irradiated GaAs solar

cells, with 4 types of multilayer-coated coverslides to reduce operating temperature, has produced some

unexpected results. Important conclusions from this study, which includes two parallel tests, are:

• All of the GaAs solar cells with multilayer-coated coverslides display UV degradation. The laboratory data,

extrapolated to 10 years in orbit, point to a significant loss mechanism from a combination of absorption and

a reduction in optical match in such coatings from this portion of the space environment.

• The effects of contamination in a vacuum system, on the measured degradation in solar-cell short-circuit

current dudng a UV test, depend upon the type of coverslide coatings present on the coverslide surfaces.

This has implications for both coated coverslides and optical solar reflectors (OSRs) in space.

• Because of the observed trends in this test and uncertainties in the extrapolation of data for multilayer-coated

coverslides, the use of any multilayer-coated coverslides for extended missions (>1 year) cannot be

recommended without prior_ testing.

Coverslide,.Coaq_ngs:

ARR =

IRR =

UVR =

BRR =

DSR =

SSR =

NOTATI O N

antireflecting

infrared (IR) reflecting

ultraviolet (UV) reflecting

blue-red reflecting (UVR on front; IRR on back)

double-sided coated coverslides (coating contains both UVR and IRR in

the same namow-ba_-pass multilayer coating)

single-sidedcoated coversiides (coating contains both UVR and IRR in

the same narmw-ba_-pass multilayer coa_ng)

-- INTRODUCTION

Five types of coverslide coatings, designed for GaAs solar cells, have been tested as part of a NASAsponsored, space-flight qualification for Blue-Red-Reflecting (BRR), multi-layer-coated, eoverslides. Covered

cells have been tested for degradation from the thermal, solar UV, and radiation environments representative of

near-earth orbits. In add'Son, humid'_y tests were performed to determine the sensitivity of the muttilayer

1 This paper is based on _ performed at COMSAT Laboratories under contract from Goddard Space Flight

Center. The final analysis and paper presented here was funded by Hix Consultants.

-257-
coatingsto pre-launchconditions.Coverslidesandsolarcellswere characterized at each step of the test to

identify the variation within the components and the reasons for changes observed with each step.

The major concern about any of these coverslides is how will they perform at end-of-mission. Electron

irradiation, which alters the spectral response of the solar cells, and UV exposure, which degrades the optical

coatings, are the tests most likely to alter the relative merits of the coverslide coatings. The electron results are

discussed elsewhere in this proceedings (1) and will only be alluded to here where necessary for completeness.

The pdmary emphasis in this paper is the UV degradation studies and their implications.

A number of unusual results Were obtained from this sedes of tests on coated coverslides for GaAs solar

cells. Since such accurate measurements of degradation from extended UV exposure to GaAs solar cells had

not been made before, several variations were introduced into the testing to avoid the pitfall of making

judgments based on incomplete data. These vadations included the UV exposure of both irradiated and

unirradiated cells, since silicon solar cells show a further decrease in electrical output when subjected to longterm UV exposure (>1000 hours) after electron irradiation (2). To further extend the effective UV exposure (or

accelerate the UV degradation rate), a number of the coverslides were inverted when mounted on the solar cells

(to expose the critical multilayer coatings to full UV without the filtedng provided by the coverslide which has

built-in UV absorption charactedstics). The effects of these variations will be described with the data from the

tests.

COVERSLIDE AND SOLAR CELL CHARACTERISTICS

The coverslides differ in the manufacturer of the ceda-doped microsheet material (Pilkington CMG and

Dow-Coming 0213) and in the types and manufacturer of coatings used. Variations in the results could not be

attributed to the coverslide material.

INTELSAT-6 silicon solar ceils (designated as I-6) provided a basis for comparison with previous UV

degradation studies. These are covered cells of a type that have provided reliable and reproducible UV

degradation data over many years. Their coverslides are ceda-doped microsheet (CMS) and are coated with a

single-layer AR coating. The cells are coated with a titanium-oxide (TiOx) AR coating. These cells are more

responsive to UV light in the solar spectrum than are GaAs cells and therefore are more sensitive to any UV

degradation of the coverslide assembly and/or contamination.

As a baseline for compadson with the multilayer-coated coverslides, standard CMG coverslides, with

only an antireflective (AR) coating on the "out" side (designated as ARR coverslides), were included. The next

level of complexity is provided by the addition of infrared-reflecting coatings on the backside ("in" side) of ARcoated coverslides. This combination is designated as IRR coversiides.

Three types of multilayer-coated, blue-red reflecting, coverslides (designed for GaAs solar cells) were

studied. The simplest of the three replaced the AR frontside coating of an IRR coverslide with a UV reflecting

(UVR) coating that compensated for the UV absorption of the ceda-doped microsheet. The UV-reflection edge of

the coating roughly matched the UV-absorption edge of the coverslide (---50% at 350 nm). This combination,

designated BRR, minimized the loss in solar-ceU short-circuit current (Isc) while still providing a significant

reduction in solar absorption of the non-useful wavelengths.

The second type of blue-red-rejecting coverslide has a combined UV/IR-reflecting coating deposited only

on the front side. This single-side reflector (SSR) has a narrower bandpass than does the BRR coverslide and

therefore a lower Isc; but, it rejects more unusable light. The last type coverslide has identical multilayer UV/1Rreflecting coatings on both sides of the coverslide. This double-sided reflector (DSR) has a somewhat narrower

bandpass than does the SSR coverslide, but it rejects more UV and IR light. Unfortunately, it also rejects more

usable light in the bandpass region. The absorption and optica! mismatch (of the DSR coatings with the

adhesive) limits the advantage provided by the grea_er rejection of both UV and infrared light relative to that of

the SSR coverslides. Therefore, even though the SSR coverslides cannot reject as much useless light as the

DSR coverslides, they are a better compromise between good cell efficiency and low operating temperature

-258-
Figure1displays the spectral reflection for the ARR, BRR, and DSR covered solar cells, to indicate the

range and nature of the coatings described above. The thermal advantage of the blue-red-reflecting coverslides

is significant. Assuming a deployed array and relative to the ARR coverslide, the reduction in cell temperatures

for production GaAs cells with BRR and DSR coverslides would be 17°C and 40°C respectively.

ILl

Z

i.g

..I

It=

I.U

1

0.9

0.8

0.7

0,6

0.5

0.4

0.3

0.2

0.1

0

0.2

f

#. •

•

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• t

• lu •

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0.4 0.6 0.8

ARR I

..... BRR I

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/ \"

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1.4 1.6 1.8 2

WAVELENGTH IN MICRONS

Figure 1. Spectral Reflection of Near-Normally Incident Ught From GaAs Solar Cells

with Three Different Coverslide Types.

Table I compares the average change in Isc when GaAs solar cells are covered with the 5 coverslide

coating types of this test. These covering factors are somewhat higherthan the results of a previous test, but the

difference is attributed to the solar cells (and perhaps to the fact that, in setting the solar simulator intensity, no

GaAs control cells were included to normalize the silicon primary standard cell to the GaAs test cells). Variations

in the solar cell AR coatings were recorded, based on color tint prior to covering, and differences in covering

factor were observed for the different tints. The cells were sorted and selected to provide each coverslide type

with a range of cell tints.

Table I. Percent Change in Isc from Covering of GaAs Cells

Coverslide ARR SSR BRR IRR DSR

% Change 4.5 2.4 2.8 2.3 -4.8

The difference between the ARR and other coated coverslides is -2% except for the DSR coverslide

which has a too narrowly defined bandpass filter and furthermore has absorption and reflection of light in the

usable wavelength region. It is clear that some penalty is paid for the attempt to reduce the infrared light that will

heat the cells without providing any electrical energy. The SSR cell covering values are very good, considering

the amount of UV and IR that they reflect. The IRR results are disappointing in that their IR reflection is similar

to that of the BRR coverslides, butthey have no UV reflectors on the front surface.

For reasons described in the Introduction, a set of the ceverslides were mounted upside down on the

GaAs solar cells. Cells with these inverted coverslides are compared in Table !1and can be compared with the

correctly mounted coverslides described above (Table I). The effects of optical mismatching are clear and these

data are Important in understanding the effects of system contamination and UV degradation.

-259-
Percent Change in Isc from "Inverted" Covering of GaAs Cells

Cover'slide ARR SSR BRR IRR DSR

% Change 1.5 2.1 2.0 -0.8 -5.0

Table II.

UV DEGRADATION

UV degradation of the various coverslides was determined with two vacuum systems - in parallel, 3000-

hour tests to explore several variables. The variables included: coating type and manufacturer;, pre-UV

exposure to 1 MeV electron irradiation; and, coverslide inversion to determine UV-induced changes in refractive

index within the coatings.

Since contamination is an ever-present threat to long-term UV tests, two techniques are used to

determine, define, and quantify this contribution to the results. The use of multiple control cells inside the

vacuum system, but not exposed to the UV source is the principal technique. Not only do these cells provide a

basis for removing systematic variations in the solar simulator, the type of sunshield used gives information on

the loss in cell Isc resulting from any contamination. An "external" light shield is placed over part of the large

fused-silic_ window of the vacuum system. This external shield prevents the UV light source from illuminating

that portion of the window over a number of the control cells and the control cells underneath the shield. It is

removed for electrical measurements with the X-25L solar simulator. An '_ntemal" shield (available on Test 1

only) allows the UV to darken any contamination on the window, but not on the cells. The internal shield is inside

the vacuum chamber to cover the control cells not protected by the external shield; but, it cannot protect the

chamber window. It is mounted on a vacuum rotary feedthru and swung aside for electrical measurements of the

control cells. The internal shield thus provides a long-term measurement of contamination buildup and the rate

and extent of UV degradation due to contamination on the window.

The second technique, to quantify contamination in the system, involves the use of a "cleanup"

procedure that measures the effect on Isc of cleaning the window and cells individually. This cleanup procedure

and results are detailed in Appendix A.

A statistical variation of 0.3% is expected for short term fluctuations in the X-25L and for the

measurement system employed in this test. The averaae, rather than the individual cell values, of the control

cells falls within this range, thus indicating longer-term ddfts in the solar simulator output intensity or spectrum.

Normalization of the test-cell data with respect to the control-cell average will remove this latter effect and

multiplicity of cells in each test-cell type and of measurements reduces the short-term statistical variations. The

ability of the externally-shielded control cells to remain at the 100% level indicated that, despite the obvious

contamination to the window (and the cells) in Test 1, if no UV exposure is experienced, no darkening results.

Test 1 Results

Figure 2 plots the UV degradation results for unirradiated solar cells from Test 1. The figure includes all

corrections for the contamination found in Test 1. Assuming that the window contamination is truly represented

by the loss in Isc of the internally-shielded ARR cell, we made a linear degradation ftt to the data for this cell.

This "window-contamination" curve indicates -4% degradation at 3000 houm. A 0% recovery in Isc, observed in

cleaning certain cells at the conclusion of the test(Appendix A), was unique to the UVR-covered cells. In this

case, losses from UV darkening of the contamination are compensated by the improved transmission over the

usable wavelength range with the addition of contamination between the UVR coating and vacuum. This

improvement, which may be provided by the low refractive index of organic contaminant deposited on top of the

UVR coating, does not appear with the AR coated coverslides. The correction used for the UVR-coated

coverslides (DSR, SSR and BRR) is based on the cleanup data and is slightly more than one half (0.6 times) that

for the AR-coated coverslides.

In a study of the normalized averages of the test cells from Test 1 with the proposed contamination

contribution removed, validity of the measurement, normalization, and contamination-correction procedures is

-260-
indicatedbythedataof theARR and I-6 cells. The average of the 4 ARR cells at each measurement point in

Figure 2 falls within 0.4% of the 99.8% line (0.998_--0.004). If we consider the 30- value to be -0.45%, then _4

~.0.15% which is 1/2 or l/sqrt(n) of the single-cell statistical variation (a1~ 0.3%), where n is the number of cells

in the set. This standard deviation is just what would be expected for the size of the set tested. Fitting the AR

covered cell data to a linear degradation curve would bring the standard deviation to G = +0.2% (i-0.002).

The standard specification for UV tests of silicon cells is that they must display less than 2% degradation

after 1000 hours of UV exposure. If.no correction for contamination were made, the raw data in this test would

dearly fail that requirement. Comparison of the contamination-corrected I-6 cells in Figure 2, with earlier

measurements (2), indicates a consistency with their ~2.5% degradation at 3000 UVSH. The I-6 cell data,

extrapolated to over 10 years exposure in space (to indicate the expected contribution to array current loss from

this mechanism), is consistent with the 4-6% extrapolated degradation observed in many other extended UV

tests on this cell type (e.g., References 2 and 3).

1.01

i 0.991

L_

0.98

O

1::

O

'- 0.97 CO

_D

0.96

0.95

10

• AR

_AR FIT

-- -- -- BRR FIT

[] IRR

.... SSR FIT

• DSR

.... DSR FIT

I-6 FIT

[]

_.o

\ . \\ N

\

\ \

\

100 1000 10000

UVSolarHoum

Figure 2. UV Degradation (Test 1) of Isc for GaAs Solar Cells With Various Coverslide Typos

The DSR covered cells show the most rapid and greatest UV degradation of the GaAs cells despite their

lowest spectral response to UV illumination. Both the SSR and the DSR covered cells displayed a quick drop in

Isc of -0.5% during the early hours of the test. Total degradation of the SSR covered cells (beyond 1000 UVSH)

is about one half that of the DSR covered cells, as could be expected for coverslideswith UV-sensitive multilayer

coatings only on one side rather than tv_. Extrapolation to 10 years in space indicates about 5% degradation to

the SSR coverstides from UV, which is comparable to that expected for silicon cells, but more than the

improvement possible from cooler operation with these coatings. Again, extrapolation with multilayer coatings

must be suspect,

The BRR cells display the least degradation in Isc of all the multilayer covered cells - - - for the first 1000

UVSH. However, beyond that exposure time the rate increases rapidly and, by the end of the test, their

degradation is exceeded only by that of the DSR cells. If normal extrapolation of the data were made to 10 years

-261-
exposure,the degradation in Isc v_uld exceed 10%. While such losses seem unlikely, the data gives great

concern about use of these coatings in space.

Cells covered with the IRR coverslides show an apparent recovery in Isc with UV and vacuum exposure

(-1% after 1000 UVSH). This may be associated with the decrease in Isc observed in these cells before 1000

UVSH, or it may be an independent effect. An early change in refractive index of one layer, which caused a

mismatch in the optical coupling, could be matched by a slower change in refractive index of a second layer,

which brought back the good coupling and/or increased the transmission through the coverslide. As indicated in

Appendix A, a reduction in the contamination correction for the AR coated coverslides v_uld bring the IRR, ARR

and I-6 data points down beyond 1000 UVSH. This could be as much as 1% at 3000 UVSH, which would

indicate a saturation of the IRR UV degradation beyond 1000 UVSH, rather than a recovery. However, it would

also indicate a higher degradation in the ARR and I-6 cells. As mentioned before, the I-6 cells should be more

sensitive to degradation from contamination than the GaAs cells; therefore, the contamination correction for

these cells should be greater than that for the ARR cells. Such a scenario is self consistent and simplifies the

conclusions. It also indicates that the contamination has contributed a systematic error of + 0.5%.

The results of the BRR ceils provides a warning that a change in refractive index, which may have

moderated apparent UV degradation in lsc for hundreds of UVSH, could continue, but to the detriment of these

cells. The selection of materials and, probably, the processing of the multilayer coatings could affect the timing

and extent of the shifts in UV induced shifts observed in the IRR and BRR coverslides. Furthermore, such

changes, observed in all of the multilayer coatings, could have a significant impact on their UV and IR rejection

capabilities.

Test 2 Results

The main points of Test 2 are illustrated in Figure 3 with data that includes the final corrections

mentioned in the appendix. Most of the cells in this test had their coverslides reversed (indicated in the figure by

-r) to determine the effects of filtering the UV light through the coverslide (the DSR coverslides did not change

with inversion); therefore, relative values, not absolute values of change are to be considered in the analysis of

this figure.

A 1% correction has been made in the initial data points (<1 hr) for the rapid darkening and saturation of

that part of the window illuminated by the UV source, and the data are represented by curves, rather than by data

points, to remove some clutter. The ARR cell, with coverslide reversed, shows no UV degradation. In this test,

the DSR cell shows a higher initial drop than does the SSR cell with the multilayer coating inside. This confirms

the Test 1 results that indicate the combined UV/IRR coating clearly has a layer sensitive to UV. The Test 2

DSR results agree with the Test 1 results (within the 0.5% offset from lower degradation in the initial Test 1 data

points), thus supporting the choice of contamination correction made in Test 1 for UVR coated coverslides. The

lower initial degradation of DSR and SSR Isc in test 1 could be a result of contamination-enhanced optical

coupling before UV darkening became significant.

The IRR cell, which showed no initial drop in isc with the multilayer coating properly placed under the

coverslide in Test 1, now displays a 0.5% drop in the first few hours when the coating is exposed directly to the

UV. The IRR covered cell, with inverted coverslide, also shows a significantly higher extended UV degradation

than the cells with correctly oriented IRR coverslides in Test 1, Figure 2 (-2% vs. 0 - 1% at 3000 UVSH). If this

result is a consequence of the unfiltered UM exposure to the multilayer coating, then the implication is that these

layers are more sensitive to energetic radiation (UV with ;¢ < 0.35 pm or 1 MeV electrons) than to the lower

energy UV that passes through the CMG coverslide. This sensitivity to energetic radiation could explain the

higher loss in Isc of the IRR covered cells from the electron irradiation than that observed for the C-,aAscells

covered with other coverslides in the same test?. There is also a hint that electron irradiation predisposes the

IRR multilayer coating to subsequent UV degradation.

-262-
0.99 --

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.... SSR-r fit \..

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I-6 fit \

0.95 ........ : ........... : ....... : '

1 10 I00 I000

\

UV Solar Hours

10000

Figure 3. UV Degradation of Reverse-Covered GaAs Solar Cells in Test 2.

The I-6 silicon cell results from Test 2 are included in Figure 3 for comparison with those data in Test 1

and to provide a visual baseline for the GaAs cell results. The curve to fit the Test 2 data is identical to that used

to fit the Test 1 I-6 data. This again indicates that the contamination correction for AR coated coverslides made

in Test 1 is appropriate.

HUMIDITY AND THERMAL STRESS RESULTS

In addition to the radiation environment testing (electron and UV) performed on the coated coverslides,

other standard tests were performed in the space qualification testing of these coverslides. Thermal-cycling tests

were of particular concern on the DSR and SSR coverslides since their multilayer coatings introduced sufficient

stress in the single-sided coverslides that they bowed; furthermore, slight edge fractures were observed in some

of eadier DSR coatings on thicker coverslides.

The cells were heated to 60°C with a flood lamp and then lowered over a liquid nitrogen bath to bring

them to -150°C. After this v,ot_l-case procedure (to slowly take the coverslide adhesive through the glass point),

the cells were dipped in the LN2 for a sligh_thermal shock. The cycle was completed with a recovery to 60°C

under the flood lamp. The operation took place in a sealed environment to prevent condensation on the cold

cells. No visual effects or changes in lso were observed from this test. No sign of bubbles, blisters,

delamlnations, or color changes associated with distress in, or caused by, mulUlayer coatings appeared. Several

cells had losses in cell fill factor, which indicated that contacts and/or cell junctions were stressed by the severity

of the test. However, no pattem of coverslide type was associated with the losses and such changes are not

generally related to coverslides.

The humidity test, which is performed primarily to detect contact corrosion effects, was expected to have

no effect on the coverslides or only an effect on the DSR and SSR coverslides which have the multilayer

coatings on the outside of the coversiide. Both electrical and reflectance measurements were made on the

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covered cells before and after a 1000 hour test with 90% humidity at 50°C. No statistically significant change in

the cell Isc was measured. However, the reflectance measurements were quite revealing.

Exterior coatings did change under extended exposure to humidity. The changes are consistent with an

increase in refractive index of one or more layers. The changes were smallest for the antireflective and UV

reflective coatings of the ARR and BRR coverslides. While the changes in the blue-red-reflecting outer layers of

the SSR coverslide were somewhat larger, losses were balanced by gains. The IRR coverslides, with their

"buded" multilayer coating, displayed the largest changes in reflectance. Despite a predicted loss in Isc of ~1%,

based on the reflectance data, no Such loss was measured. The only explanation is that the humidity test

changed not just the refractive index of one or more layers of the buried reflector, it reduced their absorption.

DISCUSSION

While variations exist in the individual cells and coverslides of this program and the sample size is

small, detailed analysis of the components and combined structures has allowed an understanding of the loss

mechanisms to be expected from the space environmental effects. There are several things to consider in the

evaluation of coated coverslides.

• Multilayer-coated coverslides do not provide as high a covering factor as AR coated coverslides. The Isc

improvement in covering of cells is 1.5 to 3% for multilayer coverslide coatings, if they are qood (i.e., low

reflection and absorption in the usable wavelength region) and if the cell AR coating is appropriately matched

for the coverslide adhesive. The improvement for coverslides with AR-coatings-only is in the 3.5 to 4.5%

range for similar conditions.

• Multilayer coatings may be degraded by either particulate (electrons and protons) or UV radiation. Since the

coatings consist of different materials with different refractive indices, an individual layer may change with

either or both radiation types. The presence of particulate damage in a layer may increase its sensitivity to

UV damage. Since multiple layers of widely varying refractive indices are desired for these coatings, the

task of finding materials that are insensitive to radiation or that respond in such a manner as to maintain their

proper relationship is more difficult than that for a single layer.

• This interdependence of the coating effectiveness on the different layers means that exposure to damaging

radiation can cause either improvements or degradation in the transmission of useful light. Furthermore, the

dominance of either effect can change with time. Absorption effects are always deleterious and therefore

predictable; refractive index changes are neither. Thus, extrapolation of experimental UV degradation data

for muit[layer-coated coverslides and cells becomes much less certain, if not impossible.

• Thermal improvements in solar-array power performance from multilayer coversiides can vary from 3 to 8%

depending on the nature of the reflectors and the extent of light rejection. Subtracting the difference in

covering factors from the thermal gain significantly reduces the beginning-of-life improvement. Thus, a 1 -

4% net improvement in beginning-of-life array performance is a more reasonable estimate for the coatings

being tested.

• The UV-plus-electron degradation of GaAs cells with multilayer-coated coverslides appears, from this study,

to be on the order of 2-4% for one year in space and 3-9% for 10 years (87600 hours) in space (compared to

the 1-4% net improvement B-O-L from above). This means that after one year in a space radiation

environment, the multilayer coated coverslides may not have any advantage over a simple AR-coated

coverslide. The uncertainty in the extrapolation to 10 years is greater than the improvement possible with

the multilayer coatings. However, beyond one year, the potential for serious losses relative to AR-coated

coverslides is high. The thermal improvement is somewhat less for the IRR coatings; but, the uncertainty in

extrapolating data appears as large as that for the other multilayer-coated coverslides and variations within

the individual coverslides and cells of a small dataset in this test are too large to predict any net benefits for

this coating for extended missions, even in a radiation-free environment.

• The cost of the coatings is not inconsequential. Nevertheless, the savings from use of these coatings for

short missions that are power and/or array-size limited could be considerable.

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Conclusion: Because of the observed trends and the uncertainty in extrapolation of data, the ability of

multilayer-coated coverslides to benefit extended missions (>1 year) is doubtful. The cost of reducing the level

of uncertainty must be weighed against the potential benefits for a particular application.

Of the five coverslide types studied in this program, the IRR coated coverslides provided the most

surpdsing results. The measured sensitivity of these IRR multilayer coatings to specific outside influences

explains a number of observations made during this report:

• The IRR coverslides initially displaYed greater absorption than expected.

• The solar-cell-covering factor was lower than that for the comparable BRR and SSR coverslides

• Degradation in Isc from electron irradiation was higher for the IRR covered cells than for any others

• The degradation of correctly-mounted IRR coverslides on unirradiated GaAs cells displayed unusual

behavior with UV exposure over time. No consistent trend was observed, although the overall losses in Isc

from UV were small (<1%). UV degradation of a cell with 'Inverted" IRR coverslide was more consistent

(nearly linear with exposure) and on the order of 2% after 3000 UVSH.

• While the other multilayer coatings displayed minimal changes from extended exposure to humidity and

heat, the IRR coatings showed major changes in reflectance and, probably, in transmittance.

Conclusion: Whether the humidity and energetic-radiation (1 MeV electrons and UV < 350 nm)

sensitivity of the IRR coated coverslides is characteristic of the materials used, or of the batch of coverslides that

we received, we cannot tell. Furthermore, the observations are from a limited database (single cells in some

cases). Nevertheless, an internally consistent picture has evolved from a data set that initially engendered

significantly less confidence than that provided from the other coverslide sets in the test.

REFERENCES

1. A. Meulenberg, =Photo-Recovery of Electron-Irradiated GaAs Solar Cells," Proceedines. of the 14th

Space Photovoltai¢ Research and Technology Conference, NASA/LeRC, Cleveland, OH, October 24-26, 1995

2. A. Meulenberg, =UVTesting of INTELSAT VII, Vila, and VIII Solar Cells.= Proceedir_s of the 13th Space

Photovoltaic Research and Technolo(:lvConference, NASA/LeRC, Cleveland, OH, June 14-16, 1994

3. A. Meulenberg, "UV Testing of Solar Cells; Effects of Anti-Reflective Coating, Prior Irradiation, and UV

Source," Proceedings of th_...l.2th Space Photovoltaic Research and Technology Conference, NASA/LeRC,

Cleveland, OH, 1992

APPENDIX A: CLEAN-UP PROCEDURES AND RESULTS

Test 1

The Post-UV Clean-up Procedure is critical to understanding the source and effects of any contamination

that might interfere with correct interpretation of the test results. The first step after the UV exposure is

terminated is to measure the cells several times over the next few days to determine the existence any light

effects such as UV "bleaching" of the cove_slides, adhesives, coatings, or contamination. If none of these effects

occur, the repeated measurements provide a good statistical base for this endpoint of the UV test and for the

beginning of the cleanup procedure. A backfill with dry nitrogen provides a basisfor any purely vacuum effects.

Exposure to air provides information on the effects of oxygen and humidity on the measured degradation.

Removal of the window for cleaning provides the first measurements of the cells, without window, since before

vacuum. The clean window is replaced to measure the effects of cleaning contamination from its inner surface.

The window is removed again and the cells arecleaned. A comparison with the prior readings without window

gives a measure of the cell surface contamination. Final measurement of the cleaned cells, behind a cleaned

window, gives a true measure of the effects of contamination.

-265-
Figure A-1 shows the effects o-f the cleanup procedure on the Test 1 solar cell short-circuit currents.

These data are normalized relative measurements [i.e., (I/Io) / (lsAso)] with the initial values measured at the

beginning of the UV test with the cells at 40"0 and under vacuum. Within the statistics (°1 ~ 0.003), there is no

recovery from exposure to nitrogen , air, or normal humidity(measurements 1 through 7). Comparison of

measurements 10 and 11 (with a clean window) to measurements 1-7 in Figure A-l, indicates a recovery of

0.025 - 0.03 (2.5 - 3%) from cleaning the window alone. Comparison of measurements 8 and 9 (contaminated

cells) with 12 - 17 (cleaned cells) indicates a somewhat smaller effect (2 + 0.5%) from cleaning the cells for 3 out

of the 6 coverslide types.

Three coverslide types show no effect of the cleaning procedure. (Remember that all cells in both figures are in

the same test chamber and randomly arranged.) The difference between the coverstide coatings is the only

possibility. The coverslide types that do not change with cell cleaning in Figure A-1 have UV reflecting coatings

and those types without UVR coatings do change. The explanation must be that contamination on the coverslide

UVR coating provides an improved optical match, which increases the light transmission into the coverslide.

Removal of the contamination reduces absorption losses, but increases the reflection by roughly the same

amount. Contamination on the vacuum chamber window does not seem to alter the optical coupling through the

window into the cell assemblies. Although such an effect could explain the difference between changes from

cleaning the window and cleaning the AR coated cells, only the difference in optical coupling resulting from UV

damage would appear in Figure A-1 since the data are normalized against control cells which are not exposed to

UV illumination.

1

_0.98

teD

o

_0.96

m

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O

(/3

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=

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• o° o-

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-- ARR AVG ," " . ,

'-. - - BRR AVG ,,_--__%_- i

--- SS_AW _' ---'-._--_--_-'_ --:.-....._ ._._......" --. --,+,v° / ...." -----"i"- i

.... DSRAW # ,.-----...... .... -.--- ..... ,,..... i

._ ---:..:...'/; / ._._

I J

---';"" -- -. ! f

0.9 , I , I , I ' t ' + ' i ' I ' : I

0 2 4 6 o 8 10 12 14 16 18 20

Measurements

Figure A-1 Change in Normalized Isc for Test I Cells during Post-UV Exposure to Dry Nitrogen (3),

Air (5), and Window (8) and Cell (12) Cleaning Procedures

The Test I results (from the cleanup procedure) indicate that the I-6 and DSR covered cells experienced

-4% degradation of Isc from UV; the SSR and BRR covered cells degraded by, ~3% the ARR covered cells

degraded by ~1%, and the IRR covered cells degraded by 0.3 - 1%. If the 0.5% recovery (seen in points 1

through 7) is in the coverslides, not the contamination, then the above values would be increased by this amount.

-266-
If the control cell values are low by 1%, then all of the above values v_uld be reduced by 1% and the

results would better fit the corrected data in Figure 2. Conversely, if the contamination correction factor for the

AR coated coverslides were reduced (e.g., by 1% at 3000 UVSH), the ARR, IRR, and I-6 degradation in Isc in

Figure 2 would be increased by 1% at 3000 UVSH and agreement with Figure A-2 would be complete, except for

the BRR ceils. This only disagreement is then resolved by noting a different annealing rate for the BRR and SSR

cells in Figure A-2.

Test 2

The Test 2 cleanup analysis, based on Figure A-2, is similar to that for Test 1, but the results are quite

different. Comparison of the cells before and after cleaning the window (measurements 1-5 vs. 9 and 10) shows

a bigger effect on the control cells (--0.5% increase in Isc) of cleaning the window than on the test cells.

Therefore there is no contamination in Test 2.

1.08

1.06

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Measurements

Figure A-2 Change in Relative Isc for Test 2 Cells during Post-UV Exposure to Dry Nitrogen (2),

Air (4), and Window (6)and Cell (11)Cleaning Procedures

A second point of interest is the fact that the test cells display a greater than 6% effect of removing or

adding the window to the system. Why is this greater than the 5 % observed in Test 1 or even than that

observed in the control cell data for Test 2?. The explanation has to do with the quality of the fused-silica

windows. If the window darkens somewhat and saturates quickly (with exposure to UV) and then recovers slowly

(with time in the dark and air after the UV test is over) then the above effect is explained. Furthermore, the 1%

drop in normalized Isc (test cell data relative to the control cell data) seen at the beginning of Test 2 in the raw

data (not shown) would be explained and is now correctable. The slow recovery with time fits the data of Figure

A-2, which indicates that no recovery in the test-cell data has occurred during the day of the cleanup procedure.

-267-
ONEYEAROFFLIGHTDATAFROMTHEPASPPLUSEXPERIMENT

HenryCurtis

NASALewisResearchCenter

ClevelandOhio

DeanMarvin

AerospaceCorp,

AlbuquerqueNM

ABSTRACT

Resultsfrom the first year of operation of the PASP Plus flight experiment are given. The experiment consists

of sixteen individual solar cell modules on twelve different panels. Both planar and concentrator technologies

are represented as well as several different cell types. The orbit is 363x2552 km at an inclination of 70 degrees.

There are two main purposes of PASP Plus, 1) to determine the interactionsbetween the space plasma and

solar arrays biased to plus or minus 500 volts, and 2) to determine the long term radiation performance of a

wide variety of solar cell types.

INTRODUCTION

The PASP Plus (Photovoltaic Array Space Power Plus diagnostics) experiment is a photovoltaic flight

experiment flying on the Air Force satellite APEX (Advanced Photovoltaic and Electronics Experiment). The

satellite was launched on a Pegasus vehicle on Aug. 3, 1994. APEX is in an ellipticalorbit with an initial apogee

of 2552 km and a perigee of 363 km. The inclinationis 70 degrees. This orbit puts the spacecratt in a wide

variety of plasma environments and is high enough so the cpacecraft receives a significant radiation dose.

The two main purposes of PASP Plus are to determine the interactions between high voltage arrays and the

space plasma, and to determine the radiation degradation characteristics of a wide variety of solar cell types.

Several of the individual modules are biased at various times at voltages up to plus or minus 500 volts. Arcing

rates and leakage currents are monitored during biasing. Radiation damage characteristics are determined by

continuous monitoring of I-V data for all the solar modules.

The purpose of this paper is to present the results of the first year of photovoltaic data for the PASP Plus

experiment. Cell performance and module thermal performance will be discussed. Comparisons will be made

with predicted (AE8 & AP8) cell degradation. On Aug. 11, 1995, after 373 days of operation, the experiment

ceased to operate. In all probability,there will be no data beyond the 373 days.

PASP PLUS DESCRIPTION

The PASP Plus experiment consists of twelve photovoltaic panels containing a total of sixteen separate cell

modules. Two of the modules are concentrators while the rest are planar. Table I liststhe different solar cell

modules. There are several different cell types on PASP Plus includingsilicon,GaAs, InP, amorphous silicon,

AIGaAs/GaAs, GaAs/CIS, and GaAs/GaSb.

As noted in Table I, not all of the modules are biased. Ten of the individualmodules are biased as part of the

plasma interactions experiment. There are three panels with more than one individual module. Module #'s 0, 1,

and 2 are all 2x4 cm silicon cell modules on the same panel, while numbers 4 and 6 are GaAs modules on the

same panel. The other panel with two modules is the GaAs/CIS panel with two (12 and 13) mechanically

stacked modules. Eight of the modules are on a deployed spacecraft panel (0 thru 7), while the other eight

-268-
modulesareonthetoppayloadshelfoftheAPEXspacecraft.Boththedeployedpanelandthepayloadshelf

are pointed toward the sun.

Table I PASP Plus Individual Cell Modules

PASP + # ._ _ Bias Cells Cover

0 Silicon 2x4 cm Planar No 20 6 mils

1 Silicon 2x4 cm Planar Yes 20 6

2 Silicon 2x4 cm Planar Yes 60 6

3 Silicon 8x8 cm Space Station Yes 4 5

5 Silicon 2.6x5 cm APSA Yes 12 2.5

4 GaAs 4x4 cm Planar Yes 20 6

6 GaAs 4x4 cm Planar Yes 12 6

8 GaAs 4x4 cm WT Planar Yes 4 6

11 GaAs 4x4 cm Planar Yes 8 6

9 Amorphous Si 4x4 cm Planar No 1 20

10 InP 2x2 cm Planar No 10 6

7 AIGaAs/GaAs Planar NO 20 24

12 GaAs/CIS 2x2 cm Planar NO 9 2

13 GaAs/CIS 2x4 cm Planar NO 3 2

14 GaAs concentrator Cassegrainian Yes 8

15 GaAs/GaSb Mini-Dome Conc. Yes 12

Two of the modules are samples of flexible arrays, Space Station and APSA. Both of these modules are

mounted on the deployed panel over a corresponding opening inthe panel. This allows these two modules to

operate not only with the top surface of the array exposed to the environment, but with the back side also open

to space. The APSA module is designed to operate in a GEO orbit, hence a thin layer of germanium was

applied to the thin film substrate of the module for atomic oxygen protection. The Space Station module is

already designed for LEO operations and the atomic oxygen environment.

In addition to the photovoltaic modules, PASP Plus has several diagnostic instruments onboard to measure the

environment through which the spacecraft is flying. Other instruments are used to determine the interactions

between the plasma and the biased modules. A Langmuir probe is used to measure the space plasma

properties; a dosimeter measures the radiation environment in several energy bands; and a set of quartz crystal

micro-balances and calorimeters are used to determine the contamination effects. A transient pulse monitor, an

electrostatic analyzer and an electron emitter are used in the plasma interaction portions of the experiment.

" RESULTS

Three hours after the launch of the APEX satellite, the PASP Plus experiment was tumed on. The experiment

is programmed to take one I-V curve every 30 seconds. Hence after eight minutes, all 16 modules have been

sampled. The process then repeats itself. This could lead to 1.05 million I-V curves per year if the controller

was operational at all times. However there were periods of non-operation for the PASP Plus experiment.

There was one very long period from Nov. 4, 1994 to Jan. 14, 1994 (70.8 days) where the controller was tumed

on only a few times for about 15 minutes. The spacecraft was having problems with the battery charging

circuits and all loads were tumed off. There were eight other smeller down periods ranging from one to 5 days.

Total down time for all periods longer than one day was about 91 days.

-269-
AlltheperformancedatapresentedinthispaperhasbeencorrectedforthevariationsintheEarth-Sundistance

(Isc&Pmax)andcorrectedto theaveragetemperatureofthefirstmonthofI-Vdata.Temperaturecoefficients

wereobtainedfromgroundtestsofthe PASP Plus experiment in the Boeing Test Chamber in Kent Washington.

It was decided to use ground measurements of the temperature coefficients because of the 5-6% variations in

flight values of Isc and Pmax due to albedo effects. (Ref. 1). Figure I shows the temperatures for each of the

panels that the data were corrected to. For most of the module performance data that follows, I-V curves were

taken from data files at approximately two hour intervals. Hence about 12 I-V curves for each day for each

module are included. This is stretched a bitfor the times when the PASP Plus experiment is off and for times

when eclipses occurred. In an effort to smooth out the albedo effects, data are averaged for 25 consecutive

pointswhich leads to a data point on the plots about every two days. For most of the data charts that follow,

there are 98 points plotted representing over 2400 I-V curves.

The orbit of PASP Plus leads to 4576 revolutions during the first year of flight. Of these, 3537 involved eclipses

while 1039 were all sun revolutions. Slightly more than 80% of the firstyear was spent in sunlight. Since the

apogee (2550 km) touches the radiation belts, we have a significant radiation dose. Calculations using AE8 and

AP8 indicate an equivalent fluence of 9.7E14 1 MeV electrons for a silicon cell with 6 mil covers and 60 mil

back shielding. This is for one year and is dominated by protons.

Figure 2 shows normalized Pmax for three different modules, silicon, GaAs, and InP. They all have six mil

coverglasses and thick back shielding. This leads to a direct comparison of the three cell types in a proton

dominated orbit. Note the significant differences between the three cell types. InP has degraded to about 92=/0

of its original value while GaAs and silicon have dropped to 87% and 82% respectively. This indicates a distinct

trend for InP to be a cell for use in high radiation orbits. How high a radiation orbit? If the data from figure 2 is

plotted on a cell efficiency basis where silicon starts at 14%, GaAs at 18.5% and InP at 17.5%, the crossover

between GaAs and InP occurs between 6 and 12 months. Hence for missions with radiation doses larger than

about one year of the PASP Plus orbit, InP becomes more attractive.

The data in figure show a variation which is due to sunlight reflected off the Earth onto the solar modules. The

best way to eliminate this albedo effect is just to plot data when the satellite has no view of the Earth. Hanscom

AFB has supplied CD-ROMs with PASP Plus data along with orbital parameters and Sun and spacecraft

position vectors. This allows us to chose data as a function of how much of the Earth is viewed. Figure 3

shows Pmax for Module 0 (Silicon) for albedo free (clear view) data only. Note that the variation are gone and

that clear view data is only available during part of the mission due to the orbit. Data with albedo for the same

module is plotted in figure 4. Here, all the data is when the satellite is between 40 and 50 degrees out of a clear

view position. Note that the variations of about 6°/=are clearly evident and comparison of figures 3 and 4 show

that the albedo free data is at the lower edge of the data with albedo.

The clear view data obviously better represents the actual performance of the module however there is no early

data. The first clear view data point is at 26 days intothe mission. This makes it difficult to obtain an accurate

initialflight value. However we do have a predicted curve based on the actual orbital parameters, AE8 and

AP8, and the known relative damage coefficients. Therefore fittingthe predicted curve onto the clear view data

should give us an accurate initial value. Figure 5 shows some problems in this approach. Here we have plotted

the clear view data with two "predicted"curves. The lower curve is based on the AE8 and AP8 models. It

dramatically overstates the degradation. The upper curve, labeled "haW, is the AE8/AP8 curve with the

degradation Cutin half. This is not mathematically correct but it does indicate the magnitude of the difference.

For all cases where good damage coefficients are available, a similar pattem occurs. AE8 and AP8 overstate

the degradation by about a factor of two. At the time of this conference, this problem is under active

investigation.

Forthe remainder of this paper we will use the data with the albedo effects included. Data with albedo effects

are available for the entire mission (clear view data forthe entire mission will be available in the future). Figure

6 shows Pmax for the two flexible array modules, Space Station and APSA. As noted before both of these

modules are open on the back to the space environment. After one year there is significant degradation in both

modules with the thin cell silicon (2.5 mils) APSA module degrading somewhat less than the thick cell silicon (8

-270-
mils) Space Station module. Even though Space Station has somewhat more shielding, the inherent radiation

hardness of the thin cells is evident. It must be noted that both of these arrays are designed for operation in

orbits with much less radiation.

The i-V parameters for the Mini-Dome concentrator module are shown in F'cjure 7. The degradation in Pmax is

only about 7=/0for the entire year, showing the effects of the shielding of the concentrator. The drop in Isc is

only 4% for a module which should be very sensitive to contamination. This, as well as data from contamination

monitors indicates a negligible amount of contamination. The concentrator only sees incoming radiation within a

small cone hence there are no albedo effects for this module. Figure 8 shows the off-pointing performance of

the Mini-Dome concentrator. Usually the spacecraft was pointed towards the sun so well that off-pointing data

was difficultto obtain. There were enough available instances to generate the data in figure 8. The module

retained 90% of its current out to about 3.5 degrees off normal. This agrees quite well with a predicted value of

4 degrees.

Data for the amorphous silicon module are shown in Figure 9. There is a significantdrop in Pmax caused to a

large part by a drop in fill factor. Isc and Voc are each down a little over 11%. Much of the degradation is

explained by the Stabler-Wronski effect but since this is a triple junction amorphous silicon cell, there may be

some current mismatch effects. The cell had a 20 mil cover so radiation damage should be small. In figure 10,

the data for the mechanically stacked GaAs/CIS module are shown. The drop in Pmax is about 8% with very

little change in fill factor. This is a low degradation for such a thin cover (2 mils). The very thin GaAs cell

(CLEFT) and the mechanically stacked configuration should both help reduce degradation.

Duringthe course of the PASP Plus mission,temperatures were obtained on a continuous basis. Figure 11

shows the temperature for the panel with the three silicon modules. Each of the four dates is during a no

eclipse period. Note the general increase in module temperature for the first few months. This occurred for all

modules. After 5-6 months the temperatures stabilized. We attribute this increase to a darkening of the Z-93

thermal control paint on the PASP Plus Deployed panel and payload shelf where the modules were mounted.

This is confirmed by the steady temperatures with time on the APEX solar panels which had no Z-93 paint.

CONCLUDING REMARKS

The PASP Plus flight experiment has completed over one year (373 days) of successful operation. Several

items of interest are evident in the data.

1) AP8 and AE8 are overestimating the amount of damage the PASP Plus modules are receiving. The elliptical

orbit is proton dominated and scrapes the bottom of the radiation belts. Further work is ongoing in this area.

2) InP cells have a much better radiation resistance than GaAs or silicon cells in a proton dominated orbit.

3) The APSA module is flying with some cracked covers and cells and is performing quite well.

4) The modules exhibited about a ten degree temperature rise during the first few months of the mission due to

darkening of the Z-93 thermal control paint:.

5) The Mini-dome concentrator module performed very well with minimal radiation degradation and an as

expected off-pointing performance.

REFERENCE

1) H, Curtis, M. Piszczor, P. Severance, D. Guidice, & D' oison, EadyResuits from the PASP Plus Flight

Experiment, First World Conference on Photovoitaic Energy Conversion, Dec. 1994, pages 2169-2172.

-271 -
, • I,M

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Figure 1. Temperatures for each panel during

first month of operation.

Figure 2. Normalized Pmax for InP, GaAs, &

Si, 6 mii covers--thick backs.

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Figure 5. Pmax (clear view) for silicon module

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Figure 4. Pmax for =1icon module #0, (40-50

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Figure 6. Normalized Pmax for flexible

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Figure 9. Normalized I-V parameters for Figure 10. Normalized I-V parameters of

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m

0

n ! J

(_)

Figure 11. Temperature of silicon panel dudng

several non-eclipse periods.

-273-
PARASITIC CURRENT COLLECTION BY PASP PLUS SOLAR ARRAYS *

V. A. Davis and B. M. Gardner

S-Cubed Division of Maxwell Laboratories

San Diego, California 92121

D. A. Guidice and P. S. Severance

Phillips Lab, Hanscom AFB, MA 01731

SUMMARY

Solar cells at potentials positive with respect to a surrounding plasma collect electrons. Current is collected by

the exposed high voltage surfaces: the interconnects and the sides of the solar cells. This current is a drain on the

array power that can be significant for high-power arrays. In addition, this current influences the current balance

that determines the floating potential of the spacecraft. One of the objectives of the Air Force (PL/GPS) PASP

Plus (Photovoltaic Array Space Power Plus Diagnostics) experiment is an improved understanding of parasitic

current collection. We have done computer modeling of parasitic current collection and have examined current

collection flight data from the first year of operations.

BACKGROUND

Solar arrays provide power for nearly all space systems. Traditionally, solar arrays have operated in the 30 V

range to avoid complex interactions with the plasma environment. As space systems become more ambitious,

more power, therefore higher voltages, is needed.

The exposed metal and semiconductor surfaces of spacecraft collect ions and electrons from the space

plasma. The potential of the spacecraft adjusts until the net current is zero. As each solar cell is at a different

potential, some cells collect ions and some collect electrons. For a conventional spacecraft design, the negative

side of each array is grounded to the spacecraft chassis. Therefore, the spacecraft body floats negative with

respect to the plasma. Figure 1 shows the various currents that contribute to the net current to a spacecraft.

As electrons are faster than ions at the same temperature, spacecraft ground is usually slightly negative.

However, it may be necessary to keep the spacecraft body near zero potential with respect to the plasma. For

example, an instrument to measure the low energy plasma environment may need to be near plasma ground.

Anodization arcing and negative potential arcing are potentially disruptive at potentials greater than 50 to 100 V

negative with respect to the plasma (refs. 1 and 2). In these cases active control is used.

At high positive potentials, typically over 200 V, the current rapidly rises due to a phenomena called snapover.

Snapover was first observed at NASA/LeRC (refs. 3, 4 and 5). Snapover can occur whenever there is a biased

surface adjoining an insulating surface and the bias is above the first crossover of the secondary yield curve of

the insulating surface. (refs. 6 and 7) This occurs when the cell potential is above the first crossover for the

coverglass or the array support structure.

4

PASP Plus is the principal experiment integrated onto the Advanced Photovoltaic and Electronics

Experiments (APEX) satellite bus (ref. 8). The experiment tested twelve different solar array designs. Parasitic

current collection was measured for eight of the designs under various operational and environment conditions.

Here we focus on the six flat designs, as the concentrators have minimal current collection as the high potential

surfaces are not exposed to the plasma. The arrays considered are listed in Table 1.

* This work is supported by the Air Force Materiel Command.

-274-
Table I

Array Number Array Type Number of Cell Size

Cells

1 Standard Silicon Module with mesh 20 2 cm x 4 cm

interconnects

2 Standard Silicon Module with mesh 60 2 cm x 4 cm

interconnects

n ,m,

3 Space Station with wrap through 4 8 cm x 8 cm

interco n nects

4 Thin GaAs/Ge with wire interconnects

5 APSA with germanium coating

6 Thin GaAs/Ge with wire interconnects

,=

8 Thick GaAs/Ge with wrap through

interconnects

11 Thick GaAs/Ge with wire interconnects

20 4 cm x 4 cm

12 2.6 cmx 5.1 cm

12 4 cmx 4 cm

4 4cmx4cm

8 4cmx4cm

CALCULATIONS

The computation of the current collected by a specific solar array can become intractable. The gab size is of

the order of tens of mils while the solar cells are a few centimeters and the entire array can be meters. Each solar

cell is at a slightly different potential. The current depends on the geometry of the gap, the geometry of the entire

array, the spacecraft, and the plasma conditions.

We are interested in improving our understanding of which aspects of the problem are most important and

developing a tool or at least an algorithm to assist spacecraft designers. Our approach was to look in detail at

current collection at a single cell gap. Using the computer we can vary each parameter independently. We then

developed a formula that estimates the current collected by a single gap. We then incorporated the formula into a

tool that adds up the current from all the gaps to give the current collected by an array. Information on the array

geometry and how it influences the current are included in the tool.

We did two-dimensional calculations for the various geometries flown. The calculations span the space of

plasma conditions, applied potential, and material parameters. We used the calculations and early flight data to

develop an analytic formula for the dependence of the current on the primary problem variables. The calculational

technique is discussed in a paper presented at the previous SPRAT conference (ref. 9).

The form chosen for the analytic fit apl_ears odd at first glance.

Sheath Area = a A(geom.)

The parameters are

potential with respect to the plasma

_=

plasma temperature

-275-
first cross over

11= potential with respect to the plasma

Z = debye length

cell thickness

The $/'q factor is proportional to the potential. The _ "qfactor modifies the dependence on the temperature and

first cross over. The exponential growth with the potential is accounted for by the exponent. The form of the

exponent allows for the increase of sheath area with debye length and a decrease in sensitivity to debye length at

larger sheath distances. The a, b, c, and d values were adjusted to fit the calculations. A is a function of the array

geometry.

The analytic formula was incorporated into the EPSAT computer code (refs. 10 and 11).

FLIGHT DATA

We examined the measured collected current as a function of the applied bias and plasma density for the first

nine months of PASP Plus data collection. We focused on measurements made in the ram and with the emitter

off. We expect that the wake measurements depend on the attitude of APEX. Generally, when the emitter is on,

the APEX floating potential is positive and an algorithm for the determination of the plasma density is needed. To

avoid these complications, we confined our early examinations to ram, emitter off measurements.

Leakage current is measured as part of a 30 second sequence of measurements. During each 30 second

sequence there are two Langmuir probe sweeps (one up and one down) with the applied bias at zero, and then

23 measurements of the leakage current with the applied bias at a constant. For each 30 second sequence, we

used the 22.nd current measurement and the plasma density and temperature from the following Langmuir probe

sweep. We divided current by the plasma thermal current to compute a collecting area for each measurement.

Collecting Area (m-2) = Leakage Current(A)

2.68 x l 0-14 Density (m -3)-_Temperatu re (eV)

In order to plot the leakage current, we binned and then averaged the data obtained over the nine months.

Lower density measurements are excluded because photoemission may play a role. As the plasma conditions

and applied bias are correlated with the time on orbit, attitude, and location within the orbit, unknown and

unaccounted for systematic factors may influence these measurements.

Several features of interest are clear on inspection of Figure 2.

Overall, the collecting area rises about two orders of magnitude as the applied bias rises one order of

magnitude. This is typical of leakage current when snapover plays a role in the current collection

process (refs. 12-14). Arrays #1 and #2, the conventional interconnect design, do not rise as quickly, particularly

at the high bias end. Also, the current collection curve for array #5, APSA, is different from all of the other arrays.

In general the collected current is several times the array area. Table 2 gives the array and panel areas for

the various test solar arrays.

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Table ri Array and Panel Areas.

Array

#1

#2

#3

#4

#5

#6

#8

#11

Array Area (m "3)

0.016

0.048

0.026

0.032

0.015

0.019

Panel Area (m-3)

0.129

0.129

0.078

0.129

0.059

0.129

0.0064 0.029

0.013 0.029

There is a minimum collecting area for each plasma density that is the same for all of the arrays. The smallest

measured PASP Plus leakage current value is 0.2 p.A. This means that the collecting area levels off at

5 x 10-3 m-2 for a plasma density of 3 X 109and at 5 x 10-4 m-2 for a plasma density of 3 X 101o. Collecting

areas near and below this value are not physically meaningful.

The collecting area does not depend strongly on the plasma density. The collecting area is larger for lower

densities (longer debye lengths). The dependence on density is stronger for lower densities.

And finally, there is a large amount of scatter in the graphs. When the several measurements in the same bin are

compared, variations of a factor of ten are common.

DISCUSSION AND COMPARISON WITH CALCULATION

In order to compare the measurements with the model described above, it is first necessary to account for the

APEX floating potential. The arrays are biased with respect to the APEX chassis. The amount of current they

collect depends on the potential with respect to the ambient plasma. Like any spacecraft, there are several

sources of current to APEX that must balance. We used the EPSAT (refs. 10 and 11) computer program to model

the various components of the current and compute the floating potential.

The electron current collected by solar cells of the power solar arrays that are at positive potential with

respect to the plasma is computed using the above model. The ion current collected by the solar cells is assumed

to be negligible. The Z-93 paint on the surfaces of the panels is taken to have a conductivity divided by thickness

of 10-6 mho m-2. The body of APEX is taken to collect ions from a sheath in the same manner as a 0.45 m

radius sphere in a flowing plasma. The photoelectron current emitted is taken to be constant at 2 x 10 -5 A m -2

when APEX is not in eclipse. And the electron current collected by the test array is modeled as above.

The floating potential of APEX is near zero when the current collected by the test array is less than the ram

ion current collected by the spacecraft body. A 0.45 m radius sphere moving at 7700 m s-1 in a 10 l° m-3 plasma

collects about 0.13 mA. This is the same as the electron thermal current for a 101o m-3, 0.1 eV plasma to a

1.5 m 2 object. The floating potential of APEX shifts when the collecting area exceeds 1.5 m 2. Therefore the

2

collecting area versus applied bias curve flattens out at 1.5 m .

-277-
Theflightmeasurements were taken over a period of months under a variety of conditions. Plasma

constituents, plasma temperature, and sunlight/eclipse condition all influence the current to the test arrays. All of

these effects are included in the EPSAT computer code. In addition, EPSAT has an orbit propagator (ORB) and a

plasma density and temperature model (IRI-86 with an extension to higher altitudes).

For each array, except #5, for each applied bias value, we computed the collected current, plasma density,

and plasma temperature at 287 times during the 9 months covered by the flight data. Keeping only the points for

which the arrays face the ram direction and APEX is below 1500 km, we computed the collecting area in the same

manner as for the flight data, binned the results by density, and averaged. Figure 3 shows the results.

A few general observations can be made regarding the comparison of the calculational results and the flight

results. At present the model has less density dependence than observed during flight. The computed current at

the higher potentials grows faster than observed. The parameters used for arrays #4 and #6 give current values

that are too low and the parameters used for arrays #8 and #11 give current values that are too high.

The current collection characteristic of array #5, APSA, is different from all of the other arrays. We believe that

this is because it is coated with a layer of Germanium, which is a semi-conductor. Current is conducted through

the Germanium coating even in the absence of plasma. This parasitic current is linear with the applied bias with a

resistance of approximately 3 M_. Current is also collected from the plasma. This current is comparable to the

current collected by an array with a low first cross over potential, F_gure 4 shows the effective circuit. Current is

collected across the entire surface of the array.

The measured current Io is given by the following:

• _bbias

1o =flp _--_-=-

The fraction f is used to account for the fact that electrons are collected by the entire surface and not just at

the array potential. When we subtract the parasitic current collected from the measured current, we get the

collecting area curves shown in Figure 5. The figure compares the experimental results with the results of

calculations that treat the array as a constant potential surface on a grounded spacecraft. The calculations were

done using the NASCAP/LEO code (ref. 15).

The adjusted flight data values are higher than the calculations. A lower resistance value might provide a

better match. Otherwise the calculations substantiate the conclusion that the measured current is the sum of the

collected current and the conducted current.

CONCLUSIONS

We examined the current collected as a function of the various parameters for the six non-concentrator

designs. The results are similar to those obtained in previous experiments and predicted by the calculations.

We are using the flight data to improve and validate the analytic formula developed. The formula can be used

to quantify the parasitic current collected. Anticipating the parasitic current value allows the spacecraft designer to

include this interaction when developing the design.

REFERENCES

1.

2-

Vaughn, J.A., et al.: Extrapolation of Electrical Breakdown Currents from the Laboratory to Space Station.

AIAA 92-0822, AIAA 30th Aerospace Sciences Meeting, Reno, Nevada, 1992.

Ferguson, D.C.: The Voltage Threshold for Arcing for Solar Cells in LEO - Flight and Ground Test Results.

NASA TM 87259, 1986.

-278-
,

,

Purvis, C.K.; Stevens, N.J.; and Berkopec, F.D.: Interaction of Large High Power Systems with Operational

Orbit Charged-Particle Environments. NASA -IM X-73867, 1977.

Stevens, J.N.: Interactions Between Spacecraft and the Charged-Particle Environment. Spacecraft Charging

Technology- 1978, NASA Conference Publication 2071, AFGL-TR-79-0082, 1978, pp. 268-294.

5. Kennerud, K.L.: High Voltage Solar Array Experiments, NASA Rep. CR-121280, 1974.

.

.

.

.

10.

11.

12.

13.

14.

15.

Chaky, R.C.; Nonnast, J.H.; and Enoch J.: Numerical Simulation of Sheath Structure and Current-Voltage

Characteristics of a Conductor-dielectric Disk in a Plasma. J. Appt. Phys., vol. 52, 1981, pp. 7092-7098.

Katz, I.; and Mandell, M.J.: Differential Charging of High-Voltage Spacecraft: The Equilibrium Potential of

Insulated Surfaces. J. Geophys. Res., vol. 87, 1982, pp. 4533-4541.

Guidice, D.A.: High Voltage Space-Plasma Interactions Measured on the PASP Plus Test Arrays, this

proceedings, 1995.

Davis, V.A.; and Gardner, B.M.: Parasitic Current Collection by Solar Arrays in LEO, NASA Conference

Publication 3278, 1994, pp. 227-235

Kuharski, R.A.; Jongeward, G.A.; Wilcox, K.G.; and Rankin, T.V.: High Voltage Interactions of a Sounding

Rocket with the Ambient and System-Generated Environments. IEEE Transactions on Nuclear Science, vol.

37, 1990, pp.2128-2133.

Gardner, B.M.; Jongeward, G.A.; Kuharski, R.A.; Wilcox, K.G.; and Rankin, T.V.: The Environmental

Workbench: A Design Tool for the Intemational Space Station. AIAA 95-0599, AIAA 33rd Areospace

Sciences Meeting, Reno, Nevada, 1995.

Grief, N.T.; and Stevens, N.J.: Plasma Interaction Experiment (PIX) Flight Results. Spacecraft Charging

Technology - 1978, NASA Conference Publication 2071, AFGL-TR-79-0082, 1979,

pp. 295-314.

Grier, N.T.: Plasma Interaction Experiment II (PIX II): Laboratory and Flight Results. Spacecraft

Environmental Interactions Technology, 1983, NASA Conference Publication 2359, AFGL-TR-85-0018,

1985, pp. 333-347.

Ferguson, D.C.: RAM/WAKE Effects on Plasma Current Collection of the PIX II Langmuir Probe. Spacecraft

Environmental Interactions Technology, 1983, NASA Conference Publication 2359, AFGL-TR-85-0018,

1985, pp. 349-357.

Mandell, M.J.; and Davis, V.A.: User's Guide to NASCAP/LEO. LEW-15641, SSS-R-85-7300-R2, 1990.

(Available as LEW-15641 from COSMIC, U of Georgia.)

-279-
Ions attracted to negatiw

ElectrOnsat_acted to positive portion of solar arrays

portionof solar arrays

Figure 1,--At the spacecraft floating potential the net current is zero.

This current has several components.

-280-
PASP Plus Array #1, Ram, Emitter Off, Day 94215 to 95134

Temperature (AII)eV Angle (All)

10

i= 1.00E+1O!

i&3.16E+10::

ie 1.00E+tl I

0.1

0.01

E

0_

.=C

o

0

0.001 ' ....

10 100 1000 10000

PASP PlusArray #2, Ram, EmitterOff, Day 94215 to 95134

Temperature (AI_)eV Angle (AI)

10

1

v

¢u

o.1

0.01

!_l.00E+10i

i&3.16E+10!

i,l.OOE÷111

i_..3.:.Ls..E..*. _?..i

0.001 ...................... '-..............

10 1O0 1000 10000

PASP PlusArray #4, Ram, Emitter Off, Day 94215 to 95134

Temperature (AIt)eV Angle (All)

10

0.1

0.01

0.001

10

o

0

6n

8,m,

J_

f

_I 00E+IO_

!dL3.16E+101

ie 1.00E+111

!_3.16E+11 ::

PASP PlusArray #6, Ram, EmitterOff, Day 94215 to 95134

Temperature (AI_eV Angle (All)

10

0.1

0.01

E

_6

_A

O_

0,001

100 1000 10000 10 100

ill 1.00E+IO i

i&3.16E+10i

i_ 1.00E+I 1 ;

;:_3.16E+11 i

1000 10000

Figure 2.--Experimental collecting area versus applied bias curves.

-281-
PASPPlusArray#3,Ram,EmitterOff,Day94215to95134

Temperature (AII_V Angle (All)

10

1

0.1

0.01

0.001 .....

10

t_

¢)

.z

o

0

,z

jil 0 _'; .

w

%

100

!_ 1.00E+I01

ij3.1SE;lOi

ie 1.00E+111

i ....

1000 1000O

PASP PlusArray#5, Ram, Emitter Off, Day 94215 to 95134

Temperature (NI)sV Angle (All)

10

0.1

0.01

1

o

o

L)

i3,1.o0E+1Oi

iA3.16E÷10i

i_,I,oOE+I I

0.001 ' -

10 100 1000 10000

PASP PlusArray #8, Ram, Emitter Off, Day 94215 to 95136

Temperature (AIt)eV Angle (A]])

10

0.1

0.01

0.001

10

F

E

"3

O

4m.

IO0

:.¢I00E+10;

!_.I.OOE+I Ii

i_._.._.6._.+.._.!J

1000 10000

PASP PlusArray #11, Ram, Emitter Off, Day 94215 to 95134

Temperature (AIl_aV Angle (/_)

10

o

0.1

0.01

/

0.001

10 100

i_ 1.00E+IO:;

iA3.16E+IO I

::_ 1.00E+I t i

i

1000 100O0

Figure 2._Experimental collecting area versus applied bias curves.

-282-
PASP Plus Array #1, EPSAT PASP Plus Array #2, EPSAT

Ev

2

c

o

10

0.1

0.01

0.001

10

/

i-_--_ E;:69i

T_ 1.00E+10i

IA3.16E+10i

ie 1.00E+11 i

100 1000 10000

E

c

o

10

0.1

0.01

0.001

10

E.f.."

I_ _ :.._ _3

.:_:_-_

i_ 1.00E+I01

!_I,3.16E+I01

::O1.00E+I I i

i_._:k6E+..!_!.!

100 1000 10000

¢u

o

0

10

0.1

0.01

0.001

10

PASP PlusArray#4, EPSAT

/

_= 1.00E+10i

i_3.16E+10_:

i_ 1.00E+ 111

E

2

o

10

0.1

0.01

PASP PlusArray #6, EPSAT

$

/

0.001

100 1000 10000 10 100

ial.00E+10i

::43.16E+101

!_ 1.00E+111

!_3.16E+111

1000 10000

Figure 3.--Computed collecting area versus applied bias curves.

°283-
10

o

O

0.01

0.001

10

PASP PlusArray #3, EPSAT

#

il,

J

100

it_ 1.00E_. 10

IJ3.16E,,lOI

io100E.111

!¢?:_6E*j_i

i

1000 10000

E

¢0

E_

0

10

0.1

0.01

0.001

10

PASP PlusArray#8, EPSAT

i= 1.00E+ 10;

ij3 leE+lO!

;_, 1.00E+ 11 !

!_:!_E+jL

d

a,=

ib

.$

/

100

<:

O

10

0.1

0.01

PASP PlusArray#11, EPSAT

/

, 0.001

1000 10000 10 100

!a 1.00E+10 i

i=_1_E,lOi

;Q 1.00E+I I i

1000 10000

Figure 3.--Computed collecting area versus applied bias curves.

-284-
(Dbim

]e (Dimly

>

T R(1-f)

(D=J=

Ip

<

Figure 4.inEffective circuit for collection of current by array 5.

PASP PlusArray #5, Ram, Emitter Off, Day 94215 to 95134

Temperature (AII)eV Angle (All)

10

"E

I

(.3

E

i 0.1

II1

I

_ o.ol

0

i";_'"_3_ .........

! _ 1.00E,,10

• 3.16E+t0

! t 1.00E+11

:: _3 3.16E+11

_-'- LEO 1,00E+10

.,_ LEO 1.00E+11

_ L..E_.... _,_ .E..,.._._

0.001 ....... ' ..................................

10 100 1000 10000

4

Figure 5.--Array 5 experimental collecting area connected for current through

Germanium coating compared with NASCAP/LEO calculations. :_

-285-
HIGH VOLTAGE SPACE-PLASMA INTERACTIONS

MEASURED ON THE PASP PLUS TEST ARRAYS

D.A. Guidice

Phillips Laboratory, Geophysics Directorate

Hanscom AFB, MA 01731

INTRODUCTION

Space systems of the late 1990s and beyond will require more efficient space-power sources. In providing

electrical power for these new systems, consideration must be given to operating photovoitaic subsystems

at higher voltage levels to minimize 12Rlosses and/or reduce cable weight. New solar-cell materials are

being developed for higher efficiency and less susceptibility to space radiation. Before using these new

technologies on operational spacecraft, various environmental interactions questions must be answered

by investigations in the actual space envlronment. The Photovoltaic Array Space P....owerPlu._._ss Diagnostics

(PASP Plus) experiment was developed and flown for this purpose. In early 1990, the Air Force's Space

Test Program (STP) offered PASP Plus a flight on a satellite put into orbit by a Pegasus launch vehicle.

PASP Plus was to be part of the APEX (A.dvanced P._hotovoltaicand E._lectronicsEXperiments) mission, set

up to fly it and two small "radiation effects on electronics" experiments, CRUX/CREDO and FERRO (1).

To make PASP Plus suitable for APEX, a controller with increased functional capability and reduced weight

was developed by Amptek, Inc. With the help of NASA Lewis personnel, twelve test arrays were mounted

to the payload shelf and one of the deployed panels of the APEX satellite. Diagnostics sensors were

incorporated into the experiment. PASP Plus successfully completed all functional and environmental

testing, including "one sun simulated" thermal-vacuum tests at the Boeing facility at Kent, WA to give us

preflight "array performance vs. temperature" characteristics for later comparison with flight data (2). The

experiment was delivered to Orbital Sciences Corp. (OSC) for integration into APEX in July 1992.

The APEX satellite was launched by a standard Pegasus rocket released from a NASA B-52 aircraft (based

at Edwards AFB) on 3 August 1994 within the Western Test Range off the coast of California. The release

from the B-52, the three stages of rocket firing, the satellite's lock-on to the sun, and the extension of its

four deployable panels all occurred without problems. A 70°-inclination, 363 km x 2550 km orbit was

achieved, satisfying PASP Plus's data-collecting requirements.

The objectives of the PASP Plus experiment were:

a. To measure the plasma "leakage" current for different kinds of arrays subjected to positive

biasing levels up to +500 V.

b. To measure the arcing parameters for different kinds of arrays subjected to negative biasing

levels up to -500 V. .,

c. To measure the long-term deterioration in the power output of arrays using different solar-cell

materials when exposed to space radiation.

In all cases, the concept was to establish cause-and-effect relationships between array interactions and

environmental conditions. This paper will discuss some of the positive and negative biasing results; another

paper at this conference will discuss the space radiation-induced deterioration aspects of the experiment.

PASP PLUS INSTRUMENTATION

The PASP Plus instrumentation consisted of four kinds of equipment:

-286-
a. a set of test arrays, some of which are divided into biased and unbiased modules.

b. experiment-control instrumentation capable of creating array or spacecraft conditions under

which measurable environmental interactions will occur.

c. interactions-measuring instrumentation that will quantify what happens when the ambient or

created conditions impact the performance of the test arrays.

d. diagnostic sensors to measure the environmental conditions affecting array performance.

Solar Array.s

To maximize the scope of the investigation, a variety of conventional and advanced-concept solar arrays

were included in the arraycomplement. As shown in Table I, twelve different solar arrays were investigated

on PASP Plus. Other than Array 0,1,2 (basic silicon, which serves as a standard), array selection were

based on the array's availability (at no cost to the experiment) and its possible use on future DoD or NASA

space missions.

Table I. PASP Plus Solar Arrays

Module Cell / Array Number Cell Size Array

No. Type of Cells (cm × cm) Developer

0,1,2 Si (Std) 8 mil 20,20,60 2 x 4 RCA / WL

3 Si 8 mil wtc [Spacesta] 4 8 x 8 Lockheed

5 Si 2 mil [APSA] 14 2.6 x 5.1 TRW

9 Amorphous Si 2 4 x 4 TRW/JPL

_6 GaAs/Ge 31/_mil 20,12 4 x 4 ASEC

8 GaAs/Ge 7 mil, wtc 4 4 x 4 Spectrolab

1..!1 GaAs/Ge 7 mil 8 4 x 4 Spectrolab

7 AIGaAs/GaAs 20 2 x 2 VS Corp.

10 InP 12 mil 10 2 x 2 Spire/NRL

12,13 GaAs/CulnSe 2 12,3 2 x 2, 2 x 4 Boeing

1__4 GaAs Mini-Cass 8 Concentrator TRW

1_55 GaAs/GaSb Mini-Dome 12 Concentrator Boeing

wtc = wrap-through connectors

underlined module = biased

A brief description of the solar arrays is given in Ref. 3. A more detailed description, including cell and

array mechanical configurations and color photographs was given in a report by Adams (4). '

Controi& Diaqnostic Instrumentation

•

The PASP Plus controller carries out all the functions needed to obtain array I-V curves, to apply the

biases to the arrays, and to process the data from the interactions-measuring and diagnostic sensors

(except the dosimeter) and send the data to the satellite (for real-time or data-dump transmittal back to a

ground station)• The dosimeter has its own separate interface (power, commands, data) with the satellite.

PASP Plus has a suite of diagnostic sensors that include the following:

Sun Sensor: To measure the alignment of the test arrays to the incident solar energy, especially important

for the concentrator arrays.

-287-
Lan.qmuirProbe (LP,: To measure low-energy plasma parameters (density and temperature).

Transient Pulse Monitor (TPM): To detect and characterize the arc-discharges that occur during the

negative-voltage biasing of the test arrays.

Electrostatic Analyzer (ESA): To measure 30 eV to 30 key electron and ion spectra, detect the passage

of APEX through an auroral region, and show the vehicle negative potential when a test array is biased

to high positive values.

Contamination Monitors:To measure the amount and/or effect of contaminants deposited on array surfaces.

Sensors include both quartz crystal microbalances and calorimeters.

Dosimeter: To measure the high-energy particle radiation flux and dose (from ions and electrons) that leads

to deterioration in solar-array power output.

APEX SATELLITE

The APEX satellite, shown in Figure 1, is in the form of a hexagonal cylinder 152 cm in height and 96 cm

across (112 cm comer to corner). On the top "Payload Shelf" we have seven of the PASP Plus test arrays

and some small sensors. Extending up from the Payload Shelf are two 61-cm booms, one holding the

Langmuir Probe sensor head and the other holdingthe APEX magnetometer (used to maintain orientation

when the satellite is in eclipse). Beneath the Payload Shelf, we have the Avionics Shelf containing the

electronic boxes for the PASP Plus instruments and the other APEX experiments, CRUX/CREDO and

FERRO. Attached to the top of four of the hexagon's sides are the four deployed panels, each 152 cm

wide by 56 cm long. Three of the panels contain the silicon arrays providing power to the spacecraft; the

other "Deployed Payload Panel" contains the remaining five test arrays and a few small sensors (3). The

PASP Plus test arrays and instrumentatior_on the Payload Shelf and Deployed Payload Panel are shown

in Figure 2. The APEX satellite is three-axis stabilized and oriented so the PASP Plus test arrays will be

sun-pointing to within +0.5 degrees (needed for operation of the concentrator arrays).

I -I.k=t=. -I II "-AVIO.,CS

SHELF

Fig. 1. Advanced Photovoltaic and Electronics Experiments (APEX) Satellite.

-288-
_PAYLOAD

LF

4, 6

S

DEPLOYED PAYLOAD PANEL

ITEM NO. INSTRUMENT

18,19,20,21 TPM E-field Sensors

23 LP Boom & Sensor

27 PASP Sun Sensor

30,31 Quartz Crystal Microbalances

32.33, 34 Calorimeters

Fig. 2. PASP Plus Instrument and Test

Array Layout

ITEM NO.

0,1.2

3

4,6

7

8

9

10

11

12.13

14

15

36(5)

PASP TEST ARRAY

St Standard

SI Wrap_Thru (Space Sta)

GaAs on Ge, 3.5 mil

AIGaAs/GaAs

GaAs on Ge,7 mil, Wrap-Thru

Amorphous Si

InP

GaAs on Ge, 7 mil

GaAs/CulnSe z

GaAs Mini-Cassegrainian

GaAslGaSb Mini-Dome

Thin Si(2mil), APSA

DATA GATHERING

The PASP Plus team undertook experiment commanding and quick-look data evaluation (using our on-site

GSE) at the Satellite Control Facility at Onizuka AFB, CA. After tum-on and checkout of the PASP Plus

instruments and some initial I-V curve taking, we began biasing of the arrays on 7 August 1994. Data

gathering involving the positive and negative biasing of the PASP Plus test arrays took place at various

times over the period from 7 Aug 94 to 1J Aug 95, with several long gaps due to APEX subsystem problems. PASP Plus biasing was halted by an APEX Battery Control Regulator problem on 4 Nov 95. After

a thorough study, a software-patch fix was developed and tested by OSC and sent up to the satellite; PASP

Plus operation was started again on 15 Jan 95. In the spring of 1995, problems with other satellite

subsystems and the time needed to find the appropriate fixes and PASP Plus operating-time limitations

caused biasing operations to be halted from 17 May to 1 Jul 95. Finally, on 12 Aug 95 a PASP Plus

controller hardware problem resulted inthe end of all biasing operations. Table II shows the data gathering

periods for positive biasing, summarizing the voltage and plasma density ranges and the environmental and

operating conditions encountered. Table III does the same for the negative biasing periods.

-289-
;I

°

,<

_" _t' I._ 1.(3 1.¢303 t_

03 03 03_ _,_

IIIIII

-290-
Z

°2 2 2

_" w w 0

mm

L_ 0 _ LO

T T '7 T

o o o .9o

> :> > >.

1.0 0 0 0

i_ ¢.0 ¢D O0

I I I

X X X X

I" I' I" I"

% % o,

g g O4x (%1 ×

(_O 0 0 0

CO_) r,.. (D o

_'9 9 T

oo o o o

O0 0 0 0

9'9' 9' 9 7

"_-- _o(/3==- o_ Z

o ,,®

(%1 _. '_" ('41 ,m.=.

I I I

=E< < "_ =

'-3 "_

(0 '," O0

¢_1 v.- O0 0

--_.-I I I T o_i

(0_ 03 ,,- 03

-291-
POSITIVE BIASING

Positive biasing of the test arrays was done to investigate the "snapovet" phenomenon associated with

operation at high positive voltages. At lower voltages, "leakage" current (from the plasma) is confined to

the very small areas of the metallic interconnects between solar cells and the very thin side edges of the

cells beneath their coverglass. At higher voltages (and plasma densities), however, secondary electron

emission from electrons hitting the coverglass cause it to become positively charged, further attracting and

collecting plasma electrons, thereby increasing"leakage" current. Eventually, collection of leakage current

takes place over the array's whole surface, a much larger area than that of the interconnects and cell sides.

It has been found that array configurations where the interconnects and/or cell-edge areas are shielded

from the space plasma (i.e., concentrator arrays or planar arrays with wrap-through connectors), have lower

leakage current than arrays without such defenses. The concentrators (#14 and #15), whose structures

shieldboth interconnects and cell side edges, have insignificantleakage currents even at high voltages and

plasma densities. Arrays #8 and #11 have the same solar-cell and coverglass material, but Array #8 with

its wrap-through connectors has lower leakage current than #11. For the arrays where snapover is well

observed, it appears to start around +100 V, although for the old-construction silicon array (Modules #t

and #2), snapover seems to be able to start as low as +50 V. See Figure 3 for the measured leakage

current for Module #1 from in-ram ($rarn= -<90°)data only. Davis (5) of S-Cubed has undertaken a detailed

study of the parasitic (leakage) current collected by the PASP Plus arrays for 0 = _<90° orientation,

comparing the results with analytic models developed using the EPSAT computermcr_de.

The leakage current under positive biasing collected by the Space Station array (#3) on APEX was much

higher than that collected for the Space Station array on NASA's SAMPLE experiment flown on Shuttle.

However, on APEX Array #3 was flown in a manner similar to how it will fly on Space Station -- stretched

out and open to space on both sides (using a cut-out in the deployed panel), thus exposing its edges to

the plasma. In the SAMPLE experiment on Shuttle, the Space-Station array sample was rigidly mounted

(mechanically, not electrically) to a base plate, yielding little edge exposure to the plasma. Ferguson (6)

investigated the leakage current discrepancy between PASP Plus and SAMPLE in light of the mountingconfiguration and other differences and has found that the results for both experiments are comparable.

o

oO

Fig. 3.

Array 1: Silicon (Si)

Start: 94215 Emitter OFF End: 95223

3 Aug 199,4 RAM Condition 11 Aug 1995

500 -

400 '

300

200 "

1oo

10 2 10 3 110 4 110 s

Plasma Density (per CC)

II

i i i i ,,

10 e

'10.4

10 -s

I 0 "qs

Leakage

Current

(Amps)

PASP Plus Measured Leakage Current [gray scale] in the Non-Wake (ram + 90 °) vs. Bias

Voltage and Plasma Density for Module #1 (Standard Si Array).

-292-
NEGATIVE BIASING

The arc rates for the ten negatively biased test arrays were measured by the PASP Plus TPM on APEX.

These arc rates have been compared to computer-generated simulations using a semi-analytic arc-rate

code developed by M.IoT. (7,8) for the experiment's conventional planar arrays. The simulations were

carried out using the measured "arc-rate determining" parameters (bias voltage, ion flux [from plasma

density and ram angle], array temperature, etc.) measured aboard APEX during negative biasing

operations. Measured and simulated arc-rate results were compared to examine cause-and-effect

relationships. Some of the PASP Plus flight results are described and interpreted below.

The measured arc rate for the biased arrays showed a strong dependency on the fabrication and mechanical configuration (i.e., metal interconnect and cell side-edgi_ exposure to the space plasma). The standard

silicon array (Modules #1 and #2) showed substantial arcing over a wide range of voltages (>160 V),

plasma densities (>109 m"-3in ram) and temperatures (in sun and eclipse). This is to be expected since

Array 0,1,2 is of old construction (exposed rough-surface interconnects). According to theory (7), if the

interconnects have many field-emission sites (microscopically jagged regions with high electric-field

enhancement factors), charging processes caused by enhanced-field electron emission can be initiated,

leading to collisional ionization of neutral gas desorbed from the cell's coverglass, eventually resulting in

an arc discharge. PASP Plus measurements also show that arrays having their interconnects and/or cell

side edges shielded from the space plasma have lower arcing rates. The concentrator arrays appear to

have almost no arcs, and the arrays with wrap-through connectors have less arcs than would be expected

based on other factors. It was also found that for arrays where matched comparisons (modules with the

same cell types and array configuration) could be made [i.e., Module #2 with #1 ; Module #4 with #6], all

other factors being equal, arc rate appears to be proportional to module area.

While the old standard silicon array exhibited much higher arc rates than the others, the arc rate of all the

arrays showed a strong dependence on bias voltage. See Figure 4 for Module #2. Arcing onset (threshold)

voltages were found to be generally in the -100 V to -300 V range; see Table IV below. For the

conventional planar arrays, there is general agreement between the PASP Plus results and the computergenerated simulations.

l

0.8

_0.6

0.4

_0.2

0

Fig. 4.

--' '°n Fii=l-el 3/m^is ?--i ' H--V---- _

0 100 200 300 400 500

Bias Voltage, -Volts

Average Arc Rate versus Bias Voltage for the Standard

Silicon Array, Module #2.

-293-
Table IV. Arcing Onset Voltages for the Biased PASP Plus Planar Arrays;

Experimental Measurements and Computer-Generated Simulations

PASP Plus Planar

Test Array

In-Space Measured

Onset Voltacle

Computer-Simulated

Onset Voltage

1,2 Silicon (Old Standard) -160 V -160 V

3 Silicon (Space Station) -160 V Not Derived

with Wrap-Through Connectors

5 Thin Silicon (APSA) -75 V to -125 V -100 V

4,6 3.5-mil GaAs on Ge -125 V -120 V

8 7-mil GaAs on Ge -260 V to -300 V Not Derived

with Wrap-Through Connectors

11 7-mil GaAs on Ge -180 V -180 V

with conventional interconnects

Ion flux (Fion = nion vim cOSeram)was also found to be an important factor governing arc rate. Access of

the plasma's positive Ions to the interactions volume between an array's solar cells is necessary for the

proper sequence of interactions (leading to arcing) to occur. In general, this requires that the array not be

in wake, in most cases this means 0ram = _-_+90 °. However, for modules prone to arc (#1 and #2) it was

observed that at high negative biases especially in eclipse (cold is another arc enhancing factor) arcing

occurred for the module near the edge of the deployed panel (Module #2) for ram angles up to nearly 120°

while Module #1 in the center of the panel would not arc until era m was <105 ° (few arcs beyond 90°).

PASP Plus also found that arcing is more prevalent under cold array conditions, especially at the ends of

eclipse periods. According to the M.I.T. developed theory (7), one would expect cold array temperatures

to favor arcing: greater desorption of the neutral gas and its staying in the localized area (having less

energy) under cold conditions. An example of the difference in arc rate between high (sunlit) and low

(eclipse) array temperatures [+38C and -42C] for Module #2 is presented in Figure 5. The experiment data

points are shown as filled circles and the simulation-generated points as open squares. For thin arrays

such as the APSA array (#5), temperature is an important arcing factor. Arcing for Array #5, which was

not allowed to be biased beyond -300 V, was confined almost exclusively to eclipse periods where its

temperature could reach -70C due to its double-sided exposure, very low mass, and thermal isolation.

'il, t

Simulated I- _ Simulated I

• Flight Data I • _ • Flight Data

I .. i 0.,t • • []

0.6 .: "=1: < 0.4 0.4-_

o .... o: !ii

0 i 00 200 300 400 500 0 100 200 300 400 500

BiasVoltage,-Volts Bias Voltage,-Volts

Fig. 5. PASP-Pius Measured and Computer Simulated Arc Rates for Module #2 (Standard Si Array) vs.

Bias Voltage at Array Temperatures of +38C [left] in Sunlight and -42C [right] in Eclipse.

-294-
CONCLUSIONS

Inspiteof launchdelaysandon-orbitsatelliteproblems,thePASPPlus experiment has provided to the

space-power community outstanding data for the examination of plasma effects on high-voltage operation

and radiation damage effects on array power output. By the summer of 1995, PASP Plus collected an

order of magnitude more data about environmental interactions on solar arrays than all previous spacebome photovoitaic experiments combined. Organizations such as the Phillips Laboratory, NASA Lewis

Research Center, Massachusetts Institute of Technology, and S-Cubed Division of Maxwell Laboratories

are investigating various aspects of space-environment effects on photovoltaic systems based on the most

extensive on-orbit data base ever acquired for this purpose.

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

REFERENCES

Guidice, D.A., "PASP Plus: An Experiment To Measure Space-Environment Effects on Photovoltaic

Power Subsystems," FTfthAnnual Workshop on Space Operations, Applications and Research

(SOAR '91), NASA Conf. Publ. 3127, pp. 662-668, Feb. 1992.

Curtis, H., Guidice, D., Severance, P., and Piszczor, M., "Resultsof Thermal Vacuum Tests for the

PASP Plus Flight Modules," Proc. of X/I Space Photovoltaic Research and Technology

Conference, NASA Conf. Publ. 3210, pp. 289-297, May 1993.

Guidice, D., Severance, P., Curtis, H., and Piszcz'or, M., "Investigation of Space-Environment

Effects on Photovoltaio Technologies by the PASP Plus Experiment", AIAA 95-0371, 33rd

Aerospace Sciences Meetina, 9-12 Jan 95.

Adams, S.F., "Photovoltaic Array Space Power Plus Diagnostics Flight Experiment: Pre-Fiight

Description of Experimental Photovoltaio Modules", WL-TR-92-2080, Wright Laboratory, Oct. 1992.

Davis, V.A., "Parasitic Current Collection by Solar Arrays in LEO", 1995 Space Power Workshop,

Phillips Lab., 18-20 Apr 95.

Ferguson, D.C., "Current Collection of Space Station Solar Arrays -- Differences between PASP

Plus-and SAMPLE', .1995 Sp.ace Power Workshop, Phillips Lab., 18-20 Apt 95.

Cho, M., and Hastings, D.E., "Dielectric Charging Processes and Arcing Rates of High-Voltage

Solar Arrays," J. Spacecraft & Rockets, VoI. 28, pp. 698-706, 1991.

Soldi, J.D., "Arc Rate Predictions and Flight Data Analysis for the Photovoltaic Array Space Power

Plus Diagnostics (PASP Plus) Experiment", M.S. Thesis, M.i.T., 1995.

-295-
PRELIMINARY CHAOTIC MODEL OF SNAPOVER ON HIGH VOLTAGE SOLAR CELLS

Willie R. Mackey

NASA Lewis Center

Cleveland, Ohio

High voltage power systems in space will interact with the space plasma in a variety of ways. One of

these, Snapover, is characterized by a sudden enlargement of the electron current collection area across

normally insulating surfaces. A power drain on solar array power systems will results from this enhanced

current collection. Optical observations of the snapover phenomena in the laboratory indicates a functional

relation between bia potential and surface glow area. This paper shall explore the potential benefits of

modeling the relation between current and bia potential as an aspect of bifurcation analysis in chaos theory.

Successful characterizations of snapover as a chaotic phenomena may provide a means of snapover

prevention and control through chaotic synchronization.

Introduction:

Nonlinear physical systems that exhibit unstable behavior have recently been reexamine for the existence

of chaotic behavior. (ref. 1) Chaotic behavior has been observed in laboratory plasmas (ref. 11-14). Chaotic

behavior in plasma has been observed within ion sheaths due to DC currents in double plasma devices.

Recently, an application of chaos theory as a control variable in nonlinear systems has received considerable

attentions (ref. 2-10). Spacecraft plasma interactions are pdmadly nonlinear and therefore may provide

opportunities for chaotic analysis. Due to the remote nature of space studies, chaotic time series signal

analysis may provide a significant means of spacecraft system control and diagnostic.

We begin the modelling process from spacecraft- plasma interactions empirical equations. Utilizing the "basis

space" (ref. 1) of model chaotic systems to acquire analytic differential equations. This in tum can lead to

predictions of experimental observable by parameter variation which alters the phase space structure. Direct

correlation of the model chaotic system with empirical equations may improve the validity of fitted models.

Bifurcation analysis:

Bifurcation analysis is concern with how steady states solutions of a nonlinear system change their

qualitative character as a parameter change. The onset of instability of a solution usually occur at the same

critical parameter value as the bifurcation, ff a sequence of bifurcations occurs as the parameter is varied then

chaotic behavior can be intiated.

Bifurcation analysis consist of the following procedures. Let's consider a first order differential equation.

dx

-B(_,x)

dt

(z)

Steady states solutions such that x(t) = X for all t exist if

-296-
Snapover:

Solar arrays consist of strips of solar cells with metallic interconnects between them. The power requirements of large space systems will require high voltages to operate economically. This is due to the necessity

to minimize the mass of space hardware. High voltage arrays exposed to the space plasma essentially acts as

biased probes attracting and repelling charged particles (ref. 21).

Snapover is a sudden enhancement of current collected from an ambient plasma attributed to secondary

electron production from insulator surfaces (ref. 23-26). Exposed solar array interconnects collect electrons

from the ambient plasma. Below a critical voltage, the bia voltage is located over the interconnect regions and

the surface potential of adjacent insulator materials is essentially zero. Above the critical voltage there is a

dramatic increase in current collection and the surface potential approaches that of a conducting plane (Fig. 2)

Figure 3 represent experimental data for a solar array section biased positive with respect to the plasma.

For voltages greater than 100-150 volts, the electron current collected by the array increases dramatically.

Even though the solar array surface is a dielectric, the surface becomes highly positive and collect current.

The plasma sheath grows over the dielectric surfaces for positive potentials. The exposed interconnects or

pinholes accelerates electrons which strikes the dielectric. Low energy secondary electrons are released which

are collected by the exposed metal. This leave the dielectric cover glass positive, allowing the plasma sheath

to grow over the solar cells. The solar array therefore collects as a conductor. This current which flows through

the plasma is not available to the spacecraft and therefore represents a power loss to the array. Comparison

of current collection between a metallic disks and a disc on "Kapton" insulation confirms that the insulator is

involve with the current enhancement (ref. 18). Typically one would expect the insulation to be at floating

potential and therefore of a negative "floating"potential which would repel electron resulting in a lower current

collection than a plain conducting metallic disk (ref. 17).

Recently Ferguson (ref. 29) and Stillwell (ref. 28) noted optical observation which may clarify the parameter

space of the snapover events. The optical data indicates that the snapover moves over the surfaces in

response to changes in surface potential due to secondary electron productions.

Snapover is observed also in pinhole current collections (ref. 26). The primary difference being that the

field distribution must expand through the pinhole emphasizing the normal distribution of the electric field. Still

the controlling factor in current collection is the superposition of the field above the insulation and the exposed

conducting surface.

Stevens et al (ref. 20-22), (Fig. 3) developed empirical relations for current collection above and below the

snapover voltage of 100 volt for the experiment. Surface voltages traces indicated that below bias voltages of

100 volts dielectric cover glass were slightly negative in potential in order to maintain current balance. Above

100 volts the cover slide potential changed to a value about 50 less than the bias voltage. Expressed as a

single formula:

 ,0o (8)

Where _Tc is the current collected, A i is the total interconnect area,

Apis the solar panel surface area and V is the bias voltage.

Let V_/2 =(1) then equ. 8 can be written;

-297-
B(_,x) =o (2)

For each value of _ ; equilibrium points are determined.

A bifurcation diagram is the plot of all curves x=X(_) in the (a, x) plane (Fig. 1). Stability of the

equilibrium point X can be ascertain by considering the behavior of the solution to small perturbations.

Performing a Taylor series about a small perturbation and neglecting higher order terms we find that stability to

first order will be determine by the growth or decay of;

exp[(dB(_'X))t] =estdx (6)

If s < 0 then all small perturbation vanishes as t increases. If s > 0, then X is unstable since small

perturbations grow.

Bifurcation analysis examines the evolution of all equilbrium or steady state solutions of by considering the

sign of VB ((_, X). A basic approach is:

Identify dissipative phenomena.

Construct dynamical equations in standard form.

Characterize equilibrium motion - fixed bifurcation points.

Characterize oscillatory motion - Hopf bifurcation points.

Perform parametric studies of the bifurcation points.

Construct a global portrayal of dynamical behavior by continuation.

Compare to test data.

Refinement of the model equations and phase space can be achieved by continuation. Laboratory and

space experiments only supply limited data points as a scalar time series in terms of a few independent

coordinates. Continuation methods will allow generation of an extensive phase space from a few points in

phase space. Identification of bifurcations from continuation generated phase spaces can enhance the

precision of engineering guidelines in terms of experimental parameters (ref. 10).

Space environment interactions offers many areas for bifurcation analysis such as transition to saturation

current as a function of potential, sheath formation as a function of potential, double layer formation, snapover

as potential jumps from one equilibrium solution to another. In the spacecraft environment we have the

following equilibria and oscillatory regions.

Equilibria: The fundamental physical process for all spacecraft charging is that of current balance, all currents

must sum to zero. Key parameters in this process can be the spacecraft potential, orbit, active sources,

surface material properties, sheath widths, _Jolar cycle activities, plasma frequency, temperature, size, length effective area.

Oscillations and Transients: Spacecraft power systems will experience arcs, discharge phenomena, currents,

plasma waves, switching and orbit eclipse. These effects correspond to periodic solutions to the dynamical

equations and therefore limit cycles bifurcation points.

Below we formulate empirical equations for snapover into standard forms for bifurcation analysis.

-298-
Zc = + 13 )+ )

Equ. 13 is in the form of bifurcation equation such as the saddle node (Fig 1). Bifurcation diagrams indicates

the points where an equilibrium solution can occur.

Experimental examination of sheath structure around pinholes was performed by Gabriel et al. The

potential structure was described by a semiempirical formula (ref. 26).

, = -- arc tan exp exp (14)

(:1)ois the potential applied to the exposed electrode in the pinhole, _ and I are empirical parameters.

The energy that electron are accelerated to the surface is determined by (I) (r, 0) . The electric field is

dominated by the x component. Carruth (ref. 25) use this field in a multiple slit model to determine where the

field is zero and reverse signs. This determines which exposed conducting surface will collect electrons. The

dominance of the horizontal component provides the basis for transforming the experimental potential into a

form suitable for a bifurcation analysis. This field distribution and its consequential derivatives can be transformed into a current potential equation similiar in form to the fold cusp bifurcations. Taking the time derivative

of Poisson equations upon equ. 14, we acquire the following form.

2

(19)

v is the horizontal velocity components of the collection current, x is the distance from the pinhole to the

leading edge of the snapover current. By performing a bifurcation analysis of this equation one can determine

the range of values for f((i)) , g ((I)) which satisfied the experimental data.

EWB a space environment interactions simulation software (ref. 6) models snapover by requiring current

balance between the incident electron current and the divergence of the current carried by the secondary

electron layer. The model assumes that secondary electron emission facilitates the spread of high voltage

from the conductor onto the insulating surface.

zv. M]I/ (21)

The secondary layer current is proportional to the incident electron current and a strong function of the normal

electric field at the surface. Where Y is the secondary electron yield and <e> is the mean secondary

electron energy. A core of the charge balance simulation scheme of EWB is an empirical based relation

between the charge density distribution and the surface potential.

p n

(23)

Given equ. 16, the simulation model therefore can be transform into a bifurcation form.

dp , _n a--E=T. eo( (24)

where (_, 13are experimental parameters containing with the "physics" of the model.

-299-
Chaotic Synchronization :

The existence of bifurcation points in spacecraft plasma dynamical equations coupled with the modeling of

nonpedodic and chaotic phenomena as a sequence of bifurcations will allow the use of chaotic synchronization

as an engineering control tool (ref. 2). The control idea is to monitorthe system until it come close to a fixed

bifurcation point and then change the parameter value until the system lies in the stable solution of the

bifurcation region (ref. 5-8). Ott et. al., (ref. 9) developed a method where control can be applied to systems

where the dynamical equations are not known. The method is to change a chaotic system into a periodic

system by means of a weak time dependent perturbations to an accessible parameter.

Carroll et. al., (ref. 8) have driven a subsystem of a chaotic system with the signal from a similar system to

synchronization. Brown et. al., (ref. 2) have demonstrated the possibility of synchronizing a fitted model to a

time series obtained via experimental observation. This method allows the generation of an ordinary differential

equation that models the dynamics of the system that produces the time series. Sproot (ref. 1) has outline 19

standard dynamical equation which demonstrate chaotic behavior. Judd and Mees (ref. 10) has examine the

problem of how to choose the best model from within a class of models to fit data. The best model captures

the essential dynamics of the time series without over fitting - including in the model aspects of the time series

that should be attributed to noise. Such a scheme potentially allows extensive remote diagnostics from a

variety of physical parameter from a single time series measurement such as plasma collection current.

Time series measurements forms the basis of the control methods (ref. 3). Experimental measurements

are gathered in the form of time series with a prescribed sampling interval or rate. Plasma currents are usually

measured by Langmuir probes and thus provide a source of time series data. Synchronization of Langmuir

time series data with a class of fitted models accords a non trivial test of the validity of the models.

Nondestructive testing via synchronization will afford the spacecraft design community a valuable tool to

spacecraft control. A possible scenario is adapted from Brown et. al., (ref. 2). A new spacecraft with a

projected operational life is placed in orbit. Prior to placement the spacecraft and its subsystems is driven by a

calibrated external driving signal (plasma chamber?) and a time series is recorded. A model of the dynamics is

constructed from part of the time series and the synchronization deviation level between the model and the

rest of the time series is recorded. Once in orbit we received time series data from the spacecraft and its

systems. We attempt to synchronize this orbital time series with the pre flight constructed model. Due to its

orbital interactions, there will be changes in the synchronization deviation level due to the orbital and plasma

dynamics. By examining and monitoring the changes in deviation we have information on spacecraft systems

dynamics from the synchronization physical model. Given the noise invariance of our model we can be

assured that our deviations are spacecraft induced and not statistical in nature.

Noise is a feature of Langmuir probe data and it is probable that certain featured of Langmuir noise has a

chaotic time signature. This would provide another region for synchronization possibilities - use of noise as a

stabilizing control signal via chaotic synchronization. Brown et. al (ref. 2, 3) indicates that synchronization can

be maintain with high noise levels.

Conclusion.

Utilizing the results of chaotic synchronization, snapover may be controllable since its empirical based model

are amendable to a bifurcation analysis. Bifurcation analysis examines the behavior of solutions to parametric

equations at bifurcations points. At bifurcation points a qualitative change in the structure of the equilibrium

solutions phase space. Changing one or more parameter might lead to instabilities. Several scheme for

instabilities and chaos have been proposed as aseries of bifurcations as a parameter is varied. Future work

would entail time series analysis of snapover data fluctuations. Transition to chaotic behavior and instabilities

can then be simply modeled as a sequence of bifurcations acquired from parametric analysis of the dynamical

equations and sychronization methods as a means of system control could be applied.

-300-
"I

I

!

01

i

I

i

/

%

u

Fig. 1 Bifurcation diagram

10"3

10-5 _--

10"6

i 10-;

10-8

10"9

Io'IC

10"11 ,o.,, j,I I , I,1,1,1I, I,_,_

10 lO0 1000 1000o

fl.ECTRO_ POT_ll_L. VOLTS

Fig. 2 Snapover ref. 16

INSULATION

AREA

Cm2'

.,..,,_ 2o

POSITIVE//" ._ TJO

f/

j -

-3

1o

_4

!

2

l_6

-- /

"- EXPERV_NTAL

DATA

--CYLINDRICAL '_ /_

COLLECTION Jl

,OAioI+/_'--__V_ 'L4-

_1 1 I

POTENTIAL (RELATIVE TO PLASMA) VOLTS

Fig. 3 F'itted mode'l re£. 22

-301-
REFERENCES:

1) J. Sproott, Some Simple Chaotic Flow, PR-B, Vol. 50, No. 2, Aug. 1994, p. R647.

2) R. Brown et. al., Modeling and Synchronizing Chaotic Systems From Experimental Data, Physics Lett. A,

Vol. 194, Oct. 1994, p. 71.

3) R. Brown and N. Rulkov, Synchronization of Chaotic Systems, Institute Nonlinear Science, Univ. California,

San Diego, 1994.

4) U. Dressier and G. Nitsche, Controlling Chaos Using Time Delay Coordinates, PRL, Vol. 68, No. 1, Jan.

1992, p. 1.

5) M. Vieira et. al., Synchronization of Regular and Chaotic Systems, PR-A, Vol. 46, No. 12, Dec. 1992, p.

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6) L. Pecora and T. Carroll, Synchronization in Chaotic Systems, PRL v.64, No. 8, pp.821, Feb. 1990.

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8) T. Carroll and L. Percora, Synchronizing Chaotic Circuits, IEEE Trans. Circyits and Systems, Vol. 38, No.

4, April 1991, p. 453.

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10) K. Judd and A. Mees, On Selecting Models for Nonlinear Time Series, Univ. of Australia, Research

Report, Dept. of Mathematic, Nov. 1994.

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Nov. 1992, p. 3573.

12) M. Kono et. al., Cascading Bifurcations to Chaos in a Current Carrying Ion Sheath, Jour. Phy. Soc. of

Japan, Vol. 61, No. 2, Feb. 1992, p. 407.

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1987, p. 551.

14) T. Braun et. al., Observation of Deterministic Chaos in Electrical Discharges in Gases, PRL, Vol. 59, No.

6, Aug. 1987, p. 613.

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Plasma Environment, NASA TMX-71554.

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D-8111, Dec. 1975.

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19)G. B. H. Hillard, Plasma Current Collection of Z-93 Thermal Paints, J. Spacecraft, Vol. 30, No. 6, p.767.

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25) M. Carruth, Plasma Collection Through Biased Slits in a Dielectric, J. Spacecraft, Vol. 24, No. 1, Jan-Feb.

1987, p. 79.

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Simulated High Voltage Solar Array, AIAA 83-0311, 1983.

27) H. Thiemann, Particle in Cell Simulation of Sheath Formation Around Biased Interconnectors in a LEO

Plasma, J. Spacecraft, Vol. 27, No. 5, Sept-Oct. 1990.

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J. Spacecraft, Vol. 22, No. 6, Nov. 1985, p. 631.

29) D. Ferguson et al, Visualizing Plasma Electric Fields, NASA Lewis DDF Report, 1995.

4

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A LINEARREFRACTIVEPHOTOVOLTAICCONCENTRATORSOLARARRAYFLIGHTEXPERIMENT

P. Alan Jones and David M. Murphy

AEC-Able Engineering Co., Inc.

Goleta, California 93117

ABSTRACT

Concentrator arrays deliver a number of generic benefits for space including high array efficiency,

low cell area cost savings, protectionfrom radiation effects, and minimized plasma interactions. The linefocus concentrator concept delivers two added advantages: 1. low-cost mass production of the lens

material and, 2. relaxation of precise array tracking requirements to only a single axis. New array designs

emphasize light weight, high stiffness, stow-ability and ease of manufacture and assembly. The linear

refractive concentrator can be designed to provide an essentially flat response over a wide range of

longitudinal pointing errors for satellites having only single-axis tracking capability. In this paper we

address the current status of the SCARLET linear concentrator program with special emphasis on

hardware development of an array-level linear refractive concentrator flightexperiment.

INTRODUCTION

High performance, light concentrating solar arrays offer spacecraft users well documented cost

and performance benefits. The SCARLET (Solar Concentrator Array with Refractive Linear Element

Technology) is the first practical concentrator array that can realistically provide such benefits. An

aggressive, 6-month development and flight validation program, sponsored by the Ballistic Missile

Defense Organization (BMDO) and NASA Lewis Research Center, will quantify and verify these benefits

with in-orbit performance measurements. Given the current economic climate, Retum On Investment

(ROI) is of primary importance to both commercial and government spacecraft users. The cost and

performance benefits to be accrued from the use of SCARLET technology directly contribute to enhanced

ROI. In this paper we introduce the basic SCARLET technology, examine its benefits to users, and

describe the first-generation flight hardware.

Simply stated, concentrator technology allows arrays to have much lower cell area for a given

power level. For instance, a concentrator array with a 15:1 geometric concentration ratio requires about

7% the active solar cell area of a traditional planar array. This equates to a direct 93% reduction in solar

cell material costs which is the largest component of total array costs. Further indirect cost benefits

accrue from the reduced active cell area. These generic cost benefits will of course favorably impact all

missiontypes.

The main technical barrier to eroploying satellites in high radiation missions is degradation of cell

energy corwersion efficiency due to electron and proton impingement. In a planar array, compensating for

cell degradation requires the use of larger, more costly arrays due to excessive cell usage. Alternately,

system costs are driven up by thick (costly and heavy) radiation protection over the entire cell area,

frontside and backside. In such missions the SCARLET array will provide significant mass savings

because only 7% of the array requires mass shielding. This mass savings can technically and financially

enable certain missions such as medium earth orbit (MEO) communication constellations or a

geosynchronous mission employing spiral-out electric propulsion orbit raising. The economic benefits of

both of these mission scenarios are well documented.

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Additionally, since only 7% of the total area need be populated, high-efficiency multijunction cells

can be more economically employed to field a reduced area array to limit aerodynamic drag and/or relax

attitude control system requirements. The mission benefits of reduced area arrays have also been well

documented (reference 1).

The SCARLET technology mitigates the debilitating effects of interplanetary distances on solar

cell efficiency. These LILT (Low Intensity, Low Temperature) effects increase the size and hence cost of

a solar array for a multi-AU (Mars and beyond) spacecraft. Although LILT effects can be minimized by cell

design, concentrator technology offers a less expensive way of addressing the issue. For instance, at 4

AU the sunlight intensity is approximately 1/16th of that at 1 AU. A 15:1 SCARLET array would bring the

sunlight intensity delivered at the solar cell back to that of the 1 AU range. The cell will then operate at

near 1 AU efficiency and will not suffer the full LILT efficiency degradation. The combination of

concentrator arrays and LILT-capable cells may enable deep interplanetary missions.

BACKGROUND

The application of concentrator arrays has historically been limited by their waning power

response to off-pointed solar vectors. From a risk point of view, the reliance on such tight pointing

accuracies is unacceptable. The SCARLET array technology utilizes pointing error tolerant refractive

Fresnel optics to achieve a 15:1 concentration of the incoming sunlightonto photovoltaicsolar cells.

The choice of refractive versus reflective optics is driven by the greater slope error tolerance of refractive

focusing. For reflective optics the high surface smoothness required to prevent scattering losses is costly

to manufacture and difficult to protect from on-orbit environmental effects. In addition, the tolerance to

shape error of the arched Fresnel lens is 300 times greater than the reflective concentrator, which has

been shown to be prone to performance losses due to thermal distortions.

The SCARLET design, utilizing the innate error tolerance of the Fresnel optics, is the first practical

concentrator solar array because it accommodates the combination of manufacturing tolerances, thermal

distortions, and jitter as well as the inevitable off-angle errors due to positioning knowledge and command

errors.

PASP+ Exoeriment

The SCARLET flight validation experiment draws directly from the flight experience and success

of the Photovoltaic Array Space Power Plus Diagnostics (PASP+) experiment which flew on the US Air

Force sponsored Advanced Photovoltaic and Electronics Experiment (APEX) spacecraft. The

concentrator portion of the PASP+ experiment is well described by Piszczor, 1991 (reference 2). The

PASP+ concentrator module, manufactured by Boeing (Figure 1), consists of 12 spot-focus, mini-dome

Fresnel lenses with GaAslGaSb mechanically stacked tandem cells. The primary optics are protected

from the AO, UV environments anticipated in the APEX orbit (350 km by 1850 km, 70° inclination) by

multi-layer optical coatings on the silicone lenses. The technology's tolerance to high radiation fluences

and the space plasma environment was also verified. Marvin, 1995, (reference 3) reported very good

experimental results for the concentrat6r module. Total power degradation is about 5% for 300 days of

elapsed rnission duration. This degradation is considered nominal and indicates the absence of any

anomalous environmental degradation mechanisms.

SCARLET/COMET FLIGHT EXPERIMENT

The SCARLET/COMET flight validation program was awarded to the team of AEC-ABLE

Engineering Company, Spectrolab, and Entech. Work was initiated in January of 1995 and the firstgeneration wing was to fly on NASA's COMET spacecraft in July 1995 (Figure 2). The launch of the

mission, renamed METEOR, has endured delays throughout the summer. The flight is currently

scheduled for late October.

-305-
FIGURE 1. PASP+ CONCENTRATOR EXPERIMENT MODULE

FIGURE 2. COMET/METEOR SPACECRAFT

-306-
The flight experiment is a six panel ABLE PUMA deployable array structure populated with

Spectrolab GaAs/Ge concentrator solar cells under Entech linear Fresnel primary and compound

parabolic concentrator (CPC) secondary optics (Figure 3). This wing will replace one of the four existing

silicon cell planar wings. The COMET spacecraft will fly in a nadir orientation most of the time and will

orient the spacecraft into a sun pointing, solar-inertial orientation for data acquisition from the array

experiment. The wing is composed of four concentrator panels and two silicon cell planar panels. The

two planar panels are required to maintain energy balance when the spacecraft is in the nadir mode as all

the wings are non-gimbaled.

FIGURE3. SCARLET/METEOR FLIGHTDEMONSTRATIONWING

Ootics System

The Fresnel primary lens uses a symmetrical refraction arched lens approach. This patented lens

provides high optical efficiency (by minimizing reflection losses), outstanding focusing properties, and

unequaled tolerance for manufacturing and operational inaccuracies. This remarkable tolerance for shape

errors eliminates potential problems due to deflections, distortions, or thermal expansion/contraction

effects, which have plagued other types of ph0tovoltaic concentrator systems (especially parabolic trough

reflector systems). A ray trace drawing for the baseline optics configuration at the specified off-pointing

tolerance of 2° is shown in Figure 4. Note the secondary concentrator at the focus of the Fresnel lens

primary concentrator. The secondary reflects rays internally by total internal reflection (TIR), and thus

requires no metallic reflector surface.

-307-
Primary Concentration: IOX

Secondary ConcentratTon: 1.6X

Overall Concentration: 16X

CMX Coverglass

C'amagin,

Solid silicone CPC secondary _

t

cell

Incident rays shown

with 2-degree lateral

pointing error

Entech Fresnel lens

50-degree rim angle

2-degree error tolerant

g energy of h[gh

frequency (UV) waves ore

refracted over top of CPC

FIGURE 4. SCARLET OPTICS LIGHT RAY TRACE

The linear Fresnel system has a different tolerance to sun vector off-pointing in each of the two

principal axes. Tolerance to errors about the longitudinalaxis is greater than the tolerance to errors about

the lateral axis as shown in Figure 5. This feature can be employed to provide optics for missions

planning only a one-axis array tracking mechanism. Longitudinalsun pointing errors of up to 23.5 ° can

easily be accommodated. The data for the particular lens design plotted in Figure 5 shows a 0.5% loss at

a 10° longitudinal pointing error and a 10% loss at a 2° lateral pointing error. In practice the desired lateral

and longitudinal tracking tolerances are traded against overall light concentration ratio in the system

design process.

FIGURE 5. OPTICAL OFF-POINTING EFFICIENCY

-308-
The current SCARLET/METEOR module design is based on GaAs/Ge concentrator cells. Each

panel has six strings, each of which are made up of four 12-cetl module subassemblies (Figure 6). The

concentrator cells, measuring 0.140 inch by 0.600 inch, are attached to the module in a sedes

interconnection made with a Kapton flex circuit. Three bypass diodes on each module are attached to the

circuitwith surface mount techniques. One blocking diode per string is similady attached to the first of the

four modules in a string.

FIGURE6. SCARLET MODULEDESIGN

The design is oriented for cost-effective mass productiontechniques. The CPC secondary optics

are molded as a single piece so that only a single secondary bond operation is performed instead of

twelve bond operations. And the cell placement and attachment is amenable to standard pick and place

elec_onic assembly operations. The cell interconnectionto the circuit is accomplished with an automated

sonic wire bonding step. Figure 7 shows a close-up photo of two flight modules mounted onto the

substrate. The cells can be seen through the CPC optics.

Deployable Optics

A critical feature of the SCARLET design is the achieved reduction in the required stowage

volume. Concentrator optics require a finite focal length that historically has defined the array's required

stowed thickness and, hence, volume. These volumes are typically too large for most missions. The

SCARLET system utilizes a deployable optics system to maintain a small stowage volume. Two patentpending approaches for lens deployment,have been defined.

One utilizes a system of lenticular spdngs to deploy the Fresnel lenses. The other uses a precision

linkage system to articulate the lenses into position. The former design is implemented in the

SCARLET/METEOR flight experiment hardware. SCARLET's tight stowage volume is shown in Figure 8.

One of the flight panels is shown in Figure 9 with its primary optics deployed above the populated

substrate panel.

-309-
FIGURE 7. SCARLET MODULES

FIGURE 8. STOWED SCARLET WING

-310-
FIGUREg. SCARLETPANELWITHDEPLOYED OPTICS

PERFORMANCE

Variousdesignand analytical trade studies were performed to assess the near-term performance

of the SCARLET technology as it evolves from the current no-frills, schedule-driven experiment design

towards application as a primary power source on a flight program. The results, which incorporate

thermal, optical, and radiation analyses, are in keeping with the technology assessments put forward by

Caveny, 1994, (reference 4) and Piszczor, 1994 (reference 5) which anticipate EOL specific powers in the

70 W/kg to 100 W/kg range. Figure i0 depicts the near-term EOL specific power trends for SCARLET

concentrator technology for three solar cell assumptions. Analyses of subsequent development indicates

EOL performance approaching 115 W/kg for a high radiation environment (also shown in Figure 10).

12o

1oo

| 80

"6 6O

E

4o

2o

o

Evolved SCARLET ....

Triple Junct/on 27% "X_ _ _ _ _ .....

I I _ Triple Junction 27%

. f _ Dual Junction 24%

"th 811111 equiv, fused silica fronUrear shielding.

i000 2000 3000 4000 5000 6000 7000 8000

System Power per Wing (EOL WANing)

FIGURE 10. SCARLET SPECIFIC POWER PERFORMANCE

-311-
CONCLUSION

The SCARLET concentrator technology offers unique capabilities to both commercial and

government spacecraft users. These include solar array cost reduction, especially when new, highperformance multijunction cells are employed; mass reduction over traditional planar arrays, especially in

high radiation missions such as MEO missions and electric powered LEO to GEO orbit raising; and

potentially reduced drag area. SCARLET has near-term applicability as it is significantly far along the

development curve as a result of the planned Fall 1995 flight validation aboard the NASA METEOR

spacecraft.

REFERENCES

.

2.

.

4.

.

Ralph, E.L., "Solar Cell Array Comparisons",Space Power Workshop, Albuquerque, N.M., 1994

Piszczor, M.F., Brinker, D.J., Flood, D.J., Avery, J.M., Fraas, L.M., Fairbanks, E.S., Yerkes, J.W., and

O'Neill, M.J., "A High Performance Photovoltaic Concentrator Array: The Mini-Dome Fresnel Lens

Concentrator with 30% Efficient GaAs/GaSb Tandem Cells", Proceedings of the 22nd IEEE

Photovoltaic Specialists Conference, 1991, pp. 1485-1490

Marvin, D., "PASP+ Flight Experiment Results", Space Power Workshop, Albuquerque, N.M., 1994

Caveny, LH., and Allen, D.M., "BMDO Photovoltaics Program Overview", Proceedings of the Xlll

Space Photovoltaic Research and Technology Conference, 1994, pp. 13-21

Piszczor, M.L., O'Neill, M.J., and Fraas, L.M., "An Update on the Development of a Line-Focus

Refractive Concentrator Array", Proceedings of the XIII Space Photovoltaic Research and Technology

Conference, 1994, pp. 313-322

-312-
Development of a Solar-cell Dust Opacity Measurement Instrument

for Mars Pathfinder

Geoffrey A. Landis

Ohio Aerospace Institute

and

Phillip_.P. Jenkins

NYMA, Inc.

NASA Lewis Research Center, mailstop 302-1

21000 Brookpark Rd., Cleveland, OH 44135

ABSTRACT

The atmosphere of Mars has a considerable load of suspended dust. Over time, this dust is

deposits out of the atmosphere. The mechanism and the temporal and geographical variation of this

deposition are not well characterized. Measurements of settling rates and dust properties are of

considerable scientific interest. Atmospheric dust affects the atmospheric solar absorption and thus the

heat balance of Mars, as well as serving as nucleation sites for water and CO 2 frost. Knowledge of dust

properties is of critical interest to design and prediction of the lifetime and power output of solar arrays,

and also to design of mechanical mechanisms and radiators.

An instrument has been designed and fabricated to measure the dust accumulation during the

course of the Mars Pathfinder rover mission. The solar-cell coverglass transmission experiment will

measure the change in optical opacity of a transparent coverglass as dust settles on the surface, and a

quartz crystal monitor will measure the mass deposited.

Background

The atmosphere of Mars contains a significant load of suspended dust. This is quite visible in the

Viking lander photographs by the light color of the sky. The amount atmospheric dust varies with season

and with the presence of local and global dust storms, but never drops entirely to zero.

Dust suspended in the atmosphere of Mars is a significant driver of the climate and meteorology of

Mars. Absorption of sunlight by dust particles is the primary driver of the thermal properties of the

atmosphere [1]. To quote from a review by Zurek et al [2]: "The effect of suspended dust on the

atmosphere are: (1) dust is an effective absorber of the incoming solar radiation and both absorbs and

emits thermal radiation; (2) the amount and possibly the optical properties of suspended dust are highly

variable in space and time; and (3) dust hazes can persist for very long periods."

Information on dust deposition rate and properties is of critical use for future mission design

[3,4,5,6]. Knowledge of how much dust deposits on solar arrays, the intensity and spectral range of

sunlight at the surface of Mars, and the size and shape of particles will be crucial elements for designing

missions that will operate on solar power for periods of several years and will have moving parts which

will be subject to degradation by dust. °

Other than the basic parameters of light scattering, however, the properties of the atmospheric dust

are almost completely unknown. The size of the particles is known only roughly (and with considerable

disagreement between models); the size distribution and the particle shapes are almost completely

unknown. While we do know that dust does settle out of the atmosphere, the mechanism, rates, and

geographic variability of dust settling out of the atmosphere are a matter of controversy. Any mission to

study the climate of Mars will remain incomplete unless a serious attempt is made to answer

fundamental questions about atmospheric dust.

A first attempt to quantitatively measure dust deposition on the surface of Mars will be our

experiments flown on the Mars Pathfinder, which will measure the mass and optical opacity of dust

settling onto the rover solar array [3,4].

-313-
Dust Properties

Settlingpropertiesof Marsdustarediscussedin earlierpapers[5,6]. Therearetwodifferent

modelsofthesizeandscatteringpropertiesof Marsdust. Theconventional modelis by Toon et a/[8],

as modified by Pollack eta/. [9], using sunlight scattering measured from the surface by Viking.

A revised model by Clancy and Lee [10] is based on infrared reflectivity measurements taken from

orbit by Viking. The Clancy and Lee model suggests that the dust is composed of much smaller

particles than the Pollack model indicates, and that the particles are less absorbing and more reflective.

The Pollack et al. analysis suggests a cross-section weighted mean radius of 2.75 lim; Clancy and Lee

analysis 0.4 lim or less. The cross-section weighted radius of 1.5 p.rnestimated by Pang and Ajello [11]

is almost halfway between that of the two other models.

Both values are consistent with estimates of the grain size of the surface drift material at the Viking

lander sites [12].

The fact that the particle size differs by nearly an order of magnitude is an indication of the difficulty

of deducing particle size from optical scattering data. The degree of apparent detail in these models

completely obscures how much is unknown. For example, the gamma distribution of particle sizes

assumed is an ad-hoc distribution function; the actual distribution of sizes is likely to be different. The

light scattering properties are significantly determined by the size and distribution of particle shapes. It

is known from the scattering that the particle shapes cannot be spherical, but the actual shapes are

impossible to determine from scattering data. Scattering of light from non-spherical particles is very

difficultto model, and if a variety of different particle sizes exists, the number of free parameters to model

makes it nearly impossible to derive particle information from scattering data. Some researchers

suggest that atmospheric dust will not be present as individual particles at all, but as fractal

agglomerates with considerable interior void fraction, it is unknown whether the particles are

homogeneous or heterogeneous in composition; for simplicity, homogeneous particles have been

assumed in modeling, but this is a questionable assumption.

The mechanism and geographical variability of dust settling is likewise unknown. Dust is likely to

deposit out of the atmosphere by gravitational settling, by vertical eddy mixing, and by serving as nuclei

for ice crystal formation. Alternative mechanisms such as electrostatic precipitation and aggregation by

electrostatic bonding may also play a role [13]. It might logically be suggested that during low-opacity

times the suspended dust will be smaller in grain size than the average. However, the Mariner 9

measurements tended to indicate that the size distribution during the major storms seems to be

essentially constant during the decay [8].

Table 1 [from ref. 5] shows the amount of degradation in solar array performance due to dust

obscuration calculated for best and worst case of a 30 day and 2 year mission. Clearly, there is

significant variation between cases. Better information on deposition rate is required if solar array

performance is to be accurately predicted.

Table 1

Total obscuration of solar array for the Pathfinder (30 day) and network (2 yr) missions

Case: obscuration obscuration

(30 day mission) (2 yr mission_

Baseline 6.6% - 77%

Best 0.5% 22%

Worst 52.2% 89%

-314-
Pathfinder Solar-Cell Experiment

Twosensors,comprising the "Materials Adherence Experiment (MAE)," will be flown to Mars on the

Pathfinder spacecraft in 1996. The Pathfinder spacecraft, built by the Jet Propulsion Laboratory (JPL), is

to be launched in December 1996, and will land on Mars on July 4, 1997. The mission includes both a

stationary lander and a small solar-powered rover, named "Sojourner." The MAE instrument is located

on the rover. It consists of a Solar Cell experiment (MAE-SC) and a Quartz Crystal Monitor. These have

been built, qualified for flight, and delivered to JPL for integration onto the spacecraft. Figure 1 shows

the location of the MAE on the Sojourner rover. The actual experiment takes up a footprint of 41 mm by

37.5 ram, including area for a JPL reference solar cell unrelated to the experiment.

Materials Adherance Experiment

.l I Sensor location

Figure 1 The MAE solar-cell and QCM experiments on the "Sojoumer" Mars Pathfinder Microrover

The Solar Cell Experiment consists of a horizontal transparent plate onto which atmospheric dust

will settle. This plate normally covers a small GaAs solar cell. A shape-memory alloy ('nitinol") actuator

temporarily moves the plate away from the photosensor, to allow a measure of solar intensity withoutthe

intervening dust settling plate; then the measurement is repeated through the dust plate, giving a

measurement of the solar intensity viewed through the dust collecting plate. The difference between

these two measurements monitors the change in opacity due to the dust settled on the plate.

The Solar Cell experiment has the only moving part inthe instrument, the actuator which moves the

transparent plate away from the sensor. For simplicity and reliability, we designed a novel rotating

actuator based on the shape-memory alloy nitinol to do this job. The actuator has only a single moving

part. The design and testing of this actuator is described in detail in the paper =A Rotating Arm Using

Shape-memory Alloy" [7]. The rotary action is shown (drawn to scale) in Figure 2.

-315-
TheMAE/SChasbeenthroughqualificationtestingforthePathfinderflightwithnofailures.

In additiontothesolarcellexperiment, the Pathfinder rover will also fly a miniature quartz crystal

monitor [3]. The QCM makes an independent measurement of the dust mass, allowing the results from

the solar cell experiment to be interpreted interms of obscuration as a function of dust mass. The quartz

crystal monitor is a vibrating quartz wafer about the size of a dime. The surface of the quartz wafer is

horizontal and exposed to the sky. As dust settles on the crystal, the frequency of vibration decreases.

This serves as a measure of the mass deposited as a function of time. QCM sensors similar to this one

have a spaceflight heritage for contamination monitors.

Quartz

Crystal Monitor

open

Circuit

Solar Cell\

Dust Cover

Experiment

/

/

Figure 2: Schematic of the Materials Adherence experiment on the Mars Pathfinder, showing the

location on the watchplate of the quartz crystal monitor,the dust cover experiment with nitinol-actuated

coverglass (shown in the "open" position), and the open-circuit voltage solar cell.

CADO Instrument Package

An advanced version of this instrument package, named "CADO" ('Characteristics of Atmospheric

Dust Observation"), is being developed for flight on a future mission, to obtain detailed information on

properties of settled dust. CADO is designed to monitor the rate of deposition of dust on a horizontal

surface on Mars, to directly observe the particle sizes and shapes, and to measure the solar insolation at

the surface in four spectral ranges.

The CADO instrument set consists of three sensors.

1. Quartz Crystal Monitor.

2. Optica/Opacity and Soat Insolation Monitor, an advanced version of the Pathfinder "Solar Cell

experiment'.

3. CCD Microscope.

The first two sensors on the CADO instrument have direct heritage from the Pathfinder

instrumentation, upgraded to make them both smaller and more capable. The third sensor of the set, the

CCD microscope, has not previously been flown. The CCD microscope has been successfully

developed through the breadboard stage under NASA Lewis discretionary funding. This project has

just completed its second year.

The CADO instrument set is described in more detail elsewhere [3].

Sensitivity

It is important that the sensitivity of tt DroDosed instruments is sufficient to actually measure the

expected properties of dust. For a baseli _caiculallon, a typical optical depth of the atmosphere is

• =1.0 is assumed, and the particles are assumed to settle out of the atmosphere with an exponential

decay time of 100 days. Assuming parameters of the Pollack model, this yields a coverage of 0.36% per

day [5,6], and an obscuration of 0.22% per day. Thus, a single day's coverage will be measurable with

-316-
theOpticalObscurationSensoraslongastheND conversion is 9 bits or more. Ten days coverage will

be detectable with 8 bits of ND accuracy. The A/D for the sensor is a 12 bit AJD (11 bits usable), and

therefore it is reasonable to expect that the dust obscuration will be visible.

The mass-weighted average particle radius is 3.9 microns. If the average particle density is 2.5

tghrams/cm,the mass deposition rate is 4.7 micrograms per cm2 per day. Since the QCM sensitivity is in

e 10 nG range, sensitivity is no difficulty.

At a cross-section weighted average particle size of 2.75 micron, the particle number density to be

expected corresponds to 1.5"104 particles/cm 2 per day. If the magnification chosen for the microscope is

20x, this will result in 3 particles being imaged per day. This is sufficientto accumulate good statisticson

particle size over an extended mission.

References

1. J.B. Pollack, "Properties of Dust in the Martian Atmosphere and Its Effect on Temperature

Structure,"Chapter III, Adv. Space Res., Vo/. 2, COSPAR, 45-56 (1982).

2. R.W. Zurek, J.R. Bames, R.M. Haberle, J.B. Pollack, J.E. Tillman and C.B. Leovy, "Dynamics of the

Atmosphere of Mars," Chapter 26, Mars, University of Arizona Press, Tucson, 1992, 835-933.

3. G. Landis, P. Jenkins, J. Flatico, L. Oberle, M. Krasowski and S. Stevenson, "Development of a

Mars Dust Characterization Instrument," presented as paper IAF-95-U.4.09, 46th International

Astronautics Federation Congress, Oslo Norway, 2-6 Oct. 1995, and at the Intermarsnet workshop,

Capri, Italy, 29 Sept. 1995.

4. G. Landis, =Instrumentation for Measurement of Dust Deposition on Solar Arrays on Mars,"

presented at Case for Mars V, Boulder CO, 26-29 May 1993

5. G. Landis, =Dust-induced Degradation of Solar Arrays on Mars," Proceedings of the 1st World

Photovoltaic Energy Conversion Conference, Hawaii, Dec. 5-9 1994, 2030-2033.

6. G. Landis, "Dust Obscuration of Mars Photovoltaic Arrays," Paper IAFo94-380, 45th International

Astronautical Federation Congress, Jerusalem, Oct. 9-14 1994.

7. P. Jenkins and G. Landis, "A Rotating Arm Using Shape-memory Alloy," presented at Space

Mechanisms Conference, NASA Johnson Space Center, Houston, TX, 17-19 May 1995.

8. O.B. Toon, J.B. Pollack, and C. Sagan, =Physical Properties of the Particles Composing the

Martian Dust Storm of 1971-1972,"/cams, Vol. 30, 663-696 (1977).

9. J.B. Pollack, D.S. Colbum, F.M. Fiassar, R. Kahn, C.E. Carlston, and D. Pidek, J. Geophysical

Research, Vo/. 84 No. B6, 1979, p. 2929-2945.

10. R.T. Clancy and S.W. Lee, =A New Look at Dust and Clouds in the Mars Atmosphere," Icarus,

Vol. 93, 135-158, 1991.

11. R.A. Kahn, T.Z. Martin and R.W. Zurek, =The Martian Dust Cycle", Mars, University of Arizona

Press, Tucson, 1992.

12. H.J. Moore, R.E. Hutton, G.D. Clow, and C.R. Spitzer, Physical Properties of the Surface

Materials at the Viking Landing Sites on Mars, U.S. Geological Survey Professional Paper 1389, pp.

117 and 130 (1987).

13. R. Greeley, N. Lancaster, S. Lee and P. Thomas, "Martian Aeolian Processes, Sediments, and

Features," Chapter 22, Mars, University of Arizona Press, Tucson, 1992, 730-766.

14. J. Appelbaum, G. Landis and I. Sherman, =Solar Energy on Mars: Stationary Collectors,,

Journal of Propulsion and Power, Vol. 11No: 3,554-561 (!995).

15. J. Appelbaum, G. Landis and I,Sherman, "Sunlight on Mars: Update i991 ," Solar Energy, VoL

50 No. 1, 35-51 (1993).

16. R.B. Singer, AGU paper 2B1214, "Minerology of High-Albedo Soils and Dust on Mars," J.

Geophysical Research, 10,159-10,168 (1982).

17. E.Ao Guinness, R.E. Arvidson, M.A. Dale-Bannister, R.B. Singer and E.A. Brukenthal. "On the

Spectral Reflectance Properties of Materials Exposed at the Viking Landing Sites," Proc. 17th Lunar

and P/anetary Science Conf., Part2, JGR Vo/. 92, No. B8, E575-E587 (1987).

*lnfor_tion on Mars Pathfinder probe can be found at the Mars Pathfinder home-page,

http'J/mpfww.jp.nasa.gov/

-317-
MultijunctionCellWorkshop

SarahKurtz

NREL

GoldenCO

HenryCurtis

NASA/Lewis

ClevelandOH

Themultijunction cell workshop met on Tuesday afternoon with over 30 attendees from a wide variety of

companies and labs. Several topics were discussed, with the most discussionon the following items.

1) What needs to be done to fly GalnP/GaAs cells inthe ENTECH linear concentrator for SCARLET i1?

2) Wrap-through or wrap-around contacts - are they available, why are they useful, and why they are not

used more.

3) Directions to consider, including use of supedattices, 3 or 4 junctions, stacked cells, etc.

4) Measurements -- does every cell need to be tested under both red and blue illumination?

The MANTECH program started in September will allow both ASEC and Spectrolab to establish a large scale

production capability for the GalnP/GaAs/Ge cells.

The New Millennium program has chosen to use multijunctionconcentrator cells in a 2 kW array. They are

expecting to use GalnP/GaAs, but have many questions regarding movingthis technology from planar to the

concentrator configuration:

-Making smaller cells will improve yield in the short term, but inthe long run this will not be a major issue.

-The existing cell designs will not need any significant modification for low concentration (10X). Small

refinements include:

The grids need to be redesigned,

The emitter may be slightly more doped,

The thickness of the top cell may need to be adjusted for the optically changed spectrum,

The distance from the lens to the cell needs to be optimized.

There was concern about the rigidityof the support structure being adequate for concentrators. In the linear

direction, off pointing of 10 to 15 degrees can be tolerated. In the other direction the structure must support off

pointing less than two degrees.

It was pointed out that concentrators pose _. serious risksince a tumbling satellite will get no power from a

concentrator system yet will get some power from a planar system. There are instances where spacecraft have

losttheir attitude control system for a period of time. A hybrid of planar and concentrator systems gives better

assurance in such cases.

Lockheed Martin is interested in using wrap-through contacts on the GalnP/GaAs on Ge cells. By using the

wrap-through contacts they were able to automate the flexible array assembly process for Si cells. For the Ge

based cells the potential gain may be even greater.

Spectrolab is patenting a wrap-through process that will work for the GalnP/GaAs and GaAs cells on Ge. They

have demonstrated their design on small volumes, but have not scaled it up. Similarly, ASEC has a wrap-

-318-
throughprocessfor the III-V cells but have not found and customers. They investigated both organic and

inorganic-based concepts but only the inorganic concept uses space qualified materials. Their yield must be

improved to make this a good manufacturing process. The wrap-through process does add some cost at the

cell level, but it's not clear how this will compare to the savings at the array level.

A number of potential directionsfor space cells were discussed. At the moment, everyone sees things headed

toward the GalnP/GaAs/Ge cell. However as production levels increase along with order volume, the

economics and marketing philosophies may change somewhat. Sometimes the most important criteria is W/kg

and sometimes W/m2 is more important. Although the customers may be asking for lower cell prices, when the

total systems costs are considered higher solar cell effic.ienciesusuallyjustify higher cell costs.

The GalnP/GaAs/Ge and Inp/GainAs systems are both lattice matches, so they don't need to use superlattice

or graded layer schemes. However, a number of uses for supedattices or graded layers were identified: 1.

Growth of InP on Ge or Si (Ge is easier than Si because of the thermal expansion differences). 2. Growth of

GalnP/GaAs on Si. 3. Superlattices can give radiation resistance if they make multiple junctions, reducing the

distance any minority carrier might need to travel before being collected. 4. Bragg reflectors. 5. Supedattices

or graded layers are beneficial to any lattice-mismatched material system.

The question was raised as to whether we really want to increase the number of junctions or, after two

junctions, the added efficiency is not worth the extra cost of generating the third (or fourth) junction. For the

case of GalnP/GaAs/Ge with an active Ge junction vs. inactive junction, it is actually easier to grow it with an

active Ge junction; no added cost is incurred by the thirdjunction but the efficiency can be increased by up to

2.3% (absolute. Clearly, inthis case, one more junction is an advantage. Other systems may or may not show

such an advantage when another junction is added. The value of the lowest junction is especially questionable

as the cell temperature is increased. However the low band gap cells have the strongest performance for outer

planet missions. AI containing materials should not necessarily be ruled out.

Stacked cells were generally considered acceptable as long as they could be connected in a two-terminal

configuration. A suggestion was made to push some array integration onto the cell vendors.) The stacked cells

have some advantages includingbeing able to be separately tested before being connected and may have

more radiation resistance.

The weight of the cells can potentially be reduced by removing the substrate. Three methods for doing this

include CLEFT, lift-off using an AlAs release layer, and sacrifice of substrates (Ge). CLEFT has the

disadvantage of needing to repolish the wafer after each use while the AlAs liftoff only works on small cells

( one dimension must be less than or equal to 1 cm). RTi found sacdrce of a cheap Ge substrate to be

cheaper than the CLEFT method. Once the substrate is removed, the device must be mounted on another

material. Mounting directly on a coverslip will not work if a metal grid is included. There was a lot of skepticism

about the reliability of these devices after being mounted to another material. Will they survive the pull strength

and thermal cycling tests?

The question of how to test multijunctionarrays was discussed (and will be addressed in the next day's

workshop). Being able to control the spectrum (relative amounts of red and blue light) is more important than

being able to mimic the exact AM0 spectrum. The needs for cell testing and array testing may be different.

There were contradictory opinions about whether one measurement can adequately characterize a two junction

device. ASEC tests every cell for both bottom and top cell performance. Spectrolab is developing statistics so

that only some fraction of the cells will need to have both junctions tested. The question of how to calibrate

reference ceils was left for discussion at the following workshop.

-319-
Report on Workshop

on

Radiation Damage

Geoffrey P. Summers

Naval Research Laboratory and

University of Maryland Baltimore County

&

Carlos Vargas-Aburto

Kent State University

Kent, OHIO 44242

ABSTRACT

The workshop met for two hours with approximately 25 participants from the conference. The discussion mainly

centered around the two issues of the availability of radiation test facilities in the U.S. and the adequacy of typical

radiation measurements to represent accurately space radiation effects. Special concern was expressed about

the accuracy of the most commonly used radiation environmental models, AE8 for electrons, and AP8 for protons.

The consensus of the workshop participants was that several recommendations be made to the conference as a

whole, as a result of the discussions.

RADIATION FACILITIES

1. Available accelerators

It was generally agreed that both electron and proton irradiation facilities were becoming progressively less

accessible in the U.S. As a first step in seeing how serious the problem had become, a list of available facilities

known to the participants was produced. This is shown in Tables 1 and 2.

Table 1.

Electron Irradiation Facilities Maximum Energy or Energy range (MeV)

Boeing Corporation 1

Jet Propulsion Laboratory (JPL) " 2.5

NASA (Goddard) 2

Naval Post Graduate School 20 120

Nat'l Institute Standards and Technology (NIST) 1.5

U. of Maryland (College Park) 10

Wayne State University 4.7

-320-
Table 2.

Proton Irradiation Facilities

Aerospace Corporation

Brookhaven

Hawa_

Nat'l Institute Standards and Technology (NIST)

Jet Propulsion Laboratory (JPL)

Naval Research Laboratory (NRL)

U. of British Columbia (*)

U. of Califomia (Davis)

U. of Michigan

Wayne State University

Western Michigan University

Maximum Energy or

0.4

200

60

10

4.5

32 500

63

4.7

10

Energy range (MeV)

* Canada

2. Alternative techniques

There was some discussion on the use of 6OCogamma rays or 241Am alpha particles as alternative radiation sources

to electron and proton accelerators. Table 3 lists the 60C0 facilities known to the participants. When 60Cogamma

rays interact with matter, they mainly generate Compton electrons in low and intermediate Z materials. In high Z

materials such as lead, photoelectrons can also become important. The Compton electrons have a spectrum of

energies up to =1 MeV with an average energy of =0.6 MeV. 60Co gamma rays therefore produce damage that is

generally similar in nature to that produced by 1 MeV electrons at an accelerator, except that the Compton

electrons are isotropic, not unidirectional. The alpha particles produced as a result of the decay of 241Am can be

used as a desktop simulation of the damage effect of protons, so long as due consideration is given to the relative

displacement damage dose deposited by the respective particles. In practice, the 241Am source is typically a small

disk which is placed =1 cm from the target. The size of solar cell that can be irradiated depends on the size of the

disk. The alpha particles lose energy passing through the air and/or the top part of the cell, and the effective

energy of the particle is degraded to =4 MeV. The problem associated with the use of both 6O(3ogamma rays and

241Am alpha particles is the general reluctance of the solar cell community to accept the data when 1 MeV

electrons and 10 MeV protons are the usual particles employed. In addition, NRC licensing is an issue for 241Am

sources used in a laboratory. Some concern was also raised about possible unusual radiation effects that might be

produced, especially by 60C0 gamma rays.

Table 3.

Cobalt-60 Irradiation Facilities Cobalt-60 Irradiation Facilities

Aerospace Corporation Nat'l InstituteStandards and Technology (NIST)

Jet Propulsion Laboratory (JPL) Naval Research Laboratory (NRL)

NASA (Goddard) U of Maryland (College Park)

-321 -
MEASUREMENTS

1. Testingprocedures.

Therearetwogeneralpurposesfor groundradiationtests. First,thereistheneedto providedatabackto the

produceras new cell technologies are being developed; and secondly, there is the need for data from which to

predict cell and array performance for proposed or actual missions. The first need can be met fairly well by

performing ground tests in the usual way. That is, the test cell is irradiated at a relatively high dose rate at room

temperature by a unidirectional, monoenergetic beam of electrons 0r protons, in the dark, and at open circuit. At

several incremental fluence levels, the irradiation is stopped and the photovoltaic parameters of the cell are

measured. However, the workshop participants repeated the concern generally recognized by the radiation

effects community that this method of performing ground tests does not adequately represent what actually

happens in space. In space, the cells are mostly irradiated at low dose rate, at temperatures that can reach =60C,

isotropically by electrons and protons with a wide spectrum of energies, close to short circuit in sunlight.

Occaslonally, the cells can als0 be plunged into the dark and drop in temperature way below room temperature. It

was not clear to the workshop which, if any (or all), of the environmental parameters really mattered, i.e., do dose

rate effects occur or does it matter whether the cell is irradiated at open circuit during tests. If these issues are

uncertain for relatively well understood cell types, they are especially a problem for new ceil technologies such as

multijunction cells and concentrator cells which are designed to operate at high injection levels.

2. Environmental Radiation Models.

The problem of whether the usual ground tests are providing satisfactory radiation data for predicting cell

performance in space is compounded by apparent deficiencies in the AP8 and AE8 models. Several space

experiments, including CRRES and PASP PLUS indicate that actual cell degradation is about a factor of two less

than the models predict. Also, the disagreement with the models was found to vary with the level of shielding on

the cells, which indicates that not only is the overall dose wrong but that the proton differential spectrum is also

wrong. These results are similar to those found in many operational spacecraft power systems. This fact has

prompted some power systems engineers to deliberately reduce the size of photovoltaic arrays below what the

models indicate are necessary for a mission, in order to save money. Clearly, this is not a satisfactory situation.

However, changing the environment models is not something that can be done easily by the space photovoltaic

community, since many other users exist for these models.

3. New Requirements

One particular concern about testing occurs for LILT systems for deep space probes. Cells in these applications

will be operating at very low light levels and temperatures. There seems to be very few institutions that have test

facilities to accommodate these requirements. In fact, only JPL and Aerospace Corporation could be named

during the workshop. There does not appear to be a facility in the U.S. with the range of capabilities similar to what

is available in the U.K., at the DRA facilities at Farnborough and HarweU. However, it was generally agreed that the

capability developed by Bruce Anspaugh at JPL needed to be supported by the radiation effects community.

Bruce's work over the years has consistently provided some of the best data taken on a variety of cell types.

Although there does not appear to be any threat that this capability will be lost in the near future, the workshop was

insistent that managements be made aware of its overall importance.

RECOMMENDATIONS

The workshop made the following recommendations to the full conference.

.

2.

3.

A comprehensive list of available radiation facilities be developed to include the types of measurements

possible, the range of energies available, the cost, the level of availability, and the point of contact.

More work be performed as soon as possible to develop acceptable test procedures for new cell

technologies such as multijunction, concentrators, and new materials.

Strong general support be given to the JPL test facility, and especially Bruce Anspaugh.

-322-
SUMMARY OF WORKSHOP ON

CHARACTERIZATION AND TESTING OF NEW CELL TYPES

Dale R. Burger

Jet Propulsion Laboratory

Pasadena, Califomia

and

David J. Brinker

NASA Lewis Research Center

Cleveland, Ohio

GENERAL

An opening statement was made to the effect that the capture of cell structure and process is an important part of

new cell testing since only by knowing what you are dealing with can you decide what a sedes of test results really

mean.

TPV CELLS

The opening statement led into a short discussion of thermophotovoitaic (TPV) cell testing. The spectrum of

visible and infrared photons from TPV system emitters will be radically different from the AM0 spectrum. Even at

this late date there is difficulty in matching the AM0 spectrum with the various light sources - how then are we to

characterize TPV cells, which may be asked to work with spectra created by sources operating at a number of

different temperatures with a number of different emitters. The statement was made that spectral response

(quantum efficiency) data was the key. This statement was somewhat challenged by the reminder that intensity

effects can create non-linear losses at the highfluence of TPV systems.

Finally,there was some discussion of the use of black body sources for characterization. The good news is that

black bodies are available which cover the temperature range of space (i.e. radioisotope) heat sources. The bad

news is that these black bodies are at most one inch (25 mm) indiameter. Cells can therefore be characterized but

strings and arrays cannot.

MULTIJUNCTION CELLS

MuitiJunction (MJ) cells was the next area'of discussion. This discussion started off with the observation that

single junctioncells were much less demanding as to spectral match of their test light source to the AMO spectrum

(already we are pining away for the good old _days,) Single junction ceil testing has the added benefit of the

existence of a large database, built up over decades of trial and error.

The MJ cell test problem is twofold. First, the test lightsource for a combined current-voltage measq_reme_ must

not contain any unwanted spikes or dips near any of the cell band edges. Second, individual adjustment of each

of the double or triple (or quadruple?) lightSources is required to _pedy characterize the performance of each

cell (or band gap). Spikes or dips in the band edge regions make proper celltesting nearly impossible. Some dual

lamp sources have been built or are being built. A spectrally matched bias light technique needs to be agreed

upon, after careful review. .....

-323-
SomeagreementwasreachedontheminimumrequirementsforMJcelltesting.Theremustbestandard cells of

identical construction (this is not as simple as it sounds as cell design and processing is constantly evolving).

Since balloon standards are typically flown only once each year, the additional use of aircraft flown standards

should be reviewed. The MJ standard cells must be accompanied by standard sub-cells which were cah'bratedat

the same time. Finally, there must be at least one impartial laboratory to act as a cleadng house, since the capital

expense is I_ely to be too high for many individual organizations to construct their own facilities.

SUMMARY

There was a strong feeling that an early definition of requirements is needed:

A joint industrylgovemment/academla approach was favored.

A high level of accuracy and confidence in MJ cell testing is required in a short time.

The approach taken for MJ cells can be applied to TPV cells but the need is not quite as urgent.

OPEN ISSUES

A review should be made of the possibility of the use of carbon arc lamps for solar simulators now that modem

electronic power conditioning and high speed data collection is available.

Cell stringsand arrays testing must also be addressed sincethe solutionsfor cell testing may notalways scale up.

How will funding be found for all of this?

-324-
Solar Power Satellites

summary of a workshop held at SPRAT-XIV

Geoffrey A. Landis

Ohio Aerospace Institute

NASA Lewis Research Center 302-1

21000 Brookpark Rd. Cleveland, OH 44135

The concept of solving the Earth's energy crisis by supplying the Earth with solar electricity

generated in space and beamed to ground receivers by microwave was proposed by Peter Glaser in

1968 [1]. The concept, referred to as the "Solar Power Satellite" or "SPS, ° was studied extensively

between about 1977 and !980, by NASA and the U.S. Department of Energy [2,3], and by the National

Academy of Sciences [4] and the Office of Technology Assessment [5]. Since these studiesTNere

completed fifteen years ago, no official reana]ysis of the concept have been clone, although there have

been a continuous (albeit unfunded) low-level of interest in the subject, as expressed by occasional

conferences and workshops [6,7]. Particularly notable (to the present author) is a recent proposed

conceptual design which uses thin-film cell technology and integral solid-state microwave elements [8].

The 1980 "reference" concept is shown in cartoon form in figure 1.

Orbit: geosynchronous (35,800 km altitude)

Array: 55 km 2, using 17% efficient Silicon cells, assumed available at 17C/watt (1980 dollars)

Transmitter:. 1 km 2 transmitting aperture; 2.45 GHz

Rectenna: 100 km 2. Microwave power density 23 mW/cm 2 center, 100 _W/cm 2 edge.

5 GW electric power produced at the busbar per satellite .....

Estimated cost: $100B first unit; $11B each additional unit (1980 dollars)

microw-,Lve transmitter

Solar army

("recteni"_")

Figure 1 -

1980 +Reference Concept for a Satellite Solar Power System [2]

The Office of Advanced Concepts at NASA recently initiated a re-analysis of the SPS concept. The

ground rule for the analysis was that a project of such size (and more particularly, cost) as the 1980

baseline concept is out of the question in today's world. The study questions: (1) have technology

improvements since the 1980 studies made SPS concepts more feasible? (2) Are new architectures or

concepts for SPS possible which would reduce the cost?

These issues were posed to the workshop conducted at the SPRAT Conference, with the intent of

soliciting input from experts on space photovoltaic technology. Note that many of the concerns,

-325-
questions, and suggestions generated echo comments from other groups. In the interest of brevity, I

report the workshop without attempting to correlate comments to references in the earlier literature.

The first issue brought up at the workshop was the size of the microwave transmitting antenna,

which is set by the wavelength and the desire to minimize receiver spot size at the distance of

geosynchronous orbit. The immediate question was asked, does transmission have to be by

microwaves? Lasers were suggested as an alternative option. Ron Cull discussed his work on

analyzing laser transmission, and said that radiators for waste heat rejection are a big problem for

lasers, which are not as efficient as microwave transmitters, and which typically require lower operating

temperatures. It was pointed out that the Goldstone demonstration of microwave power transmission at

2.5 GHz, 85% efficiency DC in to DC out, is a very impressive practical result.

A tether extending from geosynchronous orbit to the surface to serve as a transmission line was

brought up, but quickly dismissed, as the required tensile strength makes it "a very tricky materials

problem."

The next question brought up was the operating environment on the cells? The statement was

made, "If you can use low-cost screen-printed terrestrial cell technology, OK, but if you have to use

space cells, there's no way we can manufacture cells in enough quantities." This may require avoiding

orbits with significant radiation.

The question was then put forth, what is the minimum project needed to show feasibility and

affordability? What is the growth path from there?

The first suggestion was a power relay satellite. The statement was made "we don't have a power

generation problem, we have a power distribution problem." It was pointed out that if a microwave

power relay satellites could redistribute power, terrestrial arrays e.g., in Australia, or floating in the

Atlantic and Pacific Oceans, could send power from the daylit side of the Earth to the users.

The next suggestion was to use a large space mirror for lighting. 30% of the electricity used is

used for lighting, why not use sunlight directly? Perhapst not all that big a mirror is needed to do this.

On the other hand, there is a big problem if it's cloudy: you don't want a lighting system that only works

on sunny days.

The next question addressed was, is it necessary to put the array in orbit? Why not use a balloon

(or a raft of balloons) to put an array at 100,000 feet, where it will be above the clouds, and above most

of the wind This might make it possible to use a tether for the power downlink, and it would also make it

much easier to pull down, if required, for maintenance. Questions raised were, could a balloon hold up

the tether weight? Could a balloon hold up the arrayweight?

It was suggested that there would be great advantages if the solar power satellite constellation

could be made in an orbit accessible to the space shuttle. This would make the launch considerably

less expensive, and would make it accessible for maintenance. Drag is a problem; you want to put it as

high as you can, but trapped protons start getting significant at altitudes above 0.1 R e. The objective

would be "a constellation of satellites that come over and deliver power, so that there's always another

one coming over." This would need a lot of satellites, since most of the time any particular satellite is not

over the receiver The problem gets rapidly worse off the equator: the "Iridium" communications satellite

constellation, for example, needs 66 satellites to assure that one is overhead at any time for mid-latitude

users, and the communications mission allows a satellite to be usable as far as 80 ° from overhead. A

power beam probably won't be allowed to be used at such low angles. It was noted that storage would

help this situation a lot, if a satellite coyld come by and "dump" accumulated power. It was suggested,

can we make hydrogen and use that, either directly as fuel, or as a way to accumulate energy?

It would be "great" if we could configure a system to provide high-value power for peak-shaving.

This means the power maximum is at 2-4 PM in the summer (to run air-conditioners).

It was suggested that we could have many small power satellites to beam power to one big

antenna at higher orbit, which beams it back down. The advantage of this is that most of the mass is in

lower orbit, where it's accessible and repairable, with only a "few" assets in high orbit.

The final question brought up is, what are the showstoppers? The main concern was with launch

costs (and associated maintenance costs), and radiation damage. In low orbit, additional concerns

were with aerodynamic drag due to residual atmosphere, with the possibility of plasma discharge at the

presumably high voltages required for power distribution, and with the need for "lots of receivers and

-326-
lotsofsatellites."

Forall orbits,thattherecouldbeconcernsaboutmicrowavelinksafety.Whatif it misses?Itwas

notedthateven if the concern isn't a scientifically legitimate one, it still will be a factor in getting approval to do the project.

As a final note, one of the participants said that the demand for this is coming. The East Coast is

building base electrical power plants again, after a hiatus of almost 20 years due to oversupply, and

further demand will be growing down the road.

References

1. P.E. Glaser, "Power from the Sun: Its Future," Science, Vol 162, Nov. 11, 1968, pp. 857-861.

2.U.S. Department of Energy, Final Proceedings of the Solar Power Satellite Program Review, April

22-25, 1980, Lincoln Nebraska, Conf-800491; NASA TM-84183 (Document N82-22676 through 22767).

3. F.A. Koomanoff and C.E. Bloomquist, "The satellite power system: concept development and

evaluation programme." In Glaser, P.E.; Davidson, F.P.; and Csigi, K.I. (eds.); Solar Power Satellite: The

Emerging Energy Option, pp. 26-59, Ellis Horwood Limited, Chichester England, 1993.

4. National Academy of Sciences, Electric Power from Orbit: A Critique of a Satellite Power

System, National Academy Press, Washington, DC, 1981.

5. Office of Technical Assessment, Solar Power Satellites, Aug. 1981, OTA-E-144, Document N81- 77268

6. L. Deschamps (ed), SPS 91: Power from Space, Second International Symposium, Societe des

Electriciens et des Electroniciens (SEE) and Societe des Ingenieurs at Scientifiques de France (ISF),

Paris/Gif-sur Yvette, 27-30 August 1991.

7. A. Brown (ed.), Proceedings of the First Annual Wireless Power Transmission Conference (WPT '93), San Antonio, "IX 23-25 Feb. i993.

8. G.A. Landis and R.C. Cull, "Integrated Solar Power Satellites: An Approach to Low-Mass Space

Power," Space Power, VoL 11, No. 3-4, 303-218 (1992). Presented at SPS 91: Power from Space,

Second International Symposium, 27-30 August 1991; pp. 225-232.

-327-
SPRAT XIV, OCTOBER

ATTENDANCE

24-26, 1995

LIST

JIM AVERY

(JX Crystals, Inc.)

4408 287th Ave., SE

Fall City, WA 93024

ERIC CLARK

M/S 302-1

NASA LeRC

Clevland, OH 44135

PERRY BANKSTON

Jet Propulsion Lab

4800 Oak Grove Drive

Pasadena, CA 91109

NORRIS BLAKE

(Space Systems)

1078 Normington Way

San Jose, CA 95136

DAVID BRINKER

M/S 302-1

NASA/LeRC

21000 Brookpark Rd

Cleveland, OH 44135

HENRY CURTIS

M/S 302-1

NASA/LeRC

21000 Brookpark Road

Cleveland, OH 44135

RONALD F. DANIEL

(Lockheed-Martin)

1289 Balbor Ct, #149

Sunnyvale, CA 94086

MICHAEL DASHIELL

AstroPower Inc.

Solar Park

Newark, DE 19716-2000

DALE BURGER

M/S 303-308

Jet Propulsion Lab

4800 Oak Grove Drive

Pasadena, CA 91109

BILL DAVIS

Dept of Electrical Engineering

2015 Neil Avenue

Ohio State University

Columbus, OH 43210

BASAB CHATTERJEE

Dept. of Electrical Engineering

2015 Neil Avenue

Ohio State University

Columbus, OH 43210

VICTORIA DAVIS

S-Cubed

Div. of Maxwell Labs

3398 Carmel Mountain

San Diego, CA 92121

ART CHMIELEWSKI

M/S

Jet Propulsion Lab

4800 Oak Grove Drive

Pasadena CA 91109

DONALDCHUBB

M/S 302-1

NASA LeRC

21000 Brookpark Rd

Cleveland, OH 44135

RONALD DIAMOND

Spectrolab, Inc.

12500 Gladstone

Sylmar, CA 91342

PAUL DILLARD

M/S 81-25

Boeing Aerospace

P. O. Box 3999

Seattle, WA 98124

LOUIS DiNETTA

AstroPower, Inc.

Solar Park

Newark, DE 19716-2000

-328-
MARIA FAUR

DRA/MS 302-I

NASA/LeRC

21000 Brookpark Rd

Cleveland, OH 44135

MIRCEA FAUR

Dept. of

Cleveland State U.

Cleveland, OH

DALE C. FERGUSON

MS 302-1

NASA/LeRC

21000 Brookpark Rd.

Cleveland, OH 44135

NAVID FATEMI

MS 302-I

NASA LeRC/Essential Research

21000 Brookpark Road

Cleveland, OH 44135

ROBERT FLOCK

Lockheed-Martin Missiles

& Space

IIii Lockheed Way

Sunnyvale, CA 94088

CRAIG FLORA

M/S 8C-61

Boeing Aerospace

P.O. Box 3999

Seattle, WA

ALEX FREUNDLICH

Space Vacuum Epitaxy Center

M/S #724-SRI

Univ. of Houston

4800 Calhoun

Houston, TX 77204

ED GADDY

Code 734

NASA GSFC

Greenbelt, MD 20771

LINDA GARVERICK

Essential Research

Cleveland, OH

TIM GILLESPIE

Lockheed-Martin

iiii Lockheed Way

Sunnyvale, CA 94088

BRIAN GOOD

M/S 302-1

NASA LeRC

21000 Brookpark Road

Cleveland, OH 44135

CHRIS GOODBODY ....

Defense Research Agency Command

& Maritime System Group

Space Tech. Dept./Space Sector

Royal Aerospace Establishment

Farnsborough Hants GU-14 GTD

United Kingdom

CHANDRA GORADIA

Dept. of

Cleveland State University

Cleveland, OH

DONALD GUIDICE

Code PL/GPSG

Phillips Laboratory

Hanscom AFB, MA 01731-3010

LINDA HALLE

M/S 720

The Aerospace Corp.

13873 Park Center Road

Suite 18

Herdon, VA 22071

MARGARET HANNON

Astropower, Inc.

Solar Park

Newark, DE 19716-2000

PAUL HAUSGEN

ROGER HAWK

U. of Arkansas - Little Rock

Dept. of Applied Science

2801S. University Ave.

Little Rock, AR 72204

-329-
RICHARD HOFFMAN

M/S 302-1

NASA LeRC/EssentialResearch

21000 Brookpark Road

Cleveland, OH 44135

JAMES HUTCHBY

Research Triangle Institute

GEOFFREY LANDIS

M/S 302-1

NASA LeRC/OAI

21000 Brookpark Road

Cleveland, OH 44135

MIKE LARRO

optical Coatings Lab. Inst.

RAJ JAIN

M/S 302-1

NASA LeRC/U.of Toledo

21000 Brookpark Road

Cleveland, OH 44135

WILLIAM JAMES

Phillips Lab/USAF

DARREN JOHNSON

PL/VTPC

Phillips Lab/USAF

Kirtland AFB, NM 87117

DAVID KEENER

PL/VTPC

Phillips Lab/USAF

Kirtland AFB, NM 87117

LOUIS KILMER

M/S M2-275

The Aeropace Corp.

P.O. Box 92957

Los Angeles, CA 90009-2957

TED KUBRECHT

Space Vacuum Expitaxy Center

M/S S& RI, #724

University of Houston -

Houston, TX 77204-5507

SARAH KURTZ

National Renewable Energy Lab

1617 cole Blvd.

Golden, CO 80401

CAROL LEWIS

M/S

Jet Propulsion Lab

4800 Oak Grove Road

Pasadena, CA 91109

DAVID LILLINGTON

Spectrolab

12500 Gladstone

sylmar, CA 91342

ROLAND LOWE

M/S 302-1

NASA LeRC/Essential Research

21000 Brookpark Road

Cleveland, OH 44135

MIKE C. MAUK

LOUIS MAUS

Code PD-14

NASA/MSFC

Huntsville, AL 35812

SCOTT MESSENGER

SFA, Inc.

1401 McCormick Drive

Landover, MD 20785

DREW MEULENBERG

COMSAT Laboratories

22300 Comsat Drive

Clarksburg, MD 20871

-330-
SHERIF MICHAEL

Dept. of Electrical & Computer

Engineering

Code 62MI

Naval Post Graduate School

Monterey, CA 93943

JACQUES MOULOUT

Dept. of

Cleveland State University

Cleveland, Oh

DAVID MURPHY

AEC-ABLE Engineering

93 Castilian Drive

Goleta, CA 93117

JOHN MURPHY

Optical Coatings Lab. Inst.

MICHAEL F. PISZCZOR

M/S 302-1

NASA/LeRC

21000 Brookpark Road

Cleveland, OH 44135

STEVE RINGEL

Dept. of Electrical Engineering

Ohio State University

2015 Neil Avenue

Columbus, OH 43210

GEORGE RYBICKI

M/S 302-1

NASA/LeRC

21000 Brookpark Road

Cleveland, OH 44135

TATSUO SAGA

Sharp Corp.

282-1 Hagikami,

Shimjo-cho Kitukutsuragi-gum,

Nara 639-21

JAPAN

DAVID SCHEIMAN

M/S 302-1

NASA LeRC/NYMA

21000 Brookpark Road

Cleveland, OH 44135

JOEL SCHWARTZ

M/S C-369, Bldg. 925

Hughes SpaCe & Communication

P.O. Box 92919

Los Angeles, CA 90009

HARVEY SEREZE

Spire Corp.

One Patriot Park

Bedford, MA 01730

JAMES SEVERNS

Code 6615

Naval Research Labs

Washington, DC 20375

CARLA SIGNORINI

Code XPG

ESA/ESTEC

P.O. Box 299

Noordwick, 2200 AG

THE NETHERLANDS

LUTHER SLIFER, Jr.

7023 Dolphin Road

Lanham, MD 20706

BRIAN SMITH

Spectrolab

12500 Gladstone

Sylmar, CA 91342

TED STERN

Composite Optics, Inc.

9617 Distribution Ave.

San Diego, CA 92020

GEOFFREY SUMMERS

Code 6615

Naval Research Labs

Washington, DC 20375

MICHAEL TIMMONS

Research Triangle Institute

-331 -
Robert Urbe

Dept. of Electrical Engineering

Kent State University

P.O. Box 5190

Kent, OH 44242

CARLOS VARGAS-ABURTO

Dept. of Electrical Engineering

P.O. Box 5190

Kent State University

Kent, OH 44242

MAURO VIELA .

Space Vacuum Epitaxy Center

S & R1 - #724

University of Houston

Houston, TX 77204-5507

FRITZ WALD

ASE Americas, Inc.

4 Suburban Park Drive

Billerica, MA 01821-3980

ROGER WESLER

Center for Microelectronic

Materials & Structures

Yale University

P.O. Box 20824

New Haven, CT 06520-8284

DAVID WILT

M/S 30211

NASA LeRC

21000 Brookpark Road

Cleveland, OH 44135

STEVE WOJTCZAK

Spire Corp.

One Patriot Park

Bedford, MA 01730

JAMES WOODYARD

Dept. of Electrical Engineering

Wayne State University

Detroit, MI 48201

JANE YATER

MS/302-I

NASA LeRC/DRA

21000 Brookpark Road

Cleveland, OH 44135

-332-
FormApproved

REPORT DOCUMENTATION PAGE OMBNo.0;,0_o1_

-- . • o ,

1. AGENCY USE ONLY (Leave b/ank) 12. P_PORT DATE 3. REPORT TYPE AND DATES COVERED

I

October 1995 Conference Publication ,

4. TITLE AND SUBTITLE

Proceedings of the XIV Space Phomvolmic Research and Technology

Conference (SPRATXW)

e. AUTHOR(S)

Geoffrey Landis, compiler

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESSEES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

g. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(e_)

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

S. FUNDING NUMBERS

WU--506--41-11

8. PERFORMING ORGANIZATION

REPORT NUMBER

_-9943

10. SPON_,'G/MONITC-'::'O

AGENCY REPORT NUMBER

NASA CP-IOISO

11.SUPPLEMENTARY NOTES

Responsibleperson,Geoff_'eyLandis,organizadoncode 5410, (216)433-2238.

1211. DISTRIBUTIOWAVAILABILrrY STATEMENT 12b. DI_ I HiBUTION CODE

Unclassified -Unlimited

Subject Categories 76 and 20

This p_,lic, afion is av_'lahl_ from the NASA C_u-" for Amros'_rc__ Information, (301) 621--0390.

13. ABSTRACT (M_x/mum 200 words)

The Fourteenth Space Photovoltaic Research and Technology confereace was held at the NASA Lewis Research Center

from October 24-26, 1995. The abstracts presented in this volume relxxt substantial progress in a variety of areas in

space photovoltaics. Technical and review papers were presented in many areas, including high efficiency GaAs and InP

sola_ cells, GaAs/C-¢ cells as commercial items, high efficiency muldple bandgap cells, so]m"cell and array technology,

hrJ_-ocphaxia] cells, thcrmophotovollaic erergy conversion, and space radiation effects. Space flight data on a variety of

cells were also presented.

14. SUBJECT TERMS

Space power;, Phomvolta_c cells; Solar cells; Solar re'rays

17. SECURITY CLASSIFICATION' 18. SECURITY CLASSIFICATION

OF REPORT OF THIS PAGE

UncLassified Unclassified

qSN 7540-01-280-5500

19. SECU_Tf CLASSIFICATION

OF ABSTRACT

Unclassified

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337i

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