Various techniques to increase the open circuit voltage are being explored. It had been previously observed that cells made on CdS deposited from a single source gave a consistently higher V sub oc. Further tests have now shown that this effect may in fact relate to differences in source and substrate temperatures. The resulting differences in CdS structure and crystallinity are being documented. Deposits of mixed CdS and ZnS are being produced and will be initially made into cells using the conventional barriering technique. Analysis of I-V characteristics at temperatures between 25 and 110 C is being perfected to provide nondestructive analysis of the Cu2S. Changes due to vacuum heat treatments and exposure to oxygen are also being monitored by the same technique. Detailed spectral response measurements are being made

Boeer, K. W.

Meakin, J. D.

English

NASA Technical Reports Server

N76 1 2 4 9 2  
DIRECT SOLAR ENERGY CONVERSION 
FOR 
LARGE SCALE TERRESTRIAL USE 
AER 72-03478 
Formerly GI-34872 
6/1/72 - 6/30/75 
$981,000 
7/1/75 - 6130176 
$484,974 
Principal Investigators 
Karl W. BUer 
John D .  Meaki.1 
Inst i tute  of Energy Conversion 
University of Delaware 
Newark, Delaware 19711 
National Solar Photovoltaic Program Review Meeting 
Los Angeles, California 
July 22-25, 1973 
442 
https://ntrs.nasa.gov/search.jsp?R=19760005404 2020-03-22T17:55:57+00:00Z
1) 
2) 
3) 
Improve cell  performance, s t a b i l i t y  and l i f e  expectancy 
Imprwe understanding of c e l l  mechanisms u i t l i  d i r e c t  experimental v e r i f i c a t i o n  
Improve gridding and encapsulation t o  improve l i f e  expectancy 
FhST ACTlVlTIES 
have been made i n  t h e  reproducible production of CdS c e l l s ,  t h e  understandi?. 
of cell  fornat ion and operation mechanisms and t h e  understanding and cont ro l  o f  
cell degradation mechanisms. 
-. 
As a r e s u l t  o f  t he  t h r e c  year program up t o  .'..?e, 1975, major advances 
Cells can now be r e l i a b l  - made on copper/t inc,  mol), 
In  February, 1975, a run 
\urn, Fe-Ni, 
zinc y d  cadmium stannate on quartz.  60 l a rge  
(55 cm ) cells produced a y ie ld  of 85% u i t h  a mean ef f ic iency  of 4.4% and a 
maximum ef f ic iency  of 5.2%. During Yay, 1975, 20 small (4 cm2) control  cells 
were produced u i t h  a mean ef f ic iency  of 5.5% and a maximum efficicrrcy of  6.0%- 
sens i t i ve  and roof-top t e s t ing  general ly  shows a 10% enhancement. 
to  d a t e  shows a 6.8% natural  i n so la t ioc  efficic:.:y. 
Cu/Zn subs t ra tes  with a CdS layer  of 20-25 n i l r m  thickness. 
hese e f f i c i enc ie s  are under Tungsten-Lodine simulation; t he  c e l l s  are blue 
The best cell 
Cells are general ly  made on 
Procedures and contl-0,s have bet -lied t o  t h e  extent  t h a t  t r a n s f e r  
t o  o ther  groups a d  organiz .+ions can he successful ly  accomplished. 
The inf lucnce of  temperature and i l luminat ion i n t e n s i t y  on C e l l  I-V 
cha rac re r i s t i c s  have been used t o  iden t i fy  the  z f f ec t ive  b a r r i e r  height ,  to  
isolate t h e  effects of  blocking cor.tac-- and t o  Fon-destructively determine 
t h e  stoichiometry of t h e  sur face  copper su l f ide .  
A major improvement i n  reproducib i i i ty  and c e l l  performance has  been 
achieved by extensive vacuum h e a t  treatments af ter  cell  gridding and lamination. 
Degradt ior ,  due t(r in i e rac t ion  with oxygen and water vapor can be reversed by 
e i t h e r  hydrogen o r  v3:uum heat tyeatment. 
Roof top  t e s t i n g  of  ceils protected from t h e  atmosphere has show no 
de tec tab ie  degradation a f t e r  twenty months. 
a t  temperatures from 46 t o  85°C 
t o  l/e of i n i t i a l  pzrformance) i n  excess of twenty years  have observed a t  <SO"C. 
I t  is concluded that su i t ab ly  hermetically sealed c e l l s  w i l l  be adequately 
s t ab le  for terrestrial appl ica t lons .  
Accelerated t e s t i n g  has Seen corducted 
i n  var ious ambients. Lifetimes (degradation 
Quant i ta t ioe dcscr ip t ions  of t h e  short  c i r c u i t  cur ren t  have been 
developed and t h e  rpportance of  r e f l ec t ion  from t h e  subs t r a t e  i n  f r o n t  wall 
c e l l s  revealed. 
The Aninance of i n t e r f i c e  recombir t t jon  i n  cont ro l l ing  t h e  open c i r c u i t  
voltage has been iden t i f i ed .  
S t ruc tura l  s tud ies  have resul'.*d in a more complete urdersLanding of 
tiv- ce1.1. Minority c a r r i e r  d i f fus ion  
Sone changes occurring during degradation have been iden t i f i ed .  
s ~ a n c c s  have been measured i n  ac tua l  cells. 
L) r i n g  rhe  i n i t i a l  pa r t  of t h e  program, ;tudiec of t d x i c i t y  problems 
and ecmoniir assessments establish-d that ne i ther  G:. these  cc.nsi,erations presented 
l i ke ly  obstdclen t o  large sea?- u t i l i z a t i o p  of CdS ' ' ,  :r C,lls. 
4 h  
CURHkNf ACTiV IT IES 
Various techniques to increase  t h e  oycn c i r c u i t  vo l tage  ;re being 
I t  had been previously observed thqt cells wide on Tiis +posited 
The r e s u l t i n g  d i f f e rences  i n  CJS s t r u c t u r e  and c r y s t a l l i n i t y  
Deposits of mird CdS and ZnS arc b e h g  produced and 
explored. 
from a s i n g l e  source gave a cons i s t en t ly  higlicr V0c. 
shorn that t h i s  effect may i n  fact r e l a t e  to  d i f f e rcnccs  i n  zourcc ;urd s u b s t r a t e  
temperatcres. 
are being documented. 
w i l l  be i n i t i a l l y  made i n t o  cells using t h e  conventional ba r r i e r ing  technique. 
Precision lattice parameter measurcmcnts u i l l  he u s d  to  monitor composition. 
l u r t h c r  t e s t s  Imvc n o w  
Analysis of I-W char--teristics a t  *mpera tu res  bctwecn 25 and 110°C 
is being perfected to provide mn-des t ruc t i r c  a n a l y s i s  of t h e  CuzS. 
due t o  vacuum hea t  t reatments  and exposure to  oxygen are a l s o  being monitored 
by t h e  same technique. In a p a r a l l e l  O.N.R. program. detailed spec t r a l  response 
measurements are being made. 
Changes 
U1 t ra-high vacuum equipment is hcing readied t o  conduct cont ro l led  
reac t ion  experiments bctween t h e  copper-sulf ide layer  and atmospheric 
cOmpOnents, t o  be followed by vacuum heat treatments.  
w i l l  i d e n t i f y  spec ies  emitted during t h e s e  experiments. 
In  s i t u  mass spectroscopy 
Direct observat ion of t h e  Cu2S l ayer  and t h e  interface with CdS is 
under way using transmission e l ec t ron  microscopy. Atmaphere related 
degradation w i l l  also be  followed by t h i s  technique. 
FIJTUHE PLANS -
Primary t h r u s t s  Jur ing  t h e  next 6 months w i l .  be d i rec t ed  t o x i n i s  
achieving higher open c i r c u i t  v o l t q e s  and t h e  funJiunenta1 understanding of 
degradation mechnisms. Techniques t o  be attempted t o  nchicve t h e  former b i l l  
include t h e  use  of ZnS-CdS mixtures t o  g ive  a b e t t e r  l a t t i ce  and clectrm 
a f f i n i t y  match to  C112S, also t h e  e f f e c t s  of changes i n  t n e  o r i en ta t ion  of t h e  
CdS w i l l  be  erpiored. 
Both theo re t i ca l  and expcrimcntal s tud ie s  w i l l  be made t o  i d c n t i f y  
more c r i t i c a l l y  t h e  changes occurr ing during vacuum heat t reatments  and 
exposure t o  the  atmosphere. 
d i f fus ion  .in 'CdS, t h e  proner t ies  of i s o l a t e d  Cu2S films and changes i n  minority 
carrier d i f f J s i o n  i n  CuzS -.A various cells  a r e  expected to  y i e ld  valuable  rcsults. 
Support s t u d i e s  of t h e  inf luencc of doping on c*opptr 
KEY RESULTS 
l a rge  cells 155 cm2) of mcan e f f i c i ency  SSO 
Small cells (4 cp3) of mean off ic icncy  ~ b t  
Best cell 6.8% . 
t e s t i n g  
in te r face  region elucidated.  
-. 
*Re1 i a b l e  produciion technique dcvclopcd on low cos t  metallic subs t ra te .  
I 
.Usable cell l i fe  i n  excess of 20 years supported by roof-top and accc lera tcd  
.Quant i ta t ive J c s c r i p t  on of shor t  c i rcu i t  cur ren t  Role or CdS, P J ~ S  and the 
-Dominant role of in t e r f ace  rwombinstion an t', 2 es tab l i shed .  (Conventional model) 
.Stnrcture of as-formed cell  arid e f f e c t s  of degradstion substant i31 ly cstabl ishcd, 
-Regenmation accomplished by vacuiim and hytropen hea t  treatment.  
b44 
445 
Yield of 4cm2 Cells From 5 Copper Substrates 
Processed 5/19/75 to 5/29/75. 
(Measured on Pilot Line Tungsten - Iodine Simulator) 
Averoge Efficiency = 5.50% (20 Cells) 
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EFFICIENCY (%) 
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Direct solar energy conversion for large scale terrestrial use

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