Minority carrier diffusion length is one of the most important parameters affecting the solar cell performance. An attempt is made to estimate the minority carrier diffusion lengths is the emitter and base of InP/GaAs heteroepitaxial solar cells. The PC-1D computer model was used to simulate the experimental cell results measured at NASA Lewis under AMO (air mass zero) spectrum at 25 C. A 16 nm hole diffusion length in the emitter and a 0.42 micron electron diffusion length in the base gave very good agreement with the I-V curve. The effect of varying minority carrier diffusion lengths on cell short current, open circuit voltage, and efficiency was studied. It is also observed that the front surface recombination velocity has very little influence on the cell performance. The poor output of heteroepitaxial cells is caused primarily by the large number of dislocations generated at the interfaces that propagate through the bulk indium phosphide layers. Cell efficiency as a function of dislocation density was calculated and the effect of improved emitter bulk properties on cell efficiency is presented. It is found that cells with over 16 percent efficiencies should be possible, provided the dislocation density is below 10(exp 6)/sq cm

Flood, D. J.

Jain, R. K.

English

NASA Technical Reports Server

NASA Technical Memorandum 103213
Estimation of Minority Carrier
Diffusion Lengths in InP/GaAs
Solar Cells
R.K. Jain and D.J. Flood
Lewis Research Center
Cleveland, Ohio
CrLLo (?,_0._:_} I.L) i' C_;I. IOA
NgO-ZBo8'_
uncl,Js
Prepared for the
Second International Conference on Indium Phosphide and Related Materials
cosponsored by the IEEE Lasers and Electro-Optics Society and
the IEEE Electron Device Society
Denver, Colorado, April 23-25, 1990
https://ntrs.nasa.gov/search.jsp?R=19900016753 2020-03-19T22:18:56+00:00Z

Estimation of Minority Carrier Diffusion Lengths in InP/GaAs Solar Cells
R. K. Jain* and D. J. Flood
NASA Lewis Research Center
21000 Brookpark Road, Cleveland, OH 44135
Abstract Minority carrier diffusion length is one of the most important
parameters affecting the solar cell performance. In this paper an attempt is made
to estimate the minority carrier diffusion lengths in the emitter and base of
InP/GaAs heteroepitaxia] solar cells. The PC-ID computer model has been used t9
simulate the experimental cell results measured at NASA Lewis under AMO spectrum
at 25°C. A 16 nm hole diffusion length in the emitter and a 0.42#m electron
diffusion length in the base gave very good agreement with the I-V curve. The
effect of varying minority carrier diffusion lengths on cell short circuit
current, open circuit voltage and efficiency has been studied. It is also
observed that the front surface recombination velocity has very little influence
on the cell performance. The poor output of heteroepitaxial cells is caused
primarily by the large number of dislocations generated at the interfaces that
propagate through the bulk indium phosphide layers. Cell efficiency as a function
of dislocation density has been calculated and the effect of improved emitter
bulk properties on cell efficiency is presented. It is found that cells with
over 16_ AMO efficiencies should be possible provided the dislocation density
is below 106cm -2.
Introduction Minority carrier diffusion length is one of the most important
parameters affecting the solar cell performance. Unlike silicon or gallium
arsenide, indium phosphide is still not well understood, and several physical
parameters must yet be measured correctly. In this paper an attempt is made to
estimate the minority carrier diffusion lengths in the emitter and base of InP
solar cells grown by MOCVD on GaAs wafers.
Recently indium phosphide has emerged as a promising material for space
power applications due to its superb radiation resistance [I,2] and high
efficiency prospects [3,4], but the present wafer cost may limit its large scale
use. The problem is being overcome by growing single crystal indium phosphide
films on foreign substrates such as silicon and gallium arsenide [5,6]. This
approach would provide large area, light weight and mechanically strong cells.
The major problems in heteroepitaxy are lattice and thermal expansion mismatch.
InP/GaAs cells have shown better results compared to InP/GaAs/Si cells [6].
Weinberg et al. have reviewed the progress in indium phosphide solar cell
research at the previous InP conference [7].
The poor heteroepitaxial cell output is caused primarily by the large number
of dislocations generated at the interfaces that propagate through the bulk InP
layers [8]. This adversely affects the bulk minority carrier properties. The PC-
ID computer model [9,10] has been used to calculate the cell I-V results and were
compared to experimental v]lues measured on n+/p/p + indium phosphide on gallium
*This work was done while the author held a National Research Council-NASA LeRC
Research Associateship.
arsenide cells manufactured by Spire Corporation under contract to NASA Lewis.
These cells had quite shallow emitter and the size was 0.5 cm by 0.5 cm. Minority
carrier diffusion lengths have been varied to match the measured results on the
InP/GaAs cells. A 16 nm hole diffusion length in the emitter and a 0.42 #m
electron diffusion length in the base gave quite good agreement with the I-V
curve, but the matching with spectral response is not as good. In the present
paper our primary purpose has been to estimate the approximate order of the
minority carrier diffusion lengths. The effect of varying minority carrier
diffusion lengths on cell short circuit current, open circuit voltage and
efficiency has been studied. It is also observed that the variation in the front
surface recombination velocity has very little influence on the cell performance.
Minority carrier diffusion lengths could be improved by controlling the number
of dislocations generated using proper buffer layers and growth techniques. AMO
cell efficiency as a function of dislocation density has been calculated and the
effect of improvement of emitter bulk properties on cell efficiency is presented.
It is found that cells with over 169 AMO efficiencies should be possible provided
the dislocation density is below 106 cm-2. The effect of emitter thickness and
doping on cell performance has been described elsewhere [11].
ExperEmental Indium phosphide heteroepitaxial cells were manufactured by Spire
Corporation under contract to NASA Lewis Research Center. Indium phosphide layers
were grown using metalorganic chemical vapor deposition (MOCVD) technique on
gallium arsenide wafers. Figure i shows the n+/p/p+ solar cell structure. The
emitter, base and the buffer layer were 40 nm, 3 #m and i #m thick and had doping
concentration of 2xI0 I_ cm-3, 3x1016 cm-3 and Ix1018 cm-3 respectively. The front
and back contacts were made by Cr/Au/Ag and Au-Zn alloy respectively. The front
grid coverage was 59 and the cells had a double layer ZnS/MgF 2 antireflection
coating. The cell area was 0.25 cm2.
A batch of eight cells was measured at NASA Lewis Research Center under
simulated AMO spectrum (137.2 mW/cm2, 25°C) using a NASA Lear jet calibrated InP
standard cell. The average values and standard deviation of the measured short
circuit current, open circuit voltage and efficiency (total area) of the eight
cells are shown in Table I.
Results and Discussion The PC-ID computer model developed by Iowa State
University [9,10] was used to simulate the experimentally measured cell results.
It is important to mention that out of several physical parameters related to
InP, which need to be estimated and measured correctly, the exact value of the
intrinsic carrier concentration ni is still in doubt. In this work we have used
the average value of ni equals to 8xi06 cm-3 as suggested in reference 12.
Minority carrier diffusion lengths have been varied to match the calculated
values with the experimentally measured results. A 16 nm hole diffusion length
in the emitter and a 0.42 #m electron diffusion length in the base gave quite
good agreement with the I-V measurements as shown in Table I. From the analysis
it is clear that the diffusion lengths in the emitter and base of heteroepitaxial
cells are quite small. Figures 2 to 5 describe various calculated results and
it should be noted that when one parameter is changed all other parameters remain
constant.
In Fig. 2 we have plotted the effect of hole diffusion length on the cell
performance. It is observed that cell short circuit current and efficiency
improves with the increase in diffusion length, while open circuit voltage
almost remains same. Improvement in hole diffusion length is essential to achieve
better cells.
Figure 3 describes the effect of electron diffusion length on cell
performance. From this figure it is clear that base minority carrier diffusion
length has strong influence on the cell results. Cell short circuit current, open
circuit voltage and efficiency improve significantly with increases in diffusion
length. Therefore, for achieving high efficiencies, base material quality has
to be improved drastically. This in case of heteroepitaxy could be achieved by
using better annealing and growth techniques.
Figure 4 plots the effect of front surface recombination velocity (FSRV)
on cell efficiency. The FSRVhas been varied from 105 to ]07 cm/sec. FrownFig.
4 we notice a very little improvement in cell efficiency as the FSRVdecreases,
which is due to slight improvement in the short circuit current as open circuit
voltage almost remains constant. Therefore, the FSRVhas very little influence
on the cell performance, perhaps due to dominant poor bulk properties and the
fact that the cells had extremely shallow emitters (40 nm). It is felt that once
the emitter bulk properties are improved, FSRVwould certainly play its role in
controlling the cell performance.
In Fig. 5 we have plotted the cell AMOefficiency as a function of
dislocation density. The dislocation density has been calculated from the simple
relation [5] expressed as:
1 _.3 ND
4
(1)
where LD is the dislocation limited minority carrier diffusion length and ND is
the dislocation density. The electron diffusion length in the base has been taken
as the dislocation limited minority carrier diffusion length. The solid line in
Fig. 5 shows the variation of the cell efficiency as a function of dislocation
density for a hole diffusion length of 16 nm. It is quite clear that with the
decrease in dislocation density, cell efficiency improves almost linearly on a
semi-log scale. These results are further improved if the hole diffusion length
is improved to 80 nm as ShOWn by dashed curve. From Fig. 5 it is observed that
heteroepitaxial indium phosphide solar cells with over 164 AMO efficiencies are
possible provided the dislocation density is reached below 106 cm-2. It is hoped
that with the advancements in identifying proper buffer layers, suitable
annealing and improved growth techniques, it would be possible to manufacture
highly efficient, radiation hard, light weight, low cost and mechanically strong
heteroepitaxial indium phosphide cells for space power applications.
ConcZusions
Short minority carrier diffusion lengths in the emitter and base are
associated for the heteroepitaxial indium phosphide solar cells considered in
this work.
Minority carrier diffusion lengths in the emitter and base influence the
cell performance, but the base minority carrier diffusion length has a
significant effect. For improved performance it is required to enhance the base
material quality.
Front surface recombination velocity has very little influence on the cell
characteristics, perhaps due to poor emitter bulk properties.
Heteroepitaxial indium phosphide cells with over 16_ AMOefficiencies should
be possible to fabricate provided dislocation density is below 106 cm-2. This
would require improvements in lattice and thermal mismatch and advancementsin
the development of proper buffer layers and growth techniques.
For understanding fully the physics of indium phosphide solar cells, it is
highly essential to determine the various physical parameters of InP and related
materials. This study alongwith experimental measurementswould be very valuable
in pushing the indium phosphide cell technology to meet the increasing power
demandof the future space missions.
AcknowZedgements Valuable discussions with Dr. I. Weinberg and Mr. D. Wilt (,f
NASA Lewis Research Center, Dr. G. Landis of Sverdrup Technology and Dr. P. A.
Basore of Sandia National Laboratories are gratefully acknowledged.
Relerences
[11M. Yamaguchi, C. Uemura, A. Yamamoto and A. Shibukawa,'Electron Irradiation
Damage in Radiation-Resistant ]riP Solar Cells',Japan J. Appl. Phys.,23,302-
307,1984.
[2] I. Weinberg, C. K. Swartz and R. E. Hart, 'Potential for Use of ]rip S_la_
Cells in the Space Radiation Environment', Proceedings 18th IEEE Photovo|taic
Specialists Conference, 21-25,1985.
[3] C. J. Keavney and M. B. Spitzer,'Indium Phosphide Solar Cells by lo_i
Implantation', Appl. Phys. Letts.,52,1439-1440,1988.
[4] H. Okazaki, T. Takamoto, H. Takamura, T. Kamei, M. Ura, A. Yamamoto and M.
Yamaguchi,'Production of Indium Phosphide Solar Cells for Space Power
Generation', Proceedings 20th IEEE Photovoltaic Specialists Conference, 886-
892,1988.
[5] M. Yamaguchi, A. Yamamoto, N. Uchida and C. Uemura,'A New Approach for Thin
Film InP Solar Cells', Solar Cells,19,85-96,1986-1987.
[6] C. J. Keavney, S. M. Vernon, V. E. Haven and S. J. Wojtczuk,'Fabrication of
n /p InP Solar Cells on Silicon Substrates', Appl. Phys. Letts.,54,1139-
1141,1989.
[7] I. Weinberg, D. J. Brinker, C. K. Swartz and R. E. Hart,'Progress in Indium
Phosphide Solar Cell Research', Proceedings First International Conference on
Indium Phosphide and Related Materials, 434-444,1989.
[8] S. J. Pearton, K. T. Short, A. T. Macrander, C. R. Abernathy, V. P. Mazzi,
N. M. Haegel, M. M. AI-Jassim, S. M. Vernon and V. E. Haven,'Characterization
of InP/GaAs/Si Structures Grown by Atmospheric Pressure Metalorganic Chemical
Vapor Deposition', J. Appl. Phys.,65,1083-1088,1989.
4
[9] P. A. Basore, D. T. Rover and A. W. Smith,'PC-1D Version 2: Enhanced
Numerical Solar Cell Modeling',Proceedings 20th IEEE Photovoltaic Specialists
Conference, 389-396,1988.
[10] P. A. Basore, PC-ID Installation Manual and User's Guide Version 2.1, Iowa
State University Research Foundation, Ames, IA, 1989.
[11] R. K. Jain and D. J. Flood,'Effect of Emitter Parameter Variation on the
Performance of Heteroepitaxial Indium Phosphide Solar Cells', Proceedings 21st
IEEE Photovoltaic Specialists Conference, May 1990 (To be published).
[12] A. H. Yahia, M. W. Wanlass and T. J. Coutts,'Modeling and Simulation of InP
Homojunction Solar Cells', Proceedings 20th IEEE Photovoltaic Specialists
Conference, 702-707,1988.
TabZe I. Comparison of Measured and Calculated InP Cell Parameters
Measured Ave.
(Std. Dev.)
Calculated
Short Circuit Current
(mA)
6.654
(0.180)
6.657
Open Circuit Voltage(mv)
666.6
(0.037)
665.2
Efficiency
8.92
(0.138)
9.05
ZnS/MgF 2 AR Coating
Emitter n ÷
Base p 3xlO 16 cm"3 3#m
Buffer p+ IxlO 18 cm-3 l#m
GaAs Wafer
Front Contact Cr/Au/Ag
I
2xlO IB cm"3 40nm
Back Contact Au-Zn
Fig. I. Structure of a MOCVD Grown
InP/GaAs Solar Cell
720 _ , , , , , , , ,
E
_,_ 7.00
b 6.80
0
3 6.60(3
._
0
6.40
6.20 T t t t 1 t ! l
700 , , , , , , , ,
>
E
680
66O(3
>
"._ 64o(3
._.
0
E 620
Q)
Q
0
1 ! t t t I I w
10 ........
tlJ
0
(3
t-
CO
! ! f 1 ! I I !
10 20 30 40 50 60 70 80 90 100
Hole Diffusi(3n Length (riT_
Fig. 2. Effect of Hole Diffusion Length on InP Solar eel] Performance
8.CX_ i
7.,50
7.00
_) 6.50
6.00
5.5O
5.00
4.,50
(/)
4.00 1
I I I I
>
E 85o
_) 800
+- 750
0
> 700
4-'
6500
l,,,_
°m
L) 60o
C
550O.
0
5OO
16
14
12
"6
8
Ul
< 4
i 6 l
I t I
! i J
I I I I I I I
I
2 i f i I i I ,
0 1 2 3 4
Electron Diffusion Length (micron)
Fig. 3. Effect of Electron Diffusion Length on InP Solar Cell Performance
95 "'i e i i i i i i I I i i J i i i i
9.4
0
9.3
W 9.2
_ 9.1
I I I # l t ! I | I I f I I i i =
10 e 10 7
Front Surface Recomb. Velocity (cm/sec)
Fig. 4. Effect of Front Surface Recombination Velocity on Cell AMO Efficiency
18
16
0
E
°--
0
°--
Lb-
LU
0
<(
14
12
10
8
6
i i J ij i i • i i i . ij o i e i | i i ij i J i
-. Hole Diffusion Length
-.I ". 80nm
%'_ %,
I I I I I I I J I I I I t I t I l I I I i i I i i •
10 6 lO'r 10 8
Dislocation Density (cm-2)
Fig. 5. Effect of Dislocation Density on Cell AMO Efficiency
Nalional Aeronautics and
Space Administration
Repo Documentation Page
1. Repo# No.
NASA TM-103213
2. Government Accession No.
4. Title and Subtitle
Estimation of Minority Carrier Diffusion Lengths
in InP/GaAs Solar Cells
7. Author(s)
R.K. Jain and D.J. Flood
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-319l
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
3. Recipient's Catalog No.
5. Report Date
6. Performing Organization Code
8. Performing Organization Report No.
E-5624
10. Work Unit No.
506-41-11
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the Second International Conference on Indium Phosphide and Related Materials cosponsored by the
IEEE Lasers and Electro-Optics Society and the IEEE Electron Device Society, Denver, Colorado, April 23-25,
1990. R.K. Jain, National Research Council--NASA Research Associate at Lewis Research Center. D.J. Flood,
NASA Lewis Research Center.
16. Abstract
Minority carrier diffusion length is one of the most important parameters affecting the solar cell performance, la
this paper an attempt is made to estimate the minority carrier diffusion lengths in the emitter and base of
InP/GaAs heteroepitaxial solar cells. The PC-ID computer model has been used to simulate the experimental cell
results measured at NASA Lewis under AMO spectrum at 25 °C. A 16 nm hole diffusion length in the emitter
and a 0.42 /zm electron diffusion length in the base gave very good agreement with the I-V curve. The effect of
varying minority carrier diffusion lengths on cell short circuit current, open circuit voltage, and efficiency has
been studied. It is also observed that the front surface recombination velocity has very little influence on the cell
performance. The poor output of heteroepitaxial cells is caused primarily by the large number of dislocations
generated at the interfaces that propagate through the bulk indium phosphide layers. Cell efficiency as a function
of dislocation density has been calculated and the effect of improved emitter bulk properties on cell efficiency is
presented. It is found that cells with over 16% AMO efficiencies should be possible provided the dislocation
density is below 106 cm -2.
17. Key Words (Suggested by Author(s))
Indium phosphide
Solar cell
Minority carrier diffusion length
Solar power
18. Distribution Statement
Unclassified - Unlimited
Subject Category 20
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages
Unclassified Unclassified 10
NASAFOnMle2SOCTas *For sale by the National Technical Information Service, Springfield, Virginia 22161
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Estimation of minority carrier diffusion lengths in InP/GaAs solar cells

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