Encapsulant Degradation in Photovoltaic Modules - ACS Symposium

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Downloaded by PENNSYLVANIA STATE UNIV on June 19, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch024

Encapsulant Degradation in Photovoltaic Modules K. J. LEWIS and C. A. M E G E R L E ARCO Solar, Inc., Research and Development, Woodland Hills, CA 91367

The aging behavior of several encapsulant candidates for photovoltaic module designs with a plastic front surface were studied i n the field and v i a accelerated aging. Two pottant polymers and two outer cover/insulator films were tested for resistance to degradation. Test methods included dry oven aging, humidity chamber aging, field aging and accelerated outdoor weathering. Evidence of degradation included discoloration, embrittling and other changes in mechanical properties, development of opacity, changes in electrical resistivity and the appearance of polymer oxidation products observed by ESCA and multiple internal reflection IR spectroscopy. Encapsulants in a photovoltaic (PV) module provide electrical insulation and protect the metallized c e l l contacts and interconnect system against corrosion over a 20-year lifetime outof-doors. The typical environmental stresses and possible resulting failures in exposed PV modules are listed in Table I. In the case of brittle cells such as single or polycrystalline silicon, encapsulants must also provide mechanical protection for the fragile wafers and interconnect ribbons or wires. Figure 1 shows the typical layup for a plastic front design. The functions and performance requirements for the various components are described in detail in the accompanying paper in this symposium proceedings entitled "Encapsulant Material Requirements for Photovoltaic Modules" ( 1_) . The layers most vulnerable to degradation are the pottant and the flexible outer cover/insulator, the latter being particularly susceptible in the substrate design where it is on the module front and thus must be transparent. These two layers are the most easily degraded because the quantities required and the cost

0097-615 6/83/0220-03 87$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Table I·


Humiditv - Cell Metallization Delamination - Encapsulant Delamination

ultraviolet

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P r i n c i p a l Damaging Environments

Thermal C v c l i n g - Interconnect Fatigue - Encapsulant Delamination - Solar C e l l Cracking

-

IN

Optical Material Degradation Encapsulant Delamination

S t r u c t u r a l Loading - C e l l Interconnect Fatigue S t r u c t u r a l Fatigue H a i l Impact - O p t i c a l Cover Breakage - C e l l Cracking

Voltage Stress - I n s u l a t i o n Breakdown - C e l l Corrosion (Ion M i g r a t i o n )

O p t i c a l Surface

Soiling

c o n s t r a i n t s necessary f o r low cost s o l a r arrays d i c t a t e that they be upgraded, good performance m a t e r i a l s . The pottant i s the v i b r a t i o n damping, elastomeric m a t e r i a l that immediately surrounds both sides of the f r a g i l e s o l a r c e l l wafers and t h e i r e l e c t r i c a l contacts and i n t e r c o n n e c t s . I t must be s o f t , transparent, e l e c t r i c a l l y i n s u l a t i n g , weather r e s i s t a n t , chemically i n e r t and form strong and s t a b l e adhesive bonds t o the surfaces i t touches. I t p r o t e c t s the c e l l s from stresses due t o t h e r m a l e x p a n s i o n d i f f e r e n c e s and e x t e r n a l impact and i s o l a t e s them e l e c t r i c a l l y . The pottant a l s o h e l p s p r o t e c t t h e c i r c u i t m e t a l l i c contacts and interconnects from the c o r r o s i v e e f f e c t s of moisture, s a l t , smog, e t c . The o u t e r c o v e r / i n s u l a t o r must be a t o u g h , s o i l r e s i s t a n t , weather r e s i s t a n t and e l e c t r i c a l l y i n s u l a t i n g l a y e r . I t may be a f l e x i b l e o r c o n f o r m i n g p l a s t i c f i l m o r a c o a t i n g a p p l i e d from s o l u t i o n . As a f r o n t l a y e r , i t i s d e s i r a b l e that i t act as a UV s c r e e n f o r t h e p o t t a n t w h i l e i t must a t the same time be >90% transparent t o wavelengths from 0.4 t o 1.1 microns. I t must form s t a b l e bonds t o the pottant and t o other module m a t e r i a l s t o which i t seals. Most o r g a n i c m a t e r i a l s c o n t a i n s i t e s where r a d i c a l s can form more o r l e s s e a s i l y , d e p e n d i n g u p o n s t r u c t u r e . Only p e r f l u o r i n a t e d m o l e c u l e s a r e t o t a l l y f r e e of such s i t e s since r a d i c a l formation usually involves a b s t r a c t i o n of hydrogen radicals. B o t h t h e p o l y m e r and a b s t r a c t e d h y d r o g e n r a d i c a l s become s t a b i l i z e d by the i n t e r v e n t i o n of oxygen. D e g r a d a t i o n i n the form o f c r o s s l i n k i n g , c h a i n s c i s s i o n or both f o l l o w s . The

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


LEWIS AND MEGERLE

Encapsulant Degradation

Figure 1.

Module Cross S e c t i o n ,


POLYMERS

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b a s i c mechanism of t h i s process, i d e n t i f i e d over 35 years ago Bolland and Gee ( 2 ) , i s as f o l l o w s : r ^ ( r a t e of (1)

initiation)

Initiation

k (2),—fc-R- + 0

by

^

free radicals

0

fc-R0 '

2

2


propagation (3)

RÛ2* +

(4)

R02* + R02*

ROOH +

RH 2k

3'

t

^-products - termination at O2 s a t u r a t i o n

Fluorocarbon polymers are the most r e s i s t a n t to t h i s type of degradation because the c a r b o n - f l u o r i d e bond i s extremely s t a b l e , with an energy on the order of 116 kcal/mole, compared to carbonh y d r o g e n b o n d e n e r g i e s o f 91-98 kcal/mole (3., 4, 5.). F l u o r o c a r b o n s a r e , however, e x t r e m e l y e x p e n s i v e b e c a u s e t h e monomers are more complicated to synthesize and more dangerous to handle. The polymers c o s t on the o r d e r of $10-20 per pound compared with the most widely used hydrocarbon polymers which can be $1-5 per pound. S i l i c o n e polymers u s u a l l y have an a l l O-Si-O-Si-O backbone and thus do not undergo hydrogen r a d i c a l a b s t r a c t i o n i n a p o s i t i o n where i t can cause s i g n i f i c a n t c r o s s l i n k i n g or chain s c i s s i o n . The S i - 0 bond can be h y d r o l y z e d , a l t h o u g h not e a s i l y . When s i l i c o n e s do degrade, they become more h y d r o p h i l i c , allowing more moisture to reach c i r c u i t metals. They can a l s o h a r d e n i f the r u b b e r y s i d e c h a i n s a r e l o s t , thus l o s i n g t h e i r s t r e s s damping c h a r a c t e r i s t i c s . They a l s o cost about $10 per pound. The s i t e s f o r r a d i c a l f o r m a t i o n on a c r y l i c s are e i t h e r deactivated by the carbonyl group as i n ordinary a c r y l i c e s t e r s , or t o t a l l y blocked as with the m e t h a c r y l i c e s t e r s . This i s what makes them stable compared to other saturated hydrocarbon backbone m a t e r i a l s l i k e p o l y e t h y l e n e w h e r e t h e r e a r e no e l e c t r o n withdrawing groups to s t a b i l i z e against r a d i c a l f o r m a t i o n . The d e g r e e o f s t a b i l i t y depends on the s t r e n g t h of the e l e c t r o n withdrawing e f f e c t . Most a c r y l i c s a l s o e x h i b i t some h y d r o p h i l i c character, even when they have very f a t t y s i d e chains because they too can hydrolyze, however s l i g h t l y . The l e a s t s t a b l e m a t e r i a l s have u n s a t u r a t i o n i n t h e i r backbones which can be d i r e c t l y and e a s i l y attacked by oxygen and p e r o x y r a d i c a l s t o degrade a c c o r d i n g t o the mechanisms j u s t described. The l e s s s t a b l e , l e s s e x p e n s i v e m a t e r i a l s w i t h s a t u r a t e d backbones such as p o l y o l e f i n s , and e s p e c i a l l y those containing e l e c t r o n withdrawing groups such as e s t e r s , h a l o g e n o t h e r than f l u o r i n e , amides, urethane groups, e t c . to s t a b i l i z e


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Encapsulant Degradation

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them, can be d r a m a t i c a l l y u p g r a d e d w i t h a n t i o x i d a n t s a n d p h o t o s t a b i l i z e r s t o make them p o t e n t i a l l y acceptable f o r use i n photovoltaics. There a r e f i v e c l a s s i f i c a t i o n s o f o x i d a t i o n i n h i b i t o r s . These are based on d i f f e r e n c e s i n the mechanism by w h i c h they f u n c t i o n to i n t e r f e r e w i t h one or more of the r e a c t i o n s described i n the p r e v i o u s e q u a t i o n s t o p r e v e n t o r d e l a y c a t a s t r o p h i c degradation by o x i d a t i o n ( 6 ) . These c l a s s i f i c a t i o n s are:


1.

2.

3.

4.

5.

Metal d e a c t i v a t o r s , which form i n a c t i v e c h e l a t e s o r i n s o l u b l e r e a c t i o n products w i t h t r a n s i t i o n metals o r i g i n a l l y present i n a form t h a t p r o m o t e s t h e d e c o m p o s i t i o n o f p e r o x i d e s to f r e e r a d i c a l s . Examples are e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d , s a l i c y l a l d e h y d e diamine condensation products or m e t a l a l k y l dithiocarbamates such as of n i c k e l or z i n c . L i g h t a b s o r b e r s , w h i c h p r o t e c t from p h o t o - o x i d a t i o n by a b s o r b i n g the u l t r a v i o l e t l i g h t e n e r g y t h a t w o u l d o t h e r w i s e i n i t i a t e o x i d a t i o n , e i t h e r by decomposing peroxides or by s e n s i t i z i n g the o x i d i z a b l e m a t e r i a l t o oxygen a t t a c k . The absorbed energy must be disposed of by processes that do not produce a c t i v a t e s s i t e s o r f r e e radicals. Examples are 2-hydroxybenzophenones, 2-(2'hydroxyphenyl)benzotriazoles, c e r t a i n s a l i c y l a t e e s t e r s or c e r t a i n organonickel or chromium compounds. Peroxide decomposers, w h i c h promote the c o n v e r s i o n o f p e r o x i d e s t o non-free r a d i c a l products, presumably by a p o l a r mechanism. Examples a r e d i a l k y l a r y l p h o s p h i t e s , d i a l k y l t h i o d i p r o p i o n a t e s or long c h a i n alkylmercaptans. Free r a d i c a l c h a i n s t o p p e r s o r " r a d i c a l t r a p s , " w h i c h i n t e r a c t w i t h c h a i n - p r o p a g a t i n g RU2* r a d i c a l s to form i n a c t i v e products. This i s u s u a l l y accomplished by i t s donation of an Η· r a d i c a l to terminate an a c t i v e polymer r a d i c a l , i t s e l f forming a more s t a b l e one ( u s u a l l y by resonance) which w i l l not r e r e a c t w i t h the polymer (e.g., w i t h the help of s t e r i c hindrance) and w i l l e v e n t u a l l y r e l a x i t s energy through t h e r m a l i z a t i o n , fluorescence o r other innocuous means. Examples are s t e r i c a l l y hindered phenols or secondary arylamines. I n h i b i t o r regenerators. which r e a c t w i t h intermediates o r p r o d u c t s formed i n the c h a i n - s t o p p i n g ( t e r m i n a t i o n ) r e a c t i o n so as to regenerate the o r i g i n a l i n h i b i t o r o r f o r m a n o t h e r p r o d u c t c a p a b l e o f f u n c t i o n i n g as an antioxidant. Examples are d i a l k y l p h o s p h o n a t e s w i t h hindered phenols or diphenoquinones w i t h t h i o l s .

Since the cannot provide the c i r c u i t , s t a b i l i t y , and

available, inherently weather-resistant m a t e r i a l s a t o t a l l y moisture- and oxygen-free environment f o r whether o r not they t h e m s e l v e s r e q u i r e i t f o r because s t a b i l i z e r s can so d r a m a t i c a l l y improve the



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performance of much lower cost m a t e r i a l s , the lower cost m a t e r i a l s are p r e f e r r e d over s i l i c o n e s and most fluorocarbons f o r use i n low cost t e r r e s t r i a l s o l a r a r r a y s . I t i s l i k e l y that a c r y l i c s i n the $1-5 per pound range w i l l u l t i m a t e l y be the optimum m a t e r i a l f o r b o t h t h e p o t t a n t and t h e o u t e r c o v e r / i n s u l a t o r . Their a v a i l a b i l i t y i n the current market, however, i n forms s u i t a b l e f o r PV a p p l i c a t i o n i s c u r r e n t l y l i m i t e d and, f o r t h e most p a r t , unproven. The two p o t t a n t m a t e r i a l s s t u d i e d i n t h i s r e p o r t a r e p l a s t i c i z e d p o l y v i n y l b u t y r a l (plPVB) w h i c h i s e a s i l y a v a i l a b l e and used i n s a f e t y g l a s s , and a h i g h l y s t a b i l i z e d , p e r o x i d e c r o s s l i n k e d e t h y l e n e / v i n y 1 a c e t a t e (EVA) copolymer c o n t a i n i n g about 33 w e i g h t % v i n y l acetate (7.). The outer c o v e r / i n s u l a t o r m a t e r i a l s studied i n c l u d e p o l y v i n y l f l u o r i d e (PVF) and a b u t y l a c r y l a t e / m e t h y l m e t h a c r y l a t e g r a f t copolymer (BAgMMA); both are blown f i l m s . Aging Tests and Results Mechanical and O p t i c a l . C l e a r 1 5 - m i l - t h i c k f i l m s of EVA and plPVB, 4-mil PVF and 3-mil BAgMMA were exposed i n a c i r c u l a t i n g a i r oven a t about 150°C f o r periods of 0 t o 26 days. This type of t e s t i s used e x t e n s i v e l y t h r o u g h o u t t h e polymer i n d u s t r y as a screening t o o l f o r comparing the o x i d a t i v e s t a b i l i t i e s of polymers and compound formulas. A summary of the o p t i c a l t r a n s m i s s i o n changes as a r e s u l t o f the oven aging can be seen i n Figure 2. EVA e x h i b i t e d very l i t t l e y e l l o w i n g i n t h e 26 d a y s . M e c h a n i c a l l y , i t s t e n s i l e s t r e n g t h , e l o n g a t i o n a t break and permanent set decreased considerably w i t h the a g i n g . A t t h e same t i m e , however, i t s t e a r s t r e n g t h and e l a s t i c moduli a t 10% and 100% e l o n g a t i o n s , which are w i t h i n t h e regions of concern f o r PV use, remained r e l a t i v e l y constant (Table II). The plPVB d a r k e n e d r a p i d l y d u r i n g t h e o v e n e x p o s u r e , becoming too b r i t t l e t o permit t e n s i l e t e s t i n g . The BAgMMA showed no measurable y e l l o w i n g throughout the f u l l 26 days o f aging, but the l a r g e increase i n UV t r a n s m i s s i o n during the f i r s t 7 t o 10 days s u g g e s t s t h a t i t l o s e s much o f i t s UV absorber during t h i s period (Figure 3 ) . Further aging then r e s u l t e d i n a decrease i n UV t r a n s m i s s i o n . The BAgMMA f i l m was becoming n o t i c e a b l y b r i t t l e a t the 10-day sample and was q u i t e b r i t t l e a f t e r 18 days. PVF showed v e r y l i t t l e y e l l o w i n g (

Encapsulant Degradation in Photovoltaic Modules - ACS Symposium

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