Recently indium phosphide solar cells have achieved beginning of life AMO efficiencies in excess of 19 pct. at 25 C. The high efficiency prospects along with superb radiation tolerance make indium phosphide a leading material for space power requirements. To achieve cost effectiveness, practical cell efficiencies have to be raised to near theoretical limits and thin film indium phosphide cells need to be developed. The optimal design study is described of high efficiency indium phosphide solar cells for space power applications using the PC-1D computer program. It is shown that cells with efficiencies over 22 pct. AMO at 25 C could be fabricated by achieving proper material and process parameters. It is observed that further improvements in cell material and process parameters could lead to experimental cell efficiencies near theoretical limits. The effect of various emitter and base parameters on cell performance was studied

Flood, Dennis J.

Jain, Raj K.

English

NASA Technical Reports Server

NASA Technical Memorandum 103763 ...........
• __ o!
............. Optimal Design Studyof High Efficiency
" --+' " Indium Phosphide Space Solar Cells
Raj K. Jain and Dennis J. Flood
Lewis Research Center
Cleveland, Ohio
Prepared for the
5th International Photovoltaic Science-andEngifiee-ring Conference (PVSEC-5) -
sponsored by the Japan Society of Applied Physics, the Institute Of
. . Electrical Engineers of Japan, and the Foundation for the -
Advancement of International Science .....
. _:_Kyoto, Japan, November 26- 30, 1990 .....
IXlt A
(NASA-TM-I03763) OPTT_AL OESIGN STUDY OF
_ HIGH EFFICIENCY INOIu_ PH_3SPHIDE .SPACE SOLAR
CFLL5 (NASAl 7 p CSCL IOA
.................. G3/33
N91-21433
Uncl as
0008038
https://ntrs.nasa.gov/search.jsp?R=19910012120 2020-03-19T18:54:24+00:00Z

OPTIMAL DESIGN STUDY OF HIGH EFFICIENCY INDIUM
PHOSPHIDE SPACE SOLAR CELLS
Raj K. Jain* and Dennis J. Flood
National Aeonautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
Summary
Recently indium phosphide solar cells have achieved
beginning of life AM0 efficiencies in excess of 19 percent at
25 °C. The high efficiency prospects along with superb
radiation tolerance make indium phosphide a leading material
for space power requirements. To achieve cost-effectiveness,
practical cell efficiencies have to be raised to near theoretical
limits and thin film indium phosphide cells need to be
developed. Present work describes the optimal design study
of high efficiency indium phosphide solar cells for space
power applications using the PC-1D computer program. It is
shown that cells with efficiencies over 22 percent AM0 at
25 °C could be fabricated by achieving proper material and
process parameters. It is observed that further improvements
in cell material and process parameters could lead to
experimental cell efficiencies near theoretical limits. The
effect of various emitter and base parameters on cell performance
has been studied.
Introduction
Recently 4 cm 2n'pp _homocpitaxial indium phosphide solar
cells grown by MOCVD have achieved beginning of life AM0
efficiencies in excess of 19 percent at AM0 (ref. 1). This is the
highest efficiency reported for InP cells to date for cells
measured at NASA Lewis Research Center. Based on the
assumptions that all the incident photons with energies greater
than or equal to the bandgap energy generate free carriers and
that their recombination is entirely radiative, an upper limit for
conversion efficiency of 29 percent for a semiconductor with
bandgap of 1.30 eV has been obtained (ref. 2). The indium
phosphide bandgap (1.35 eV) is quite close to the bandgap
(1.30 eV) considered in the maximum conversion efficiency
calculations (ref. 2). The high efficiency prospects along with
superbradiation tolerance (refs. 3 and 4) make indium phosphide
a leading material for space power requirements. To achieve
wide spread use, however, practical cell efficiencies have to be
raised to near theoretical limits wlfile maintaining high radiation
resistance and cells utilizing thin layers of indium phosphide
only for active regions need to be developed. Current
heteroepitaxial indium phosphide cells on gallium arsenide or
silicon substrates have low efficiencies (ref. 5) due to large
numbers of dislocations and their effect on cell performance
has been described elsewhere (ref. 6).
The present paper describes an optimum design study of high
efficiency indium phosphide solar cells for space power
applications using the PC-ID computer program (ref. 7). It is
shown that cells with efficiencies over 22 percent AM0 at 25 °C
could be fabricated by achieving proper material and process
parameters. It is observed that further improvement in cell
material and process parameters could lead to experimental cell
efficiencies near theoretical limits. The effects of various
emitter and base parameters (minority carrier diffusion lengths,
thickness, grid shading, cell series resistance, front/back surface
recombination velocities, etc.) on cell AMO efficiency have
been studied.
Design Approach
The PC-1D is a one-dimensional computer program for
investigating semiconductor devices. The program accurately
solves the device transport equations based on the isolated-
*NationalResearchCouncil -- NASA Research Associate at Lewis Research Center.
element approach. A number of researchers have used the
PC-1D for analyzing various semiconductor device problems
as well as for solar cell modeling.
Based on a set of optimally designed cell material and
process parameters, the current-voltage characteristics of the
indium phosphide solar cell are calculated. The cell
performance parameters are extracted from these calculations.
The effect of various emitter and base parameters on cell
efficiency is studied by varying the parameter of interest and
keeping other parameters constant.
In the case of indium phosphide, the exact value of the
intrinsic carrier concentration is still in question. In this work
we have used the average value (Sx106 cm -3) of the intrinsic
carrier concentration as suggested in reference 8.
Results and Discussion
Figure 1shows the calculated current-voltage characteristics
of an optimally designed n'p indium phosphide solar cell for the
parameters given in Table 1. This design yields with the cell
open circuit voltage of 922 mV, short circuit current density
over 40 mA/cm 2, fill factor of 0.84 and AM0 efficiency in
excess of 22 percent at 25 °C. This design study shows what
must be done to achieve improvement in cell efficiency over
current state-of-the-art indium phosphide cells. Some of the
cell material and process parameters of Table I have already
been achieved and other parameters, especially the minority
carrier diffusion lengths and surface recombination velocities
must be realized to obtain high efficiency cells.
In figures 2 and 3 the effects of emitter minority carrier (hole)
diffusion length and base minority carrier (electron) diffusion
length respectively on cell efficiency have been studied. The
efficiency initially improves rapidly and afterwards saturates
with increasing minority carrier diffusion lengths. The hole
diffusion length affects the cell current only, while the
electron diffusion length affects both the cell current as well
as the voltage. The results shown in figures 2 and 3
emphasize the requirement of long diffusion lengths for
obtaining high efficiencies.
Figure 4 plots the variation of cell efficiency with emitter
thickness from l0 to 100 nm. From these results it is clear that
cell efficiency improves significantly with the decrease in
emitter thickness. This is due to reduced recombination, which
improves the current, while the voltage remains almost
unchanged with the reduction in emitter thickness. Fabrication
of thin emitters free from other process interactions is a
challenge in itself, but emitters around 20 nm should be targeted
for better results.
In figure 5 we have demonstrated the influence of front
contact grid shading on cell efficiency. A reduction in grid
shading increases cell current, which has a net effect on the cell
efficiency. However the grid coverage has to be optimized
considering its series resistance, cell contact fabrication
technique used and the cell application (flat plate/concentrator).
In the optimal design study 5 percent grid coverage has been
considered. It has been possible to effectively reduce the grid
shading losses by the use of prismatic covers on cells (ref. 9).
In figure 6 we have studied the effect of cell series resistance
on cell efficiency. The series resistance of a solar cell has to be
reduced to its lowest possible value to keep the power losses at
a minimum. A full discussion of the problem is beyond the
scope of the present paper. In the design study we have
considered 0.5 fl-cm 2 as the cell series resistance. A lower
value would improve efficiencies still further.
Figures 7 and 8 show the effect of front and back surface
recombination velocities on cell efficiency respectively. Lower
values of SRV's would help in achieving high cell efficiencies.
Due to the absence of passivation layers, present indium
phosphide cells have high front SRV's. Therefore proper
passivating layers need to be developed. The back SRV's could
probably be lowered by suitable BSF and BSR layers and
require to be further investigated. Calculations have shown that
the front SRV affects the cell current only, while the back SRV
affects the voltage as well as the current. This is contrary to the
results found in cells with poor emitter quality where the front
SRV has no effect of efficiency (ref. 10).
From the results described in figures 2 to 8, it is clear that
further increase in cell efficiencies up to 25 percent AM0
could be achieved by improvements in cell material and
process parameters.
Conclusions
An optimal design study of indium phosphide solar cells has
shown that cell efficiencies in excess of 22 percent AMO at
25 °C are possible. The effect of various emitter and base
parameters on cell efficiency has been studied. Further
improvements in cell material and process parameters could
lead to experimental cell efficiencies near theoretical limits.
Acknowledgments
This work was done while one of the authors (RKJ) held a
National Research Council-NASA Lewis Research Center
Research Associateship.
References
1. Keavney, C.J.; Haven, V.E.; and Vernon, S.M.: Surface
Recombination and High Efficiency in haP Solar Cells.
Proceedings 2nd International Conference on Indium
Phosphide and Related Material, April 23-25, 1990,
pp. 435-438.
2. Ruppel, W.; and Wurfel, P.: Upper Limit for the Conver-
sion of Solar Energy. IEEE Trans. Electron Dev., vol.
ED-27, no. 3, Apr. 1980, pp. 877-882.
3. Yamaguchi,M.;etal.: Electron Irradiation Damage in
Radiation-Resistant InP Solar Cells. Jpn. J. Appl. Phys.,
vol. 23, no. 3, Mar. 1984, pp. 302-307.
4. Weinberg, I., Swartz, C.K.; and Hart, R.E.: Potential for
Use of InP Solar Cells in the Space Radiation Environ-
ment. 18th IEEE Photovoltaic Specialists Conference,
IEEE, 1985, pp. 1722-1724.
5. Keavney, C.L; et al.: Fabrication ofn+p InP Solar Cells on
Silicon Substrates, Appl. Phys. Lett., vol. 54, no. 12,
Mar. 20, 1989, pp. 1139-1141.
6. Jain, R.K.; and Flood, D.J.: Effect of Dislocations on the
Open Circuit Voltage, Short-Circuit Current and Effi-
ciency of Heteroepitaxial Indium Phosphide Solar Cells.
Proceedings 5th Int. PVSEC, Nov. 26-30, 1990,
Kyoto, Japan, pp. 85-88. (NASA TM-103762).
7. Basore, P.A.; Rover, D.T.; and Smith, A.W.: PC-ID
Version 2: Enhanced Numerical Solar Cell Modeling.
20th IEEE Photovoltaic Specialists Conference, vol. 1,
IEEE, 1988, pp. 389-396.
8. Yahia, A.H:; Wanlass, M.W.; and Coutts, T.J.: Modeling
and Simulation of InP Homojunction Solar Cells. 20th
IEEE Photovoltaic Specialists Conference, vol. 1, IEEE,
1988, pp. 702-707.
9. Zhao, J.; Wang, A.; and Green, M.A.: An Optimized
Prismatic Cover Design for Concentrator and
Nonconcentrator Solar Cells. J. Appl. Phys., vol. 68,
no. 3, Aug. 1, 1990, pp. 1345-1350.
10. Jain, R.K.; and Flood, D.J.: Estimation of Minority Carrier
Diffusion Lengths in InP/GaAs Solar Cells. Proceedings
2nd International Conference on Indium Phosphide and
Related Materials, April 23-25, 1990, Denver, USA,
pp. 439-446. (NASA TM-103213).
Table I. Parameters of an Optimally Designed lnP Solar Cell
Emitter thickness, rtm............................................................................... 20
Emitter doping, cm-3........................................................................ 2x10 It
Hole diffusion length,/am ..................................................................... O.1
Front surface recomb, velocity, cm/sec ................................................ 104
Front grid coverage, percent .................................................................... 5
Cell series resistance, f_-cm2 ................................................................. 0.5
Base thickness, _ma................................................................................... 5
Base doping, cm-3............................................................................. 5x10I+
Electron diffusion length, larn................................................................. 20
Back surface recomb, velocity, cm/see ................................................. 10s
45 D
30
15
I::
0
Open clrcut voltage= 0.9223 v
Shortcircutcurrentdensity= 40,22 mA/cm2 L
Fill factor = 0.84 _/
Efficiency = 22.72% AMO (25 "C)
o I I 1
0.00 .25 .50 ,75
Voltage, v
Figure 1.--Calculated I-V characteristics of an
optimally designed n+p Indium Phosphide
solar cell.
I
1.00
23
c
.8
_. 22
_ 21
20
m
I 1 ]
0 50 100 150
Hole diffusion length, nm
Figure 2.--Effect of emitter minority carrier
(hole) diffusion length on cell efficiency.
I
20O
23
E
8 21
O.
15 [ 1 1 I I I
5 10 15 20 25 30
Electron diffusion lenght, Fm
Figure &--Effect of base minority carrier
(electron) diffusion length on cell efficiency.
23
22
21
_ 2o
8
19 I I I I I ] 1 I ]
10 20 30 40 50 60 70 80 90 100
Emitter thickness, nm
Figure 4._Effect of emitter thickness on cell
efficiency.
25 23
23 ._ 22
21 21
0 2 4 6 8 0 .50 1.00 1.50 2.00
Front grid shading, percent Cell series resistance, ohm-cm2
Figure 5.--Effect of front contact grid shading Figure 6.--Effect of cell series resistance on
on cell efficiency, cell efficiency.
23
_. 22
8
u
m
20 I ilihh] I Ilihhl I illhh] I ilihh]
103 104 105 106 107
Front surface recombination velocity, cm/sec
Figure 7.--Effect of front surface recombination
velocity on cell efficiency.
23--
8
21
20 I illhli[ I illhh[ /,,l,Jilihi 1 Ilihhl
10 3 10 4 105 106 107
Back surface recombination velocity, cm/sec
Figure 8.--Effect of back surface recombination
velocity on cell efficiency.
N/ A
National Aeror_u'lJcs and
Space Administration
1. Report No_ 2. Government Accession No.
NASA TM - 103763
4. Title and Subtitle
Optimal Design Study of High Efficiency Indium Phosphide Space
Solar Cells
7. Author(s)
Raj K. Jain and Dennis J. Flood
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135 - 3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546 - 0001
Report Documentation Page
3. Recipient's Catalog No.
5. Report Date
6. Performing Organization Code
8. Performing Organization Report No.
E - 6025
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 5th International Photovoltaic Science and Engineering Conference (PVSEC- 5) sponsored by the Japan
Society of Applied Physics, the Institute of Electrical Engineers of Japan, and the Foundation for the Advancement of
International Science, Kyoto, Japan, Novemeber 26-30, 1990. Raj K. Jain, National Research Council - NASA Research
Associate at Lewis Research Center; Dennis J. Flood, NASA Lewis Research Center. Responsible person, Raj K. Jain
(216) 433- 2227.
16. Abstract
Recently indium phosphide solar cells have achieved beginning of life AM0 efficiencies in excess of 19 percent at 25 °C.
The high efficiency prospects along with superb radiation tolerance make indium phosphide a leading material for space
power requirements. To achieve cost-effectiveness, practical cell efficiencies have to be raised to near theoretical limits
and thin film indium phosphide cells need to be developed. Present work describes the optimal design study of high
efficiency indium phosphide solar cells for space power applications using the PC-1D computer program. It is shown that
cells with efficiencies over 22 percent AM0 at 25 °C could be fabricated by achieving proper material and process
parameters. It is observed that further improvements in cell material and process parameters could lead to experimental
cell efficiencies near theoretical limits. The effect of various emitter and base parameters on cell performance has been
studied.
17. Kay Words (Suggested by Author(s))
Indium phosphide
Space solar ceils
Optimal design
High efficiency
18. Distribution Statement
Unclassified - Unlimited
Subject Category 33
19. Security Classif. (of the report) 20. Security Classif. (of this page) 21. No. of pages
Unclassified Unclassified 6
NASAFORM1e26OCT86 *For sale bythe NationalTechnicalInformation Service,Springfield,Virginia 22161
22. Pdce °
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