The reliability of crystalline silicon modules has been brought to a high level with lifetimes approaching 20 years, and excellent industry credibility and user satisfaction. The transition from crystalline modules to thin film modules is comparable to the transition from discrete transistors to integrated circuits. New cell materials and monolithic structures will require new device processing techniques, but the package function and design will evolve to a lesser extent. Although there will be new encapsulants optimized to take advantage of the mechanical flexibility and low temperature processing features of thin films, the reliability and life degradation stresses and mechanisms will remain mostly unchanged. Key reliability technologies in common between crystalline and thin film modules include hot spot heating, galvanic and electrochemical corrosion, hail impact stresses, glass breakage, mechanical fatigue, photothermal degradation of encapsulants, operating temperature, moisture sorption, circuit design strategies, product safety issues, and the process required to achieve a reliable product from a laboratory prototype

Ross, R. G., Jr.

English

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N86- 1 W 6 0  
CRY STALLINE-S ILICON RELIABILITY LESSONS FOR 
T' ! I N  -F I LM MODULES 
Ronald G. Ross, Jr. 
Je t  Propul s i  on Laboratory 
Pasadena, C a l i f o r n i a  91 ;09 
In t roduc t i on  
During tne Clast 10 years the r e l i a b i l i t y  o f  c r y s t a l l i n e - s i l i c o n  modules 
has been brought t o  a h igh l eve l  f i t h  l i f e t i m e s  approaching 20 years, and 
exce l len t  i ndus t r y  c r e d i b i l i t y  arld user s a t i s f a c t i o n .  With the emergence o f  
t h i n - f i l m  power modules i t  i s  important t o  review the  lessons learned from t h e  
c r y s t a l  1 ine-Si  product development h i s t o r y  and apply the technology base, 
Hhere appl icable,  t o  enhsnce the  development o f  t h i n - f i l m  modules. 
The t r a n s i t i o n  from c r y s t a l l i n e  modules t o  t h i n - f i l m  modules i s  compdrable 
t o  the  t r a n s i t i o n  from d i s c r e t e  t r a n s i s t o r s  t o  in tegra ted  c i r c u i t s .  New c e l l  
ma te r i z l s  and mono l i th ic  s t ruc tu res  w i l l  r equ i re  new device processing 
techniques, but  the  package func t i on  and design w i l l  evolve t o  a lesser  
extent .  Although there  w i l l  be new encapsulaits optimized t o  take advantage 
o f  t h e  mechanical f l e x i b i l i t y  and low-temperature processing fea tures  of 
t h i n - f i l m s ,  the r e l i a b i l i t y  and l i f s -deg rada t ion  stresses and mechw :sms w i l l  
remain most ly  unchanged. Key r e l i a b i l i t y  t. chnologies i n  comnon between 
c r y s t a l l i n e  and t h i n - f i l m  modules inc lude t-ot-spct heating, ga lvanic  a w l  
e l i x t rochemica l  corros ion,  ha i l - impact  stresses, glass breakage, mechanical 
f a t i gue ,  photnthermal degradation o f  encapsi lants, opera t ing  temperature, 
mois t l j re  sorpt ion,  c i r c u i t  design s t ra teg ies ,  product s a f e t y  issues, and the  
process requi red t o  achievc a r e l i a b l e  product from a labora tory  prototype. 
Crystal l ine-; i  Rezearch - Object ive  and&* ?acJ 
Before exam-' ;ng the lessons learned from the c r y s t a l  1 ine-Si  module 
development ef f , . : t  i t  i s  i n s t r u c t i v e  t o  review b r i e f l y  i t s  ob jec t i ve  and 
wproach. 
Increased ar ray  l i f e  and r e l i a b i l i t y  d i r e c t l y  inf luer.. :  the economic 
v i a b i l i t y  o f  photovo l ta ics  as an energy source by c o n t r o l l i r l g  t h e  t o t a l  number 
and s i ze  o f  revenue paynients received from f u t u r e  sales o f  e l e c t r i c i t j .  A f t e r  
cons iderat ions o f  prescxt  value d iscount ing and esca la t ion  o f  t he  worth o f  
e l e c t r i c i t y  i n  f u t u r e  years, a 30-year PV p lan t ,  f o r  example, i s  worth 25 t o  
30 percent more than a 20 -yea r - l i f e  p lan t .  
t o  p l a n t  l i f e ,  a j u - y w -  l i f e  was chosen as the t a r g e t  o f  the c r y s t a l l i n e - S i  
module development e f f o r t  (F ig .  1) (1 ) .  
Based on t h i s  economic s e n s i t i v i t y  
TU achieve t h i s  h igh l eve l  of re1  i abi 1 i t y  a systetllat i c  re1  i abi 1 i t y  program 
(F ig.  2) was undertaken i n  1975 by t h e  Je t  Propuls ion Laboratory F la t -P la te  
Solar Array Project. t o  develop t!ie techno!ygy base requ i red  ( 2 ) .  F igure 3 
l i s t s  the  p r i n c i p a l  f a i l u r e  mechanisms f o r  c r y s t a l  l i ne -S i  modu:es and notcs 
the economic importance o f  each and the ta rge t  a l l oc3 t i o t i  l e v e l  f o r  esch which 
i s  cons is tan t  w i t h  achieving a 30-year lif, ( 3 ) .  
i l l u s t r a t e  the h i s t o r s  o f  occurance of c r y s t a l l i n e - S i  f i e l d  r e l i a b i l i t y  
problem1 3nd the  research developments over the  pa;t 10 ypars which have led  
t o  the ' ~ 1  Tent h igh r e l i a b i l i t y  o f  c rys ta l l i ne -S '  nodules. 
The next t h ree  f i g u r e s  
3 
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Lessons Learned 
The remainder of the f i g u r e s  sys temat ica l l y  sumnarize the key r e l i a b i l i t y  
lessons learned from the  10-year c r y s t a l  l ine-S i  module development e f f o r t .  
For convenience the lessons ar2 subdivided i n t o  f i v e  t o p i c  areas: 
o Module Re1 i a b i  I i t y  Lessons 
o Re? i abi 1 i t y  Research Lessons 
o Module Qua: i f i ca t ion  Experience 
o Q u a l i f i c a t i o n  Test Experience 
o F i e l d  Test Experience 
Concl u r i  ons -- 
An important lesson from the c r y s t a l l i n e  program (and the nuclear program) 
i s  t h a t  honest conscientious working o f  r e l i a b i l i t y  and s a f e t y  issues can 
s i g n i f i c a n t l y  a f f e c t  the economic v i a b i l i t y  and pub l ic  acceptance o f  the 
product. Resolving the  issues i s  not cheap, and cannot be accomplished 
overnight. 
product design t o  successful passing o f  product q u a l i f i c a t i o n  s p e c i f i c a t i m s  
f o r  a c r y s t a l l i n e - S i  module. 
For example, i t  s t i l l  takes approximately 2 years from ' n i t i a l  
As w i t h  your f a m i l y  car, i n i t i a l  cost  and e f f i c i e n c y  are d i r e c t l y  
measurable; l i f e t i m e  and r e l i a b i l i t y  are the greatest  areas o f  user r i s k  and 
p l a y  a key r o l e  ir :  purchase decisions. 
References 
1. Five Year k;earch P?an, 1984-1588, Nat ional  Photovol t a i r s  Program, O f f  i c e  
--- 
o f  So?ar E l e c t r i c  Technologies, U.S .  Dersrtmept o f  Energy, May 1983. 
2. Ross, R.G. J r . ,  " R e l i a b i l i t y  Research Toward 30-year - l i fe  Photovol ta ic  
Modules", - Proceedings o f  the 1 s t  In te rna t iona l  -- Photovol ta ic Science and 
Engineering Conference, Kobe., Japan, November 75-18, 1984, pp. 331-340. 
3. Ross, R.G. Jr .  "Technology Developments Toward 30-year - l i fe  o f  
Photovo! t a i c  Modules" -- P r o c e e d i n g s f  - ~ -  the 17th I E E E  Photovol ta ic  
Specia l is ts  --Conference, Orlando, FL, May 1-4, 1985, pp. 464-472. 
4 
Figure 1 . Crystalline-Silicon Reliability Objective 
NORMALIZED 
POWER 
OUTPUT 
0 -  
- - - - - - 
1 I 1 
Figure 2. Reliability Research Elements 
Establishment of me:’lanism-specific reliability goals 
Identification of key degradation mechanisms 
Determination of system energy-cost impacts 
0 Allocation of si z tern-level reliability 
Quantification of mechanism parameter dependencies 
Governing materials parameters 
Governing environmental-stress parameters 
Qualitative understanding of mechanism physics 
Development of degradation prw!ktion methods 
Quantitative accelerated tests 
Life-prediction ir. odels 
Identification of cost-effective solutions 
Component design features 
Circuit redundaycy and reliability features 
Testing and failure bnalysis of trial solutions 
5 
Figure 3. Life-Cycle Cost Impacts and 
Allowable Cegradation Levels 
Component 
failures 
Open-circuit cracked cells - %/yr T O 8  013 i 0.005 1 Energy 
%lyr 0.24 1 0.40 1 0.050 1 Energy Short-circuit cells 
'Interconnect oDen circuits I %/vr' 1 0.05 1 0.25 ! 0.001 Enarav 
Power 
degradation 
'k = Discount rate 
Cell gradual power loss %/yr 1 0.67 ! 1.15 1 0.20 Energy 
Module optical degradatio; %/yr I 0,: I 1,; 1 0;:; 1 Energy F 
Front surface soiling 
, Module glass breakage f r 7  0.33 1.18 0&M 
% i Energy 
Module open circuits -- 1 %/yr [ 0.33 1.18 , 0.1 i O&M 
Figure 4. Module Reliability Lessons 
%/yr 0.33 
Most module reliability problems are related t o  the encapsulant system 
1.18 1 0.1 1 O&M 
O&M 
Soiling 
Cracking 
%/yr' ' 0.022 
%/yr' 0.022 
of life , 
Years 27 
Accelerated corrosion 
Voltage breakdown 
- 
0.122 ! 0.01 ' O&M 
0.122 0.01 O&M 
End of 
life 20 35 
Yellowinr, Leminating (processing) stresses 
Delaminating Lblfferential expansion stresses 
Primary function of encapsulant is structural support and electrical 
isohtion for safety reasons. The secret is  t o  perform these functions 
while not degrading the intrinsic reliability of the cells themsslves 
Second most frequent module reliability problems are related t o  
circuit integrity 
Fatigue due to  differential expansion stresses 
Poor solder joints 
Crystalline-Si cell reliability prct lems are most often related to  cell 
cracking, metallization adherart1:elseries resistance and durability of 
anti-reflective coatings 
6 
Figure 5. Reliability Research Lessons 
Failure mechanisms fall into two broad classes: generic and statistical. 
Generic problems must be solbed by design or process changes; 
statistical failures are effectiveh solved through redundancy and 
quality control 
The physics of most failure mechanisms is poorly understood. This 
requires a high reliance on emperical characterization and testing 
Increased tovperature is an excellent universal accelerator of chemical 
degradatioi, mechanisms. Typical acceleration is Arrhenius with a 
factor of 2 increase per 10°C 
Figure 6. Module Qualification Experience 
0 Qualification testing is a cost-effective way to identify obvious reliability 
problems: should be used during development as well as for 
design verification 
New designs almost never pass .he Qual tests on the first try 
Corollary: Great political pressisre to field unquadied hardware generally 
results in disaster 
Slipped schedules, cost overruns 
Early application retirement 
Minimal learning 
Decreased credibility 
* Qual tests must be periodically uydateL to reflect field experience with 
previously tested modules 
Long-term life testing at parametric stress levels is required for 
quantitative correlation to extemled field performance 
7 
Figure 7. Qualification Test Experience 
Temperature cycling and humiciity tests are workhorse tests with good 
correlation to  field failures; the) are generally the most difficult t o  pass 
Hot-spot testing is controversial, but correlates well t o  field experience. 
Its complexity requires a high skill and knowledge level 
Mechanical loading, twist, and hail tests are effective design 
requirements and generally straightforward t o  meet 
Voltage standoff (hipot) requirements require great care in design and 
are troublesome to meet 
Photothermal testing (UV) is en remely complex with poor correlation 
with field results (no Qual test exists) 
Soiling evaluatior, is best done in field tests, but is highly site-dependent 
(no Qual test exists) 
Figure 8. Field Test Experience 
Most problems are not acceptec' as problems until encountered in :arge 
operating systems 
Large statisticai sample sir 3 aids quantification 
Operational user-interface stresses are present 
iorollary: Good module not prcven good until tssted in large 
operating system 
Corollary: Operational interactkm of module with user system is 
important source of ;nodule stress 
Test-stand aging only useful fcr very generic problems; sample sizes too 
limited for statistical failures; many user interface stresses not present 
in test.stand tests 
e heliance on field-failure data places requirements on system 
experiments: 
To obtain quantitative da-z on failu;es 
To have failure containment features 
To have failure contingency plans 
8 
Figure 9. Conclusions 
Crystalline-Si and thin-film modules are expected to have much in 
common Vvith respect to reliability problems, methods and solutions 
New materials and processes in thin-film modules will require a diliquent 
reliability program 
Establishment of mechanism-tcecific reliability goals 
Quantification of mechanism pwameter dependencies 
Prediction of expected long-term degradation 
0 Identification of cost-sffective solutions 
Testing and failure acalysis of trial solutions 
DISCUSSION 
YERKES: Can you guess where problems misht be different from those with the 
modules we have been doing for 10 years, in the new thin-film 
modules? Some insight on your part, from early examination. 
ROSS: Host of our testing indicates that thin-film modules and crystalline 
modules are quite similar; things like hot-spot heating modes are 
almost identical. The cell-breakage problem is a big difference. 
Crystalline-silicon cell cracking was one of the mcre formidable 
problems, long-term, in differential expansion stresses and 
processing yields, and thin-film modules will have a different 
type of processing-yield problem, I'm sure. Crystalline cells 
allow series-paralleling to solve the cell-shorting mismatch type 
of problems. I ' m  not sure if that is going to be a problem with 
thin-film modules; it depends on how uniformly the deposition can 
occur. It may be a different problem with stainless-steel-backed 
modules; they may go through the same kind of cell shorting that 
crystalline modules do. Corrosion-type issues are clearly going 
to be different, although the data we have in our electrochemical 
corrosion studies indicate there is not much difference between 
the resistances of thin-film modules and those of crystalline 
modules. When you have a micrometer o r  so of material and lose a 
half micrometer you have lost a cell. On a crystalline cell you 
can lose a lot of the metallization system; there are bulk amounts 
of material that you can corrode away and still leave an active 
solar cell. The crystalline cells are less fragile in terms of 
mechanical damage. With thin-film cells, if you penetrate the 
back side with something, you could poke a hole right through the 
cell. At the same time, tne thin-film cslls are very resistant. 
If you lose a part of the cell they typically don't shunt and the 
lateral series resistance is such that a small area of damage 
doesc't seem to spread across the total ceil in terms of total 
electrical effect. There are differences. 
10 


Crystalline-silicon reliability lessons for thin-film modules

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