NASA Conference Publication 3324 

Proceedings of the SPRAT XIV Conference 

held at and sponsored by NASA Lewis Research Center 

Cleveland, Ohio 

October 24-26,1995 

National Aeronautics and 

Space Administration 

Ofice of Management 

Scientific and Technical 

Information Program 

1996 

Space Photovoltaic Research and Technology 1995 

Table of Contents 

... Contents 111 

Foreword vi 

Session 1: Invited papers 

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

G. Strobl et al. 

G.P. Summers 

The Irving Weinberg Award 1 

Planetary and Deep-Space Requirements for Photovoltaic Solar Arrays 2 

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

Naval Research Laboratory’s Programs in Advanced Indium Phosphide Solar Cell Development 21 

C. Goodbody and R. Kimber 

T. Saga, Y Kiyota, T. Matsutani, A. Suzuki, 0. Kawasaki, T. Hisamatsu and S. Matsuda 

The Defense Research Agency Photovoltaic Programme 

Japanese Photovoltaic Power Generation for Space Application 

24 

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 Developmzilt at 

57 

Spectrolab 47 

Status of Multijunction Solar Cells 

YC.M. Yeh and C.L. Chu 

M.F. Vilela et al. 

InP tunnel Junctions for InGaAshP Tandem Solar Cells 65 
Session 3: Radiation Damage 

G.P. 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 

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

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

95 

G. Rybicki et al. 

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

S. Wojtczuk et al. 

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

Diffusion Lengths in Irradiated n/p InP or, Si Solar Cells 

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

100 

A. Meulenberg 

Session 4: Cell Technology 

L.C. DiNetta et al. 

Photo-Recovery of Electron-Irradiated GaAs Solar Cells 106 

Initial Results for the Silicon Monolithically Interconnected Solar Cell Product 

114 

L. DiNetta and M.H. Hannon 

Monolithically Interconnected GaAs Solar Cells: A New Interconnection 

Technology for High-Voltage Solar Cell Output 122 

127 

137 

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

R.E. Welser and L.J. Guido 

J. Moulot et al. 

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

The Growth of Low Bandgap InAs on (1 1 l)B GaAs Substrates 

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

Chemical Oxide as First Layer 

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

Progress and Results Update 150 

142 

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

Session 5: Measurements 

J.R. Woodyard 

Laboratory Instrumentation and Techniques for Characterizing Multi-junction Solar 

Cells for Space Applications 158 

Solar Cell Angle of Incidence Corrections 168 

Low Intensity Low Temperature <) Measurements and Coefficients on New 

Photovoltaic Structures 178 

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

Measurement Techniques 186 

D.R. Burger and R.L. Mueller 

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

D.J. Brinker etal. 

-iv- 
Session 6: TPV and advanced concepts 

D. Chubb et al. 

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

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

S. Wojtczuk 

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

Session 7: Assembly and Panels 

M.R. Brown et al. 

E.L. Ralph and E.B. Linder 

A. Meulenberg 

Review of Recent Thermophotovoltaic Research at Lewis Research Center 

Thermopho tovoltaic Generators using Selective Metallic Emitters 

InGaAsS b/GaS b Thermophotovoltaic Cells 

Multijunction InGaAs Thermophotovoltaic Power Converter 

Gallium Phosphide Energy Converters 

191 

208 

215 

223 

23 1 

Characterization Testing of MEASAT GaAs/Ge Solar Cell Assemblies 237 

Advanced Solar Panel Designs 247 

Space Quallfication of W and IR Reflecting Coverslides for GaAs Solar Cells 

257 

Session 8: Flight Experiments and Space Plasma 

H. Curtis and D. Marvin 

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

D.A. Guidice 

W.R. Mackey 

P.A. Jones and D.M. Murphy 

G.A. Landis and P. Jenkins 

One Year of Flight Data From the PASP-Plus Experiment 268 

Parasitic Collection by PASP-Plus Solar Arrays 274 

High Voltage Space Plasma Interactions on the PASP-Plus Test Arrays 286 

Preliminary Chaotic Model of Snapover on High Voltage Solar Cells 296 

A Linear Refractive Photovoltaic Concentrator Solar Array Flight Experiment 304 

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 

3 18 

320 

323 

325 

List of Attendees 328 

-V- 
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 community from the U.S. , Europe, and Japan. In attendance were about 100 

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

government 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 in particular, than with the 

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

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

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

the conventional Si technology. This fact gave more urgency to the discussions held 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 invited papers, 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: publications chair, Geoffrey Landis; logistics chair, George Rybicki; social events chair, Karen Wester; 

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

for his mentorship. 
The Irving Weinberg Award 

The fourteenth Space Photovoltaic Research and Technology executive committee has established the 

Irving Weinberg Award, in memory of Dr. Irving Weinberg, a leading contributor to 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 contributions to the field of space 

photovoltaics. This award is meant to be inclusive of 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 mission applications. 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 first five years, his research was 

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

Multijunction silicon solar cells. Subsequently, inspired by and in collaboration with 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 resistivity conventional silicon nip cells, space GaAs concentrator 

cells, CulnSe2 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 Goradia has published quite widely in the semiconductors and solar cells area and is the 

principal author or joint author of Over seventy-five publications in journals and conference proceedings. 

Professor Goradia is also a Senior Member of the Institute of Electrical and Electronics Engineers (IEEE). 

-1 - 
Planetary and Deep Space Requirements for Photovoltaic Solar Arrays' !" 

C. P. Ban\stonj R. B. Bennett, and P. M. Stella 

* JY#Propulsion Laboratory 

Cakfornia Institute of Technology 

Pasadena, CA 91 109 

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 photovoftaic 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 landers 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 in the 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 with the Same programmatic requirements 

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

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

integration of the spacecraft. Launch services costs must be kept to a minimum. light funding 

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, light weight 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 schaled within three 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 comoact packaging and low mass 

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

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

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

me work described in this paper was perfonned by the Jet propulsion Laborarmy, 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. ile 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 bet and the ne4 for rapid, low cost implementation with the best ter or ma^^^, lo 

most 0- rnpact packaging. 

ercuivy is now being planned for launch aroun 

~a~ing a sepa~~~~ lander package. The orb 

t&~ile the lander wrii end to the surface a 

e the subsolar temperat 

e lander temperature 

nslruction along with 

cha~cteflzatio~ of 

smry. Solar radiation 

levels will require the use of heavy shielding. Note that a sotar electric ~ropul~jo~ option is being 

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

discussion below. 

erformance of th 

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

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

ass array would be required d 

This array would also be on the cruise inward to a 

array will be ~etermin~ 

array must perform under low in 

provide pointing capabilii on 

mass designs will be 

baselined concept. This ar 

second smaller panel for the re 

survivability. This array will ai 

the second array may not be necessary if the primary amy 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. 

-3 - 
OUTER PLANETS 

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 intensity will require very large amy areas. For example, the solar intensity at 5 AU would be 

only 54 W/m2 and even if as much as 25 percent can be converted (a high value) to electrical 

power, this would yield only 13.5 W/n?of array area. As a result, such missions will demand 

extremely compact packaging and low mass structures to conform to launch vehicle constraints. 

In turn, the large area, low mass designs will impact allowable spacecraft loads. Inflatable array 

structures, with and without concentration, 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 50-1 OOW at Jupiter, equivalent to at least 

2500 W at 1 AU. If radiation damage, either from solar flares, or trapped radiation in 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 perfonance under low intensity, low light 

(LILT) conditions. ’Ihe Ganymede lander will require only a few watts, but must operate in the 

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

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

exceptionally large solar array in comparison to the lander. Furthermore, the difficulty in 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 IO 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, significant technology 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 constraints dictate a solar array performace target of >200W/kg at 1 

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

technology is presently baselined. 

MARS, COMETARY, AND ASTEROID LANDERS AND PROBES 

Most sfudies 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 dayhight cycle. On the other hand, conddions on a comet or 

asteroid may provide different challenges. The surface density and composition of a cornet 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 condtions, particularly the likelihood of dust storms. Power levels vary from 

as low as milliwatts in a quiescent mode to near 1 watt for brief periods for a so-called 
rnicrolander/penetrator, up to 100 watts for more complex rovers or other science packages. 

Surface temperatures will likely range between 150K (night time at 70' 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 

loadings 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 possibility of 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 

up to 6 are possible in low to middle latitudes, thus reducing the solar flux by as much as 85 

percent. The Mars PathfinderfRover mission will 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(l9'N, 32'W). 

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

proposed lander for the Rosetta mission to the comet 'Wirtanen" would require 1-1 0 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 occur far 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 amy 

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 missions the 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 benefits arise 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 rely only on PV for propulsion, with no 

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

significant fraction 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 insufficient for propulsion stage operation, it is in most 

cases more than sufficient to 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 flight test, it will be critical to 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. 

-5 - 
EXAMPLE SEP BENEFITS 

An example of the advantages attributable to the use of SEP is in the Mercury Orbiierkander 

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 including an "advanced SEP" design which incorporates 'TAL" 

(anode layer) thrusters. 

Unlike the all chemical propulsion design, the maximum landed mass for an SEPdeliiered system 

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

circularize the orbii at 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 orbiier propellant savings associated with using a 

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 orbiiterllander 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 Pathfinder lander and rover mission, the Mars Global 

Surveyor orbiting mission, the Mars 1998 orbiier 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 GaAdGe solar cells. The Applied Physics 

Laboratory's NEAR will be the first NASA planetary mission to launch with GaAdGe. The NEAR 

array will provide 1800 W at 1 AU and 400 W at 2.2 AU on its trajectory to the asteroid Eros. In 

addition, all of the planned Mars missions will utilize GaAdGe solar cells for the orbiter or lander, or 

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

available sutface area. This marks a clear transition to the acceptance of GaAdGe solar arrays for 

planetary missions. 

The Mars Global Surveyor orbiier, to be launched in November of 1996,empIoys 2-GaAdGe and 

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

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

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

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

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

mission is to be the first of a series of missions to 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 rover array 16 W. For the Mars 1998 mission, now in the early phase 

of development, the baseline now includes GaAdGe on an orbiier (1440 w), GWGe 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 

GalnPIGaAdGe, 2-junction, multi-bandgap solar cells. This mission will 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 
New Millennium spacecraft for engineering and science functions. The combined demonstration 

of concentrator array, multi-bandgap cells, and solar electric propulsion will 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 missions face special thermal 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 mass 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 

reaiization of such innovative architectures will, of course, depend heavily on p 

other spacecraft technologies. Spacecraft pointing, attitude control, thermal 

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

Earth 

PrimaryBalteries I 

\ 

Figure 1. Solar Probe Spacecraft Concept 

-7- 
Figure 2. Kuiper Express Concept 
lm 

Figure 4. Comet Lander Concept 

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

GStrobI’, P.Uebele’, R.Kern’, KRoy’, C.Flores2, R.Campesato2, C.SignorinP, K.Bogus3, P.C0z4 

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

Roma Italy. 

1 

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 related 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 

carried out in Europe by ASE (Germany) and CISE (Italy) in the frame of an €SA programme, aimed to 

provide the photovoltaic generators for ROSETTA: 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 -1 50 

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 efficiencies over 25 o/o at an iliimination 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 Oh, that is the 

highest value ever measured on a single junction solar cell. 

A large quantity of both “HI-ETAINR-LILT ‘‘ silicon and “GaAslGe-LILT” solar cells are presently under 

fabrication and they will be submitted to a qualification test 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 scientific project. 

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 verification tests 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.& 0. programme has 

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

in 2003. The ROSETTA baseline mission foresees 10 years lifetime and two options power requirements 

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

consequently adapted to the ROSElTA 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 carried 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. 

-1 0- 
2. Silicon solar cell develownent 

2.1 HI-ETA1NR-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 [l]. 

With the newly developed HI-ETA/NR-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-ETA/NR-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-ETAINR-LILT solar cells will be presented and another inherent design feature of HI-ETNNRLILT solar cells, namely an integrated Zener diode (IZD) for by-pass shunting will be discussed. 

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

technology, such as 10 Qcm/CZ base material, n+/pp+ 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-ETAINR-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-ETAMR-LILT silicon solar cells as measured under an 

insolation intensity of 0.1 1 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 5 and a reduction in minority carrier lifetime t [2]. The voltage strongly 

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

square of the intrinsic carrier 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 u=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 

Rgure 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] ('Wat spot" effect). 

2.2 lntearated Zener Diode ClZD) 

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 pn-junction 

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-ETAINRLILT solar cells, displayed in Figure 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- 
behaviour of HI-ETA/NR-LILT silicon solar ells in reverse bias is characterised by an integrated Zener 

diode (IZD) for by-pass shunting. The equivalent circuit can be inspected in Figure 5. 

3. Ga As solar cell develooment 

3.1. LILT GaAs solar cell structure 

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 

- introduction of a i-layer into the junction 

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

Low dooina 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 [71, 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 2~10’~cm 

was chosen for the base of LILT solar cells. 

Hiqh doping level of the caD. 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 2~10’~crn~ 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 carriers 

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 1 reports the 

performances in AMO, 0.1 1 - 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 I, 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 -1 50 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 ClSE [4 1; the introduction of the i-layer lowers the tunneling saturation current of one 

order of magnitude. 

-1 2- 
3.2 ComDarison between GaAsIGaAs and GaAslGe 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 GaAsIGe 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 lsc for the GaAs and the Ge junctions. Moreover an 

active GaAs/Ge cell shows a knee in the 1-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 GaAs/GaAs 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 1-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 PM 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 %, AMO, the cell area is 2x4 cm2. At low temperature (-150 C), 0.11 suns, the 

efficiency of this cell was 24.5 Oh. 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. Figure 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 GaAslGe solar cells while it is not 

noticeable in GaAs/GaAs 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 temerature measurements 

The 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. 

-1 3- 
Table Ill 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 Technoloav 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 1015e-/ have been performed at the Interfaculty Reactor Institute of the Technical university in Delftrrhe 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 carried 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 Figures 8 to 11 the relative degradation data of HI-ETA/NR-LILT SCAs measured during this electron 

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

insolation intensity of 0.1 1 AM0 and temperatures ranging from +25”C to -1 75°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, according to the ROSETTA mission requirements. 

The availability of two technologies will minimize the risk for the project. 

With the newly developed HI-ETAINR-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 GaAslGe structure seems to be 

slightly affected by the lattice mismatch between the substrate and the grown 1ayers.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 experiments 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 ROSElTA project. 

-1 4- 
TABLE I: LILT performances of bare GaAs/GaAs solar cells 

2s 

0 

-50 

-1 00 

-1 30 

-1 50 

951 \ 29.71 0.755) 17.59 867 8.64) 0.774 16.74 

1000/ 29.25' 0.788, 19.02 942 j 8.581 0.790 18.43 

1110/ 28.85 I 0.8291 21.91 1 042 1 8.51 0.815 20.83 

1211: 28.31 0.847' 23.93 11471 8.431 0.834, 23.29 

1269 i 28.1, 0.8481 24.93 12101 8.281 0.831 j 24.04 

1300 27.8' 0.851 i 25.37 1251 ' 8.13' 0.833: 24.46 

-1 5- 
TABLE 11: GaAslGe solar cell LILT characteristics 

TABLE 111 : Comparison of the predicted and measured values of a GaAs LILT solar cell 

I I I I I 

0.03 6UW I I I I 

-1 6- 
FiQure I : Illuminated I-V characteristics of bare HI-E3A/NR-LILJ silicon solar cells (3.78cm*6.i9cmf as

measured under an insolation intensity of 0.1 1 SC and 0.037% in the temperature range of +25"C and - 

150°C. 

HI-€TA/NR-LILT 

Silicon Solar Cell 

Voltage V 

Fiqure 2: Measured dark J-V characteristics of HI-ETAMR-LILT silicon solar 

function of temperature with theoretical fit curves of double diode modeling. cell (3.78cm*6.19cm) as a 

Voltage v 

Fiqure 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 modeling. 

-1 7- 
DARK +\\ I 

n+ emitter 

- 

-0.2 - +25% 

-0.4 -c- - -50% 

-100% -0.6 - -150% 

E -0.8 

E! 5 -1.0 

-1.2 

-1.4 

-1.6 

n+ 

emitter 

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

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

p base 

I gen 

RP 

- 

I 

Fiaure 5: Equivalent circuit for I-V characteristics of HI-ETAINR-LILT silicon solar cell with integrated 

Zener diode (IZD) for by-pass shunting. 

Fiaure 6: Ga As LtLT solar cell structure. -I a- 
Fiaure 7: Dark I-V curves for GaAs/Ge and GaAs/GaAs solar cells. 

a 

Fiaure 8: Degradation of open-circuit-voltage V,of HI-ETA-NRILILT SCAs (3.78cm*6.19cm) with lOOpm 

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

. 

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

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

-1 9- 
ly.V*.drmh.dbda,~1mommpp.. wtm 

Fiaure 10: Degradation of maximum power density P,,, of HI-ETA-NR/LILT SCAs (3.78cm*6.19cm) with 

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

Fiaure 11 : Degradation of fill factor FF of HI-ETA-NR/LILT SCAs (3.78cm*6.19cm) with 100pm thick CMX 

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

References 

[l] P.M.S€ella, F.S.Pool, M.A.Nicolet, P.A.lles, ,,PV technology for low intensity, low temperature (LILT) 

applications", 1 WCPEC - World conference on photovoltaic energy conversion (1 994) 

[2j GStrobl, C.Flores, P.Uebele, R.Kern, R.Campesato, K.Roy, M.Martella, 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] GStrobl, P.Uebele, R.Kern, K.Roy, C. flores, R.Campesato, CSignorini, K-Bogus, Silicon and 

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

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

[4] G.Strobl, P.Uebele, R.Kern, K.Roy, R.Carnpesato, C. flores, C.Signorini, )<.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 

161 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.,Vo1.53,No.8, (1982), p.5926 

[7'j 

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

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

cells", IEEE Transaction on Electron Devices, Vo1.38, n.6, (June 1991). 

R.Campesato, C. flores, '%a As solar cells for low temperature, low intensity applications", 

-2 0- 
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 21 228 

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 

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

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

radiation resistant, high efficiency, planar cells known. 

nsored InP solar cell technology in 1986, when 

w g/w cells by liquid phase epitaxy. NRL's i 

e results ~re~ented by Vamaguchi and his co-workers i 

iation resistance of cells grown by diffusion of S in 

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

annealed by sunlight. Relatively large quantities of 1x2 cm2 diffused junction cells 

and were use SES-A and the EXOS-D satellites. These cells were alsG available in the 

US. through d were studied at NRL and elsewhere. Workers at NASA Lewis became 

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

cells also had beginning of life (BOL) efficiencies - 

SINGL D TAN CELL 

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

year contract from RL to Spire Corporation in 1988-89. The sals 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 

in this program enabled cells to be made with AM0 efficiencies >19%. 

were eventually produced, many of hich have been fl 

PLUS on the APEX satellite. The& jests have confi 

eriments such as PASP 

iation resistance of InP 

f these cells and also on radiationDuring the late 1980s 

for the development of ta 

(E,-1.32 eV) on a lattice m 

3- and 2-terminal versions 

work led in 1991 to a thr 

(2x2 cm2), 2-terminal cel 

-21- 
epitaxially grown, single junction InP cells. During thg course of this program, several cells 

with AM0 efficiencies >22% were produced, 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 subcells 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 >lo16 cm-2. 

Five 2x2 cm2 tandem cells were provided to the U.K. DRA for the STRV lb 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 lnP 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 

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 conciusions of this study, which was 

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

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

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 peaormance 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-termina1, 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 

roposed global satellite communication systems such as Ellipso and Odyssey. 

-2 2- 
emitter, RTI have produced single junction p/n InP cells with efficiencies >16%. It is thought 

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

The proton response of InP and GalnAs single junction cells is the same for both p/n 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 gr n if required. Finally, of course, cell structures have to be developed that can 

be p~~d~c~d in uantities at a competitive cost. 

-2 3- 
The Defence Resdarch Agency Photovottaic Programme 

C. Goodbody and R. Kimber 

Defence Research Agency 

Space Department 

Farnboroug h 

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, 5-1 0 

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 estimates 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 111- 

V high efficiency cells. The study%howed that at the power level of 3kW the mass of a rigid array 

using the Ill-V cells were substantially lower than for Silicon (55k9, GalnP/GaAs against lOOkg, Si 

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

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

ultrathin Ill-VI multijunctions and thin film low efficiency CIS, CdTe and a-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. 

-2 4- 
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 lo9 protons cm-2 s-', 

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 keV. The proton intensity distribution across the target plate is currently uniform to 

+lo%. 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 23.3 "C to f 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 11Omm 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 OC; -100 

& 2OC. 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 1 MeV 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 keV protons, which can be 

characterised, but not discrete energies. 

FLIGHT EXPERIMENTS 

UoSAT-5 

UoSAT-5 satellite was launched into 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 200pm CMG coverglass and the other with the same 

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

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

-2 5- 
STRV- 1 AIB 

STRV-1 A/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-1B 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 celllpanel 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 requirements. We are 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 of the basic simulator design, 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 Arravs 

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. 

-2 6- 
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. 

Fliaht Exoeriments 

The flight of STRV-1 C/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 estimates 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. 

La boratorv 

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 MOD 

customer. Further activities are planned in these areas to meet the future requirements of MOD 

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 1 995. 

C. Goodbody, R. Kimber, "Achieving the optimum solar array size for spacecraft", 29th 

lntersociety Energy Conversion Engineering Conference, Monterey, CA, USA, Aug 1994. 

C. Goodbody, "The UoSAT-5 Solar cell experiment - First year in orbit", Proc. XI1 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 1 993. 

@ British Crown Copyright, 1995 /DRA. 

Published with permission of the Controller of Her Britannic Majesty's Stationary Office. 

-2 7- 
Mart and Cost for 8.7kW Flexible Amy (Slngie Wing) 

SilOOhmCm BlOOhman Si WTA G&Ge GaInPffiaAffie 

BSR B8R PU 

Figure 1. Comparison of Array Mass & Costs for LEO Flexible Array 

Mass and Cost for 8.7kW Rigid Array (Single Wing) 

Figure 2. Comparison of Array Mass & Costs for LEO Flexible Array 

Vacuum gauge heads 

Port for mlar simulatio 

dunng irradiation Vacuum shaft seal & bearing - top and bottom 

Access d 

Spximm mounting plate 

1 LN2 Sprays 

Figure 3. Schematic of Proton Irradiation Chamber. 

-2 8- 
Pott wih dccmcal lcadlhmu& 

Speamm munnng plafe Adjustable plane mrmi 

-c, Filter w 300W xmon arc lam 

Bm 

I 3mw For illumination during irradiation xenon arc lamp 

For in-situ measurement 

Figure 4. Schematic of Proton Irradiation Chamber. 

99 

0 05 I 15 2 25 3 

Dm Si= Lu.rb 

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-1A. 

-2 9- 
SIRV-1B SOUR PAhW 

104 

102 

100 

= 98 s 

*% d 

u 

m 

i94 

c. * 92 s 

90 

88 

86 

0 50 100 150 200 250 300 350 400 450 500 

mv si= nua am maveragedRlery5daYs 

Figure 7. STRV-1B Solar Panels : Degradation of the Current generated at 28V 

with time in orbit, Normalised to the Average Day 3 value. 

2.5 

2 

A 

3 

1 = 1.5 

0 

Figure 8. Spectrum of AM0 Class A simulator in development 

-30- 
JAPANESE PHOTOVOLTAIC POWER GENERATION 

FOR SPACE APPLICATION 

T. Saga Y. Kiyota T Matsutani. A S&i. 0. Kawasaki*, T. Hisamatsu* and S. Matsuda* 

SHARP Corporation 

282-1 Ha j ikami, Shin jo-cho, Ki takatsuragi-gun, Nara 639-21, Japan 

* National Space Development Agency of Japan(NASDA1 

2-1-1 Sengen, Tsukxba-shi. Ibaraki-ken 305. Japan 

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 

efficiency than /LEI? 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 section. our future plan for the solar cell calibration is presented. 

0 and 28°C conditions. After electron irradiation. NRS/IBSF cells s 

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 compared 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. NASDA and SHARP have continued to develop Si solar cells with higher 

efficiency, higher radiation resistance and lower mass. Ye developed the ultrathin 50~ BSFR 

cells in 1980’s. The ultrathin solar cells have been used for JERS-1 and En-VI and the 

solar cell monitor (Sclrr) on ETS-V showed their excellent radiation performance (Ref. 1). 

However the spacecraft have made a demand of higher power from year to year. GaAs solar 

cells have been expected to supplant Si solar cells. Although the GaRs solar cells have high 

efficieny and high radiation resistance, they have also the disadvantages of high mass, high 

fragility and high material coit resulted from the source limits which will become very 

important factor considering the future large scale space application. for example. space 

stations or space platforms. We thought the high efficiecy Si 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 lOOp thick NRS/LBSF (Eon Beflective S_urface/-Localized Back surface Field) 

cells with about 18% efficiency, NES/BSF (&Jon Beflective Surface/&ck Surface Eield) cells 

with about 17% and NRS/LBSF & NRS/BSF cells with IBF. First. the solar cell applications 

for NASDA satellites are introduced, 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 

This table 

for space. 

introduced 

product ion 

adoped for 

historical solar cell applications for NASDA satellites are shown in Table 1. 

shows the application and trends in development of several kinds of solar cells 

The solar cells except high efficiency Si solar cells shown in Table 1 were 

at the SPRAT XI[ (Ref. 2). The 10Qcm NBs/B!T cells have been brought into mass 

for ADEOS-II program and the 2Qcm NBS/BSF cells with IBF are studying to be 

OICETS program. The NBS/BSF cells will use the blue red reflective (BE) 

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 NBS/LB!3F cell and NBs/BSF cell are shown in Figure 1 and 

Figure 2 respectively. Their designs are the same except two differences. The NES/LBSF cell 

has a locally diffused P' back surface field layer and a planar PN junction. On the other 

hand the NES/BSF has a normal P' back layer and a normal PN junction Both solar cells have 

2Op squared inverted pyramids on the cell active area to reduce light reflection loss and 

the Si02 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 string. The NBS/WF cell was made 

from only 2Qcm Si substrates because the fill factor of the 1OQcm NBS/LBSF cell was lower 

than the 2Qcm NES/LBSF cell (Ref. 3). The NRS/BSF cells were made from 2Qcm and 1OQcm 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 NBS/LBSF cells showed the highest efficiency (average 

18.0%) of three kinds of cells. The NWBSF cells showed lower efficiency (amage 17.0 to 

17.3%) than the NES/LBSF cell. We measured reverse saturation currents JO 1 and JOZ of these 

cells using two diodes model and ND filters methd The NBS/LBSF cell gave lower JOI (6X 

10-13A/~m2) by about one third and a little lower JOZ (7x10-9A/cm2) than the NBS/BSF cell 

(JO 1 : 1.8xlO-' 2A/cm2, JOZ : 1. 0x10-*A/cm2). In generally the localized BSF is neccessary to get 

high open circuit voltage (low JOI) and the planar PN junction is neccessary to get large 

fill factor (low Joz). But the*planar PN junction did not improve the fill factor of the NBS 

/BSF cell and was not neccessary for them. !e found that the planar PN junction decreasedJ0z and was effective to increase the fill factor of the NBs/LBsF cell with low JOI but not 

effective for the NRS/BSF cell with relatively high JOI. 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 Si02 passivation back layer. 

The results of lMeV electron irradiation to these solar cells are shown in Figure 3. 

The NRS/LBSF cells showed larger degradation than the NRS/BSF cells and conventional BSFE 

-3 2- 
cells with the same thickness in the low fluence range (less than lx10'4e/cm2). On the other 

hand the NRS/BSF cells showed smaller degradation rate than the conventional BSFB cells. 

The 2Qcm NES/BSF cells showed higher power at lower fluences than 1x10'4e/cm2 compared to 

the 1OQcm NBS/BSF cells. The lOS2cm NBs/BSF cells showed the highest power at higher 

fluence range than lX10L4 e/cm2. These results suggest the radiation characteristics of NBs/ 

BSF cell is dominated mainly by the bulk damage and the damage of Si/SiOz interface is added 

to the bulk damage in the NRS/LBSF cell (Ref 4). The back Si/SiOz interface of the NRS/LBSF 

cell is susceptible to the electron irradiation and increase JOI rapidly and decrease open 

circuit voltages of the NBS/LBSF cells. The radiation characteristics of NBs/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 200j.m BSB cells used for geostationary satellites 

at their EOL (1MeV electron fluence of 1x10' 5e/~m2). 

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 4. 

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 interconnection. It needed neccessarily high cost 

and did not become pogular~ The IBF (integrated bypass function) is a simple idea to protect 

the solar cell from the cell failure due to shadowing. The IBF cell s many small dots of 

P' diffused layer in the front N' diffused layer. The P' dots are formed by the same boron 

diffusion as the back 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 current at low 

reverse voltage (about -3V). the IBF cell does not suffered a high reverse voltage and 

can survive from shadowing As the reverse current increases in proportion to the total 

length of P'N' junctions, we can easily control the teristics of the cell 

without a remarkable power loss (Ref.5). Consideri ture effects of the reverse 

I-V chracteristics (the reverse current decreases ture increase), we think the 

avalanche breakdowns of P'N' junctions are king i ltages in the IBF cells. 

The typical reverse I-V characteristics of a 2X2cmz IBF cell is s 

The NRS/LBSF and NRS/BSF cells with IBF were developed and qualified for 

The electrical parameters of them are shown in Table 3. 

cea and NBS/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 A per 2X2cm2 at the reverse voltage of -3 V using the electrical and thermal 

-3 3- 
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 hardly changed by 1 MeV electron irradiation to 3x10' 

e/cm2. The temperature coefficients of the electrical parameters and solar absorptivities o 

them were almost the same as those of the same kinds of usual cells. 

The reverse bias tests using NES/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 cel' 

shadowing problems. 

FUTURE PLAN FOE 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 advantage: 

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 contamination. 

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 flighl 

calibration value was 1.1% to 2.2 % using spectral irradiance of WO (World Meteorogical 

Organization). And we caluculated confidence intervals from these results to analyze the 

uncertainty of this method. A very small value of total confidence interval (0.25%) 

indicates that the solar simulator method is highly accurate systematically. The accuracy oJ 

the solar simulator method is comparable with the balloon flight calibration method. Ye 

belive that the solar simulator method can be applied to the primary calibration of Si sola1 

cells and are going to prepare the IS0 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'e use and their characteristics were clarified. The NRS/LBSF 

cell gave the highest efficiency at BOL condition, But the NES/BSF cells showed higher 

radiation resistances than the NBS/LB!jF cells. Both cells gave about 20% higher 

efficiencies than 200~ thick BSB cells used for geostationary satellites at their EOL 

condition (lMeV electron fluence of lx1OL5e/cm2), 

The design, characte&tics and test results of the IBF cells were presented. By 

forming small 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 

-3 4- 
realize the solar array with a simple design, high reliability and less cost. 

The advantages of the solar simulator calibration method were presented. We are 

going to prepare the IS0 draft standards and expect this method will be accepted as primary 

calibration of Si solar cells by IS0 in future1. 0. Kawasaki et al, "NASDA's Space Solar Cell -Ground Test Data and Flight Data", hoc. 

2. S. IYatsuda et al. "NASDA Activities in Space Solar Power System Research Development and 

1994 IEEE 1st World Conference on Photovoltaic Energy Gmversion", 1994. P.2224 

Applications", k- of the XU Space Photovoltaic Research and Technology Conference 

(SPBSAT a), 1992. P. 318 

International Photovoltaic Science and Engineering Conference, 1993. P. 573 

Power Conference, 1995. P. 371 

3. T. Hisamatsu et al. "Electrical Properties of Thin Silicon Space Solar Cells", Proc. 7th 

4, T. Hisamatsu et al. "Advanced Thin Silicon Space Solar Cell", Pro. 4th European Space 

5. T. ktsu et al. "Development of High Efficiency Silicon Space Solar Cells". Proc. 1994 

rld Conference on Photovoltaic Energy Conversion", 1994. P. 2133 

6. EL S. Rauschenbach, "Solar Cell Array Design Handbook". Van Norstrand Reinhold Company. 

1980, P.300 

Table 1 Solar Cell Applications of NASDA Satellites 

*t : main array 

r2 : charge arrw 

-3 5- 
P-Type Si I i con 

2 P" Diffused La er 

\ Sio, Layer 

Fig. 1 Schematic Draw i ng of NRS/LBSF Ce I I 

N-contact Gr id I i ne N' Diffused Layer \ 

P-Type S i I icon 

\ sioz Layer 

Fig. 2 Schematic Drawing of NRS/BSF Cel I 

36- 
Table 2 Typical Eiectr ical Parameters and Absorptivities 

Note : AM0 , 135. 3mW/cm2 , 28°C 

Cell Size 2cmx2cm , 100,um thick 

100 

90 

80 

70 

60 

50 

40 

n 

3= 

E 

x 

m 

E 

e 

0 1 014 1015 1 

I ectron F I uence [e/cm'] 

-3 7- 
Fi E. 4 Schemat i c Drawing of NRS/BSF Ce I I with 1 BF 

0.5 

2 v 0.4 

+J 

E 

L 

3 

0 

Z 0.3 

(D 0.2 0 

L 

Q 

> 

g 0. 1 

0 

0 1 2 3 4 

Reverse Voltage (VI 

Fig. 5 Typical Reverse I-V Character istics of 2x2cm2 IBF Cel I 

Table 3 Typical Electrical Parameters and Absorptivities 

I NRS/BSF with IBF I 10 191 I 0.16 I 90.3 I 16.7 I 0.85 I 

Note : AM0 , 135. W/cd , 28°C 

Cell Size 2cmx2cm , lOOflm thick 

-3 8- 
Module Simulator 

-1 .--------------- I 

I 

I 

I 

1 

I 

I 

I 

I 

I 

1. 

I 

I 

I 

I 

I 

I 

I 

I 

I 

1 

I 

I 

I 

I 

1 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I I 

I 

Standard 

-3 9- 
COST TRADE BETWEEN MULTI-JUNCTION, GALLIUM ARSENIDE, 

AND SILICON SOLAR CELLS 

t$oddard,Space Right Center 

Gydnbelt, MD 20771 

,+ - . Edard M. Gaddy 

Abstract 

Multi-junction (MJ),' gallium arsenide (GaAs), and silicon (Si) solar cells have respective test efficiencies of 

approximately 24%. 18.5% and 14.8%. Multi-junction and gallium arsenide solar cells weigh more than silicon solar 

cells and cost approximately fwe times as much per unit power at the cell level? 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 support3 science in the case of the spacecraft equipped with silicon 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 reduction is shown to derive from the smaller area 

of the array and hence 

If the silicon solar cells are changed out for multi-junction solar 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-junction cells 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 $1 80,000 per kilogram. 

This is still much less than the original $552,000 per kilogram to launch and service the science. Data and 

qualitative factors are presented to show that these figures are subject to a great deal of uncertainty. Nonetheless, 

the benefit of the higher efficiency solar cells for TRMM is far greater than the uncertainties in the analysis. 

uctions in the weight of the array substrate 2nd supporting structure. 

ult~-Junction, GaAs, and Silicon Solar Cells 

The TRMM spacecraft has actually already chosen gallium arsenide cells. These were selected in 1991 , before 

multi-junction cells were remotely available, and just as adequate technical and price information on gallium 

arsenide 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 in this 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 originally estimated for silicon 2nd gallium 

arsenide solar cells, with the estimates for the multi-junction cdls scaled from the estimate for the gallium arsenide 

cells. Subsequently, the weight for the selecfed gallium arsenide array has increased due to an increase in the 

weight of the array mechanical systems. At least an equal, and probably a 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 in this 

paper and that the resulting trade between the cells, computed below, predicts a too small advantage for the more 

efficient cells. 

The TRMM spacecraft is planned to be earth pointing and to fly at an initial altitude of 350 km and an inclination of 

35". For any of the solar cells, the spacecraft has two sdar array 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 with four panels or two panels per wing. The wings for the more 

efficient mufti-junction cells are of course smaller than the wings for the gallium arsenide solar cells. The silicon 

40- 
solar array needs twelve panels or six per wing. This means of course that the weight of the silicon panels is 

significantly greater. All of this is summarized in Table 1. 

Table I 

TRMM Array Comparisons for a 2674 Watt at End of Life Array 

I Parameter Si GaAs MI 

Weight of Cell Stack, Wiring, Connectors & 48 kg 47 kg 38 kg 

MiSC. 

Array Area 26.2d 18d 13.4m2 

Array Temperature 74C 87C 87 C 

BOL Efficiency at Operating Temperature 1 1.3% 15.8% 21.4% 

EOL Efficiency at Operating Temperature 9.4% 13.3% 18.0% 

Number of Panels 12 4 4 

umber of Panel Hinges 20 4 

2 0. 0 

12 2 /- 

The cost of thes 

Table II 

MM Solar Array Cost Compar~ 

Component Si Cost ($) GaAs Cost ($) cost ($1 

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 

Depbyment Test 150,000 75,000 75,000 

Total 3,329,600 4,655,500 3,998,833 

41- 
Table It does not show the price advantage of the multi-junction and gallium arsenide solar arrays to the spacecraft. 

This is because these arrays offer substantial weight reductions, see Table I, and some of this reduction can 

increase the amount of science that the spacecraft can cany hence increasing the cost effectiveness of the 

spacecraft. Below, this advantage is estimated first for the gallium arsenide and then the multi-junction cells. 

Value of GaAs Array Weight Reduction on Spacecraft Science 

From Table I, the gallium arsenide array is 51 kilograms less than the silicon array. Not all of the 51 kilograms can 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, those whose size is 

primarily determined 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 silicon array. The data in Tables I 

and I1 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 §A are respectively the weights of the scientific instruments; command and data handling, 

communications, high gain antenna, electrical, power exclusive of the solar array, and solar array systems on the 

spacecraft with the silicon array. The variables ACDH, ACOMM, AHGAS, AELEC, APWR and ASA are the increases 

in the weights of the respective systems as a resuft of the additional capability4hey must have to sewe the 

addiiional science when the silicon array is replaced by a gallium arsenide array. The variable CF is the fraction of 

the command and data handling system that is used to support science. This fraction is obtained because about 

13.85 kb/sec are used for the spacecraft while about 169.80 kb/sec are used for the instruments. This same 

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 363% for every 1.0% increase in the array's power 

producing capability. The six equations imply that the capability of the spacecraft systems is proportional to weight. 

Although this is a reasonable approximation, it is not necessarily the case. For the 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 not available and the approximation used is good enough. 

(1) CF*CDH (Sl-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = ACDH, 

(3) CF*HGAS (51-ACDH-ACOMM-APWR-AELEC-AHW-ASA)/INSTR = AHGAS, 

(4) PF*ELEC (51-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = AELEC, 

(5) PP*PWR (Sl-ACDH-ACOMM-APWR-AELEC-AHW-ASA)/INSTR = APWR, 

(6) SF*PF*SA (Sl-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = ASA, 

(2) CF*COMM (S~-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = ACOMM, 

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 = .E63 

-42- 
Solving these equations yields the following results: 

ACDH = 3.74 kg, 

ACOMM = 1.46 kg, 

AHGAS = 3.60 kg, 

AELEC = 5.25 kg, 

APWR = 3.43 kg, 

ASA = 2.43 kg, 

Total = 20.0 kg. 

~igur@ must be increas 

The scientific instruments weigh 633 kilograms (The TRMM spacecraft itself will weigh approximately 3,512 

kilograms). This means that the launch costs for the original scientific payload a 

ree, the value of the galliu 

of the spacecraft orb 

longer necessary to meet mission requirements. 

-43- 
Table 111 

Cost of Spacecraft Systems for TRMM in Millions of Dollars 

hem (An asterisk indicates that subsystem price Cost With Si Cost With 

changes with solar cell price) 

Test 

Structure 

Deploya bles 

Optical Alignment 

Gimbal 

ACS 

'Power (Exclusive of Array) 

Thermal Design 

Thermal Contamination 

Thermal Coatings 

Reaction Control System 

"Electrical including I & T 

"C & DH 

Software 

'Communications 

ubtotal 

Software Management 

System Engineering 

Project Support 

Instruments ($73.2) 

P.A. 

Ground Systems($25.2) 

Mission Operations($3.7) 

MPS (Center Tax-Overhead) 

Subtotal 

Contingency 

Civil Servants 

Total 

4.7 

12.6 

7.0 

.7 

2.8 

15.2 

7.3 

3.3 

3.5 

1.6 

.3 

5.3 

21.4 

11.0 

2.1 

6.0 

104.8 

1.8 

4.9 

4.1 

NIA 

5.5 

NIA 

NIA 

26.9 

148.0 

9.0 

75.7 

232.7 

GaAsArray 

4.7 

12.6 

7.0 

.7 

2.8 

15.2 

7.4 

4.7 

3.5 

1.6 

.3 

5.3 

21.8 

11.5 

2.1 

6.3 

107.6 

1.8 

4.9 

4.1 

N/A 

5.5 

N/A 

N/A 

26.9 

150.8 

9.0 

75.7 

235.5 

Cost Wth 

MJ Array 

4.7 

12.6 

7.0 

.7 

2.8 

15.2 

7.5 

4.1 

3.5 

1.6 

.3 

5.3 

22.0 

11.7 

2.1 

6.4 

107.4 

1.8 

4.9 

4.1 

NIA 

5.5 

NIA 

NIA 

26.9 

150.6 

9.0 

75.7 

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 siliin array. Again, nd all of the 73 kilograms can go into science. 

The spacecraft system weights will increase per the equations below. The variables have the same meaning and 

the constants have the same values as in equations (I) through (6) except for SA which is given its value below. 

CF*CDH (73-ACDH-ACOMM-APWR-AELEC-AHG&S-ASA)/INSTR = ACDH, 

CF*COMM (73-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = ACOMM, 

CF*HGAS (73-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = AHGAS, 

PF*ELEC (73-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = AELEC, 

PF*PWR (73-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = APWR, 

SF*PF*SA (73-ACDH-ACOMM-APWR-AELEC-AHGAS-ASA)/INSTR = ASA, 

SA = 119 kg 

Solving these equations yields the following results: 

ACDH = 5.39 kg, 

ACOMM = 2.10 kg, 

AHGAS = 5.31 kg, 

AELEC = 7.57 kg, 

APWR = 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 ohit, support it, and send raw data back are computed as $235 million in Table 111. 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. 

Sensitivity Analysis 

That the cost of the first several multi-junction solar arrays would be significantly greater than that quoted in Table I1 

and Table 111 was mentioned earlier. Making the assumption that this price is twice that given for gallium arsenide 

cells in Table 111 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 ceils should be quite close to that 

of gallium arsenide solar The capital equipment used to p 

arsenide cells. The labor to produce multi-junction cells may be slightly greater than gallium arsenide cells in 

that the multi-junctions m longer in the reactor that grows additional cell layers . The major difference may 

well be that the multi-junction cells will have a 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 

array is priced at twice that of the gallium arsenide array as 

science, it will cost $9.6 million rather than the $4.1 million sh 

project will rise $5.5 million to $8.1 million from the $2.6 mill 

uce them should be the same as for gallium 

in array price will change the analysis as follows. If the multi-junction solar 

to support addiional 

receding analysi 

-45- 
per kilogram; still substantially less than the $552,000 per kilogram cost to launch and service the science with a 

siliin array. 

Range of Validity of the Analysis 

This results of this analysis depend on the higher efficiency solar cells to reduce the mechanical weight of the solar 

anay substantially. This is true with an array such as TRMM that has relatively heavy substrates, and mechanical 

deployment and positioning systems. The analysis may lose validity if the cells can no longer leverage their 

efficiency advantage to cut array mechanical systems weight. 

There are two types of arrays where the analysis in this paper may not hold true. The first is a body mounted array 

on spinning spacecraft. If such a spacecraft can obtain adequate power with the area available on its body with the 

use of silicon arrays, the use of higher efficiency cells would be counterproductive because the higher efficiency 

cells are 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)' or Solar Array Flight Experiment (SAFE)' or the Hubble Space 

Telescope Array'. For these arrays, the reduction of weight with the 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 little effect on reducing this mechanical weight. This issue 

is quantitatively discussed by Ralphe who uses only the launch cost per unit weight. In this case, the analysis still 

favors the higher efficiency cells. 

~ci~sion~ 

From this study, both gallium arsenide and multi-junction solar cells offer significant cost advantages to spacecraft 

Gallium arsenide solar cells offer increases in the scientific payload at about $90,000 per 

n solar cells offer increases in the scientific payload at about $58,000 per kilogram. This 

that costs $533,000 to launch and support. The magnitude of this advantage is 

ependent on the higher efficiency cells to reduce the mechanical and structural weights of the solar array 

through a reduction in the array's area. 

P. K. Chiang, et al, "Large Area GalnPJGaAs Tandem Cell Development for Space Power Systems," TwentyThird Photovoltaic Specialists Conference, 1 993, pp. 659-664. 

Gene Ralph, "High Efficiency Solar Cell Arrays System Tradeoffs," E€€ First World Conference on Photovoltaic 

Energy Conversion, 1994, Hawaii, Figure 1, p.1998. 

Throughout this paper the term "support" means to provide the scientific instrument with environmental 

protection from 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. 

Gene Ralph, "High Efficiency Solar Cell Arrays System Tradeoffs," p. 1998. 

Paul Stella and Richard Kuriand, "Thin Film GaAs for Space-Moving Out of the Laboratory," Twenty-Third 

"Solar Array flight Experiment," Final Report, LMSC-F087173, under Contract NAS-8-31352, April 1986. 

Lothar Gerlach, "The Solar-Power Generator for the Hubble Space Telescope," €SA Journal, vol. 14, no. 2, 

Gene Ralph, "High Efficiency Solar Cell Arrays System Tradeoffs," Figure 7, p. 2000 

Photovoltaic Specialists Conference, 1993, pp. 21 -26. 

1990, pp. 149-168. 
THE PROGRESS OF LARGE AREA GalnP2/GaAs/Ge TRIPLE-JUNCTION CELL DEVELOPMENT AT 

SPECTROLAB' 

P. K. Chiang, D. 0. Krut and 8. T. Cavicchi 

Spectrolab Inc., 

Sylmar, California 

ABSTRACT 

This paper describes the successful demonstration of high efficiency, large area monolithic triplejunction, n on p, GalnPt/GaAs/Ge cells. The highest open circuit voltage and cell efficiency (cell size: 2 cm x 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-jur)ction 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-interconned-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 

obselved. 

INTRODUCTION 

The GalnPdGaAdGe monolithic multiijunction cell is an attradive technology for space applications due to 

its high efficiency and radiation hardness. Ge is a law cost (compared to GaAs or InP) substrate available in 100 mm 

wafer diameters sufficiently rugged for thin, large area cell fabrication. In past years, Spedrolab has demonstrated 

dual-junction cell efficiencies up to 24.2% (cell size: 0.5 cm x 0.5 cm) under lx, AM0 illumination on Ge substrates [l]. 

Concurrently, we have modeled a triple jundion version of the cell utilizing the Ge as a votage booster. Model results 

indicated with the inclusion of a third junction in the Ge substrate, the cell efficiency will have an additional 2% to 2.5% 

(absolute efficiency) increase. 

Under this U.S. Air Force development contract, the objective has n to demonstrate proof-of-concept triple 

junction devices and to deliver cells with a goal of 24% efficiency. The ultimate objectives are (1) to demonstrate a 

26.5% cell efficiency and (2) to extend the growths to larger reactors capable of supporting volume produdion. In this 

work, we have utilized a multiple wafer reactor to grow the triple-jundion GalnP$GaAslGe cells. The hghest efficiency 

achieved to date is 23.3% for a cell measuring 2 cm x 2 cm. We have also achieved a very uniform dslribution of sell 

efficiencies aauss 3- diameter wafers. Average cel efficiency of 22.8% across the 3" diameter wafer indicates 

large area cells, up to 4 x 0 cm2 across 3'wafer are possible. Cells have been characterized by electron inadia- 

'I This Work is supported by the Department of the Air Force and Managed by Phillips Laboratory, Space Power and 

Thermal Management Division, under Phillips Laboratory contract no. F33615-91-2146. 

-4 7- 
tion stability (1 MeV), and temperature coefficient measurements for both dual and triple-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 ClCs (CIC size: 2 cm x 2 cm) 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 fabricated into 

CICs. Several ClCs 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. 

GalnPdGaAslGe TRIPLEJUNCTION CELL RESULTS 

GalnPdGaAslGe TripleJunction Performance Modeling 

In this work we first performed modeling to calculate the minimum achievable efficiency in the triple-junction 

cell in the high volume production environment. Our approach has been to combine what we have already 

demonstrated on the dual-junction with conservatively modeled performance for the Ge bottom cell. The triple-junction 

cell structure shown in Fig. 1 consists of a dual-junction GalnPdGaAs interconnected to the Ge bottom cell through a 

second GaAs tunnel junction. The resultant I-V curve for a triplejunction GalnPgGaAdGe cell is shown in Fig. 2. The 

expected minimum average efficiency at beginning-of-life (BOL) is 26.5%. Previous work on the eledron inadiation of 

Ge cells demonstrated a low rate of current and voltage loss up to high fluence levels (2,3]. At an EOL fluence of 1 x 

1 MeV electrons/cm2, the Ge cell will continue to function as a voltage booster and the triple-junction cell will 

have an efficiency greater than 21 %. 

alnPdGaAs/Ge TripleJunction Solar Cell Fabrication 

The cell structure utilized in this work has been shown in Fig. 1. The thickness of the emitter and base layers 

in GalnP2 cell were 0.1-0.15 um and 0.4-0.48 um, respedively. The carrier concentration in the emitter of both celi: 

was 1 -3~10" cm'. The base of the top cell was doped to a level of 1 -3x1 017 cm', while a base doping of the 3-6 x 10 

cm3 was targeted in the GaAs cell to maximize EOL current collection. A high bandgap AllnPz layer was used to 

passivate the front of the GalnP2 cell; the back surface was passivated in this cell with an AlGalnP layer. A GalnP2 

layer was used for the window on the GaAs cell. An AlGaAs (or GainP2) was used for the GaAs cell back surface 

passivation. The Ge bottom cell active junction was formed by As diffusion into a ptype Ge substrate. 

The GainP2/GaAs/Ge triple-junction cells were grown on binch diameter Ge substrates. The grown 

wafers show mirror like shiny surface morphology. Cells were then processed into a 2 cm 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 Ga1nP.L) standard. 

The highest efficiency measured to date is 23.3% (AMO, 28 "C) for a 2 em x 2 cm cell. As shown in Fii. 3 

the open circuit votsage (Voc), short Circuit current (Isc), and fill factor (FF) are 2.573 V, 58.06 mA and 86.1%. 

respectively. To our knowledge this is the first, and the highest efficiency reported for, an n/p GalnPz/GaAs/Ge triple 

junction cell. 

-4 8- 
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, AMO), 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 external 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. 

GalnPdGWGe TripleJunction 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/cm2, 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 summarized in Table 1. Vocl, Iscl, FF1 and Effl are the 

open circuit voltage, short circuit current, fill factor and efficiency, respectively, for BOL. Voc2, Is@, FF2 and 

Eff2 are the results for EOL. Average Voc, lsc, FF and Eff ratios for cells measu at EOL to BOL are 92.4%. 

81.1%, 99.2% and 74.4%, respectively. Since the ratio of lsc is very close to that measured single 

junction cells, we concluded that the EOL performance of triple-junction cell is ~~~~i~@

by the tion of 

GaAs cell. In order to verify this, we performed spectral response measurements on these cell own in 

Fig. 6 the external Q.E. for both GalnP2 and GaAs cells degraded at EOL. The ratio of integrated current for 

EOL to BOL are ap~r~~jfflately 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. 

Ga~n~~G~s/Ge TripleJunction 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 absorption edge of three cells. The rate of 

increase of short circuit current density with temperature is 0.0162 Wm2/"C. 

The efficiency decreases with increasing temperature for both dual and triple-junction cells are shown in 

Fig. 9. The rate of decrease of Eff with temperature are 0.040 and 0.053 %/"C (absolute), respectively, for dual 

and triple-junction cells. This figure also clearly shows the difference tween these two devices is 2% at an 

operating temperature of 54 "C, as we previously reported [l]. 

-4 9- 
GalnPdGaAslGe TripleJunction 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 GalnPz 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 

measurements, Cell 2T110A4 was mounted onto the standard JPL balloon package and 

oon flight. The results and temperature corrected results are shown in Table 2. 

Measurement differences b en 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. 

GalnPdGaAslGe TRIPLEJUNCTION 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 

to the top metal contact of the cells. Three mil thick, CMX glasses were cut to the size 

to the surfaces of the cells. After fabrication, light I-V were re-measured. The highest 

0,28 "C) for a 2 cm x 2 cm CIC. The Voc, Isc, FF are 2.564 V, 57.76 mA 

e efficiency for these 50 CICs is 22.0%. which is very comparable to the 

before they were fabricated into the ClCs. 

Several ClCs 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 "Clmin 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 

GalnPz/GaAs/Ge triple-junction cells have been successfully demonstrated. The best cell efficiency of 23.3% 

AMO, 28 "C) is the highest reported efficiency for this device. Wrth optimization, these cells are capable of providing a 

minimum average efficiency of 26.5% in volume produdion. The electron irradiation of first few cells yielded 

average P/Po at 0.744. This performance is limited by the GaAs Tiddle cell. with optimization the PPo in this 

device is expected to improve to 0.80 at 1.0 MeV (lEl5 dm ) 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 ClCs were successfully fabricated 

indicating the "transpanw of the product to standard Spectrolab's welded panel fabrication process. The best 

CIC (2 cm x 2 cm) efficiency is 23.2%. No degradation in ClCs was observed after thermal cycle testing. 

-5 0- 
REFERENCES 

Spectrolab Measurement JPL Balloon 

(ASTM AMO) 

Measured at 28OC Measured at 53.5"C I Corrected to 

(11 P.K. Chiang et. al., Urge area GalnPz/GaAs/Ge multijundion solar cells for space applications', Proc. lstwodd 

[2] R. Venkatasubramanian et. ai., "High temperature performance and radiation resistance of high efficiency Ge and 

[3] D. D. Kiut et. al., "The development of Ge bottom cell for monolithic and stacked muhijunction applications", Pa. 

conf. on PV energy conversion, 1994, pp. 2120-2123. 

SicnG@93 solar cells on lightweight Ge substrates", Proc. 22nd IEEE PVSC, 1991, pp.85-89. 

22nd IEEE PVSC, 1991, pp. 90-92. 

NREL 

(WRRL AMO) 

Measured at 

Table 1 Light I-V test results for BOL and EOL 

Vocl Voc2 Ratio 1x1 lsc2 Ratio FF1 FF2 Ratio Effl Eff2 Ratio 

oo (W) (mA) (%) (%) 1 (%I (%) 

2TlOl A-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 

2T11OA-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 
Fig. 1. Cross-section of a triplejunction &ll 

- Performance - Jsc(mA/cm*) Voc(V) FF(%) Eff(%) - GalnPJGaA8 15.96 2.39 86.7 243 

5.0 

- I - 

_.. ..................................... ...i... I ................ - 

- I 

i 

l*.,*ic! _I I 

3.0 

Voltage (V) 

iii. 2. Modeled BOL perfOrtnaW of proposed nlp C8lnPzK;aAslGe monoliii ~ple-junCtion cell (AM0 28 'C) 

-5 2- 
IV PLOT 272226 I 04 I 08/04/95 I 12: 16 

06 

05 

04 

< 

V 

Y 

L 

- 03 

3 

02 

01 

0 

0 5 1 15 2 25 

Voltage (VI 

Fig. 3. I-V curve for the highest efficiency GalnPz/GaAs/Ge tiiple jundion e61 

23.0% 23.3% 

22.7% 22.2% 

22.8% 22.5% 

Strb: 3' Diameter p-Ge 

(Cell Area: 4.078 cm2) 

Fig. 4. Efficiency distribution of triple-junction cells across a 3" diameter substrate 

-5 3- 
Ext. & Int. QE (%) Reflectance (94) 

. . . . , . . . . 

350 550 750 950 1150 1350 1550 1750 

100 

80 

60 

40 

20 

0 

Wavelength (nm) 

Fig. 5. External quantum efficiency and reflectance of a triplejunction cell 

External Q.E. (96) 

_--_ 

340 400 450 800 550 600 650 700 750 800 850 900 

Wavelength (nm) 

Fig. 6. External quantum efficiency of a GalnPZ/GaAslGe triple junction cell measured before and after electron 

irradiation 

-5 4- 
Open Circuit Voltage (V) 

2.7 III ................................. TI (Cell Area: 4.078 ern3 1 0 0 =r- - - - -. 

0 

0 0 50 - ................................................................. 0 

0 

0 

0 10- 

40 - ................................ .'... ...... / - c 0 

0 0 

0 30 _................ ................................................. - 

- 

Temperature Coefficient: 0.0160 mA/crna*C 0 0 4 1 

- 0 20-..--. ..# .......................... 0 

- - 10 _........... ....................................................... 

0 1.1.1.1~1.1.1.1.. 

L 

28 ---- 

2.5 -- - - 

- 

- 

2.4 -... 

2.3 -.._ 

2.2 -- - - 

- 

2.1 .- 

............ ................................. 

..................... 

................................................ 

I ~ -1 

2 

0 

- 

10 20 30 40 50 60 70 80 90 

Temperature (C) 

Fig. 7. Open circuit voltage of dual and triple-junction cells as a function of temperature 

Fig. 8. Short circuit current of triple junction cell as a fundion of temf~erature. (note: vertical bar in each 

temperature represents the range of measurements in three cells) 

5 5- 
Efficiency (%) (@ lx AMO) 

................................................ 

......... ........... 

........................ ................... 

................. 

.......... 

................ 

......................................................... L 

- 17 ............................................................. - 

Id.''. -- .. .I.' . I. "'(. .. 'I. .. 'I. '.'" ... . 

0 10 20 30 40 50 60 70 80 SO 

Temperature (C) 

Fig. 9. Efficiency of dual and tripie-junction cells as a function of temperature 

5 6- 
STATUS OF MULTJJUNCTION SOLAR CELLS 

Y.C. M Yeh and C.L. Chu 

Applied Solar Energy Corporation 

15251 Don Julian Rd. 

City of Industry, CA 91 745-1 002 

I NTRO D UCTl 0 N 

This paper describes Applied Solar's present activity on Multijunction space 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 status of this cell design will be reviewed here. 

Purpose of Work 

MJ cells are in demand to provide satellite power because of the acceptance of the owemhelming 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 production of GaAslGe cells, with efficiencies 18.5-1 9%. When users realized that 

MJ cells could provide higher efficiencies (from 22% to 26%) with only fractional increase in costs, the demand 

for production MJ cells increased rapidly. 

The main purpose of the work described is to transfer the MOCVD growth technology of MJ high efficiency cells 

to a production environment, providing all the space requirements of users. 

TECHNICAL APPROACH 

The feasibility of the high efficiency MJ cells considered here, was demonstrated primarily by an NREL group 

under Or. Jerry Olsen. The challenge is to transfer this MJ cell to production status. This required validation that 

large area MJ cells could be fabricated evaluation of the relative merits of P/N and N/P configurations, and 

demonstration that Ge substrates could 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 procedures required for uniform multilayer growth over substrates. 

The approach used is to systematically vary the various layers using design 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-clock operation, to meet user delivery schedules. 

There is one other important factor in control of the growth procedures. 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 subcells at different rates, (usually the 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 procedures to 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 prediction and monitoring tests that ensure than large numbers of MJ cells have 

consistent EOL performance, meeting specific flight conditions. 

-5 7- 
For MJ cells, the contact requirements are not excessive. The higher internal impedance of the cell allows very 

low series resistance (high CFF) to be obtained with low metal shading (- 3%). The requirements for bondability 

contact adhesion and minimization of interaction between the 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 resulting from the larger grid path 

length on larger size substrates. 

The AR coatings need modification, to ensure the correct match for 2J (and 3J cells). 

To control and monitor MJ cell performance, we have developed a set of inline characterization tests. A major 

requirement is that the solar simulators used can replicate the AM0 spectrum for all cells in the stack, and over 

the whole cell area. We are using two-light simulators, one an existing Hoffman simulator, the other a Xe light 

simulator, with added optical filters. We are also modifying a large area pulsed solar simulator for testing strings 

and panels made from MJ cells. The simulator coefficient is checked by radiometers, and also using balloonflown subcells or 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 morphology of MOCVD grown layers. (This follows procedures developed 

- Detailed analysis of illuminated I-V performance 

- Spectral analysis of individual cell output 

- C-V Polaron profiling, to derive the impurity concentration and thickness of the main layers, usually 

- X-ray diffraction to check the lattice spacing and strain 

- Spectral reflectance to check solar absorptance and AR coatings 

successfully with GaAs/Ge cells) 

the emitter and base of the subcells. 

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 production MOCVD reactor. 

As mentioned above, the major factor in achieving high efficiency for large area MJ cells is not to 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 shows some experimental radiation data for dual junction 

cells, plotted with the model predictions. 

Figures 6 and 7 shows some in-line characterization results, the measurement of the spectral response of each 

subcell, and the deviation of the bandgap of the GainP2 cell. 

These MJ cells have successfully passed the space qualification tests shown in Table1 . 

Present efforts continue to increase the yield for cells made under production conditions. 

5 8- 
DEVELOPMENT WORK 

In addition to the on-line yield improvements, several other areas of MJ cell development are proceeding at 

Applied Solar, to meet future production requirements. 

Enhanced Dual Junction Cells 

Work is proceeding to develop dual-junction cells 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 

output cells (minimum average goal 24%) will also ensure that the cells meet specified radiation exposures. 

Triple-Junction Cells 

The user-demands for GaAs/Ge cells led to the requirement that the GaAs/Ge interface should be 

photovoltaically inactive, and the growth procedures were defined to tender this interface inactive. 

For MJ cells, it is easier to provide matching current in a Ge cell under a 2J cell, and work has proceeded to add 

a third (Ge) cell under the dual junction cell. 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 AlGaAs &or 

GalnAsP) cells grown on Ge cells. We.are re-evaluating the options for forming Ge cells, which retain best 

performance after exposure to the full growth sequence 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 rowth, or by ion implantation or diffusion. We 

have established that high emitter doping density (>5 x 10’’ cm5 is needed to ensure effective PN junctions in 

P/N structures after As in-diffusion occurs during growth of the GaAs cell. 

We are also studying the many impurity interactions that take place across the GaAs/Ge interface during the 

growth schedule. 

We have established that the quality of present N-Ge substrates can provide matching current when 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 RT1) on an NRL contract, to investigate MJ cells (InPAnGaAs) grown on 

Ge substrates. The goal of this contract is to provide high EOL performance. The main technical emphasis is to 

optimize the ceil performance by growing intermediate layers with increasing lattice spacing on the Ge substrates 

to reduce the effects of the lattice mismatch between Ge and the other two cell materials. 

We are also continiung to explore the mahods developed with RTI, on mechanically stacked MJ cells with 

improved ceii-cell bonding methods. This option allows cells with suitable bandgaps but widely varying lattice 

spacings to be combined effectively. 

SUMMARY 

This paper described Applied Solar‘s current activity in MJ cells. The goals are to meet increased user demands 

for higher efficiency large area space cells, at production levels. We have described the production build 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-growth process methods have been demonstrated. 

-5 9- 
We outlined work to enhance DJ cells, and to add a third junction. We also described work on some associated 

MJ cell designs. 

Table I Space Qualification Tests Completed 

TEST 

PARTICULATE RADIATION 

UV RADIATION 

HUMIDITY 

OPTICAL PROPERTIES 

WELDABI UTY 

SOLDERABILITY 

ANGLE OF INCIDENCE 

REVERSE BIAS SOAK 

REVERSE BIAS CYCLING 

TEMPERATURE COEFFICIENTS 

THERMAL CYCLING 

CONDITIONS 

PROTON ENERGY: 

200 KeV 

FLUENCES 

1 E + 1 1 TO 1 E + 13 e/cm2 

ELECTRON ENERGY: 

1-MeV 

FLUENCES 

3E+13 TO 3E + 15 dun2 

550 - SUN-HOURS AT 1-SUN AM0 

95% AT 45°C FOR 30 DAYS 

ABSORPTIVITY = 0.89, EMISSIVITY = 0.85 WITH OCLl0213 

GLASS COVERSLIDE 

FRONTBACK CONTACT 45" PULL, 350 GRAMS 

FRONTBACK CONTACT 45" PULL, 600 GRAMS 

0" TO 80" SUN ANGLE AT ROOM AMBIENT 

30-MINS DARK REVERSE AT 120°C,1 .67 X 1 

> 25,000 CYCLES AT 120"C, 1 67 X 1s 

28°C TO 120"C, 0 TO 3E +15,1-MeV ELECTRONS 

5,000 CYCLES FROM -1 10°C TO 120°C 

-6 0- 
.14 

Prn 243.4 rnW 

- Vm = 2091.1 mV 

Irn = 116.4 rnA 

Jl~l~llll~Illl~lIll~llll~llll~lll~~llll~III~~IIII~I

n .12 

VI 

.4 

.2 

Q) 

a) 

a 

.10 L 

L. 

- Pm 

- 

- 

- 

- 

- Isc = 532.3 mA 

- Voc = 2394 mV 

- Cff 81.1 % 

- Eff = 21.4 % 

- Prn = 1033.5 mW 

- Vrn = 2077.7 mV 

- Irn = 497.4 rnA 

IIIIIIIII IIIIIIIII IfIIIIIII IIIIIIIII ,rIIIItII 111 

- 

- 

- 

- 

- 

- 

- 

- 

- 

.08 E 

Q 

.06 

-04 

.02 

c 

S 

9) 

L 

L 

3 

0 

v 

0 

L 

Q, 

Q 

Q 

E 

c 

S 

9) 

L 

L 

3 

0 

v 

(Voltage, Volts) 

Figure 1 I-V Data (AMO, 28OC) for 

DualJunction Cell, Area 8 cmz 

(Voltage, Volts) 

Figure 2 I-V Data (AMO, 28OC) for 

Dual Junction Cell, 

-61- 
Jsc Ratio (GaPLslGaIaP1) 

Figure 3 Jnfluence of Current Mismatch on AM0 

Efficiency of a Dual Junction Cell Exposed 

to Several 1 MeV Electron Fiuences 

2 1.0 

E 0.9 

s 

.- 

C 

(0 

Q) 

a 

3 0.8 

0 

a 

d 0.7 

ZE 

0 

.- 

c 

0.6 c 

e 

0 

l.L 0.5 

E! 

0.7 0.8 0.9 1.0 1.1 12 1.3 1.4 

Js'c Ratio (GaAs/GalnP2) 

igure 4 lnfluence of Current Mismatch on Fraction 

ower Remaining, for a Dualdunction Cell 

Exposed to Several 1 MeV Electron Fluences 

-62- 
- 

23 

22 

21 

20 

19 

18 

17 

16 

15 

14 

13 

12 

11 

10 

0 

MISMATCH RATIO 

El = MEASURED 

... I....I....~....I..~,..,I,,..I.~..I..~~I...~I..~J 

0 1 oi3 10" 1015 1Ol6 

1 MeV Electron Fluence (cm2) 

Figure 5 Radiation Performance of DualJunction 

Cell [Modeled and Experimental] 

W8VdeWh (MI) 

Figure 6 Spectral Response of DualJunction Cell 

-6 3- 
0.7 2 

0.70 

0.68 

0.66 

3 

' 0.64 

"'1 0.60 

0 

a 

0 

0.58 1 

0 2 4 6 8 IO 12 

QE'" 2 

Computed bandgap - i ,807 eV 

Figure 7 Bandgap of GalnPl Derived 

From Spectral Response 
InP tunnel junctions for InPIInGaAs tandem solar 

M. F. VILELAa, N. MEDELCI’, A. BENSAOULA’, A. FREUNDLICHa, and P. RENAUDa a Space Vacuum Epitaxy Center, University of Houston, Houston, TX 77204-5507 

Also International Stellar Technologies, Inc.. 9209 Hilldale St.. Houston TX USA 77055 b 

We report, for the ’rst time, an epitaxially grown InP p+/n++ tunnel junction. A diode with peak current 

densities up to 1600 Ncm and maximum specific resistivities (Vp/lp - peak voltage to peak current ratio) in 

the range of 10-4~.~m2 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. 

4 

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/lnGaAs 

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/lnGaAs. 

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 ln0.53Ga0.47As 

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 [I]. However, InGaAs tunnel diodes absorb a 

significant portion of the low energy photons thus limiting the efficiency of the tandem devic In this work we 

report the first realization of an InP tunnel junction with a peak current density of 1600 Ncm’. 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-Indium (TMI), Triethyl-Gallium 

(TEG), and pre-cracked Arsine (AsH3) and Phosphine (PH3) as growth precursors. Solid Beryllium and Silicon 

Be-doped p-InGaAs with net hole concentrations varying froq,2 x -30 cm tq,2 x >O cm and Sidoped n-InGaAs with net electron concentrations varying froni 4 x 10 cm to 2 x 10 cm were achieved; 

more details on these results can be found in reference [2]. 

were used as p and n doping sources respectively. 17 -3 20 -3 

(1) This work was supported by the following State of Texas Ad\ :ed Technology and Research Programs: 

#93-03652-224, #93-03652-236, #93-03652-243, #93-03652-260. , he work at I.S.T. Inc. was supported by 

NASA SBlR Program #NASW-4093. 
The InP layers were grown at temperatures ranging from 753K (480°C) to 803K (530%) and growth 

rates garyiw from 0,s pq/hr to 1 pm/hr. Be-doped p-lnP with net hole concentrations varyigg ftpm 

2 x cmm3 to 1 x 10 cm and Si-doped n-lnP with net electron concentrations varying from 1 x 10 cm to 

4 x 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 lpm/hr and a growth temperature of 783K (510aC), the maximum doping level reached was for a 

Be source at 1203K (930%). Using a growth rate of 0.75 pm/hr and the same growth temperature of 783K 

(51OoC), the maximum doping level was reached for a Be source at 11 53K (880%). 

Fig. 1 shows that the carrier concentration is more sensitive to growth temperature than to growth rate. 

For instance, a change in the grqyth .$ate from I,&o 0.75 pm/hr (2.78 to 2.08 &s) only slightly increases the 

hole concentration, from 4 x 10 cm to 5 x 10 ~rn-~; this difference is within the experimental error. 

However, if the growtt?,temqerature is,decr%ased by 45 K (from 828K (555°C) to 783K (510%)) the doping level 

increases from 5 x 10 cm to 1 x 10 cm . The Be source activation energy was determined to be 3.0 eV. 

m 

0 

n 1El 

3 

E 

0 

c 

0 

CU 

Q) 

0 

c 

0 

0 

v 

.- 

c 

1El 

.- F 

0" 1El D 

Q, m rate = 1 jlrnIh, Tg = 783 K 

9? A rate = 0.75 ltm Ih, Tg = 783 K 

% - a rate = 0.75 jtm lh, Tg = 828 K 

1El 

0.80 0.85 0.90 0.95 1 .oo 

Be effusion cell temperature (1 OOO/T(K)) 

Fig. I: 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- 
IEIC f I I I I I I 

0.68 0.70 0.72 0.74 0.76 0.78 0.80 

Si effusion cell Temperature (1 000/T(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 InP 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 In0 53Ga0 47As tunnel junctions with peak current densities of 

up to 1015 Ncm2 using CBE. In0 53Ga0 47As tunnel junciions grown on GaAs and Si substrates which yield 

peak current densities as high as 560 Ncm2 were also demonstrated [l], 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 pm) 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 

lnGaAs Tunnel Diodes 

substrate peak current resistivity annealing 

InP 1015 2.5 No 

GaAs 452 4 No 

Si 560 4.8 No 

InP* 580 3.14 No 

InP** 860 2.9 2 hr (> 56OOC) 

Afcm2 (XI 04)n-cm2 

*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. 

-6 7- 
Our preliminary work on the growth of InP and InGaAs solar cells resulted in photovoltaic converters with 

AM1.5 efficiencies equal to 18% for InP and 10.2% for ln0.53Gao 47As. We have also fabricated InP/lnGaAs 

tandem devices which were grown using a 0.1 pm 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 a1 [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 '1 

(AM1.5 values) V mA/cm2 % YO 

1nP 0.81 28 80 18.1 

InGaAs 0.25 60 62 10.2 

InP/lnGaAs tandem* 1.2 60 82 

InP/lnGaAs tandem 1 .o 8 76 6.3 

--- 

* - under concentrated light; Voc - open circuit voltage; Jsc - short circuit current density; FF - fill factor; 

q - 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 [SI). The growth is terminated with an ln0.53Ga0.47As p-type contact layer. The complete structure is 

shown in Fig. 3. For all sam les analyzed, the n-doped InP layer characteristics were kept constant, with the Si 

doping at N, - N, = 1 x 101gcm-31 a thickness of 750 A, a growth temperature ( Tg) of 783K (510'C), and a 

growth rate (r,) of 1 @hr. Only the p+ layer growth parameters were varied. rig. 4 shows the I-V 

characteristics of a device, CBE 399, exhibiting a peak current density of 1,600 Ncm . This tunnel junction 

imposes a voltage drop on the tandem device in the microvolt range, for a sunlight concentration of 1 OOx (AM0 

spectrum). 

The Be-doped InP layer for this device was grown under the following conditions: T = 773K (5OOOC); 

measurements, is N,- N = 7 x 10 em . To our knowledge this is the first demonstration of an 

epitaxially-grown InP tunnerdiode. 

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), weincrease the growth temperature by 30K (from 773K (5OOOC) to 813K 

(53OoC)), or if we increase the growth rate by 33% (from 0.75 to 1 pm/hr (2.08 to 2.78 h)), 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 InP. 

The doping levels of both layers com osing tunnel diode CBE-399, as deducey9 from our Hall 

measurement calibration graphs, were: 7 x lO''~m-~ for the p layer (see Fig. 1) and 1 x 10 cm-3 for the n 

layer see Fig 2 . Those doping concentrations give an effective doping level, N*= NAX No/ (NA + No), of 

4 x 10 cm . The theoretical expression developed by Kane [6] (see Vilela et al [Z]), correlating the effective 

doping level and peak current in tunnel diodes, predicts a peak current of less than 200 A/cm2. ffowever, the 

rg = 0.75~m/hr; and TBe = 1153K,d880P&). The doping level in this layer, as extrac 0 ed from the Hall 

(8 -3 - ) 

-6 8- 
2 

1,600 Ncrn reak-2urrent density exhibited by CBE-399 demands an effective doping level of 8 x 10'' cmT3, at 

least 2 x IO' cm for each adjacent layer in the CBE 399 device. In other words, the theoretical calculatioq 

implies that one or both doping layer levels in that tunnel junction are underestimated. Also, the 1,600 Ncm 

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 93K (530°C) for 20 minutes, as CBE-399 was, 

would show peak current densities much lower than 1,600 Ncrn . 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 iirn/h "C A/crn2 

CBE 399 500 0.75 880 1,600 

CBE 469 500 1 880 No (B) 

CBE 449 530 0.75 880 No (a 

CBE 407 500 0.75 920 No (S) 

n - M 

For all the samples the n++doped (silicon) InP layer characteristics were fixed at a No - NA = 1 x 10 

thickness of 750 A, a growth temperature ( Ts) of 783K (51 0°C) and a growth rate (r,) of 1 pm/hr. 

No - means no tunnel characteristics presented. 

(Z) means Zener, (B) means Backward or (S) means Simple diode. 

em , 

I I no. 5 9G ao. 4 7As: Be 

p +=lxlOB 

0.05 pm 

InP:S 

Fig. 3: Schematic representation of the InP tunnel diode studied in this work. 

-6 9- 
-W characteristic of an InP tunnel junction (mesa 200 pm in diameter) fabricated on InP 

his tunnel diode has a cap layer simulating the InP solar cell growth during the tandem 

fabrication ( 20 min 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 Ncrn2). This peak current is one of the highest ever reported for 

tunnel junctions fabricated with Ill-V compounds. This realization should allow the growth of InP/lnGaAs 

tandem solar cells with optically transparent and electrically compatible interconnects for concentrator 

applications. This tunnel junction induces a voltage drop in the microvolt range for a concentration of 1OOx 

AMO. We are currently working toward the fabrication of such rnonolithically integrated InGaAdlnP 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/lnP tandem solar cells. 

The authors would like to thank Alex lgnatiev for helpful discussions, David Moore for the legibility of this 

text and Ping-chi (Pablo) Chang for his assistance during CBE growth. 

6. References 

A. Freundlich, M. F. Vilela, A. Bensaouia and N. Medelci, Proc. 2grd IEEE-PV Conf. (1993) 644. 

M. F. Vilela, A. Freundlich, A. Bensaoula and N. Medelci, 5th International Conference on inP and Related 

rance, lEEE Catalog #93CH3276 (1993) 227. 

Panish, R. N. Nottenburg, Y. K. Chen, and Humphrey; Appl. Phys. Lett. 

ard, K. A. Emery and T. J. Coutts, Proc. 23'tl IEEE-PV Conf. (1993) 621. , A. Freundlich, N. Medelci and A. Bensaoula; Proc. of the XI1 Space Photovoltaic 

Conference (SPRAT XIl), Cleveland (1994) 1 

-7 0- 
THE ROLE OF RADIATION HARD SOLAR CELLS IN MINIMIZING THE COSTS OF GLOBAL SATELLITE 

COMMUNICATION SYSTEM!3 

Geoffrey P. Summers 

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

Department of Physics 

University of Maryland Baltimore County, Baltimore, MD 21 228 

Robert J. Walters 

Code 661 5, Naval Research Laboratory, Washington, DC 20375 

Scott R. Messenger and Edward A. Burke, 

SFA, Inc., Landover, MD 20785 

INTRO D UCTl ON 

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 

104krn) 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 lnP/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 $1 6 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 connection to mobile handsets was possible where cellular systems 

were available. The new systems are designed to provide glabal 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 krn). 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 ME0 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 [i 1. 

There are a number of other reasons why a ME0 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 (31. 

The major obstacle to establishing cost-effective ME0 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 ME0 operation. 

SELECTING AN OPTIMUM SOLAR ARRAY FOR A ME0 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 [SI. 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 etuatorial 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 

Handbook [SI. Solar flare effects can also be included. 

rogram can be used to determine the optimum configuration for any cell technology 

of a particular orbit and a specified mission life. Consequently, it is possible to 

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 

-7 2- 
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 

lnP/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 1 for the particular mission used for Figs 3 - 6. 

Table I 

Percentage Contributions of Major Elements to the Total Array Weight 

--- _1-1_- ------- 

InP/Si WGe Si 

12 mil glass 30 mil glass 20 mil glass 12 mil cell 8 mil cell 4 mil cell 

-------- ----- - 

14% BOL 18% BOL 14% BOL 

~._---------I------------I_--------~--- 

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 EO1 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. 

1. 

2. 

3. 

4. 

REFERENCES 

T. Logston, "Mobile Communication satellites", McGraw-Hill, 1995. 

W.W. Wu et at., "Mobile Satellite Communications", Proc. IEEE, 82, 1431-1448, 

1994. 

N.D. Hulkower,"An Update on-The Mobile Communications Scene", IEEE Communications 

Chapter Boston, MA, 14 Sept. 1995. 

S. Bailey and G. Landis, "Use of Advanced Solar Cells for Commercial Communication 

atellites", IEEE Proc. 1 st World Conference on Photovoltaic Energy Conversion, 

. Summers, R.J. Walters, M.A. Xapsos, E.A. Burke, S . Messenger, P. Shapiro and 

a Statler, "A New Approach to Damage Prediction for ar Cells Exposed to Different 

ec. 1994, pp 2002-2005. 

-74- 
Radiations", IEEE Proc. 1 st World Conference on Photovoltaic Energy Conversion, 

Hawaii, Dec. 1994, pp 2068-2073. 

6. 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 

100 

m 

E. 80 

0 

c 

P 

2 

60 C 

v) 

- 

H 

.= 40 

m 

m 

c 20 

0) 

a, 

(I) 

- 

c 

- 

e 

0' 

_ ......... 

-. ......... 

-.._... .... 

-..... 1 .... 

- 

..... 

..... 

-.... 

\ 

// 

4.0 

3.5 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

0.0 

0- 

E 

.- 

s 

1,000 

- 

IRIDIUM 

- 

- 

LLOBALsTAR MSAT 

- 

- 

ODYSSEY m ELLIPSO 

- 

- 

I I 

- Launch Cost 

+Total Mass 

- 100 

80 

c 

f 

E 

60 

0 

0 

?? 

40 2 

!e. X 

20 

n 

10,000 100,0'00 

Orbital Altitude (km) 

Fig.1 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 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. 

-7 6- 
EOL SOLAR ARRAY POWER DENSllY vs ALTITUDE 

10 Year Mission - Circular Orbit - 60" Inclination 

JU 

40 A 

E 30 

t 20 

ts 

g 

8 10- 

H 

v) 

C 

L 

-I 

0- 

60 

12 mil coverglass 

8 mil thick cells 

2 kgJm 2Al-Honeycomb 

- 

. -__ ------_____ 

- GaAsIGe - - - - - - - - - - _*_ - \ __)* 

0 

r ................... .......... ...... .-'I 

.......... - _-- / / ........ .e- / 

I si .* / 

I 

I BOL Efficiency 

1 si 14% I InP/SI 14% 

GaAsIGe 18% 

I 

I 

I 

I I I I I I I I I I I 

..... BOL Efficiency 

InP/SI 14% 

GaAs/Ge 18% 

Si 14% 

n I I I I I I , I I I Y 

0 2 4 6 8 10 12 14 16 18 20 22 

Altitude ( lo3 kin) 

Fig. 3 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 600 angle of 

standard AI honeycomb array material has been used in each case. 

EOL POWER DENSITY vs COVERGUSS THICKNESS 

5 Year Mission - 7400 km Circular Orbit - 60" Inclination 

-7 7- 
EOL POWER DENSITY vs BOL EFFICIENCY 

5 Year Mission - 7400 km Circular Orbit - 60" Inclination 

60 

50 - 12 mil coverglass 

I E 30 mil coverglass m 

30 - GdS/G6 _________________----------------. \ 

d _--- 20 mil coverglass z --- .-.- ................................................................... 

L 0 20 - si / 

A 

0 w 

10 - 8 mil thick cells 

2 kg/m 2Al-Honeycomb 

"V 

50 

h 

40- 

- 2. 

6 

8 20 

a 

30 

8 3 

10 

01 I 

I 1 

10 15 20 25 

EOL Efficiency (%) 

Fig. 5 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 600 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" lnclination 

- 2 kg/m 2Al-Honeycomb 

- 

InP/SI 7- - 

- ---_ 

- 

-------____ -----___ ------____ 

GaAs/Ge / - - - - - - - - _. ............................................ ........................... / 

SI - 

BOL Efficiency 

sr lnP/Sl 14% 1 4% 

GaAsiGe 18% 

- 

Solar Cell Thickness (mils) 

Fig. of life (EOL) power density vs cell thickness for arrays of InP/Si, GaAdGe and Si 

solar cells, for a 5 year mission in a 7,400 km circular orbit with a 600 angle of 

inclination. 

-7 8- 
EOL SOLAR ARMY POWEA DENSIN vs ALTITUDE 

10 Year Mission - Circular Orbit - 60" Inclination 

Flex Array - 0.2 kg/m 2 

30 mil coverglass __-- c - 

, 

, 

/ 

, 

. 00L Efficiency 

InP/Si 14% 

Altitude ( lo3 hm ) 

ig. 7 

flex array material has 

e effect of !he ligh~er 

-79- 
High and Low Energy Proton Radiation Damage 

in pin InP MOCVD Solar Cells 

312 

George Rybicki and IN 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 

InP p+/n/n+ solar cells, fabricated by metal organic chemical vapor deposition, (MOCVD) were 

irradiated with 0.2 MeV and 10 MeV protons to a fluence of 1 Ol3/cm? 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 in the recombination currents in the 0.2 MeV proton-irradiated solar cells. 

The low energy proton irradiations also 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 irradiations and even increased during irradiation with 0.2 MeV 

protons. In a deep level transient spectroscopy (DLTS) study of the irradiated samples, the minorii carrier 

defects H4 and H5 at Ev + 0.33 and Ev c 0.52 eV and the majority carrier defects E7 and El 0 at &- 0.39 

and &-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 El 0, observed here after 

irradiation, has been shown to act as a donor in irradiated n-type InP and may be responsible for obscuring 

carrier removal. The results of this study are consistent with the much greater damage produced by low 

energy protons whose limited range causes them to stop in the active region of the solar cell. 

Jntroductioa 

A near record AM0 efficiency InP solar cell of 19.1% was achieved through MOCVD growth of a 

homoepitaxial InP p+/n/n+ structure.(l) This type of structure has the potential to simplify the growth of 

heteroepitaxial InP cells on Si by eliminating the problem of counter doping of the base region in the InP 

on Si n on p cells. Electron irradiation testing of MOCVD InP p/n cells has also shown that they have 

superior radiation resistance to MOCVD InP n/p cells. (2) a 

Several phenomena are observed in irradiation studies of n-type InP that may contribute to its improved 

radiation resistance. It has been observed that carrier 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 with a specific defect through the use of DLTS. (3) In contrast, it has 

been shown that the carrier 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 interest to observe the 

effects of carrier enhancement on the performance of the solar cells. 

The diodes and solar cells used in this 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 laboratory of the Photovoltaic Branch at NASA Lewis, 

using a Spectrolab X-25 xenon arc solar simulator and a flightcalibrated InP standard cell. The low energy 

proton irradiations were performed at the University of Michigan’s Ion Beam Laboratory and the high 

energy proton irradiations were performed in the tandem Van de Graaff accelerator of Western Michigan 

University. Spectral response and IV measurements were made prior to the irradiations and at each step 

throughout. Carrier concentrations were measured using the capacitance-voltage (0 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 Frenkel-Poole effect. (9) 

Results and Discussion 

The pre-irradiation performance parameters of the p on n cells are given in Table 1. 

Table 1. Pre-irradiation cell pe~ormance parameters 

The performance of these cells was clearly not state of the art. As indicated before, cells of up to 19.1 % 

have been fabricated in this configuration.(l) The cells used in this study were produced as part of a 

development program: therefore, growth and cell design parameters had not yet been optimized. The 

post irradiation cell 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. 

Energy (MeV) 0.2 02 0.2 10 10 10 

Fluence cm-2 efficiency % lsc (ma) Voc (mv) efficiency % Isc(ma) Voc (ma) 

0 12.41 23.6 851 1 1.95 22.5 855 

10” 8.37 19.8 772 1 1.81 22.4 852 

10Q 4.85 148 649 10.45 21.4 828 

2.75 10.8 545 7.90 19.0 755 

Table 2. Pre and Post Irradiation cell performance parameters 

The dramatic difference in damage rates is immediately evident. The low 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 urn and will pass 

-81- 
completely through the active region of the cell. The 0.2 MeV proton however has a projected range of 

1.56 urn 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 in this 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 in open circuit voltage and short circuit current in the 10 MeV 

irradiated cells. This is also related to the differences in damage production of the two particles. Although 

the 0.2 and 10 MeV irradiations both 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 in the external quantum efficiency curves given in Figure 3. 

Figure 3 shows the external quantum efficiency of the unirradiated and irradiated cells. 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 in the 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 carriers. 

The 10 MeV proton radiation resistance of the pMn+ MOCVD solar cells versus n/p MOCVD solar 

cells (2) and diffused junction n/p solar cells(1 l), is shown in Figure 4. The radiation resistance of the 

p/n/n+ cells in this 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. (1 2) 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 junction cells 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 not observed, or at least not to the same extent. (12) The 

variation in radiation resistance 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 p type materials. (13,14) An absence of radiation induced carrier removal may also help 

improve the radiation resistance in n type lnP.(3) 

Carrier concentration data for the irradiated samples are presented in Figure 5. It can be seen that not 

only is carrier removal not observed after proton irradiation but that the carrier concentration increases 

slightly in the case of 10 MeV irradiations, and strongly in the 0.2 MeV irradiations. The lack of carrier 

removal has been previously observed in n-type InP and has been correlated with a radiation induced 

defect. (3) As the 0.2 MeV protons have a higher defect production rate than the 10 MeV protons, they 

produce a larger increase in carrier concentration. 

IV curves were also taken throughout-inadations and 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 irradiations produce a larger increase in the 

recornbination currents in the solar cells. The differences produced in series resistance as shown in 

Figure 6, became more pronounced at higher fluences. 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 diffusion current 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 irradiations the 

degradation in the recombination current is very limited, but much greater in the diiusion 

-8 2- 
Energy (MeV) 

Fluence (cm-2) 

10x3 

02 0.2 0.2 

JOI Wd) Jo~ WC~) Rs (ohm) 

3.2 x 10-17 5.3~10-14 0 .# 

1.85~10-11 1.08~10-9 - 

1.5 xl0-11 5.5 xl0-8 0.56 

1.2 xlO-9 1.75~10-8 1.7 

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 resutt of the high damage 

production rate of the low energy protons which stop in the active region of the solar cell as opposed to 

the high energy protons which pass through the cell or produce damage deeper in the base of the cell. 

The series resistance of the solar cells was also more strongly affected by the low energy protons but 

the effect was pronounced only at the highest fluence. This effect occurs at the same point where the 

carrier concentration increase was observed in the 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 in the 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 introduction rates in the 

samples. The DLTS spectra are shown in Figures 7 through 11. A preirradiation analysis showed that no 

deep levels were present in the samples. The defect concentrations were measured throughout the 

experiments and the introduction rates calculated. The data appears in Tables 4 and 5. 

3.08~ 1014k0.28 I - 

NT (cm-3) E10111 I - I- I- I 4.88 x 1015 k0.41 

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.) 

-8 3- 
Table 5. Defect Concentration and lntroduction Rates in 10 MeV Proton Irradiated InP 

(Numbers in parenthesis are a least squares fit of all Nt vs. I$ data.) 

The post irradiation DLTS analysis revealed the presence of two deep levels in the minority carrier 

spectra and three deep levels in the rnajorii carrier spectra. Only two of the rnajorii levels could be 

resolved in both the high and low energy proton irradiations, the first, 6-0.37 eV is referred to in the 

literature as E7.(5) The second level was found considerably deeper in the gap at &-0.74 eV, and is 

known as E10 or El 1 , ( 45). An additional level, E9 at Ec-0.64 eV was observed after the 10 MeV proton 

irradiations but it could not be 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, (Vln-Pln).(l7) The defect 

E1011 1 is correlated with a displacement in the P sublattice of InP and thought to be a complex between 

this displacement induced defect and an intrinsic defect or impurity. (3) 

The relative concentrations of the two defects in the 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 El 0/11 is the 

defect with the largest introduction rate.@) 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 I3 configuration, which results from 

cooling the sample under reverse bias.(l7) 

The minority carrier spectra in both high and low energy proton irradiations showed two deep levels at 

Evt.0.29 and E,+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 PI, . H5 is thought to be a complex of a defect and a 

dopant impurity, VI,-Zn or P1n-ZnI as evidenced by the strong dependence of its introduction rate on the 

dopant concentration. (1 8) The minority carrier spectra shown here in which H4 is the defect with the 

highest introduction rate, is in agreement with the spectra published in the Iierature.(l5) 

The defect introduction rates were calculated from a least squares fi 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 fi of ail data up to the highest fluence 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 particular problem in the case of minority carrier analysis after 0.2 MeV irradiations at the 

highest fluences. high concentration of defects can affect the quality of a DLTS analysis by introducing 

leakage currents, increasing series resistance or by producing very high defect concentrations. (16) In the 

-8 4- 
case of the lowest fluence 10 MeV irradiations the defect concentrations were below the detection limit of 

our instrument. The variation in defect introduction rates calculated with each additional data point is 

illustrative of the error which might be introduced by estimating introduction rates from a single data point, 

a practice which is common in the literature. This estimation however was necesmy in this case for the 

calculation of the defect El 0/11 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 radiation damage. 

The defect introduction rates measured here are very much higher in the case of the 0.2 MeV protons 

than for the 10 MeV protons and both are considerably higher than those measured for 1 MeV electrons. 

(6,13) An attempt was made to correlate the defect introduction rates with the energy loss rates of the 

particles in the material. The great majority of the energy loss of a high energy particle incident on a 

material is by ionization of the lattice atoms, but a small fraction is lost producing lattice atomic 

displacements. The energy lost by atomic displacement can be estimated using TRIM. (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 relationship in the introduction rates of El 0/11 in the two cases 

above, the prediction is a ratio of 25 but the observed ratio is 63. 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 fortnation of El 0/11 requires the participation of Zn in the n-type material from the 

diffusion front tail produced during the formation of the pn junction. (5) The introduction rate of €1 0/11 

was also found to depend strongly on electron energy, leading one to believe that Zn atoms ejected into 

or implanted into the n region of the diode by proton irradiation may enhance the formation rate of El all. 

(5) In this case the low energy proton irradiations may have caused a large redistribution of 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 El 0/11 by implanting more Zn into the n-type region of the diodes. 

The superior radiation resistance of the p/n-type InP solar cells may be related to the defect 

introduction rates. In this study the introduction rate of the dominant majority carrier defect €1 0 in n-type 

InP was about 8 per 10 MeV proton. The defect introduction rate for the dominant majorii carrier 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 InP may contribute to enhanced radiation resistance in p/n InP solar cells. This finding is an 

agreement with previous authors who found the same relationship in electron irradiated lnP.(13,14) 

Conclusions 

The 0.2 and 10 MeV proton irradiations produced dramatically different rates of degradation in the 

p/n/n+ InP solar cells studied. The relative rates of damage were correlated with the different ranges and 

damage production rates of the two patticles. The low energy protons stopped in the active region of the 

cell and produced a dramatic decrease in efficiency, dominated by a decrease in the diffusion length of 

photogenerated carriers. The 10 MeV protons produced degradation dominated by an increase in dark 

currents in the cell. The MOCVD p/n/n+ InP solar cell configuration studied here showed better radiation 

resistance than the MOCVD nlp configuration in the literature. Studies of the radiation damage showed 

that the lack of carrier removal in n-type InP was not the reason for the 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 significant differences in the defect spectra generated by low 

and high energy proton irradiation, or differences with the 1 MeV electron irradiation spectra in the 

literature, but that the low energy protons produced 15 to 60 times as many defects as the high energy 

protons. The defect introduction rate for the dominant majorii carrier defect in n-type InP was found to be 

-85- 
lower than that in p-type InP, in agreement with the findings on electron irradiated materials. This fact may 

contribute to the superior radiation resistance in n-type InP materials. 

Acknowiedclements 

The authors would like to acknowledge the help of Dr. Stephen Ferguson, from Western Michigan 

University and Dr. Victor Rotberg of the Michigan Ion Beam Laboratory of the University of Michigan for 

their help with the proton irradiations reported in this work. One of us (C.V.-A.) would also like to 

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. 

1. C. Keavney, V. Haven and S. Vernon, Proc. 21 St. IEEE Photovoltaics Specialist Conference, 

2. C. Keavney, R. Walters and P. Drevinsky, J. Appl. Phys., 73,60 (1 993) 

p.141 (1990). 

ibile, Appl. Phys. Lett 

einberg, G. Rybicki, C. Jain and D. Scheirnan, Proc. Of the Xlll Space 

Photovoltaics Research and Technology Conference, p. 147 (1994). 

5. J. Benton, M. Levinson, A. Macrander, H. Ternkin 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 (1 982). 

9. G. Vincent, A.Chantre and D. Bois, J. Appl. Phys., 50, 5484 (1979). 

10. J. Ziegler, J. Biersack and U. Littmark, ‘The Stopping and Range of Ions in Solids”, 

1 1. 1. Weinberg, C. Swartz, R. Hart and M. Yamaguchi, Proc. Of the 5th. European Symposium 

12. R. Walters, S. Messenger, G. Summers, E. Burke and C. Keavney, Proc. 22nd. IEEE Photovoltaics 

13. M. Levinson, J. Benton, H. Temkin and L. Kimmerling, Appl. Phys. Lett., 40, 990 (1983). 

14. M. Levinson, H. Ternkin and W. Bonner, J. Elec. Matls. ,12, 423 (1983). 

15. A. Sibile and J. Bourgoin, Appl. Phys. Lett., 41, 956 (1982). 

1 6. D. Schroder, Semiconductor Material and Device Characterization, 

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). 

(Pergamon Press, New York, NY,1985). 

on Photovoltaic Generators in Space” , ESA SP267, p. 415 (1986). 

Specialist Conference, p. 1560 (1991). 

(Wiley Interseience, NY, NY, 1990), p. 336. 

-8 6- 
Normalized Efficiency as a 

Function of Proton Ruence 

p+ 2 x 1018 (zn) 

n 3.2 x l0l6 (Si) 

n+ 5 x 1018 (Si) 

n 5x1018 (SI 

Figure 1. Details of InP cell configuration 

1 

0.8 

0.6 

0.4 

02 

0 

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 

Wavelength urn 

0.25 urn 

1.5 urn 

0.5 urn 

Fgure 3.l3kmal quantum efficiency of cells, 

uninadiied and inadiied to1 01Yd 

5.0 10'~ 9 

10'6 ~ 

t 10 MeV I 

lox6 

10" 

id6 _- 

10" f0'2 10" Proton Fluence cd ) loto 

Fgure5. Canierconcentration asa 

functionof fluence. 

h 

1 

x 

0 

Y 

v) 

9) 

9) 

v) 

.- 

L1 

0.8 

0.6 

0.4 

0.2 

0 

1010 10" 10'2 1013 

Figure 2. Normalized efficiency as a 

furdionofpratonfluence. 

Fluence (cm") 

1 

0.95 

0.9 

0.85 

0.8 

0.75 

0.7 

0.65 

0.6 

10'0 10" 10f 10" 

Fluence (an ) 

Fgure 4. Normalied efficiency of various 

cells after 10 MeV irraditions. 

1.8 

1.6 

1.4 

1.2 

1 

0.8 

0.6 

0.4 

lO'O lo" 10t2 10" 

Fluence (cmJ 

Fgure 6. Seties resistance as a function 

ofpiutonfluence. 

-8 7- 
-60 

<'50 

a 

2" 

Y 

8 -30 

f = -20 

a 

n -10 

0 

0 

200 250 m 350 400 

Kdvin 

Fgure 7. Majority canier DLTS spectrum of 

02 MeV proton irradiaded InP. 

-35k .I....I I ...,....I.... j 

1 

a = -30 c 

a -25 

Y -20 g 

0 

- i -15 

g -10 

4-5 

0 

200 250 300 350 400 

Kelvin 

FQU~ 9. Majority carrier DLTS spednrm of 

10 MeV praton irradiated InP. 

n 9.0 lo" 

3 

g 8.010~ W V 

7.0 lou 

4 LL 6.0 lou a 

e 

c 6 3.0 loG 

3 5.0 lou 

8 4.0 io1' 

* 20 10'5 U c 1.0 loi5 

cl 

2 10'' 4 10" 6 810'' 1 lou 110" 

Proton Huenee (em-' ) 

figure 1 1. Defect introduction rates in 0.2 MeV 

pruton iadated InP. 

0 

E -5 

a -10 

4 -15 

I 

c 

Y 

8 -20 

5 -25 

g, -30 

G -35 

c 

0 

- 

100 150 200 250 300 

Kolvin 

figure 8. Minority carrier DLTS spedmm of 

0.2 MeV proton inadiated InP. 

Fwrel 0. Minority her DLTS spectrum of 

10 MeV pruton irradiied InP. 

n 6.0 IO" 

e 

(D 

2 4.0 io1' LL 

I; 

LL 

U 

e 

5 3.0 ioL4 

6 20 1014 

p 1.0 io1' 

d 

LL 

L. 

ElW.8 

210" 410" 6 IOu 810" 1 IOu 1 

Proton Huenee (em-' ) 

10" 

figure 12. Defect intduction rates in 10 MeV 

pmton iradateed InP 

-a 8- 
Correlation of Electron and Proton Irradiation-Induced 

Damage in InP Solar Cells 

Robert J. Walters 

Naval Research Laboratory, Code 66 15, Washington, DC 203 75 

Geoffrey P. Summers 

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

Department of Physics, University of Maryland Baltimore County, Baltimore, MD 2 1228 

Scott R. Messenger and Edward A. Burke 

SFA, Inc., Landover, MD 20785 

ABSTRACT 

The measured degradation of epitaxial shallow homojunction n+p InP 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 analyzed as a finction 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 ofken 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.l 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 Co60 gammas. Summers et a1.2 have used this 

method to accurately predict the data measured by Anspaugh3 on GaAs/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. 

-8 9- 
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 GMGe cells. The results show Id? to be inherently 

more resistant to displacement energy deposition than WGe. 

EXPERIMENTAL NQTES 

The 1 MeV electron irradiations were performed using the Van De Graff 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 lOOOW 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 3pm 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 cm2 was used in all calculations. 

CORRELATING 1 MeV ELECTRON AM) 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 EL, 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 NlEL 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 3.348~10-5 (MeV cm2/g) and that for 3 MeV protons is 2.031~10'2 (MeV 

cm2/g). The resultant plot of the degradation of Pmm vs displacement damage dose is shown in figure 2. 

As can be seen in figure 2, analyzing the degradation data as a fbnction 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 hP). is defined as the ratio of the 

dose along the 1 MeV electron degradation curve to the dose along the 3 deV proton degradation curve 

-9 0- 
which cause equal cell degradation. For the present data, = 4.12. By dividing the dose values of 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 fbnction of 

displacement damage dose (i.e why Rep f 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(l+ -) 

D. 

D, = displacement damage dose (1) 

A,C,D, = constants 

which is essentially the degradation equation given in the Solar Cell Radiation Handbook4 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, = 

1.79~109 (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 of p-type InP, the damage coefficients have been found to vary 

lineraly with NIEL5’7. Therefore, the degradation of these InP cells under any proton irradiation can be 

calculated by simply dividing the dose range of 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 finction of displacement 

damage dose (figure 2) indicates that the electron damage coefficients in p-type Id? do not vary linearly 

with NIEL. It has been found in p-type Si and GaAs that the electron damage coefficients vary with the 

square of the NIEL1,5. Preliminary results indicate the same to be true for p-type InP8, 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 PERFORhlANCE OF InP AND GaAdGe 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 GaAdGe solar cells can be compared with the response 

of the InP cells studied here. In reference 2, a Characteristic degradation curve was determined for 

GaAdGe solar cells based on measurements made by Anspaugh3. This curve is reproduced in figure 4 

along with the characteristic degradation curve for InP solar cells derived here. The InP curve lies above 

-91- 
the GaAs/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 GMGe solar cells. The comparison shows these InP 

cells to be inherently more resistant to displacement energy deposition than GaAs/Ge cells. 

FERENCES 

1 

Radiation Effects", Radiation Measurements, 24, No,. 1, 1-8 (1995). 

G.P. Summers, E.A. Burke, and M.A. Xapsos, "Displacement Damage Analogs to Ionizing 

2 G.P. Summers, R.J. Walters, 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", IEEE 

Proc. 1st World Conference on Photovoltaic Energy Conversion, Waikoloa, Hawaii, Dec 5-9, 1994, p. 

2068-2073. 

3 

22nd Photovoltaic Spec. Conf., Las Vegas, NV, Oct., p1593 (1991) 

B.E. Anspaugh, "Proton and Electron Damage Coefficients for GaAs/Ge Solar Cells", IEEE Proc. 

H.Y. Tada, J.R. Carter, B.E. Anspaugh, and R.G. Downing, "The Solar Cell Radiation 

Handbook", JPL Publication 82-69, 1982. 

G.P. Summers, E.A. Burke, P. Shapiro, S.R. Messenger, and R.J. Walters, "Damage Correlation 

in Semiconductors Exposed to Gamma, Electron, and Proton Radiations", IEEE Trans. NUC. Sci., 40, 

1300-1306, D~C (1993) 

6 T. Takamoto, H. Okazaki, €I. 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, Japan, p547, (1 990) 

7 M. Yamguchi, 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. 22nd Photovoltaic Spec. Conf, Las Vegas, NV, Oct., (1991) 

8 

Semiconductors Devices", Ph.D. Dissertation, Univ. of Maryland Baltimore County, 1995 

S.R. Messenger, "Electron and Gamma - Induced Displacement Damage Effects in InP 

-9 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; 

10 

I I llllll~ I I111111( I I lllllll I I111111~ I 1 llmr - - 

- e lSUn 

- e AM0 - 

- e 298K 

- 

- m - 

- m - 

- e - 

- e - 

- e - 

- 

1 MeV electrons ~1 - 

- 

- 

- - 

I Illlllll I I1111111 I I1111111 I Illlllll I IlllU 

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 hnction 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 hnction of displacement damage dose brings the degradation curves 

much closer together. 

-9 3- 
epitaxialn pInPhP NE 20 

15 

t 11 ~~,~~ \ -I 

Characteristic degradation curve 

10 

1 07 

m 

108 109 10'0 10" 10'2 

Displacement Damage Dose (MeV/g) 

GURE 3: Correlation of the measured 1 MeV electron and 3 MeV proton irradiation damage. By 

applying the 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). 

value of 4.12 to the data of figure 2, the degradation data is correlated. 

c1 

a 

25 

20 

15 

10 

5 

108 18 10'0 10" 

Displacement Damage Dose (MeV/g) 

4: Comparison of the radiation response of GaAs/Ge and Inl? solar cells. Since the 

displacement damage dose damage correlation method allows the solar cell degradation under irradiation 

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 displacement damage deposition than WGe. 

-9 4- 
DIFFUSION LENGTHS IN IRRADIATED N/P InP-on-Si SOLAR CELLS 

Steven Wojtczuk and Claudia Colerico P fA <*: 

1’ Spire Corporation 

Geoffrey P. Summers and Robert J. Walters 

Naval Research Laboratory 

Washington, DC 20375 

Bedford, MA 01730-2396 99 

$ ?$+ 

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 properties of the InP solar cell and the light weight 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 

NIP InP/Si cells’ of sizes up to 2 cm by 4 crn with beginning-of-life (BOL) AM0 efficiencies over 13% (onesun, 28C). These InP/Si cells have higher absolute efficiency and power density after a high radiation dose 

than gallium arsenide (GaAs) or silicon (Si) solar cells after a fluence of about 2el5 1 MeV electrondcm2. 

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 measured AM0 photocurrent (30.1 mNcm2). 

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 electrondcm2 (the 12% BOL 

cell was 8% after the final irradiation). By comparing the measured photocurrents with the predicted 

photocurrents, base diffusion lengths were 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 mNcm2) agreed with the simulator-measured photocurrent after irradiation (25.7 mNcm2). 

QUANTUM EFFICIENCY MODEL 

The quantum efficiency model used, described well by Hovef , breaks up the 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 limit light absorbed in the emitter which is subjected to a high front surface recombination loss (le7 cmls). 

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 3OOA emitter thickness. This non-sensitivity of the 

photocurrent to emitter diffusion length makes the study of the base diffusion length much easier. 

-9 5- 
The second component that contributes to the quantum efficiency is the NP junction depletion space 

charge region (SCR). In the NP InP cell the emitter is very heavily doped (> leWcm3) so that a one-sided 

step junction approximation is used to calculate the zero-bias SCR width in the base (doping 3el7/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 diffusion lengths. 

The third model component is from the base region of the solar cell, and the model is essentially 

similar to that of the emitter (but of opposite polarity and minority carrier types). The surface recombination 

velocity at the back of the 3pm thick cell was taken to be le4 cm/s, but the results are very insensitive to this 

value, since the base diffusion lengths are all less than 1 pm and very little is collected from 3pm away from 

the junction. 

Figure 1 shows (black dots) the measured quantum efficiency of a 1 cm2 12% InP/Si cell before 

irradiation. The AM0 photocurrent from the measured quantum efficiency and I-V measurements at one-sun 

on a sun simulator (set with a NASA-calibrated InP reference cell) agreed (30.1 mA). The quantum efficiency 

data were non-linear least squares fitted to the model using the Marquandt-Levenberg algorithm. The model 

fit is shown as the uppermost solid line, and when integrated against the AM0 power spectrum, gives a 

photocurrent of 30.3 mA, in close agreement with the measured data. The base (electron) diffusion length 

extracted at this point was 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 in the heteroepitaxial InP/Si cell. 

1.01 ' I ' I ' I ' I ' I 1' 

0 measured QE of 12% BOL InP/Si 

0.9 1 (30.1 mA sun-simulator Jsc) 

2 0.8 

'3 0.7 E 

0.6 E - 0.5 

6 0.4 

E 0.3 

- E 0.2 

E 

Q) 

=I 

c 

(P 

- 

Q) 

CI 

Le > We 

Sb: le4 cmls 

- Lb: 0.8pm 

- 

- cell 5854-3533 

*'- 1- SCR (10.1 mA) - - - - - - 

400 500 600 700 800 900 1000 

Wavelength (nm) 

Measured (black dots) and model (solid line) quantum eficiency before irradiation for a 12% 

WP InP/Si cell. Solid line (equivalent to 30.3 mA AMO) is the sum of the three dotted lines, 

representing contributions from the base (16.6 mA), depletion space-charge region (SCR) 

(70.1 mA), and the emitter (3.6 mA). An electron base diffusion length of 0.8pm fit the data. 

-9 6- 
DIFFUSION LENGTH DETERMINATION 

Using the above quantum efficiency (QE) model, the predicted photocurrent was obtained versus 

the base diffusion length (Figure 2). This curve is 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). 

-; 35 

$ 

E 

2 

Y 

c 

r 

30 

0 

0 

s 

c 

n 

a 

I" 

ii! 

@ 

% -

!$ 20 

T 25 

w 

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

Base (Electron) Diffusion Length (prn) 

Figure 2 Model AM0 photocurrent for an WP /nP/Si cell vs. base electron diffusion length. 

The AM0 I-Vs for the 12% AM0 BOL N/P InP/Si cell were measured from no irradiation to an 

equivalent fluence of 7.7616 1 MeV electrondcrn2, here 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 Ruence using the non-ionizing energy loss (NIEL) 

For the source-cell distance used, the 3.9 MeV alpha flux was calculated as 1.036 a/crWs. The 

calculated equivalent 1 MeV electron flux was 6.45e18 electrondcm2/s. Table 1 shows the equivalent 1 MeV 

fluence, the measured photocurrent, and the base diffusion lengths obtained from Figure 2. 

Table I FIuence vs. Measured AM0 Photocutrent and Estimated Base Diffusion Lengths 

-9 7- 
A standard empirical model for diffusion length versus fluence5 was fit using Table I data (Fig.3). 

The model parameters are the unirradiated electron diffusion length Lo (0.8pm, Fig. 1) and K, the damage 

coefficient (48-8). The fit is good except at the highest fluence; this may be due to radiation damage carrier 

removal effects changing the width of the space charge region; this effect was not included in our modeling. 

1012 10'3 1014 1015 1016 1017 

Eqv. 1 MeV Electron Ruence 4 (#/cm2) 

Figure 3 Estimated base electron diffusion length vs. fluence for an NIP InP/Si cell 

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 the 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. 

mkasured QE 0112% B~L inphi 

now 8%(25.7 mA sun-simulator Jsc) 

after 7.7e16 1 MeVlelelcm2 

400 500 600 700 800 900 1000 

Wavelength (nm) 

igur@ 4 Measured (black dots) and model (solid line) quantum efficiency a..d 7.7e MeV 

electrons (12% BOL N/P InP/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 (1 1.9 mA), 

depletion space-charge region (SCR) (10.1 mA), and the emitter (3.6mA). An electron base 

diffusion length of 0.3pm fit the data. 

-9 a- 
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 0.3pm after an extremely high 7.761 6 1 MeV electron fluence (measured 8% AM0 efficiency, 25.7 mA 

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 photocollection from the base of the cell. 

REFERENCES 

1. S.J. Wojtczuk, N.H. Karam, P. Gouker, P. Colter, S.M. Vernon, G.P. Summers, R.L. Walters 

and R. Statler, 'Development of InP Solar Cells on Inexpensive Si Wafers," Proc. of Ist 

WCPEC (24th lEEE PVSC), 1994, pp. 1705-1 708. 

2. 

3. 

H.J. Hovel, Solar Cells, Vol.11 Semiconductors and Semimetals, 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", I€€€ Trans. Nuclear 

Science, 40, 1993, pp. 1372-1 379. 

4. G.P. Summers, R.J. Walters, 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", 

Roc. of Ist WCPEC (24th /E€€ PVSC), 1994, pp. 2068-2073. 

5. H.Y. Tada, J.R. Carter, B.E. Anspaugh, and R.G. Downing, Solar Cell Radiation Handbook, 3rd 

Ed., JPL Publication 82-69, p. 3-1 9. 

-9 9- 
ELECTRON AND PROTON DAMAGE IN In,~,Ga,As SOLAR CELLS 

HAVING AN InP WINDOW LAYER 

Scott R. Messenger 

SFA, lnc. 

Landover, MD 20785 

Hector L. Cotal and Robert J. Walters 

Naval Research Laboratory 

Code 661 5, 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/ln,,,,Ga,.,,As 

tandem solar cells, ln0.53Gao.47A~ (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 ,urn 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 InP/ln,.,Ga,,As 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, AMO, conversion efficiencies as high as 21.1 % and 22.2% at 25OC were measured for monolithic, twoterminal tandems with total areas of -4 cmz and - 1 cm2, respectively, for unoptimized cells(ref.1). The 

efficiency could be increased to -26% when fully optimized(ref.1). Several radiation studies have shown that 

the InP/lnGaAs tandem cell displays superior radiation resistan~e(refs.2~3). Anew program is now underway 

involving NRL, ASEC, RTI and NREL to grow the InPllnGaAs 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. 

-1 00- 
To optimize the radiation degradation of InPllnGaAs, 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/lnGaAs 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 cm2 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,ll). 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 = 1 3.6 mm2) were fabricated by the Research Triangle Institute (RTI) 

while the p/n cells (A = 25 mmZ) were grown by the National Energy Renewable Laboratory (NREL). Both cell 

types had a base doping concentration of 2xlO” cm-’. 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 GPlB operation with a 

QuickBasic program. The measurement errors are expected to be less than 1 %. 

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 2.88~10-~ and 2.33xlO-’ MeVcm2/g, 

respectively. The method of calculating NIEL values has been outlined in several prior publications(refs. 1 1,121. 

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 

-1 01 - 
determine the electron to proton dose equivalency ratio (Rep) as described in reference 9. Rep is defined as the 

ratio of the dose along 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, Re, 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 (q) as an example): 

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 (%I, and D,= 1 .433x108 (MeV/g). The solid line in Figure 2 shows the characteristic 

degradation curve for n/p InGaAs given by Eq. (1 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 (2x1 O'7cm-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.13). 

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 NlEL of the cell material for the particle and energy of interest. 

A spectrum of particle energies, such as that produced by a Co*O 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 Walters et ai. 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/lnGaAs tandem cell. The-polarity independence gives us considerable flexibility in optimizing 

the final design for the InP/lnGaAs tandem cell. 

-1 02- 
REFERENCES 

1. M.W. Wanlass, J.S. Ward, K.A. Emery, A. Duda, and T.J. Coutts, "Improved Large-Area, Two-Terminal 

InPln,~,,Ga,,,As Tandem Solar Cells", Proceedings of the 1 st IEEE World Conference on Photovoltaic Energy 

Conversion, Waikoloa, HI, 1994, pp. 171 7-1 9. 

2. H.L. Cotal, R.J. Walters, S.R. Messenger, and G.P. Summers, "Performance of Proton- and ElectronIrradiated, Two-Terminal, Monolithic InP/ln,,Ga,,As Tandem Solar Cells", Proceedings of the 1 " IEEE World 

Conference on Photovoltaic Energy Conversion, Waikoloa, HI, 1994, pp. 2088-95. 

3. 

Monolithic InP/ln,.,,Ga,,,,As Tandem Solar Cells", J. Appl. Phys. 77, 21 73 (1 9951. 

H.L. Cotal, R.J. Walters, G.P. Summers, and S.R. Messenger, "Electron Irradiation of Two-Terminal, 

4. 

Effect of Dopant Density and Cell Thickness", J. Appl. Phys. 73, 60 (1993). 

C.J. Keavney, R.J. Walters, and P.J. Drevinsky, "Optimizing the Radiation Resistance of InP Solar Cells: 

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, 1 989, pp. 

149-58. 

6. S.R. Messenger, R.J. Walters, M.J. Panunto, and G.P. Summers, "Radiation Effects on p'n Junctions 

Grown by MOCVD", Proceedings of the 1 3'h Space Photovoltaic Research and Technology Conference, NASA 

Lewis Research Center, Cleveland, OH, 1994, pp. 167-80. 

7. R.J. Walters, G.J. Shaw, G.P. Summers, and S.R. Messenger, "The Effect of Base Dopant Level and 

Thickness on the Radiation Response of Ga,ln,~,As Solar Cells", Proceedings of the 5'h IEEE International 

Conference on Indium Phosphide and Related Materials, Paris, France, 1993, pp. 600-4. 

a. S.R. Messenger, R.J. Walters, H.L Cotal, and G.P. Summers, "Radiation Degradation in In,Ga, 47A~ 

Solar Cells", Proceedings of the 6'h IEEE International Conference on Indium Phosphide and Related Materials, 

Santa Barbara, CA, 1994, pp. 504-7. 

9. G.P. Summers, R.J. Walters, 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, 1 994, pp. 2068-75. 

10. G.P. Summers, E.A. Burke, P. Shapiro, R. Statler, S.R. Messenger, and R.J. Walters, "The Use of 

Displacement Damage Dose to Correlate Degradation in Solar Cells Exposed to Different Radiations", 

Proceedings of the 1 3'h 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. Walters, G.J. Shaw, P. Shapiro, R. Statler, and S.R. Messenger, "A 

General Method for Predicting Radiation Damage to Solar Cells in the Space Environment", Proceedings of the 

23rd 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. Walters, "Damage Correlations in 

Semiconductors Exposed to Gamma, Electron and Proton Radiations", IEEETrans. Nucl. Sci. 40, 1372 (1 9931. 

1 3. 

Devices", Ph.D. Dissertation, University of Maryland Baltimore County, 1995. 

S.R. Messenger, "Electron and Gamma-Induced Displacement Damage Effects in InP Semiconductor 

-1 03- 
1 MeV Ele 

3 MeV Protons 

t i 

Particle Fiuence (particle/crn") 

Figure 1 Degradation in n/p JnGaAs solar cell efficiency (1 sun, AM01 due to 3 MeV proton and 1 MeV electron 

irradiation as a function of particle fluence. 

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 R,, and a characteristic radiation curve is obtained. 

-1 04- 
n 

s 

Y 

I..., I I I !,I I 

1 014 10’~ 1 o’6 

1 MeV Electron Fluence (ekrn2) 

Figure 3 Efficiency degradation of n/p and p/n InGaAs solar cells. The degradation is shown to be polarity 

independent. 

-1 05- 
Photo-Recovery of Electron-Irradiated GaAs Solar Cells1 

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 coverslides to 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-circuit current (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 flight data. 

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 

SSR = single-sided coated coverslides (coating contains both UVR and IRR in the 

same narrow-band-pass multilayer coating) 

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-layercoated, 

coverslides. Covered cells have been tested for degradation from the humidity, thermal, solar UV, and radiation 

environments representative of nearearth 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 W test, several covered INTELSAT-6 silicon solar cells (designated as 1-6) were included. 

Solar cells change their spectral response with particulate irradiation. In the case of GaAs cells, the 

response degradation is highest at 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-radiationdamage 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 

Flight Center. The final analysis and paper presented here was funded by H?Consultants. 

This paper is based on work performed at COMSAT Laboratories under contract from Goddard Space 

-1 06- 
Silicon solar cells have demonstrated an effect called "Photo-Redegradation." This effect shows up in 

cells that have been electron irradiated and then exposed to light for an extended period: the cells degrade 

beyond the point that was measured right after irradiation. When first discovered and studied in depth (in the late 

 O OS), the effect was determined to saturate rather quickly and to appear primarily in float-zone refined material. 

The procedure developed at the time was to anneal and stabilize the cells after irradiation with an overnight bake 

at 60°C under flood-lamp illumination. 

In recent years, photo-redegradation has also been observed in cruciblegrown 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 carried out at the National Institutes of 

Standards and Technology. Cells were irradiated, in various combinations, at 2, 4, and 4~10'~ e-/cm2 to provide 

data points at 2, 4, 6, 8, and ?OX?O'~ e-/cm2 levels. Selected cells were removed from the test at different points to 

provide UV-irradiation test samples at the radiation levels more likely to be encountered in common missions (4- 

6~10'~ e-/cm2). The primary purpose of the electron irradiation was 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. 

0.95 

0.9 

0.85 

0.8 

0.75 

0.7 

0.65 

I \ I '. '. 

II 

1 MeV Electron Fluence (xle141cm2) 0 

+ IRR avg (4,2: 

SSRavg 

(4s3) 

bare avg (5) 

- - - - barel 980's 

cel Is, J P L 

Hand book 

Figure 1 Relative Degradation of Short Circuit Current for 1 MeV 

Electron Irradiated GaAs Cells 

-1 07- 
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 1014 e-/cm2 respectively (e.g., 5.2 indicates 2 out of 5 cells were irradiated past 4 x 1014 e-/cm2). The 

data plotted at "4.5" and "8.5" x 10" e&m2 are values remeasured after a -24 hour, 6VC, thermal anneal 

following the irradiations to 4 and 8 x I 014 e-/cm2 respectively. 

The dashed curve in Figure 1 is taken from the JPL Solar Cell Radiation Handbook, Addendum 1: 1982- 

1988 and represents bare GWGe 'solar cells from this period. The curve is quite different from the present 

coveredcell 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 (4 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 correction is 

inadequate at the higher fluences (the difference in slopes also eliminates a simple dosimetry error). Furtnermore, 

bare cell results from this batch, irradiated at 4E14, also were significantly lower 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 highdegradation 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 1014 e-tcm2, two cells at 4 x 10" ekm2, and one cell at 4 and 8 xlO'* dcm2, is internally consistent and clearly different from all but one individual 

cell in the 4 other groups of cells. Values of Ise for the three cells, irradiated to 4 x 1014 e-/cm2, are within f 0.03. 

Cell variation is therefare not enough to explain the difference with the other coverslide types. The data does not 

depend upon coverslide orientation. 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 / lo, where lo 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 (Mo / Idlco) 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, lo, is amcially placed 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 prior to 

beginning the UV test exposure.) 

Most of the cells in this second of two W 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. 

-1 08- 
The most unexpected feature of Figure 2 is the recovery in Isc 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 lsc for the pre-irradiated cells. 

I a, 

0-l 

t 2 0.98 

0 

a, 

> 

'E 0.97 

- m 

2 

Oag6 i 

0 

8 

*o* 

0 0 

* 

A 

A A 

56B2-r (SSR, 4E14) A 

A 

A A 

b 0 56A2-r (SSR, no rad) i'A 58B-r (DSR at 4E14) 

A 58A-r (DSR, no rad) 

* :j 

4 * 

0 4 

A 

0.95 , I , 6 # .$ ,",", 

1 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 ERR 

cells from the same test. This time, the unirradiated cell data are represented by faed 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 wellcharacterized in past UV tests. 

it is clear that qualitatively the pre-irradiated 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" 

cunre, 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 ERR covered cells would be close to that 

of the irradiated "SSR + recovery" curve, since the BRR and SSR W 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 1 and Test 2 for unirradiated-ERR cells. These cells therefore appear to have an 

unusually high recovery from electron irradiation (45% at 3000 UVSH, rather than 3%). 

-1 09- 
1.03 

1.02 

1.01 

- 1.00 at 

fY 

at 

0 

0 

- v) 

.- 9 

.Id 

-5 0.99 

0.98 -I 

0.97 

5 

0 

v) 

c 

_. 

.- 

- 

-1 

-: 

-1. 

-1 

-1 

I 

I 

"i. \ 

cr 

\ \ 0.96 1 I I I , .,. - I . 1 10 100 1000 10000 

UV Solar Hours 

- GaAs recovery/ 

i BRR-r4E14 

- - BRR-r no rad 

SSR-r4E14 

- - SSR-rnorad 

- - -SSR-r + recov 

i 

A DSR-r4E14 I 

! - - DSR-rnorad ' 

! - - . - DSR-r + recov j 

! - - - 1-6 no rad I 

Figure 3. UV Degradation and Electron-irradiation Recovery for BRR, SSR, and DSR 

Covered GaAs Solar Cells in Test 2. 

1.04 . 

1.03 j~ 398 (ARR at 6E14) 

- . - - - ARR norad + recovery 

g) 1.02 

5 

p 1.01 

d 

c 

(zI 

.- 

.Id 

- lu 

1 

0.99 

. 

8 en' ' 

C: 

Figure 4. UV Degradation and Electron-Irradiation Recovery for ARR 

Covered GaAs Solar Cells in Test 2. 

-1 10- 
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 1 data suggests 0.2% loss for a set of ARR 

cells). The ARR covered cell (with reversed coverslide) irradiated tcr 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). 

0 

v) 

c 

al 

0) 

r 

0 

al 

> 

9 0.98 

- 

.- 

6 0.99 

*- 

.cI 

8 

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 hours2. 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 

* 

** 

3-4 

80 

* * 

l-:+ A * 

eo 00 * 

0 * 

0 0 

I 0 0 

o 49A-r (IRR, no rad) 

31B (IRR at 6e 14) 

0 

-- 

c 

r 

0.97 1 1 I I , i 

1 10 100 1000 10000 

UV Solar Hours 

Figure 5. UV Degradation and Electron-Irradiation Recovery for 1 RR 

Covered GaAs Solar Cells in Test 2. 

The irradiated IRR covered cell in Figure 5, with correctly oriented coverslide at 3000 UVSH, shows only a 

0 - 1.5% increase in lsc relative to the unirradiated IRRcovered cells in Test 11. In that test, the unirradiated IRR 

data varied between 0 and 1% below the initial value. The 'IRR at 6E14" cell mimics the "ARR + 6E14" data of 

Figure 4 in that it displays an initial recovery 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 W degradation. 

The unirradiated IRR cell in Figure 5, with inverted coverslide, shows a significantly higher UV degradation 

than do the cells with correctly oriented IRR coverslides in Reference 1 (2% vs. 0 - 1% at 3000 UVSH). If this 

result is a consequence of the unfiltered W exposure to the multilayer coating, then the implication is that these 

layers are more sensitive to energetic radiation (UV with h 0.35 pm or 1 MeV electrons) than to the lower energy 

2 

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. 

The deviation begins earlier, if the 1000 UVSH data is lowered by 0.5%, or later, if the data cluster at 1500 

-1 11- 
UV that passes through the CMG coverslide. This sensitivity to energetic radiation could explain the higher loss in 

isc of the IRR-mvered cells from the electron irradiation than 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 this correction, 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 initial point. 

0 

m 0 

m f 

I 0 SSR6E14 

DSR 6E14 I@ I x 1-6 

X 

X 

+ 

X 

X 

0 

0 08O 

OT +. 

+ 

xx 

X 

X 

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 1-6 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 1-6 

degradation had been attributed to scattered UV light affecting the sensitive silicon cell optical stack. 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 possibilities to be considered. . 

If thermal vacuum recovery is real for the electron-irradiated GaAs cells, then, the normalization procedure 

should be based on the 1-6 cell rather than the irradiated GaAs cells in Figure 6. the consequences of such a 

renormalition 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 turnover, 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 minority carriers. 

-1 12- 
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. Nevertheless, there are two things to consider 

in this postulation of photo-recovery for electron irradiated GaAs solar cells 

1. 

expected. 

0 

The measured electron-induced damage in the GaAslGe solar cells involved in this test was higher than 

The degradation in Isc of the solar cells in this test was significantly higher than that seen in prior data (>14% 

vs. -1 1 % at 4 x 1 0'4 1 MeV electrons/cm* and >22% vs. 16% at 8 x 1 014 /cm2). 

The covered cells degraded further than did the bare cells of this test (2 - 8% more in Isc I lsco at 8 x 

ioi4rcm*). 

2. 

mind, the data is limited and statistics are poor. Nevertheless: 

0 

Since the test from which the photo-recovery data has been extracted was not designed with this study in 

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: 

0 

0 

The long-term photo-recovery is a significant portion of the total electron damage (-3 out of 15%). Nevertheless, 

it has so far only brought the 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 effect. 

The initial electron degradation for Isc of these recovering cells was -15%. 

No cells from this batch showed >I % recovery after irradiation when exposed to a floodlamp and 

60°C for -24 hours. 

REFERENCES 

1. A Meulenberg, 'Space Qualification of UV and IR Reflecting Coverslides for GaAs Solar Cells," Prac. of 

the 14th Soace Photovoltaic Research and Technoloav Conference, NASA I LeRC, Cleveland, OH, October 24- 

26,1995 

A Meulenberg, "UV Testing of INTELSAT VII, Vila, and Vlll Solar Cells," Proceed inas of the 13th Space 

Photovoltaic Research and Technoloav Conference, NASA I LeRC, Cleveland, OH, June 14-16,1994 

2. 

3. A Meulenberg, 'UV Testing of Solar Cells; Effects of Anti-Reflective Coating, Prior Irradiation, and UV 

Source," Proceedinas of the 12th Space Photovoltaic Research and Technoloav Con ference, NASA I LeRC, 

Cleveland, OH, 1992 

-1 13- 
INITIAL RESULTS FOR THE SILICON MONOLITHICALLY INTERCONNECTED SOLAR CELL PRODUCTt 

1. 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 pm. 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 28mAlcm2 at 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 

J( Minority carrier diffusion length required to be two times the layer thickness permitting efficient 

operation from poor quality or degraded lifetime material. 

I( The thin device design opportunity allows high open-circuit voltage. 

3 Light trapping leads to good light absorption. 

)t Good carrier collection leads to high short-circuit current. 

RESULTING IN 

d High Specific Power 

J High Efficiency 

J 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 12R losses, and 

minimization of the impact of shorts and opens in a deployed array. 

This research is supported by BMDO and managed by USASSDC through a current SBIR program, contract 

#DASG60-95-C-0007. 

-1 14- 
HIGH PERFORMANCE DESIGN 

its efficiency. They include: a thin base layer, high base doping, light trapping and front and back surface 

passivation. The thin base layer can be highly doped thereby increasing the output voltage without paying a 

penalty in collection efficiency; Le. current is maintained while the voltage is increased. This is due to the reduced 

dependence of collection efficiency 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 InP 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 [I, 21. 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 

plasmaenhanced 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 "V-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 bbproper 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 IO%.) 

This thin electrostatically bonded solar cell structure incorporates a number of features that will enhance 

-1 15- 
HIGH RADIATION TOLERANCE 

degradation mechanism of silicon solar 

cells deployed in space and results in 

reduced performance with time and 

depends on the particular 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 p 

m due to handling and yield concerns. 

silicon solar cells can occur if the active 

layers are thinner (20-35 pm). 

Modeling and experimental data 

showing the efficiency degradation of 

Radiation damage is the primary 

riprovements in the radiation tolerance of 

f % 

20 urn Si. 

0.7 theoretical 

--*.-- 

d 100 um silicon, ' 

theoretical \ 

experimental 

x ',A 

c 

.- A 55-76 urn silicon, 

w 0.4 x GaAs/Ge, 

al 

> 

rn 

.- - 

- 

2 

experimental 

InP. experimental ::: 

0.1 

X 

00 

1.00E+12 1.00E+13 1.00E+14 1 .OOE+15 1.00E+16 

1 MeV Electron Fluence 

Figure 7: Radiation resistance of space solar cells [3, 4, 51. 

candidate space solar cells as a function of 1 MeV electron fluence is shown in Figure 1. Thin, light trapped, 

silicon solar cells have a theoretical radiation resistance similar to InP solar cells and better radiation resistance 

than GaAs/Ge solar cells. Experimental and theoretical values for a 4-mil 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-canier lifetime caused by irradiation. The efficiency of the array is roughly independent of the minoritycarrier diffusion length until it is less than the thickness of the absorber layer. For silicon base layers on the order 

of 25 pm or less, this is equivalent to a minority-carrier lifetime of 250 nanoseconds (as-grown, non-irradiated 

silicon typically has a lifetime greater than 10 microseconds). In contrast, present high-performance silicon solar 

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 

attainable by the monolithicall 

interconnected solar cell the I R 

array losses will be minimized for 

any bus voltage when compared 

to conventional silicon solar cells. 

Utilizing a higher voltage also 

results in a weight savings at the 

Due to the higher voltage 

Y 

5 

i 

0 LL - 

12 24 36 48 60 

Bus Voltage 

(volts) 

- Conventional (0.5 volts) - MlSC (12 volts) 

Figure 2: Solar cell voltage effect on power loss. 

-1 16- 
array level from 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 [6], and monolithically interconnected solar cell arrays offers potentially higher 

yield compared to conventional array manufacturing processes such as welding [i'l 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, high voltage 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 

Corning glass. This device consists of twelve monolithically 

series interconnected segments. The I-V curve is shown in 

Figure 3. 

resistance and low shunt resistance. Shunt resistance is 11. 

approximately 20 ohm/cm2 and has limited both J,, and V,, of 

this device. High series resistance is the other limiting factor 

of this device. Figure 4 shows the gray 1-V response. It is 

III Fill - -, - -- - , - - - - _, factor is limited to 61.5% by the high series 

Figure 3. Light cuNe #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 V,. It is estimated at 

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,,, value. Lower resistivity bulk 

material is needed, in addition to a lower Rshaat 

emitter value. The device operation will be 

optimal when bulk material resistivity is 

approximately 0.3 ohm-cm, and emitter Rshcet 

value is approximately 45 - 50 ohmslsquare. 

A second device, #28, is 28-microns 

thick and is processed similarly to the previous 

I i , -I' ,I 

I i 

I I 

I 

10 - 

1- 

I , 

0.1 

h 

8. 0.01 

$ 0.001 

E 

m 

CI 

- 

0.0001 

0.00001 

0.000001 

240 ohm resistor 

0 0.12 0.24 0.36 0.48 0.6 0.72 

voc (Volts) 

Figure 4. Gray I-V curve (#56). 

-1 17- 
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 

1 E+01 

1 E+OO 

1 E-01 

1 E-02 

1 E-03 LI 

1E-04 

u f 1E-05 

$ 1E-06 

1 E-07 

1 E-08 

1 E-09 

1 E-1 0 

7 

,' , 1 I I 1E-11 ' , 1 1 

0.000 0.120 0.240 0.360 0.480 0.600 0.720 

voc (volts) 

characteristics. Fill factor has improved to 

64.5% due to reduced 12R losses in the base. 

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 Rshaet is nearly the same as 

#56 at 130 ohms/square, which is nearly three times the ideal value. The gray I-V curve, Fi ure 6, indicates good 

shunt resistivity (23k ohms total) and reasonable Jol and Jo2 values of 5 x 10l2 and 1 x 10- , respectively, for a 

silicon diode. The series resistance has improved to 110 ohms. Total series resistance is still limiting the fill-factor 

of this device, 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 quality would 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 900OC. Experiments are being conducted to reduce the temperature of 

this processing step 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 diffusion length, which is derived from the long 

wavelength response is 195 microns and 93 microns. This demonstrates a light trapping coefficient of greater 

than 7 and 3 respectively. Inconsistencies in the effective diffusion length can be remedied by optimizing the solar 

cell processing and carefully controlling the s@rting material specifications as related to specific process 

parameters. 

Figure 6. Gray I-V cuwe (FZ28). 

9 

-1 18- 
_-_-._.___ 

. - - . . . . - . - . - 

Solar Cell Thickness = 27pm 

Effective Diffusion Length = 195 pm 

- - 

350 450 550 650 750 850 950 1050. 

Wavelength (nanometers) 

I - -External QE - Internal QE - Reflectance I 

Figure 7. Spectral response curves of MlSC device #56 

s 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

100 

90 

80 

70 

8 60 5 

50 8 

5 

40 d 

s 

30 

20 

10 

0 

*I. 

100 

90 

80 

70 

30 

20 

10 

o! , 1 I ‘0 

350 450 550 650 750 850 950 1050 

Wavelength (nm) 

t- -€E;:’--nal QE - Internal QE - Reflectance I 

Figure 8. Spectral response curves of MlSC device #28 

-1 19- 
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 electrostatically 

bonded silicon to Coming's 1737 glass structures. Interfacial 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. 

available space quality ceria doped glasses and other glass compounds including a custom formulated ceria 

doped glass. Ceria 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 ceria, although imparting some initial tint 

to these glasses, makes them more stable and inhibits the formation of color centers. 

A high temperature survivable, custom ceria doped glass was formulated successfully by Sem-Com 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 Corning 

1737 glass we used to successfully fabricate the 10.3% and 1 1 .O% devices. The TCE (thermal expansion 

coefficient) of this glass is more closely matched to silicon at 33.5 x 1 O-7/0C than the 1737 glass which has a TCE 

of 37.8 x 1 O-7/0C. 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. 

High temperature survivability and optical transmission tests were conducted using various commercially 

100 

I 

I 

I 

90 

-o- Sem-Com Ceria doped glass 20.5 

80 .- 

ln .- 

E, 

w E 

+Sem-Corn Ceria doped glass 50.5 mil 

-w- Pilkington CMZ glass 8 mil 

+Corning 1737 glass 26.5 mil 

+ CMZ 80 glass 3 mil 

+Ce doped Corning 213 glass 5 mil 

-Coming 7740 glass 73 mil 

c 

s 70 

60 

Coming 7059 glass 20.5 mil 

50 

400 500 600 700 800 900 

wavelength (nm) 

Figure 9. Optical transmission properties of varioue glasses 

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. 

The change in output power of MlSC u.i?vice #CZ55 over 100 cycles from -80°C to +130°C was measured. 

-1 20- 
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 prototype of a lightweight, high-efficiency, 

monolithically interconnected solar cell array with the previously defined operating parameters. 

The advantages of utilizing this technology are: 

J High efficiency thindevice structure and light weight result in high specific power 

J Higher output power and better stability than amorphous or polycrystalline solar arrays 

Higher Reliability 

J High reliability interconnections 

I/ Radiation hard 

8 Higher Design Flexibility 

t/ Any size or voltage array can be fabricated cost effectively 

J High power, large area, cost-effective, array technology 

J Current and voltage configurable 

t/ Cost effective for high voltage in a small area 

J Arrays are practical for charging battery banks, including applications of voltages of 1 to 500 volts 

J Per-watt cost is independent of the number of interconnections and the array size 

t/ No labor/material cost as associated with cut and tabbed cells less than 1 00cm2 

FUTURE PLANS 

A preliminary process outline has been designed for production of large quantities of MISC devices for the 

terrestrial applications market. Various size wafers can be utilized and these cut to size in the final fabrication step 

to obtain the desired currenffvoltage output. This process has been cost analyzed and the initial results are quite 

favorable. AstroPower will direct market the space power applications. The use of high voltage solar cell arrays 

has been identified as a critical need for electric propulsion. This solar cell technology can enable electric 

propulsion for orbit raising missions with a considerable savings in launch cost. Secondly, the use of MISC 

technology for applications in concentrator solar cell arrays is very interesting due to the lower 12R 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. 

8 Higher Performance 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

REFERENCES 

Rand, R.B. Hall, and A.M. Barnett, "Light Trapping in Thin Crystalline Silicon Solar Cells", 21st IEEE 

Photovoltaic Specialist Conference, May 1990 page 263. 

Rand and P.A. Basore, "Light Trapping Silicon Solar Cells, Experimental Results and Analysis", 22nd IEEE 

Photovoltaic Specialists Conference, October 1991. 

Tada, J.R. Carter, B.E. Anspaugh, and RG Downing, "Solar Cell Radiation Handbook", JPL Publication 82-69, 

1989. 

Weinberg, D.J. Brinker, C.K. Schwartz, and R.E. Hart, "Progress in Indium Phosphide Solar Cell Research," 

SPlE Indium Phosphide and Related Materials for Advanced Electronic and Optical Devices Conference, 

1989, page 434. 

Anspaugh, "Solar Cell Radiation Handbook, Addendum l", 1989. 

J.R Woodyard, "Thin Film Cell Development Workshop Report", Proceedings of the Space Photovoltaic 

Research and Technology Conference, May 1991 I page 48-1. 

Smith and B.T. Cavicchi, "GaAs Solar Cell Circuit Assembly", 19th Photovoltaics Specialists Conference, New 

Orleans, May, 1987, page 448. 

-121- 
MOI OLlTHlCALL INTERCONNECTED GAAS SOLAR CELLS: 

A NEW INTERCONNECTION TECHNOLOGY FOR HIGH VOLTAGE SOLAR CELL OUTPUT 

L.C. 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 thin GaAs solar cell and epitaxial lateral 

overgrowth technologies with the application of monolithically interconnected solar cell (MISC) techniques. This 

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 12R losses, 

while properly designing the device allows for minimal shading and resistance losses. It is possible to obtain open 

circuit voltages as high as 67 voltslcrn of solar cell length with existing technology. The projected power density 

for the high performance device is 5 kW/m2 for 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. 

array will enjoy widespread application for space based solar 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. 

ISC array has several advantages which make it ideal for space concentrator systems. These are high system 

It is anticipated that the high efficiency, GaAs monolithically interconnected linear concentrator solar cell 

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 12R losses, while properly 

designing the device allows for minimal shading and resistance losses. 

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 

A schematic of the device is shown in Figure 1. This device is fabricated on a semi-insulating or surrogate 

-1 22- 
interconnect metal 

(base, emitter contact) 

GaAs or surrogate PECVD deposited Bragg reflector 

substrate / 

Figure I. 

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 12R array 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 [ 11, 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. 

GaAs monolithic array for linear concentrators. 

! 

0. - - - - - - - 4 - - 24 3s 48 60 72 84 96 108 120 

Qua Voltage 

(volts) 

Figure 2. Solar cell voltage effect on power loss. 

-1 23- 
Another reliability benefit from the 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 .O volt GaAs solar cell and a 12 volt rnonolithically 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: 

automated. 

e The cost of fabricating the array interconnections is reduced and the process can be 

e 12R losses are minimized 

e 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 

e 

0 

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 cm2 GaAs 

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 

zero grid width 

(prismatic cover) 

I/ 

600.00 

;. 500.00 O* --O -+ ---*--- 0- 

-0 -_ ,x/--x 7-X-X --x--x-- e- -0 _... x-- -x /’ 

50 micron grid width 

5 400.00 - 

3 300.00 - / E 

5 

-x Y I 

L 

0 0.05 0.1 

Active Area Width [cm] 

Figure 3. Predicted monolithic array performance. The 0% loss line 

is an ideal case which includes no shading or series 

resistance losses (AM, 15x1. 

considerable reduction of the 

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 mW/cm2 is 

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 voltdcm, and an active area 

width of 0.1 cm results in 

10 voltsfcrn. Higher voltages are 

feasible using photolithography to 

reduce the grid width. 

-1 24- 
The other cell parameters for this model are: base doping of 3x1 Oi7/cm3, base thickness = 1 micron, and emitter 

sheet resistivity of 18 ohmla. 

t trapping benefits of an 

ultra-thin GaAs solar cell b r. This reflector consists of 

alternating layers of silicon e silicon/silicon dioxide 

system offers significant ad 

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. 

and number of layers. Reflectivity data of a typical silicon/silicon dioxide Bragg reflector is shown in Figure 4. 

The monolithically interconnecte trator solar cell incorp 

ragg reflectors grown by MOCVD or MBE, foremost of 

The reflectivity spectrum of a Bragg reflector can be tuned for high reflection depending on the thickness 

100 - 

0 ----J 

350 450 550 650 750 850 950 1050 1150 

Wavelength (nm) 

Figure 4. Reflectivity of a two pair silicon/silicon 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. monolithicaily interconnected concentrator device, the interdevice semi-insulating regions can be 

situated directly over both the growth front and the via openings (see Figure 'I). 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. 

and dielectrics. Figure 5 shows a cross sectional photomicrograph of consecutive AlGaAs 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. 

AstroPower has extensive experience with the lateral overgrowth of GaAs and AlGaAs over both metals 

-1 25- 
A1.60Ga.4&S-) 

overgrowth layer 

AlGaAs DH 

LED structure 

Figure 5. Cross-section of AlGaAs layers laterally overgrowing a tungsten mask (1 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 6.T. CAVICCHI, “GA& SOLAR CELL CIRCU~T ASSEMBLY”, 19TH PHOTOVOLTAICS SPECIALISTS 

CONFERENCE, NEW ORLEANS, MAY 1987, PAGE 448. 

-1 26- 
HYDROGEN PASSIVATION OF NP AND P'N HETEROEPITAXIAL 

InP SOLAR CELL STRUCTURES' 

B. Chattejee, W.C. Davis and S.A. 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 limit the 

performance of heteroepitaxial InP cells by introducing shunting 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'n InP 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 both n'pjnd p'n heteroepitaxial InP 

cell structures from asgrown values of 5-7x10" an3, down to 3-5~lO'~cm . 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+n structures. In addition to being passivated, dislocations are 

also shown to participate in secondary interactions during 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 Zndoped InP to 1.12 eV for heteroepitaxial 

Zndoped 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 InPIGe and InP/GaAs, typically result in 

threading dislocation densities in tge range 1-10x108 ans within the InP layers, much higher than the 

theoretically predicted value of 10 crn" 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 andlor the electrical adivity of dislocations, including 

compositionally graded buffer layers, thermally cycled growth and hydrogen passivation (refs. 3.4). 

and Ge substrates demonstrated the effectiveness of a post-growth plasma hydrogenation treatment 

Our earlier work on hydrogen passivation of heteroepitaxial InP test structures grown on GaAs 

' Work supportedby NASA grant no. NAG3-1461. 

-1 27- 
which reduced the dislocation related deep level concentration from - 6~10'~ an3 to - 3~10'~ cm" in the 

InP layer (refs. 2,5,6). This passivation was stable up to - 550 'C, which combined with dopant 

reactivation occurring at -400 "c, opens a 150 'c 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 pointdefect-like behavior, significantly 

reducing the role of disloqtions 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 heteroepitm'al 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 InPAnP 

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 stmctures were grown by low pressure MOCVD in a reactor of 

our own design and construction as previously described [ref. 71. Precursor species of phosphine. 

trimethylindium (TMln), silane and diethylzinc were injected into the hydrogen camer gas. Reactor 

chamber pressure was maintained at 150 torr throughout the growth process. A typical InP on GaAs 

heteroepitaxial growth included a five minute, 620°C anneal in 1 .5 torr partial pressure phosphine prior to 

nucleation of a thin layer of InP at 550°C. The susceptor temperature was then raised to 620°C forth: 

remainder of the growth of the 34 pm th,'ck heteroepitaxial layers. A TMln flow rate of 14.5 pmol min' 

resulted in a growth rate of 0.61 nm sec- . Doping of the InP layers was achieved by injection of silane 

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 floy rate of semiconductor grade H2, chamber pressure of 750 mTorr, and 

power density of 0.08 W/cm . Substrate temperatures were varied from 250° C 275' C. Hydrogen 

exposure time was varied from 1.5 - 2 hours. Immediately prior to 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 during 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 metallization. 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 "C for 5 minutes. Metallization was performed in an e-beam evaporator 

using AuGeNi for n-type ohmic contacts and AgRdAg for p-type contacts (ref. 8). Test devices were 

isolated by mesa etching to an active junction area of 0.785 mm2. 

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 plnP might be expected to impede the indiffusion of hydrogen into 

the base region of the heteroepitaxial cell. Hence, hydrogen passivation was performed on cell 

-1 28- 
structures of both types to compare passivation effectiveness. Figure 2 shows dark I-V plots for the two 

cell types prior to hydrogen processing (as-grown) and after 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 st~ctures, indicating that V, for the former should be improved. 

Furthermore, in both cases, hydrogen is seen to significantly improve the reverse leakage current 

characteristics, shown in table 1. The 400 C anneal is required to reactivate the Zn dopants in both cell 

structures (confirmed by C-V measurements). 

Table 1. Reverse bias leaka e currents for various hydrogenation anneal conditions. 

Device areas are 0.785 mm . 

I, @ -lv n'p devices p+n devices 

As grown 1.39 mA 1.llmA 

hydrogen + 400 'C anneal 10.74 pA 5.21 pi4 

hydrogen + 602 'C anneal 1.25 mA 1.79 mA 

9 

hydrogen + 512 'C anneal 198 pA 991 pi4 

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 after annealing in excess of 500 C does the 

leakage current increase toward its original, non-passivated value. What is dea 

that neither the direction of the built-in electric field nor preferential trapping of h 

si~~i~cant~y imped desired indiffusion of hydrogen. However, table I indicates th 

exhibited a greate f thermal degradation. While the reasons for is are unclear, one possibility 

might be that Zn gettered by threading dislocations as inte which are s~unting the 

junction. We ha ed in an earlier work using photoluminescence that interstitial Zn is indeed 

gettered by dislocations in heteroepitaxial InP and we are currently pe ng SlMS and polaron 

estigate this possibility in these samples (ref. 9). suggests that a~emat~ve 

appropriate for heteroepitaxial p'n cells. 

Figure 3 shows the corres ing DLTS measurements for both the p+n and n"p struct 

confirm that in both cases, hyd passivating deep levels by lowering their concentration 

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 Ti (comprised of subpeaks T1A and T4B) that we have reported previously for heteroepitaxial p 

InP test structures as related to extended defects resulting from lattice-mismatch (either dislocation cores 

or point defect dusters) (ref. 2). The dislocation-related hole trap previously labeled T2 in the earlier 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 InP/GaAs interface. The DLTS spectra of the p+n structure indicates passivation 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 thebLTS spectra in detail to determine the relationship of these 

states with the presence of dislocations and associated defects, and with our results on heteroepitaxial p 

type InP. 

One concern 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 period studied here. Light soaking 

experiments will be performed on actual cells (Le. with a grid pattern) that are currently being fabricated. 

-1 29- 
3.2 Hydrogen diffusion in InPlGaAs and InPllnP cell structures 

structures. Our earlier work on specially designed test st~dure~ which incorporated a pn fundion buried 

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 heteroepitaxial 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. IO). 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 acceptors, before and affer 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 passivation than its homoepitaxial 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). 

One of the key issues for hydrogen passivation is the depth of hydrogen diffusion into the cell 

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 InP, Zn 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 bond 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 (ref. 9). 

Zn-H reactivation kinetics for homoepitaxial and heteroepitaxial ptype InP are compared below. 

In the absence of an applied electric field, hydrogen-zinc complexes dissociate at temperatures 

close to 40OoC. Such high temperatures are necessary to (I) 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 series 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 temperature (70 - 120 "C) 

and time (5-30 min ). Following the analysis of Zundel et ai., the inactive carrier concentration, NI, which 

is the concentration of carriers removed by passivation, is plotted in figure 6 according to 

NI = NI - N~exp(-vd) 

where Ni is the initial carrier concentration of the as grown sample, NH is carrier concentration at the 

chosen depth after hydrogenation, vd 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 

where EI, is the dissociation energy of the zinohydrogen complex First, the annealing experiments were 

done for different time periods at a fixed annealing temperature. Then these experiments were repeated 

-1 30- 
for other temperatures generating figure 7. The slopes of the lines in figure 7 when plotted against the 

annealing temperatures gives the dissociation energy 6 which is shown in figure 8. Anaiysis of the 

homoepitaxial data gives a value of 1.19 eV for EO., consistent with previous reports by other 

investigators for zinc acceptors in homoepitaxial InP (ref. 12). However for heteroepitaxial Zndoped InP, 

we calculate ED to be 1.1 2 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 electric and strain fields surrounding the dislocations aid the removal process of hydrogen 

from the acceptors once hydrogen is thermally liberated, thereby lowering the apparent dissociation 

energy. DLTS measurements shown in figure 9 support this notion, where additional reduction, or 

passivation, of the Tl (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 ea$ 

deep levels was detected for at least 24 hours due to heating at 80 

of hydrogen diffusion, and Zn reactivation kinetics with regard to th 

performed. It was found that dislocations aid the hydrogen passivation p 

diffusion paths and by lowering the apparent dissociation energy o 

processing hydrogen-passivated cells using the procedures present 

future. 

reactivation of 

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 Passivation and its Effects on Carrier 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, "Fabrication of n'/p InP 

Solar Cells on Si Substrates," Appl. Phys. Lett., vol. 54, pp. 11 39-1 141, 1989. 

4. 1. Weinberg, C.K. Swartz and D.J. Brinker, "Heteroepitaxial InP Solar Cells on Si and GaAs 

Substrates," Tech. Digest Int. PVSEC-5, pp. 81-84,1990. 

5. 6. Chatterjee, S.A. Ringel, R. Sieg, R. Hoffman and 1. Weinberg, "Hydrogen Passivation of 

Dislocations in InP on GaAs Heterostructures," Appl. Phys. Lett., vol. 65, pp. 1-3, 1994. 

6. S.A. Ringel, 6. Chatterjee, R.M. Sieg, E.V. Schnetzer and R. Hoffman, Jr., "Material Quality and 

Dislocation Trapping in Hydrogen Passivated Heteroepitaxial InP/GaAs and InP/Ge Solar Cells,' Proc. 

1st IEEE WCPEC, pp. 2204-2207,1994. 

7. "High Efficiency InP Solar Cells from Low Toxicity Tertiatybutylphosphine and Trimethylindium by 

OMVPE", R.W. Hoffman Jr., N.S. Fatemi, D.M. Wilt, P. Jenkins, D.J Brinker and D.A. Scheiman, Proc. 

1st lEEE WCPEC p. 1882,1994. 

-131- 
8. N.S. Faterni, V.G. Weizer, D.M. Wilt and R.W. Hoffman, "A Simple, Low Resistance Contact System 

for Shallow Junction p'nn' InP Solar Cells That Preserves Emitter Integrity During Sintering, Proc. IEEE 

9. R.M. Sieg, B. Chatterjee and S.A. Ringel,"Evidence for Enhanced Zinc Interstitial Concentration in 

Strain-Relaxed Heteroepitaxial Indium Phosphide, "Appl. Phys. Lett. vol. 66, pp. 3108-31 10,1995. 

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 Si," J. Appl. 

11. W.C. Dautremont-Smith, J. Lopata, S.J. Pearton, LA. Koszi, M. Stavola and V. Swaminathan, 

"Hydrogen Passivation of Acceptors in p-InP," J. Appl. Phys. vol. 66, pp. 1993-1996,1989. 

12. S.J. Pearton, W.S. Hobson and C.R. Abemathy, "Dissociation Energies of Acceptor-Hydrogen 

Complexed in InP,' Appl. Phys. Lett. vol. 61 ,pp. 1588-1 590,1992. 

13. T. Zundef and J. Weber, 'Dissociation Energies of Shallow-Acceptor-Hydrogen Pairs in Silicon," 

Phys. Rev. B. vot. 39, pp. 549-552.1989. 

WCPEC, pp. 2129-2132,1994. 

Phys. VOI. 65,pp. 347-353,1989. 

z .- 

t 

m 

I I 

* electron VI 

* 1E+17 4 I 

0 0.5 1 1.5 2 2.5 3 3.5 

Depth [ urn] 

la) 

n+-BSF I n-base n+contact 

layer 

-* 

......... 0. 

. ............. 1 

0 0.5 1 1.5 2 2.5 3 

Depth [urn] 

Figure 1. Electrochemical C-V profiles of carrier concentration for (a) n*/p/p* and (b) p+ /n/n*/n' cell 

structures grown on p'-GaAs substrates used in this study. 

-1 32- 
Figure 2. Dark l-V characteristics of (a) np and (b) p*n structures. For each case, ad denote as-grown, 

1.5 hr. H-passivaion + 400 "C anneal, 1.5 hr. H-passivation + 51 2 "C anneal, and 1.5 hr. H passivation + 

602 "C anneal conditions. All anneals were for 5 minutes. 

100 150 200 250 300 350 400 

Temperature [KJ 

100 150 200 250 300 350 400 

Temperature [KJ 

Figure 3. DLTS spectra of for (a) n7p and (b) p*/n InP/GaAs device structures in the asgrown state and 

after a 2 hr. H-passivation exposure plus dopant reactivation anneal. 

-1 33- 
hydrogen. no REA 

1.00€+13 

0 

F 

5 

E 

2 

F 1*00E+'8 

LL Eo 

5 

c 

1.00E+17 

c 

E 

c 

1.00E+16 0 

ki 

'E 

(0 

1.00E+15 1 

1 .WE+ 12 

0 5 10 15 20 25 

AnnealingTime [hours] 

Figlrre 4. Stability of passivated deep levels against long term annealing in the dark at 80' C as obtained 

from DLTS peak heights of the annealed samples. 

--+ n+ emmer hydrogenated n+/p cell 

-- 

-- 

- homoepi 

I 

h rn 

Fiure 5. Electrochemical C-V Zn-acceptor profiles comparing passivation depth in heteroepitaxial and 

homoepitaxial n+/p structures after a 2 hour hydrogen plasma exposure. 

./ 

V; Bv ,-II 

T=lrn°C 

j - H2+ io mtn REA 

n+/p call 

?.00E+17 

c 

3 

E 

;-Asgrown 

1-2- 2 hr H2. no RBA 

.. I . ;-H2+5mtnRBA , E 

L 

'0 

0 

0 0.1 0.2 0.3 04 0.5 

bpih luml 

Figure 6. C-V doping profiles of heteroepitaxial no/ p InP/GaAs structures, for as grown and 

hydrogenated ( 2 h, 275'C) samples annealed at 100' C under -8V reverse bias for various time periods. 

The plots were obtained from C-V data. 

-1 34- 
.- Ei 

Y 

V 

LL 

g! 0.1 

L 

3 

a 

8 0.01 a 

Q, 

.- % 

U 

0 

m = 0.001 

0 5 10 15 20 25 30 

Annealing Time jmin] 

Figure 7. Inactive aEeptor 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. 

-1 35- 
1 OOO/T(K) 

Figure 8. Arrhenius plots of the dissociation frequencies of Zn-H complexes in homoepitaxial and 

heteroepitaxial n*/ p devices. The dissociation energies are indicated in the figure. 

7.00E+12 T 

6.00E+ 12 

5.00E+ 12 

e 4.00E+12 22d 

Z 

n 

c3 

- 3.00E+12 

2.00E+12 

1 .OOE+ 12 

O.OOE+OO 

hydrogen, no RBA 

hydrogen + 5 min REA 

hydrogen + 10 min REA 

hydrogen + 30 min RBA 

1 05 150 200 250 300 350 400 

Temperature [I(] 

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'l p structures. 

-1 36- 
THE GROWTH OF LOW BAND-GAP lnAs ON (1 1 l)B GaAs SUBSTRATES' 

R. E. Welser and L. J. Guido 

Yale University 

New Haven, Connecticut 06520-8284 

Summary 

Growth on the (1 1 l)B orientation exhibits a number of advantageous properties as compared to the 

(100) during the early stages of strained-layer epitaxy. In accordance with a developing model of 

nucleation and growth, we have deposited thin (60 8, - 2500 A), fully relaxed lnAs films on (1 1 l)B GaAs 

substrates. Although thicker lnAs films are subject to the formation of twin defects common to epitaxy on 

the (1 1 l)B orientation, appropriate control of the growth parameters can greatly minimize their density. 

Using this knowledge base, InAs films up to 2 pm in thickness with improved morphology and structural 

quality have been grown on (1 1 l)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 [l], long wavelength light 

sources, detectors, and thermophotovoltaics. 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 Ill-V 

semiconductor heteroepitaxy, one must contend with the strain induced by both lattice constant 

mismatch at the growth temperature anddifferences in the rates of mechanical deformation during the 

cool-down .cycle. Several experimental techniques have been developed to minimize the impact of these 

phenomena (Le., compositional grading, strained-layer superlattices, and high-temperature annealing). 

However, in highly strained systems such as InAs-on-GaAs, three-dimensional (3-D) island formation and 

large defect densities (2 1 O8 cm'*) tend to limit their appiicabifity. In these particular cases, the surface 

morphology and defect density must be controlled during the initial stages of nucleation and growth. 

1 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). 

-1 37- 
Thin Film Evolution 

At the last SPRAT conference, we reported on a study of the evolution of lnAs islands on (100) and 

(1 1 l)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 (2-0) 

wetting layer is followed by the formation of discrete 3-0 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), TMln flow (fTMln), and AsH3 flow (fAsH3). 

Moreover, the density, geometry, and defect structure of the 3-0 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 3-0 

islands (Figure-1). Specifically, a 2-0 film forms more quickly with higher Ni and the larger w/h. 

2 U 

1 

1 o4 

000 

100 . 

10 

io5 io6 10' io8 io9 iolo lo1 

Island * Density (mi2) 

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 (k) at 

which this transition takes place can be expressed as = N-1'2(w/h)-1. The solid line assumes a wk - 7 

for the (100) orientation, while the dashed line takes wk - 100, which is more appropriate for the (1 ll)B 

substrate. 

-1 38- 
Featureless 250 8, 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 (1 1 l)B substrates can be 

attributed to the lower density, flatter geometry, and fewer threading defects of the initial islands on that 

orientation. In particular, (1 1 l)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 8, films 

on the (1 1 l)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 lnAs films on 

the (1 1 l)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 (1 1 l)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 

(1 1 l)B substrates. 

~~ ~~ ~~ ~ 

Ts (OC) fTEGa (seem> fAsH3 (seem) Hillock Density (cm-2) 

525 23 100 5 x 106 

600 23 100 5 x 106 

650 23 100 5x 103 

700 23 100 50 

600 140 100 1 x 108 

600 23 100 5 x 106 

600 23 500 1 x 106 

600 140 500 1 x 107 

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. 

-1 39- 
2-Step Approach 

With this in mind, we have implemented a 2-step procedure for the growth of thicker inAs films on 

(1 1 l)B GaAs substrates. In the first step, optimized conditions for the deposition of a specular, strainrelaxed 250 8, film of lnAs are employed. Next the growth is interrupted, and the conditions altered to 

approximate those ielding specular, lattice-matched GaAs films. In this way, the hillock denstty on latticemismatched 2500 x lnAs films has been reduced by over an order of magnitude. 

lnAs films grown on (1 1 l)B GaAs substrates employing this 2-step procedure still exhibit similar 

improvements in structural properties compared to lnAs films on (100) GaAs. Figure-2 shows the x-ray 

diffraction peaks from 2500 A lnAs films on (100) and (1 1 l)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 roughly twice that of the 

substrate, indicating a substantial degradation in structural quality. On the other hand, the film peak 

FWHM on the (1 1l)B is nearly the same as that of the substrate, suggesting an improved structural quality. 

Indeed, cross-sectional TE of these films indicates at least an order of magnitude lower threading 

dislocation density in the (1 1 l)B film. 

Conclusions 

We have produced atomically terraced, fully relaxed lnAs films on (1 1 l)B GaAs substrates with misfit 

dislocations largely confined to the epilayer / substrate interface. The advantages of the (1 1 l)B 

orientation stem from the shape of the initial lnAs islands, their density (which can be manipulated by 

adjusting growth conditions), and the nature of their defect structure. Thicker lnAs layers on the (1 1 l)B 

are susceptible to growth defects similar to those observed for GaAs homoepitaxy on (1 1 l)B substrates. 

However, by implementing a 2-step growth approach, thicker layers of lnAs films with reduced growth 

defect density have been grown on the (1 1 l)B orientation. Thus, the use of (1 1 l)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 (1 995). 

2. R. E. Welser and L. J. Guido, Proc. d SPRAT Xlli, 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. 1. Wang, Appl. Phys. Lett. 48,s (1986). 

6. P. Chen, K. C. Rajkumar, and A. Madhukar, Appl. Phys. Lett. 58,1771 (1991). 

-1 40- 
3 

h 

W 

cd 

.d u 

u 

H c 

A(20) (degrees) 

Figure-2 X-ray diffraction data from 2500 A films giown on (a) (100) and (b) (1 1 l)B GaAs substrates. 

-141- 
DESIGN OF A THREE-LAYER ANTIREFLECTION COATING FOR HIGH 

EFFICIENCY INDIUM PHOSPHIDE SOLAR CELLS 

USING A CHEMICAL OXIDE AS FIRST LAYER 

Jacques Moutot 

Cleveland State University, 

Cleveland, Ohio 441 15 

Mircea Faur 

Cleveland State University, 

Cleveland, Ohio 441 15 

Maria Faur 

NASA Lewis Research Center 

Cleveland, Ohio 44135 

Chandra Goradia 

Cleveland State University, 

Cleveland, Ohio 441 15 

Manju Goradia 

Cleveland State University, 

Cleveland, Ohio 441 15 

Sheila Bailey 

NASA Lewis Research Center 

Cleveland, Ohio 441 35 

ABSTRACT 

It is well known that the behavior of Ill-V compound based solar cells is largely controlled 

by their surface, since the majority of light generated carriers (63% for GaAs and 79% for InP) 

are created within 0.2 ym 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 pn 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 Ill-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 antireflection (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 HNO3, o-H3P04 

and H202. This oxide has the unique properties of passivating the surface as well as serving as 

a failly efficient antireflective layer yielding a measured record high AMO, 25OC, open-circuit 

voltage of 890.3 mV on a thermally diffused InP(Cd,S) solar cell. Unlike conventional single 

layer AR coatings such as ZnS, Sb2O3, Si0 or double layer AR coatings such as ZnSIMgF2 

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 ln(P03)3 grown oxide as the first layer and AI203 , MgF2 or ZnS, MgF2 

as the second and third layers respectively, so as to yield an overall theoretical reflectance of 

less than 2%. 

Since chemical oxides are readily grown on Ill-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 Ill-V compound-based solar cells. 

INTRODUCTION 

Calculations show that the majority of light generated camers in direct bandgap Ill-V 

compound semiconductor based solar cells, are created within 0.2 pm of the illuminated surface 

of the cell(1). For such materials, surface recombination velocity (SRV) and p/n junction depth 

-1 42- 
considerations are of critical importance in the design of high efficiency solar cells. Even though 

published values of SRV on InP and GaAs vary over a wide range, depending on the surface 

preparation and the techniques used for measurement(2), it is estimated that SRV values can be 

as high as 1.8~10~ ~m/s(~-~), even for epitaxially grown cells. Therefore, thin emitters and 

reduced SRV appear to be necessary conditions for the fabrication of high efficiency Ill-V 

compound based solar cells. A good surface passivation layer, ideally a grown oxide, will cause 

a significant reduction of the SRV without adding semiconductorloxide interface traps and oxide 

bulk defects which are inherent to deposited oxides, thus improving the overall performance of 

the Ill-V compound based solar cell. Yet another obvious but not often properly 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 

ln(P03)3/A1203/MgF2. This ln(PO3)~ oxide, which is described in detail elsewhere(3), is 

primarily designed as a surface passivating layer, and has yielded a record high AMO, 25OC 

open circuit voltage Voc of 890.3 mV, measured on a thermally diffused p'n InP(Cd,S) solar 

cell@). We also investigate a three-layer AR coating composed of In(P03)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 Ill-V compound semiconductor based solar cells, since 

chemical oxides are readily grown on most Ill-V semiconductor materials. 

EXPERIMENTAL 

The 0.8 cm x 0.6 cm p+n InP solar cells were fabricated usin the closed ampoule 

diffusion source. Diffusion temperatures were from 560° to 660 OC. The substrates were 

Czochralski (LEC) grown with EPD of about 5-7 x lo4 cme2. Diffusions were performed through 

chemically grown P-rich oxide cap layers. 

technique to diffuse Cd into n-lnP:S (ND-NA = 3.5 ~1016 to 3.1 x1017 cm- 1 ) with Cd3P2 as the 

Au-Ge was used for the back contact. The Au-Zn (0.1 8 to 0.3 pm thick) front contact grid 

was deposited by evaporation and defined using existing photolithography masks, designed for 

the n/p cell configuration. Because Au based contacts are known to penetrate into InP during 

sintering at 430 OC, 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 p3.5 pm) 

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-H~PO~, and H202, 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(P03)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 HNO and HF based etchants were found to 

InP, after rinsing the substrates in a HNO3 (1 5%) solution(4). 

Using the PNP etch, from low frequency EG-V measurements, we recorded a surface 

state density minimum (Nss) at the Cd-diffused p+-lnP/passivating layer interface as low as 2 

decrease the SRV to below 5 x lo5 cm/s, e.g. 1.7 x 10 3 for n+-lnP and 4.7 x lo5 cm/s for p+- 

-1 43- 
xl0lo emm2 eV" VI. About 40 nm was removed from the surface of a the p+n InP structure 

diffused at 660 OC (surface acceptor concentration: - 4 x 10l8 ~m-~; junction depth: - 3.5 pm). 

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 elsewherem. 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)g and has a bandgap of 6.8 i0.2eV(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, Sb2O3, MgF2 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 (1 988)(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 

external 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/MgF2, deposited on p+n InP solar cell surface leads to a net decrease of the open circuit 

voltage (Vo,-.) of the cell by as much as 50 mV due to an increase in SRV('). For that reason, we 

recommend a three-layer AR coating with ln(PO3)~ passivating oxide as the first layer, followed 

by either ZnS/MgF or A1203/MgF2. The ln(P03)3/A1203/MgF2 is a new optical system we are 

proposing for the p n InP solar cell. AI203 is known for its stability, its high dielectric constant, 

and its radiation tolerance (higher than that of commonly used Si02). Also, AI203 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/? .43, microsheetladhesive 1.53/1.43). 

% 

RESULTS AND DISCUSSION 

Table 1. and Table 2. give optimum designs of three-layer AR coatings using 

In(P03)3/ZnS/MgF2 and ln(PO&/AI203/MgF2 for different thicknesses of the first layer 

In(PO3)g and a AM0 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 

-1 44- 
ICL= 1- ; and IRL='I a 

4 4 

Jsc.I?=o(~)a~ J'a 

The results suggest that the ln(P03)3/A1203/MgF2 coating gives a lower ICL than 

In(P03)3/ZnS/MgF2 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)g oxide (300A) is about 

25% (curve b). In Fig. 3, it can be seen that a thick two-layered oxide 

In20 (-9OOA)/ln(PO3)3(-3OOA) gives a lower overall reflectance than a single layer of Si0 

(8001 thick). Its blue reflectance is lower than either SiO, Sb2O3 (750A thick), or a double layer 

of ZnS/MgF2 (not shown here). However, Sb2O3 has lower mid-spectrum reflectance. Our 

experiments show that single layers of SiO, Sb2O3 as well as a double layer of ZnS/MgF2, 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(q. 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)g (400A), AI203 (549A), and MgF2 (71a). 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 (-9OOA 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 Si0 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 n-cm2. The projected Jsc 

and Voc values of 37 mA/cm2 -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 InP solar cells where in 

addition to using the designed 3-layer AR coating, reducin the emitter junction depth to less 

efficiency, q=21.3% achie~able(~). 

than 0.3 pm, the specific contact resistance to less than 10' B n-cm, should make a solar cell of 

-1 45- 
CONCLUSIONS 

The dual problem of surface passivation and antireflection coating design of Ill-V 

compound based solar cells have been addressed. We have designed a three-layer optimized 

AR coating for p+n InP solar cell, which brings the average reflectance down from about 40% to 

less than 2% while at the same time significantly improving the Jsc and Vo, by passivating the 

top surface of the emitter. This has been accomplished by using a chemically grown ln(PO3)~ 

rich passivating oxide layer as the first layer of a 3-layer AR coating, with AI203 and MgFZ as 

the second and third layers. We believe that the significant front 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 Ill-V compound solar cells as well. 

REFERENCES 

M.B. Spitzer, B. Dingle, J. Dingle, and R. Morrison, "A review of Indium Phosphide 

Space Solar Cell Fabrication Technology", Proceedings of the 21 st IEEE Photovoltaic 

Specialists Conference, 1990, pp. 196-206. 

J.F. Wager and C.W. Wilmsen, Physics and Chemistry of Ill-V compound 

Semiconductor Interfaces, edited by Carl W. Wilmsen, Plenum Press, 1985, pp. 165- 

211. 

M.Faur, M.Faur, P.Jenkins, M.Goradia, S.Bailey, D.Jayne, 1.Weinberg and C.Goradia, 

"Study of Surface Passivation of InP," Surface and Interface Analysis, 15, 745 (1 990) 

P.Jenkins, M.Goradia, M.Faur, S.Eailey and M.Faur, "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 its Applications in the Fabrication of InP Diffused Junction Solar Cells," Surface 

and Interface Analysis, 21, 110 (1994) 

M.Faur, M.Faur, D.J.Flood, C).J.Brinker, C.Goradia, S.Bailey, LWeinberg, 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.Femt and M.Pitval, "The Passivation of InP by 

ln(PO3)~ 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.1 1 , 1 June 1988, pp. 2320-2332 

-1 46- 
Table I: Summary of 3-layer ARC design with ln(P03)3/A1203/MgF2 

Table 2: Summary of 3-layer ARC design with In(P03)3/ZnS/MgF2 

-1 47- 
Table 3: AMO, 25OC performances of selected p+n InP diffused solar cells measured at NASA 

LeRC. 

C 

parameters for the cell in row 6. 

Three-layer AR Coating Design 

with ln(P03)3 oxide as first layer 

20 

16 

I: ln(P03 13 -2nS-MgF2 

12 

b ln(P03)3 -AI203 -MgF2 

8 

4 

a 

aoa 400 = 500 600 700 800 so0 

Wavelength (nm) 

Fig. 1. Reflectance on optimized Three-layer AR coating using: a)ln(P03)3/ZnS/MgF2 b) 

ln(P03)dA1203/MgF2 
Reflectance on InP Surface 

60 

A8 

36 

24 

12 

0 

300 400 500 600 700 a00 800 

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, ln(P03)3/A1203/MgF2. 

Surface Reflectance of InP Solar Cell 

with one layer ARC 

70 

66 

A2 

28 

14 

0 

300 400 500 600 700 800 eo0 

Wavelength (nm) 

Fig. 3: Surface reflectance vs. wavelength of p+n InP solar cells using single layer AR coating of 

the passivating chemical oxide (ln203/ln(P03)3 -1 100/400~); Si0 (800A); and Sb2O3 (750A). 

-1 49- 
LIGHTWEIGHT, LIGHT-TRAPPED, THIN GaAs SOLAR CELL FOR SPACECRAFT APPLICATIONS: 

PROGRESS AND RESULTS UPDATE' 

b9 M.H. Hannon, M.W. Dashiell, L.C. DiNetta, and A.M. Barnett 

AstroPower, Inc. 

Newark, DE 1971 6-2000 IN 

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% (AMO, 1X) have 

been demonstrated for a 3 pm thick, 1 cm2 solar 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/rn2. Device parameters were 1.01 5 volts open circuit voltage, 80% 

fill factor, and a short-circuit current density of 29.54 mA/cm2. 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 sol; 

cell prototypes have been fabricated demonstrating an open circuit voltage of 1 volt for a cell base thickness of 1 

pm with a 0.5 pm 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. 

bonding agent 

dielectric 

GaAs emitter, 0.5~ 

GaAs base, 1.0 prn 

back surface 

mtlectorln-type contact 

AlGaAs window layers 

Figure 1. Ultra-thin GaAs solar cell with light trapping. 

'This research was supported in part by the Department of the Air Force and managed by Phillips Laboratory, 

Space Power and Thermal Management Division under SBlR contract #F29601-93-C-0188. 

-1 50- 
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 756°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. 

diffusion front 

Glass superstrate 

dielectric A 

p-type contact lines or 

\ 

p-bus barlreflector 

/ 

n-bus barlreflector 

,/ 

n-type contact lines or dots 

Figure 2. Elecfrosfafically bonded, all back contact, ultra-thin GaAs solar cell. 

The benefits of this device technology include the following: 

0 

0 

0 

0 

0 

0 

0 

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 

-1 51 - 
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 easil! 

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 thc 

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. 

.I 

I.. 

*I 

.. 1 Horz: 0.2 V/div Vert: 5.0 mA/div 

Figure 3. Current voltage measurement for ultra-thin device G 73907A. 
100 

Sylgard Silicone Adhesive (I-mil) 

Gallium Arsenide 

90 -- 

80 -- 

70 -- 

60 -- 

50 -- 

40 -- 

30 -- 

20 -- 

lo -- 

0- 

350 450 550 650 750 850 950 

WAVELENGTH (nm) 

0.9g/cm3 

5.32g/cm3 

350 450 550 650 750 850 950 

WAVELENGTH (nm) 

Figure 4. External quantum efticjency measurement for ultra-thin device G 13901A. 

Table 1. Weight contribution of the major solar cell components for the ultra-thin device (G13901A). 

Total Weight/cm2 I Density I I Weight 

Material Percentage of Total Cell 

Pilkington CMG glass (3-mil) I 2.554g/cm3 I 19.46mg 1 82% 

2.29mg 

1.85mg 

10% 

8% 

Typical values for the dark diode reverse saturation current densities for the GaAs solar cells are 

3x1 0-’9A/cm2 and 5x1 O~”A/cm2 for the diffusion and depletion region recombination components respectively. 

These current densities provide an indication of the junction quality, minority carrier 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 demonstratell 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/cm2. 

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. 

-7 53- 
90 , 

85 

80 

75 

lo 

65 

60 

Figure 5. 

-._ -._ ~________.__..._____.____.-..-_-~..--.... ---___ --..__ -- 

.- 

-- 

-. 

-~ 

.-.__. on GaAs substrate 

-thinned. vnth reflector 

External quantum efficiency of sample G 134058. 

Photographs of the ultra-thin, light trapped 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. 

Figure 6. Photograph of the front (a) and back (b) surface of the ultra-thin, light trapped GaAs solar cell. 

fabricated on the GaAs substrate is shown in Figure 7. Similar devices are being processed as ultra-thin, fight 

trapped solar cells. The results of six 1 cm2 devices processed from one 2x4 cm2 LPE growth are shown in 

Table II. The performance of these devices demonstrates the capability of the material to support large area 

devices. 

Fabrication of large area (8 cm2) devices is underway. A photograph of an 8 cm2 GaAs solar cell 

-1 54- 
Figure 7. Photograph of a large area (8cm2) GaAs solar cell. 

Table 11. Current-voltage characteristics of six 1 cm2 devices fabricated from one large area (8cm2) LPE 

growth. 

Efficiency (AMO, IX, 25°C) 

on GaAs substrate (%) 

17.29 

17.89 

16.98 

16.64 

18.20 

16.33 

17.14 

Efficiency (AMO, IX, 25°C) 

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 ultra-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: 0.89-3.92~10’~/cm~) areshown 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(Q-crn)-’). These measurements indicate that 

the resistance of the zinc diffused regions is minimal and the width of the back contact fingers can be reduced to 

less than 25 microns without hindering the performance of the solar cell. 
17- 

'do 05 1'0 1'5 20 215 io 3'5 To apn frml 

Figure 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 890'6, and a CTE of 6.0x104/K. The annealing point of this glass is approximately 650°C. 

Void-free, 6 cm2 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 voll 

have been demonstrated for a cell base thickness of 1 .O micron with a 0.5 micron emitter. In addition to 

completing 16 and 25 cm2 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% (AMO, 1X) have been demonstrated resulting in a specific power of 1020 Wlkg (with a 

3-mil cover glass) and a power density of 240 W/m2. The incorporation of light trapping has increased the extern; 

quantum efficiency of these solar cells in thebng wavelength range. Large area, electrostatically bonded, ultrathin GaAs solar cell structures have demonstrated survivability to 75OoC, 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 cm2 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. 

-1 56- 
REFERENCES 

1 Kelly Gaffney, “Air Force Activities in Space Photovoltaic Power System Technology”, Proc. NASA SDace 

Photovoltaic Research and Technoloov Conference, Cleveland, Ohio (1994). 

2. G. Lush and M. Lundstrom, “Thin film approaches for high efficiency Ill-V cells”, Solar Cells, 30 (1991). 

3. C.B. Honsberg and A.M. Bamett, “Light Trapping in Thin Film GaAs Solar Cells”, Proc. 22nd IE 

Photovoltaic Specialists Conference, Las Vegas, Nevada, (1 991). 

4. P.A. lles and F. Ho, “Technology Challenges for Space Solar Cells”, Proc. 24th IEEE Photovoltaic SDecialists 

Conference, (1 994). 

5. L.C. Olsen, G. Dunham, F.W. Addis, D. Huber, and D. Daling, “Electro-optical Characterization of GaAs Solar 

Cells, Proc. 19th IFEE Photovoltaic S~ec ialists Co nference , (1987). 

6. P.D. DeMoulin, C.S. Kyuno, M.S. Lundstrom, and M.R. Melloch, “Dark IV Characterization of GaAs p/n 

Heteroface Cells, Proc. 19th IEEE Photovoltaic SDec ialists Conference, (1 987). 

7 S.A. Ringel, A. Rohatgi, and S.P, Tobin, “An Approach Toward 25% Efficient GaAs Heteroface Solar Cells, 

IFEE Transactions on Electron Devices, vol. 36, No. 7, July 1989. 

-157- 
LABORATORY INSTRUMENTATION AND TECHNIQUES FOR CHARACTERIZING 

MULTJUNCTION SOLAR CELLS FOR SPACE APPLICATIONS* 

James R. Woodyard 

Wayne State University 

Detroit, Michigan 48202 

ABSTRACT 

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 with a calibration based on NlST scales. The quantum efficiency apparatus 

includes a photodiode calibrated using NlST scales and a monochromatic probe beam. The apparatus is used to 

measure the dependence of the absolute external quantum efficiency of solar cells at various forward-bias voltages 

including the maximumpower point under AM0 light bias. Quantum efficiencies of multi-junction cells were measured 

with both spectral-light bias and AM0 light bias. Measured spectral irradiances of the dual-source simulator were 

convoluted with cell quantum efficiencies to calculate cell currents as function of voltage. The calculated currents agree 

measured currents at the 1 % level. 

INTRODUCTlON 

MuH@mction solar cells are attractive for space applications because they can be designed to convert a larger 

fraction of AM0 into electrical power than single-junction cells. The performance of multi-junction cells is much more 

sensitive to the spectral irradiance of the illuminating source than single-junction cells. The design of huh efficiency 

multi-junction cells for space applications requires matching the optoelectronic properties of the junctions to AM0 

spectral irradiance. Unlike single-junction cells, it is not possible to determine the quantum efficiency of multi-junction 

cells using only a monochromatic probe beam. It is necessary to use a light bias because of the series nature of the 

cell structure. Burdick and Glatfelter (1) reported a measurement technique for multi-junction cells which employs a 

spectral light-bias technique. While the method is useful for understanding and improving multi-junction solar cells, it 

does not yield the absolute quantum efficiency under AM0 light-bias. Because of the non-linear nature of multi-junction 

cells, current-voltage characteristics under AM0 conditions cannot be calculated from measurements under non-AM0 

conditions using spectralcorrection methods. 

The goal of the research is to develop a solar simulator which approxjmates AM0 spectral irradiance, and 

laboratory instrumentation and techniques, for use in measuring the quantum efficiency and I-V characteristics of multijunction solar cells under AM0 powergenerating 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 

calibration traceable to National InStiMe of Science and Technology (NIST) scales. The calibrated spectral radiometer 

is used to measure the spectral irradiance of the light-bias beam and obtain an integral fit of it in two spectral regions 

to the World Radiation Laboratory (WRL) AM0 spectral irradiance data. The solar simulator produces a iight-bias beam 

which is used for current-voltage and external absolute quantum efficiency measurements. The quantum efficiency 

apparatus includes a photodiode calibrated using NIST scales and a monochromatic probe beam. The apparatus 

used to measure the absolute external quantum efficiency of triple-junction solar cells at various fomrard-bias voltages 

under both spectral-light and AM0 light-bias conditions. 

SOLAR SIMULATOR DEVELOPMENT 

The spectral irradiance of the solar simulator plays an important role in the characterization of multi-junction 

solar cells. Characterization techniques such as light I-V and light-biased quantum efficiency measurements require 

-1 58- 
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 monochromator and detector; the detector is a silicon photodiode and temperature stabilized. The 

instrument is computer interfaced for control purposes, and data acquisition, display and analyses. The spectral 

radiometer was calibrated with a type FEL 1000 W quartz tungsten-halogen standard lamp traceable to NIST scales; 

calibrations were carried out using the procedure specified in MBS Special Publication 250-20. The accuracy of 

measurements in the 350 to 900 nm range is believed to be better than 4%; the precision of consecutive spectral scans 

is better than 1 %. 

A model SSlOOO solar simulator manufactured by Optical Radiation Corporation (ORC) was used in this work. 

The light source in the simulator is a xenon high-pressure discharge lamp. The spectral 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 purposes in the figure by the solid 

line. The spectral irradiance of the 

simulator differs from 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 simulator is dominated by xenon 

lines. The differences in spectral 

irradiances of the ORC solar simulator 

when compared to AMO, while generally not important in characterizing 

single-junction solar cells, can introduce major errors in light I-V and 

quantum efficiency measurements of 

multi-junction cells. 

The solar simulator was modSpectral lrradlance of Slngle-Source Solar 

600 

460 

400 

E 360 

c 

T 

9 300 

3: 200 

0 E 260 

Gi 100 

j, - 160 

60 

0 

300 400 600 600 700 800 900 1000 1100 

Wavelength (nm) 

Simulator Cornparedto WRL AM0 1 - . !wRL+MO! . 1 . ‘1 . { 

and Podlesny-(2). In order to obtain a better match with the AM0 spectral irradiance at wavelengths above 700 nm, a 

600 W tunasten-halogen lamp and cold mirror were added to the solar simulator. A diagram illustrating the optics of 

the modi& simulatoci show in Figure 

2. The design includes two elliptical 

mirrors and a flat cold mirror to focus 

illumination from the tungsten-halogen 

and xenon lamps on an optical integrator; the design produces a hoiizontal 

light beam which is compatible with the 

hobntal optics of the spectral radiometer and quantum efficiency apparatus. 

The cold mirror was custom fabricated 

to transmit wavelengths greater than 

750 nm and reflect shorter wavelengths. The original folding mirror in 

the simulator was replaced with the 

cold mirror; it is mounted at forty-five 

degrees with respect to both the xenon 

and tungsten-halogen lamps. The cold 

mirror serves two purposes. it transCold Mirror 

/ 

TungstenHalogen 

Lamp 

L I 

Figure 2 Optics of DualSource Simulator 

-1 59- 
mits the xenon spectrum at wavelengths greater than 750 nm and reflects wavelengths less than 750 nm. The effec 

of the mirror on the spectral irradiance of the xenon lamp is to attenuate the xenon lines shown in Figure 1. The mirrc 

transmits the Spectral irradiance of the tungsten-halogen lamp wh wavelengths greater than 750 nm while reflectin1 

wavelengths Jess than 750 nm. 

The optics of the solar simulator, as delivered by ORC, incorporated a second folding mirror, Vicor beam spliie 

and Pyrex collimating lens. The Wcor beam splitter was used to produce an optical signal for the feed-back circu 

which stabilizes the xenon lamp current. The folding mirror and collimating lens produce a vertical beam focussed o 

a horizontal work surface. The folding mirror and lens were removed to produce a horizontal light beam. The Vicc 

beam sprier was replaced with a hghqualii quartz beam sprier. The modification in the optics also increased th 

W throughput of the simulator. Additional air cooling capacity was added in order to dissipate the power produced b 

the tungsten-halogen lamp. The electronics supplied by ORC with the simulator were used to power and control th 

xenon lamp. A D.C. power supply regulated at the 0.01 % level was added to the system for powering the tungster 

halogen lamp. The stability of the dual-source light beam intensity is at the 0.1 % level. 

The Spectral irradiance of the duakource solar simulator was fit to the VVRL AM0 spectrum by integrating an 

comparing the two spectral irradiances in two regions. The simulator and WRL spectral irradiances were integrate 

in two regions and compared. One region employed wavelength limits of 350 and 750 nm; the other region had limil 

of 750 and 900 nm. The limits of 350 and 900 nm were selected because the quantum efficiency of the triple-junctio 

solar cells investigated in thii work is negligible outside thii wavelength range. The wavelength limit of 750 was selecte 

because it is the bandpass of the cold mirror. Selection of the two regions in thii manner made it possible to obtai 

integrated spectral irradiance fits in each region almost independently of each other by adjusting the current in th 

corresponding lamp; most of the spectral irradiance in the 350-750 nm range is due to the xenon lamp while th 

tungsten-halogen lamp produces most of the spectral irradiance in the 750-900 nm range. 

The fit of ihe solar simulator spectral irradiance to the WRL AM0 spectrum was carried out using a procedur 

which included calibrating the soectral radiometer; adjusting the currents in the xenon and tungsten lamps; measurin 

the simulator spectral irradiance; integrating the measured and WRL AM0 

spectral irradiances in the 350 to 750 

nm and 750 to 900 nm ranges; calculating the percentage error in the integrated measured and WRL AM0 

spectral irradiances in the two wavelength regions: and repeating the 

process until the error in each of regions was less than 1 %. The spectral 

irradiance of the dual-source simulator cornpared to the WRL AM0 spectrum is shown in Figure 3 by the filled 

circles. Comparison of the spectral 

irradiances in Fgures 1 and 3 shows 

the dual-source solar simulator produces a spectral irradiance which is in 

significantly better agreement with the 

WRL AM0 specbum. The percentage 

difference between the integrated 

spectral irradiance of dual-source simulator compared to WRL AM0 in Faure 3 is +0.06% in the 350 to 750 nm 

wavelength range and -0.9% in the 

750 to 900 nm range. 

Spectral lrradlance of Dual-Source Solar 

Simulator Compared to WRL AM0 

300 

260 - 

7 g 200 

L, 100 

3 

150 

v 

-... 

60 

0 

300 400 500 600 700 800 900 1000 1100 

Wavalength (nm) 

Figwe 3: Spectral Irradiance of dual-source solar simulator compared 

with World Radiation Laboratory AM0 spectral irradiance. 

QUANTUM EFFICIENCY SYSTEM DESCRIPTION 

The quantum efficiency (QE) system was designed and constructed to cany out measurements undc 

three condib'ons: dark, spectrablight bias and AM0 light bias. A second personal computer is used to control the Q 

system, as well as data acquisition, display and analyses. The design permits quantum efficiency measurements d 

an accuracy better than 2% over the 400 to 1000 nm wavelength range. The probe beam intensity and calibrate 

-1 60- 
detector response result in reductions in accuracy outside this wavelength range. The major components of the system 

include two computerinterfaced Scientific Measurement System, Inc. Mon 27 monochromators; two computerinterfaced motorized filter wheels with filters for order sorting and spectral- as; Wgrade fused silica lenses and 

beam splitter; two magnesium fluoride coated AI mirrors; computer interfaced Stanford Research Systems model 

SR830 DSP lockin amplifier and chopper; 60 watt quartz-tungsten-halogen (QTH) lamp; and a computerinterfaced 

Hewlett-Packard model 6038A power supply for the QTH lamp. 

The two monochromators were mounted in tandem and used with the QTH lamp to produce a 

monochromatic probe beam for QE measurements of solar cells. Two monochromators are employed to reduce the 

stray-light level in the probe beam. Each monochromator has a ruled 1200 groovedmm grating blazed at 500. The 

subtractive mode is used for the physical configuration of the monochromators along with 4 mm slit widths to optimize 

light throughput The measured resolution of 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 

located next to the input optics is referred to as monochromator #.i in the following discussion; the other monochromator 

contains the exit slit and is referred to as monochromator #2. 

Optical components located in front of the entrance silt of monochromator #1 serve to focus an A.C. 

light beam on the entrance slit and provide order-sorting capability. The optics include a QTH lamp, collimating lens, 

beam chopper and filter wheel. The beam chopper and filter wheel are mounted next to the entrance slit of the first 

monochromator. The beam chopper is used in conjunction with the lock-in amplifier to detect the response of the test 

solar cell to the monochromatic A.C. probe beam. The filter wheel contains four long pass filters. The filters provide 

ordersorting of the light-beams passing through the monochromators. The filters serve to reject the nth order beams 

of wavelength Mn in the QE probe; the higher order beams can introduce large errors in QE measurements. 

The optical components at the exit slit of monochromator #2 produce two light beams from the A.C. 

monochromatic probe beam which passes through the exit slii. The beams are used to measure tCle absolute external 

QE of a test solar cell. The configuration of the optical components is shown 

in Fgure 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 exit slit of monochromator #2 

is split into two separate light beams by 

the beam splier. The light beam 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 

reference detector for the QE measurements. The photodiode calibration is 

traceable to NlST scales and permits 

determining the absolute number of 

photons incident on the test cell. The 

second light beam transmits the beam 

splitter and is incident on the flat mirror; 

it is reflected onto the concave mirror 

which focusses the beam on the test 

cell. The optics do not permit mea- 

\ Light-Tight BOX Cooling Air Exhaust Port n/ 

Test-Cell Holder 

D.C. tight Beam 

Concave Mirror 

Monochromator '#2 I / I\ 

Cooling Air 

Inlet Port A.C. Monochromatic - Probe Beam 

Solar Simulator tight Beam 

igure 4: Top view of optical components located at the exit slii of 

monochromator #2. 

surement ofhe reflected-light from the test cell. The QE values measured by the system and reported in this paper 

are absolute external quantum efficiencies. 

A D.C. light beam originating from &e solar simulator and passing through the filter wheel shown in Figure 4 

is incident on the test cell; it is coincident with the optical axis defined by the test cell and filter wheel. The spectral 

content of the light beam is determined by the filter wheel, Three positions on the filter wheel are used to hold filters 

which pass spectra for measuring QE of triple-junction cells under spectral-light bias condib'ons (1); each filter passes 

-161- 
a portion of the simulator spectrum for 3uming-on" two of the three junctions of a test cell. One position on the filter 

wheel is open and used for AM0 light-bias measurements; another position contains 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 

temperature air on the back of the cell mounted in the test-cell holder. Figure 4 shows the cooling air inlet port which 

is mounted oppb the testall holder. Air is exhausted through the cooling air exhaust port which is baffled to prevent 

ambient light from entering the light-tight box, The air flow is set to insure the active junction of the test solar cell is 

maintained at room temperature; the junction temperature is monitored by measuring the open circuit voltage of the 

test cell. Temperature probes are included in the system to monitor the ambient air temperature in the light-tight box 

and the temperature of the test-cell holder. 

The system was designed to minimize electrical noise. Electrical connection of the test and reference cells to 

a multiplexer and lockin-amplifier are made with BNC cables and connectors. Ground loops are minimized through 

the use of a single common ground for all the electrical components; all electrical components are also electrostatically 

shielded. The background current levels are less than 1 E-12 A and the AC. monochromatic probe currents of the order 

of 1 E-6 A The wavelength is scanned by stepping both of the monochromators in 20 nm intervals, and measuring and 

logging ten values of the current at each wavelength. The standard deviation of the ten current values is typically of 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 using a multiplexing circuit and lockin amplifier. 

Calibration of the QE system is accomplished following alignment of the optical components and carrying out 

two scans. One Scan is carried out with the calibrated detector in the position shown in Figure 4; a second scan is done 

with the calibrated detector positioned in place of the test-cell holder. Computer sofhware is used to calculate a 

calibration vector which is used in subsequent scans to determine the absolute external QE of solar cells mounted in 

the test-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 monitor in 

graphical form and in tabular form on the printer. The tabular data are also saved on the computer hard drive for 

archival purposes and subsequent analyses. Computer control of the fitter wheels, monochromators, QTH lamp power 

supply, and lockin amplifier makes it possible to measure QE over a selected wavelength range in about twenty 

minutes. The stability of the system for successive scans is at the 0.1% level. 

SlNGLEJUNCllON QE RESULTS 

The insbumentation 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 riiht bias at 24 "6 under 

short-circuit conditions are shown in 

Fgure 5 by the 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 p 

doped layer. The reduction in QE 

above 700 nm is due to the band gap 

of the intrinsic layer. The details of QE 

in the 400 to 700 nm range reflect the 

roles of the doped layers, intrinsic 

layer thickneSs and carrier transport. 

QE was measured at 24 OC using the 

AM0 light bias shown in Figure 3; the 

results are shown in Figure 5 by the 

open triangular symbols. The effect of 

AM0 light bias on QE is not dscemible 

on the graph. The measurements 

show that changes in the occupancy 

of the sub-bandgap states resulting 

from hsh carrier injedion levels do not 

81ng IeJu nction Q E VB W avelen gth 

1 .oo 

3 0.80 

c 

0 

0 E 0.60 

UJ 

- 

c 0.40 * 

a= 

=3 

a 

0 0.20 

0.00 

300 400 600 800 700 800 BOO 

Wavelength (nm) 

igure 5: Quantum Efficiency of a single junction a-Si:H alloy solar cell 

under dark and light-bias conditions at 24 "6, and under dark conditions at 

55 OC. 

-1 62- 
have a major effect on carrier transport and collection in single-junction aSi:H alloy solar cells. The light-tight box 

shown in Fqure 4 was heated to 55 "c and QE measured under dark conditions. The results of the measurements are 

shown bythe inverted triangular symbols in Figure 5. QE differences of th of a few percent are dscemible and 

may be attributed to the temperature coefficients of both the test cell and ca detector. The 55 "C measurements 

show the techniques employed are not critically sensitive to room temperature variations of the order of a few degrees. 

The effect of forwadbias voltage on QE for a singlejunction a-Si:H alloy solar 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 Figure 6. 

The measurements show QE is insensitive to forward bias from 0.0 to 0.5 V. 

The peak value in QE is 0.8 and occurs at 580 nrn. At a forward bias of 

0.6 V, approximately the madmumpower 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 opencircuit voltage. At 0.8 V, the peak in 

QE is 520 nm and the value is 0.08. 

An interpretation of the wavelength 

shii in the peak of QE shown in Figure 

6 is as the forward bias voltage is 

increased, the trapped charge in the 

intrinsic layer plays an increasing important role in skewing the electric 

field distribution towards the front of 

the cell; thii in turn shifts the peak in 

QE to a lower wavelength, and influences carrier transport and decreases 

carrier collection. 

Slngle-Junction QE Dependence on Voltage Bla8 

Wavelength (nm) 

igure 6: Three dimensional view of the quantum efficiency dependence 

on forward-bias voltage for a singlejunction aSi: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-circuit current, lsc (calc). The value of the 

calculated short-circuit current was obtained by convoluting the measured quantum efficiency values, QE (A , ), shown 

in Fiure 5 with the measured spectral irradiance values, SI( A, ), shown in Figure 3. The convolution was carried out 

over the 350 to 900 nm wavelength range. The calculated short-circuit current was obtained using the expression: 

Equation 1 I A Cou/o~s 

1239 Jou/esxnm /sc(ca/c)=- xi Q€( A, ) x S/( Ai ) x Ai x M [ 

where 81 is the wavelength, M the monochromator step width and A the cell area. The results of the convolution are 

shown in Table 1. 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 with NIST scales referenced to NIST publications 

a52194 and a51357; Isc (calc) was calculated using 

measured QE( A ,) and SI( h values traceable to NIST 

scales, as indicated in the preceding section. The agree- Cell Isc(meas) Isc(calc) Alsd Isc 

result from using good measurement techniques and X302 6.315 6.353 

instrumentation calibrated with scales traceable to NIST. 

The measurements charactetize the behavior of a singlejunction aSi:H cell under forward and light bias. They also demonstrate the validity of the measurement techniques 

for characterizing single-junction solar cells. These observations alos prove useful in understanding the behavior of 

triple- junction solar cells under various biasing conditions. 

Table I 

(%I 

0.6% 

ment between Isc (calc) and Isc (meas) is believed to (ma) (W 

-1 63- 
SPECTRAL-BIAS TRIPLEJUNCTION QUANTUM EFFICIENCY RESULTS 

QE measurements of triple-junction a-Si:H cells under short-circuit conditions were carried out using thc 

spectral light-bias technique (1). The structure of the triple-junction cells has been previously discussed (3). The lighl 

bias technique is based on selectively injecting carriers in cell junctions, and "turning-off' and "turning-on" junction3 

A junction is referred to as "turned-off when there is relatively lie optical injection of carriers in the junction; it i 

"hrmed-on" when there is a relatively large optical injection of carriers. When one junction is "turned-off, and the othe 

two junctions are "turned-on", the "turned-off junction limits the current in the cell. QE of a junction limiting the ce 

current can be determined using an A.C. monochromatic light beam, referred to as the probe beam. The carrier 

injected by the probe beam in the 7umed-off" junction increase the photoconductivity and produce an A.C. current whicl 

characterizes QE of the iunction under the conditions of the measurement. The injection of carriers in the junctions i 

determined by the sdal irradiance of 

the D.C. light beam and the optoelectronic properties of each of the three 

junctions. The spectral irradiance of 

the D.C. light beam used in the spectral 

light-bias technique is varied to selectively "tum-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 "turnssn" two of the three junctions. The first filter "turns-on" the middle and bottom junctions, the second 

filter "turns-on" the top and bottom 

filters, and the third filter 'Yurns-on" the 

top and middle junctions. Thus the first 

filter makes it possible to measure QE 

of the top junction, 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 aSi: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 arcles. The peak in QE 

of the top junction is about 0.54 at 440 

nm. QE ofthe middle junction is shown 

by the closed squares; it peaks at 600 

nm with a value of about 0.53. QE of 

the third junction 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 

investigated 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 intensities of the 

three D.C. spectral light-bias beams. 

Spectral-Light Bias TripleJunctIon Solar Cell QE 

1 .oo 

% 

4, 

0 

L: 0.60 

w 

E 

3 0.40 

r 

0 

a 

u 0.20 

0.80 

- 

- 

0.00 

400 500 600 700 800 900 

Wavelength (nm) 

Figure 7: Spectral light-bias quantum efficiencies of the top, middle and 

bottom junctions for an a-Si:H triple-junction solar cell. 

Effect of Spectral-Light BI.8 Intensity on 

1.00 I 1 I I I TripleJunctIon Solar Cell QE 

6 

c 0.80 

m 

e 5 0.60 

E 

a 

- 

3 g 0.40 

3 

0.20 

0.00 

400 500 600 700 800 900 

Wavelength (nm) 

Figure 8: Effect of spectra-light bias intensity on QE for an a-Si:H alloy 

triple-junction solar cell. 

-1 64- 
The role of the relative beam intensities was evaluated by measuring QE with an attenuated probe beam and comparing 

it with QE values obtained with an unattenuated probe beam. The results of measurements with a 50% probe beam 

attenuation are shown in Fgure 8. The symbols used to represent the various junctions are the same as Fiure 7. Filled symbols represent QE measurements with the unattenuated probe beam; the QE values measured with the 

unattenuated beam are referred to as 100% intensity in Figure 8. The open symbols in the mure are plotted over the 

closed symbols and correspond to QE values measured with the probe beam attenuated 50%. Fiiure 8 shows QE of 

the middle and bottom junctions is not influenced by the probe beam intensity; the top junction shows a reduction in 

measured QE values of the order of a few percent when the probe beam intensity is attenuated by 50%. Hence it may 

be conduded the QEvalues obtained with the spectral light-bias technique are not significantly influenced by the probe 

beam intensities used in these measurements. 

The measurements on the biple-junction solar cells show the top junction is effective in absorbing the shorter 

wavelengths of an AM0 spectrum, while the bottom cell absorbs the longer wavelengths, and the middle cell the 

intermediate wavelengths. Since the junctions are in series, the photocurrent in each junction is the same under powergenerating conditions. The design of a triple-iunction cell requires optimal junctions to convert the largest fraction of 

the AM0 spectrum into electrical eneig). I1 the cell design is not optimal, then one of the junctions may limit the 

photocurrent, and canier recombination in the other two junction will result in lower cell efficiency. The structure of the 

quantum effidency of a triplejunction cell measured under AM0 D.C. light bias is useful in evaluating the design of the 

cell. If the measured QE resembles one of the curves in Figure 7, the results will suggest cell performance is limited 

by the junction which corresponds to the curve. 

AM0 UGHT-BlAS TRlPLEJUNCTlON QUANTUM EFFlClENCY RESULTS 

The quantum efficieny of 

triple-juunction a-Si:H alloy solar cells 

were measured using a D.C. light-bias 

beam produced by the solar simulator 

adjusted to produce the spectral irradiance shown in Fgure 3. The measurements were carried out with the cells 

under short-drcuit 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 cum in Fgure 9 has approximately the same shape as the curve in 

Faure 7 for the topjunction of the cell. 

Fgure 7 shows the peak value is 0.54 

at 440 nm for the top junction of the 

cell measured. The results suggest 

the Performance of the triplejunction 

cell under AM0 light bias is limited by 

the top junction. QE of three triplejunction cells were measured; the 

results of the measurements were 

convoluted with the solar simulator 

spedrai irradiance using Equation 1 in 

TtipleJunction QE VI Wavelength 

0.50 

* 0.40 0 

c 

I 

8 

2 0.30 

5= 

W 

g 0.20 

g 0.10 

.). 

c 

a 

0.00 

400 500 600 700 800 90 

Wavelength (nm) 

igure 9: Quantum efficiency of aSi:H alloy -.- triple-juction solar cell under 

AM0 light bias. 

order to obtaii a calculailed blue for the short-circuit current. 

The results of the calculations are shown in Table II. 

Isc(calc) is within about 1% of Isc(meas) for the three cells. Cell l=(meas) Isc(ca1c) Ald Isc 

(d) 

technique for determining QE of triplejunction cells. ST05 2.338 

Table It 

(ma) (%I 2.361 1 .O% 

The calculations confirm the validity of the D.C. light-bias 

The role of forward bias on QE of a triple-junction ST17 2-36 2.395 0.4% 

solar cells under AM0 D.C. light bias was investigated. A cell ST38 2-253 22?9 1.4% 

was maintained at 24 OC and QE measured with forward 

biases ranging between 0 and 1.82 V. The voltage at the 

-1 65- 
maximum-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 measurements are shown in Figure 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 conditions 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 junctions are 

limiting the operation of the cell. 

The response of the triplejunction 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 

WRL AM0 shown in Figure 3. As the 

lamp current was decreased, the 

spectral irradiance beyond 750 nm 

decreased. Figure 11 shows the 

behavior of QE. The quantum e6 

aency with a lamp current of 5.0 A is 

the same as the values plotted in 

Figure 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 

cutve produced by the bottom juno 

tion shown in Finure 7. Thus as exTrlple-Junctlon QE Dependence on Voltage Blas 

Under AM0 Llght Bias 

I 

Figure 10: The dependence of the quantum efficiency of a triple-junction 

a-Si:H alloy solar cell on forward-bias voltage for 

Trlple-Junctlon QE ~ependence on 

Slmulator Spectral lrradlance 

* 

0 

c 

e 0.4 

I * Ob 

E 0.3 

E 0.2 

3 E 0.1 

2 0.0 

a 

I 

Figure 11 : Dependence of triple-junction quantum efficiency on spectral 

irradiance of solar simulator. The spectral irradiance of the solar 

simulator was vaned by changing the filament current in the tungstenhalide lamp. 

pected from the Ganges in the spectral 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 section 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 multi-junction cells. The accuracies will depend, among other things, on the number of junctions 

in the multi-junction cells and the optoelectronic properties of each of the junctions. The work to determine the 

-1 66- 
specifications for AM0 solar simulators can be carried out using small area cells, Le., cells of the order of 1 .O cm2 in 

area. While device simulation studies are useful for designing cell structures and defining the optoelectronic of each 

junction, laboratory charademation of cells should be carried out both to facilitate solar cell development and optimize 

the limited resources available for space testing. 

SUMMARY 

An integrated system was 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 was 

calibrated with a bungstewhalogen standard lamp. The quantum efficiency apparatus employed a calibrated reference 

detector which was used in measuring the absolute external quantum effiaency of triple-junction aSi:H ally solar cells. 

The calibrations of the lamp and photodiode, as well as the source measurement unit used to measure cell currents 

were based on NlST scales. Quantum efficiencies were measured using both the spectral-light bias and AM0 light-bias 

techniques. Quantum efficiencies measured with the AM0 light-bias technique were shown to be dependent on 

foward-bias voltage and the spectral irradiance of the AM0 light-bias beam. Measured spectral irradiances and 

quantum effiaenues were convoluted to calculate cell short-circuit cell currents. Calculated currents compared with 

measured shortcircuit currents at the 1% level . 

REFERENCES 

* Appreaation is expressed to Kenneth R. Lord It, Michael R. Walters and Fazal UrRahman Syed for their contributions 

1. Joseph Burdick and Troy Glatfelter, Solar Cells 18 (1 986) page 301. 

2. M. Bennett and R. Podlesny Proceedings of the 21st IEEE Photovoltaic Specialists Conference (1990) page 1438. 

3. Kenneth R. Lord II, Michael R. Walters and James R. Woodyard, Proceedings of the 23st IEEE Photovoltaic 

to this work. 

Specialists Conference (1993) page 1448. 

-1 67- 
SOLAR CELL ANGLE OF INCIDENCE CORRECTIONS' 

bY 

Dale R. Burger and Robert L. Mueller 

Jet Propulsion Laboratory, California Institute of Technology 

Pasadena, California 91 109 

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 I1 -4) and 

no tabular data. Reference 1 lists the following possible causes of off-normal array power loss other 

than the cosine correction: 

a. 

b. 

C. 

d. 

"Optical effects relating to the first-surface reflectance of the cover slide; 

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; 

Edge effects relating to refraction, scattering and additional light collection by solar cells and 

covers, especially by thicker covers: and 

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. 

terrestrial concerns): 

f. 

g. 

paper. 

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 

Shadowing by particulate soiling; and 

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 

'The 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. 

-1 68- 
DATA COLLECTION 

The Large Area Pulsed Solar Simulator (LAPSS) is a research tool at JPL and was used for 

creating and collectmg all of the data in this paper. The LAPSS has been described in detail elsewhere 

(51 and thus will not be described here. The data used in this paper was contributed by Applied Solar 

Energy Research, City of Industry, California and is gratefully acknowledged. 

CELLS 

The 28 cells used in this study were from seven different lots made with standard processes 

but were considered to be research cells and thus no effort was made to compare any cell performance 

values except the response of their short circuit current (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 MgF, 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 

Cosine - The cosine correction is due to the change in effective array collection area due to the 

Fresnel Reflectivity - Fresnel reflection is due to the interface between two optical media having 

dependent effects were examined. 

projection of the off-normal array surface onto a plane normal to incident sunlight. 

different indices of refraction. The reflected portion of the incident light is given by: 

where cp, is the incident beam angle and the refracted beam angle,@,, is: 

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 16). 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. 

-1 69- 
EXTREME ANGLE EFFECTS 

There is a possibility of trapping light or shadowing a cell when very large incidence angles are 

encountered. These effects are difficult to quantify but have been reponed [31. 

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). 

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 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. 

nown that the front surface was coated with magnesium fluoride ( 

Table I - Prediction Errors 

Cell 

No. 

7 

2 

3 

4 

5 

6 

7 

8 

9 

10 

10 

-0.00063 

-0.001 66 

-0.00027 

-0.00299 

-0.00080 

-0.00049 

-0.00363 

-0.00051 

+0.00018 

-0.00055 

Angle of Incidence 

20 30 

+0.00121 -0.00064 

-0.0049 5 -0.00222 

-0.00020 -0.00033 

-0.00402 -0.00363 

+ 0.00027 -0.00045 

-0.00468 -0.00722 

-0.00323 -0.00471 

-0.00202 -0.001 68 

-0.001 67 -0.00262 

+0.00171 +0.00084 

40 

-0.00290 

-0.00375 

-0.00243 

-0.00565 

-0.0001 0 

+0.00031 

-0.00927 

-0.00541 

-0.00586 

-0.001 51 

65 

-0.00756 

-0.00742 

-0.00878 

-0.01 030 

-0.00796 

-0.00704 

-0.01 534 

-0.01 001 

-0.01 372 

-0.0028 1 

-1 70- 
11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

-0.00086 

-0.00382 

-0.00 1 4 1 

-0.001 43 

-0.001 35 

-0.00261 

-0.001 08 

-0.00045 

-0.00 1 5 1 

-0.001 75 

-0.00094 

-0.00456 

-0.00209 

-0.001 02 

-0,0021 7 

-0.00376 

-0.00322 

-0.005 1 7 

-0.00624 

-0.00230 

-0.0041 1 

-0.00225 

-0.00404 

-0.00043 

-0.0003 2 

-0.001 87 

-0.003 76 

-0.00326 

-0.00740 

-0.00287 

-0.00305 

-0.00027 

-0.00661 

-0.00697 

-0.0042 1 

-0.0092 1 

-0.005 1 3 

-0.00345 

-0.00267 

-0.00604 

-0.00284 

+0.00059 

-0.003 1 7 

-0.00490 

-0.005 82 

-0.007 5 8 

-0.00224 

-0.00206 

-0.00538 

-0.00737 

-0.0071 7 

-0.00637 

-0.01 081 

-0.0063 1 

-0.00474 

-0.00497 

-0.00930 

-0.0041 7 

-0.00408 

-0.00440 

-0.00856 

-0.0071 7 

-0.00742 

-0.00388 

-0.00486 

-0.00782 

-0.0085 1 

-0.00598 

-0.01 178 

-0.01 434 

-0.01 056 

-0.0081 3 

-0.00608 

-0.01 696 

-0.01 1 13 

-0.01 258 

-0.01 308 

-0.01 828 

-0.01 259 

-0.00995 

-0.00935 

-0.0091 9 

-0.01 326 

-0.01 402 

-0.01 447 

Note: the expected random error for Isc values taken with the LAPSS is no greater than +/- 

0.003. 

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 II. 

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 -0.001 7 -0.0029 -0.0038 -0.0054 -0.01 10 

Error 

Std. Dev. 0.001 3 0.0025 0.0027 0.0027 0.0035 

Max. Error 0.0002 0.001 7 0.0008 0.0003 -0.0028 

Min. Error -0.0046 -0.0074 -0.0092 -0.01 08 -0.01 83 

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 7.81 5 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. Onset of the expected extreme angle improvement; 

b. Improved cell performance due to the MgF, 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 n, were assumed as 1.25, 1.30, and 1.35 with results as shown 

in Table 111. 

Table Ill 

Index 

n2 

Angle of Incidence 

10 20 30 40 65 

1.25 -0.001 75 -0.00260 -0.005 2 7 -0.00593 -0.00223 

1.30 -0.001 41 -0.00230 -0.0051 3 -0.0063 1 -0.01 056 

1.35 -0.001 59 -0.00250 -0.00548 -0.007 1 3 -0.01 788 

Table Ill 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 

-1 72- 
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 decollimation 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%. 

A/D Count - The data acquisition system used on the LAPSS is typical in that it uses if 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 .O% 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 

-1 73- 
Using the combined cosine and Fresnel corrections gave very goad predictions of the measured 

An empiricatly derived value of the coverglass front surface index of refraction should be used 

There is probably an effect from increasing angle of incidence which could not be explained 

data. For a typical example see Figure 1. 

if possible rather than a published material value. 

in detail. 

REFERENCES 

1. Solar Cell Array Design Handbook, 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. ai., "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 1 13-1 15. 

3. Opiorden, R. W., "Solar Cell Optical Design Considerations," Conf. Rec. of 9th tEEE PVSC, Silver 

Springs, MD, May 2-4, 1972, pp 164-1 73. 

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 1, pp 423-426. 

5. 

A, July 15, 1994. 

Mueller, R. L., "The Large Area Pulsed Solar Simulator (LAPSS)," JPL Publication #93-22, Rev. 

6. Heavens, 0. S, "Optical Properties of Thin Solid Films," Butterworths Scientific Publications, 

London, 1955, pp. 208-215. 

-1 74- 
0 

0 * 

0 

b 

0 

(D 

0 

rn 

0 

d0 

rs) 

0 

0 

M 

0 

0 

(\I 

-1 75- 
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-1 77- 
LOW INTENSITY LOW TEMPERATURE (LILT) MEASUREMENTS AND COEFFtClENTS 

ON NEW PHOTOVOLTAIC STRUCTURES 

David A. Schefman and Phflllp P. Jenkins 

NYMA Inc. 

Brook Park, Ohio, 44142 

David J. Brinker Joseph Appelbaum 

NASA LeRC Tel-Aviv University 

Cleveland, Ohio 441 35 Tel-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 alternate 

energy sources, some of them being photovoltaics (PV) and thermophotovoltaics (TPV). Both of these alternate 

energy sources convert light/thermal energy directly into electricity. In order to create a viable PV data base for 

planetary mission planners and cell designers, we have compiled low intensity low temperature (LILT) I-V data on 

single junction 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 (I*& 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 variations 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/lnP, InP/lnGaAs/lnP, 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 experimental results and are intended to provide the user with approximate numbers. 

BACKGROUND 

With increasing concerns over the safety and cost of RTGs, alternate power sources are being sought. 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 (AMO, 436.7 mW/cd) and moderate 

temperatures (20" to 80°C). As space exploratory missions extend beyond earth's otbii, 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. Intensity changes inversely as the square 

of the distance. Temperature calculations are 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/cd 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. Early LILT data was also 

performed using older techniques with limited temperature and intensity regulation, and less sensitiive 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 V, 

will increase linearly, and short circuit current I,will decrease due to a shift in bandgap (the absorption coefficient 

-1 7a- 
also decreases with temperature). Fill Factor 

will tend to increase proportionally with voltage 

but there are many other mechanisms that 

The most important effect is that the dark 

current I, decreases as temperature decreases. 

The temperature effects on voltage and current 

can be seen in the following equations (3,4): 

contribute to its temperature dependence (2). 1 

I 

(1) 8 *' v, = ykT (+ + I] 

- i3 

c 

3 .01 

- 

9 

e [ - 11 (2) /(V) = /,- I, 

Q 

(3) 1 -E* 

lo a Pewhere T is temperature, y is the ideality factor, 

typically between 1 and 2, k is Bottzman's 

charge on an electron. As temperature 

material increases. This decreases the FIgure 1: Solar intensity vs. Distance From the Sun 

spectrum which can be absorbed and reduces 

the photocurrent. 

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 in temperature and intensity, these changes can 

be nonlinear. Cells may become shunted and/or carriers 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). 

z 

Uranus 19.19A. 

.ool 

constant, Go is the bandgap, and q is the 10 20 30 40 

decreases, the bandgap of the semiconductor Distance From the Sun (A.U.) 

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. 

- InP/lnGaAs two-terminal monolithic tandem grown on InP. 

0 GaAs/Ge (passive Ge), GaAs grown on Ge. 

Si 2 Qcm with BSF. This a production cell. 

- .72 eV InGaAs (InP window, InP substrate) 

GaSb (bottom cell of GaAs on GaSb tandem stack) 

- GalnP cell on GaAs (inactive) 

- GaAs cell with a GalnP window layer. 

- InP cell 

- InGaAs cell with a InP window and grown latticed matched on InP 

I 

InP MOCVD 

TEST DESCRIPTION 

The test consisted of measuring IV curves of solar cells at varying light intensities and temperature. The 

-1 79- 
temperatures ranged from 25°C to -185°C. The 

intensities ranged from I sun down to -03 suns, or 

equivalent distances of 1 to 6 au. I-V curves were run 

every 25°C at 2.8, 4.7, 11.5, 46, and 136.7 mW/crr? 

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 correct for flicker in the arc 

lamp light source. 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. 

Quartz Window hf2 I 

Figure 2 LILT Test Setup 

A single thermocouple embedded in the test plate is used for temperature control. Additionally, four witness cells 

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. Intensity was decreased by using metal screens, which lower the amount of light on 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. 

Galn P/GaAs 

The GalnPlGaAs 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 temperaturelintensity dependence to about 

-90°C, with peak efficiency at around -50°C. Below -90°C, the cell voltage 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. 

-1 80- 
In PA nGaAs 

The InP/lnGaAs cell is a monolithic tandem cell consistina of series-connected current matched cells. This cell 

also had typical temperatureAntensity dependence to abou<-9O0C. 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 Figure 4. The vohage 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 

temperaturefintensity dependence over the entire range of measurements. The voltage slope of both cells tends 

to lessen below -90°C. The current of the lnGaAs cell changes very little with temperature. This is due to the shift 

at both ends of the spectrum. The InP window layer is shifting along 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). 

12 - a 

3 

2 1.1 . 

.9 

.8 

50 0 -50 -100 -1 50 -200 

Temperature ("C) 

Figure 3 GalnP/GaAs at 1 Sun 

t 

1.50 - 

v) N 

0 

1.25 . 

3 

..................... 50 0 -50 -1 00 -1 50 -200 

Temperature ("C) 

Figure 4 InPAnGaAs at 1 Sun 

Si Cells 

The Si cell is a mcrn cell with a BSF. The 1 sun temDerature data is shown in Fioure 5. Below -100°C the 

voltage slope is much lower. This cell had typicai temperatureAntensity 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 AMO. 

InP Cell 

This InP cell had typical temperaturelintensity dependence over the entire range of its measurements. The 

voltage slope did change at temperatures below -75OC, but the change was not as much as seen on the previous 

cells. The efficiency on this cell continued to rise over the entire temperature range, increasing by 30% from room 

temperature down to -180°C. 

InGaAs Cell 

The InGaAs cell is grown lattice matched7.72 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 -125OC, seen as a bend in the IV curve near V,. 

Low Intensity measurements were conducted on all cells at every temperature recorded above. The behavior 

of I,, and V, 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 vaned with the linearly 

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 I, as voltage is defined in 

logarithm of I,,. The Fill Factor tended to follow V,. 1 J 

9 1.5 

- equation 1. 0 3 1.3 

B TEMPERATURE CORRECTION 

= 6 - 

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: 

1.1 

a -. .= 

50 0 -50 -1 00 -1 50 -200 1 dprm 1 dr, 1 dv, -- = -- + -- + 1- (4) 

CmtaI dr i,dT V-ar FdT Temperature ("C) 

From the above equation, which is based on the Figure 5 Si at 1 Sun 

maximum power point, temperature correction can be 

applied directly. Simpler techniques apply correction 

to V, I,, and P, (or FF), then use curve fitting 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. 

and 

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 in this paper presents a brief overview of the temperature and intensity characteristics 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 showgthat 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 coeff icients; voltage increases and arrent decreases with lowering 

temperature; V, is proportional to the log of intensity, current is directly proportional to intensity, and fill factor tends 

to drift up to a peak and drop down. 

Although multi-junction cells 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 

-1 82- 
temperatures, where changes in temperature cause a 

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 V, vs temperature. This characteristic is 

indicative of a change in the I, as well as I,. Different 

recombination mechanisms affect different voltage 

ranges and temperatures, Le., 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 fill factor peak could 

be observed: this required a second order equation for 

curve fitting. 

The plots shown in Figure 6 indicate that the voltage 

coefficients tend to increase linearly as a function of the 

01 

o^ -.005 

2 

L 

G 

b 

- % 

-.010 . 

*--a InGaAsBottom 

m- --. GalnP/GaAs 

---T InP 

,/ 

.P 

/' 

Relative Intensity (1 =AMO) 

Figure 6 V, Coefficient vs Log(1ntensity) on Cells 

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., Bartels, F. T. C., and Kirkpatrick, A. R., 1970, "Solar Cell Low Temperature, Low Solar Intensity 

2) Hovel, Harold J.,"Semiconductors and Semimetals, Volume 1 1 , Solar Cells", Academic Press, 1975, pp 166-180 

3) Hu, Chenming, and Whae, Richard M., 1983, "Solar Cells, From Basic to Advanced Systems", McGraw-Hill , pp 

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 Iles, 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, Phillip, Scheiman, David, and Brinker, 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 

Operation". Conference record of the 8th lEEE Photovoltaic Specialists Conference, pp. 150-154. 

57-63 

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-1 85- 
A SUMMARY OF THE INTERNATIONAL WORKSHOPS ON SPACE SOLAR CELL 

CALIBRATION AND MEASUREMENT TECHNIQUES 

D.J. Brinker, H.B. Curtis and D.J. Flood 

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 international workshops 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 international 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 1 st 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 

-1 86- 
Table I - Affiliations of 1st Workshop Participants 

Japan 

Adv. Eng Services Co. 

Hi-Reliability Comp. 

I SAS 

JQA 

NASDA 

Opt0 Research Corp. 

Sharp Cop. 

Wacom R&D Cop. 

M. Wantanabe & Go. 

United States Europe 

Hughes Space & Com. 

NASA LeRC DRA 

NREL EEV Ltd. 

USAF/Phillips Lab EST1 

Deutsche Aerospace 

Spectrolab ESA-ESTEC 

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. Concerns 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 

ultijunction solar cells present pew problems because of the requirement for current matching of the 

pee 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 generaily 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 t erature coefficients is also required, with 

the different subcells having different temperature coefficients. 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 

-1 87- 
uncertainty analysis to allow extrapolation of taboratory 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: 

Cell tvDe lnadiited Jamn Eurme United States 

Silicon No X X X 

Silicon Yes X X X 

GaAs No X X 

GaAs Yes X X 

Hi Eff. Si No X 

Hi Eff. 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 11. 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, AM0 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 workshop 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 

altiiude 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- 
Table II - Affiliations of 2nd Workshop Participants 

Japan United States Eurooe 

Adv. Eng Services Co. CNES 

Hi-Reliability Comp. Deutsche Aerospace 

ISAS EEV Ltd. 

NASDA China Acad. Space Tech. Fraunhofer Institute 

Opt0 Research Corp. INTNSpasolab 

Sharp Corp. 

NASA LeRC 

JQA China ESA-ESTEC 

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 OC, 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 will test the cells on the 

Lear aircraft (both Isc and IV measurements) and distribute them to the following possible collaborators: 

United States Eume Jawn 

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 

results will 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. 

-1 89- 
The 2nd Workshop ended with a plenary session at which time the four working groups reported their 

results. Preliminary wriien 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 

discussion of 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 International Photovoltaic Science and Engineering 

Conference. The workshop will be hosted by NASDA. 

CONCLUSION 

The first two of a planned series of International 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 cells and arrays. The objectives of the workshops include 

agreement on standardized values of the (AMO) 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. 

-1 90- 
Donald L Chubb, Brian S. Good and David M. Witt \\ *! 

NASA Lewis Research Center 

Cleveland, OH 44135 

and 

old A. Lowe, Fatemi and Richard H. Hoffman 

Research, Inc. 

Cleveland, OH 

and 

David Meiman 

e 1980's is reviewed. 

Is and rare earth - 

dies. An emittance theory 

measured for this cell 

uits for erbium Er-YAG and 

.75 eV lnGaAs PV cells 

-191- 
energy in a single bandgap-matched emission band, or a bandpass fitter - grey body emitter combination, 

which results in a single bandgap-matched emission band 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. 

qth (thermal eflidency) x flEf (emitter -filer eficiency) x ‘Iw (W efiamy) = q~ (system efiaercy) 

combusion 

4- 

II) 

Filter 

have a single emission band with an 

outside the emission band. For the 

atoms are compressed to so 

ion as with an isolated atom, 

transitions from th 

have been extens 

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 pm) rare earth oxide fibers similar to the 

construction of the Welsbach mantle used in gas lanterns. The very small characteristic dimension of these 

-1 92- 
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, it was the small 

characteristic dimension that made the fibrous emitters efficient. Another geometry for achieving a small 

characteristic dimension and also easily coupling to any thermal source is a thin film. A thin film containing a rare 

earth on a bw emittance substrate can be easily attached to any thermal source. In addition, a thin film is more 

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 successful thin film emitter is yttrium aluminum 

laces the yttrium ion, is grown as a single crystal and then cut and polished to the desired thickness. A low 

ittam?, opaque substrate must be placed between the emitter and thermal source to block the grey body 

ed with any of the rare earths most of our research has been confined to Er - 

emission bands at & -1.55 pm and & =I .95pm, respectively, they are 

e theoretical analysis of the thin film emitter is based on one 

13,14) that includes scattering, as well as, emission and absorption. If 

complicated. However, neglecting scattering allows an analytic 

met (YAG, Y$415012) doped with rare earths (9). The rare earth doped YAG, where the rare earth ion 

Where E= is the substrate emittance, a is the reflectance at the emitter surface, is emitter material 

index of refraction, KM = akd is the emitter optical depth where ax is the extinction coefficient (= absorption 

coefficient for no scattering) and d is the emitter thickness. Other quantities appearing in eq. (1) are the 

(5) p:= 1--1/n, 2 

E3(x) = Jue-""du =ye Exponential Integral of order 3 (s) 

1 

1 -312' 

0 

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 p,, = cos S, ,where 0, is the maxhum angle of incidence at the emitter surface for radiation to escape 

the film (sin e,, = l/Wfrom Snell's Law for refraction). 

The dependence of qon Kkd is approximately 1 - exp (-3Ku /2) since 4 (x) = 1/2 exp (-3x/2). 

Equation (1) also shows the importance of substrate emittance. For regions outside the emission band where 

ax and thus Ku is small (& (0) = 1/2) the emittance will be dominated by the substrate emittance, eb . Thus it 

is important to have low substrate emittance to minimize the emission outside the emission band and thus 

maximize the emitter efficiency. Low E& also implies the reflectance (1 - e&) at the substrate approaches 1 

which means that a significant copribution (1 - 

the substrate. Therefore, low substrate emittance minimizes emission outside the emission band where ax is 

term in eq. (1)) to the emission band results from reflection at 

-1 93- 
e emission band where a is large. 

e, a, the emitter eff 

where e~, is the black body emissive power and A,, is the short wavelength limit of the emiqsion band and & is 

the long wavelength limit of the emission band. 

And h is Planck‘s constant, c,, is the vacuum speed of light, k is Boltzmann’s constant and TE is the 

can be split into 4 bands. For the 

the emisshn band (A,, 5. XI &) the 

emitter temperature. We have found (1 1,12) that the spect 

region 0 s Is A, the spectral emittance is nearly a constant, 

emittance shows some variation bul can also be approximated as a constant, e+, . For & s Xs & the 

emittance is also a constant, y , where & = 5 pm for YAG. For ;hc 

large so that the emittance, & -* 1 .O. 

equation (7) becomes the following. 

X the extinction coefficient for YAG is 

re, if Q is approximated by the four band model just described 

I rtE= 

Where, 

dx O1 x3 

G,(u,)= Ia. ex- 1 

(9) 

As equation (9) shows the emitter efficiency depends on the ratio of the emittances outside the 

emission band (eu, 4, k) to the emittance for the emission band, e+, , as well as, the emitter temperature, TE , 

and the emission band IocWon (b & ) 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 (1 1,12), 

et ai. have experimentally found that the optimum TE for Ho-YAG with an emission band centered at X = 1.95 

pm is approximately 1650K. For Er-YAG, which has an emission band at & = 1.55 pm the opthum TE will be 

higher. Theoretically (7,8), it has been found that for the case where y = e,, = Q, maximum q~ occurs when 

& / k TE = 4, where 6 = hc, / 

band. 

= (he, / 2) (1 + 1 fi) Is the photon energy at the center of the emission 

-1 94- 
As mentioned earlier, the rare earth oxide fibrous emitter is efficient because of the small dimension of 

the fibers. In the care of the thin film emitter there will be an optimum thmess for maximum efficiency. This 

can be seen by considering equation (9). For a given TE and emission band, the efficiency will be a maximum 

when the emittance ratbs y / a, a, /a and E / are a minirmm. As already mentioned in discussing the 

spectral emittance (eq. (1) ), the emittance has nearly an exponential dependence 1 - exp (-3Ku 12) on optical 

depth, Ku (=wd). Therefore, for d = 0, Q- e& and therefore y - e,, - e,, - q, . As d increases Q will increase 

much faster than 4, a, and ~c since >> cy, % and - constant. As a result the emittance ratios will decrease 

and q~ will increase to a maximum for some optimum d. Ford greater than the optimum value the emittance 

asymptotic value for KW -- while 4 and EU will 

12), et al. experimentally found the optimum 

rge dimension for a thin film a more appmpnate 

ive emitter rather than thin film selective emitter. 

ntal results for Ho-YAG and Er-YAG selective emitters. The first parameter of 

is the extinction coefficient ax since the spectral emittance depends 

rature was calarlated using measured values 

2 shows the extindion coefficients of Ho (25%) -YAG 

age of Ho and E3 ions that replace the yttrium ions in 

coefficients within the emission bands are krge (> 10 cml) 

8 where ax e 1 .0 cm1. Therefore, we would expect the same 

are compared to the spectral emittances outside 

25 

0 

1700 iaoo 1900 2000 2100 2200 1300 1400 1500 1600 1700 

Wavelength. li nm Wavelength, li nm 

Fgure 2(a) Ha (25%) - YAG Fgwe 2@) Er{40%0) -YAG 

Figure 2 Extinction Coefficient from Measured Transmission and Reflectance 

(a) Ho(Z%)-YAG (b) €r(Wh)-YAG. 

-1 95- 
Equal aamling 

and e- 

.....-. * ...*.-.... --------* 

I 

1400 1600 1800 2000 2200 2400 

Wavelength, A nm 

Figure 3 

1200 1400 1600 1800 2000 2200 2400 

Wavelength, X nm 

Figure 3 (a) Ho (25%)-YAG Figure 3(b) Er (40%)-YAG 

Theoretical and experimental spectral emittance e% with platinum foil substrate. Theoretical 

calculations assumed constant substrate emittance (e =.2) and index of refraction for YAG (nf -1.9). 

(a) Ho(25%)-YAG,.65mm thick; front surface temp. = 1380K, back surface te 

temp. = 1500K. (b) Er(40%)-YAG, 1.04mm thick, front surface temp. =1398K, back surface temp. = 

1606K, average temp. = 1500K. 

. =1616K, average 

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 corn ring the theoretical 

and experimental EX results, several features should be noted. First, the emittance in the emission bands is 

large. Second, results with small scattering (ax 5.5) agree more closely with the measurements. Third, the 

measured emittance outside the emission band is considerably higher than the theoretical values. Part of this 

discrepancy results from experimental enor. In wavelength regions of !ow emittance, background radiation 

becomes significant so that the measured emittances are larger than the actual values (9,lO). Finally, the 

theoretical emittance outside the emission band is smaller than the assumed platinum substrate emittance 

(Q = .2). This result occurs because part of the substrate emission is totally reflected at the emitter surface due 

to the refractive limit (k2term in h-term of eq. (1). 

To summarize the selective emittel discussion the following points should be emphasized. First of all, 

the thin film Ho-YAG and Er-YAG selective emitters have latge emittances (2.6) within their emission bands. 

However, the emittance outside the emission bands is also larger than predicted. For 1 > 5pm YAG has a large 

extinction coefficient therefore the emittance for X > 5pm will be large. For X < 5 pm 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, !ow substrate emittance (high substrate 

reflectance) increases the emittance within the 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. 

-1 96- 
GU AWF PHOTOVOI TAIC CFI IS 

The TPV need for a low bandgap energy photovoltaic (PV) cell is the main reason the Lewis research 

program (1516) on indium gallium arsenide (In, Gal-x As) FV cells was initiated. In, Gal-, As is a direct bandgap 

semicondudor material that has a bandgap ranging from .35eV to 1.42eV depending on the lWGa ratio. It is of 

interest for tandem solar celL (13, as well as for TPV. Besides Lewis, In, Gal-, As research is being camed out at 

several laboratories including NREL (17,18,19), Spire Corporation (20) , Research Triangle Institute (21) and 

Rensselaer Polytechnic Institute (22). 

In, Gal, 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 first cell grwwn was In.= Ga.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 incorporating step 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 Figure 4. 

AuKj, F~mt Meialization 

TaQ AR Coatina 

0.17 pm p InGaAa (0.65 eVB.69 ev) 

a15 pm p InOaAs (0.70 eV10.72 ev) 

Figure 4 Lattice matched (0.75ev) and mismatched (0.6010.66eV) InGaAs device structures. 

(1 -1 Oalcrn2). Therefore it is critical 

ped InP window 

e between the abso 

optimum window thi 

cells. Antireflecting (AR) coatings 

een the current 

ction in series r 

re~~~n in the wavele~h regions near the cell bandgaps. 

InGaAs cells are given in references 15 and 16. In this paper we 

nt-voltage relations (dark diode data). In Figure 5 

be seen the latticed matched, .75eV cell attains the 

OeV cells have lower quantum efficiency and also show a "roll 

. Because of the many dislocations in lattice 

s fact coupled with longer absorption lengths for 

at the long wavelengths. Optimization of base thicknesses, 

Id improve the long wavelength response. 

-1 97- 
1 

g 0.8 

c 

- Q) 

8 0.6 

E 

a 

3 0.4 

0 

U 

r 

- 

2. 4 0.2 

W 

0 

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

Wavelength (lun) 

Figure 5 External Quantum Efficiency measurements of InGaAs 

phototwoltaic devices with anti-reflective coatings. 

Table 1 InGaAs photovoltaic device diode characteristics for three different bandgaps. 

The second column ("A') is the diode ideality factor, JO1 is the reverse saturation 

current (dark current), Rs is the series resistance and Rsh is the shunt resistance. 

In Table I the dark diode data for the three cells is shown. As can be seen the dark saturation current, j 01 

increases considerably in going from the .75eV lattice matched cell to the .66eV and .60eV lattice mismatched 

cells. There is also a significant decrease in the shunt resistance, Rh, in going from -75eV to .60eV. Also note 

that the diode ideality factor A4 for each sf the cells so that recombination current in the depletion region is 

negligible. Fmally, the series resistance, R,, shows a small increase in going from the .60eV cell to the .75eV cell. 

mismatched cells result mainly because of dislocations. Currently, buffer layer design and dislocation passivation 

techniques are under development to alleviate these problem. In addition a new cell structure that addresses two 

important TFV issues is being fabricated. First of all, photons reaching the W cell with energy below the bandgap 

energy are a loss to 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 law bandgap energy cells necessary for TPV. By using an array of series 

connected InGaAs cells the total current can be reduced and outpn voltage increased thus reducing resistive 

The large dark saturation currents and "roll off" in quantum eff iciency for long wavelengths for the lattice 

-1 98- 
losses. The cell array stntdure that uses series connection and also a backside reflector is the monolithically 

integrated module (MIM) device shown in Fgure 6. 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 SOW thd< semi-insulating InP wafer with polished surfaces and gold on one side is shown in 

Figure 7.Also shown is the reflectance of a MlM cell without an AR coating. As can be seen 

> 80% reflection is achieved for the long wavelength (low energy) photons. 

Figure 6 Mon~lithiilly Integrated Module (MIM) InGaAs device design. 

100 

8 *O 

p 60 

0 

6) 

cg 40 

0.75eV hGaAs MIM Structure & 

20 

0 

1000 I500 2000 2500 

elength (nm) 

ure 7 ra9 tefiectances of thick inP wafer with 

M P~~ll 

antum efficiency has 

mismatched cells 

s for TPV applications 

nier lifetimes and 

-1 99- 
A computer mdel for a general TPV system has been developed at Lewis to determine overall TPV 

System performance (23). The model has been applied specifiilly to a solar driven themphotovoltaic (STPV) 

System (24) in addition to 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 

system obtained using the systems model. 

An important assumption of the model is that the wavelength range (OS AS ) is divided into four bands. 

Within each 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 .O 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 

Where as discussed earlier, x(, is the long wavelength cutoff for the emitter emission band or the fitter bandpass. 

We also assume a constant dimensionless bandwidth A Eb / & for ?he emitter emission band or ?he filter bandpass. 

Where Eb is the photon energy at the center of the emitter emission band or the fit er bandpass and E" is the high 

energy cutoff for the emission band or filter bandpass. 

The overall effiency 9~ for a general TPV system is defined as follows. 

Where the thermal efficiency, qh, emitter-fitter efficiency, qEf, and PV efficiency, qpv, are defined as follows. 

power input to emitter 

qth= power input to thermal some 

input power to PV cells for h s h, 

power input to emitter qEf= 

electrical power output firrnn PV cells 

input power to PV cells for h s h, rlPv= 

For comparing selective emitter and filter TPV systems the thermal efficiency, qh, need not be considered 

assuming it is the same for both systems. Therefore, the systems model is used to calculate 9Ef and qpv for a 

given emitter temperature, TE, with bandgap energy Eg being the independent variable. 

cell bandgap energy for maximum efficiency and output power density. This is illustrated In Figure 8 where results 

for a selective emilter system based on the rare earth-YAG emitter at TE = 1500K am shown. For this case the 

The first thing to note about a TPV system is that for a given emitter temperature there is an optimum PV 

-200- 
assumed emittances for the emitter are % = .75,4 = 2, Q, = .75, e,, = .2 and a dimensionless bandwidth, 

Ab / 6 = .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 fern for all wavelengths. As can be seen from Fwre 8 an 

optimum bandgap energy occurs because t\~ decreases while qw, increases with increasing Eg. Also, Eg for 

maximum power output is slightly lower than Eg for maximum efficiency. Atthough not shown in the figure, as TE 

increases the optimum values for Eg move to larger values. 

Ob 

02 

ai 

0 

(a) Emitter Efficiency, qEf,PV Effiaency,qpv, 8 Product qm qw @) Output Power 

Fire 8 Periorm%nce of Rare Earth-YAG selective emitter TPV system at TE = 1500K. 

Properties based on expimental data (q=~ = 2, q, =e= .=,A 61 Et, = .15). 

kj=Jw-(cP-um = 1 .). 

Now consider the 

The Wo filter systems are 

r system and two filter systems. 

(25) and a combination 

) . The dimensionless bandwidth 

ion of .8 can be obtained (25). 

ex of refractiin 

is A& / Et, .6. For the 

= q = Q = .9. Atso, we assumed 

ission. This is a rather 

outside the emission 

-201- 
compensating for the significant emittance (4 = e,, = .2, ~6 5.75) outside the emission band is to use a PV cell that 

has lafge reflectance outside the emission band and thus reflect this outof-the emission band radiation back to the 

emitter. The MIM lnGa As cell discussed earlier is just such a W cell. Therefore, for eompan'ng the selective 

emitter system and the filter systems we assumed a reflecting PV cell for the outof-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; 

f"=fb=.o3, ri= rc =.8, which should be representative of the MIM InGaAs structure. 

In Figure 9, the product of efficiencies, qEf qp~ 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 efficiency followed 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 efficiency than the selective emitter system. 

First of all, as discussed earlier, the filters have been assumed to have no absorptance so the calailated effiiiency 

is larger than for a real system. Second, the large reflectames for regions outside the bandpass region in the 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 proportional to the following quantities. 

Therefore, it is the large dimensionless bandwidth (AEb / Eb = .6) and large transmission-emittance product in the 

bandpass region (Tb q, i= .63) that accounts for the P1F system having the largest output power. Even though the 

SER system has a much smaller dimensionless bandwidth (Ah / & = .15) than the RAF system (Ab / 6 = .4), the 

low bandpass transmission-emittance product (7b q, = .a) for the AAF system resutts 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 

smaller than the optimum Eg for maximum efficiency. This resuit was pointed out earlier in discussing Figure 8. 

Ob 

Figure 9 Efficiency comparison of bssless filter and Rare Earth-YAG selective emitter TPV systems at TE =1500KFiIter 

pr0perties;PIF (4-4-e.9, r1-.9.r,-r~.8,rb-.3,~-1 -r,AE~=.6).RAF(~~~-~.9,r~-ru-.95,r~.971r~-.5,~=l -r, 

AE~ /Eb=.4).Selective emitter properties;SER(q-q,=.2,4,=~.75, ru=rb=.03,q=r~.8. AEb Eb =.lfi)Meal PV cell model 

used for all cases. 

-202- 
I- plasmallnterference mer pq 

output Power 

Wlm2 

Finer WF) Rare Earth-YAG Selective emitter with 

3- 

0.5 - 

t 

0.2 0.4 0.8 0.8 1 1 .P 1.4 

PV Cell Bandgap En-. eV 

Fgure 10 Power output comparison of lossless filter & Rare Earth-YAG selective 

e~ef TPV Systems at TE = 1500K. Same conditions as Figure 9. 

s 9 and 10 it would appear that the filter systems will perform better than the 

. However, as already me~ion~ if filter absorptance were included the 

be reduced. Also, improveme~s to the selective emitter efficiency can be 

utside the emission 

large reflectance in the 

proved by increasing the 

e (q, =.5). Power output of the SER system can also be increased 

YAG emitter with two rare earths with contiguous emission bands 

present time there is rto clear r in choosing between filter and 

issues plot yet considered, reii and cost, will probably be the 

system becomes a viable e 

s and increasing the PV cell refledan 

ems efficiency will be more difficult since th 

However, the power output of the RAF s 

nverter for space or 

l~~i-~~ Pv cells (27). 

cell efficiency is therefore given by the following expression. 

-203- 
, measuredmaximumoutputpower P, 

qw= measumi total radiant input power - P, 

-- 

The maximum output power, PEL, was determined from measured current-voltage curves. It should be pointed out 

that q'pv given by equation (21) is not the same as qw given by eq. (19). The radiation in the denominator of eq. 

(19) includes only photons with energy greater than the bandgap energy (Ash) whereas Pin in equation (21) 

includes all photon energies. As a result, (qpv 2 q'pv). 

figure 1 1 Schematic diagram of test apparatus used to measure PV efficiency. 

Properties of the Er-YAG emitter and In.mGa-47As W cell used in the experiment have already been 

described. For these experiments the Er-YAG emitter was .4mm thd< with a platinum foil substrate backed with 

silicon carbide (Sic). SIC has large emittarice (= absorptance) so that there is thermal coupling between the 

furnace and the emitter. This good thermal coupling reduces the temperature ent across the emitter. The 

.75eV InGaAs cell used in the experiment had a thinner (.OS-.lpm) InP window layer than the cell shown in Figure 4 

and an area of I&. 

Two systems were experimentally considered. The first system consisted of the Er-YAG emitter and the 

In.mGa.47As W cell. The second system consisted of the same emitter and cell an interference 

shortpass IR filler between the emitter and PV cell. The filter had a cutoff at Xu 1 X >l .6pm the filter has 

greater than .9 reflectance. For XI; 1.6 the filter transmission .7. Therefore, most all radiation with 1 > 1 .- will 

be reflected back to the emitter when the filter is used. (In that case q'pv = qw). 

The I-V curves and cell performance parameters obtained for the two systems operating with TE = 1373K 

are shown in Figure 12. Obviously, the addaion of the filler greatly improves q'pv (going from 2.3% to 13.2%). 

However, the output power (PEL= JscVwFF) is reduced by a factor of two with the filter because of the filter 

transmission loss. Use of the MIM cell structure with the badc surface gold reflector will give the efficiency 

improvement possible with the filter but not suffer the redudion in power resulting from the transmission loss. 

-204- 
2 so 

200 

- 100 

50 

0 

I I 

Jsc = 220 rnA/cd 

0 100 200 300 400 

v [mV) 

(a) Without Fitter 

(b) With Fitter 

0 100 200 300 400 

v (mv) 

Fylure 12 I-V curves for the 0.75 eV InGaAs PV AR coated cell coupled to the Er-YAG 

seledive emitter at TE = 1 100° C (a) without filter, (b) with filter. 

Ie TPV research has been carried out at Lewis beginning in the late 1980’s. The work has 

on thin film rare earthVAG selective emitters and In,Ga~-,As PV cells, as well as, theoretical and 

emitters with excellent emission band emittance (> .7) 

outside the emission band. 

r emittance substrates. 

excellent quantum efficiency. 

g from the lattice mismatch cause reduced quantum efficiency and 

mitter and short pass fitter. However, for the lattice 

rated module (MIM) structure is being fabricated that is 

better suited for TPV appiiiions. 

A TPV systems model developed at Lewis is able to model any possible TPV system. ?he model has 

-205- 
been used to compare filter and selective emitter TPV sydled. However, since fitter absorptance Josses were not 

included in the model it is not possible to say whch system will yield the larger efficiency. With tossless fitters the 

madel predicts that the filter systems will yield the largest efficiency. 

REFERENCES 

1. R. A. Lowe, C. Goradia, M. Goradia and 0. L Chubb, J. Appl. Physics, 68,5033 (1990). 

2. G. H. Dieke, "Spectra and Energy Levels of Rare Earth ions in Crystals" (Interscience, New Yolk, 1968). 

3. 6. E. Guazzoni, Appl. Spectra. 26,60 (1972). 

4. R. E. Nelson, in Proceedings of the 32nd International Power Soutces Symposium (Electrochemical Society, 

Pennington, NJ, 1986) pp 95-101. 

5. C. R. Parent and R. E. Nelson, in Proceedings of the 21st lntemdety Energy Conversion Engineering 

Conference (American Chemical Society, Washington, D. C., 1986), Vol. 2, pp 1314-1317. 

6. P. L Adair and M. F. Rose, in Proceedings of The First NRU Confere 

of UectriCity, (American Institute of Physics Conference Proceedings 321 , 1994), pp 245-262. 

7. D. L Chubb, in Pnoceedings of the 21 st Photovottaic Specialists Conference (IEEE, New Yo&, 1990) 

8. D. L Chubb and R. A. Lowe, J. Appl. Phys., 74,5687 (1993). 

9. R. A. Lowe, D. L Chubb, S. C. Farmer, and B. S. Good, Appl. Phys. Lett. 64,3551 (1994) 

on The~ophotovott~~ GeRf3ratkm 

p~ 1326-1342; also NASA TM - 103290,1990. 

10. 

11. 

12. 

13. 

14. 

15. 

1 6. 

17. 

R. A. Lowe, 0. L Chubb, and 8. S. Good, in Proceedings of The First NRU. Conference on 

Themphotovoltaic Generation of Electricily, (American institute of Physics Conference Procedi 

1994), Pp 291-297. 

R. A. Lowe, €3. L. Chubb, and B. S. 

Energy Conversion, (IEEE, 19%). 

, in Proceedings of the First Wo Conference on P~tovott~ 

R. A. Lowe, 6. S. Good and 0. L Chubb, in Pro 

51 1-515. 

En0 neeriRg 

Conference Engineering Conference (IECEC), I. 2, 

D. L Chubb, R. A. Lowe and 8. S. Good, in Proceedings of The Fist NREL Conference on 

Themphotovottaic Generation of Electricity, (American I of Physics Conference Proceedings 321, 

1994), Pp 229-244. 

B. S. Goob, D. t Chubb and R. A. Lowe, in Proceedings of The First NREL Conference on 

Themphotovoltaic Generation of Electricity, (American institute of Physics Conference Proceedings 321, 

1994), pp 263-275 

D. M. Wilt, N. S. Fatemi, R. W. Hoffman, Jr. et al., Appl. Phrys. Lett 64,2415 (1994). 

D. M. Wilt, N. S. Fatemi, R. W. Hoffman, Jr. et al., in Pmceedings of The First NREL Conference on 

Themphotovoftaic Generation of Electricity (American Institute of Physics Conference Proceedings 321, 

1994), pp 21 0.220. 

M. W. Wanlass, J. S. Ward, K. A. Emery and T. J. Coutts, in Conference Record of the 23rd IEEE Photovol!ab -206- Sp&&i!S ConferenCe, 1993, Pp 621-627. 
18. 

19. 

20. 

21. 

22. 

23. 

24. 

25. 

26. 

27. 

J. S. Ward, M. W. Wanlass, X. Wu and T. J. Corn, in Procsedings of The First NREL Conference on 

Themphotovoltaic Generation of Uectrici (American Institute of Physics Conference Procsedings 321, 

R. K. Ahrenkiel, T. Wangansteen, M. M. AIJasaim, M. Wanlass and T. J. Coutts, in Proceedings of The First 

1994), pp 40441 1. 

NREL Conference on Thermophotovoftaic Generation of Electricity (American Institute of Physics 

Conference Proceedings 321,1994), pp 412-424. 

S. Wojtczuk, E. Gagnon, L Geoff roy and T. Parodos, in Proceedings of The First NREL Conference on 

Themphotovoltaic Generation of Electricity (American institute of Physics Conference Proceedings 321, 

9994), pp 177-187. 

P. R. Sharps, M. L. Timmons, R. Venkatasubramanian, J. S. Hills, 6. O'Quinn, J. A. Hutchby, P. A. llles and 

C. L. Chu, in Pm ings of The First NREL Conference on Themphotovoitaic Generation of Electricity 

(American Institute of Physics Conference Proceedings 321,1994), pp 194-201. 

H. Ehsani, 1. Bhat, D. M~~, G. Nichols, J. Bonego, J. Parrington and R. Gutmann, in Proceedings of The First 

REL Conference on ~e~~hotovo~a~ Generation of Electricity (American Institute of Physics 

ings 321,1994), pp 188-1 93. 

B. S. , D. L. Chubb and R. A. Lowe, in Proceedings of The Second NREL Conference on 

Themphotovoltaic Generation of Eledricity (American Institute of Physics Conference Proceedings 1995). 

K. W. Stone, E. F. Leingang, R. E. Drubka, D. L Chubb, 6. S. Good and Q. M. Wilt, in Proceedings of 30th 

lntemiety Energy Conversion Engineering Conference (IECEC), (American Society of Mechanical 

Engineers, 1995), vol. 1, pp 713-718. 

A. &hock and V. Kumar, in Proceedings of The Fist NREL Conference on Themphotovoltaic Generation 

of iiiedricity (American Institute of Physics Conference Proceedings 321,1994), pp 139-152. 

P. F. Baldasam, E. J. Brown, D. M. De , in Proceedings of The First 

NREL Co~e~~e on Themp Institute of Physics 

Conference Proceedings 321,1994), pp 29-43. 

N. S. Fatemi, R. H. Hoffman, D. M. Wilt, R. A. Lowe, L. M. Garverick Q. Meiman, in Proceedings of The 

reme on Themphotovoltaic Generation of Electricity (American Institute of Physics 

ings, 1995). 

-207- 
Thermophotovoltaic Generators Using Selective Metallic Emitters 

Lewis M. Fraas, John E. Samaras, James E. Avery 

JX Crystals Inc. 

. ",/ Issaquah, WA \I 

and 

Richard Ewell 

Jet Propulsion Labs 

Introduction 

In the literature to date on thennophotovoltaic (TPV) 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 lantern 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 thennophotovoltaic generator, a photovoltaic cell typically converts infrared 

radiation to electricity out to some cut-off wavelength. For example, Gallium Antimonide (GaSb) 

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-YAG 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 seleeivity 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- 
(Watts) Efficiency 

~~) ~~) 

1 324 1298 273 250 34-4 327 13.1% 

1325 1299 293 250 31.9 30.3 12.1 % 

JX Crystals has been independently developing hydmarbon fired TPV generators for 

applications here on earth. After the space wntmct had terminated, personnel at JX 

Crystals disGovered that the short wavel bias of the tungsten emitter could be generalized 

to other metals. The Fgure 1 wrve "B" metal contains a common oxidiing metal catalyst. We 

demonstrated the usefulness of this curve "B" metal by fabricating a hydmca-n fKed TPV 

generator m-ng GaSb cells and a Bunsen bumer. Our 2 Watt demonstration unit is shown in 

Fgure 5 operating an AM I FM Itape "boom box". This demonstration unit consists of a ppane 

battle conneded to a Bunsen bumer with a coil of the curve "B" metal catalyst immersed in the 

flame senring as the IR emitter. The 20 GaSb cell circuit shown in Fgure 6 is then wrapped 

-209- 
around the IR emitter converting the infrared radiant eneqy into electric power. An illuminated 

current vs voltage curve for a GaSb cell circuit in a Bunsen bumer 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 1 to 5 microns. Note that the spectra from the blue flame from a Bunsen burner without 

an emitter inserted shows peaks at 4.5 microns and 2.7 microns. These emissions are 

associated with C02 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 failing 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 Cuwe "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 sewing 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 

1. 

2. 

3. 

4. 

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. 99-1 05, 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 Institute of Physics Conference Proceedings 321, pp. 291-297, 

1994. 

Fraas,L.M., Ballantyne, R.J., Samaras, J.E., Sea1,M.R. 'I Electric Power Production 

Using New GaSb Photovoltaic Cells with Extended Infrared Response," The First NREL 

Conference on Therrnophotovoltaic Generation of Electricity, American Institute of 

Physics Conference Procetklings 321 , pp. 44-53, 1994. 

Ewell,R., Chimielewski,A., Nesbith,W. and Sercel,J. "Therrnophotovoltaic Power System 

Final Report," JPL D-10814, May 1993. 

-21 0- 
0.4 

0.3 

0. 

0 

B ) catalytic metal 

6 8 

a vel ength (m i cron s) 

10 

e 1. EmissiWty of Metal Emitters 

-21 1- 
1 

0 

Temperature = 1300 K 

metal emitter 

1.74 micron peak 

0 2 4 6 8 10 

wavelength (micrometers) 

Figure 3. Spectral Emissive Power Shift with Metal Emitter 

1 

0.8 

0.6 

0.4 

0.2 

O 

a, 

0 

c: 

.I 

E 

e! 

I0 2 4 6 8 10 

Wavelength ( microns ) 

Figure 4. Near Term Dielectric Bandpass Filter 

-21 2- 
-21 3- 
0.3 

cn 

0. E 0.2 

4 

0. I 

0 

Pmax = 1.7 Watts 

voc = 7.5 Volts 

0 2 4 

Volts 

Figure 7. GaSb TPV Circuit with Gas Flame / Metal Mantle Light Source 

1 2 3 4 6 

Wavelength (mlcrometem) 

Figure 8. Emissive Power of Proprietary Emitter 

-21 4- 
InGaAsSblGaSb ERMOPHOTOVOLTAIC CELLS 

ZA. SHEULMBARGER, M.G. MAUK. and LC. D~NE~TA 

AstroPower, Inc. 

Solar Park, Newark, DE 19716-2000 

G.W. CHARACHE 

P.O. BOX 1072, SChmeCtady. MY 12301-1072 

S RY 

Seweral ~eQr~~~i studies have indicated Sb quaternary alloy 

GaSb substrate 

-21 5- 
bandgap of the quaternary alloy ranges from approximately 0.3 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 

absorption edge of 2380 to 2480 nanometers. 

It is worth emphasizing that the use of the quaternary 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 supetiattices, 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 wellestablished 

technology for Ill-V compound semiconductor devices. A major advantage of LPE for this application is the high 

material quality, and more specifically, the long minority carrier 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 chemical vapor deposition (MOCVD). Another major advantage 

is that LPE is a simple, inexpensive, and safe method for semiconductor device fabrication. Significantly, the LPE 

process does not require or produce any highly toxic or dangerous substancesin contrast to MOCVD. Also, the 

epitaxial growth rate with InGaAsSb LPE is -2 microndminute which is ten to hundred times faster than MOCVD 

or MBE. We have successfully scaled up the LPE process for epitaxial growth in a semicontinuous mode 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 lowcost. 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 InGaAsSb 

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 quart! tube placed in a microprocessorcontrolled, programmable, 

three-zone tube furnace. The growth ambient is palladiumdifised hydrogen at atmospheric pressure with a Row 

rate of 300 ml/min. 

The substrates are 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). 

Substrates are doped to 3-5 x 1017 cm" with tellurium. The substrate resistivity is 9 x lo4 Rcm, and the average 

etch-pit density is approximately 1000 cm-'. 

The growth solutions are indium (x,,=0.59), gallium (x,=O. 19), antimony (xsb=0.21), and arsenic 

(x,=O.Ol). 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 lnAs polycrystalline material. The total weight of the melt is about 10 g. 

Prior to growth, the melts are baked out at 700 "C for fifteen 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 = 

doping, a weighable amount of Te is added as 100 to 200 mg of Tedoped GaSb (&=lo'' ern-'). Tin is added to 

is problematic. For reproducible 

-21 6- 

4. TPV DEVICE EVALUATION 

We present external and internal spectral response and current-voltage characteristics for 1 em x 1 cm 

p-lno.,,Gao 92Aso o7Sb9 ,:Ge I n-ln0.08G~.92A~0.07Sb0.g~: Te (or Sn) epitaxial cells on an n-GaSb:Te substrate 

produced as described above. The external spectral response of a typical InGaAsSb TPV cell is shown in 

FIGURE 4. FIGURE 5 shows the corresponding internal spectral response. The lower external spectral response 

is due to grid shading and reflection of incident light from the uncoated InGaAsSb emitter surface. The grid 

shading is 18.2%. The absorption edge implied by the spectral response measurements of a number of samples 

ranged from approximately 2200 to 2250 nm. At a wavelength of 2000 nm, internal quantum efficiencies as high 

as 95% have been measured, and at a wavelength of 1 micron, internal quantum efficiencies of almost 55% have 

been observed. The internal quantum efficiency averaged over the spectral region from 1 to 2 microns 

wavelength is 60%. (It should be noted that for the intended TPV applications, the response of the cell for 

wavelengths less than 1.5 microns is not important.) 

The 1 cm x 1 cm InGaAsSb TPV cells were tested under simulated infrared light using a ZnSe-filtered 

tungsten source (Carley Lamps, lnc., Torrance, CA) with a spectral emission in the 800 to 3000 nm wavelength 

range. Under an illumination intensity corresponding to a short-circuit current density of 2 Nun2, open-circuit 

voltages as high as 0.260 volts have been measured. FIGURE 6 shows the current-voltage characteristics of a 

1 cm x 1 cm InGaAsSb TPV cell under an infrared illumination intensity that yields a short-circuit current density of 

62.4 Mcm2 and a open-circuit voltage of 0.178 V. The fill-factor is 0.57. To date, the best fill-factors observed 

are less than 0.6. We believe that one cause of the somewhat low fill-factors is series resistance, which is 

discussed further in the next section. FIGURE 7 shows open-circuit voltage vs. short-circuit current for varying 

light intensity. The open-circuit voltage increases logarithmically with illurnination intensity and an open-circuit 

voltage of -0.250 V is reached for current densities of 1 A/cm2. The diode ideality factor in the voltage range of 

0.1 to 0.25 V is close to 2, implying that high injection is dominant in this voltage range. 

5. CONCLUSION AND DISCUSSION 

Our results to date have demonstrated the potential of InGaAsSb TPV devices made by liquid-phase 

epitaxy. We believe there is still room for substantial efficiency enhancements in these devices by optimization of 

the doping levels and layer thicknesses. Further improvements might include wide bandgap lattice-matched 

AlGaAsSb window layers for front surface passivation, and AlGaAsSb back-surface field cladding layers to reduce 

the reverse saturation current and thereby increase the open-circuit voltage. Highly doped contact layers will 

provide lower series resistance, as will substrate thinning. Lower series resistance will lead to higher fill factors. 

Thinning the substrate will also improve heat sinking of the device. 

The required performance of a TPV device is dependent on its system application. Spectral control of 

thermal emitters, the use of selective filters and reflectors, heat transfer, and photon recycling effects need to be 

included in the device design and system optimization. These considerations are not usually relevant for 

conventional photovoltaic devices and therefore the design and optimization rules for TPVs will be significantly 

different than those for solar cells. For example, grid obscuration and reflection are not necessarily losses in TPV 

systems if photons reflected from the front surface are re-absorbed by the emitter. Our next generation of 

InGaAsSb TPV devices will incorporate design features to fully exploit photon recycling effects. 

REFERENCES 

A. ANDASPAEVA,.N. BARANOV, A. GUSEINOV, A.N. IMENKOV, L.M. LITVAK, G.M. FIIARETOVA, and Y.P. YAKOW. 

"Highly Efficient GalnAsSb Light-Emitting Diodes (A = 2.2 m, q=4%, T = 300K)" Soviet Technical Pbysies Letters 

14,s (1988) 377-378. 

-21 8- 
M. AsnEs, H. HILL, A.J. WILLLAMS, P.J. WRIGHT, and M.L. YOUNG, "Studies of the Ga,,ln~s,.,Sb, Quaternary 

Alloy System I. Liquid-Phase Epitaxial Growth and Assessment" J. Electronic Materials 15,1 (1986) 4149. 

A.N. BARANOV, A.M. LITVAK, 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 Ga,ln,.,As,Sb,, Solid Solutions from the Liquid" Russian J. Physical Chemistry 65,12 (1991) 1713-1 716. 

C. CANEAU, A.K. SRIVASTAVA, A.G. DENTAI, J.L. ZYSKIND, and M.A. POLLACK, "Room-Temperature 

GalnAsSblAIGaAsSb DH Injection Lasers at 2.2 prn- Electronics Letters 21, 18 (1985) 815-817. 

J.C. DEWINTER, M.A. POLLACK, A.K. SRIVASTAVA, and J.L. ZYSKIND, "Liquid Phase Epitaxial Ga,.,ln,As,Sb,., 

Lattice-Matched to (100) GaSb over the 1.17 to 2.33 urn Wavelength Range" J. Electronic Materials 14, 6 (1985) 

729-747. 

A.E. DRAKIN, P.G. ELISEEV, B.N. SVERDLOV, A.E. BOCHKAREV, L.M. DOLGINOV, and L.V. DRUZHININA, "InGaSbAs 

Injection Lasers" I€€€ J. Quantum Electronics QE-23, 6 (1987) 1089-1 094. 

A. JOULLIE, F. JIA HUA, F. KAROUTA, and H. MANI, *LPE Growth of GalnAsSb/GaSb System: The Importance of 

the Sign of the Lattice Mismatch" J. Crystal Growth 75 (1986) 309-318. 

H. KANo, S. MIYAZAWA, and K. SUGIYAMA, "Liquid-Phase Epitaxy of Ga,,ln~s,Sb,.x Quaternary Alloys on GaSb" 

Japanese J. AppliedPhysics 18,ll (1979) 2183-2184. 

F. KAROUTA, A MARBEUF, A. JOLLIE, and J.H. FAN, 'Low Temperature Phase Diagram of the Ga,.,ln,AsySb,, 

System" J. Crystal Growth 79 (1 986) 445450. 

N. KOBAYASHI, Y. HORIKOSHI, and C. UEMURA, "Liquid-Phase Epitaxial Growth of lnGaAsSb/GaSb and 

InGaAsSb/AIGaAsSb DH Wafers" Japanese J. Applied Physics 18,ll (1979) 2169-2170. 

J.B. MCNEELY, M.G. MAUK, and L.C. DINETTA, "An InGaAsSb/GaSb Photovoltaic Cell by Liquid-Phase Epitaxy for 

Thermophotovoltaic (TPV) Application" Proc. 7st NREL Conf on Thermophotovoltaic Generation of Electricrty, 

1994 T.J. Courrs and J.P. BENNER, eds.. (New York: American Institute Physics. 1995) 221-225. 

K. NAKAJIMA, K. OSAMURA, K. YASUDA, and Y. MURAKAMI, "The Pseudoquaternary Phase Diagram of the 

Ga-in-As-Sb System" J. Crystal Growth 41 (1977) 87-92. 

N. KOBAYASHI and Y. HORIKOSHI, 'Pseudoquaternary Phase Diagram Calculation of In,Ga,As,.,Sb, Quaternary 

System" Japanese J. Applied Physics 21,1 (1982) 201-202. 

K. NAKAJIMA, K. OSAMURA, K. YASUDA, and Y. MURAKAMI, "The Pseudoquaternary Phase Diagram of the Ga-lnAs-Sb System J. Crystal Growth 41 (1977) 87-92. 

E. TOURNIE, F. PITARD, A. JOUUE and R. FOURCADE. "High Temperature Liquid Phase Epitaxy of (100) Oriented 

GalnAsSb Near the Miscibility Gap Boundary" J. Crystal Growth 104 (1990) 683-694. 

M.M. WANLASS, J.S. WARD, K.A. EMERY, and T.J. Courrs, "Ga,ln,,As Therrnophotovoltaic Converters" /E€€ 

1st Wodd Conference on Photovoltaic Energy Conversion (New York: IEEE Press, 1994) 1685-1 691. 

JTCZUK, E. GAGNON, L. GEOFFROY. and T. PARODOS, "In,Ga,,As Therrnophotovoltaic Cell Performance vs. 

Bandgap" Proc. Ist NREL Conf on Thermophotovoltaic Generation of €lectric@, 7994 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 em InGaAsSb TPV Cell. 

7 AuIZnlAu p-type Contact 

0 3-05 pin Thick p-type inGaAsSb Emitter 6/-- 

t-- 3-5 pm Thick n-type InGaAsSb Base 

GaSb Substrate 

AulGelAuJNi n-type Contact 

IGURE 2: InGaAsSb/GaSb p-n Junction Thermophotovoltaic Cell Design. 

FIGURE 3: SlMS Depth Profile of Doping. 

-220- 
loo 

80 

60 8 

a 

B 

w40 

W 

Ix 

20 

0 

loo0 1200 1400 l6W t600 2000 2200 2400 

WAVELENGTH 

FIGURE 4: External Spectral Response of InGaAsSb/GaSb pn Junction Thermophotovoltaic Cell. 

aoo 1000 iu~l il~a MOO iaoo 2000 zoo 2400 

WAVELENGTH 

qGURE 5: Internal Spectral Response of InGaAsSb/GaSb p-n Junction Thermophotovoltaic Cell. 

-221- 
ii 

-25 

-1.0 -0.8 -0.8 -0.4 02 

It 

iV*ge M 

FIGURE 6: CL Tent-Voltage characteristic of 1 crn x 1 crn InGaAsSb TPV Cell. 

m ! 

1 .WE-05 

0 0.05 0.10 0-15 020 015 

V= M 

FIGURE 7: Open-circuit Voltage vs. Short-circuit Current for inGaAsSb TPV Cell under Varying Illumination 

Intensity. 

-222- 
MULTIJUNCTION InGaAs THERMOPHOTOVOLTAIC POWER CONVERTER 

t 

** r, 

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 98OC blackbody irradiation is reported. Eight InGaAs PN 

junctions grown epitaxially on a semi-insulating wafer were monolithically integrated in series to boost the 

- 0.4 V photovoltage per typical InGaAs junction to over 3 volts for the 1 cd 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 

multijunction TPV device. This is not a traditional tandem cell in which the junctions are stacked vertically. 

Eight 1 rnm long by 1 cm wide junctions are laterally connected across the device area. This multijunction 

design has the potential for lower 12R power loss since the smaller PN junction area limits the current to oneeighth that of the equivalent surface area. In essence, the current is traded for voltage to avoid the f R loss, 

analogous to the way power utlities avoid 1% 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 rnultijunction TPV 

does not yet work at the 100W/cm2 level; the figure is only meant to illustrate the concept. 

aedral Output sow fff 

-0.5vx 10 =sv &Iars Iooxh I for the same resiance 

Area of LPC 

Broken Into 10 

SeriesConneded 

Junctions 

Figure 1 Illustration of how ohmic power loss can be great& reduced with a multijunction TPV device 

-223- 
DEVICE DESCRIPTION 

The epilayer 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 monolithically. 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 grid to lower its series resistance, is of the 

lower resistivity N 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, heavilydoped N-lnP back surface fieldetch stop is grown. This acts as a minority carrier 

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 recombination velocity increases 

the photocurrent noticeably. This base layer, also N-type InGaAs, is more lightly doped than the buried layer 

(by two orders of magnitude) to increase the diffusion length. Low resistivity is not crucial since the current 

path is vertical (only 3pm long) unlike the buried N-InGaAs where current flows lateraliy along 1000pm. 

Finally, a heavily doped (1 el 9/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 (1 OX 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 mult.i]unction TPV device. Contact grid too fine too 

see; thin lines parallel to busbars are the interconnects between the junctions. 

-224- 
IV CHARACTERISTICS 

Figure 3 shows a dark and illuminated I-V curve for the multijunction TPV under a weak (i.e. small 

area) 980C Infrared Associates blackbody originally made for 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 

multijunction short-circuit photocurrent was 6mA (each junction is o.lcm2 so the current density per junction 

was 60mNcm2). 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 0.7& V multJunction TPV converter under weak 980C 

blackbody irradiation. Vertical scale is 2mNdiv and horizontal is 5OOm Vldiv. 

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 (“Rl”) situated two-inches Ch”) from the TPV device. 

In order to use an analytical view factor formula for two coaxial circular disks, the rectangular TPV device 

(1 cm by 0.8 cm photoarea) is approximated as a 0.2-inch radius (“W) circular area (equal to 0.8cm2, same 

as the original photoarea). The view factor “F for this arrangement is? 

h2 + R12 + R22 

2RI R2 

F=- R2 ( X-4- ) where X = 

RI 

or F is 0.009 (Le. the TPV receives -1% of the light emitted by the blackbody). The amount of power from 

the 980C blackbody above the 0.74eV bandgap(l.65vm 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 multijunction converter could convert. The amount of power incident on the device 

was then (0.009x5.9) 55mW. The power the device developed was (2.3Vx0.006Ax0.65) 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 

L.4 

300 

U 

a 0 200 

0 

c 

0 

s 

P 

100 

5745-2948-1 

0 

0 

e 

a 

a 

0 

e 

a 

a 

a 

e 

0 

0 

e 

0 

0 

0 

0 

a 

e 

a I- a 

O 

0.0 0.5 1 .o 1.5 2.0 2.5 3.0 

PHOTOVOLTAGE (V) 

Figure 4 IVs obtained with the mult@nction TPV cell at various illuminations with a solar simulator 

-226- 
OHMIC POWER LOSS CALCULATIONS 

Figure 4 above shows that this first prototype multijunction TPV 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: 

12R loss in buried N-type InGaAs lateral conduction layer 

1% 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/cm2 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: 

\ ++-E -4 

P lnGaAs Emitter tp, P p 

/ 

N InGaAS Buried layer tN, PN 

Figure 5 Definition of terms used in calculating loss in one-junction of eight in the TPV. 

Loss in buried W lnGaAs - The TPVs are on a semi-insulating InP substrate. The N+ buried layer 

must conduct the photocurrent to 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: 

Maximum photocurrent from the incident optical power (100 W/cd), 7.6 A, is at X = L - IC, where L is the 

junction length (1 mm), and "IC" is the interconnect width (100 pm). Here q is the external quantum 

efficiency (0.8) at an assumed narrow selective emitter peak wavelength A (1.315 pm), q (1.602~10~'~ C), h 

(6.626~10~ J-s) and c (3x108m/s) are constants, and W is the junction width (1 cm) of the multijunction TW. 

-227- 
The 12R power loss in the lower N' InGaAs layer is then given by: 

PN dx q2q242A2WPN L-IC L-IC 

Puss, = 1 Z2(X) dR = 1 Z2(X) - - 

0 0 'N 3h2c2tN 

where dR is the incremental resistance, & is the resistivity (1 04 R-cm) of Spire's N-type InGaAs (N - 3x1 Olg 

cm3 , mobility - 2000 cd N-s), and t, (4 pn) is the thickness of the N InGaAs buried layer. P- is 

0.45W/junction. The available incident optical power on a single junction is: 

PA", is low, so that FN , the ratio of 1% power loss in the N' buried InGaAs layer to the available 

optical power is 4.5% and is given by: 

Loss in upper P lnGaAs - The upper P InGaAs emitter is doped (- 3~10'~ ~m-~) simiiar 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 (900vm), 

with a contact grid the current now only flows across half of the gridline-to-gridline spacing S (100 vm), or 

50 urn. This 18X shorter distance than the flow in the 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) = 4t1W L y 

hc 

The maximum photocurrent at Y = S/2 is 0.042 A. The 1% power loss, P,,, , is: 

SI2 SI2 Pp 4Y Wq2q202k2LPp s2 

12 h 2c2tp -s I 2 

S 

-s I 2 

where W/S is the number of gridlines per junction, p,, is the resistivity (0.003 Q-cm) of Spire's P-type InGaAs 

(- 3x10'' cmS doping, mobility - 80 cdN-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%. P-, is then 0.52 W. Fp , the ratio of 12R power loss in the emitter sheet resistance to the available 

-228- 
optical power is 5.2% and is given by: 

Loss in fop contact grid metal - The top contact grid also has I Q loss. Gold has a resistivity p 

.of - 3x106 hm. The thickness (height) tu of the evaporated gold is 3 pm. The width "d" of a gold gridline 

is 5 pm. The power loss P, ,0.34 W per junction is given by: 

- 1 1 12(X) dR = - w L-Ic I 12(X) - Pm ak 

L-IC 

S 0 'M 

GRID s 

0 3h2e2dt, 

F,Ri, , the ratio of 1% loss in the grid to available optical power is 3.5% and is given by: 

Shadow Loss from Grid and Interconnect- We are now planning to use an interconnect distance 

"IC" of 100 pm out of a total junction width of "L' of 1000 pm. This means 10% of the total area is "dead" 

area (Le. IC/L). In addition, we are now using a top contact grid with gridline width "d" of 5 pm on 100 pm 

spacings "S", for an additional shadow loss of 5% (as). Total shadow loss (the largest loss mechanism) 

is 1 .SW/junction or 15% and is given by: 

I-AVA 3 

SUMMARY 

We have demonstrated a monolithic eight-junction 0.74eV InGaAs thermophotovoltaic converter with 

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 

e blackbody emission wavelengths above the material bandgap (1.65 ym cutoff wavelength). We 

ussed the oh~ic wer losses in the device. We calculated that an upper limit for 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- 
Table I Losses in P-on-N multijunction TPV design under 100 W/m2 of 1.3 pm light. 

Intrinsic 38% Cell Efficiency 

(loss due to Eg 0.74eV while 

REFERENCES 

1. S. Wojtczuk, T. Parodos, and G. Walker, ‘PIN In(AI)GaAs Multijunction Laser Power 

Converters,” Proc. of XI11 SPRAT NASA Conf. Pub. 3278, 1994, pp. 363-371. 

2. M.F Modest, Radiative Heat Transfer, McGraw, 1993, pp. 166-173. 

-230- 
d 

'* 

>."/ 

GALLIUM PHOSPHIDE ENERGY CONVERTERS 

P. E. Sims, L. C. DiNetta, K. Dugan Cavanagh, and M. A. Goetz 

AstroPower, Inc. 

Newark, DE 19716-2000 

SUMMARY 

reliability. Results are presented for GaP devices powered by Ni-63 and tritiarated phosphors. Leakage currents 

as low as 1.2~10~'~ Ncm2 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. 

Betavoltaic power supplies based on gallium phosphide can supply long term low-level power with high 

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 63Ni as a power source. Figure 3 shows data for an indirect conversion array. 

For the direct conversion system, the short-circuit current density is 1 .9x108 A/cm2. The short-circuit current 

density for the indirect conversion system is 2.4~1 0-7 Ncm2, better than an order of magnitude improvement over 

the 6JNi 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. 

InStnsneWrn 

, ..O..O..c.. % usm p- radiatm 

I :I I1 I1 11- 

@)a TritkKn Phosphor &$@ power poww 

ccmvmm 

FIGURE I. Indirect betavoltaic conversion. 

RESULTS 

Actual two-step tritium-fueled power generation has been accomplished since the last report [l] 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 700pL. 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 nNcm2. 

The average open-circuit voltage was 0.997 Wcell. 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 500pL 

and there are no mirrors on the tubes. The best short-circuit current achieved with this configuration was 19nA, 

which corresponds to 95 nNcm2 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 

-0 I 

--u 2 

r> -"i 

,- -03 

E, -05 

>- -u.c 

:t? -0 9 

SZ -1 

FE -1 1 LE-1 2 

f -1 3 " -1 4 5 -1 5 

4 -0.7 

-0.8 

- 

-1 .B 

-I 7 

-1.8 

-1 9 

-2 

0 0.2 04 0.ri 0.8 0 1 2 3 4 6 

ASRAY VOLTAGE IV) Cell V9ltOrJ.e (9 

FIGURE 2. GaP betavoltaic device fueled by direct FIGURE 3. GaP betavoltaic array fueled by two 

conversion of 63Ni beta-electrons [a]. custom-made T,-ZnS:Ag light sources 

FIGURE 4. Photograph of commercial tritium lamps (scale is in inches). 

-232- 
-14 

-111 

-38 

0 1 2 5 4 5 6 

VOLTAGE (V) 

FIGURE 5. GaP betavoltaic array fueled by three commercially available T,-ZnS:Ag light sources. 

I 

I I /I I 

i 

1 

I 

- ' 

The cell characteristics for the 12cell 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 eiements, a realistic prediction of the possible 

current generation density for this type of device is 720 nAlcm2 for the bright light configuration. The first factor is 

confirmed by the last section of Table 1, 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 

+ 

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 12x1 0-'7 Ncm2 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 11, very little current is required to 

generate high voltages with these devices. 

-233- 
Table 1. 

Cell data for array elements illuminated by = 8.24 pW/cm2 ZnS:Ag light [Ref. 1) 

A1 61 14-14-05 0.99 51 0.8 45 45 0.72 4.37 

A2 8114-10-05 1.03 54 0.8 48 48 0.69 4.66 

A3 61 14-10-07 0.95 52 0.8 48 48 0.78 4.66 

A4 8114-12-05 1.00 51 0.9 44 49 0.77 4.81 

A5 6114-10-04 1.03 51 0.8 50 50 0.76 4.86 

A6 8114-14-01 1.04 53 0.8 50 50 0.73 4.86 

81 8114-12-04 1.06 52 0.9 46 51 0.75 5.03 

82 6114-12-02 1.01 52 0.9 47 52 0.80 5.14 

93 6114-12-01 1.12 53 0.9 53 52 0.76 5.46 

64 6114-14-02 1.14 54 1 .o 50 53 0.81 6.07 

65 6114-10-01 1.08 55 0.9 52 54 0.79 5.68 

86 6114-10-03 1.16 55 1 .o 51 55 0.80 6.19 

6.27 1 4Ciff 5.25 116 121 @*4.8V 0.56 5.52 

R115-02-Ol t.19 106 0.9 tQO 104 0.71 10.93 Improved 

hiah-current 

GiP device 

61 15-05-02 1.1 1 111 0.9 108 109 0.79 11.8 Improved 

hgh-cunent 

GaP device 

1 cm2 GaAs~~ll 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 

v, 1, Jo2 

fnA) Wcm3 

8126-07-06 1.302 252 1.2~1~’~ 

Device (volts) 

8123-15-17 

6123-06-15 

8126-04-19 

81 23-1 4-22 

6123-05-16 

6124-01-09 

91 26-05-02 

8124-05-1 4 

8123-03-23 

1.288 

1.272 

1.281 

1.276 

1.266 

1.230 

1.212 

1.153 

1.154 

234 

300 

373 

344 

343 

273 

259 

207 

320 

-234- 
Figures 7, 8, and 9 display measured values for the dark current, quantum efficiency, and shunt 

resistance of a GaP device. These results are 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 duration to about 400°C. The high values of 

shunt resistance which have been achieved are particularly critical for this application. 

Figure 7. Temperature dependent leakage current of GaP p/n junction diodes [3]. 

400 500 600 

WAVELENGTH (nm) 

Figure 7. Temperature dependent external quantum 

efficiency of a GaP photodiode (no AR) [3]. 

0 100 200 300 400 500 

TEMPERATURE (C) 

Figure 10. Estimated shunt resistance of a GaP 

device at different temperatures compared 

to a commercially available silicon device 

[31. 

The experimentally determined power density for a direct conversion 63Ni fueled system is 

1.35~10" W/cm2, while the indirect conversion system power density was measured as 1.9~10-~ W/cm2. 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 tritium, 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 tritium light sources to develop safe, high output light sources 

for a variety of applications. AstroPower is collaborating with Encapsulite, Inc. (Maplewood, NJ) to develop 

prototype betavoltaic generators using Gap-based devices and microsphere illumination technology. The 

microsphere concept, which is the high pressure encapsulation of tritium and phosphors in small (4 mm dia.) 

spheres, holds promise for the achievement of significantly brighter light sources. Encapsulite Inc. believes that 

luminance’s of 1 to 10 FtL are possible using this approach (1 FtL = 753 pL). These light sources will enhance the 

power generating ability of the betavoltaic power conversion system by greatly increasing the incident light flux. 

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 statistics of the beta emitter. Useful generator lifetimes of 20-years are expected using tritium as a 

fuel. 

CONCLUSIONS 

Betavoltaic power generation using GaP devices and tritium powered light sources is technically feasible. 

A “realistic” power supply is proposed as having a Vmp of 5 V and an I,of 20 pA, which would require a converter 

area of approximately 140 cm2 using a projected current generation density of 720 nA/cm2. 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 missions as a housekeeping power supply. This device could both store power for periodic burst 

transmissions and comrnand/control receptions. It is also possible that this type of device could switch on a more 

powerful power source as the spacecraft reaches a vicinity where data acquisition activities are planned. 

References 

[I} P.E. Sims, L.C. DiNetta, and A.M. Barnett, ”High Efficiency GaP Power Conversion for Betavoltaic 

Applications”, Proceedings of the Xlll SPRAT Conference, June 1994. 

[2] G.H. Negely, “Gallium Phosphide Convefiers Optimized for use in Beta-Cells”, Final Report for Contract # 

09-2988, Sandia National Laboratories, Sept. , 1990. 

P.E. Sims, L.C. DiNetta, M.A. Goetz, and J.B. McNeely, “Embedded High-Temperature Optical 

Transducer for Real Time Combustion Controf, Workshop on High Temperature Paver Electronics for 

Vehicles, Fort Monmou€h. NJ, April 1995. 

[3] 

-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 

>. 

L\ 

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 

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 [l]. 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 GWGe solar cells on 

commercial spacecraft both feasible and 

Fixed solar 

Figure 1. M EASAT HS-376 Spacecraft desirable. The first commercial satellite to use 

GaAs/Ge cells is the MEASAT spacecraft 

(shown in Figure l), 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 began in 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. 

Type 

CMG/AR 

Test Art i c I e s 

Description Thickness Manufacturer I Quantity Of ClCs 

CMG glass with anti-reflective front 80 pm PST I 100 

?he 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 ClCs (cell-interconnect-coverglass 

assemblies). These assemblies were manufactured by Spectrolab, Inc.. The interconnects consisted of 

25 pm 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 Corning 93-500 adhesive. 

Several coverglass configurations were tested during the characterization test program. These 

included CMG coverglass [2] supplied by Pilkington Space Technology and Corning 021 3 coverglass [3] 

supplied by Optical Coating Laboratories Inc.. The coverglass types used in the test program are listed in 

Table 11. For the MEASAT flight panels, 150 pm thick CMGnRR was chosen. This covergiass 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 (Le. 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 pm. 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 pm. 

Table II. Coverglass Types 

-238- 
Table 1. MEASAT Solar Cell Assembly Characterization Tests 

I TEST 

Electrical Characterization 

Electrical output 

........................................................................... 

Covering gain/loss 

........................................................................... I 

Angle of incidence 

............................................................................ 

Temperature profile 

Spectral response 

Reverse bias 

............................................................................ 

............................................................................ 

........................................................................... 

Proton radiation 

........................................................................... 

Ultraviolet radiation 

........................................................................... 

Reverse bias after radiation I 

........................................................................... 

Solar absorptance 

Off-angle solar absorptance ........................................................................... 

Contact integrity 

Bending test ...................... n ................................................ t 

I RANG WDURATION I RESULT 

80° and at 85O 

1 Irradiation with 1 MeV electrons at 4 I Characterized performance 

(see Figure 6) 

Degradation less than JPL 

fluences from o to 1015 e/cm* 

Irradiation with 9 MeV protons at 

3.3301 O1 and 5.4301 O1 p/cm2 model prediction 

31 00 sun-hours 

Reverse bias at maximum voltage after 

1 MeV electron irradiation at 2.501 014 

e/cm* 

........................................................................................................................................................................ Degradation less than 1 Ol0 

Passed 

....................................................................................................................................................................... 

measurements at wavelengths 

between 0.25 to 2.5 pm. 

Measure solar absorptance at angles 

of incidence fro m 0 to 71O. 

0.77 with CMG/IRR coverglass 

eighted average over ........................................................................................... ..................................................... 

curved panel; 1600 cycles from -1 50 

curved panel; 1200 cycles from -1 50 

-239- 
For thermal cycling tests, solar cell assemblies were welded into strings, and bonded onto solar panel 

coupons. The first coupon had dimensions 30 cm x 30 cm, and contained 20 solar cell assemblies with 80 

pm thick CMG/IRR covers. The second coupon had dimensions 30 cm x 30 cm and contained a total of 1 2 

solar cell assemblies; four cells had 150 pm thick CMG/IRR covers, and the remainder had 80 pm thick AR 

coated covers (CMG and Corning 0213). Two of the cells on the second coupon with 80 pm 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 pm thick CMG/iRR covers and the remainder 

had 80 pm thick AR coated covers (CMG and Corning 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 111 for each 

configuration tested. 

3.459 x 6.904 

* Based on nominal bare cell dimensions (shown in Table) and a solar constant of 135.3 mW/cm2. 

'* Based on nominal coverglass dimensions (shown in Table) and a solar constant of 135.3 mW/cm2. 

Covering gain tests were performed by measuring the change in short-circuit current (Isc) obtained 

when placing acoverglass on acell, 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 

-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 85O. 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: 

where F(lrnp) = deviation from cosine factor for Imp 

Imp = current at maximum power measured at angle of incidence 8 

and the integral is taken over the range of angles from -9OO to +goo. 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/cm2 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 of 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, 1 014, 5.1 Oi4 and 1 015 e/cm2. 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. 

1.30 

1.20 

1.10 

s - 1.00 

8 0.90 

> 

0.80 

0.70 

0 fluence --.- 

- 1 E14 e/cm2 

5E14 e/cm2 

___O__ 1 El 5 e/cm2 

-*- 

Figure 2. Open-circuit Voltage Versus Temperature 

-241- 
7.2 

1.1 

0 

- UJ 

ut 

Q, 

N .- - 

E 0.9 L 

0 

z 

0.8 

0.7 

............................................................ T Irradiated at 1 E15 e/cm2 

-150 -100 -50 0 50 100 150 

Temperature, deg C 

Figure 3. Normalized Short-circuit Current Versus Temperature 

Temperature (deg C) 

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 GaAdGe 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 -- 

0.5 --- 

5 0.4 --- 

3 0.3 --- 

- 

E 

0 

P 

K 

g 0.2 --- 

0.1 --- 

0 Fluence -m- 

-1E14 

5E14 - 1E15 

-+- 

300 500 700 900 

Wavelength, nm 

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 Ise. 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 1 MeV electrons at a fluence of 2.5-1014 

e/cm2. 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 Res u Its 

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, lof4, and lOI5 e/cm2. 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 e/cm2 was 0.765. This 

value is slightly less severe than the value of 0.74 reported in the Solar Cell Radiation Handbook [7]. 

-243- 
Figure 6. Radiation Degradation With 1 MeV Electrons 

15 

Proton radiation was performed at the Caltech Van de Graff accelerator. Small area bare solar cells 

were irradiated with 9 MeV protons at fluences of 3.33010~1, and 5.43010~~ p/cm2. 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 181, the equivalence factor for 

conversion from 10 MeV protons to 1 MeV electrons [8] and the electron degradation cuwe shown in 

Figure 6. The predicted degradation factors were 0.83 and 0.80 for the two fluences 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. at. 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 torr. GaAdGe test samples included 4 cells with CMG/AR 

covers, 4 cells with CMG/IRR covers and 4 cells with AR coated Corning 0213 covers. Control samples 

included 4 silicon cells with fused silica covers and 4 silicon cells 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 within 1200 hours of exposure. The results are summarized in Table 

V. All GaAs/Ge solar cell assemblies showed a net degradation of less than 1%. 

Radiometric Prooerties 

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 Eiectro-Optical Systems Segment. The results for normal emissivity solar absorptance are 

summarized in Table VI. 

-244- 
Table V. Ultraviolet Exposure Results (3100 UV hours) 

Average loss in Isc, 

GaAs/Ge cells with GaAs/Ge cells with GaAs/Ge cetls with 

CMG/AR covers CMGIIRR covers AR coated Corning 

0213 covers 

1.4% 1.6% 1.5% 

raw data I I I 

Measured system I 0.9% 0.9% 0.9% 

effects 

Net loss in Isc 

(raw data less 

systems effects) 

0.5% 0.7% 0.6% 

GaAs/Ge cells with 

CMG/AR covers 

GaAs/Ge cells with GaAs/Ge cells with 

CMGIIRR covers AR coated Corning 

0213 covers 

I absorptance I I I 1 

Normal emissivity 

Normal solar 

Solar absorptance was also measured as a function of the angle of incident light for GaAs/Ge cells with 

CMGIIRR 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 71 O. To determine the effective solar absorptance with CMG/IRR 

covers on a spinning spacecraft, the cosine weighted average absorptance was calculated as follows: 

j a(e)cosede 

Effective solar absorptance = 

cos 8 de 

I 1 - - - - - - - - - - A 

0.84 I 0.84 I 0.84 

0.90 0.77 0.90 

where a = solar absorptance and 0 = 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 f 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 .O%; 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- 
Bending strength was measured by subjecting assethblies to bending around 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 pm thick covers and four cells with 150 pm thiek covers were tested. Cracking was first observed at 

a bending radius of 18 cm for assemblies with 150 pm thick covers, and at 15 cm for assemblies with 80 pm 

thick covers. 

Thermal cycling was performed on the 3 solar panel coupons described above. Coupons #1 and #2 

undercnrent 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 

(IAPSS). 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 no change (0.0%) measured 

in average power output. 

Summary 

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 

[l] P.A. lles 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. 11, 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. Gasner, 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 11 th Photovoltaic Specialists Conference, IEEE, 1975, p.184-9. 

-246- 
ADVANCED SOLAR PANEL DESIGNS' \P 

E. L. Ralph and E. B. Linder 

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 (Wlkg) 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 deveioped 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 GaAdcarbon 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 GaAsjcarbon mesh coupons have been fabricated and tested to successfully 

demonstrate critical performance parameters and results are also provided here. 

2.0 GaAs ISOGRID S LAR PANEL ASS~MBLY 

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 soler 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 srnallsat 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 primary 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/GaAdGe solar cells on carbon mesh substrate. 

-247- 
the solar cells from the substrate. Integral wiring reduces array assembly labor for reduced cost and schedule 

and improves reliability. 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 spacecraft 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 OC, Air Mass Zero (AMO) 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 fowarded to the 

smallsat developer who needs low-cost and high-performance solar arrays. Based on these benefits, it is 

concluded that GaAs composite isogrid 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 em 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 cell. 

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 

lsogrids are lightweight shell structures stiffened by triangular stiffening ribs integrally co-cured or bonded to the 

shell. Carbon fiber/polymer composite isogrid 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 spacecraWsolar 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 reductions combine with improved manufacturing processes to 

result in lower panel cost. Composite isogrid specific weight as a function of panel size is shown in Figure 2.2-2. 

.O GalnP2/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 GalnPz/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 -1 7s x 5P wired in 

parallel which provides 119 W at 34 V (AM0 28%). This represents a 67% increase in power output BOL (28OC) 

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. C X coverglass, 0.010 cm 

(4 mil) 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 pm silver-plated lnvar 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 lightweight design fabricated by Ketema Corporation. It uses a one half inch 

thick aluminum honeycomb core with high strength lightweight 0.005 inch thick T300/934 graphite 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/m2 (0.19 Ib/ft2)/ 

J 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 couton, one then gets a demonstrated specific power value of 105 W/kg (9.5 

kg/kW) and a value of 230 Wlm 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%. 

ltijunct~on 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- 
collection efficiency for each bandgap. Electrical data of a solar cell used in the panel coupon 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 production runs will start out with 2 cm x 4 cm by 200 pm 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 both 80 and 11 5 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 providing cost 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 multijunction solar cell cost be slightly higher (1520%) 

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, considering 

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 .O-1 Detailed weight estimates for each design appmach 

number of cells on panel 

verglass thickness (pin) 

efficiency (28 "C, AMO) 

Characteristics 

output voltage 01) 

output current (A) 

output power 0 

ne1 Substrate Characteristics 

Mass Analysis (all units in grams) 

gallium arsenide 

5.5 cm x 6.5 cm 

86 

64 cm x 55 cm 

75.1 W 

.94 kg 

80 Wlkg 

gallium arsenide 

5.5 cm x 6.5 cm 

139.7 

100 

19% 

0.86 

1.069 

0.91 9 

3.13 

43 

2 

4% 

0.36 

35.1 4 

2.1 4 

75.13 

graphite epoxy isogrid 

0.863 cm 

graphite epoxy plane 

508 pm 

1.3 kg/m2 

64 cm x 55 cm 

269.1 8 

71.19 

21.23 

12.9 

27.49 

0.8 

4.57 

9.0 

1.2 

457.6 

35.0 

Growth matgin 30.0 

Total Estimated Mass 940.1 5 

Phase II MJlMesh Array 

GalnP21GaAs/Ge . 

5.0 cm x 4.9 cm 

170 

50.8 cm x 88.9 cm 

119 w 

1.04 kg 

11 5 Wlkg 

GalnP2lGaAslGe 

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 

11 8.94 

AI honeycomb 

1.27 cm 

graphite epoxy mesh 

127 pm 

0.9 kglm' 

50.8 cm x 88.9 cm 

365.5 

95.8 

28.56 

17.0 

37.24 

1.6 

9.14 

9.0 

2.4 

406.45 

35.0 

30.0 

1,037.68 

-25 1 - 
Figure 2.0-1 The baseline GaAs composjte isogrid solar may provides 75 Watts and weighs less than 1 

ki/ogram. 

- b 

0 

v) 

v) 

6\ 

-252- 
Figure 2.0-2 Proof-Of-Concept GaAs composite isogrid panel coupon measurees 38.1 cm on a side 

-253- 
Figure 2.1-1 19% conversion efficiency solar cells are the most frequently produced by current production. 

J 

..... I ... . . . ...... . . . . . ... 

. . . . 

.... 

. . . . . . . . . . ...... 

r.*..::.-...\ 

,...... * 

-.-.:.5* 

1995 GaAslGe CELL PRODUCTION 

(36 cm ') 

.*.*:' 

APPLIED SOLAR 

OlSTRlBUflON (X) 

TOTAL POWER 10 KW 

*O i 

30 1: AVERAGE 10.5% 

I 

0 

16.5 16.8 17.1 17.3 17.6 17.9 18.1 18.4 18.7 18.9 19.2 19.5 

CELL EFFICIENCY ("!) 

Figure 2.2-1 Composite isogrid array technology uses a low-density triangular structure for high strength and 

stiffness to weight ratios. 

-254- 
Figure 2.2-2 Panels with edge lengths greater than 50 cm derive the most beneM from composite isogrid 

technology 

3 

0 

u 

a, 

g= 1.5 .- 

a 

a1 

E 

n. 

9) 

-4=+ 

0.5 

3 

cn 

0 

0 20 40 60 80 100 120 140 160 180 200 

Square Panel Side Length (cm) 

Figure 3.0-1 Multijunction carbon mesh panel provides 115 Wkg 

h 

b 

a 

E 

E 

00 

s 

889 mrn (35") 

Multiiunction Cell Panel 

Cell: GaInP~lGaAdGe 25 22%, 5.5 mil 

5.0 crn x 4.9 crn, 6 mil CMG cover 

Panel: 17s x 1 OP, 170 celldpanel, 1 I9 W at 34 V (AM0 28°C) 

-255- 
Figure 3.0. 

Figure 3.1-1 A 22.2% (Best) 2 cm x 4 cm 2J-GaAs solar cell used on the demonstration coupon 

n 

v) 

L 

(u 

(u -12 

a -[ € 

.08 

Q -06 

- 

c 

(u 

L 

L -04 

?j -02 Y 

0 

SERIRL NO. 44 Pm 

Isc = 128.R mA I V0.c = 2413.5 mV 

Cff = 82.3 i: I Eff = 22.2 2 \ 

-256- 
Space Qualification of UV and IR Reflecting Coverslides 

for G~AS Sotar Cells1 

. Andrew Meulenberg (301-926-3609) 

Consultant 

Gaithersburg, MD 20855 Q5 

ABSTRACT 

As part of the space qualification effort for blue-red reflecting coverslides designed for use with GaAs 

solar cells, the first long-term (3ooo hours) UV testing of unirradiated and 1 MeV electmn-ifradiated GaAs solar 

cells, with 4 types of multilayeraated coverslides to reduce operating temperature, has produced some 

All of the GaAs solar cells vifh rnultilavercoated coverslides display UV degradation. The laboratory data, 

extrapolated to 10 years in ortit, 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 solarcell shorturnit 

current during 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 multilayercoated 

coverslides, the use of g multilayercoated coverslides for extended missions (~1 year) cannot be 

recommended without prior flm testing. 

nsxpected results. Important conclusions from this study, which includes 1\M, parallel tests, are: 

e 

0 

NOTATION 

Coverslide Coatinas: 

ARR = 

IRR = 

UVR = 

BRR = 

DSR = 

SSR = 

antirefleding 

infrared (IR) reflecting 

uttravidet (UV) reflecting 

Mdreflecting(uvRonf~ IRRmback) 

double-sided coated coverslides (coating Contains both WR and IRR in 

single-sided coated coverslides (coating contains both UVR and IRR in 

the same nanwu-band-pass multilayercoating) 

the same narrowbandpass multilayercoating) 

I NTRODUCTWN 

Five types of coverslide coatings, deslgned for GaAs sdar cells, have been tested as part of a NASAspa , spaceflight quatification for Blue-RebReflecting (8RR). rnuWayewoated, coverslides. Covered 

cells have been tested for degradation from ~e thermai, sdar W, and radiation environments representative of 

near- txbb. In addiion, humidity tests were perfomred to determine the sensitivity of the multilayer 

This paper is based on VIlOrk performed at COMSAT Lahatoties under wnfracf from Goddard Space flight 

Center. The final analysis and paper presented hem ~lils funded by ttixCansulbnts. 

-257- 
coatings to pre-launch conditions. Coverslides and solar cells wre 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 primary emphasis in this paper is the UV degradation studies and their implications. 

A number of unusual results were obtained from this series 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 variations 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 (>loo0 hours) after electron irradiation (2). To further extend the effective UV exposure (of 

accelerate the UV degradation rate), a number of the coverslides wre inverted when mounted on the solar cells 

(to expose the critical multilayer coatings to full UV without the filtering provided by the coverslide which has 

built-in UW absorption characteristics). The effects of these variations will be described with the data from the 

tests. 

COVERSUDE AND SOLAR CELL CHARACTERISTICS 

The coverslides differ in the manufacturer of the ceriadoped microsheet material (Pilkington CMG and 

DowComing 0213) and in the types and manufacturer of coatings used. Van'ations in the results could not be 

attributed to the coverslide material. 

lNTELSAT-6 silicon solar cells (designated as 1-6) 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 ceriadoped 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 spedrum than are GaAs cells and therefore are more sensitive to any UV 

degradation of the coverslide assembly and/or contamination. 

As a baseline for comparison with the multilayercoated coverslides, standard CMG coverslides, with 

only an antireflective (AR) coating on the 'but" side (designated as ARR coverslides), wre included. The next 

level of comptexity is provided by the addition of infrared-reflecting coatings on the backside ("in" side) of ARcoated coverslides. This combination is designated as IRR coverslides. 

Three types of multilayer-wated, blue-red reflecting, coverslides (designed for GaAs solar cells) wre 

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 ceria-doped miausheet. The UV-reflection edge of 

the coating roughly matched the UV-absorption edge of the coverslide (-50% at 350 nm). This combination, 

designated ERR, minimized the loss in solar-cell shortcircuit cwrent (1%) Mile still providing a significant 

reduction in solar absorption of the non-useful welengths. 

The second type of blue-red-rejecting coverslide has a combined UVAR-reflecting coating deposited only 

on the front side. This single-side reflector (SSR) has a naTFcMRr bandpass than does the ERR coverslide and 

therefore a lower Isc; but, it rejects more unusable light. The last type coverslide has identical multilayer UVARreflecting coatings on both sides of the coverslide. This double-sided reflector (DSR) has a somewhat namr 

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 optical mismatch (of the DSR coatings with the 

adhesive) limits the advantage provided by the greater 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 
Figure 1 displays 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 coverstides wuld be 17% and 40°C respectively. 

1 

0.9 

0.8 

0.7 

0.6 

o 0.5 

o 

Iu 2 0.4 

0.3 

u 

0.2 

0.1 

0 

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

WAVELENGTH IN MICRONS 

Figure 1. Spectral Reflection of Near-Normally Incident tight From GaAs Solar Cells 

with Three Different Coverstide Types. 

Table I compares the average change in 1% when GaAs solar cells are covered with the 5 coverslide 

coating types of this test. These covering factors are somewhat higher than 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 1. Percent Change in Isc fmm Covering of GaAs Cells 

Coverslide ARR SSR BRR IRR DSR 

% Change 4.5 2.4 2.8 2.3 -4.8 

The difference beheen the ARR and other coated coverslides is -2% except for the DSR coverslide 

y 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, but they have no UV reflectors on the front surface. 

ich has a too na 

For reasons described in the Introduction, a set of the coverslides were mounted upside down on the 

GaAs solar cells. Cells with these inverted coverslides are compared in Table II and 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- 
Table II. Percent Change in Isc from 'Inverted" Covering of GaAs Cells 

Coverslide ARR SSR BRR t RR DSR 

'30 Change 1.5 2.1 2.0 -0.8 -5.0 

UV DEGRADATION 

UV degradation of the various coverslides was determined with tm 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, tw 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-siiiw 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 "internal" 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 Weanup" 

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 drifts in the solar simulator output intensity or spectrum. 

Normalization of the testell data with respect to the controlcell 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 unimdiated 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 intemally-shielded ARR cell, we made a linear degradation fit to the data for this cell. 

This '%ndowcontamination" cuwe indicates -4% degradation at 3000 hours. A 0% recovery in Isc, observed in 

cleaning certain cells at the conclusion of the test(Appendix A), was unique to the UVRGovered 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 betveen 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 UVRGoated 

coverslides (DSR, SSR and BRR) is based on the cleanup data and is slightly more than one half (0.6 times) that 

for the ARGoated 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 contaminationcorrection procedures is 

-260- 
indicated by the data of the ARR and 1-6 cells. The average of the 4 ARR cells at each measurement point in 

figure 2 falls within 0.4% of the 99.8% line (0.998k0.004). If we consider the 30 value to be -0.45%, then crq 

-.0.15% which is 1/2 or l/sqrt(n) of the single-cell statistical variation (a, - 0.3%), where n is the number of cells 

in the set. This standard deviation is just what wuld be expected for the size of the set tested. Fitting the AR 

covered cell data to a linear degradation curve wuld bring the standard deviation to (J = *0.2% (k0.002). 

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 vvould 

clearly fail that requirement. Comparison of the contaminationcorrected 1-6 cells in Figure 2, Mth earlier 

measurements (Z), indicates a consistency with their -2.5% degradation at 3000 UVSH. The 1-6 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 66% extrapolated degradation observed in many other extended UV 

tests on this cell type (e.g., References 2 and 3). 

5 0.99 

5 

0 

'3 0.98 2 

i5 

C 

5 0.97 

, ,\ , , , , ,) 0.95 ! I 1 

10 100 1000 1 OOOO 

UV Solar Hours 

Figure 2. UV Degradation (Test 1) of 1% for GaAs Solar Cells With Various Coverslide Types 

The DSR covered cells show the most rapid and greatest UV degradation of the GaAs cells despite their 

lowest spectml response to UV illumination. Both the SSR and the DSR covered cells displayed a quick drop in 

Isc of 4.5% during the early hours of the tgst. Total degradation of the SSR covered cells (beyond lo00 UVSH) 

is about one half that of the DSR covered cells, as couid be expected for coverslides with UV-sensitive multilayer 

coatings only on one side mther than two. EWmpolation to 10 years in space indicates about 5% degradation to 

the SSR coverslides 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 lo00 

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 wuld exceed 10%. While such losses seem unlikely, the data gives great 

concern about use of these coatings in space. 

Cells covered hth the IRR coverslides show an apparent recovery in isc with UV and vacuum exposure 

(-1% after lo00 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 muld bring the IRR, ARR 

and 18 data points down beyond lo00 UVSH. This could be as much as 1% at 3000 UVSH, which crvould 

indicate a saturation of the IRR UV degradation beyond lo00 UVSH, rather than a recovery. However, it muld 

also indicate a higher degradation in the ARR and 1-6 cells. As mentioned before, the 1-6 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 

condusions. It also indicates that the contamination has contributed a systematic error of 5 0.5%. 

The results of the BRR cells provides a wming that a change in refractive index, which may have 

moderated apparent UV degradation in Isc 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 

hth 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 (el 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 UVARR 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 lover degradation in the initial Test 1 data 

points), thus supporting the choice of contamination correction made in Test I for UVR mated coverslides. The 

Imr 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 shovved no initial drop in Isc with the multilayer coating properly placed under the 

coverslide in Test 1. now displays a 0.5!/0 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 mrredly oriented IRR coverslides in Test 1, Figure 2 (-2% vs. 0 - 1% at 3000 UVSH). If this 

result is a consequence of the unfiltered UU exposure to the multilayer coating, then the implication is that these 

layers are more sensitive to energetic radiation (UV With h e 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 GaAs cells 

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- 
1- 

0.99 -- 

5: 

2 

- 

a, 

> E 0.98 -- - 

C 

co 

C 

m 

.- 

Q, 0.97 

5 

0.96 

0.95 

ARRfit I 

-- 

-- I-16fit 1 \ 

\ 

I I \, , ,,,, I I I I 

99 - IRR-r fit 

--- ERR-r fit 

-*- - SSR-r fit 

----DSR-rfitI '- \: \ 

\ 

\ 

Figure 3. UV Degradation of Reverse-Covered GaAs Solar Cells in Test 2. 

The 1-6 silicon cell results from Test 2 are included in Figure 3 for comparison i4th those data in Test 1 

and to provide a visual baseline for the GaAs cell results. The wtve to fit the Test 2 data is identical to that used 

to fit the Test 1 1-6 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 environmen4 testing (electron and UV) performed on the coated coverslides, 

other standard tests were performed in the space qualification testing of these coverslides. Thermalcycling tests 

were of particular concern on the DSR and SSR coverslides since their multilayer watings introduced sufficient 

stress in the single-sided coverslides that they bowed; furthermore, slight edge fractures were observed in some 

of earlier DSR coatings on thicker coverslides. 

The cells were heated to 60°C with a flood lamp and then lowred over a liquid nitrogen bath to bring 

them to -150°C. After this vvorstese procedure (to slowly take the coverslide adhesive through the glass point), 

the cells were dipped in the LNz for a sligN 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 sign of bubbles, blisters, 

delaminations, or color changes associated with distress in, or caused by, multilayer coatings appeared. Several 

cells had losses in cell fill factor, which indicated that contacts andlor cell junctions were stressed by the severity 

of the test. However, no pattern of coverslide type was associated with the losses and such changes are not 

generally related to coverslides. 

No visual effects or changes in Isc were observed from this test. 

The humidity test, which is performed primarily to detect contact comsion 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 coverslide. Both electrical and reflectance measurements were made on the 

-263- 
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, v4th their 

"buried" multilayer coating, displayed the largest changes in reflectance. Despite a predicted loss in Isc of 4%, 

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. 

Multilayercoated 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 Mn'th ARcoatings-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 vvith 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 alwys deleterious and therefore 

predictable; refractive index changes are neither. Thus, extrapolation of experimental UV degradation data 

for multilayer-coated coverslides and cells becomes much less certain, if not impossible. 

Thermal improvements in solar-array power performance from multilayer merslides 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 24% for one year in space and 3-9% for 10 years (87600 hours) in space (compared to 

the 14% net improvement B-O-L from above). This means that after one year in a space radiation 

environment, the multilayer coated coverslides may 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 ARGoated 

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 multilayeraated merslides 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 r and/or array-size limited could be considerable. 

-264- 
Conclusion: Because of the observed trends and the uncertainty in extrapolation of data, the ability of 

multilayer-wated coverslides to benefit extended missions (>l 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 

surprising results. The measured sensitivity of these IRR multilayer coatings to specific outside influences 

explains a number of observations made during this report: 

0 The IRR coverslides initially displayed greater absorption than expected. 

0 The solarcellcovering factor vas lower than that for the comparable BRR and SSR coverslides 

0 Degradation in Isc from electron irradiation was higher for the IRR covered cells than for any others 

0 The degradation of correctly-mounted IRR coverslides on unirradiated GaAs cells displayed unusual 

behavior with UV exposure over time. No consistent trend ws observed, although the overall losses in Isc 

from UV were small (4%). 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. 

0 

REFERENCES 

1. A. Meulenberg, 'Photo-Recovery of Electron-Irradiated GaAs Solar Cells," Proceedinas. of the 14th 

Space Photovoltaic Research and Technoloav Conference, NASA LeRC, Cleveland, OH, October 24-26, 1995 

2. A. Meulenberg, "UV Testing of INTELSAT VII, Vlla, and Vlll Solar Cells," Proceedinas of the 13th Space 

Photovoltaic Research and Technolwv Conference, 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," Proceedinas of the 12th Soace Photovoltaic Research and Technoloav Conference, NASA LeRC, 

Cleveland, OH, 1992 

APPENDIX A. CLEAN-UP PROCEDURES AND RESULTS 

Test 1 

The Post-UV Cleanup 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 coveklides, 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 prwides a basis for 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 are cleaned. 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 of the cleanup procedure on the Test 1 solar cell short-circuit currents. 

These data are normalized relative measurements [Le., (In,) I (Idso)] with the initial values measured at the 

beginning of the UV test with the cells at 40°C and under vacuum. Within the statistics (al - 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-1, 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 f 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 coverslide 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 tu'thout 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 

~ndow 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 mould appear in Figure A-1 since the data are normalized against control cells Mich are not exposed to 

UV illumination. 

lT 

.c) 0.98 

t 

s 

0 

.- c( 

$0.96 

.- 

0 

-g 0.94 

- N 

E 

0 

0.92 

0.9 

ARR AVG 

- - - BRR AVG 

IRR AVG 

--- SSXAVG 

----- 

1-6 AVG 

0 2 4 6 8 10 12 14 16 18 20 

Measurements 

Figure A-1 Change in Normalized Isc for Test 1 Cells during Post-UV Exposure to Dry Nitrogen (3), 

Air (5). and Window(8) and Cell (12) Cleaning Procedures 

The Test 1 results (from the deanup procedure) indicate that the 1-6 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 -l%, 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 muld be reduced by 1% and the 

results wuld 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 1-6 degradation in Isc in 

Figure 2 would be increased by 1% at 3000 UVSH and agreement with Figure A-2 wuld be complete, except for 

the ERR cells. This only disagreement is then resolved by noting a different annealing rate for the ERR and SSR 

cells in Figure A-2. 

Test 2 

The Test 2 deanup 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 -- 

c 

c 

1.04 -- 

3 

2 1.02 -- 

i3 

.- c 

3 

0 1 2 3 4 5 6 7 8 9 10 11 12 

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 (1 1) 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 contml cell data for Test 23 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) muld 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- 
ONE YEAR OF FLIGHT DATA FROM THE PASP PLUS EXPERIMENT 

Henry Curtis 

NASA Lewis Research Center 

Cleveland Ohio 

Dean Manrin 

Aerospace Corp. 

Albuquerque NM 

ABSTRACT 

Results from 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 interactions between 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 elliptical orbit with an initial apogee 

of 2552 km and a perigee of 363 km. The inclination is 70 degrees. This orbit puts the spacecraft 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. 1 1 , 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 photovottaic panels containing a total of sixteen separate cell 

modules. Two of the modules are concentrators while the rest are planar. Table I lists the different solar cell 

modules. There are several different cell types on PASP Plus including silicon, GaAs, InP, amorphous silicon, 

AIGaAdGaAs, GaAdCIS, and GaAdGaSb. 

As noted in Table I, not all of the modules are biased. Ten of the individual modules are biased as part of the 

plasma interactions experiment. There are three panels with more than one individual module. 

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 GaAdCIS 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- 
modules are on the top payload shelf of the APEX spacecraft. Both the deployed panel and the payload shelf 

are pointed toward the sun. 

Table I PASP Plus individual Cell Modules 

eesl2d 

0 

1 

2 

3 

5 

4 

6 

8 

11 

9 

10 

7 

12 

13 

14 

15 

CellTvDe 

Silicon 2x4 cm 

Silicon 2x4 cm 

Silicon 2x4 cm 

Silicon 8x8 cm 

Silicon 2.6~5 cm 

GaAs 4x4 cm 

GaAs 4x4 cm 

GaAs 4x4 cm WT 

GaAs 4x4 cm 

Amorphous Si 4x4 cm 

InP 2x2 cm 

AIGaAdGaAs 

GaAdCIS 2x2 cm 

GaAslClS 2x4 cm 

GaAs concentrator 

GaAslGaSb 

AlraYu= 

Planar 

Planar 

Planar 

Space Station 

APSA 

Planar 

Planar 

Planar 

Planar 

Planar 

Planar 

Planar 

Planar 

Planar 

Cassegrainian 

Mini-Dome Conc. 

Bias 

No 

Yes 

Yes 

Yes 

Yes 

Yes 

Yes 

Yes 

Yes 

No 

No 

No 

No 

No 

Yes 

Yes 

G.& 

20 

20 

60 

4 

12 

20 

12 

4 

8 

1 

10 

20 

9 

3 

a 

12 

Gover 

6 mils 

6 

6 

5 

2.5 

6 

6 

6 

6 

20 

6 

24 

2 

2 

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 in the 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 orbii, 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 turned 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 turned 

on only a few times for about 15 minutes. The spacecraft was having problems with the battery charging 

circuits and all loads were turned off. There were eight other smaller down periods ranging from one to 5 days. 

Total down time for all periods longer than one day was about 91 days. 

-269- 
All the performance data presented in this paper has been corrected for the variations in the Earth-Sun distance 

(Isc & Pmax) and corrected to the average temperature of the first month of I-V data. Temperature coefficients 

were obtained from ground tests of the 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 1 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 bit for 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 

points which 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 first year was spent in sunlight. Since the 

apogee (2550 km) touches the radiation belts, we have a sign.ificant radiation dose. Calculations using AE8 and 

AP8 indicate an equivalent fluence of 9.7E14 1 MeV electrons for a silicon cell with 6 rnil covers and 60 rnil 

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% 

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 lnP 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-RO s with PASP Plus data along with orbital Parameters and Sun and spacecraft 

position vectors. This ai s 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 yrifh 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 into the mission. This makes it difficult to obtain an accurate 

initial flight value. However we do have a predicted curve based on the actual orbital parameters, AE8 and 

AP8, and the known relative damage coefficients. Therefore fitting the 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. me upper curve, labeled "half", is the AEWAP8 curve with the 

degradation cut in half. This is not mathematically correct but it does indicate the magnitude of the difference. 

For all cases where good damage coeff ici 8 and AP8 overstate 

the degradation by about a factor of two. 

investigation. 

For the remainder of this paper 

are available for the e 

6 shows Pmax for the 

modules are open on 

modules with the thin cell silicon (2.5 mils) APSA module degrading somewhat less than the thick cell silicon (8 

s are available, a similar pattern occurs. 

he time of this conference, this problem i 

-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 anays are designed for operation in 

orbis with much less radiation. 

The I-V parameters for the Mini-Dome concentrator module are shown in Figure 7. The degradation in Pmax is 

only about 7% for 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 difficutt to 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 significant drop 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 

m-ne current mismatch effects. The cell had a 20 mil cover so radiation damage should be small. In figure 10, 

"le data for the mechanically stacked GaAsiCIS module are shown. The drop in Pmax is about 8% with very 

tAtle 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. 

During the 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 2-93 

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 2-93 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 orbii. 

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 2-93 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. Olson, Early Results from the PASP Plus Flight 

Experiment, First World Conference on Photovoltaic Energy Conversion, Dec. 1994, pages 21 69-21 72. 

-271 - 
-1 

uFigure 1 - Temperatures for each panel during 

first month of operation. 

4 100 xa 3110 

Day8 from Launch 

Figure 5. Pmax (clear view) for silicon module 

#o with predicted curves. 

I 

0.1 1 0 100 a00 3110 00 

Days from Launch 

InP Silicon 0 GaAs 

Figure 2. Normaiiied Pmax for InP, GaAs, & 

Si, 6 mil covers-thick backs. 

Figure 4. Pmax for siiiwn module #0, (40-50 

degree view). 

tm 

I 

an 

aY 

I 

0 IOD ma m 

D8ySfromLWCh 

m SprCeStation e APSA 

Figure 6. Normdied Pmax for flexible 

modules. 

-272- 

40 
Figure 7. Normalized I-V parameters for MiniDome concentrator module. 

1 I 

f 09 

3 

YI 

im 00 3m 

DayafmmLauach 

.I% +VOC *Rnu xFiU 

Figure 9. Normalized I-V parameters for 

amorphous silimn module. 

I 100 PD 300 40 

Tkna (min) 

Figure I 1. Temperature of silicon panel during 

several non-ecliise periods. 

1.1 

I 

Q) 

u 

ai 

!as 

103 

0.4 

03 

01 

ai 

4 

I 

I 1 I 1 

off-pointing Angle (&g) 

Figure 8. Off-pointing performance of MiniDome concentrator module. 

Lu yI, a YI) PI 3m 40 

D.yrfmrn- 

~Irc +VOC ~kxPii 

Figure 10. Normalized I-V parameters of 

GaAs/CIS module. 

-273- 
I ‘3 .-’6?C)l 1 (><”%“”) * 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 

---a. 

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 (PUGPS) 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 interac ns 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. 

Snapover was first observed at NASNLeRC (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. (re&. 6 and 7) This occurs when the cell potential is above the first crossover for the 

coverglass or the array support structure. 

At high positive potentials, typically over 200 V, the current rapidly rises due to a phenomena called snapover. 

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 

4 

5 

6 

~ ~~ 

Thin GaAs/Ge with wire interconnects 

APSA with germanium coating 12 2.6 cm x 5.1 cm 

Thin GaAs/Ge with wire interconnects 

20 4 cm x 4 cm 

12 4 cm x 4 cm 

8 

11 

CALCULATIONS 

~~ ~~ ~~ 

Thick GaAs/Ge with wrap through 4 4CMX4Cm 

interconnects 

Thick GaAs/Ge with wire interconnects 8 4cmx4cm 

The computation of the current collected by a specific solar array can become intractable. The gap 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 appears odd at first glance. 

+0.35 

,,0.65 Sheath Area = a A(geom.) - (+ exp 

The parameters are 

potential with respect to the plasma 

plasma temperature 9= 

-275- 
first cross over 

potential with respect to the plasma q= 

debye length 

cell thickness A= 

The $/q factor is proportional to the potential. The Q q factor 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 22nd 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. 

Leakage Current (A) 

2.68 x 1 0-14 Density (~n-~),/Temperature (V) 

CollectingArea m-2 = ( 

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 appiied bias rises one order of 

magnitude. This is typical of leakage current when snapover plays a role in the current collection 

process (refs. 12-1 4). 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. 

the various test solar arrays. 

In general the collected current is several times the array area. Table 2 gives the array and panel areas for 
Table 11 Array and Panel Areas. 

I ~~ I #8 I 0.0064 I 0.029 

I I 0.013 I 0.029 I 

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 pA. This means that the collecting area levels off at 

5 x 10 for a plasma density of 3 X 10’and at 5 x lo4 m-2 for a plasma density of 3 X 10”. 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. 

-3 -2 m 

DISCUSSION AND COMPARISON WITH CALCULATION 

In order to compare the measurements with the model described above, it is first necessaw 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. 

respect to the plasma is computed using the above model. The ion current collected by the solar cells is assumed 

to be negligible. The 2-93 paint on the surfaces of the panels is taken to have a conductivity divided by thickness 

of 10 mho rn-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 phgtoelectron current emitted is taken to be constant at 2 x lo4 A mq2 

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-’ in a 10 lo m3 plasma 

collects about 0.13 mA. This is the same as the electron thermal current for a 10” m3, 0.1 eV plasma to a 

1.5 m2 object. The floating potential of APEX shifts when the collecting area exceeds 1.5 m2. Therefore the 

2 collecting area versus applied bias curve flattens out at 1.5 m . 

The electron current collected by solar cells of the power solar arrays that are at positive potential with 

-6 

-277- 
The flight measurements were taken over a period of months under a variety of conditions. Plasma 

constituents, plasma temperature, and sunlightleclipse 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 MR. 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. Figure 4 shows the effective circuit. Current is 

collected across the entire surface of the array. 

The measured current lo is given by the following: 

I,=fIp+- bbias 

R 

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. Vaughn, J.A., et al.: Extrapolation of Electrical Breakdown Currents from the Laboratory to Space Station. 

AlAA 92-0822, AlAA 30th Aerospace Sciences Meeting, Reno, Nevada, 1992. 

2. Ferguson, D.C.: The Voltage Threshold for Arcing for Solar Cells in LEO - Flight and Ground Test Results. 

NASA TM 87259,1986. 

-278- 
3. Purvis, C.K.; Stevens, N.J.; and Berkopec, F.D.: Interaction of Large High Power Systems with Operational 

Orbit Charged-Particle Environments. NASA TM 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. 

Kennerud, K.L.: High Voltage Solar Array Experiments, NASA Rep. CR-121280, 1974. 

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. Appl. Phys., vol. 52, 1981, pp. 7092-7098. 

4. 

5. 

6. 

7. 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. 

8. Guidice, D.A.: High Voltage Space-Plasma Interactions Measured on the PASP Plus Test Arrays, this 

proceedings, 1995. 

9. Davis, V.A.; and Gardner, B.M.: Parasitic Current Collection by Solar Arrays in LEO, NASA Conference 

Publication 3278, 1994, pp. 227-235 

10. 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. 

11. Gardner, B.M.; Jongeward, G.A.; Kuharski, R.A.; Wilcox, K.G.; and Rankin, T.V.: The Environmental 

Workbench: A Design Tool for the International Space Station. AIAA 95-0599, AlAA 33rd Areospace 

Sciences Meeting, Reno, Nevada, 1995. 

12. Grier, 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. 295-31 4. 

13. Grier, N.T.: Plasma Interaction Experiment II (PIX 11): Laboratory and Flight Results. Spacecraft 

Environmental Interactions Technology, 1983 , NASA Conference Publication 2359, AFGL-TR-85-0018, 

1985, pp. 333-347. 

14. Ferguson, D.C.: RAMMAKE Effects on Plasma Current Collection of the PIX II Cangmuir Probe. Spacecraft 

Environmental Interactions Technology, 1983, NASA Conference Publication 2359, AFGL-TR-85-0018, 

15. 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.) 

1985, pp. 349-357. 

-279- 
lons attracted to nagativt Electrons attracted to positive portion of solar arrays portion 

lons attracted to 

spacecraft body 

Figure 1 .-At the spacecraft floating potential the net current is zero. 

This current has several components. 

-280- 
PASP Plus Array #I, Ram, Emitter Off, Day 9421 5 to 95134 

Temperature (AbV Angle (All) 

10 

1 

N 

E 

E 

1 

m 

a 0.1 

3 

ol C 

1 0 

- Q) 

0.01 

0.001 

..................... 

ie3.16E+09: 

jSl.OOE+lOi 

iA3.16E+10: 

jBl.OOE+ll j 

iC3.16E+11 i .................... 

10 100 1000 10000 

PASP Plus Array #4, Ram, Emitter cjff, Day 9421 5 to 951 34 

10 

1 

h 

Iu 

E 

E 

u. 0.1 

- 

m 

0) C 

.- + 

- 8 

6 

- 

0.01 

0.001 

.................... 

ie3.16 E+09 j 

iSl.OOE+lO; 

jA3.16€+10; 

j&l.OOE+ll; 

!^3.16E+111 ................... 

PASP Plus Array #2, Ram, Emitter Off, Day 94215 to 95134 

Temperature (&V Angle (A4 

10 

1 

N 

E 

?? 

4 0.1 

m 

tJI 

.- 

.d 

- k! - 

0 

0 

0.01 

0.001 

$3.16E+11 .................. ; 

.................... 

i&16E+09 i 

lBl.OOE+lO: 

:A3.16E+10: 

:*l.OoE+ll' 

10 100 1000 10000 

PASP Plus Array #6, Ram, Emitter Off, Day 9421 5 to 95134 

Temperature (AWV Angle (All) 

10 

1 

N 

E 

E 

1 

m 

a 0.1 

- 8 

8 

ol K 

.- + 

- 

33 16E+09 

81 00E+10 

A3.16E+10 

$1 00E+11 

..3 16E+ll 

10 100 1000 10000 10 100 1000 10000 

Figure 2.-Experirnental collecting area versus applied bias curves. 

-281- 
PASP Plus Array #3, Ram, Emitter Off, Day 94215 to 95134 

Temperature (AWV Angle (All) 

10 

1 

P 

E 

2 

a 0.1 

- 

m 

.- P 

s 

L 

0 

- a 

0.01 

0.001 

*3.16E+09 

I l.OOE+lO 

43.1 6E+10 

1.00E+11 

:3.!6E+ll, 

10 100 1000 10000 

PASP Plus Array #8, Ram, Emitter Off, Day 9421 5 to 95136 

Temperature (AWV Angle (All) 

10 

1 

0.1 

0.01 

................... 

%3.16E+09; 

;E l.OOE+lO; 

143.1 6 E+ 1 0 ; 

j& l.OOE+ll! 

;:;3.16E+lll 

................... 

0.001 

10 100 1000 10000 

PASP Plus Array #5, Ram, Emitter Off, Day 9421 5 to 95134 

Temperature (AllbV Angle (All) 

10 

1 

- 

N 

E 

2 

- 

m 

a 0.1 

s 

C 0) 

0 

.- 

L 

- m 

0.01 

0.001 

.................... 

i*3.16E+09 i 

jBl.OOE+lO! 

h3.16E+lO 1 

ie t.OOE+ll; 

%3.16E+ll i .................... 

10 100 1000 10000 

PASP Plus Array #11, Ram, Emitter Off, Day 9421 5 to 95134 

Temperature (All)eV Angle W) 

10 

1 

N 

E 

2 

4: 0.1 

m 

c 0) 

.- 

w 

- % 

0" 

- 

0.01 

0.001 

.................... 

113.1 6E+09 j 

jll.OOE+lO: 

!/3.16E+10 

i%l.OOE+lli 

i83.16E+1 1 

.................... 

10 100 1000 10000 

Figure 2.-Experimental collecting area versus applied bias curves. 

-282- 
PASP Plus Array #I, EPSAT 

c.. ............... 

{+3.16E+09i 

jOll.OOE+lO: 

!&3.16€+10: 

Iel.OOE+ll' 

.................... 

gp'c'i" 

&< 

PASP Plus Array #2, EPSAT 

10 

1 

N 

E 

2 

- 

rn 

a 0.1 

UJ c 

0 m 

.- - 

- 

3 

0.01 

0.001 

10 100 1000 10000 

10 

1 

A 

cu 

E 

m 

Y 

0 

B 0.1 

.- P 

- 8 

0" 

- 

- 

0.01 

0.001 

10 100 1000 10000 

PASP Plus Array #4, EPSAT 

r.. ................ 

:.3.16E+09! 

is 1 .OOE+ 10: 

iA3.16€+10; 

jBl.OOE+ll: 

!::3.16E+11 i * ................... 

10 

1 

- 

P 

E 

E 

cn 

- 

m 

a 0.1 

c 

0 m 

0 

0 

.- 

L 

- 

0.01 

0.001 

10 100 1000 10000 

PASP Plus Array #6, EPSAT 

93.16 E+09 

a1 00E+10 

A3.16E+10 

(B 1 00E+11 

:::>3.16E+ll 1 ..................... 

,e- .* J 

0.001 f 

10 100 1000 10000 

Figure 3.-Computed collecting area versus applied bias curves. 

-283- 
PASP Plus Array #3, EPSAT 

10 

1 

0.1 

0.01 

................... 

ie3.16E+09: 

iLl.OOE+lOi 

:&3.16E+10/ 

ir 1 .OOE+ll i 

i%3.16E+ll I ................... 2 

0.001 

10 100 1000 10000 

10 

1 

0.1 

0.01 

PASP Plus Array #8, EPSAT 

.................... 

:+3.16E+09: 

iP 1.00Ei.10; 

j43.16E+lO: 

jB l.OOE+ll; 

!'.3.16E+1 lj .................. 

0.001 

10 100 1000 10000 

10 

1 

F 

E 

Y 

m 

E! 

a 0.1 

0.01 

PASP Plus Array #I 1, EPSAT 

_.........___m__. 

.................... 

i*3.16€+09 i 

j%ll .OOE+lOj 

:/3.16E+10! 

ifll.OOE+ll i 

0.001 

10 100 1000 10000 

Figure 3.-Computed collecting area versus applied bias curves. 

-284- 
plasma 

Figure 4.-Effective circuit for collection of current by array 5. 

PASP Plus Array #5, Ram, Emitter Off, Day 94215 to 95134 

Temperature (AWV Angle (All) 

10 

3 16E+09 ' 

a 1.00E+10 F A 3.16€+10 

- E so 100E+ll - .3 3.16E+ll 

2 ---LEO 1.00€+10 

51 

0 

a 

E 

5 

8 

I 

E 

K 

fl: 

5 0.1 .- 

u) 

m '"T 

$ 0.01 

s 

m 

rn 

.- c 

- 

0.001 

10 100 1000 10000 

Figure 5.-Array 5 experimental collecting area connected for current through 

Germanium coating compared with NASCAPILEO calculations. 

-285- 
HIGH VOLTAGE SPACE-PLASMA INTERACTIONS 

MEASURED ON THE PASP PLUS TEST ARRAYS 

D.A. Guidice S3b 

Phillips Laboratory, Geophysics Directorate 

Hanscom AFB, MA 01 731 

);q ---5OG 2- 

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 photovoltaic subsystems 

at higher voltage levels to minimize 12R losses and/or reduce cable weight. New solarcell 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 environment. The Photovoltaic Array @ace Power pius Diagnostics 

(PASP Plus) experiment was developed and flown for this purpose. fn 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 Bdvanced Photovoltaic and Electronics Emeriments) mission, set 

up to fly it and two small "radiation effects on electronics" experiments, CRUWCREDO 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 8-52, 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 datacollecting 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. 

Q 

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 

6. interactions-measuring instrumentation that will quantify what happens when the ambient or 

d. diagnostic sensors to measure the environmental conditions affecting array performance. 

which measurable environmental interactions will occur. 

created conditions impact the performance of the test arrays. 

Solar Arrays 

To maximize the scope of the investigation, a variety of conventional and advanced-concept solar arrays 

were included in the array complement. 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 1. PASP Plus Solar Arrays 

Module Cell / Array Number Cell Size Array 

- No. Type of Cells Jcm x cml Developer 

0 J,Z Si (Std) 8 mil 20,20,60 2x4 

- 3 Si 8 mil wtc [Space Stal 4 8x8 

- 5 Si 2 mil [APSA] 14 2.6 x 5.1 

9 Amorphous Si 2 4x4 

2- 46 GaAs/Ge 3% mil 20,12 4x4 

- 8 GaAs/Ge 7 mil, wtc 4 4x4 

- 11 GaAs/Ge 7 mil 8 4x4 

RCA I WL 

Lockheed 

TRW 

TRW/J PL 

ASEC 

Spectrolab 

Spect rolab 

7 AIGaAdGa As 20 2x2 vs Corp. 

10 InP 12 mil 10 2x2 Spire/N RL 

12,13 GaAs/CulnSe2 12,3 2x2,2x4 Boeing 

- 14 GaAs Mini-Cass 8 Concentrator TRW 

15 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). 

Control & Diaanostic 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- 
Lanamuir Probe (LPI: To measure low-energy plasma parameters (density and temperature). 

Transient Pulse Monitor (TPMh To detect and characterize the arc-discharges that occur during the 

negative-voltage biasing of the test arrays. 

Electrostatic Analvzer (ESAL: To measure 30 eV to 30 kev 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 (1 12 cm corner 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 holding the 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, CRUWCREDO 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 instrumentation 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 k0.5 degrees (needed for operation of the concentrator arrays). 

Fig. 1. Advanced Photovoltaic and Electronics Experiments (APEX) Satellite. 

-288- 
ITEM NO. INSTRUMENT 

18.1 9,20,21 TPM E-lieM Sensors 

23 CP ~oorn & Sensor 

27 ?ASP Sun Sensor 

30.31 Quartz Crystal Microbalances 

32-33. 34 Calorimeters 

Fig. 2. PASP Plus Jnstrument and Test 

. Array Layout . 

ITEM NO. 

0,l.Z 

3 

4.6 

7 

8 

9 

10 

I1 

12.13 

14 

15 

3615) 

$ASP TEST ARRAY 

St Standard 

Si WrapThru (Space Sta) 

GaAs on Ge. 3.5 mil 

AlGaAslCaAs 

GaAs on Ge,7 mil, Wrap-TJm 

Amorphous Si 

InP 

GsAs on Ge, 7 mi) 

GaAslCulnSe, 

G&s Mini-Cassegrainian 

GaAs/GaSb Mini-Dome 

Thin Si(2mit). APSA 

DATA GAMERING 

The PASP Plus team undertook experiment commanding and quM-look data evaluation (using our on-site 

GSE) at the Satellit@ Control Facility at Onjzuka Am, CA. Atter tum-on and checkout of the PASP Plus 

instruments and some initial t-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 13 Aug 95, with several bng 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 sta&xf again OR 75 Jan 95. In the spring of 1995, problem with other satellite 

subsystems and the time needed to find the appropriate fixes and PASP Plus operatingtime limitations 

caused biasing operatbns.to be halted from 17 May to i Jul 95. Enally, on 12 Aug 95 a PASP Plus 

controller hardware problem resulted in the 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 111 does the Same for the negative biasing periods. 

-289- 
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-290- 
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-291- 
POSITIVE BIASING 

Positive biasing of the test arrays was done to investigate the "snapover" 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 (Le., 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 

shield both interconnects and cell side edges, have insignificant leakage 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 +lo0 V, although for the old-construction silicon array (Modules #1 

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 (era,,, = <goo) data only. Davis (5) of S-Cubed has undertaken a detailed 

study of the parasitic (leakage) current collected by the PASP Plus arrays for €Irarn = 190" orientation, 

comparing the results with analytic models developed using the EPSAT computer code. 

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 SAMPIE 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 SAMPIE 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 SAMPIE in light of the mountingconfiguration and other differences and has found that the results for both experiments are comparable. 

400 - 

W 300 - m 

m, 

-v) 

o= 

>o 

m 

- 

In? 

* 200- 

.- 

100 - 

.. 

Start: 9 

3 Aug 1 

50° I0 

4215 

I994 

- 

- Current 

- (Amps) I I I 

Array 1: Silicon (Si) 

Emitter OFF 

RAM Condition 

End: 9 

11 Aug - 

15223 

10-3 

10-4 

1 o-= 

-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.I.T. (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 (Le., metal interconnect and cell side-edge 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 (>lo9 m-3 in 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 [Le., 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 1V below. For the 

conventional planar arrays, there is general agreement between the PASP Plus results and the computergenerated simulations. 

1 

U 8 0.8 1 

0 

0 100 200 300 400 500 

Bias Voltage, -Volts 

Fig. 4. Average Arc Rate versus Bias Voltage for the Standard 

Silicon Array, Module #2. 

-293- 
Table IV. Arcing Onset Voltages f4r *e Biased PASP Plus Planar Arrays; 

Experimental Measurements and Computer-Generated Simulations 

p- 1.2: 0 

1: 

r 0.8 : 0 

PASP Plus Planar 

Test Array 

In-Space Measured Computer-Simulated 

Onset Voltaae Onset Voltaqe 

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 

with conventional interconnects 

11 7-mil GaAs on Ge -180 V -180 V 

Ion flux (Fion = nion vion cosc), 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 e,, = BOo. 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 c),, was 5105' (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 -7OC due to its double-sided exposure, very low mass, and thermal isolation. 

B 

2 

5 ~~1 0.2 

0 100 200 300 400 500 ~ 

Bias Voltage, -Volts 0 

0.6 

0.4 

0.2 

0 

0 100 200 300 400 500 

Bias Voltage, -Volts 

Fig. 5. PASP-Plus Measurcd and Computer Simulated Arc Rates for Module #2 (Standard Si Array) vs. 

Bias Voltage at Array Tempcratures of +38C [Icft] in Sunlight and -42C.[right] in Eclipsc. 

-294- 
CONCLUSIONS 

In spite of launch delays and on-orbit satellite problems, the PASP Plus 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 spaceborne photovottaic 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 photovottaic systems based on the most 

extensive on-otbit data base ever acquired for this purpose. 

REFERENCES 

Guidice, D.A., "PASP Plus: An Experiment To Measure Space-Environment Effects on Photovoltaic 

Power Subsystems," Fifth Annual Workshop on Space Operations, Applications and Research 

(SOAR '971, NASA Conf. Publ. 3127, pp. 662-668, Feb. 1992. 

Curtis, H., Guidice, D., Severance, P., and Piszczor, M., "Results of Thermal Vacuum Tests for the 

PASP Plus Flight Modules,'' Proc. of XI1 Space Photovoltaic Research and Technology 

Conference, NASA Conf. Publ. 3210, pp. 289-297, May 1993. 

Guidice, D., Severance, P., Curtis, H., and Piszczor, M., "Investigation of Space-Environment 

Effects on Photovoltaic Technologies by the PASP Plus Experiment", AIM 95-0371, 

Aerospace Sciences Meetinq, 9-12 Jan 95. 

Adarns, S.F., "Photovoltaic Array Space Power Plus Diagnostics Flight Experiment: Pre-Flight 

Description of Experimental Photovoltaic 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 SAMPIE", 1995 Space Power Workshop, Phillips Lab., 18-20 Apr 95. 

Cho, M., and Hastings, D.E., "Dielectric Charging Processes and Arcing Rates of High-Voltage 

Solar Arrays," J. Spacecraft & Rockets, Vol. 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. 2-1 0). Spacecraft plasma interactions are primarily 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 turn 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. tf 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 

-e = B (a, x) dt 

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 100-1 50 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 

I 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"potentia1 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 a1 (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: 

Where I, is the current collected, A, is the total interconnect area, 

AJs the solar panel surface area and V is the bias voltage. 

Let V1I2 =a then equ. 8 can be written; 

-297- 
B(a,X) =o 

For each value of a ; equilibrium points are determined. 

A bifurcation diagram is the plot of all curves x=X( cc ) in the (a I 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; 

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 (a I 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 oscillatov 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, Solar 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- 
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). 

@(r,z) 2% =- 7c 

(14) 

@o is the potential applied to the exposed electrode in the pinhole, 01 and 3t are empirical parameters. 

The energy that electron are accelerated to the surface is determined by @ (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. 

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 (4) , g(@) 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. 

E.n={<e> YV.E,]1'2 (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 =F(@) =E f&" (23 1 

Given equ. 16, the simulation model therefore can be transform into a bifurcation form. * =c f,(Cr, p>w (24) dt 

where 01 , p are 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 

nonperiodic 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 monitor the 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 niethod is to change a chaotic system into a periodic 

system by means of a weak time dependent perturbations to an accessible parameter. 

Carroll et. at., (ref. 8) have driven a subsystem of a chaotic system with the signal from a similar system to 

synchronization. Brown et. at., (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. IO) 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. ai., (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 sy,nchronization. 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 a series 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 

ions and sychronization methods as a means of system control could be applied. 

-300- 
‘A 

Fig. 1 Bifurcation diagram 

Fig. 2 Snapover ref. 16 

1 

- 

10 Id 103 

POTENTIAL ~REIATIVE TO PLASM1 VOLTS 

Fig. 3 Fitted model ref. 22 

-30 1 - 
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. Dressler and G. Nitsche, Controlling Chaos Using Time Delay Coordinates, PRL, Voi. 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. 

R7359. 

6) L. Pecora and T. Carroll, Synchronization in Chaotic Systems, PRL v.64, No. 8, pp.821, Feb. 1990. 

7) L. Pecora and T. Carroll, Driving Systems with Chaotic Signals, PR-A, v.44, No. 4, pp 2374, Aug. 1991 

8) T. Carroll and L. Percora, Synchronizing Chaotic Circuits, IEEE Trans. Circyits and Systems, Vol. 38, No. 

4, April 1991, p. 453. 

9) E. Ott et. al., Phys. Rev. Lett. Vol 64, 1990, p. 1196 

10) K. Judd and A. Mees, On Selecting Models for Nonlinear Time Series, Univ. of Australia, Research 

Report, Dept. of Mathernatic, Nov. 1994. 

11) A. Komori et. al., Bifurcation and Chaos in a Current Carrying /on Sheath, Phys. Fluids B, vol. 4, No. 11, 

Nov. 1992, p. 3573. 

12) M. Kono et. al., Cascading Bifurcations to Chaos in a Current Carrying /on Sheath, Jour. Phy. SOC. of 

Japan, Vol. 61, No. 2, Feb. 1992, p. 407. 

13) P. Cheung nad A. Wong, Chaotic behavior anh Period Doubling in Plasma, PRL, Vol. 59, No. 5, Aug. 

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. 

15) C. Purvis, et at, lnteraction of Large, High Power Systems with Operational Orbit Charged Particle 

Environments, NASA TM-73867. 

16) S. Domitz and N. Grier, The Interaction of Spacecraft High Voltage Power Systems with the Space 

Plasma Environment, NASA TMX-71554. 

17) N. Grier and S. Domitz, Current From A Dilute Plasma Measured Through Holes In Insulators, NASA TN 

0-81 11, Dec. 1975. 

18) N. Grier and D. McKinzie, Current Drainage To A High Voltage Probe In A Dilute Plasma, NASA TM X67890, Jan. 1972 

-302- 
19) G. B. H. Hillard, Plasma Current Collection of 2-93 Thermal Paints, J. Spacecraft, Vol. 30, No. 6, p.767. 

20) N. Steven, et. al, lnvestigation of High Voltage Spacecraft System lnteracfions with Plasma Environments, 

AlAA paper 78-672, 4125-27178. 

21) N. Steven, et. al, Large Space System - Charged Particle Environment lnferaction Technology, AlAA 79- 

0913. 

22) N. Stevens, Review of Bias Solar Array-Plasma lnferaction Studies, NASA TM 82693, 1981. 

23) M. Mandell and I. Katz, High Voltage Plasma lnteractions Calculations Using NASCAPLEO, AIAA paper 

90-0725, 118-1 1/90. 

24) M. Carruth and C. Purvis, Space Test Program of High Voltage Solar Array/Space Plasam lnteractions 

25) M. Carruth, Plasma Collection Through Biased Slits in a Dielectric, J. Spacecraft, Vol. 24, No. 1, Jan-Feb. 

1987, p. 79. 

26) S. Gabriel, et. ai., Experimental Measurement of the Plasma Sheath Around Pinhole Defects in a 

Simulated High Voltage Solar Array, AlAA 83-031 1, 1983. 

27) H. Thiemann, Particle in Cell Simulation of Sheath Formation Around Biased lnterconnectors in a LEO 

Plasma, J. Spacecraft, Vol. 27, No. 5, Sept-Oct. 1990. 

28) R. Stillwell et al, Current Collection from the Space Plasma Through Defects in Solar Array Insulation, 

J. Spacecraft, Vol. 22, No. 6, Nov. 1985, p. 631. 

29) D. Ferguson et al, Visualizing Plasma Necfric Fields, NASA Lewis DDF Report, 1995. 

-303- 
A LINEAR REFRACT WE PHOTOVOLTAIC CONCENTRATOR SOUR ARRAY FLIGHT EXPERIMENT 

P. Alan Jones and David M. Murphy 

AEC-Able Engineering Co., Inc. 

Goleta, California 931 17 

ABSTRACT 

Concentrator arrays deliver a number of generic benefits for space including high array efficiency, 

low cell area cost savings, protection from 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 flight experiment. 

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, Return 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. 

Svstem Benefits 

Simply stated, concentrator technology allows arrays to have much lower cell area for a given 

power level. For instance, a concentrator array with a 151 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 

mission types. 

The main technical barrier to ergploying satellites in high radiation missions is degradation of cell 

energy conversion 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. 

-304- 
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 andlor 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 151 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 emf tolemnf refractive 

Fresnel optics to achieve a 151 concentration of the incoming sunlight onto photovoltaic solar 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+ Experiment 

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 concentratt? module. Total power degradation is about 5% for 300 days of 

elapsed mission duration. This degradation is considered nominal and indicates the absence of any 

anomalous environmental degradation mechanisms. 

The SCARLETEOMET 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 (Figure2). The launch of the 

mission, renamed METEOR, has endured delays throughout the summer. The flight is currently 

scheduled for late October. 

-305- 
SP+ CONCENTRATOR 

FIGURE 2. COMEVMETEOR SPACECRAIT 

-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. 

FIGURE 3. SCARLETlMETEOR FLIGHT DEMONSTRAT~ON WING 

Pptics Svsterll 

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 expansionkontraction 

effects, which have plagued other types of photovoltaic 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- 
I 

Primary Concentration: lGX 

Secondary Concentration. 1 GX 

Overall Concentration: 1GX 

CMX Coverglcss 

Domoging energy of high 

frequency (Uv) woves are 

Solid siliccne CPC secondary 

1 

/ refroctea over top of CPC 

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 longitudinal axis 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. Longitudinal sun 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 IO" 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 

FIGURE 4. SCARLET OPTICS LIGHT RAY TRACE 

design process. 

FIGURE 5. OPTICAL OFF-POINTING EFFICIENCY 

-308- 
Module Design 

The current SCARLETIMETEOR module design is based on GaAslGe concentrator cells. Each 

panel has six strings, each of which are made up of four 12-cell module subassemblies (Figure 6). The 

concentrator cells, measuring 0.140 inch by 0.600 inch, are attached to the module in a series 

interconnection made with a Kapton flex circuit. Three bypass diodes on each module are attached to the 

circuit with surface mount techniques. One blocking diode per string is similarly attached to the first of the 

four modules 

FIGURE 6. SCARLET MODULE DESIGN 

The design is oriented for cost-effective mass production techniques. 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 

electronic assembly operations. The cell interconnection to 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. 

Dedovable 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 springs 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 

SCARLETMETEOR flight experiment hardware. SCARLETS 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 

-31 0- 
FIGURE 9. SCARLET PANEL WITH DEPLOYED OPTICS 

PERFORMANCE 

Various design and analytical trade studies were performed to assess the near-term performance 

of the SCARLET technology as it evolves from the current no-frills, scheduledriven 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 10 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 IO). 

120 

$ 80 

a 

s 

Q 60 

E 

.p! 40 -- 

u) 

-- 

u 

-- 

3 

3 

f 20 -- U 

with 811111 equiv. fused silica f’ronffrear shielding. 

0 

0 I000 2000 3000 4000 5OOO 6000 7000 8OOO 

System Powerper Wng (EOL WMngj 

Evolved SCARLET ------- 

Triple Junetfon 27% 

/ 

T‘ple Junction 27% 

Dual Junclion 24% 

GaAMGe 21.7% 

EOL data for 8000 km, I = 60’, 5 year mission, 

FIGURE IO. SCARLET SPECIFIC Porn PERFORMANCE 

-31 1- 
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 ME0 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 

1. 

2. 

3. 

4. 

5. 

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, L.H., 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., ONeill, M.J., and Fraas, L.M., "An Update on the Development of a Line-Focus 

Refractive Concentrator Array", Proceedings of the Xlll Space Photovoltaic Research and Technology 

Conference, 1994, pp. 31 3-322 

-31 2- 
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, 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 [l]. To quote from a review by Zurek et a/ [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 . e do know that dust does settle out of the atmosphere, the mechanism, rates, and 

geographic tY 0 settling out of the atmosphere are a matter of controversy. Any mission to 

study the climate of 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]. 

-31 3- 
Dust Properties 

Settling properties of Mars dust are discussed in earlier papers [5,6]. There are two different 

models of the size and scattering properties of Mars dust. The conventional model is by Toon et a/ [8], 

as modified by Pollack et a/. [9], using sunlight scattering measured from the surface by Viking. 

A revised model by CIancy and Lee [lo] 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 a/. analysis suggests a cross-section weighted mean radius of 2.75 prn; Clancy and Lee 

analysis 0.4 pm or less. The cross-section weighted radius of 1.5 pm estimated by Pang and Ajello [ll] 

is almost halfway between that of the two other models. 

Both values are consistent with estimates of the grain size of the surface drii material at the Viking 

lander sites [121. 

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 

difficutt to 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 181. 

Table 1 [from ref. 51 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 7 

Total obscuration of solar array for the Pathfinder (30 day) and network (2 yr) missions 

Case: 

Baseline 6.6% 77% 

Best 0.5% 22% 

worst 52.2% 89% 

obscuration obscuration 

130 dav mission1 12 vr mission) 

-31 4- 
Pathfinder Solar-Cell Experiment 

Two sensors, 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 Cetl 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 mm, including area for a JPL reference solar cell unrelated to the experiment. 

Materials Adherance Experiment 

Sensor location 

Figure 1 The MAE solar-cell and QCM experiments on the "Sojourner" 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 without the 

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 in the instrument, the actuator which moves the 

transparent plate away from the sensor. For simplicity and reliabiiity, 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. 

-31 5- 
The MAUSC has been through qualification testing for the Pathfinder flight with no failures. 

In addition to the solar cell experiment, 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 in terms 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 heriiage for contamination monitors. 

Crystal 

Monitor 

open 

Circuit 

Solar Cell 

Quartz 

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. Qua& Crystal Monitor. 

2. Optical Opacify and Solar lnsolafion Monitor, an advanced version of the Pathfinder "Solar Cell 

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. 

experiment". 

The CADO instrument set is described in more detail elsewhere [3]. 

Sensitivity 

It is important that the sensitivity of the proposed instruments is sufficient to actually measure the 

expected properties of dust. For a baseline calculation, 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 

-31 6- 
the Optical Obscuration Sensor as long as the A/D conversion is 9 bits or more. Ten days covera e will 

be detectable with 8 bits of A/D accuracy. The AID for the sensor is a 12 bit AID (1 1 bits usableg, 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 

rams/cm3, the mass deposition rate is 4.7 micrograms per cm2 per day. Since the QCM sensitivity is in 

At a cross-section weighted average particle size of 2.75 micron, the particle number density to be 

expected corresponds to 1.51 O4 particles/cm2 per day. If the magnification chosen for the microscope is 

20x, this will result in 3 particles being imaged per day. This is sufficient to accumulate good statistics on 

particle size over an extended mission. 

t 7l e 10 nG range, sensitivity is no difficulty. 

References 

1. J.B. Pollack, "Properties of Dust in the Martian Atmosphere and Its Effect on Temperature Structure," Chapter 111, Adv. Space Res., Vo/. 2, COSPAR, 45-56 (1982). 

2. R.W. Zurek, J.R. Barnes, R.M. Haberle, J.B. Pollack, J.E. Tillman and C.6. 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 lnternational 

Astronautics Federation Congress, Oslo Noway, 2-6 Oct. 1995, and at the lntermarsnet 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 Ist World 

Photovoltaic Energy Conversion Conference, Hawaii, Dec. 5-9 1 994, 2030-2033. 

6. G. Landis, "Dust Obscuration of Mars Photovoltaic Arrays," Paper IAF-94-380, 45th International 

Astronautical Federation Congress, Jerusalem, Oct. 9-1 4 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. 0.8. Toon, J.B. Pollack, and C. Sagan, "Physical Properties of the Particles Composing the 

Martian Dust Storm of 1971 -1 972," /carus, Vol. 30,663-696 (1 977). 

9. J.B. Pollack, D.S. Colburn, F.M. Flassar, R. Kahn, C.E. Carlston, and D. Pidek, J. Geophysical 

Research, Vol. 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," /carus, 

11. R.A. Kahn, T.Z. Martin and R.W. Zurek, "The Martian Dust Cycle", Mars, Universlty 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. Greeiey, 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 1. Sherman, "Solar Energy on Mars: Stationary Collectors," 

Journal of Propulsion and Power, Vol. 77 No. 3,55661 (1995). 

15. J. Appelbaum, G. Landis and I,Sherman, "Sunlight on Mars: Update 1991," Solar Energy, Vo/. 

16. R.B. Singer, AGU paper 281214, "Minerology of High-Albedo Soils and Dust on Mars," J. 

Geophysical Research, 10,159-10,168 (1982). 

17. E.A. Guinness, R.E. Awidson, 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. 77th Lunar 

and Planetary Science Conf., Part2, JGR Vol. 92, No. 88, E575-E587 (1987). 

VO/. 93, 135-158, 1991. 

50 NO. 1,%-51 (1 993). 

"Information on Mars Pathfinder probe can be found at the Mars Pathfinder home-page, 

httpJ/mpW. jp.nasa.g ovl 

-31 7- 
Muitijunction Cell Workshop 

Sarah Kurtz 

NREL 

Golden CO 

Henry Curtis 

NASNLewis 

Cleveland OH 

The muhijunction 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 discussion on the following items. 

1) What needs to be done to fly GalnP/GaAs cells in the ENTECH linear concentrator for SCARLET II? 

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? > 

production capability for the GalnP/GaAs/Ge cells. 

The New Millennium program has chosen to use multijunction concentrator cells in a 2 kW array. They are 

expecting to use GalnPIGaAs, but have many questions regarding moving this technology from planar to the 

concentrator configuration: 

-Making smaller cells will improve yiekl in the short term, but in the long run this will not be a major issue. 

-The existing cell designs will not need any significant modification for low concentration (IOX). 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 rigidity of 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 a serious risk since 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 

lost their 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- 

-31 a- 
through process for the Ill-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 directions for 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 Wkg 

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 efficiencies usually justify higher cell costs. 

The GalnP/GaAs/Ge and Inp/GalnAs systems are both lattice matches, so they don’t need to use superlattice 

or graded layer schemes. However, a number of uses for superlattices 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. Superlattices 

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 third junction but the efficiency can be increased by up to 

2.3% (absolute. Clearly, in this 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 twoterminal 

configuration. A suggestion was made to push some array integration onto the cell vendors.) The stacked cells 

have some advantages including being 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 AIAs lift off only works on small cells 

( one dimension must be less than or equal to 1 cm). RTI found sacrifice 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 multijunction arrays 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 spectrutn. 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 cells was left for discussion at the following workshop. 

-31 9- 
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 

le, as a result of the discussions. i 

\ 

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. 

rable I. 

Electron Irradiation Facilities Maximum Energy or Energy range (MeV) 

Boeing Corporation 1 

a 2.5 

2 

Jet Propulsion Laboratory (JPL) I 

NASA (Goddard) 

1 Naval Post Graduate School I20 120 

'Nat'l Institute Standards and Technology (NIST) I 1.5 ' U. of Maryland (College Park) 10 

1 Wayne State university 4.7 

320- 
rable 2. 

Proton Irradiation Facilities 

Aerospace Corporation 10.4 

Brookhaven 

Harvard 

Nat’l Institute Standards and Technology (NIST) 

Maximum Energy or Energy range (MeV) 

200 I 

I 6o 

Jet Propulsion Laboratory (JPL) 110 

Naval Research Laboratory (NRL) 14.5 

I ? 

U. of British Columbia (*) 132 500 

* 

Cobalt-60 Irradiation Facilities 

U. of California (Davis) I63 

Cobalt-60 irradiation Facilities 

U. of Michigan 13 

Wayne State University 14.7 

Western Michigan University 

Canada 

2. Alternative techniques 

There was some discussion on the use of 6OCo gamma rays or 24lAm alpha particles as alternative radiation sources 

to electron and proton accelerators. Table 3 lists the 6OCo facilities known to the participants. When SOCo gamma 

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. COCO 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 6OCo gamma 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 241Arn 

sources used in a laboratory. Some concern was also raised about possible unusual radiation effects that might be 

produced, especially by 6OCo gamma rays. 

-32 1 - 
MEASUREMENTS 

1. Test in g procedures. 

There are two general purposes for ground radiation tests. First, there is the need to provide data back to the 

producer as 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 or 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. 

Occasionally, the cells can also 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 cell 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 ridiation 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 Harwell. 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. 

1. 

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 li 

Dale R. Burger 

Jet Propulsion Laboratory 

Pasadena, California 

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 series of test results really 

mean. 

TPV CELLS 

The opening statement led into a short discussion of thermophotovoltaic (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 high fluence 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) in diameter. Cells can therefore be characterized but 

strings and arrays cannot. 

MULTl JUNCTION CELLS 

MultiJunction (MJ) cells was the next area*of discussion. This discussion started off with the observation that 

single junction cells were much less demanding as to spectral match of their test light source to the AM0 spectrum 

(already we are pining away for the good old days.) Single junction cell 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 light source for a combined current-voltage measurement 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?) light sources is required to properly characterize the performance of each 

cell (or band gap). Spikes or dips in the band edge regions make proper cell testing 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- 
Some agreement was reached on the minimum requirements for MJ cell testing. There must be standard 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 calibrated at 

the same time. Finally, there must be at least one impartial laboratory to act as a clearing house, since the capital 

expense is likely to be too high for many individual organizations to constmct their own facilities. 

SUMMARY 

There was a strong feeling that an early definition of requirements is needed: 

A joint industry/govemment/academia 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 modern 

electronic power conditioning and high speed data collection is available. 

Cell strings and arrays testing must also be addressed since the solutions for cell testing may not always 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 [l]. The concept , referred to as the "Solar Power Satellite" or "SPS," was studied extensively 

between about 1977 and 1980, by NASA and the US. Department of Energy [2,3], and by the National 

Academy of Sciences [4] and the Office of Technology Assessment [5]. Since these studies were 

completed fifteen years ago, no official reanalysis of the concept have been done, 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 [a]. 

The 1980 "reference" concept is shown in cartoon form in figure 1. 

Orbit: geosynchronous (35,800 km altitude) 

Array: 55 km2, using 17% efficient Silicon cells, assumed available at 17Glwatt (1980 dollars) 

Transmitter: 1 km2 transmitting aperture; 2.45 GHz 

Rectenna: 100 km2. Microwave power density 23 mW/cm2 center, 100 pW/cm2 edge. 

5 GW electric power produced at the busbar per satellite 

Estimated cost: $1006 first unit; $11 6 each additional unit (1 980 dollars) 

icmmve transmitter 

("rectenna" ) 

Figure 1 

798O'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,. 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- 
lots of satellites." 

For all orbits, that there could be concerns about microwave link safety. What if it misses? It was 

noted that even 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 Sate//ites, 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 lngenieurs at Scientifiques de France (ISF), 

ParisIGif-sur Yvette, 27-30 August 1991. 

7. A. Brown (ed.), Proceedings of the First Annual Wireless Power Transmission Conference (WPT 

'93), San Antonio, TX 23-25 Feb. 1993. 

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 1 991 ; pp. 225-232. 

-327- 
SPFLAT XIV, OCTOBER 24-26, 1995 

ATTENDANCE LIST 

JIM AWRY 

(JX Crystals Inc. 1 

4408 287th Ave., SE 

Fall City, WA 93024 

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 

DALE BURGER 

M/S 303-308 

Jet Propulsion Lab 

4800 Oak Grove Drive 

Pasadena, CA 91109 

BASAB CHATTERJEE 

Dept. of Electrical Engineering 

2015 Neil Avenue 

Ohio State University 

Columbus, OH 43210 

ART CHPlIELEWSKI 

Jet Propulsion Lab 

4800 Oak Grove Drive 

Pasadena CA 91109 

M/S 

DONALD .CHUBB 

M/S 302-1 

NASA LeRC 

21000 Brookpark Rd 

Cleveland, OH 44135 

ERIC CLARK 

NASA LeRC 

Clevland, OH 44135 

M/S 302-1 

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 

BILL DAVIS 

Dept of Electrical Engineering 

2015 Neil Avenue 

Ohio State University 

Columbus, OH 43210 

VICTORIA DAVIS 

S-Cubed 

Div. of Maxwell Labs 

3398 Camel Mountain 

San Diego, CA 92121 

RONALD DIAMOND 

Spectrolab, Inc. 

12500 Gladstone 

Sylrnar, CA 91342 

PAUL DILLARD 

Boei ng Aerospace 

P. 0. Box 3999 

Seattle, NA 98124 

LOUIS DiNETTA 

AstroPower, Inc. 

Solar Park 

Newark, DE 19716-2000 

M/S 81-25 

-32 8- 
MARIA FAUR 

DRA/MS 302-1 

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 302-1 

NASA LeRC/Essential Research 

21000 Brookpark Road 

Cleveland, OH 44135 

ROBERT FLOCK 

Lockheed-Martin Missiles 

& Space 

1111 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 GARVERICR 

Essential Research 

Cleveland, OH 

TIM GILLESPIE 

Lmkheed-Martin 

1111 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 

Space Tech. Dept./Space Sector 

Royal Aerospace Establishment 

Farnsborough Hants GU-14 GTD 

United Kingdom 

& Maritime System Group 

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 

2801 S. University Ave. 

Little Rock, AR 72204 

-329- 
RICHARD HOFFMAN 

M/S 302-1 

NASA LeRC/Essential Research 

21000 Brookpark Road 

Cleveland, OH 44135 

JAMES HUTCHBY 

Research Triangle Institute 

RAJ JAIN 

M/S 302-1 

NASA LeRC/U.of Toledo 

21000 Brookpark Road 

Cleveland, OH 44135 

WILLIAM JAMES 

Phillips Lab/USAF 

DARREN JOHNSON 

Phillips Lab/USAF 

Kirtland AFB, NM 87117 

PL/vTPC 

DAVID KEENER 

Phillips Lab/USAF 

Kirtland AFB, NM 87117 

PL/vTPC 

LOUIS KILMER 

The Aeropace Corp. 

P.O. Box 92957 

Los Angeles, CA 90009-2957 

M/S M2-275 

TED KUBRECHT 

Space Vacuum Expitaxy Center 

MIS S& R1, #724 

University of Houston 

Houston, TX 77204-5507 

SARAH KURT2 

National Renewable Energy Lab 

1617 Cole Blvd. 

Golden, CO 80401 

GEOFFREY LANDIS 

M/S 302-1 

NASA LeRC/OAI 

21000 Brookpark Road 

Cleveland, OH 44135 

MIKE LARRO 

Optical Coatings Lab. Inst. 

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. MAW 

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 

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STEVE RINGEL 

Dept. of Electrical Engineering 

Ohio State University 

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GEORGE RYBICKI 

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TATSUO SAGA 

Sharp Cow. 

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Shirnj o-.cho Ki tukutsuragi -gum, 

Nara 639-21 

JAPAN 

DAVID SCHEIMAN 

M/S 302-1 

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JOEL SCHWARTZ 

Hughes Space & Communication 

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HARVEY SEREZE 

Spire Corp. 

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BRIAN SMITH 

Spectrolab 

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TED STERN 

Composite Optics, Inc. 

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GEOFFREY SUMMERS 

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Research Triangle Institute 

-33 1 - 
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CARLOS VARGAS -ABURTO 

Dept. of Electrical Engineering 

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Kent State University 

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MAURO VIELA 

Space Vacuum Epitaxy Center 

University of Houston 

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ROGER WESLER 

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-332- 
REPORT DOCUMENTATION PAGE 

Space Photovoltaic Research and Technology 1995 

14. SUBJECTTERMS 

Space powec Photovoltaic cells; Solar cells; Solar arrays 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 

OF REPORT OF THIS PAGE OF ABSTRACT 

Unclassified Unclassified Unclassified 

6. AUTHOR(S) 

Geoffrey Landis, compiler 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

9. SPONSORlNGlMONlTORlNG AGENCY NAME(S) AND ADDRESS(ES) 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

15. NUMBER OF PAGES 

339 

A15 

16. PRICE CODE 

20. LIMITATION OF ABSTRACT 

Form Approved 

OMB NO. 0704-0188 

mference Publication 

5. FUNDING NUMBERS 

WU-506-41-11 

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

E-10127 

10. SPONSORINGMAONITORING 

AGENCY REPORT NUMBER 

NASA CP-3324 

I 

11. SUPPLEMENTARY NOTES 

This document supersedes CP-10108. Responsible person, Geoffrey Landis, organization code 5410, (216) 433-2238. 

12a. DISTRIBUTIONIAVAILABILITY STATEMENT 12b. DISTRIBUTION CODE 

Unclassified -Unlimited 

Subject Categories 76 and 20 

This publication is available from the NASACenter for Aerospace Information, (301) 621-0390. 

13. ABSTRACT (Maximum 200 words) 

The Fourteenth Space Photovoltaic Research and Technology conference was held at the NASALewis Research Center 

from October 24-261995. The abstracts presented in this volume report substantial progress in a variety of areas in 

space photovoltaics. Technical and review papers were presented in many areas, including high efficiency GaAs and In€' 

solar cells, GaAs/Ge cells as commercial items, high efficiency multiple bandgap cells, solar cell and array technology, 

heteroepitaxial cells, thermophotovoltaic energy conversion, and space radiation effects. Space flight data on a variety of 

cells were also presented.