NASA Technical Memorandum 102093 

NASA Advanced Space Photovoltaic 

Technology-Status Potential and 

Future Mission Applications 

Dennis J. Flood and Michael Piszczor, Jr. 

Lewis Research Center 

Cleveland, Ohio 

Paul M. Stella 

Jet Propulsion Laboratory 

Pasadena, California 

and 

Gary L. Bennett 

NASA Headquarters 

Washington, D. C. 

Prepared for the 

sponsored by the European Space Agency 

Madrid, Spain, October 2-6, 1989 

P European Space Power Conference 

. 

[IASA-TH-l02093) NASA ADVANCED SPACE H09-27705 

PEOTOVOLTAIC TECHNOLOGY-STATUS, POTENTIA L 

AND FUTURE MXSSION APPLICATIONS [NASA, 

Lewis Research Center) 8 p CSCL 10B Unclas ~ 

G3/20 0325967 
NASA ADVANCED SPACE PHOTOVOLTAIC TECHNOLOGY - 

I 

w 

STATUS, POTENTIAL AND FUTURE MISSION APPLICATIONS 

Dennis J. Flood and Michael Piszczor, Jr. 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 USA 

and 

Paul M. Stella 

Jet Propulsion Laboratory 

Pasadena, California 91109 USA 

and 

Gary L. Bennett 

National Aeronautics and Space Administration 

Washington, D.C. 20546 USA 

SUMMARY 

The NASA program in space photovoltaic research and development encompasses a wide range of emerging options for future space power systems, and 

includes both cell and array technology development. The long range goals are 

to develop technology capable of achieving 300 W/kg for planar arrays, and 

300 W/m2 for concentrator arrays. InP and GaAs planar and concentrator cell 

technologies are under investigation for their potential high efficiency and 

good radiation resistance. 

gram is a near term effort aimed at demonstrating 130 W/kg beginning of life 

specific power using thin (62 pm) silicon cells. It is intended to be technology transparent to future high efficiency cells and provides the baseline 

for development of the 300 W/Kg array. 

The Advanced Photovoltaic Solar Array (APSA) proINTRODUCTION 

NASA, through its Office of Propulsion and Power, conducts a continuing 

space energy conversion research and technology program of wide ranging scope 

and content. The primary objective of the program is to provide a broad 

technology base to meet power system requirements for future space missions, 

including growth space station, advanced Earth orbiting satellites, lunar and 

planetary bases, and solar system exploration. Power system requirements for 

those future missions will span the range from a few kilowatts to megawatts, 

and operating lifetimes will vary from 2 years to perhaps multiple decades. 

In some mission scenarios, such as a lunar or Mars base, a new planning element will be introduced: the mission capabilities will evolve over time from 

the initial outpost with intermittent operation to a permanent base with continuous operation. The impact on power system requirements and capabilities 

is just beginning to be assessed, but it is clear that an evolutionary technology development strategy will be needed to address this most ambitious of 

all manned space mission scenarios. As in any program of limited resources, 
it is essential to address those technologies that represent mission critical 

capabilities, and which have the broadest application across the entire mission set. 

The photovoltaic element of the space energy conversion program is 

designed to provide the technology for improved conversion efficiency, reduced 

mass, reduced cost, and increased operating life of solar cell cells and 

arrays. At present, the program is divided into a generic base R&T effort, 

and a more focused effort for lunar or Mars surface power systems under Project 

Pathfinder. This paper will discuss some of the key technologies under developement in the base R&T program and how they are expected to benefit future 

Agency missions. The program is divided roughly into two elements (that have 

synergistic overlap), each of which is managed by a separate research center. 

The Lewis Research Center is responsible primarily for research and development of advanced space solar cells and for investigating issues relating to 

advanced blanket technology. The Jet Propulsion Laboratory is primarily concerned with the development of high performance array technology, and for 

maintaining engineering data on the radiation damage resistance of cells that 

are available from commercial suppliers. The two centers also maintain facilities for direct cell calibration at high altitude, and both provide sec- .. 

ondary standard measurements for flight programs and for assessing the 

progress of industrial and university laboratory advanced cell development 

efforts. 

A specific long range goal of the base R&T program is to develop the 

technology base for photovoltaic arrays with a specific power of 300 W/kg at 

beginning of life (BOL). Achievement of this technology goal will improve the 

payload fraction of future earth orbiting spacecraft, and extend the potential ly useful range of photovoltaic power systems on interplanetary spacecraft. 

The increased sophistication and long lead time of future missions, along with 

anticipated launch vehicle constraints, will place a premium on system reliability, lifetime and mass. For many current spacecraft almost half of the 

total spacecraft mass is taken up by the auxiliary propulsion and power subsystems, while the payload accounts for about one-fourth of the total. 

Reducing the mass of the nonpayload portion of the spacecraft by one-third 

would permit a doubling of the mass allocation for the payload. 

allocation could be used for the addition of equipment to enhance operations, 

autonomy, reliability, redundancy and lifetime of the payload. 

The extra mass 

The overall thrust of the NASA space energy conversion research and 

technology program is aimed at achieving these mass and reliability benefits. 

For example, current solar arrays have specific powers in the range from 15 to 

30 W/kg. NASA-sponsored technology has already shown that 66 W/kg is achievable, and the program is now aimed toward a proposed demonstration of 130 W/kg, 

using the NASA-developed 62 pm thick high efficiency silicon solar cell. The 

preceeding work is described in the section on high performance arrays. 

Achievement of the goal mentioned at the outset, i.e., 300 W/kg, especially 

when coupled with improved radiation resistance, will require not only improvements in array structures and blankets, but new, ultrahigh specific power solar 

cells as well. Some of the work on advanced cell technology is described in 

the next section of the paper. The successful realization of all the required 

technology elements will provide spacecraft designers with a ten-fold increase 

in solar array specific power compared to present practice, and open up new 

mission opportunities previously thought inaccessible to photovoltaic power 

sys terns. 

2 
ADVANCED SOLAR CELL TECHNOLOGY 

The research on high performance solar cells conducted by NASA Lewis is 

focused on radiation tolerance and high efficiency, with special emphasis on 

indium phosphide (InP) and gallium arsenide (GaAs) devices. 

three major thrusts: 

20 percent AM0 efficiency with 1 percent or less degradation in power after 

10 years in GEO; (2) heteroepitaxial growth of InP on alternate substrates such 

as silicon or germanium; and (3) demonstration of high efficiency, ultrathin 

InP cells with improved radiation resistance compared to GaAs and silicon. 

The papers by Weinberg, et al. (ref. 1) and by Brinker, et al. (ref. 2) in this 

volume provide an excellent update on some recent results and the potential of 

this exciting new cell technology. Other details appear in the paragraphs 

below. 

of high efficiency concentrator cells; (2) demonstrating feasibility of the 

point contact junction geometry in GaAs; and (3) development of a V-grooved 

cell geometry with improved efficiency and radiation resistance compared to 

the standard cell geometry. The paper by Bailey, et al. (ref. 3) describes 

the latter in more detail. 

The InP work has 

(1) demonstrating a cell structure capable of achieving 

Work on GaAs cells similarly has three major thrusts: (1) development 

In addition to the areas mentioned above, the Agency has a growing interest in the so-called thin-film cell technologies such as amorphous silicon and 

copper indium diselenide. 

(1) the potential radiation resistance of these ultra-thin devices, and 

(2) the possibility of their extremely low cost. Internal Agency assessments 

indicate that these cell types will only be of value if they can be incorporated on very lightweight, flexible solar array blankets (such as Kapton) that 

can be easily deployed or erected, and which have a large volumetric power density when stowed on a spacecraft during launch and transport. 

basic stability issues associated with these cell types are being addressed in 

the Surface Power program, which has been discussed elsewhere (ref. 4). 

There are two primary reasons for such interest: 

Some of the 

Work on InP solar cells has resulted in an achieved efficiency of 18.8 

percent in laboratory devices (ref. 5) in an n/p homojunction structure. A 

second cell type, produced by sputtering IT0 onto InP (ref. 6>, has produced 

efficiencies in excess of 16 percent and with a level of radiation resistance 

equivalent to that observed in the OMCVD-produced homojunction device (ref. 7). 

Efforts to fabricate InP cells on alternate substrates have been directed 

entirely toward silicon thus far. Progress to date has been encouraging, and 

despite the large lattice mismatch that exists between silicon and InP, it is 

felt that epitaxial growth can be accomplished with sufficiently high epitaxial layer quality that efficiencies above 19 percent can be realized. The 

first devices were produced with unoptimized growth conditions and structures, 

and had efficiencies above 9 percent (ref. 8). The intent is to incorporate 

the optimized high efficiency structure that will be developed in thrust one 

on an alternate substrate to improve ruggedness and reduce cost. 

third area, the ultrathin InP cell, has just begun, and will utilize the CLEFT 

process developed by John Fan and co-workers to produce high specific power 

GaAs cells (ref. 9). Once achieved, the CLEFT device will be mechanically 

bonded to a low cost, rugged, lightweight substrate and tested for its spaceflight worthiness. Both planar and concentrator cell structures will be 

investigated i n a l l of the above work. 

Work in the 

3 
Significant progress on GaAs concentrator cells has been achieved 

recently, both in understanding its radiation resistance, and in improving its 

performance. Employing a prism cover on the cell allows for a much simpler 

and more effective parallel gridline pattern to be used and still maintain 

good current collection. 

trator array on orbit is expected to be 100 OC. Prism covered cells operating 

at 100 suns and 100 OC have exceeded 22 percent efficiency in laboratory measurements at NASA Lewis. The radiation resistance of the concentrator cell 

structure is not fully characterized, but early results suggests that it is 

not much different from that observed in p/n,,planar cells. Although the 

reflective concentrator optics that have been developed by NASA and the Air 

Force should provide significant shielding from the natural radiation background, they also restrict the specific power of such arrays to 30 W/kg or 

less. Use of a lightweight refractive concentrator such as the domed miniFresnel lens under developement by NASA and the SDIO can raise the specific 

power by a factor of two or three, but will require the cell to be significantly more radiation resistant than in the former case. 

The equilibrium operating temperature of a concenThe fundamental requirements that must be met to demonstrate a high efficiency point contact GaAs cell are reasonably well understood (ref. 10). The 

structure should have a p-type, high resistivity base with fully passivated 

front and back surfaces, and the total junction areas should be on the order 

of 1 percent of the cell area. If the cell is to have good radiation resistance, the base diffusion length should be on the order of 100 pm, and the 

surface passivation should be unaffected by bombardment of charged particle 

radiation. Putting the junction areas on the back of the cell, as in the silicon point contact cell (ref. 11). implies that ultrathin cells with light trapping may be required for maximum efficiency. 

avai 1 able at present. 

No experimental data are 

Development of a high efficiency, lightweight, rugged, radiation resistant solar cell is an essential element of the NASA program to demonstrate the 

technology needed to achieve an array specific power of 300 W/kg for a deployable planar array. The Agency also has the goal to demonstrate technology for 

achieving 300 W/m2 at specific powers approaching 100 W/kg in concentrator 

arrays. Excellent progress toward both sets of goals continues to be made. 

HIGH PERFORMANCE SOLAR ARRAYS 

The key element in the present JPL high performance array development work 

is the thin (62 pm) silicon solar cell. The presently available device represents the results of many years development, from initial device research, 

to pilot line operation, and finally to circuit assembly and test. The lightweight cell, in combination with a thin protective cover, allows the assembly 

of very low mass circuits and blankets. The reduction of the blanket mass then 

allows the mass of the support structure to be significantly reduced, yet 

still provide sufficient stiffness and strength. 

In 1985, the program emphasis on the development of advanced array components was reduced in order to initiate the design and fabrication of a prototype wing that would demonstrate the integration of various array elements, 

such as thin cells and covers, and lightweight substrates and deployment systems. That effort, APSA (Advanced Photovoltaic Solar Array), is presently in 

4 
hardware fabrication phase and is expected to demonstrate the capabi 1 i ty for 

achieving greater than 130 W/kG for a 5 kWe (BOL) deployable wing (ref. 12). 

The wing design is scalable over a range of approximately 2 to 13 kW by changing width and length. Since certain elements such as the blanket housing and 

deployment motor do not change linearly with array power, the specific performance will range from approximately 90 to 150 W/kg from smallest to largest 

power levels. These high specific power values were achieved by commitment to 

low array mass and avoiding the tendency to increase mass by the addition of 

features that were not absolutely necessary. For example retraction is not a 

feature of the APSA design. JPL and TRW surveys of potential users indicated 

that although many endorsed array retraction and restowage, few could actually 

cite a mission or requirement that would actually require retraction. Consequently the present APSA design will not retract and relatch without manual 

assistance. At the same time the design can be modified at a modest mass 

penalty to provide automated retraction and relatching, if required. 

Although APSA is based on the use of thin silicon cells, it is felt that 

more advanced cell types can be readily used when available. Initial prospects include thin GaAs and InP based devices, most likely on a rugged substrate. Due to the high density of the 111-V materials, it is necessary that 

their thickness be on the order of a few mils for that array specific performance to complete with values obtained with thin silicon. However, the 

higher available cell efficiencies will allow for a reduction in required 

array area, a feature which can be important in some applications. In contrast, a number of thin film materials such as CuInSe2 and amorphous silicon, due to their extreme thinness, offer the potential for very high specific 

performance, even at low overall efficiency (9-10 percent). Typically, a large 

increase in array area will be required when compared to the single crystal 

devices. However, if encouraging preliminary results on radiation resistance 

can be maintained for higher efficiency spaceworthy configurations, thin film 

array areas for severe radiation environments may be comparably small. At 

this point in time these thin film materials present many difficulties before 

being capable of space applications. These include problems of uniformity, 

stability, and the lack of availability in the form of a large area flexible 

film blanket. 

As the development of the APSA prototype nears completion, the advanced 

array development program will then address ground based testing, and ultimately a space based test of the electrical and dynamic characteristics. At 

the same time emphasis will again be placed on array component technology 

development including evaluation of environmental, electrical and mechanical 

properties of the next generation of solar cells. Structure development will 

address questions of increased strength for enhanced spacecraft maneuverability 

and for lunar and planetary applications. 

cell blanket configuration will be examined to ascertain alternative stowage, 

deployment, and structural concepts. Array design for power levels beyond the 

APSA limits will be examined for potential SEP missions. The long term goal 

at this time still is to achieve extremely high specific performance (300 W/kg> 

for a wide range of mission applications. 

The impact of a truly "flexible" 

CONCLUSIONS 

The major requirements for space photovoltaic solar arrays and cells are 

high efficiency, low mass and survivability in the naturally occurring charged 

5 
particle space radiation environment. The NASA space photovoltaic program is 

directed at providing the technology base for achieving such performance, and 

will provide a variety cell and array options for future mission planners to 

consider. 

REFERENCES 

1. Weinberg, I., Swartz, C.K., and Hart, R.E., 1989, InP Solar Cell Research 

in the Unites States - Comparison with Non-Photovoltaic Power Sources, 

European Space Power Conference, Madrid, Spain, Oct. 2-6. 

2. Brinker, D.J., 1989, On Orbit Results of the LIPS III/InP Homojunction 

Solar Cell Experiment, European Space Power Conference, Madrid, Spain, 

OCt. 2-6. 

3. Bailey, S.G., Fatemi, N., Wilt, D.M., Landis, G., and Thomas, R.D., 1989, 

GaAs Solar Cells with V-Grooved Emitters, European Space Power Conference, 

Madrid, Spain, Oct. 2-6. 

4. Flood, D.J. and Appelbaum, J., Photovoltaic Power System Considerations for 

Future Lunar Bases, Proceedings of the 4th Photovoltaic Science and Engineering Conference, Sydney Australia, to be published. (NASA TM-102019). 

5. Keavney, C., Spitzer, M.B., Vernon, S.M., Haven, V.E., and Augustine, G., 

1975, InP Shallow-Homojunction Solar Cells, Proceedings, 10th Space Photovoltaic Research and Technology Conference, p. 27. 

6. Coutts, T.J., Wanlass, M.W., Li, X., and Gessert, T.A., 1989, A Review of 

Recent Progress in InP Solar Cells for Space Application, Proceedings of 

the 4th Photovoltaic Science and Enqineering Conference, Sydney Australia, 

to be published. 

7. Weinberg, I., Swartz, C.K., and Hart, R.E., Jr., and Coutts, T.J., 1988, 

Radiation Resistance and Comparative Performance of ITO/InP and n/p InP 

Homojunction Solar Cells, Twentieth IEEE Photovoltaic Specialists Confer- 

=, Vol. 2, IEEE, Piscataway, NJ, pp. 893-897. 

8. Vernon, S. and Spitzer, M., 1988, Indium Phosphide Solar Cells on Silicon 

Substrates, NASA Contract NAS3-25283, Spire Corp., Bedford, MA. 

9. Fan, J.C.C., McClelland, R.W., and King, B.D., 1984, GaAs Cleft Solar Cells 

for Space Applications, Seventeenth Photovoltaic Specialists Conference, 

IEEE, Piscataway, NJ, pp. 31-35. 

10. Welter, W.G., and Godlewski, M.P., 1985, Effect of Solar-Cell Junction 

Geometry on Open-circuit Voltage, J. Appl. Phys., 57, pp. 2292-2294. 

11. Slnton, R.A. and Swanson, R.M. 1987, An Optimization Study of Si PointContract Concentrator Solar Cells, Nineteenth IEEE Photovoltaic Specialists 

Conference, IEEE, Piscataway, NJ, pp. 1201-1208. 

gram, European Space Power Conference, Madrid, Spain, Oct. 2-6. 

12. Kurland, R.M. and Stella P., 1989, Advanced Photovoltaic Solar Array Pro6 
m National Aeronautics and Report Documentation Page 

1. Report No. 

NASA TM-102093 

2. Government Accession No. 3. Recipient's Catalog No. 

I I 

4. Title and Subtitle 1 5. Report Date 

7. Author@) 

Dennis J. Flood, Michael Piszczor, Jr., 

NASA Advanced Space Photovoltaic Technology-Status, Potential and 

Future Mission Applications 

8. Performing Organization Report No. 

E-4856 

2. Sponsoring Agency Name and Address 

Paul M. Stella, and Gary L. Bennett 

Technical Memorandum 

10. Work Unit No. 

9. Security Classif. (of this report) 

Unclassified 

506-41-1 1 

9. Performing Organization Name and Address 

11. Contract or Grant No. 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 13. Type of Report and Period Covered 

20. Security Classif. (of this page) 21. No of pages 22. Price' 

Unclassified 8 A02 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

14. Sponsoring Agency Code 

5. Supplementary Notes 

Prepared for the European Space Power Conference sponsored by the European Space Agency, Madrid, Spain, 

October 2-6, 1989. Dennis J. Flood and Michael Piszczor, Jr., NASA Lewis Research Center; Paul M. Stella, 

NASA Jet Propulsion Laboratory, Pasadena, California 91 109; Gary L. Bennett, NASA Headquarters, 

Washington, D.C. 20546. 

6. Abstract 

The NASA program in space photovoltaic research and development encompasses a wide range of emerging options 

for future space power systems, and includes both cell and array technology development. The long range goals are 

to develop technology capable of achieving 300 W/kg for planar arrays, and 300 W/m2 for concentrator arrays. 

InP and GaAs planar and concentrator cell technologies are under investigation for their potential high efficiency 

and good radiation resistance. The Advanced Photovoltaic Solar Array (APSA) program is a near term effort 

aimed at demonstrating 130 W/kg beginning of life specific power using thin (62 pm) silicon cells. It is intended 

to be technology transparent to future high efficiency cells and provides the baseline for development of the 

300 W/kg array. 

7. Key Words (Suggested by Author@)) 

Solar cells; Solar arrays; Space power; InP; GaAs; 

Concentrators 

18. Distribution Statement 

Unclassified - Unlimited 

Subject Category 20 

'For sale by the National Technical Information Service, Springfield, Virginia 221 61 \SA FORM 1626 OCT 86