The effect of series and voltage-matched configurations on the performance of multijunction solar cells in a radiation environment was investigated. It was found that the configuration of the multijunction solar cell can have a significant impact on its radiation tolerance characteristics

Curtis, Henry B.

Gee, James M.

English

NASA Technical Reports Server

N89-24737 
A Comparison of the Radiation Tolerance Characteristics 
of Multijunction Solar Cells with Series and 
Voltage-Matched Configurations* 
James M. Gee 
Sandia National Laboraiories 
Albuquerpue, NM 
Henry B. Curtis 
NASA Lewis Research Cenier 
Cleveland, OH 
Summary 
The effect of series and voltage-matched configurations on the performance of multijunction 
solar cells in a radiation environment was investigated. It was found that the configuration of the 
multijunction solar cell can have a significant impact on its radiation tolerance characteristics. 
Introduction 
Multijunction (MJ) solar cells have the potential for extremely high efficiencies (>30%). Such 
cells consist of several photovoltaically active junctions (subcells) with different bandgaps stacked in 
optical series. This arrangement essentially splits the broad solar spectrum into portions to  which the 
individual subcells are better matched. MJ cells are under consideration for space applications where 
high efficiency is important. In order to  be useful for space applications, the radiation tolerance of 
MJ cells needs to be addressed. 
The radiation tolerance of an MJ solar cell is determined by several factors. The first factor is the 
radiation characteristics of the individual subcells. The degradation characteristics of an individual 
subcell are expected to  be similar (after accounting for the shielding of any overlying material) 
to  a single-junction cell fabricated from the same material and with the same cell structure. The 
radiation tolerance of single-junction solar cells has been extensively studied and documented [ref. 
11. 
A second factor that influences the radiation tolerance characteristics of an MJ cell is its module 
configuration. Module configuration refers to  the electrical circuit in which the subcells of the MJ 
cell are wired. The degradation characteristics of one subcell may affect the power available from 
the other subcells through limitations imposed by the electrical circuit. In this paper, we report 
results of a study concerning the effect of the module configuration on the radiation tolerance of an 
MJ cell. 
* This work was partially supported by the U.S. Department of Energy under contract DE- 
AC04-76DP00789. 
332 
https://ntrs.nasa.gov/search.jsp?R=19890015366 2020-03-20T01:42:52+00:00Z
MJ Cell Configurations 
The simplest module configuration for an MJ cell has the subcells connected in series. This 
cell requires only two terminals. The current from a series string of cells is limited by the cell with 
the lowest current. The bandgaps of the subcells for a series-configured MJ cell should therefore 
be chosen for matched photocurrents. In a radiation environment, the bandgaps should be chosen 
for matched currents at  end-af-life (EOL). Since many cells degrade more rapidly in current rather 
than voltage, series operation could impose severe limitations to the radiation tolerance of MJ cells 
with a series configuration. 
MJ cells whose subcells can be wired in various series/parallel circuits have been recently de- 
scribed [ref. 21. The voltage of cells in parallel is limited by the cell with the lowest voltage. 
Hence, these MJ cells require matched voltages between subcell circuits for efficient operation and 
are referred to as having a voltage-matched (VM) configuration. An example of a two-junction, 
four-terminal tandem cell wired in a voltage-matched configuration is given in figure 1. VM circuits 
have also been described for both two- and three-junction tandem cells with three terminals, so 
that the VM configuration may be used with monolithic MJ cells [ref. 21. Figures 2 and 3 show the 
effect of the module configuration on the efficiency versus bandgap relationship for series and VM 
configurations. (These efficiencies were calculated using the model of reference 2.) An advantage 
of the VM configuration compared to the series configuration is that it allows a wider selection of 
bandgaps for a given efficiency. 
Experiment and Calculations 
In general, the voltage and current of a solar cell degrade at  different rates with irradiation. 
Hence, the radiation tolerance is expected to be influenced by the module configuration. For this 
study, we used the measured radiation characteristicsof AlGaAs (1.72 eV), GaAs, and InGaAs (1.15 
eV) concentrator cells presented in reference 3. The AlGaAs and InGaAs cells have appropriate 
bandgaps for use with both the series and VM configurations. The initial device characteristics 
are presented in table 1 and the degradation characteristics under 1-MeV electron irradiation are 
presented in figures 4, 5, and 6. Note that the maximum power (Pmax) of the InGaAs cell degrades 
very rapidly due to the rapid degradation of the current. 
The expected performance of an AlGaAs/InGaAs tandem cell was calculated using the following 
procedure. Each illuminated current-voltage (IV) curve was fitted to a lumped parameter model 
consisting of a current source, two diodes (n=1 and n>l ) ,  and a shunt and series resistance. No 
physical interpretation was attached to these fitted parameters; the purpose of the exercise was 
to  allow addition of IV curves for tandem cell modeling. Next, the tandem cell performance for 
independent, series, and VM configurations was calculated using the lumped parameter model for 
the AlGaAs and InGaAs subcells. (“Independent” configuration refers to operation of each subcell 
independently.) For this calculation, the photocurrent of the InGaAs subcell was set equal to the 
photocurrent of the AlGaAs subcell at  beginning-of-life (BOL); i.e. we have assumed that the 
photocurrents are matched at BOL for an optimized cell. The photocurrents from the AlGaAs and 
InGaAs cells were assumed to degrade at  the measured rates given in figures 4 and 6. The data 
of figure 6 was taken with full spectrum illumination while the InGaAs cell in an AlGaAs/InGaAs 
tandem cell will only be illuminated by a filtered spectrum. Our spectral response data indicates 
that the InGaAs cell degrades more rapidly in the blue, so that our calculations may overestimate 
slightly the current degradation expected from an InGaAs cell in the stacked configuration. 
333 
h s u l t s  of the calculations are presented in figure 7. The rapid degradation in I,, for the 
InGaAs cell is seen t o  have a substantial effect on the series-configured MJ cell. The  P,, of the 
series-configured tandem cell, in fact, becomes less than that of a single-junction GaAs cell a t  high 
fluences despite the much higher BOL efficiency. The P,,, degradation of the Vhl configuration 
is much less than that of the series configuration for the AlGaAs/InGaAs tandem concentrator cell 
since voltage degrades much less rapidly than current for these particular subcells. Initially, the Vhl 
configuration produces about 3% less power a t  BOL than the series or independent configurations 
because the voltages of the subcells are slightly mismatched. However, the subcell voltages become 
better matched as they degrade with irradiation. In fact, P,,, of the VM configuration is nearly 
the same as the independent configuration and 39% greater than the series configuration a t  EOL. 
Conclusions 
We have calculated the expected performance of an AlGaAs/InGaAs tandem cell as a function 
of 1-MeV electron fluence with series and voltage-matched configurations. I t  was shown that  the 
module configuration can have a significant impact on the radiation tolerance of an hlJ cell due to  
the different rates of degradation for voltage and current of the individual subcells. 
References 
[ 1.1 H .  Y. Tada, et al., Solar Cell Radiation Handbook, 3rd Ed.,  J P L  Publication No. 82-69 
(November 1982). 
[ 2.1 James M. Gee, “Voltage-Matched Configurations for Multijunction Solar Cells,” Pmc. 19th 
IEEE Photo. Spec. Conf., pg. 536 (1987). 
[ 3.1 H.B. Curtis, C.K. Swartz, and R.E. Hart, Jr . ,  “Radiation Performance of AlGaAs and InGaAs 
Concentrator Cells and Expected Performance of Cascade Structures,” Proc. 19th IEEE Photo. 
Spec. Conf., pg. 727 (1987). 
Table 1. Initial IV data at 100 suns, AM0 and 25OC. 
AlGaAs GaAs InGaAs 
J,, (A/cm2) 1.961 3.174 3.579 
V, (volts) 1.367 1.139 0.859 
Fill Factor 0.835 0.799 0.794 
Efficiency (%) 16.5 21.3 18.1 
334 
ttom 
Figure 1. 
cell. 
A voltage-matched circuit for a four-terminal, two-junction tandem 
In the above circuit, the toD subcells are wired in parallel with two 
series-connected bottom subcells. 
Series Volt age-Ma t c hed 
Figure 2. 
of top and bottom subcell bandgaps (lX, AMO). 
Iso-efficiency curves for a two-junction tandem cell as a function 
335 
I Volt a go-Ma t c h ed Serles 
Figure 3 .  
function of top and middle subcell bandgaps (lX, N O ) .  The bandgap of the 
bottom subcell is optimized each pair of top and middle subcell bandgaps. 
Iso-efficiency curves for a three-junction tandem cell as a 
N 
01 N AIGaAs (1 .72  e V )  
"1 
I 1 1 I I I l l  I I v 1 1 1 1 1  I 1bP 1b4 lb6 a+-(( I ' 1 ' 1 1 1  1 1  0 
1 - M e V  E l e c t r o n  F l u e n c e  
Figure 4 .  
A l G a f i s  (1.72 eV) concentrator cell as a function of 1-MeV electron fluence 
(lOOX, AMO, 2 5 ° C ) .  
Ratio of degraded/initial values for Voc, Isc, and Pma, of an 
(The units of fluence are ern'*.) 
336 
N 
.- 
:{ GaAs 
1 - M e V  E l e c t r o n  F l u e n c e  
Figure 5 .  
( 1 . 4 2  eV) concentrator cell as a function of 1-MeV electron fluence (loox, 
AMO, 25°C) .  
Ratio of degraded/initial values for Voc, Isc, and P,, of a GaAs 
(The units of fluence are ern-*.) 
7 I InGaAs (1.15 e V )  
"1 
1 - M e V  E l e c t r o n  F l u e n c e  
Figure 6 .  
InGaAs (1 .15  eV) concentrator cell as a function of 1-MeV electron fluence 
(lOOX, AMO, 25°C) .  (The units of fluence are 
Ratio of degraded/initial values for Voc, Isc, and P,, of an 
337 
. 
s e r i e s '*e, 
. 
1 - M e V  E l e c t r o n  F l u e n c e  
Figure 7. 
concentrator cell and an AlGaAs/InGaAs tandem concentrator cell with a series, 
an independent, and a VM configuration (lOOX, A M O ,  25°C). 
with respect to the initial Pmax of the AlGaAs/InGaAs tandem cell with an 
independent configuration. 
Pmax at as a function of 1-MeV electron fluence for a GaAs 
Pma, is normalized 
(The units of fluence are cm-* .) 
338 


A comparison of the radiation tolerance characteristics of multijunction solar cells with series and voltage-matched configurations

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