The tandem cells consisting of GaAs single crystal and CuInSe2 polycrystalline thin films are being developed under the joint program of the Boeing Co. and Kopin Corp. to meet the increasing power needs for future spacecraft. The updated status of this program is presented along with experimental results such as cell performance, and radiation resistance. Other cell characteristics including the specific power of and the interconnect options for this tandem cell approach are also discussed

Gale, Ronald P.

Kim, Namsoo P.

Mcclelland, Robert W.

Stanbery, Billy J.

English

NASA Technical Reports Server

Development of Tandem Cells Consisting of GaAs Single Crystal 
and CuInSe2/CdZnS Polycrystalline Thin Films 
Namsoo P. Kim and Billy J .  Stanbery 
Boeing Electronics Co. 
Ronald P. Gale and Robert W. McClelland 
Kopin Corp. 
Summary 
The tandem cells consisting of GaAs single crystal and CuInSez polycrystalline thin films are 
being developed under the joint program of the Boeing Co. and Kopin Corp. to  meet the increasing 
power needs for future spacecraft. The updated status of this program is presented along with 
experimental results such as cell performance, and radiation resistance. Other cell characteristics 
including the specific power of and the interconnect options for this tandem cell approach are also 
discussed. 
Introduction 
Energy demands for future spacecraft are expected to  increase significantly. The development of 
high-efficiency, light weight, and radiation resistant photovoltaic arrays is required to  meet. these de- 
mands. The  Boeing Co. and Kopin Corp. have been jointly developing mechanically stacked tandem 
cells based on GaAs(A1GaAs) top cells and CuInSez(C1S) lower cells t o  address these requirements. 
The  thin film GaAs(A1GaAs) has been chosen as top cell due to  its high efficiency and good 
radiation resistance. The CIS cell has been chosen as bottom cell due to its excellent radiation 
hardness, high optical absorption coefficient, and optimal bandgap value in combination with an 
AlGaAs top cell. Since both cells are incorporated as thin films, the specific power of this tandem 
is expected to  be extremely high. In addition, this tandem cell approach provides interconnect 
flexibility to  permit more efficient arrays since each cell is fabricated and tested independently and 
can be selectively matched. In this paper, we discuss the characteristics of these cells and present 
the current status of this program. 
Tandem Cell Fabrication 
The structure of the tandem cell under development is shown schematically in Figure 1. It 
consists of a double heterostructure GaAs/AlGaAs CLEFT top cell mechanically stacked on a 
CdZnS/CuInSez thin film bottom cell. Fabrication of the cell is accomplished in a similar man- 
ner to  the one described elsewhere [ref.l]. 
The very thin GaAs single crystal film (10 p m  thick) is fabricated by the CLEFT technique 
[ref.2] and the layers of p+ AIGaAs, p GaAs base, n GaAs emitter, n+ AlGaAs window, and n+ 
GaAs cap are grown by OMCYD [ref.3] to  form a double-heterostructure. After the front face of 
138 
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the cell is processed, a support is mounted to  the cell and the thin film is separated from the bulk 
GaAs. Grid deposition and anti-reflection coating, and etch removal of inactive GaAs film complete 
the top cell fabrication. 
The CIS lower cell is fabricated by sequential deposition directly onto a glass substrate of a Mo 
electrode, CuInSez absorber, and CdZnS window and metallic contact grid. The CuInSez film is 
deposited by a simultaneous elemental coevaporation process and the CdZnS film is deposited by 
coevaporation of the CdS and ZnS binary compounds [refs. 4 and 51. 
Mechanical stacking of the top cell and the bottom cell is accomplished using a space qualified 
optical adhesive to  form the final structure consisting of a coverglass and the subcells. Interconnect- 
ing the top cell grids to  contact pads on the CIS cell substrate completes fabrication of the tandem 
cell. 
Tandem Cell Characteristics 
Efficiency 
The best performance of this tandem approach that we have achieved so far is 20.3%* (20.5%) 
AM0 for a four terminal 1 cm2 cell design. The results of this cell are shown in Figure 2. The 
subcell efficiencies were 18.1% and 2.2%* (2.4%) for the GaAs top cell and the CIS bottom cell 
respectively. Measurements were conducted at  28OC, with a Xenon solar simulator intensity setting 
of 137.2 mW/cm2. The external quantum efficiency measured with the same cell is shown in Figure 
3. Exact optical loss mechanisms for the bottom cell are being investigated and the tandem stack is 
being further optimized. Details of this work will be published at a later date. 
Earlier projections using the combination of bandgap values of AlGaAs(1.75 eV) and CIS(l.0 
eV) predicted the maximum efficiency of 32.9% [ref.S]. Using a realistic tandem cell model, we expect 
to achieve 22.7% AM0 cells with our current cell structure(GaAs/CIS) in the very near future. When 
combined with a high quality AlGaAs top cell, further improvement of efficiency up to 26% AM0 is 
anticipated. 
Radiation Hardness 
Radiation resistance of this tandem cell is expected to be superb. In order to  assess the radiation 
effects on these cells, bare CIS and bare GaAs solar cells were fabricated, and submitted for radiation 
experiments using facilities at  Boeing Radiation Effects Laboratory(BREL). 
*Adjusted for white light response of the lower cell (suspected to be due to  light trapping in 
the layers between cells) 
139 
The experimental results on the normalized power degradation for CLEFT GaAs and CIS cells 
as a function of irradiation fluence are shown respectively in Figure 4 and Figure 5. These results 
reaffirmed our previous report [refs.l and 71 that  the CIS cells have superior radiation hardness 
when compared to Si and GaAs solar cells. The illuminated I-V curves measured on the 1.0 and 
2.0 MeV electron radiation samples of CIS exhibited a negligible degradation at all fluences. The  
CIS cells showed much more resistance to proton radiation than GaAs or Si cells as shown in Figure 
6. We also observed a recovery of the CIS performance t o  near pre-irradiation values with samples 
stored at room temperature and ambient pressure. The  results of radiation experiments on the 
GaAs cells showed comparable values to  the ones that  have been reported with bulk. We expect, 
however, somewhat better performance with CLEFT GaAs film on radiation hardness due to  its 
n-on-p structure, the shallowness of junction in these CLEFT cells, and to  the thinness of the film. 
Radiation results on GaAs cells from various reports are plotted for relative comparison in Figure 7. 
Since radiation effects depend on parameters such as cell structure, operating temperature, initial 
performance or device parameters and many others, direct comparison is not easily possible. Thus, 
we have t o  consider the present comparison of GaAs radiation hardness results as being more relative 
than absolute. 
Based on these results combined with our enhanced solar cell modeling program, however, 
further improvement is expected with GaAs cell structure optimization. Even further improvement 
of radiation hardness is expected when an AlGaAs top cell currently being evaluated, is included in 
the tandem stack. 
Weight 
One distinct advantage of this approach is found in weight consideration. Compared to  con- 
ventional approaches involving cell growth on a bulk substrate, this tandem utilizes the CLEFT 
technique to  fabricate a very thin GaAs film. Combined with the thin-film CIS cell directly de- 
posited onto thin glass, we expect the weight of a 4 cm2 tandem cell including coverglass to be 190 
mg, as shown in the Table 1. With the efficiency of 20.5% already achieved, this provides specific 
power of 590 W/kg. When a projected efficiency of 26% is used, we expect specific powers upto 750 
W/kg in these cells. 
Interconnect Options 
Since this tandem cell is mechanically stacked, it provides considerable interconnection flexibil- 
ity. Using the four terminal approach, each cell can be fabricated with high processing yields, 
optimized independently and then selected for array level optimization. Since it is not series- 
interconnected, degradation of each cell due t o  radiation and/or operating temperature would have 
less impact on the other, especially when the array is configured in voltage matching condition. It is 
known [ref. 81 that  voltage matching offers advantages such as less sensitivity to  radiation damage, 
and spectral variation and wider selection of bandgaps for optimal performance. I t  was found that 
a configuration of three series connected CIS cells in parallel with a GaAs cell is suitable for voltage 
140 
matching and this configuration can be realized at either the module (tandem cell) level or array 
level. 
Tandem Cell Optimization 
In addition to our current effort toward improving the GaAs/CIS tandem cell efficiency by 
minimizing optical transmission losses, evaluation of AlGaAs/CIS tandem has been undertaken for 
the purpose of achieving higher EOL efficiency. Also being developed is a larger area(4 cm2) tandem 
cell. Masks for both top and bottom cell have been fabricated and initial device fabrication has 
shown promising results, 
Conclusion 
The updated status of our GaAs/CIS tandem cell program has been summarized, and the 
characteristics of cells have been discussed. This tandem cell approach utilizing thin film GaAs 
single crystal and polycrystalline CIS thin film cells is viable, mature and nearing readiness for 
implementation in high-performance space power systems. 
References 
[ 11 B.J. Stanbery, J.E. Avery, R.M. Burgess, W.S. Chen, W.E. Devaney, D.H. Doyle, R.A. Mick- 
elsen, R.W. McClelland, B.D. King, R.P. Gale and John C.C. Fan, Conference Record 19th IEEE 
Photovoltaic Specialists Conference (IEEE, New York, 1987), p. 280. 
Specialists Conference (IEEE, New York, 1981), p. 666. 
[ 21 John C.C. Fan, C.O. Bozler, and R.W. McClelland, Conference Record 15th I E E E  Photovoltaic 
[ 31 R.P. Gale, John C.C. Fan, G.W. lhrner,  and R.L. Chapman, Conference Record 18th I E E E  
Photovoltaic Specialists Conference (IEEE, New York, 1985), p. 296. 
nece (IEEE, New York, 1982), p.781. 
[ 41 R.A. Mickelsen, and W.S. Chen, Conference Record 16th IEEE Photovoltaic Specialists Confer- 
[ 51 R.A. Mickelsen, W.S. Chen, B.J. Stanbery, H. Dursch, J.M. Stewart, Y.R. Hsiao, and W. De- 
vaney, Conference Record 18th IEEE Photovoltaic Specialists Conference (IEEE,New York, 1985), 
p. 1069. 
[ 61 John C.C. Fan, B-Y. Tsaur and B.J. Palm, Conference Record 16th I E E E  Photovoltaic Specialists 
Conference (IEEE, New York, 1982), p.692. 
[ 71 H. Dursch, W.S.Chen and D. Russell, Proceedings of Space Photovoltaic Research and Technology 
1985,  NASA Conf. Pub. 2408, p.165. 
1987), p.536. 
[ 81 J.M. Gee, Conference Record 19th IEEE Photovoltaic Specialists Conference (IEEE, New York, 
141 
Table 1 .  Weight of Tandem Cell (4 cm2) 
Component 
Coverglass 
Th tckness Weight 
2 mil 53.1 mg 
Top adhesive 
GaAs CLEFT 
I Bottom adhesive I 2mi l  I 21.6mg I 
~~ ~ 
2 mil 21 6 m g  
10 um 21 2 m q  
CulnSe2 
Glzss substrate 
~~ 
6 um 1 3 0 m g  
2 mil 53.1 mq 
I I- 
Others 
Total 
I I -  
3.0 mg 
188.0 mg 
1 -  
n n n n n n  n n n 
I- 
Vertical dimensions not t o  scale 
(see Table 1 for details) 
Cove rglass 
Top adhesive 
GaAs (or AIGaAs) Cell (1 0 pm) 
Bottom Adhesive 
CdZnS/CulnSe2 Cell (6 pm) 
Glass substrate 
Figure 1 .  Schematic of Tandem Cell Structure 
142 
0 I I 1 I I I I 1  I 
CulnSez bottom cel l  I 
Efficiency = 2.4% (2.2%') I 
Jsc = -12.6 mNcm2 I 
I 
Voc = 0.382V I 
I .  
I FF = 63.0% 
Air mass = 0 (GaAs filtered) I 
Cellarea = 1 cm2 I 
-5 - 
-10 - 
0.6 
0.4 
0.2 
0 
-1 5 
Current 
-20 
-2 5 
-30 
I 0 0 
I I 
I 1 7  \ 
\ 
I 
- I 
f 
I 
\ 
\ 
\ 
\ 
I 1 ,  
'I 
1, 
I 
I - I 
I 
- I  
I i 
- 1 
- I  
I 
I 
c 
/ n  '\ I >--- I I 
Re I a tive 
quantum 
efficiency 
t 
I 
I 
I 
I 
/CLEFT GaAs top ce 
Total efficiency = 20.5Oh (20.3%') 
Cell area = 1 cm2 
-3 5 1 1 1 I I 1 I I I I 1 
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 
Voltage, V 
*Corrected for white light response of lower cell 
--I 
\--- 
0.8 i / - - '  I 
Figure 2 .  I-V Measurements of Tandem Cell (*corrected for white light response of lower cell) 
7 
Wavelength (pm) 
Figure 3. Spectral Response of  Tandem Cell 
143 
1.2 
0.4 
0.2 
t 
L 
- 
- 
0.8 1 
1.0-  
0.8 - 
0.6- 
Normalized 
power, 
Prnax/Po 
0.4 - 
0.2- 
0 
power, 0.61 PrnaxIPo 
0 1.0 MeVe- 
0 2.0 MeV e- 
0 l . O M e V p +  
m 0.8 MeV p + 
x 0.4 MeV p + 
b 0.2 MeV p + 
I I I I I 
1010 1011 1012 1013 1014 1015 1016 
\ 
\ 
Normalized t 
\ 
A 0.2 MeV p + 
(shallow homojunction) \ 
0 0.2 MeV + \ 
(double Reterostructure) \ 
0 1.0 MeV e- (shallow homojunction) 
m 1 .O MeV e- (double heterostructure) 
- 0 1.0 MeV p + (shallow homojunction) 
@ 1.0 MeV p (double heteros,tructure)b I I I 
0 2.0 MeV e- (shallow homojunction) 
1011 1012 1013 1014 1015 10 0 1010 
Particle fluence, p/cm2 
Figure 4. Normalized Power Versus Particle Fluence Results of Irradiations of CLEFT GaAs Solar 
Cells by Protons and Electrons 
6 
Figure 5. Normalized Power Versus Particle Fluence Results of Irradiations of CulnSe2 
Cells by Protons and Electrons 
144 
Pm 
Pmo 
-
Fluence, protonslcmz 
Figure 6. Degradation of Thin-Film CdSICulnSe2 Solar Cells as Function of 1 MeV 
Proton Fluence (ref. 7) 
0 
\ 
\ 
\ 
P \ 
\ /* 
Si'icon Handbook 
GaAs bulk I 84-61 Ref *JPL 
GaAs thin film 
(s ha I low homoiu nction) 
GaAs thin film 
(double heterstructure) 
CulnSe2 
I I 1 
0.10 1 .o 10.0 
Proton energy, MeV 
Figure 7. Critical Fluence vs. Proton Energy for Various Semiconductors 
145 


Development of tandem cells consisting of GaAs single crystal and CuInSe2/CdZnS polycrystalline thin films

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