A computer program was used to analyze the illuminated I-V characteristics of four sets of gallium arsenide (GaAs) solar cells irradiated with 1-MeV electrons and 10-MeV protons. It was concluded that junction regions (J sub r) dominate nearly all GaAs cells tested, except for irradiated Mitsubishi cells, which appear to have a different doping profile. Irradiation maintains or increases the dominance by J sub r. Proton irradiation increases J sub r more than does electron irradiation. The U.S. cells were optimized for beginning of life (BOL) and the Japanese for end of life (EOL). I-V analysis indicates ways of improving both the BOL and EOL performance of GaAs solar cells

Heulenberg, A.

Kinnison, J. D.

Maurer, R. H.

English

NASA Technical Reports Server

N91-30242
I-V ANALYSIS OF IRRADIATED GALLIUM ARSENIDE SOLAR CELLS
A. Meulenberg
COMSAT Laboratories
Clarksburg, Maryland
and
R. H. Maurer and J. D. Kinnison
The Johns Hopkins University Applied Physics Laboratory
Laurel, Maryland
While a spacecraft designer may only be interested in end-of-life (EOL) solar cell output power, analysis of
the full I-V characteristics of unirradiated and irradiated cells can benefit both the solar cell designer and
manufacturer, as well as those responsible for determining solar cell acceptance criteria. COMSAT Laboratories
has used a computer program to analyze the illuminated I-V characteristics of four sets of gallium arsenide
(GaAs) solar cells irradiated by the Johns Hopkins University Applied Physics Laboratory (APL) with 1-MeV
electrons and 10-MeV protons.
The illuminated I-V characteristic provides the values (under operating conditions) for series resistance (Rs),
minority carrier lifetime, internal electric fields, and other cell parameters which may depend on photo-generated
carrier concentrations. A multi-regression fit ot these data is made to the following equation:
where
I=
Ire and Ido =
IL=
V=
Rsh =
I= Iro [eq(V'lRs)/2kT-1 ]+ Ido[eq(V'lRs)/kT-1 ]-(V-IRs)/(Rsh) - IL (1)
output current
coefficients of the junction recombination and bulk dark currents, respectively
photo-generated short-circuit current
cell voltage (V - IRs = Vj, the junction voltage)
junction shunt resistance.
The short-circuit current, IL, is subtracted and the data are fitted (with Rs being stepped to obtain the best
correlation) to determine values for Rs, Rsh, Ido, and Iro. In the simplest analysis, these are all constants. The
two currents separate the equation into contributions from bulk regions, Jd, and junction regions, Jr. (Current
densities are used to remove area effects.) More detailed analysis (ref. 1) employs the full Sah-Noyce-
Schockley formulation for Jro, which allows determination of non-midgap defect levels, and band gap narrowing
due to the Franz-Keldysh effect.
*This work was sponsored in part by the Communications Satellite Corporation and in part by NASA.
39-1
https://ntrs.nasa.gov/search.jsp?R=19910020928 2020-03-19T17:03:07+00:00Z
I-Vanalysishasbeenusedforsiliconsolarcells(ref.2) to measuretheeffectivenessof thefollowing:
• p+backcontactreatments.
• Treatmentsto reducesurfacerecombination.
• Dotcontactsvsgrids.
• Alteredjunctionprofiles.
• Gettedngto increasediffusionlengths.
• Manyotherprocessingchanges.
Thismethodhasalso been used for analyzing silicon solar cells to determine the following:
• Band gap narrowing from heavy doping (ref. 1),
• Band gap nan'owing from intense electric fields (ref. 1).
• The presence of A-centers in the junction, resulting from oxygen diffusion during n+ layer formation (un-
published data).
• The presence and nature of junction leakage paths caused by heavy-ion bombardment (ref. 3).
• The best approach for improving beginning-of-life (BOL) or EOL performance (ref. 4).
Since I-V analysis had been successfully applied to silicon cells (ref. 2), it was thought that it might be
equally beneficial when used for GaAs cells, such as those irradiated by APL (ref. 5, 6). The analysis of
representative unirradiated GaAs cells in this experiment indicates that the junction recombination current density
(Jr) dominates the contributions to the dark current. At 0.82 V, which is the voltage at maximum power, the bulk
dark-current density (Jd) is not even 10 percent of Jr. Below this voltage, Jd is even less important. The
dominance of Jr is presumably due to defects initially present in the junction region of the cell. The Jr term
increases linearly with junction thickness (Wj) and exponentially with the electric field in the junction, Ej. Since Ej
= Vj/Wj, Jr is high for both thick and thin junctions, with an optimum thickness that is between the two.
Figure 1 is characteristic of unirradiated high-quality [18- to 19-percent air mass zero (AM0)] GaAs solar
cells. The three current contributions in this semi-log plot add up to the calculated cell current (LIDC) for
comparison with the experimental data (LIDX). Beyond the illuminated data, the cell area (8 cm2), base doping
(3 x 1023/m3), and principal junction defect level (with respect to mid-gap) are included. For simplicity, a trap
gap of 0.12 eV is selected to keep the Jr line straight at this stage. The correlation coefficient of 0.99983 is
weighted heavily by the higher density of points at high voltages. The apparently poor tit at low voltages results
from a nonlinear shunt current which was fitted with a linear term. The actual difference between the data and
the model is less than 1 mA (out of an Isc of 251 mA). The current resolution is seen to be approximately
±0.2 mA.
For solar cells that have been irradiated, the contribution of Jr remains greater than that of Jd except near
the maximum voltage, where the two terms are approximately equal. Figure 2 is a typical analysis result for
electron-irradiated GaAs solar cells. Note that both Jd and Jr have increased so that the relative contributions of
the terms are similar to those from the unirradiated cells. For cells from U.S. vendors, this trend continues to at
least 101Se/cm 2. The correlation coefficient and best value for series resistance are nearly unchanged from
those of the unirradiated sample; however, the diode current data and calculated shunt current at low voltages
have both increased slightly.
Figure 3 shows the effects of proton irradiation on GaAs solar cell current contributions. Two major
differences from electron-irradiated cells are seen: the bulk current contribution is now negligible, and the series
resistance has increased significantly (from 0.001 to 0.041 _). Comparison of electron- and proton-irradiated
cells, degraded to the same power, indicates more degradation in Jr (and Rs) and less in Jd from protons. The
relative effect on cell characteristics from protons is a lower Voc and higher Isc. Study of these combined effects
provides two possible explanations. One is that protons damage junctions more than do electrons, and that they
lower the bulk carrier concentration more with less change in carrier lifetime. The second explanation is that the
higher reduction in carder concentration increases the junction field-region thickness (and thereby Jr) and
provides a drift field to increase carrier collection (greater effective diffusion length) and Isc.
39-2
In Figure 4, the same data as in Figure 3 have been reanalyzed; however, the expression for shunt
current has been changed in the model. The modification is based on earlier studies which indicated that most
of the observed shunt currents result from a few individual leakage paths through the junction. These paths are
shortened as the junction depletion layer collapses under forward bias. Thus, the effective resistance decreases
with increasing junction voltage. As a second approximation, a fixed value was maintained for Rsh, but
(V - IRs)2/Rsh was substituted for the shunt term in equation 1. The results in Figure 4 are a closer fit to data
in the low-voltage region, but provide a lower correlation coefficient than in Figure 3. The other terms are not
significantly changed except for Jd, which is at the noise level. This exercise demonstrates that, while the fit can
be improved at low voltages, the effect on the critical currents (Jr in this case) is quite small.
Figure 5 is an example of the large differences noted in analyses of Mitsubishi GaAs cells compared to
those from U.S. manufacturers. At 101S/cm2 1-MeV electrons, the results appear much closer to the proton
results of Figure 3 than to the electron results of Figure 2. The proton irradiation results for the Mitsubishi cells
are similar to those from the U.S. cells, but with a higher series resistance (0.27 vs ~0.04 _) and higher Isc.
The conclusion is that the Mitsubishi cells have lower doping in either (or both) the emitter or base layers.
Radiation reduces the carrier concentration further, and the junction field reaches further into the low doped
layer(s).
Analysis of GaAs-on-germanium cells (ref. 4) indicates that the principal difference between these cells and
GaAs/GaAs is at BOL, where a higher level of junction defects is proposed. With exposure to radiation, the
difference in Jr diminishes as radiation-induced defects come to dominate the results. Table I gives typical
values for cell characteristics before and after irradiation.
Table I. Results of I-V Analysis on GaAs Solar Cells
i I i
Jd Jr Rs
Condition (A/cm 2) (AJcm2) (Q)
GaAs
Initial 2 x 10-19 1 x 10-l° <0.01
10Is e/crT_ 4 x 10"18 6 x 10"1° <0.01
10TM e/crn2 4 X 10 "17 2 x 10-9 <0.01
1013 p/crn2 3 x 10-18 8 x 10.9 0.04
GaAs/Ge
Initial 5 X 10 .2o 4 x 101° <0.01
1015 e/crT# 4 x 10-18 1 x 10.9 <0.01
10le e/crr# 5 x 10-17 2 x 10.9 <0.01
10_3e/cm 2 5 x 10-17 1 x 10.8 0.04
The separation of cell I-V curves into junction and bulk contributions, and the differences observed in
degradation rates for Voc and Isc from protons, and electrons, provide a basis for understanding why a distinction
must be made between cell I-V degradation parameters for proton-irradiated solar cells. The greater effect of
protons on Jr, relative to that from electrons, increases GaAs solar cell degradation because the Jr term is
dominant here, while Jd is dominant in silicon solar cells. The I-V analysis technique provides not only a tool in
the design of solar cells for benign and radiation environments, but also a means of studying radiation damage
at the defect level.
It can be concluded from this study that Jr dominates nearly all GaAs cells tested, except for unirradiated
Mitsubishi ceils, which appear to have a different doping profile, irradiation maintains or increases the dominance
by Jr, and proton irradiation increases Jr more than does electron irradiation. The U.S. cells have been
optimized for BOL and the Japanese for EOL. I-V analysis indicates ways of improving both the BOL and EOL
performance of GaAs solar cells.
39-3
B-1
-2
-3
O
..J
-4
-5
e
-1
-2
H
_-3
o
_J
-4
-5 __ ¢,
I
.'I
1 '1 I [ I | I "| I ! I | I I | | I
CELL • :SGE5
Voc : .921
Imc : .229
Pm,,x : .165
RS : .BBI
NR : 3.E+23
R : 8
Cop : .9997?
Tr IpgL p :
3RC : 5.39E-18
Rah : 84.3
l/
°o " ////
o
/ / NOTES :/ /
./ / : SD
," /
/ : LIDC
/ oooo(3oo • LZDX
I • _ I I l I ,a I _ I i I •
.2 .3 . ,5 .6 ,7 .!1 .9
VOLTRGE
Figure 2. Spectrolab GaAs Solar Cell After 1015/cm 2 1-MeV Electrons
]1 I ' I I I 1 I l II I I I"' I _ I I " I' 1 --
CELL (, :SGP5
Voc : .Bet
I=c : .]e7
RS : .e4t
NR : 3.E+23
R :B
Cot : .9994
Trmpgap: .12
3. : 3.32E-IB _.._'1_'___. .....
3RC : B.3E-9 w..-,_ _'r_o /
Rsh : 35 I
---'""" o
/
/
o /
o /
/
o /
/
/
/
/
/
/
/
I * , l , * I j I I •
.2 .3 .4 .5 .6
NOTES :
3D
JR
V/RSH
: LIDC
: LIDX
i . f , i I .
.7 .B .9
VOLTRGE
Figure 3. Spectrolab GaAs Solar Ceil After 1013/cm 2 10-MeV Protons
39-4
-1
H- 2
Lg
O
._J-3
-4
-5
-!
-2
G-S
o
.J
-4
-5
6
I I I I I I I I I I I I I
CELL _ :SGP5
- Vec : .B61
Isc : . IB7
Pmax : . IB4
_ RS : . O6 j,c'r*"
NRR : 3. E+23 _,'/
: 8 _... /
Cot : .99399 / /
- Trapgap: .12 _. /
JD : 1.62E-17 /'--_ _--
JRC : 7.55E-9 /.---'_" .....--__
- Rsh : 34.3 _,._'...--- """ _/
_..I ._' / /
-- _I ._" o / /
__ y o //// /
/ / NOTES : /
/i /
/ ---- : IO/
/ -- -- : 3R_
---- : V^,E/RSH
- / -- : LZDC
ooooooo LIDX
I I I I I I I I I I I I I I
.1 .2 .3 .4 .5 .G .7 .B
VOLTRGE
Figure 4. Proton-Irradiated Spectrolab Cell With V 2 Dependence In Shunt Term
! f f I
CELL I :MGE5
Voc : .845
Zmc : . 166
Pmmx : . 162
RS : .623
NR : 3. E+23
R : 8
Cor : .99924
Tripgip: . 12
ID : 3. 19E-19
IRC : 3.31E-9
R=h : B7.2
!
I I I I + I I I l T----lr--F I '
/
/
/
/
/
/
o /
/
/
/
/
/
/
/
.2 .3 .4 .5 .G
N_TES :
-- -- : 3R
: V/RSH
: LIDC
: LIDX
.-_ | a I +
.7 .8 .9
VOLTRGE
Figure 5. Mitsubishi GaAs Solar Cell After 1016/cm 2 1-MeV Electrons
.9
39-5
REFERENCES
1. Rittner,E.S.; Meulenberg,A.;andAllison,J.F.:"Dependenceof Efficiencyof ShallowJunctionSiliconSolar
CellsonSubstrateDoping,"Journal of Energy, Vol. 5, No. 1, January-February 1981, pp. 9-14. [See also:
Ritlner, E. S, "An Improved Theory for the Si p-n Junction Solar Cell," Journal of Energy, VoI. 1, No. 1,
January-February 1977, pp. 9-17.]
2. Meulenberg, A., Jr.: "Developments Toward an 18% Efficient Silicon Solar Cell," Final Report for Contract
NAS-3-22217, NASA CR 168141, April 1983.
3. Meulenberg, A.: "Evidence for a Permanent Single-Event Upset Mechanism," IEEE Transactions on Nuclear
Science, Vol. NS-31, No. 6, December 1984, pp.1280-1283.
4. Meulenberg, A.; and Rittner, E. S.: "Limiting Processes in Shallow Junction Solar Cells," 3rd Solar Cell High
Efficiency and Radiation Damage Meeting, NASA Lewis Research Center, June 1979.
5. Herbert, G. A., et aL: "Electron and Proton Displacement Damage in Production Line Quality Silicon and
Gallium Arsenide Solar Cells," 4th International Photovoltaic Science and Engineering Conference, Sydney,
Australia, February 1989.
6. Maurer, R. H., et aL: "Gallium Arsenide Solar Cell Radiation Damage Study," IEEE Transactions on Nuclear
Science, Vol. NS-36, No. 6, December 1989, pp. 2083-2091. (See also these Proceedings.)
-1
-2
G-a
O
J
-4
_11 ,,
-5
t i 1 I i
CELL # :RGCI
Voc : t
Isc : .25!
Pmlx = .2BI
RS : .OS1
NR : 3.E÷23
R :B
Cor : .99983
Trlpgap: .12
3D = 1.7E-19
JRC : 1.3IE-IB
Rsh : 132
.2 .3
I I I' -" I
/
/
.4 .5 .6
VOLTAGE
l l ! 1 t ! i || j
../'/
/
: 30
: ER
: V/RSH
...... : LIBC
ooooooo : LIDX
.7 .B .9
Figure 1. Unirradiated ASEC GaAs Solar Cell
39-6


A I-V analysis of irradiated Gallium Arsenide solar cells

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