Very high-efficiency GaAs Cassegrainian solar cells have been fabricated in both the n-p and p-n configurations. The n-p configuration exhibits the highest efficiency at concentration, the best cells having an efficiency eta of 24.5 percent (100X, AM0, temperature T = 28 C). Although the cells are designed for operation at this concentration, peak efficiency is observed near 300 suns (eta = 25.1 percent). To our knowledge, this is the highest reported solar cell efficiency for space applications. The improvement in efficiency over that reported at the previous SPRAT conference is attributed primarily to lower series resistance and improved grid-line plating procedures. Using previously measured temperature coefficients, researchers estimate that the n-p GaAs cells should deliver approximately 22.5 percent efficiency at the operating conditions of 100 suns and T = 80 C. This performance exceeds the NASA program goal of 22 percent for the Cassegrainian cell. One hundred Cassegrainian cells have been sent to NASA as deliverables, sixty-eight in the n-p configuration and thirty-two in the p-n configuration

Grounner, M.

Hamaker, H. C.

Kaminar, N. R.

Kuryla, M. S.

Ladle, M. J.

Liu, D. D.

Macmillan, H. F.

Partain, L. D.

Virshup, G. F.

Werthen, J. G.

English

NASA Technical Reports Server

N89-24733 
25%-Efficient GaAs Cassegrainian Concentrator Cell 
H. C. Hamaker, M. Grounner, N. R. Kaminar, M. S. Kuryla, M. J. Ladle, 
D. D. Liu, H. F. MacMillan, L. D. Partain, G. F. Virshup and J. G. Werthen 
Varian Research Center 
Polo Atto, California 9/303 
J. M. Gee 
Sandia National Labomiories 
Albuquerque, New Mezico 87185 
Very high-efficiency GaAs Cassegrainian solar cells have been fabricated in both the n-p and p-n 
configurations. The n-p configuration exhibits the highest efficiency at concentration, the best cells 
having an efficiency q of 24.5% (lOOX, AMO, temperature T = 28 OC). Although the cells are designed 
for operation at  this concentration, peak efficiency is observed near 300 suns ( q  = 25.1%). To our 
knowledge, this is the highest reported solar cell efficiency for space applications. The improvement 
in efficiency over that reported at  the previous SPRAT conference is attributed primarily to lower 
series resistance and improved grid-line plating procedures. Using previously measured temperature 
coefficients, we estimate that the n-p GaAs cells should deliver approximately 22.5% efficiency at  
the operating conditions of 100 suns and T = 80 OC. This performance exceeds the NASA program 
goal of 22% for the Cassegrainian cell. One hundred Cassegrainian cells have been sent to NASA as 
deliverables, sixty-eight in the n-p configuration and thirty-two in the p-n configuration. 
Introduction 
Solar energy systems using GaAs cells in concentrator arrays offer the potential for very high 
conversion efficiency along with low array costs. Several concepts for light-weight, radiation-resistant 
space concentrators have been proposed, such as the point-focus miniature Cassegrainian array [ref. 
11 and the parabolic trough design [ref. 21. The advantages of higher cell efficiency and reduced 
array costs in a concentrator system are balanced against optical losses of the reflecting mirrors and 
the need for greater accuracy in aiming the arrays at the sun. Since the cell comprises such a small 
fraction of the weight and area of the array, the most important of the cell parameters - cost, 
weight, etc. - is the conversion efficiency r). GaAs devices have a greater potential for this appli- 
cation than their Si counterparts due to their more optimal band gap, superior radiation resistance 
and lower temperature coefficient. The last factor enables the GaAs cells to operate efficiently at 
moderate to extreme solar concentrations, for which the operating temperatures T of the solar cells 
are higher than observed in flat-plate systems. In this paper, we report on improved performance 
of Varian’s Al,Gal-,As/GaAs heteroface solar concentrator cells grown by metalorganic chemical 
vapor deposition (MOCVD) for use in Cassegrainian arrays. 
Cell Design and Fabrication 
The structure of the concentrator cells described in this paper is very similar to that reported 
at  the previous SPRAT conference[ref. 31. The design conditions for the Cassegrainian cells were 
292 
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defined to be 100 suns, AM0 at T = 8OoC, with a circularly illuminated area of 0.126 cm2 (4-mm 
diameter) on a 5 x 5 mm2 die. As before, the cells were designed with the aid of a computer model 
simulating the performance of Al,Gal-.As/GaAs heteroface solar cells [ref. 41. The structure of a 
typical p-n cell is shown in Fig. 1. In addition to the emitter and base, the devices include a highly 
doped buffer layer to minimize surface recombination effects at the back of the base and to provide 
a smooth surface upon which the overlying structure is grown. The Ab.9Gao,lAs window layer 
passivates the front surface of the emitter and, in combination with the single-layer antireflection 
(AR) coating, minimizea the reflectance. Finally, the GaAs cap layer enables excellent ohmic contact 
by the top grid pattern while simultaneously minimizing the possibility of the grid metallization 
diffusing into the p-n (or n-p) junction during processing. The cap layer is selectively etched away 
between the grid lines prior to the application of the AR coating. For n-p cells, the emitter thickness 
is approximately 0.2 pm. 
The multilayer structure was grown in a horizontal rf-heated MOCVD reactor at 73OoC, as 
described in Fkference 5. Growth rates were 0.1 pm/min, and zinc and selenium were used as the 
p- and n-type dopants, respectively. Conventional photolithographic techniques were used to lay 
down the top grid pattern. Metallizations, which were deposited by evaporation, were Pd/Au and 
Au/Ge/Ni/Au for p- and n-type GaAs ohmic contacts, respectively. The SiO,N, AR coating was 
deposited by plasma deposition at  300 OC. 
To raise the cell conversion efficiency, improvements were sought in the grid-line quality and the 
series resistance R, of the cell. To accomplish the first task, a superior technique for electroplating 
the front grid metallization was developed. This procedure, which uses thick photoresist as a plating 
mask, significantly reduced the mushrooming effects associated with plating. Since the Cassegrainian 
mask requires numerous narrow grid lines (- 3 pm wide) for optimal efficiency, the reduction in the 
amount of mushrooming greatly lowered the obscuration and raised the spectral response. 
The series resistance problem is a more difficult matter, however. The primary factors con- 
tributing to  R, are the contact resistance R,, the sheet resistance Ra, and the grid-line resistance 
Rg. With the good ohmic contact afforded by the highly doped GaAs cap layer, R, does not signif- 
icantly influence r], even for concentrations 2 1000 suns. Any improvement in R,, therefore, must 
come from lowering either Ra or R,. Recently, we have attempted to lower the value of R ~ J  by 
reexamining the effects of the doping level in the emitter. Using the original material parameters 
included in the computer model, the calculated value for the optimal doping level in the emitter 
NE was approximately 1 x 1 O l a  ~ r n - ~  in both the p-n  and n-p configurations. This value resulted 
from the conflicting objectives of maximizing the diffusion length L in the emitter for high quantum 
efficiency (low doping condition) and lowering Rn (high doping). Recent data [ref. 61 for L in GaAs 
indicate that the values originally used in the computer model [ref. 41 were unduly pessimistic. As 
a result, the optimum value for NE is higher than the computer model predicted, since satisfactory 
quantum yield may be attained at higher doping levels. Consequently, in the cells presented here, 
we have empirically increased the value for N E  in both the p-n and n-p configurations until the 
optimal value was obtained. 
Testing 
One-sun efficiencies were determined using Spectrolab XT-10 simulators both a t  Varian and 
Sandia National Laboratories. For AM0 measurements, the simulator intensity was set using balloon- 
flight-calibrated GaAs solar cells which had similar spectral responses to the appropriate concentrator 
293 
cells. A xenon-lamp flash tester was used to determine device performance under concentration. The  
incident power upon the cell in the flash tester was determined by assuming that  the short-circuit 
current I,, was linearly dependent upon the solar concentration. The exposed sawing streets at 
the edges of the cell were masked off from any light t o  assure the accuracy of the measurements. 
Efficiency calculations were based on the total illuminated area, and no correction was made for 
obscuration. 
Results and Discussion 
Figure 2 plots the external quantum yield of an n-p spectral response sample, in which the grid 
obscuration has been eliminated. In this case, NE = 2 x 10la ~ m - ~ ,  and at this doping level, the 
sheet resistance of the emitter is not a significant factor in limiting cell performance, i e . ,  the power 
loss associated with R n  is much smaller than that associated with R,. I t  is readily apparent that  
in spite of the increased doping level in the emitter, the cell still has excellent quantum efficiency. 
Nearly unity quantum yield is observed a t  the center of the spectrum, and most of the losses in the 
red and blue are due t o  a less-than-optimal AR coating on this particular sample. This demonstrates 
that  the material quality is sufficiently high that low series resistance and high values for I,, are not 
mutually exclusive. For p-n cells, 2 x 10la cm-3 also appears to be the optimal value for N E .  Due 
to the lower mobility of the holes in this structure, Rm is not negligible. At  the solar concentrations 
relevant t o  the Cassegrainian module, however, the series resistance problem is not serious, as will 
be shown below. 
Figure 3 shows the current-voltage ( I -V)  characteristics of an n-p Cassegrainian cell a t  a solar 
concentration of 333 suns and T = 28OC. Although the cell is intended for use a t  100 suns, the 
efficiency remains very high ( q  = 25.1%). The open-circuit voltage V,, = 1.18V, and the current 
density J, ,  corresponds t o  33.6 mA/cm2/sun. The high value for the fill factor (FF  = 0.862) indicates 
that  series resistance is not a significant parameter limiting cell performance. Curve fitting of the 
I-V da ta  yields an estimate of R, - 3mO. Figures 4-6 plot the dependence upon concentration 
of '1, V,, and FF,  respectively. The efficiency peaks near 300 suns with a value of 25.1%; a t  100 
suns, '1 = 24.5%. The drop in efficiency above - 400 suns results from the increasing importance of 
losses due to series resistance, as indicated by the decreasing fill factor. The p-n cells show slightly 
lower efficiencies, primarily due to  lower values of Z,,. The advantages of the n-p configuration 
may be attributed to tenfold higher values of mobility for electrons us holes in GaAs. By making 
electrons the majority carrier in the emitter, lower sheet resistance Rm may be obtained without the 
disadvantages of either overdoping or thickening the emitter. The lower value of Ra leads directly 
t o  the generally higher values for F F  in the n-p case. The higher values for I,, are the result of the 
thinner emitter coupled with the long diffusion length in the p-type base. 
Comparing these results with those which were presented a t  the previous SPRAT conference 
[ref. 31, it is plain that the improvements in the plating procedures and sheet resistance have 
significantly improved cell performance. The more recent cells exhibit better values for I,, (33.6 
ws 32.4 mA/cm2), presumably due t o  the reduced obscuration due to  mushrooming. The series 
resistance is also less of a problem, as indicated by the increase in the concentration for which the 
efficiency peak is observed (300 us 180 suns). Even higher efficiencies may be attained by lowering 
R,, since this is the only remaining source of significant series resistance. This may be accomplished 
by widening and thickening the grid lines or lowering the resistivity of the grid-line metallization. 
Since increasing the grid-line width decreases I,,, improvements must come from the latter two 
294 
factors. At this time, we are developing an improved metallization process to address this minor 
problem. 
Although we have not yet measured the temperature behavior of the cell examined in Figs. 3-6, 
we can use the previously determined value for the temperature coefficient [ref. 31 (-O.O36%/OC 
for a 23.40eefficient cell at T = 28OC and 90 suns) to estimate the performance of the cell under 
the intended operating conditions of 100 suns and T = 8OOC. Such an estimate predicts that the 
most recent cell would be 22.50eefficient under these conditions, which exceeds the NASA-program 
goal of 22%. The program also calls for the delivery of 100 Cassegrainian cells to NASA, so we have 
delivered sixty-eight n-p cells and thirty- two p-n cells. 
Summary 
Cassegrainian cells with a conversion efficiency near 25% have been fabricated in both the n-p 
and p-n configuration. To our knowledge, this is the highest reported cell efficiency for space appli- 
cations. The improved performance is the result of new grid-line plating procedures and lower series 
resistance associated with higher doping in the emitter. Under the probable operating conditions of 
100 suns and T = 80 OC, the estimated efficiency is 22.5%. To meet program goals, we have delivered 
sixty-eight n-p cells and thirty-two p-n cells to NASA. 
References 
[ 11 R. E. Patterson, H. S. Rauschenbach and M. D. Cannady, Conference Record of the 16th 
IEEE Photovoltaic Specialists Conference, IEEE, New York, 1982, p. 39. 
[ 21 T. G. Stern and E. W. Hayes, Conference Record of the 17th IEEE Photovoltaic Specialists 
Conference, IEEE, New York, 1984, p. 326. 
[ 31 J .  G. Werthen, G. F. Virshup, H. F. MacMillan, C. W. Ford and H. C. Hamaker, Space 
Photovoltaic Research and Technology 1986, NASA Conference Publication 2475, 1986, p. 25. 
[ 41 H. C. Hamaker, J. Appl. Phys. 58, 2344, 1985. 
[ 51 C. R. Lewis, W. T .  Dietze and M. J .  Ludowise, J .  Electron. Mater. 12, 507, 1983. 
[ 61 R. K. Ahrenkiel, D. J. Dunlavy, D. Greenberg, J. Schlupmann, H. C. Hamaker and H. F. 
MacMillan, Appl. Phys. Lett. 51, 776, 1987. 
295 
FRONT CONTACT 
/ AR COATING 
0.6 p+ GaAs p n  . /  
300 A p AbgGaO.1As WINDOW 
0.6- p G d t  E M l l T E R  
1.0 
0.9 
E ::: 
€ :I 
9 
Q 0.4 
BACK CONTACT I 
I 
- 
1 
- 
- - 
Fig. 1. Schematic structure of a typical p-n GaAs 
concentrator cell. 
[;I, I , I ,  I ,  I ,  I ,  I ,  I ,  
8.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 
PHOTON ENERGY Wl 
Fig. 2. External quantum efficiency us photon en- 
ergy for an n-p GaAs spectral response 
sample. The scale on the top axb shows 
the wavelength in nm. 
Fig. 3. Current 041 voltage for an n-p GaAs Casse- 
grainian cell under a simulated 333 x , AM0 
spectrum. The cross indicates the maxi- 
mum power point. 
26 I ' ' " ' 1 1 ' 1  ' " r " l  
- 
211 10 100 1000 
' ' " " " 1  ' ' " " " 1  ' ' "" ' '  
CONCENTRATION 
Fig. 4. AM0 efficiency us solar concentration for 
the n-p GaAs cell shown in Fig. 3. 
296 
1.20 
1.18 - 1 = 280C 
- 
- 
.. 
CONCENTRATION 
Fig. 5. Open-circuit voltage u8 solar concentration 
for the n-p GaAs cell shown in Fig. 3. 
Fig. 6. Fill factor u8 solar concentration for the n-p 
GaAs cell shown in Fig. 3. 
297 


The 25 percent-efficient GaAs Cassegrainian concentrator cell

The 25 percent-efficient GaAs Cassegrainian concentrator cell

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