Sputtering of glass coatings on cadmium sulfide thin film solar cell

Hill, E. R.

Schaefer, J. C.

English

NASA Technical Reports Server

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(NASA CR OR TMX OR A D  NUMBER) (CATEGORY1 
THIRD QUARTERLY REPORT 
ON 
DEVELOPMENT OF OPTICAL COATINGS 
FOR 
CdS THIN FILM SOLAR CELLS 
B Y  
J . C .  S C H A E F E R ,  E . R .  H I L L  
P r e p a r e d  F o r  
NATIONAL AERQNAUTICS AND SPACE AOMlNlSTRATlON 
CONTRACT NAS3 6Lc6Lc 
THE H A R S H A W  C H E M I C A L  co. 
https://ntrs.nasa.gov/search.jsp?R=19650025471 2020-03-17T01:11:35+00:00Z
N O T I C E  
T h i s  r e p o r t  was p r e p a r e d  a s  a n  a c c o u n t  o f  G o v e r n m e n t  
s p o n s o r e d  work .  N e i t h e r  t h e  U n i t e d  S t a t e s ,  n o r  t h e  
N a t i o n a l  A e r o n a u t i c s  a n d  S p a c e  A d m i n i s t r a t i o n  (NASA), 
n o r  any p e r s o n  a c t i n g  on b e h a l f  o f  NASA: 
A , )  Makes a n y  w a r r a n t y  o r  r e p r e s e n t a t i o n ,  
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a c c u r a c y ,  c o m p l e t e n e s s ,  o r  u s e f u l n e s s  o f  
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or t h a t  t h e  u s e  o f  a n y  i n f o r m a t i o n ,  
a p p a r a t u s ,  m e t h o d ,  o r  p r o c e s s  d i s c l o s e d  
i n  t h i s  r e p o r t  may n o t  i n f r i n g e  p r i v a t e l y  
owned r i g h t s ;  or 
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t h e  u s e  o f ,  or f o r  d a m a g e s  r e s u l t i n g  f r o m  
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m e t h o d ,  o r  p r o c e s s  d i s c l o s e d  i n  t h i s  r e p o r t .  
As u s e d  a b o v e ,  "person a c t i n g  on b e h a l f  o f  NASA" i n c l u d e s  
a n y  e m p l o y e e  or c o n t r a c t o r  o f  N A S A ,  or e m p l o y e e  o f  s u c h  
c o n t r a c t o r ,  t o  t h e  e x t e n t  t h a t  s u c h  c o n t r a c t o r  p r e p a r e s ,  
d i s s e m i n a t e s ,  or p r o v i d e s  a c c e s s  t o ,  a n y  i n f o r m a t i o n  
p u r s u a n t  t o  h i s  e m p l o y m e n t  or c o n t r a c t  w i t h  N A S A ,  o r  
h i s  employmen t  w i t h  s u c h  c o n t r a c t o r .  
R e q u e s t s  for c o p i e s  o f  t h i s  r e p o r t  s h o u l d  b e  r e f e r r e d  t o :  
N a t i o n a l  A e r o n a u t i c s  a n d  S p a c e  A d m i n i s t r a t i o n  
O f f i c e  o f  S c i e n t i f i c  a n d  T e c h n i c a l  I n f o r m a t i o n  
A t t e n t i o n :  AFSS-A 
W a s h i n g t o n ,  D.C. 2 0 5 4 6  
Y 
THIRD QUARTERLY REPORT 
DEVELOPMENT OF OPTICAL COATINGS 
FOR 
CdS THIN FILM SOLAR CELLS 
by 
J. C. Schaefer, E. R. H i l l  
Prepared For 
NATIONAL AERONAUTICS ANI SPACE ADMINISTRATION 
June 1, 1965 t o  August 1, 1965’ 
Contract NAS 3-646b 
Technical Management 
NASA Lewis Research Center 
Cleveland, Ohio 
Space Power Sys terns Division 
Cl i f ford  Swartz 
Harshaw Chemical Company 
Crys tal-S o l id  S t a t e  Division 
1945’ E. 97th S t r e e t  
Cleveland 6, Ohio 
This repor t  was prepared by The Harshaw Chemical Company, Crystal- 
Sol id  S t a t e  Division under Contract NAS 3-6464 for the  NASA L e w i s  Research 
Center. Dr. A. E. Po t te r  and Mr. C. Swartz have served as Technical Advisor 
and Project  Manager respectively.  
The Harshaw Sol id  S t a t e  Laboratory is d i rec ted  by D r .  J. M. McKenzie, 
Technical Director. Project  direct ion is  provided by Mr. E. R. H i l l  with 
M r .  J. C. Schaefer serving as Project Supervisor. The following Harshaw 
Chemical Company personnel have contributed t o  t h i s  program: 
R. J. Humrick and A. Hartzler. 
R. W. Olmsted, 
iv 
TABLE OF CONTENTS 
PAGE 
S W R Y  ................................................... 1 
INTRODUCTION .............................................. 1 
COATING D E P O S I T I O N  ........................................ 3 
REFLECTANCE AND EMITTANCE MEASUREMENTS ..................... 4 
FUTURE WORK ............................................... 7 
V 
LIST OF FIGUElES 
1 
2 
3 
4 
5 
6 
TITLF: -
Specular Reflectance Spectrum of 
Polished Alumina 
PAGE 
10 
Specular Reflectance Spectrum of CdS Cell 
Specular Reflectance Spectrum of CdS Cell 
11 
12 
Over c oa ted wi th  Aluminum 
Specular Reflectance Spectrum of Silicone 
Primer on Aluminum 
13 
Emittance Measurement Apparatus 14 
Measuring Circuit - Power & Temperature 15 
vi 
DEVELOPMENT OF OPTICAL COATINGS 
FOR 
CdS THIN FILM SOLAR CELLS 
//#7\ By J. C.  Schaefer and E. R. H i l l  
SUMMARY 350722 
Suitable  g lass  coatings have been sputtered on C d S  so la r  c e l l  surfaces.  
Addition of oyygen t o  the sputtering gases has produced s t ab le  f i l m s  of low 
reflectance.  Ef for t s  a r e  being made t o  increase the r a t e  of sput ter ing.  A 
f o u r  electrode t a rge t  electroded on the r ea r  surface of the g lass  t a rge t  
indicates  increased ra tes .  
L- 
/1 
t \ Measurements of spec t ra l  ref lectance were made on a number of samples, 
including a s i l i cone  primer pa in t .  
Apparatus has been assembled f o r  measurement of emittance a t  so la r  
i r r a d i a t i o n  power levels .  Emittances have 
and a number of other surfaces. 
been determined for bare c e l l s  / 
INTRODUCTION 
The purpose of t h i s  program is t o  develop and apply op t i ca l  coatings t o  
CdS so la r  c e l l s  f o r  the purpose of increasing the power conversion eff ic iency.  
It is desired t o  maximize the amount of l i g h t  entering the c e l l  i n  the region 
of the spectrum where the c e l l  i s  sensi t ive,  r e j e c t  a l l  others,  and reduce the  
operating temperature of the c e l l  in  outer space. 
independent s ince a coating which r e f l e c t s  strongly in  the inf ra red  w i l l  a l s o  
reduce the emissivity i n  the region of  maximum c e l l  reradiat ion.  
s t a t e d  can be reached by developing a coating with a high transmission t o  wave 
lengths  l e s s  than about 1 micron, has a re f rac t ive  index of about 1 . 2  t o  1.3 i n  
t h i s  same range and absorbs r e l a t ive ly  strongly (about 
These conditions a re  not a l l  
The object ives  
t o  10-4 cm-l) in  the  
1 
* 
' range of 5 t o  microns. These propert ies  a r e  s a t i s f i e d  by ordinary glass 
and consequently has been the first material  t o  be investigated t o  any extent.  
Other obvious properties t o  be desired a r e  compatibil i ty with the  device, 
mechanics and chemical s t a b i l i t y ,  and res i s tance  t o  environmental e f f ec t s .  
I n  addition, it is  desirable  t o  have the coating s e a l  the c e l l  f rom atmospheric 
water vapor. 
2 
COATING DEPOSITION 
The primary purpose of the deposition work during t h i s  pas t  quarter  was 
the development of a method of applying the coating a t  a higher r a t e .  
higher r a t e  was necessary t o  produce a coating of su f f i c i en t  thickness within 
a p rac t i ca l  period of t i m e .  
The 
Ea r l i e r  i n  the contract  it had been observed tha t  use of the  R.F. electrode 
i n  contact with the  d i e l e c t r i c  target  f ace  resu l ted  in Ifhot spots.I1 
were white-hot pinpoints which quickly heated the surrounding pyrex t o  a red 
heat and, i n  some cases, caused the  g lass  t o  bubble. 
t h a t  the hot spots formed a t  the points of mechanical 'contact between the  electrode 
and the pyrex. 
electrode spaced 1-2mm behind the d ie lec t r ic .  
en t r i e s  of Table I. 
intimate contact with the  ta rge t  d i e l ec t r i c  over a l a rge  area would r e s u l t  i n  
be t t e r  transfer of power t o  the d ie lec t r ic .  
glass  fr i t  was f i r e d  onto the back of a four  inch diameter pyrex disc.  
t h i s  t a rge t  was used, a yellow color, indicat ive of sodium, appeared on the 
t a rge t  surface and in the plasma column a t  a r e l a t i v e l y  low ne t  RF power. 
"hot spotfl did, however, develop. 
t o  a point  of contact between the s i l ve r  layer  and the back-up electrode. It was 
apparently a r e s u l t  of an inhomogeneity i n  the g lass  and the negative temperature 
coeff ic ient  of the  g lass  e l e c t r i c a l  r e s i s t i v i t y .  The pyrex cracked i n  t h i s  area. 
These 
Their small s i z e  suggested 
It had a l so  been found tha t  sput ter ing could be achieved with the 
Refer t o  the four th  and f i f t h  
It w a s  thought, however, t h a t  placing the electrode i n  
For t h i s  purpose, conductive s i lver -  
When 
A 
This hot spot was cooler and d id  not  correspond 
A smaller (two inch diameter) disc of pyrex w a s  coated with the  s i l ve r .  
Effective power transfer was again achieved with t h i s  ta rge t ,  but there  appeared 
t o  be l e s s  tendency f o r  hot spots t o  develop. A t a rge t  of t h i s  approximate s i z e  
is  compatible with the square four-target array. 
3 
I n s t a l l a t i o n  of the four-target array has brought about a number of 
mechanical problems. None of these a r e  of concern. 
A s  might be expected, the  use of  a thinner  d ie lec t r iccont r ibu tes  t o  a 
This e f fec t  was  observed experimentally during t h i s  higher deposit ion rate. 
p a s t  quarter.  
A secondary effort, during t h i s  p a s t  quarter,  was directed toward 
increasing the mechanical s t a b i l i t y  of t he  coating. This w a s  necessary 
because ce r t a in  e a r l i e r  coatings spontaneously peeled o r  b l i s t e r ed  from the  
c e l l s  on which they were deposited. 
ind ica te  t h a t  the addi t ion of oxygen t o  the  sput ter ing atmosphere permits the  
formation of coatings with reduced s t r e s s .  The use of ambient laboratory a i r  
gave rise t o  coatings of unsat isfactory c l a r i t y  and color ,  
w a s  very low when oxygen alone was used. 
r e s u l t s  was a mixture of argon and oxygen. 
produced with 60% of Ar, 40% of 02 and 80% of Ar, 20% of 02 respectively.  
c e l l  produced with only 20% of 02 showed a s l i g h t  tendency t o  pee l  i n  humid air .  
Table I i s  a l i s t i n g  of representative runs made throughout the quarter.  
Experiments have been conducted which 
The deposit ion r a t e  
The atmosphere showing the most promising 
Examples of t h i s  combination were cells 
The 
The e f f ec t  of pure oyygen i n  reducing the sput ter ing r a t e  is seen. Also,  
the  r e l a t i v e l y  high r a t e  produced by the s i lvered  t a rge t  is seen i n  t h a t  a 
normal rate i s  observed f o r  a ra ther  low input power. 
REFLECTANCE AND EMITTANCE MEASUREMENTS 
Spect ra l  ref lectance measurements provide an easy means of measuring 
emittance when the ref lectance spectrum i s  simple i n  s t ruc ture .  However, 
the presence of absorption and re f lec t ion  bands i n  the inf ra red  complicate 
ca lcu la t ion  of emittance a t  l o w  temperatures. I n  t h i s  case, it i s  simpler t o  
assemble a r ad ia to r  from the material t o  be studied and measure its temperature 
a t  a known power diss ipat ion.  
shown schematically i n  Figures 5 and 6. 
An apparatus has been assembled to  do t h i s ,  
Ir 
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A vacuum stand with a blackened 14 inch glass  b e l l  jar provides an iso- 
thermal room temperature enclosure, 
mater ia l  i n  question i s  suspended in  t h i s  space. 
a 1-1/2 x 1-1/2 x 1/16 inch flat  co i l  of #36 copper wire with a room 
temperature res is tance of about 50 ohms. The sample mater ia l  i s  mounted 
t o  both faces  of the  heater and the temperature of the heater  determined 
A thin,  f l a t  heater  coated with the 
The heater i s  made from 
from the resis tance 
is: 
where : 
dT 
d t  
- 
P 
Pl 
p2 
T 
C 
A 
6- 
e 
of  the copper co i l .  The thermal equation f o r  t h i s  system 
heater power 
power contributed t o  the sample by rad ia t ion  from the 
enclosure and by sample radiaLion re f lec ted  back t o  the 
sample. 
power l o s t  by conduction and convection. 
sample surface temperature. 
heat capacity of the sample. 
surface area o f  t he  sample. 
Stefan-Boltzmann constant. 
t o t a l  emittance of the sample. 
A t  a steady-state condition the thermal equation reduces to: 
0 = P + P1 - P2 - eA.(T4 
The quant i ty  P1 can a l so  be minimized by keeping the w a l l s  o f  the  chamber a t  a 
s i g n i f i c a n t l y  lower temperature than the  sample surface and by blackening the 
chamber w a l l s  t o  eliminate, o r  a t  l e a s t  g rea t ly  reduce, the amount of r e f l ec t ion  
from the  walls.  This w i l l  reduce P1 from 
I P = Aa6T:+A<T4Rei I-- 1 1 E - ( l - ~ i ) R  
t o  : Pl T 
4 Aa6 To 
6 
P;! i s  made small by evacuating t h e  chamber and by using l i g h t  lead  wires. 
A = surface area of sample 
a = absorptance of sample 
To = wall temperature 
R = wall reflectance 
We then ar r ive  a t  the re la t ion :  
0 = P +AadT: - e ~ 6 ~ ~  
Measuring power and temperature a t  three points  allows the determination of 
A, a, and e from three simultaneous equations. 
in te res ted  i n  temperatures under solar  i r rad ia t ion ,  emittance w a s  determined for 
power d iss ipa t ion  i n  the neighborhood of 70 mw/cm2. Table I1 shows the steady 
s t a t e  power and temperatures f o r  various material  tes ted.  
Since w e  a r e  pr imari ly  
Figure I is  the 
spec t r a l  ref lectance of polished aluminum, showing the  emittance t o  be 0.05 
a t  a maximum, agreeing qui te  well  with the  measured value of 0.02. The bare 
C d S  c e l l  emittance was measured a t  about 0.2. 
measured ref lectance is very close t o  80$, making the emittance about 0.2. 
F ina l ly ,  the  emittance of the s i l icone primer, GES5-4044 was measured t o  be 
0.26. 
aluminum. 
of 0.58, o r  an emittance of 0.42. 
emittance of 0.22. 
Camparing Figures 2 and 3, the  
Figure 4 is the ref lectance spectrum of t h i s  primer on polished 
Integrat ion and averaging of t h i s  curve gave an average ref lectance 
The same primer on a c e l l  gave a measured 
FIJ’IVRE WORK 
It is expected tha t  the four  target  a r ray  w i l l  soon be operating more 
effect ively,  producing more uniform coatings a t  a higher deposit ion r a t e .  
The r e l a t ion  of e lectron emission r a t e  from the  sput ter ing u n i t ’ s  hot 
Possibly, a be t t e r  e lectron cathode t o  the deposition r a t e  w i l l  be checked. 
emi t te r  can r a i s e  deposition rate i n  two ways. The first way would be by 
7 
TABU I1 
pa te r j  al, 
Alumina 
Alumina w/cover g lass  
Aluminum Sheet 
Aluminum Sheet coated 
W/GE s i l i c  one primer 
GE -S S - 4044 
Aluminum She e t  covered 
w / f l a t  black pa in t  
CdS c e l l  
CdS c e l l  coated 
w/s il icone primer 
Pmer Sample Surface mw/cm2 Temperature OK 
62 3880 
50 247 O 
13 5 3 3 O  
26 LlOOO 
15 380' 
27 46L0 
17 415O 
Emittance 
(Calc . ) 
.73 
.83 
.02 
.26 
.20 
.18 
.22 
8 
increasing the ion density in the plasma providing more power concentrations in 
the target. The second way would be the permitting of the use of a lower 
gas pressure, thereby, reducing the scattering of the material particles. 
The substrate cooler will be used to avoid temperature degradation of 
the cells during deposition. 
Alternate target materials now on hand will be evaluated. 
Sample cells will be sent to NASA periodically. 
Measurements of emittance and reflectance on samples will continue, 
specifically on coated C d S  cells. 
Operating CdS cells w i l l  be coated and tested. 
9 
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(blackened) - 
I 
Sample - with in t e rna l  
heater  
Evacuated Chamber 
(blackened in t e r io r )  
Suspension 
System 
Small Wire Hea 
Leads 
I 
I 1 I J Base 
I 
Discharge Evacuation 
Gauge O r i f  i c e  
.ter 
P la te  
Figure 5’. Emittance Measurement Apparatus 
12v 
Battery 
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I t T 2 r 
+ g  Td 
V 0  
L & N  , Switch 4 
Pot. 
Heater 
Figure 6. Measuring Circuit - Power & Temperature 


Development of optical coatings for cos thin film solar cells third quarterly report, jun. 1 - aug. 1, 1965

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