Vacuum Lamination of Photovoltaic Modules - ACS Publications

25 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch025

Vacuum Lamination of Photovoltaic Modules D A L E R. BURGER and EDWARD F. CUDDIHY California Institute of Technology, Jet Propulsion Laboratory, Pasadena, CA 91109

Vacuum lamination of terrestrial photovoltaic modules is a new production process requiring special equipment and a significant material development effort. Equipment design studies resulted in improved control and lower costs when using a double-chamber vacuum laminator. Application testing of new materials and primers showed the feasibility of two different back sheet materials, one encapsulant, two new primers for polymers and one primer for metals. A few years ago, most terrestrial photovoltaic (PV) modules were assembled by casting the cells in a transparent silicone substance, using a metal substrate for support. When this approach was reviewed by the Flat-Plate Solar Array Project (FSA), development was begun on new materials that would reduce the cost and quantity of material required for encapsulation. During development of these materials, the PV industry improved module design by eliminating metal substrates and incorincorporating glass superstrates to provide a hard, easily cleaned top surface. Use of glass superstrates created new material problems. Bonding some encapsulating materials proved to be a difficult problem. Elimination of visual defects, such as voids and bubbles, became necessary because of increased market sophistication. Long-term corrosion concerns became important now that thin, water-permeable polymer materials were being used to reduce cost. Discussion of these problems will start with equipment development, because improved equipment was necessary to the subsequent materials development effort. While the development was being pursued, some module design and processing problems became evident. Because these problems have a bearing upon the reliability of the lamination process, they will also be discussed. 0097-6156/83/0220-0407$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, Charles G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch025

Equipment

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Development

When modules were b e i n g a s s e m b l e d by c a s t i n g w i t h s i l i c o n e s , m a t e r i a l and l a b o r c o s t s were h i g h and equipment c o s t s were l o w . M a t e r i a l - c o s t r e d u c t i o n was s o u g h t i n t h e d e v e l o p m e n t o f new materials. L a b o r - c o s t r e d u c t i o n , h o w e v e r , i s dependent upon t h e d e v e l o p m e n t o f b e t t e r p r o c e s s e s and t h e i n t r o d u c t i o n o f e q u i p m e n t s p e c i f i c a l l y d e s i g n e d t o use t h e new m a t e r i a l s and p r o c e s s e s . The p r o c e s s t h a t seemed most s u i t a b l e f o r t h e new m a t e r i a l s was a lamination process. I n i t i a l FSA i n v o l v e m e n t w i t h l a m i n a t i o n o c c u r r e d l a t e i n 1978 a t RCA L a b o r a t o r i e s . Early Designs. RCA d o u b l e - g l a s s lamination experiments showed t h a t an a u t o c l a v e was e x p e n s i v e and o f t e n i n t r o d u c e d a i r i n t o module edges d u r i n g cooldown ( J J . L a t e 1978 was a l s o t h e t i m e o f c o n t r a c t i n i t i a t i o n w i t h ARCO S o l a r , I n c . , t o d e v e l o p a l a m i n a t o r t h a t w o u l d r e d u c e t h e n e a r - t e r m c o s t s o f PV m o d u l e s . By J u l y 1979 a d o u b l e - c h a m b e r vacuum l a m i n a t o r was d e v e l o p e d , t e s t e d and p u t i n t o p r o d u c t i o n {2). T h i s l a m i n a t o r used a p r o v e n t h e r m o p l a s t i c , p o l y v i n y l b u t y r a l (PVB), which r e q u i r e s humidityand t e m p e r a t u r e - c o n t r o l l e d s t o r a g e and h a n d l i n g . Other problems w i t h PVB were h i g h c o s t and h i g h v i s c o s i t y a t t h e 150°C p r o c e s s temperature. An answer t o t h e s e p r o b l e m s was s o u g h t i n a m a t e r i a l t h a t would cure at process t e m p e r a t u r e s . One p o s s i b l e s o l u t i o n was t o develop a t h e r m o s e t t i n g polymer, but t h e s e polymers p l a c e more s t r i n g e n t t e m p e r a t u r e r e q u i r e m e n t s on a l a m i n a t o r . The o r i g i n a l ARCO S o l a r l a m i n a t o r u s e d l o n g , s l e n d e r t u n g s t e n - f i l ament lamps as a h e a t s o u r c e and s h i n y aluminum s t r i p s t o adjust for uniformity. T h i s a p p r o a c h works w e l l w i t h a t h e r m o p l a s t i c s u c h as PVB, but was c o n s i d e r e d t o be t o o v a r i a b l e and l i m i t e d f o r r e s e a r c h i n t o t h e r m o s e t t i n g m a t e r i a l s and s u b s t r a t e module d e s i g n s . A r e s i s t i v e s t r i p h e a t e r was c o n s i d e r e d as a p o s s i b l e i n e x p e n s i v e , improved heat s o u r c e . Wire-wound s t r i p h e a t e r s a r e commonly made w i t h s u r f a c e - t e m p e r a t u r e v a r i a t i o n s o f l e s s t h a n _+ 2% (when measured on t o p o f a 0 . 1 2 5 - i n . - t h i c k g l a s s p l a t e ) . A n o t h e r w e l l - c o n t r o l l e d h e a t s o u r c e w o u l d be a h e a t e d o i l s y s t e m . T h i s s y s t e m a l s o w o u l d have t h e c a p a b i l i t y o f c o o l i n g down t h e product b e f o r e l a m i n a t o r i s opened. Small Research Laminator. The P r o c e s s Development A r e a (PDA) o f FSA r e c e i v e d two ARCO S o l a r - d e v e l o p e d l a m i n a t o r s a t t h e end o f t h e c o n t r a c t . One o f t h e s e l a m i n a t o r s was m o d i f i e d f o r use w i t h a s t r i p h e a t e r . This c o n f i g u r a t i o n , with a boosta n d - b u c k a u t o t r a n s f o r m e r power s u p p l y , was used s u c c e s s f u l l y f o r some p r o c e s s v e r i f i c a t i o n and m a t e r i a l s s u r v e y e x p e r i m e n t s . A s s e s s m e n t o f f u t u r e r e s e a r c h needs and p r e s e n t equipment c a p a bilities led to additional m o d i f i c a t i o n s of the l a m i n a t o r .

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


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E x p e r i e n c e w i t h t h e l a m i n a t o r showed t h a t t h e heavy aluminum base p l a t e , a l a r g e thermal mass, caused c o n t r o l - r e s p o n s e problems. The 0 . 2 5 - i n . T r a n s i t e p l a t e a l s o c a u s e d some c o n t r o l p r o b l e m s , b e c a u s e i t was c l o s e t o t h e c o n t r o l l e r t h e r m o c o u p l e . Thermal mass keeps t h e chamber t e m p e r a t u r e h i g h d u r i n g t h e u n l o a d - l o a d c y c l e , which can s t a r t t h e cure c y c l e e a r l i e r than desired. The c o n t r o l l e r t e m p e r a t u r e must be m a n u a l l y a d j u s t e d to prevent the temperature of the laminant adhesive-encapsulant from o v e r s h o o t i n g . An i n e x p e n s i v e , m e c h a n i c a l l y s t i f f t h e r m a l i n s u l a t i o n system t h a t would not outgas d u r i n g exposure t o p r o c e s s i n g t e m p e r a t u r e s a s h i g h as 175°C was r e q u i r e d . Figure 1 shows t h e l a m i n a t o r m o d i f i c a t i o n s r e q u i r e d t o a c h i e v e t h e d e s i r e d thermal i s o l a t i o n and i m p r o v e d c o n t r o l l a b i l i t y . An u n u s u a l m a t e r i a l a p p l i c a t i o n was t h e u s e o f g l a s s m a r b l e s as i n s u l a t i o n . T h e r e was an improvement i n warm-up t i m e and i n t r a c k i n g . This d e s i g n p r e s e n t s a n e a r l y b a l a n c e d t h e r m a l l o a d above and below t h e s t r i p h e a t e r t h a t s h o u l d a l l o w good t r a c k i n g , r e g a r d l e s s o f the desired time-temperature c y c l e . Additional (1)

(2)

l a m i n a t o r changes t h a t were i m p l e m e n t e d

included:

A p p l y i n g a vacuum t o t h e t o p chamber o f a d o u b l e chamber vacuum l a m i n a t o r j u s t b e f o r e r a i s i n g t h e u p p e r chamber ( l i d ) t o remove a c o m p l e t e d m o d u l e . This a l l o w s t h e s i l i c o n e - r u b b e r d i a p h r a g m t o be a t t a c h e d t o t h e u p p e r c h a m b e r , w h i c h r e d u c e s h a n d l i n g and keeps the diaphragm weight o f f t h e l a m i n a n t s t a c k d u r i n g e v a c u a t i o n o f t h e lower chamber. A t t a c h i n g t h e c o n t r o l l e r thermocouple t o t h e bottom o f the 0.125-in. aluminum p l a t e n by l a m i n a t i n g . This was d o n e , and t h e i m p r o v e d t h e r m a l c o u p l i n g enhanced the performance of t h e l a m i n a t o r .

One measure o f t h e u t i l i t y o f t h e p r e s e n t l a m i n a t o r i s i t s a c c e p t a n c e by o t h e r r e s e a r c h e r s . T e s t programs now s c h e d u l e d on the laminator include: (1) (2) (3) (4) (5) (6) (7)

New e n c a p s u l a t i n g m a t e r i a l s e v a l u a t i o n . Substrate encapsulation studies. P r e p a r a t i o n of water permeation study samples. E v a l u a t i o n o f module d e s i g n d e v e l o p e d u n d e r J P L c o n t r a c t . E n c a p s u l a n t - t o - m e t a l primer-compound r e s e a r c h . Preparation of e l e c t r o s t a t i c t e s t samples. Preparation of u l t r a v i o l e t t e s t samples.

L a r g e New L a m i n a t o r . B e c a u s e o f t h e equipment d e v e l o p m e n t s u c c e s s , a new, l a r g e r l a m i n a t o r i s b e i n g d e s i g n e d t o e x p l o r e p r o b l e m s i n h e r e n t i n f a b r i c a t i n g t h e l a r g e r PV modules e n v i s i o n e d f o r the mid-1980s. A 4 - f t - s q u a r e laminator area i s expected; i t w o u l d be c o m p a t i b l e w i t h t h e 1.2-m s q u a r e d e s i g n s o r any



Figure 1.

Lamina t o r With Improved Thermal I s o l a t i o n .


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smaller configurations. The most e x p e n s i v e p a r t o f a l a r g e l a m i n a t o r i s t h e chamber t h a t must w i t h s t a n d t h e a t m o s p h e r i c p r e s s u r e l o a d . A t p r e s e n t t h e l o w e s t - c o s t vacuum chamber t h a t has been c o n s i d e r e d u s e s s t a n d a r d h e m i s p h e r i c p r e s s u r e v e s s e l - e n d c a p s c o s t i n g a b o u t $600 e a c h ( i n c l u d i n g a 2 - i n . - w i d e f l a n g e ) . A l e a d a l l o y c o u n t e r w e i g h t w i l l p e r m i t t h e o p e r a t o r t o r a p i d l y and s a f e l y l o a d and u n l o a d t h e l a m i n a t o r . S u p p o r t and i n s u l a t i o n o f t h e p l a t e n was a l s o a p r o b l e m , and t h e u s e o f m a r b l e s seems t o be an i n e x p e n s i v e c h o i c e b e c a u s e a b o u t $700 w o r t h o f m a r b l e s w o u l d p r o v i d e s u p p o r t and t h e r m a l i s o l a t i o n , and has t h e added a d v a n t a g e s o f e a s y t r a n s p o r t and m o d i f i c a t i o n o f t h e chamber. M a r b l e s a l s o r e d u c e t h e volume o f t h e vacuum chamber and t h e r e b y r e d u c e pump-down t i m e and e n e r g y . C o n t r o l o f t h e l a m i n a t o r c y c l e w i l l be v e r y f l e x i b l e due t o u s e o f f o u r i n d e p e n d e n t l y a d j u s t a b l e t i m e r s and a 24 s t e p i n d u s t r i a l c o n t r o l l e r w i t h 10 i n d i v i d u a l s w i t c h m o d u l e s . Materials

Research

D e s i g n o f a PV module t h a t w i l l w i t h s t a n d 2 0 y e a r s o f e x p o s u r e t o a v a r i e t y o f t e r r e s t r i a l e n v i r o n m e n t s c r e a t e s many problems. An FSA c o s t a l l o c a t i o n o f $14/m f o r e n c a p s u l a t i o n , s u p e r s t r a t e o r s u b s t r a t e and e d g e - s e a l / g a s k e t p l a c e s an a d d i t i o n a l b u r d e n on t h e e n c a p s u l a t i o n m a t e r i a l s , b e c a u s e t h e g l a s s s u p e r s t r a t e a l o n e has a p r o j e c t e d c o s t o f a b o u t $10/m2. D e t a i l s and b a c k g r o u n d on e a r l y m a t e r i a l s r e s e a r c h e f f o r t s have been p u b l i s h e d ( 3 - 7 ) . A d e t a i l e d discussion of present e n c a p s u l a t i o n m a t e r i a l s w i l l be p u b l i s h e d soon {8). This report c o v e r s t h e a p p l i c a t i o n t e s t i n g o f d e v e l o p e d m a t e r i a l s and o t h e r r e q u i r e m e n t s f o r s u c c e s s f u l vacuum l a m i n a t i o n . 2

Ethylene Vinyl Acetate System. The f i r s t new l a m i n a t i o n m a t e r i a l d e v e l o p e d by FSA was compounded by S p r i n g b o r n L a b o r a t o r i e s I n c . ( S L I ) f r o m an e t h y l e n e v i n y l a c e t a t e (EVA) f e e d s t o c k a v a i l a b l e f r o m Du P o n t . Compared w i t h P V B , EVA c o s t s a b o u t o n e - t h i r d a s much, has much l o w e r v i s c o s i t y a t p r o c e s s t e m p e r a t u r e , and has no h u m i d i t y - c o n t r o l r e q u i r e m e n t d u r i n g p r o c e s s i n g . E a r l y l a m i n a t o r e x p e r i e n c e uncovered problems w i t h c u r i n g and a d h e s i o n o f EVA. The o r i g i n a l m a t e r i a l f r o m S L I a l s o w o u l d block (adhere t o i t s e l f ) . Subsequent m a t e r i a l d e l i v e r e d from S L I a n d Du Pont d i d n o t b l o c k . The Du Pont m a t e r i a l had one w a f f l e d s u r f a c e , w h i c h enhanced a i r removal d u r i n g vacuum pumpdown T h e r e i s more t h a n one c o r r e c t c u r e c y c l e f o r EVA. Like most p o l y m e r s w i t h p e r o x i d e p r o m o t e r s , i t i s good p r a c t i c e t o r a i s e t h e b o n d - l i n e t e m p e r a t u r e r a p i d l y t o a v o i d p e r o x i d e decomp o s i t i o n b e f o r e an a d e q u a t e c u r e has been o b t a i n e d . One c y c l e t h a t has been p r o v e n u s e s two s t e p s , one a t 100°C f o r e v a c u a t i o n and a d h e s i o n , t h e o t h e r a t 150°C f o r l o n g - t e r m oven c u r e . This c y c l e p r o v i d e s a h i g h t h r o u g h p u t w i t h o n l y one l a m i n a t o r .


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A d e s c r i p t i o n of t h e c u r e c y c l e used f o r m a t e r i a l s t e s t i n g at our l a b o r a t o r y i s : E v a c u a t i o n f o r 5 m i n , t h e n 25 mi η o f c u r e w i t h t h e t o p chamber b l e d t o a t m o s p h e r e . During the 25-min c u r e , t h e f i r s t 8 - 1 0 min i s r e q u i r e d t o r a i s e t h e e n c a p s u l a n t t e m p e r a t u r e t o 1 6 0 ° C , where i t i s m a i n t a i n e d t o t h e end o f t h e cycle. Modules a r e t h e n removed w i t h o u t b e i n g c o o l e d . Modules f a b r i c a t e d w i t h t h i s c u r e c y c l e show even and c o m p l e t e c u r i n g and no b u b b l e s . A d h e s i o n i s a more d i f f i c u l t p r o b l e m . T h e r e a r e many c h e m i c a l l y d i f f e r e n t i n t e r f a c e s i n a laminant stack: glass-EVA, E V A - s o l a r c e l l s u r f a c e ( o x i d i z e d s i l i c o n o r some a n t i r e f l e c t i o n (AR) c o a t i n g ) , cell back s u r f a c e m e t a l l i z a t i o n - E V A , EVA-back s h e e t , and EVA-bus b a r s ( c o p p e r o r t i n n e d c o p p e r ) . Each o f t h e s e i n t e r f a c e s i s i m p o r t a n t , b e c a u s e m e c h a n i c a l l y good a d h e s i v e bonds w i l l o f t e n f a i l by d e l a m i n a t i o n a f t e r e x p o s u r e t o h u m i d i t y i n the f i e l d . Water v a p o r w i l l permeate t h r o u g h p o l y m e r s a n d , i f t h e r e i s a n o n - c h e m i c a l l y bonded s u r f a c e , w a t e r may c o l l e c t and c a u s e f a i l u r e by d i s p l a c e m e n t . T a b l e s I, I I , and I I I p r o v i d e d e t a i l s o f some o f t h e r e s e a r c h e f f o r t s i n a d h e s i o n . A m a t e r i a l s u r v e y was made u s i n g EVA and e t h y l e n e m e t h y l a c r y l a t e (EMA) e n c a p s u l a n t s w i t h K o r a d 6 3 0 0 0 , S c o t c h p a r 20CP and T e d l a r 200BS 30WH as back s h e e t s . T a b l e I shows t h e d e t a i l e d r e s u l t s of ths survey. P r i m e d and u n p r i m e d s u r f a c e s and a new Du Pont a d h e s i v e , 6 8 0 4 0 , were i n v e s t i g a t e d . T h i s s u r v e y showed good g l a s s b o n d i n g w i t h SLI P r i m e r Al 1 8 6 1 - 1 , (Dow C o r n i n g s i l a n e Z - 6 0 3 0 , 9 p a r t s ; N, N - d i m e t h y l - b e n z y l ami n e , 1 p a r t ; L u p e r s o l 101, 0.1 p a r t , and methanol 8 9 . 9 p a r t s ) . The o n l y back s h e e t t h a t a d h e r e d t o EVA was T e d l a r w i t h A d h e s i v e 6 8 0 4 0 . Earlier t e s t s showed good m e c h a n i c a l b o n d i n g t o u n t r e a t e d T e d l a r b u t poor humidity performance. B e c a u s e t h e g l a s s - t o - E V A i n t e r f a c e b o n d i n g p r o b l e m seemed t o be s o l v e d when a l l s a m p l e s e x h i b i t e d a d h e r e n t and p e r s i s t e n t b o n d s , t h e f o c u s o f t h e e f f o r t was s h i f t e d t o b a c k - s h e e t a d h e s i o n . An a d d i t i o n a l s e r i e s o f t e s t ( s e e T a b l e I I ) c o n f i r m e d t h e good r e s u l t s o f T e d l a r w i t h t h e Du P o n t a d h e s i v e 6 8 0 4 0 . K o r a d 63000 may be a u s e f u l m a t e r i a l , b u t c u r e t e m p e r a t u r e s d u r i n g l a m i n a t i o n c a u s e d some d e g r a d a t i o n . A d d i t i o n a l t e s t s on t h i s a c r y l i c s h e e t may be r u n . A p o l y e s t e r f i l m , S c o t c h p a r 2 0 C P , was i n t e r e s t i n g , b e c a u s e i t w o u l d be l e s s e x p e n s i v e t h a n a p o l y v i n y l f l u o r i d e f i l m , s u c h as T e d l a r . T h i s t e s t s e r i e s showed t h a t a new p r i m e r o r a d h e s i v e was needed f o r t h e S c o t c h p a r f i l m . F o r t u n a t e l y , E. P. Plueddemann o f Dow C o r n i n g C o r p . had a l r e a d y developed a primer f o r p o l y e s t e r f i l m s . The p r i m e r c o n s i s t s o f A m e r i c a n Cyanamid Cymel 3 0 3 , 90 p a r t s ; Dow C o r n i n g s i l a n e Z - 6 0 4 0 , 10 p a r t s ; and m e t h a n o l , 300 p a r t s . Peel t e s t s of S c o t c h p a r bonded t o EVA u s i n g t h i s p r i m e r were e x c e l l e n t . Unfor t u n a t e l y , t h i s system d i d not perform w e l l i n t h e 7-day c o l d w a t e r soak t e s t as shown i n T a b l e I I I .


In Polymers in Solar Energy Utilization; Gebelein, Charles G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. A l l 861-1 P r i m e r None A l l 861-1 P r i m e r None A l l 861-1 P r i m e r 68040

A l l 861 -1 None A l l 861 -1 None A l l 861 -1 None

K o r a d 63000

S c o t c h p a r 20CP




T e d l a r 200BS

EMA

EVA

EVA

EMA

EMA

EVA

EVA

EMA

EMA

4

5

6

7

8

9

10

11

12

30WH

T e d l a r 200BS

30WH A l l 861 - 1

None

T e d l a r 200BS

30WH

A l l 861 - 1

T e d l a r 200BS

A l l 861-1 Primer

68040 and

68040

A l l 861-1 P r i m e r

68040 and

Not

None

None

K o r a d 63000

EMA

3

30WH

10,000

Al 1861-1 P r i m e r

A l l 861 -1

K o r a d 63000

EVA tested

tested

tested

tested

tested

7,000

Not

5,100

Not

7,000

Not

4,300

Not

2,900

tested

2

Not

None

None

K o r a d 63000

EVA

EncapsulantGlass P e e l , g

1

Back-Sheet Ahesive

Glass Primer

Back Sheet

Encapsulant

M a t e r i a l Survey

Sample No.

T a b l e I.

but

4,400;

peel

peel

peel

broke

couldn't

Adherent;

couldn't

Adherent;

couldn't

g

brittle

broke Adherent;

3,200;

110

590-680

55

Adherent,

0

370

185

Back S h e e t Encasulant P e e l ,


In Polymers in Solar Energy Utilization; Gebelein, Charles G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 68040

A l l 861 -1

EMA

A-10

EVA

EVA

A-l 3

EMA

A-8

A-9

EVA

EVA

A-7

EVA

Gel

EMA

A-6

A-l 2

Gel

EVA

A-l Ί

K o r a d 63000

EMA

A-4

A-5

Combination

test

Combination

K o r a d 63000


Gel

EVA

Tedlar/68040/

EVA

Tedlar/68040/

Test

Test

30WH

T e d l a r 200BS

30WH

T e d l a r 200BS

K o r a d 63000

EVA

A-3

A l l 861 -1

A l l 861 -1

None

A l l 861 -1

A l l 861 -1

None

None

A l l 861 -1

Al 1861 -1

1359

1359

(68040)

(68040)

68040

68040

68040

68040

A l l 861 -1


EMA

A-2

A l l 861 -1

68040


Sheet A l l 861 -1

Adhesion Back-Sheet Adhesive

Back-Sheet Glass Primer

Back

EVA

Encap sulant

A-l

Sample No.

Table I I .

Adherent,

0

couldn't

Adherent;

couldn't

Adherent;

couldn't

Adherent;

couldn't

Adherent;

Adherent,

Adherent,

0

0

Peel

but b r i t t l e

peel

peel

peel

peel

but b r i t t l e

but b r i t t l e

Back-Sheet Encapsulant



b

a

No.

10

parts;

Peels Sample g i v e n away Peeled a f t e r cure Peeled a f t e r cure Adherent Adherent Adherent Adherent Adherent Adherent Adherent Adherent Adherent Peels Brittle Peels Peels Peels Peels a f t e r cure Peels Peels

Results]!

Du Pont Z - 6 0 4 0 ,

S c o t c h p a r 20CP S c o t c h p a r 20CP K o r a d 63000 K o r a d 63000 T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS S c o t c h p a r 20CP Acrylar Acrylar Acrylar Acrylar T e d l a r 100BG 30UT T e d l a r 200BS T e d l a r 200BS

Cymel Cymel Cymel Cymel 68040 68040 68040 68040 68040 68040 68040 68040 68040 Cymel Cymel A - l 1861-1 Cymel A - l 1861-1 68040 68040 68040

EVA EVA EVA EVA EVA EVA EVA EVA EVA EVA EVA EVA EVA EMA EVA EVA EMA EMA EMA None None

Sheet

Back

7-Day Water Soak T e s t

PrimerfL

of

Encapsulant

Results

P r i m e r c o n s i s t s o f Cymel 303 ( A m e r i c a n C y a n a m i d ) , 90 p a r t s ; m e t h a n o l ; 300 p a r t s . A f t e r 7-day soak u n l e s s o t h e r w i s e n o t e d .

C-l C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-ll C-l 2 C-13 E-l E-2 E-3 E-4 E-5 E-7 E-8 E-9

Coupon

Table III.



416

POLYMERS

IN

SOLAR E N E R G Y

UTILIZATION

A n o t h e r p r i m e r s y s t e m was s u g g e s t e d . T h i s p r i m e r was made f r o m Monsanto Resimene 7 4 0 , 2 3 . 5 p a r t s ; D o w - C o r n i n g S i l a n e Z - 6 0 4 0 , 1 . 2 5 p a r t s ; and a n h y d r o u s i s o p r o p a n o l , 75 p a r t s . The i n i t i a l p e e l t e s t s were e x c e l l e n t and r e s i s t a n c e t o 7-day c o l d w a t e r soak was f a i r t o g o o d . F u r t h e r d i s c u s s i o n w i t h Plueddemann l e d t o t h e a d d i t i o n o f 1.25 p a r t s o f D o w - C o r n i n g S i l a n e Z - 6 0 3 0 t o p r o v i d e d o u b l e bond f o r i m p r o v e d a d h e s i o n t o t h e EVA. Again, i n i t i a l p e e l t e s t s were e x c e l l e n t . Water soak r e s i s t a n c e was good t o v e r y good on t h o s e s a m p l e s where t h e p r i m e r had been d i l u t e d 10 t o 1 w i t h a n h y d r o u s i s o p r o p a n o l . Additional tests a r e p l a n n e d w i t h t h e p o s s i b l e m o d i f i c a t i o n o f a s m a l l amount o f L u p e r s o l 101 t o i m p r o v e t h e EVA bond a g a i n . EVA does not bond w e l l t o a l l m e t a l s . Copper i s p a r t i c u l a r l y d i f f i c u l t , which poses a problem: copper i s the best c a n d i date f o r low-cost p h o t o v o l t a i c c e l l m e t a l l i z a t i o n . Some p r i m e r t e s t s were run u s i n g two p r i m e r s . The f i r s t was z i n c c h r o m a t e p o w d e r , 10 p a r t s ; D o w - C o r n i n g S i l a n e Z - 6 0 3 0 , 9 . 9 p a r t s ; N, N - d i m e t h y l b e n z y l ami n e , 0 . 1 p a r t ; and m e t h a n o l , 30 p a r t s . Because t h e z i n c chromate i s opaque and d i f f i c u l t t o keep i n s u s p e n s i o n , the second primer o m i t t e d i t . The p r i m e r w i t h o u t t h e z i n c c h r o m a t e gave e x c e l l e n t i n i t i a l adhesion i f thoroughly wiped a f t e r a p p l i c a t i o n . Additional t e s t s were made a f t e r 10 t o 1 d i l u t i o n w i t h m e t h a n o l . Again w i p i n g was r e q u i r e d and i n i t i a l p e e l t e s t s were e x c e l l e n t . A d h e s i o n a f t e r w a t e r soak was e x c e l l e n t . T e s t s were a l s o made on c o p p e r w h i c h had been f u s i o n s o l d e r p l a t e d , z i n c p l a t e d , or n i c k e l p l a t e d . None o f t h e s e showed good i n i t i a l adhesion. Ethylene Methyl A c r y l a t e System. EMA, new e n c a p s u l a n t a d h e s i v e , i s under development. P r e l i m i n a r y work showed e x c e l l e n t a d h e s i o n o f EMA t o g l a s s when t h e g l a s s i s p r i m e d w i t h Al 1 8 6 1 - 1 . Long-term soaking i n c o l d water reduced the a d h e s i o n . Additional work and samples a r e n e e d e d . T e s t s w i t h T e d l a r and A d h e s i v e 68040 showed a d h e s i o n t o EMA. However, a d h e s i o n a f t e r c o l d - w a t e r soak was p o o r . T h i s problem i s being i n v e s t i g a t e d . The Cymel p r i m e r t h a t was used t o bond EVA t o a p o l y e s t e r was a l s o t r i e d i n b o n d i n g EMA t o a p o l y e s t e r . T h i s system a l s o degraded a f t e r l o n g - t e r m s o a k i n g i n c o l d w a t e r . K o r a d 63000 has a d h e r e d t o EMA, but t h e r e s u l t i n g back s h e e t was b r i t t l e . EMA a d h e s i o n t o c o p p e r w h i c h has been p r i m e d as above was e x c e l l e n t b o t h i n i t i a l l y and a f t e r t h e 7-day w a t e r soak t e s t . Testing

Methods

A g e l t e s t , recommended by S L I , was made on EVA coupon s a m p l e s p r o d u c e d when t h e o r i g i n a l f o u r modules were made. B e c a u s e u n c r o s s l i n k e d EVA i s s o l u b l e i n t o l u e n e , w e i g h e d s a m p l e s



25.

BURGER AND CUDDIHY

Vacuum Lamination

417

were p l a c e d i n 60°C t o l u e n e f o r 2 h , and t h e r e s u l t a n t s o l u t i o n and sample was p o u r e d t h r o u g h w e i g h e d f i l t e r p a p e r . After f i l t r a t i o n , t h e samples were d r i e d i n a 90°C c i r c u l a t i n g - a i r oven f o r 5 h . The p e r c e n t a g e o f EVA r e m a i n i n g i s a measure o f t h e degree o f g e l a t i o n or c r o s s l i n k i n g d u r i n g c u r e . SLI s p e c i f i e s a n o m i n a l 80% g e l w i t h 65% as t h e l o w e r l i m i t . The g e l t e s t on t h e s a m p l e s p r o d u c e d above showed b e t t e r t h a n 95% g e l a t i o n . P e e l - t e s t s a m p l e s were p r e p a r e d by c u t t i n g t h r o u g h t h e l a y e r t o be t e s t e d u s i n g a 0 . 2 5 - i n . - w i d e t e m p l a t e . The d e s i r e d l a y e r was t h e n p e e l e d back by c u t t i n g when n e c e s s a r y . Peel s t r e n g t h was measured u s i n g a U n i t e k M i c r o p u l l I , Model 6 - 0 9 2 . Several a d h e r e n t s a m p l e s had a c o h e s i v e s t r e n g t h above t h e 5 - 1 b l i m i t o f t h e t e s t e q u i p m e n t , s o t h e s e s a m p l e s were t e s t e d u s i n g a c a l i brated spring s c a l e . In c a s e s where t h e a d h e s i o n i s e x c e l l e n t i t was n o t e d t h a t a p e e l - t e s t sample c o u l d n o t a l w a y s be p r e p a r e d . These s i t u a t i o n s a r e n o t e d i n t h e a t t a c h e d t a b l e s . P e r f o r m i n g a p e e l t e s t a f t e r l a m i n a t i o n s h o u l d be c o n s i d e r e d as o n l y a good s c r e e n i n g t e s t ; i t i s not s u f f i c i e n t f o r m a t e r i a l selection. Plueddemann recommends a 7 - d a y soak i n r o o m - t e m p e r a t u r e w a t e r a s an a d d i t i o n a l t e s t , w i t h f i n a l p e e l t e s t s demon s t r a t i n g cohesive f a i l u r e r a t h e r than adhesive f a i l u r e (Reference 9). A l l o f t h e l a m i n a n t s made a t J P L have been s u b j e c t e d t o t h e 7-day room-temperature water-soak t e s t . T a b l e I I I summarizes t h e r e s u l t s o f 7-day r o o m - t e m p e r a t u r e w a t e r - s o a k t e s t s . Other

Lamination-Related

Efforts

The f i r s t l a m i n a t i o n e f f o r t s were m e c h a n i c a l l y s u c c e s s f u l b u t v i s u a l l y u n s u c c e s s f u l . Many b u b b l e s and v o i d s were f o u n d t h a t were r e l a t e d t o s o l d e r j o i n t s . A n o t h e r v i s u a l p r o b l e m was c e l l m i s a l i g n m e n t and p o o r p l a c e m e n t . Both o f t h e s e problem a r e a s were n o t c a u s e d by t h e l a m i n a t i o n m a t e r i a l s b u t w o u l d have a p r o f o u n d e f f e c t on t h e m a r k e t a b i l i t y and f i e l d s e r v i c e l i f e of the f i n a l laminated product. S o l d e r - F l u x Removal. Removal o f s o l d e r i n g f l u x r e s i d u e s i s an e s t a b l i s h e d p r o c e s s i n t h e p r i n t e d - c i r c u i t - b o a r d and e l e c tronic-assembly industries. The q u a l i t y o f t h e l a m i n a t i o n p r o c e s s i s dependent upon c h e m i c a l b o n d i n g o f a l l s u r f a c e s w i t h i n t h e l a m i n a n t , so v e r y c l e a n c e l l - s t r i n g a s s e m b l i e s a r e r e q u i r e d . P r o p e r removal o f f l u x r e s i d u e s r e q u i r e s s o l v e n t s t h a t c a n remove b o t h p o l a r and n o n - p o l a r s o l u b l e c o n t a m i n a n t s , s o u s e o f p r o p r i e t a r y f l u x - r e m o v a l s o l v e n t s was i n d i c a t e d . B e c a u s e c e l l i n t e r c o n n e c t s p r o v i d e f l u x t r a p s ( e x p e c i a l l y t h e M o t o r o l a I n c . and ARCO S o l a r c o m b i n a t i o n b u s - b a r - i n t e r c o n n e c t d e s i g n s ) , i t was d e c i d e d t o t r y u l t r a s o n i c c l e a n i n g f o l l o w e d by v a p o r d e g r e a s i n g . S i x c e l l - s t r i n g a s s e m b l i e s f o r m i n i m o d u l e s and f o u r a s s e m b l i e s f o r 1 χ 4 - f t modules were f i r s t c l e a n e d i n K e s t e r 5345 R o s i n R e s i d u e Remover u s i n g a S o n i x IV Model S S - 1 0 4 U l t r a s o n i c C l e a n e r .



418

POLYMERS

IN

SOLAR

ENERGY

UTILIZATION

S u b s e q u e n t l y , t h e s e same c e l l - s t r i n g a s s e m b l i e s were c l e a n e d i n K e s t e r 5120 v a p o r d e g r e a s i n g s o l v e n t u s i n g an E l e c t r o v e r t , Inc., D e g r e s t i l Model L C D - 1 8 v a p o r d e g r e a s e r . The a s s e m b l i e s were f i r s t i n t r o d u c e d t o t h e v a p o r z o n e , t h e n were d i p p e d i n t h e c o l d s o l v e n t t a n k and f i n a l l y were removed s l o w l y t h r o u g h t h e v a p o r z o n e . These c e l l s t r i n g s showed no d e l a m i n a t e d a r e a s o r b u b b l e s a f t e r b e i n g l a m i n a t e d w i t h EVA. C e l l s t r i n g s t h a t were o n l y swab c l e a n e d f o r f l u x removal showed b o t h b u b b l e s and d e l a m i n a t i o n when l a m i n a t e d u s i n g i d e n t i c a l p r o c e s s p a r a m e t e r s . B e c a u s e f l u x i s s u c h a c o n c e r n , one c o n t r a c t o r i s e x p l o r i n g u l t r a s o n i c b o n d i n g ( 1 0 ) u s i n g p r e p u n c h e d aluminum i n t e r c o n n e c t s t h a t are attached t o e l e c t r o p l a t e d copper c e l l m e t a l l i z a t i o n w i t h a seam w e l d e r . Others are examining f l u x l e s s bonding con c e p t s , s u c h as v a p o r - p h a s e s o l d e r r e f l o w . Conclusions The (1)

(2)

(3)

(4) (5)

(6)

f o l l o w i n g c o n c l u s i o n s can be drawn f r o m e f f o r t s

to

date:

Vacuum l a m i n a t i o n i s an a c c e p t a b l e p r o c e s s f o r m a n u f a c t u r i n g v o i d - f r e e PV m o d u l e s , i f matched w i t h c o r r e c t m a t e r i a l s and u s e d w i t h a q u a l i f i e d c u r e c y c l e . C o n c e p t u a l d e s i g n o f a l a r g e (4 χ 4 - f t ) vacuum l a m i n a t o r i n d i c a t e s t h e p o t e n t i a l f o r an i n e x p e n s i v e p i e c e of c a p i t a l equipment. M a t e r i a l r e s e a r c h by t h e E n c a p s u l a t i o n Task o f FSA has been a p p l i e d t o a c t u a l l a m i n a t e d s y s t e m s w i t h good results. One l a m i n a n t s y s t e m has been d e v e l o p e d t h a t shows e x c e l l e n t a d h e s i o n and r e s i s t a n c e t o d e l a m i n a t i o n a f t e r being soaked f o r 7 days i n c o l d w a t e r . Another system i s n e a r i n g f i n a l a c c e p t a n c e . Gel t e s t s a r e u s e f u l i n d e t e r m i n i n g p r o p e r c u r e c y c l e s . P e e l t e s t s as a measure o f l a m i n a t e a d h e s i o n a r e o n l y partially useful. Most l a m i n a n t s y s t e m s e x h i b i t e i t h e r v e r y low o r v e r y h i g h a d h e s i o n a f t e r a 7-day soak i n cold water. The soak t e s t may n o t be a s u f f i c i e n t p r e d i c t o r f o r 2 0 - y r s e r v i c e l i f e ; h o w e v e r , i t may be c o n s i d e r e d as a s c r e e n i n g t e s t f o r s y s t e m s t h a t s h o u l d receive additional e f f o r t . C o m p l e t e removal o f s o l d e r f l u x i s c o n s i d e r e d n e c e s s a r y t o ensure long-term laminant a d h e s i o n . A process change t o a v o i d s o l d e r f l u x i s t h e r e f o r e e n c o u r a g e d .

Acknowledgments The e f f o r t r e p o r t e d i n t h i s p a p e r i s b a s e d upon many c o n c e p t s and m a t e r i a l s d e v e l o p e d by t h e E n c a p s u l a t i o n Task o f FSA. Paul W i l l i s of S p r i n g b o r n L a b o r a t o r i e s , I n c . , c o n t r i b u t e d b a c k g r o u n d i n f o r m a t i o n on t h e p r o c e s s i n g and t e s t i n g o f t h e m a t e r i a l s developed at Springborn.


25.

BURGER AND CUDDIHY

Vacuum Lamination

419

E. P. Plueddemann o f Dow C o r n i n g C o r p . c o n c e i v e d t h e c o u p l i n g a g e n t s and c h e m i c a l - b o n d i n g p h i l o s o p h y so v i t a l t o t h i s effort. Many o t h e r i n d u s t r y t e c h n o l o g i s t s a l s o c o n t r i b u t e d .


The r e s e a r c h d e s c r i b e d i n t h i s p u b l i c a t i o n was c a r r i e d o u t by t h e J e t P r o p u l s i o n L a b o r a t o r y , C a l i f o r n i a I n s t i t u t e o f T e c h n o l o g y , and was s p o n s o r e d by t h e U n i t e d S t a t e s Department o f Energy t h r o u g h an agreement w i t h t h e N a t i o n a l A e r o n a u t i c s and Space Administration.

Literature Cited 1. 2. *3. *4. *5.

*6. 7. 8. *9.

10.

D'Aiello, R. V., Quarterly Report No. 5, RCA Laboratories, DOE/JPL-954868-79/2, March 1979. Somberg, H., Quarterly Report No. 2, ARCO Solar, Inc., DOE/JPL-955278-79/2, July 8, 1978. Cuddihy, E., Encapsulation Material Trends Relative to 1986 Cost Goals, JPL Internal Document No. 5101-61, Pasadena, California, April 13, 1978. Maxwell, H . , Encapsulation Candidate Materials for 1982 Cost Goals, JPL Internal Document No. 5101-72, Pasadena, California, June 15, 1978. Cuddihy, E. (JPL), Baum, B., and Willis, P. (Springborn Laboratories, Low-Cost Encapsulation Materials for Terres trial Solar Cell Modules, JPL Internal Document No. 5101-78, Pasadena, California, September 1978. Cuddihy, E., Encapsulation Materials Status to December 1979, JPL Internal Document No. 5101-144, Pasadena, California, January 15, 1980. Bouquet, F . , Glass for Low-Cost Photovoltaic Solar Arrays, JPL Document No. 5101-147, Pasadena, California, February 1, 1980. (JPL Publication 80-12, DOE/JPL 1012-40). Photovoltaic Module Encapsulation Design and Material Selec tion, JPL Document NO. 5101-177 (in press). (JPL Publication 81-10, DOE/JPL 1012-60). Plueddemann, E. P., Dow Corning Corp., Chemical Bonding Technology for Terrestrial Solar Cell Modules, JPL Internal Document No. 5101-132, Pasadena, California, September 1, 1980. Rose, C. Μ., Quarterly Report No. 1, Westinghouse Electric Corp., AESD, DOE/JPL-955909-81/1.

RECEIVED November 22,1982


Vacuum Lamination of Photovoltaic Modules - ACS Publications

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