The electrical behavior of 100 micron thick tandem junction solar cells manufactured by Texas Instruments was studied as a function of 1 MeV electron fluence, photon irradiation, and 60 C annealing. These cells are found to degrade rapidly with radiation, the most serious loss occurring in the blue end of the cell's spectral response. No photon degradation was found to occur, but the cells did anneal a small amount at 60 C

Anspaugh, B. E.

Miyahira, T. F.

Scott-Monck, J. A.

English

NASA Technical Reports Server

ELECTRON IRRADIATION OF TANDEM JUNCTION 
SOLAR CELLS* 
Bruce E. Anspaugh, Tetsuo F.  Miyahira and John A.  Scott-Monck 
J e t  Propul s i  on Laboratory 
California I n s t i t u t e  of Technology 
SUMMARY 
The e l ec t r i c a l  behavior of 100 micron thick tandem junction so l a r  ce l l  s 
manufactured by Texas Instruments has been studied a s  a function of 1 MeV 
electron fluence, photon i r r ad i a t i on ,  and 60°C annealing. These c e l l s  a r e  
found t o  degrade rapidly w i t h  r ad ia t ion ,  the  most serious loss  occurring in 
the blue end of the  c e l l ' s  spectral  response. No photon degradation was 
found t o  occur, but the  c e l l s  did anneal a smal i amount a t  60°C. 
INTRODUCTION 
The Texas Instrument tandem junction so l a r  c e l l  ( r e f .  1 ) i s  constructed 
with a junction on the  f ron t  surface and a junction on the  rear  surface i n  an 
n+pn+ configuration. The f ron t  surface,  usually textured,  has no grid l i ne s  
or  contacts of any kind. An n' layer  i s  diffused i n  a f inger l ike  pattern on 
the rea r  surface which leaves both n and p material exposed f o r  purposes of 
contact deposition. This construction,  w i t h  contacts only on the  rear  surfaces 
permits simplif ied ce l l  interconnection during panel construction. An addi- 
t ional  advantage i s  t ha t  these c e l l s  have worked qu i te  well a t  thicknesses 
down t o  100 microns. Their performance a f t e r  1 MeV electron i r r ad i a t i on ,  
annealing, and photon exposure i s  the  subject  of t h i s  paper. 
We have i r rad ia ted  and measured three  TI tandem junction c e l l s  using the  
JPL Dynamitron as  a source of 1 MeV elect rons .  I-V curves and spectral  response 
measurements were made as a function of f l  uence, photon i r rad ia t ion  and 60°C 
anneal. A1 1 three  ce l l  s had textured,  contact1 ess f ron t  surfaces.  They were 
furnished mounted on t h i n  ceramic subs t ra tes .  Cell 20-2 was made of 3 ohm-cm 
CZ s i l i con ,  100 microns th ick,  area 4 cm2, and c e l l s  34-1 and 38-1 were 6 ohm- 
cm CZ s i l i c o n ,  110 microns th ick ,  area 4 cm2. P contacts were A1-Ti-Pd-Ag 
and n contacts were Ti-Pd-Ag. All junction depths f ron t  and back were reported 
to  be approximately 0.25 t o  0.3 microns deep. 
*The research described i n  t h i s  paper was carr ied out a t  the  J e t  Propulsion 
L.aboratory , Cal i fo rn ia  I n s t i t u t e  of Technology, under NASA Contract NAS7-100. 
https://ntrs.nasa.gov/search.jsp?R=19790024486 2020-03-21T21:02:28+00:00Z
IRRADIATION 
The resu l t s  of the irradiated data are plotted in figures 1 to  3, showing 
Is,, Voc and Pmax vs. 1 MeV electron fluence. A1 1 measurements are AMO, 28"C, 
made w i t h  an Aerospace Controls Model 302 f i l t e red  xenon simulator. The ce l l s  
were annealed for  16 hours a t  60°C a f t e r  the cumulative fluence reached 
2x1014 e/cm2. The values a f t e r  anneal in are  used i n  the plots. The changes 8 in Isc and Pmax a r e  dramatic. After 101 e/cm2, ce l l s  which began with e f f i -  
ci encies of 11.84%, 8.96%, and 13.47.% have been reduced to 0.67%, 1.99%, and 
0.81%, respectively. Cell 20-2 made of 3 ohm-cm material i s  seen to be the 
most vulnerable. I t  dropped to  an efficiency of 1.69% a f t e r  2x1013 e/cm2, 
then decreased in output slowly with fluence. The two 6 ohm-cm ce l l s  did not 
decrease as rapidly until exposed to  somewhat greater fluences, b u t  when they 
f e l l  the i r  output coincided with that  of the 3 ohm-cm c e l l .  The Voc behavior 
of a l l  three ce l l s  has roughly the same trend as shown by Figure 2, b u t  the 
undulations in the cel l  38-1 curve could possibly be due to  a thermal contact 
problem involving the cell/ceramic structure.  
ANNEALING A N D  PHOTON EFFECTS 
All three c e l l s  were annealed a t  60°C a f t e r  higher fluence exposures. All 
ce l l s  showed positive annealing a f t e r  t h i s  treatment in a l l  cases, with the 
6 ohm-cm c e l l s  showing a greater e f fec t  than the 3 ohm-cm cell ( t h i s  duplicates 
the behavior of conventional structure c e l l s ) .  The annealed 6 ohm-cm ce l l s  
showed improvements in Isc of between 7 and 27%, in Pmax between 9 and 37% and 
in Voc between 0.7 and 4.8%. The annealing in the 3 ohm-cm cell  was between 
1.5 to  5% in Isc, 5 to  17% in Pmax and 1.2 to  2.5% in V o c .  
Cell 20-2 was subjected to  a 3-day exposure to  tungsten l ight  following the 
1015 e/cm2 fluence and to a 35 hour l i gh t  exposure following the 1016 e/cm2 
fluence. No reverse annealing was observed, b u t  there was evidence of damage 
recovery of roughly 2% in Isc and Voc and 5-1 0% in Pmax. 
SPECTRAL RESPONSE 
The spectral response measurements are  taken using a chopped monochromatic 
l igh t  beam as an excitation source. Cell output goes to a lock-in amplifier 
tuned to  the l ight  chopper frequency. This permits flooding the solar cell  
under t e s t  with a dc l igh t  level to check for  injection level effects .  
(Standard space quality ce l l s  do not normally show injection level effects  
either before or a f t e r  electron i r rad ia t ion) .  
Figures 4 and 5 show the spectral responses of ce l l s  20-2 and 38-1 as a 
function of radiation fluence. Both ce l l s  also exhibit  a bandwide injection 
dependence which, although not shown, nearly disappears a f t e r  exposure t o  
fluences greater than 1x1014 e/cm2. Figure 6 shows the spectral response of 
cell  34-1. I t s  injection de endence i s  remarkably strong and remains important i unti 1 f l  uences of nearly 101 e/cm2 have been reached. A1 1 curves shown in 
figure 6 would be shifted upward b u t  by a decreasing amount as the fluence 
increases. I t  appears then quite probable tha t  these ce l l s  have large concen- 
t ra t ions of trapping levels.  
Figures 4-6 show tha t  as radiation fluence i s  increased, spectral response 
i s  diminished a l l  across the band, b u t  most importantly they exhibit  complete 
loss in the blue end of the spectrum a f t e r  fluences of only 1014 e/cm2. This 
contrasts strongly with the behavior of standard ce l l s  where the loss occurs 
only in the red end of the spectrum and the blue response remains unchanged. 
CONCLUSIONS 
While i t  i s  usually dangerous to  draw far-reaching conclusions based on 
so small a sample s ize ,  certain general statements can reasonably be made con- 
cerning the ce l l s  we have studied. 
1 .  A1 though these cell  s are  a t t rac t ive  for  the i r  possible ease in panel 
assembly, they are not yet suitably developed for  use in a radiation 
envi ronment. 
2. Varying b u t  strong concentrations of trapping levels appear to  be 
introduced in these ce l l  s .  
3. TI has shown that  the presence of the front  junction i s  necessary for  
the high i n i t i a l  performance of these ce l l s .  The mechanism introduced 
by the junction must ac t  to  force e i ther  majority or minority car r ie rs  
toward the rear surface of the cell  for  collection by the rear junc- 
t ion. Whatever the force i s ,  i t  i s  readily destroyed by relat ively 
low electron fluences. One possible mechanism for  producing such a 
force i s  the existence of charged surface s t a t e s  on the front surface. 
These surface s t a t e s  are  highly affected by 1 MeV electron i r radiat ion 
and will probably be affected by electrons having energy below the 
displacement threshold. The surface s t a t e  hypothesis may easi ly  be 
tested by i r radiat ion with electrons having energies below 100 key. 
4. Following i n i t i a l  loss  of the force mechanism, the ce l l s  exhibit  a 
more gradual loss in spectral response (primarily blue) which i s  con- 
s i s t en t  with a decreasing diffusion length and loss in the a b i l i t y  
of car r ie rs  to  traverse the thickness of the c e l l .  
REFERENCES 
1. Chiang, S., Carbajal, B. G . ,  and Wakefield, G . F . :  Thin Tandem Junction 
Solar Cell. Thirteenth IEEE Photovoltaic Specialists Conference, June 1978, 
p.  1290. 
FIGURE 1. ISC VS 1 MeV ELECTRON FLUENCE FOR 
TANDEM JUNCTION SOLAR CELLS (AMO, 28°C)  
1 MEV ELECTRON FLUENCE, e/cm 2 
134 
I  1 1  1 1 1 1 1 ~  I 1 1 1 1  1 1 1 1  I 1 1 1 1  1 1 1 1  I 1 1 1 1  1 1 1 -  
50 - - 
- 
- 
- 
10 - 
1 MeV ELECTRON FLUENCE, e/crn2 
FIGURE 2. VOC VS 1 MeV ELECTRON FLUENCE FOR 
TANDEM JUNCTION SOLAR CELLS (AMO, 28°C) 
600 S 1 1 l 1 1 1 1 1 1  I 1 1 1 1 1 1 1 1  I  I  1 1 1  1 1 1 1  I  I I  1 1  1 1 1 1  
\ 
h 
560- \ - 
520 - - 
% 
480 - 
>" 
440- 
400 - 
I 1 1 1 1  1 1 1 1  I  I  1 1 1  1 1 1 1  I 1 1 1 1 1 1 1 1  I  I 1 1 1  1 1 1  
o 1013 loJ4 loT5 lo16 
F I G U R E  3 .  PMAX VS 1 MeV ELECTRON FLUENCE FOR 
TANDEM J U N C T I O N  SOLAR C E L L S  (AMO, 28 " C )  
h 
'i I 1 1  1 1 1 1 1 ~  I 1 1 1 1  1 1 1 1  I I 1 1 1 1 1 1 ~  I 1 I 1 1  1 1 1  
20- 
- 
- 
X 
8 - - 
6 - 
4 - 
- 
2 - 20-2 
I I I 1 1  1 1 1 1  I 1 1  1 1 1 1 1 1  I I I i i ; : h  
o 1013 1014 1015 1o16 
F I G U R E  4.  SPECTRAL RESPONSE O F  CELL 20-2 
- I I I I I I I - 
- 
- 
- 
- 
- PREiRRADIATION + BIAS LIGHT 
7 
- 
- 
- 
PRE-IRRADIATION 
- 
- 
- 
- 
- 
- 
- 
- 
- 
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 .O 1.1 
WAVELENGTH (MICROMETERS) 
13 5 
FIGURE 5. SPECTRAL RESPONSE OF CELL 38-1 
- I I I I I I I 
- 
- 
- 
- PRE-IRRADIATION + BIAS LIGHT 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- - 
- - 
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 .O 1.1 
WAVELENGTH, MICRONS 
F IGURE 6. SPECTRAL RESPONSE OF CELL 34-1 
- I I I I I I I 
- 
- 
- 
- 
- PRE-IRRADIATION + BIAS LIGHT 
- 
- PRE-IRRADIATION 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
I 
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 .O 1.1 
WAVELENGTH MICRONS 
136 


Electron irradiation of tandem junction solar cells

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