Encapsulant Material Requirements for Photovoltaic Modules

23 Encapsulant Material Requirements for Photovoltaic Modules Downloaded by NORTH CAROLINA STATE UNIV on October 4, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch023

K. J. LEWIS ARCO Solar, Inc., Research and Development, Woodland Hills, CA 91367

Encapsulants are necessary for electrical isolation of the photovoltaic c i r c u i t . They also provide mechanical protection for the solar cell wafers and corrosion protection for the metal contacts and circuit interconnect system over the 20-year design life of a photovoltaic array. The required components include the solar cell circuit, the rigid or structural member, the pottant, and the outer cover/insulator. Surface modifications may be needed to develop strong, stable bonds at the interfaces in the composite. If the module is to be framed, edge sealants may also be required. The functions of the individual components and the performance requirements as they are now known are described. Costs are compared where possible and candidate materials identified. In the next few years, as lower-cost solar c e l l s are developed, encapsulation materials will become a dominant cost in a finished photovoltaic (PV) module. Encapsulants are necessary for electrical isolation, mechanical protection of the cells, and corrosion protection of the metal contacts and interconnect system for more than 20 years of outdoor exposure in even the most severe terrestrial climate. All PV systems, regardless of how inert, tough, pliable, and weatherable the actual cells themselves are, need encapsulation. While improvements in encapsulant materials cannot yield orders of magnitude in cost reduction, judicious engineering design and materials development can r e s u l t i n significant cost savings and performance improvements. It is only with such cost-reduced arrays that the domestic photovoltaic rooftop market has a high probability of developing. Figures 1 and 2 compare a commercial module and an advanced rooftop module design. Each design has a minimum of three components in addition to the cell circuit. They are the rigid or 0097-6156/83/0220-0367$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.

Downloaded by NORTH CAROLINA STATE UNIV on October 4, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch023

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Commercial Module,

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

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w i n d - l o a d - b e a r i n g member, 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 . I n a d d i t i o n , i f s t r u c t u r a l elements i n the plane o f t h e c i r c u i t a r e metal o r i f c i r c u i t connections must o v e r l a p , additional non-softening e l e c t r i c a l l y i n s u l a t i n g layers a r e needed. A d h e s i v e s , p r i m e r s o r o t h e r s u r f a c e m o d i f i c a t i o n s may a l s o be needed t o d e v e l o p s t r o n g , s t a b l e bonds a t t h e v a r i o u s i n t e r f a c e s i n t h e composite. Edge s e a l a n t s may be needed i f the module i s t o be framed.


R i g i d Member The r i g i d o r w i n d - l o a d - b e a r i n g member o f t h e c o m p o s i t e prevents f l e x u r e o f b r i t t l e c e l l s beyond the f r a c t u r e p o i n t . I n a d d i t i o n t o r e q u i r i n g a h i g h f l e x u r a l modulus (> ΙΟ**, p r e f e r a b l y > 10? p s i ) t h e r i g i d e n t i t y s h o u l d be l o w c o s t (< $ 1 . 0 0 / f t , p r e f e r a b l y < $ 0 . 5 0 / f t ) and o f minimum weight. I t must be weather r e s i s t a n t (> 20 y e a r s ) and must have a r e l a t i v e l y l o w t h e r m a l expansion c o e f f i c i e n t t h a t i s as near as p o s s i b l e t o t h a t o f the c e l l material. This i s i n order not t o put e x c e s s i v e ( f a t i g u i n g l e v e l ) m e c h a n i c a l s t r e s s on the c e l l s u r f a c e e l e c t r i c a l c o n t a c t s from the f o r c e s o f d i f f e r e n t i a l thermal expansion over t h e d a i l y thermal c y c l i n g o f a PV a r r a y . The thermal expansion c o e f f i c i e n t of c r y s t a l l i n e s i l i c o n i s q u i t e low a t 3 x 10"^ °C~ . Optical Weight and P e r m e a b i l i t y R e q u i r e m e n t s . I f the s t r u c t u r a l member i s the f r o n t cover o f t h e module ( s u p e r s t r a t e c o n f i g u r a t i o n ) , i t must be o p t i c a l l y c l e a r Ο 9 0 Î t r a n s m i s s i o n ) through the s o l a r spectrum o f i m p o r t a n c e t o a b s o r p t i o n by t h e s o l a r c e l l ( 0 . 4 - 1 . 1 m i c r o n s ) . I t must a l s o be r e l a t i v e l y hard (> 90 shore A durometer), s o i l r e p e l l e n t , p r e f e r a b l y UV absorbing below 0.36-0.37 microns, and non-permeable t o oxygen, water vapor, and atmospheric p o l l u t a n t s . I f t h e r i g i d member i s t h e back cover, i t may be opaque. C a n d i d a t e s , Cost. Tempered g l a s s i s t o date the best known material f o r a r i g i d f r o n t cover f o r s i l i c o n c e l l s . I t costs ~ $ 0 . 7 5 - $ 1 . 2 5 / f t f o r t h e l o w - i r o n c o n t e n t g l a s s i d e a l f o r PV a p p l i c a t i o n s . F o r a r i g i d back c o n f i g u r a t i o n , s u r f a c e p a s s i v a t e d s t e e l p r e s e n t l y a p p e a r s t o be t h e optimum choice f o r use w i t h s i l i c o n c e l l s , considering i t s thermal expansion c o e f f i c i e n t , w e a t h e r r e s i s t a n c e , a n d c o s t f o r t h e w e i g h t and s t i f f n e s s required. I t can range from $ 0 . 3 0 / f t f o r z i n c g a l v a n i z e d t o $3·00 for porcelainized. P a s s i v a t e d s t e e l i s a l s o non-permeable. Glass r e i n f o r c e d concrete, sealed hardwood, and aluminum are a l s o l o w - c o s t p o s s i b i l i t i e s , b u t each has s i g n i f i c a n t disadvantages compared t o s t e e l i n t h e a r e a s o f w e i g h t o r t h e r m a l e x p a n s i o n coefficients. Weather and C o r r o s i o n R e s i s t a n c e Requirements. There are many commercial ways t o " r u s t - p r o o f " s t e e l t o make i t weather resistant. They range from p a i n t , which i s u s u a l l y the cheapest but l e a s t e f f e c t i v e method, t o chromeplating, which i s one o f the most e x p e n s i v e and e f f e c t i v e methods. I n between, i n both cost 2

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and performance, are p o r c e l a i n enameling and v a r i o u s l o w m e l t i n g m e t a l d i p c o a t i n g s such as t i n , z i n c , aluminum, o r combinations thereof. P o r c e l a i n i z i n g i s on the expensive side (over $ 1 . 0 0 / f t , even i n l a r g e volume) b u t i s one o f the o l d e s t , most e f f e c t i v e ways t o p r o t e c t the s t e e l . Zinc g a l v a n i z i n g i s t h e most common lower cost method. M o s t o r d i n a r y " r u s t p r o o f " s t e e l i s n o t adequate as a substrate f o r solar c e l l s . The p r o t e c t i v e l a y e r must s t o p n o t o n l y g r o s s r u s t i n g , which would cause t o t a l delamination, but i t must a l s o h a l t even the s l i g h t e s t progress o f c o r r o s i o n o f e i t h e r the s t e e l o r any a c t i v e c o a t i n g metals on i t once the piece i s laminated t o the s o l a r c e l l s . T h i s i s because e v e n s l i g h t c o r r o s i o n g e n e r a t e s s m a l l amounts o f hydrogen long before the c o r r o s i o n l a y e r b u i l d s up s u f f i c i e n t l y t o cause adhesive f a i l u r e . I f hydrogen i s generated f a s t enough and i f the pottant l a y e r i s r e l a t i v e l y s o f t , t h e gas c o l l e c t s as bubbles behind t h e impermeable c e l l s o r g l a s s f r o n t . These bubbles not only become s t r e s s p o i n t s on the c e l l s l e a d i n g t o w a f e r f r a c t u r e , b u t t h e y a l s o reduce the d i e l e c t r i c standoff of the i n t e r v e n i n g 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 s and become peel s t r e s s p o i n t s f o r delamination propagation w i t h thermal c y c l i n g . Thus, no c o r r o s i o n can be t o l e r a t e d . P o r c e l a i n i z i n g i s t h e b e s t method f o r e l i m i n a t i n g s t e e l c o r r o s i o n . I t can be used only on r i g i d s u r f a c e s , however, s i n c e i t cracks w i t h very l i t t l e f l e x i n g . I t s e l e c t r i c a l p r o p e r t i e s a r e i n t r i n s i c a l l y very good but are f r e q u e n t l y degraded by c r a c k s , d i r t and p i n h o l e s . I t must a l s o be p r o p e r l y f i r e d s i n c e f i r i n g temperatures a f f e c t i t s s u r f a c e c h a r a c t e r i s t i c s f o r b o n d i n g t o s o l a r c e l l p o t t i n g polymers. I n s u f f i c i e n t l y f i r e d f i l m s are b a s i c enough t o r a p i d l y h y d r o l y z e t h e v i n y l a c e t a t e e s t e r g r o u p s i n e t h y l e n e / v i n y l acetate. Adhesion i s l o s t very q u i c k l y , w i t h r a p i d generation o f a c e t i c a c i d a t the i n t e r f a c e . P a i n t over z i n c g a l v a n i z i n g has been found t o b l i s t e r e a s i l y when exposed t o high humidity (100$ a t elevated temperatures up t o 100°C). P l a s t i c f i l m l a m i n a t e d t o z i n c g a l v a n i z i n g does not b l i s t e r as e a s i l y , but o u t g a s s i n g s t i l l occurs r e a d i l y . B l i s t e r i n g s t a r t s a t t h e c e l l o r module edges and moves i n . E v e n t u a l l y enough "white r u s t " develops t o produce d e l a m i n a t i o n . The q u a l i t y o r t y p e o f z i n c g a l v a n i z i n g such as d e g r e e o f p a s s i v a t i o n (chromate coatings, etc.) o r g r a i n s i z e t o a f f e c t the z i n c c o r r o s i o n r a t e s , w h i c h a r e q u i t e r a p i d i n even the best cases. Zinc/aluminum a l l o y s and aluminum coatings s t i l l corrode, but not as much as z i n c alone on s t e e l . Thin F i l m C e l l s . Future, l o w e r c o s t s o l a r c e l l m a t e r i a l s w i l l l i k e l y be more f l e x i b l e t h a n c r y s t a l l i n e s i l i c o n and t h e r e f o r e may not r e q u i r e a r i g i d member i n t h e module l a y up. They w i l l s t i l l need e l e c t r i c a l i s o l a t i o n and p r o t e c t i o n from abrasion and c o r r o s i o n , however, and w i l l thus s t i l l need p o t t a n t or p r o b a b l y t h i n n e r a d h e s i v e l a y e r s as w e l l as o u t e r covers/insulators.


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Pottant The p o t t a n t i s t h e s o f t , e l a s t o m e r i c , vibration-damping m a t e r i a l that immediately surrounds both s i d e s o f f r a g i l e s o l a r c e l l w a f e r s 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 p r o t e c t s t h e c e l l s from s t r e s s e s due t o t h e r m a l expansion d i f f e r e n c e s and e x t e r n a l impact. I t i s o l a t e s them e l e c t r i c a l l y and helps p r o t e c t t h e i r m e t a l l i c contacts and i n t e r c o n n e c t s f r o m corrosion. O p t i c a l Requirements, Cost. The p o t t a n t must be h i g h l y transparent (> 90$ from 0.4 t o 1.1 μ ) , s e r v i n g a s an o p t i c a l c o u p l i n g medium t o provide maximum l i g h t t r a n s m i s s i o n t o the c e l l surface. Because i t i s used i n a f a i r l y t h i c k l a y e r f o r b r i t t l e c e l l s (10-20 m i l s on each c e l l s i d e ) , the pottant must be very inexpensive ( < $ 0 . 3 0 / f t , p r e f e r a b l y < $ 0 . 2 0 / f t ) . At 30 m i l s t o t a l , t h i s t r a n s l a t e s t o between $1.00 and $2.00/lb, i n c l u d i n g compounding and f a b r i c a t i n g i n t o PV f a c t o r y u s a b l e f o r m . No i n h e r e n t l y weather r e s i s t a n t m a t e r i a l i s i n t h i s p r i c e range, but s e v e r a l moderately s t a b l e m a t e r i a l s which can be upgraded w i t h s t a b i l i z e r s a r e i n t h i s range. Mechanical Requirements. The pottant m a t e r i a l should have a r e l a t i v e l y low modulus (< 2000 p s i a t 2 5 ° C ) . The maximum t o l e r a b l e m o d u l u s d e p e n d s on t h e d i f f e r e n c e i n e x p a n s i o n c o e f f i c i e n t s o f t h e c e l l s and t h e r i g i d member a n d o n t h e t h i c k n e s s o f t h e l a y e r between them. R e l a t i v e l y high modulus rubbers could be used but would r e q u i r e i n o r d i n a t e l y t h i c k and thus expensive l a y e r s t o damp out the expansion d i f f e r e n c e s . For example, w i t h an 1 / 8 - i n . - t h i c k g l a s s s u p e r s t r a t e and s i l i c o n c e l l s , which w i l l take 5000 p s i maximum l i n e a r s t r e s s , a pottant o f 1000 p s i modulus needs t o be a minimum o f o n l y 1 .5 m i l s each s i d e f o r a 1:1 s a f e t y f a c t o r i n s e r v i c e ; with a 2500 p s i modulus the minimum i s - 3.5 m i l s , e t c . A s a f e t y f a c t o r higher than 1:1 i s highly desirable. F a b r i c a t i o n technique i s a l s o a f a c t o r . For example, whether the c e l l s t r i n g s are pressed a g a i n s t the pottant while i t i s s t i l l c o l d and must cushion the c e l l s under pressure, or whether i t i s squeezed only w h i l e i t i s molten, or not a t a l l , can determine the minimum t h i c k n e s s t o l e r a n c e s f o r module f a b r i c a t i o n . The minimum usable pottant thickness can a l s o be l i m i t e d by the green s t r e n g t h of the m a t e r i a l i t s e l f i f i t i s f a b r i c a t e d as a c a s t sheet. If i t i s e x t r u s i o n - c o a t e d on t h e s u p p o r t and c o v e r m a t e r i a l s , t h e pottant can be l e s s tough and t h e r e f o r e t h i n n e r , but must be f r e e o f p i n h o l e s and o t h e r f l a w s t o p r e v e n t e l e c t r i c a l leakage. E x p e r i e n c e suggests a minimum thickness o f 10-15 m i l s t o achieve the n e c e s s a r y freedom f r o m f l a w s f o r s u f f i c i e n t electrical i n s u l a t i o n p r o p e r t i e s as w e l l as ease o f handling. The p o t t i n g m a t e r i a l must have a g l a s s t r a n s i t i o n temperature b e l o w t h e l o w e s t t e m p e r a t u r e e x t r e m e t h e PV module m i g h t experience, which i s — 4 0 ° C . The m a t e r i a l must remain rubbery i n o r d e r t o damp i m p a c t s a n d v i b r a t i o n o f t h e f r a g i l e c e l l s . 2

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S i m i l a r l y , i t must e x h i b i t no s i g n i f i c a n t mechanical creep at the upper o p e r a t i n g t e m p e r a t u r e extreme o f 90°C i n o r d e r f o r the layup, c e l l p o s i t i o n s , e t c . to r e m a i n i n t a c t when t i l t e d a t an angle f a c i n g the sun. The pottant must e x h i b i t strong, m o i s t u r e - r e s i s t a n t adhesion (>10 l b / i n . peel strength) to a l l the s u r f a c e s i t must bond t o , over a 20-year l i f e t i m e . The 10 l b / i n . peel strength may decrease to 5 l b / i n . while the p o t t a n t or a d h e s i v e s a r e s a t u r a t e d w i t h water i n a non-hermetic design as long as i t recovers to w i t h i n 10$ o f the o r i g i n a l v a l u e when r e d r i e d a t up t o the n o m i n a l o p e r a t i n g c e l l temperature of the p a r t i c u l a r design. Moisturer e s i s t a n t adhesion i s tested mostly by exposure to 100°C/100$ RH. Ten months a t these c o n d i t i o n s would equal 20 years at 70°C, 50$ RH, but u s u a l l y the f i r s t week or two w i l l separate the good bonds from the poor ones. Exposure to b o i l i n g water f o r a few hours or overnight i s a l s o a good i n i t i a l screening technique. I n a vacuum l a m i n a t i o n p r o c e s s , the s t e e p e r the m e l t v i s c o s i t y / t e m p e r a t u r e curve f o r sheet p o t t a n t m a t e r i a l , the better. The l a y e r s need t o be dry and n o n - t a c k y d u r i n g t h e i n i t i a l evacuation step so as not to trap a i r between them. At the same time, the pottant must then melt to as f l u i d a s t a t e as p o s s i b l e i n o r d e r t o e f f e c t i v e l y p e n e t r a t e and wet a l l t h e i r r e g u l a r i t i e s o f the c e l l c i r c u i t . Block or g r a f t thermoplastic elastomers with r e l a t i v e l y low m o l e c u l a r w e i g h t amorphous segments o f a weather r e s i s t a n t saturated backbone have the p o t e n t i a l of being s u p e r i o r p o l y m e r s for potting solar cells. The c r o s s l i n k f o r m i n g c r y s t a l l i n e segments make r e l a t i v e l y s o f t , low m o l e c u l a r w e i g h t , r u b b e r y polymers handle w e l l . They e x h i b i t h i g h cohesive strength or toughness and low s u r f a c e tack when the c r y s t a l l i n e domains a r e s o l i d i f i e d (see Figure 3 ) . Candidates, Free r a d i c a l polymerized vinyl or a c r y l i c / e t h y l e n e copolymers made i n h i g h p r e s s u r e p o l y e t h y l e n e r e a c t o r s have been shown by E. Cuddihy to be block polymers of pure c r y s t a l l i n e homopolyethylene and amorphous high v i n y l acetate (- 70 weight $) or methyl a c r y l a t e - c o - e t h y l e n e segments. When the c r y s t a l l i t e s are submicron i n s i z e as i n DuPont's Elvax 150, they do not s i g n i f i c a n t l y s c a t t e r the i n c o m i n g l i g h t . A number of l a b o r a t o r i e s have shown, however, t h a t even when some l i g h t s c a t t e r i n g o c c u r s , i t does not n e c e s s a r i l y decrease s o l a r c e l l output. For example, i t was found that e n c a p s u l a t i n g a 3 - m i l t h i c k non-woven g l a s s mat i n 15 m i l s o f e t h y l e n e / v i n y l acetate (EVA) drops the t r a n s m i s s i o n by 60$. The same c o m p o s i t e when fused to the f r o n t of a textured ( a n t i r e f l e c t i v e treated) s i l i c o n s o l a r c e l l does not drop the output a t a l l unless d i s c o l o r i n g from d e g r a d a t i o n t a k e s p l a c e . Aromatic c r y s t a l l i n e segments such as polystyrene are undesirable even i n i t i a l l y because the degradation products are l i g h t absorbing. A G u l f O i l ethylene/methyl a c r y l a t e rubber of 20 weight $ EMA which i s not n e a r l y as t r a n s p a r e n t as E l v a x 150 EVA i s b e i n g 1

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Advantages •

Reversible cure by simple melting

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Fast processing, no cure time

• Steep viscosity vs temperature curve •

High ceiling temperature for processing

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No outgassing from decomposing peroxides

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Low (room temp) tack

Unknowns

Figure 3 ·

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Availability of materials of proven weather resistance

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UV sensitivity of aromatic crystalline blocks

Structures o f Thermoplastic Elastomers*


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e v a l u a t e d by a number of l a b o r a t o r i e s , and appears t o o f f e r some a t t r a c t i v e p r o p e r t i e s . I t i s more thermally s t a b l e and does n o t appear t o need a s much c r o s s l i n k i n g t o prevent creep. It i s considerably l e s s transparent, however, p a r t i c u l a r l y when uncured. Lower m e l t i n d e x E l v a x e s a r e a l s o p o s s i b i l i t i e s f o r a noncuring t h e r m o p l a s t i c elastomer p o t t a n t . 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 (PVB) i s easy t o p r o c e s s because i t i s t h e r m o p l a s t i c r a t h e r than thermosetting, but has the disadvantage o f c o n t a i n i n g p l a s t i c i z e r (see below). There are some other advantages o f t h e r m o p l a s t i c e l a s t o m e r s as p o t t i n g m a t e r i a l s f o r s o l a r c e l l s besides t h e steep melt v i s c o s i t y curve. They c r o s s l i n k by c o o l i n g so t h a t l o n g c u r e t i m e s a r e u n n e c e s s a r y and t h e c u r e i s r e v e r s i b l e t o help t h e recovery o f f l a w e d p a n e l s . They have no c e i l i n g f a b r i c a t i o n t e m p e r a t u r e above which the cure w i l l s e t o f f nd/or outgassing can occur. Thus t h e r m o p l a s t i c elastomers a l l o w more l a t i t u d e than i s a v a i l a b l e w i t h a thermosetting m a t e r i a l i n u s i n g temperature t o a d j u s t v i s c o s i t y f o r optimum l a m i n a t i o n processing. P l a s t i c i z e d PVB, used e x t e n s i v e l y i n laminated s a f e t y g l a s s , has been s t u d i e d . I t i s expensive and e a s i l y degraded when n o t h e r m e t i c a l l y sealed.3 This can be compensated f o r by s e a l i n g i t between a g l a s s f r o n t and metal f o i l back cover i n a s u p e r s t r a t e design module. E l e c t r i c a l Requirements, The p o t t a n t s h o u l d c o n t a i n no p l a s t i c i z e r because p l a s t i c i z e r can reduce the volume r e s i s t i v i t y of a polymer d r a s t i c a l l y . I t reduces t h e r e s i s t i v i t y of PVB by 5 orders o f magnitude i n some f o r m u l a t i o n s . PVB w i t h 40$ d i e s t e r p l a s t i c i z e r measures only 1 0 ohm-cm i n laminated form a t room temperature whereas i t measures 1 0 ^ ohm-cm w i t h the p l a s t i c i z e r d r i v e n o u t . Volume r e s i s t i v i t i e s o f 1 0 ohm-cm or l e s s w i l l conduct s m a l l amounts o f c u r r e n t f a i r l y r e a d i l y , a l b e i t s l o w l y . ( F o r example, a r e s o l v e d 5 l i n e pair/mm charge image has been observed t o b l u r w i t h i n t h e f i r s t few seconds when placed on the s u r f a c e o f a f i l m o r immersed i n a l i a u i d o f 1 0 - 1 0 ohm-cm resistivity. The same image on o r i n 1 0 ^ ohm-cm m a t e r i a l w i l l not b l u r f o r s e v e r a l hours. On a 1 0 ^ " ^ ohm-cm m a t e r i a l an image w i l l l a s t unblurred from weeks t o months.) Because i o n i c i m p u r i t y m o b i l i t y determines r e s i s t i v i t y i n a polymer r a t h e r than e l e c t r o n m o b i l i t y as i n metals, higher module o p e r a t i n g t e m p e r a t u r e s drop r a t h e r t h a n r a i s e the r e s i s t i v i t y because o f the v i s c o s i t y drop w i t h t e m p e r a t u r e . The v i s c o s i t y d r o p i n p l a s t i c i z e d PVB w i t h t e m p e r a t u r e i s e x t r e m e l y steep. Indeed, a f t e r o n l y a day o f d r y oven a g i n g a t 150°C, an open p l a s t i c i z e d PVB f i l m i s b r i t t l e from t o t a l l o s s o f the p l a s t i c i z e r and already s i g n i f i c a n t l y d i s c o l o r e d from o x i d a t i v e d e g r a d a t i o n . The volume r e s i s t i v i t y , however, r i s e s t o 1 0 ^ ohm-cm from t h e o r i g i n a l 1 0 ohm-cm by the removal of p l a s t i c i z e r . As an added d i f f i c u l t y , the p l a s t i c i z e r s s o l v a t i o n e f f e c t i n PVB a p p e a r s t o e n h a n c e t h e p o l y m e r v i s c o s i t y d r o p w i t h temperature. U n f o r t u n a t e l y , t h i s r a i s e s t h e leakage current o f a


1

1 1

1

1 2

1 1

1

1

1

1

1 1

f


1 2

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module a t 1.5 kV by over an order o f magnitude with an o p e r a t i n g t e m p e r a t u r e r i s e of only 25°C when an u n p l a s t i c i z e d , high volume r e s i s t i v i t y b a r r i e r l a y e r i s not present between the c i r c u i t and any grounded m e t a l s u r f a c e s i n c l o s e proximity (see Figure 4 ) . 25°C i s t h e normal r i s e f o r a module between d a r k and f u l l y i l l u m i n a t e d a t a f u l l sun f l u x o f 100 mW/cm . The volume r e s i s t i v i t y o f an u n p l a s t i c i z e d p o t t a n t m a t e r i a l such as EVA i s 1 0 ^ ohm-cm. The module current leakage a t 1.5 kV with EVA i s an order o f magnitude lower than with p l a s t i c i z e d PVB a t 25-30°C and t h e r e a p p e a r s t o be no r i s e i n leakage a t ~ 5060°C. (See F i g u r e 4 and Table I.) S i m i l a r l y , the c u r r e n t leakage o f modules c o n t a i n i n g p l a s t i c i z e d PVB c a n be b l o c k e d by t h e i n s e r t i o n of an a d d i t i o n a l h i g h volume r e s i s t i v i t y l a y e r such a s p o l y e t h y l e n e t e r e p h t h a l a t e f i l m as d i s c u s s e d below, which i s r e s i s t a n t to the s o l v a t i o n e f f e c t of d i e s t e r p l a s t i c i z e r s . 2


1

Table I· Volume R e s i s t l T i t y o f Pottant M a t e r i a l s Volume r e s i s t i v i t y values Material

Measured Initial

PVB* EVA PVB (Tedlar) PET (Mylar) *Plasticized **Most l i k e l y PVB

(Ω-cm):

1 1

10 10 10 *** 10 ? 1 4

1i

Literature

Dry Oven Aged (150°C) 10 10 5 x 10

1 6

1 4

1 4

1

reduced

when " e f f e c t i v e l y " laminated

101* 1018

to p l a s t i c i z e d

The p o t t a n t s h o u l d have a d i e l e c t r i c s t r e n g t h o f a t l e a s t 400-500 v o l t s / m i l , w h i c h i s t y p i c a l f o r u n o r i e n t e d amorphous polymers. S i n c e t h e p o t t a n t i s d e s i g n e d t o flow, however, i t cannot be r e l i e d upon t o provide s u f f i c i e n t d i e l e c t r i c s t a n d o f f by itself. I t w i l l tend t o move o u t o f t h e a r e a s where i t i s mechanically s t r e s s e d (squeezed), u n f o r t u n a t e l y , those areas are u s u a l l y a l s o t h e a r e a s o f highest e l e c t r i c a l s t r e s s s i n c e f i e l d l i n e s a r e the densest around i r r e g u l a r i t i e s i n the geometry o f the c i r c u i t m e t a l , e.g., i n t e r c o n n e c t r i b b o n or wire kinks, excess s o l d e r beads, e t c . A n o n - s o f t e n i n g , h i g h volume r e s i s t i v i t y i n s u l a t o r l a y e r i s thus needed t o guarantee c i r c u i t i s o l a t i o n . Chemical Requirements. The pottant must be s t a b l e ; that i s , chemically r e s i s t a n t to o x i d a t i o n and h y d r o l y s i s unless p r o t e c t e d i n a hermetic package, t o r e d u c t i o n by metals, and t o o u t g a s s i n g o f d i s s o l v e d g a s e s o r l i q u i d s o r d e c o m p o s i t i o n products under normal o p e r a t i n g c o n d i t i o n s o f -40°C to +90°C f o r 20 y e a r s . The need f o r chemical s t a b i l i t y i s e s p e c i a l l y s t r i n g e n t when a lower cost non-hermetic design i s used. Even when a hermetic package i s



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Figure 4 .

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Module Leakage Current vs.Temperature.



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u s e d , however, some s t a b i l i t y i s r e q u i r e d because t h e s m a l l amounts of d i s s o l v e d oxygen and moisture w h i c h w i l l be f o u n d i n any a m o r p h o u s , r u b b e r y m a t e r i a l c a n s t i l l cause n o t i c e a b l e d i s c o l o r a t i o n of a p o t t a n t , p a r t i c u l a r l y when c a t a l y z e d by c e r t a i n metal o x i d e s i f present i n the metallized c e l l contacts.3 Degradation i n a hermetic package can a l s o occur from the extreme heat t h a t can develop under "hot spot" c o n d i t i o n s which can occur i n i s o l a t e d areas of a module when e l e c t r i c a l mismatching of c e l l output i n a s e r i e s s t r i n g without diodes has occurred such as may be i n i t i a t e d by shadowing of c e l l s . T h e i d e a l m a t e r i a l would t o l e r a t e a t l e a s t i n t e r m i t t e n t e x c u r s i o n s up t o the m e l t i n g p o i n t of s o l d e r (~ 1°0°C) without d i s c o l o r i n g , e m b r i t t l i n g or r e v e r t i n g , outgassing, or breaking down e l e c t r i c a l l y . O x i d a t i v e breakdown of polymers can f o l l o w one o r more paths of change i n p h y s i c a l and chemical p r o p e r t i e s . O x i d a t i v e a t t a c k can be c a t a l y z e d by h e a t , UV l i g h t , c e r t a i n m e t a l s o r m e t a l o x i d e s , s o m e t i m e s m o i s t u r e , e t c . N e v e r t h e l e s s , t h e same f u n c t i o n a l weaknesses i n a g i v e n polymer w i l l be a t t a c k e d by oxygen r e g a r d l e s s o f the s p e c i f i c mechanisms by which the energy i s a c t u a l l y absorbed. The r e s u l t i s u s u a l l y e i t h e r embrittlement, c h a i n s c i s s i o n , h y d r o l y s i s , or combinations o f these. Embrittlement comes from e x t e n s i v e 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 r e v e r s i o n r e s u l t s i n e x t e n s i v e molecular weight l o s s and both c h a i n s c i s s i o n and h y d r o l y s i s r e s u l t i n t h e f o r m a t i o n o f h y d r o p h i l i c end g r o u p s . When both embrittlement and r e v e r s i o n occur s i m u l t a n e o u s l y , t h e r e s u l t i s t h a t t h e o v e r a l l p h y s i c a l p r o p e r t i e s , which depend mostly on molecular weight, o f t e n remain u n c h a n g e d f o r some t i m e u n t i l o n e m e c h a n i s m dominates. Embrittlement w i l l a l l o w i n c r e a s e d mechanical s t r e s s t o reach the s o l a r c e l l s , and i n c r e a s e s t h e p o s s i b i l i t i e s o f i n t e r f a c e d e l a m i n a t i o n from s t r e s s concentrations. Reversion allows d i s t o r t i o n or creeping of the l a y e r s , l e s s mechanical p r o t e c t i o n of c e l l s , and e a s i e r bubble formation from outgassing. The c h e m i c a l c h a r a c t e r o f a polymer o f t e n c h a n g e s more r a p i d l y w i t h o x i d a t i v e breakdown than the mechanical p r o p e r t i e s . H y d r o p h i l i c group formation changes the m o i s t u r e a b s o r p t i o n and p e r m e a b i l i t y o f t h e m a t e r i a l , a problem when p r o t e c t i n g metals from c o r r o s i o n as i n a PV module. H y d r o p h i l i c group f o r m a t i o n r e d u c e s t h e m o i s t u r e r e s i s t a n c e o f adhesive bonds, changing the a c i d - b a s e c h a r a c t e r i s t i c s o f t h e b o n d e d i n t e r f a c e s . The d e v e l o p m e n t o f c o n j u g a t e d c a r b o n - c a r b o n and c a r b o n - o x y g e n unsaturated color c e n t e r s changes the o p t i c a l a b s o r p t i o n p r o p e r t i e s o f the m a t e r i a l , reducing the amount o f l i g h t r e a c h i n g the s o l a r c e l l s and t h u s r e d u c i n g t h e i r e l e c t r i c a l o u t p u t . Reversion o r h y d r o l y s i s can a l s o generate v o l a t i l e s p e c i e s o f very low molecular weight, which can evolve from the polymer, l e a v i n g v o i d s which degrade v o l t a g e s t a n d o f f , reduce the o p t i c a l c o u p l i n g of l i g h t t o the c e l l s and create s t r e s s p o i n t s f o r c e l l f r a c t u r e . The e x a c t amount o f d i s s o l v e d v o l a t i l e s p e c i e s t h a t can be t o l e r a t e d i n a n e n c a p s u l a t e d PV system depends o n t h e v a p o r



378

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p r e s s u r e of the p a r t i c u l a r d i s s o l v e d s p e c i e s i n the p a r t i c u l a r pottant medium and on the v i s c o s i t y of t h a t p o t t a n t a t module o p e r a t i n g temperatures. The i m p o r t a n c e o f m i n i m i z i n g such d i s s o l v e d v o l a t i l e s was d i s c o v e r e d by ARCO S o l a r i n o u t d o o r t e s t i n g o f t h e f i r s t s u b s t r a t e d e s i g n r o o f t o p module. The encapsulated c e l l s began b u l g i n g up and c r a c k i n g as the a r r a y r e a c h e d summer o p e r a t i n g temperatures f o r the f i r s t time. The p o t t a n t was a s t a b i l i z e d t r a n s p a r e n t e t h y l e n e / v i n y l a c e t a t e pottant based on Elvax 150.7 The outer c o v e r / i n s u l a t o r l a y e r was a f l e x i b l e a c r y l i c copolymer f i l m . The b u l g i n g c e l l s r e s u l t e d from a c o m b i n a t i o n o f poor adhesion between unprimed s u r f a c e s and outgassing of the v o l a t i l e d e c o m p o s i t i o n p r o d u c t s o f the l a r g e amount of peroxide used to c r o s s l i n k the EVA. The a c r y l i c copolymer used melts under normal l a m i n a t i o n c o n d i t i o n s so the edges became buried and a peel was d i f f i c u l t to s t a r t . Although other i n v e s t i g a t o r s ? t & have reported that a c r y l i c f i l m and EVA c o - c r o s s l i n k , we found the peel strength to be very weak (~ 1-2 l b / i n . ) w i t h o u t u s i n g p r i m e r s . A large amount o f 2 , 5 - d i m e t h y l - 2 , 5 - b i s - ( t - b u t y 1 p e r o x y ) hexane c r o s s l i n k i n g agent ( L u p e r s o l 101 by Pennwalt) was used by the designers to i n s u r e the m a t e r i a l would s u f f i c i e n t l y c r o s s l i n k even when heated very slowly. Lupersol 101 forms s i g n i f i c a n t amounts o f a c e t o n e and t - b u t a n o l when i t decomposes i n a d d i t i o n t o methane, ethane, and e t h y l e n e (see F i g u r e 5 ) . Acetone and t b u t a n o l a r e not e f f e c t i v e l y removed from EVA during most vacuum p r o c e s s i n g and r e l i q u i f y u p o n c o o l i n g t h e m o d u l e t o room temperature. When the modules i n the outdoor array began to reach summer operating temperatures of 75-80°C, the v a p o r i z i n g t r a p p e d l i q u i d s began to b u i l d up pressure behind the c e l l s from the l a r g e volume i n c r e a s e of v a p o r i z a t i o n . When the vapor p r e s s u r e became g r e a t e r t h a n the combined adhesive strength of the bonded l a y e r s and the f l e x u r a l strength of the c e l l s , delamination began, w i t h the c e l l s b u l g i n g and c r a c k i n g . The problem was solved by a c o m b i n a t i o n o f a l t e r i n g the c u r i n g s y s t e m and r a i s i n g the adhesion. The p o t t a n t must be c h e m i c a l l y i n e r t i n that i t must not r e a c t with the metals or other s u r f a c e s i t bonds t o . Related to t h i s i s the need f o r i t to e x h i b i t l i t t l e or no water absorption to c o r r o d e m e t a l s bonded t o i t or t o r e d u c e i t s volume resistivity. The l a r g e f r a c t i o n of homopolyethylene i n the EVAs we are studying make them q u i t e i n e r t and low i n water absorption. As e x p l a i n e d above, the pottant should c o n t a i n l i t t l e or no p l a s t i c i z e r s i n c e i t can generate e l e c t r i c a l problems. The l a s t chemical requirement f o r the pottant i s t h a t i t s melt e q u i l i b r i u m contact angle with a l l the surfaces to which i t bonds be as low as p o s s i b l e below 90°C. This speeds p r o c e s s i n g as w e l l as maximizing adhesion and minimizing the c o l l e c t i o n of water and oxygen a t the i n t e r f a c e s t o r e d u c e m e t a l c o r r o s i o n and metal oxide c a t a l y z e d polymer changes to form c o l o r centers. T h i n F i l m Systems, As p r e v i o u s l y mentioned, the same types



23.

LEWIS

CH

I

w

I

H C — C — Ο

Ι

CH

CH

cm

3

I 3

379


«(CH ), 2

Ι

3

I

-ο-

«ε3

CH

3

Ι

I

CH-

CH

CH,

3

Δ

CH H C»

I

Ο

3

II • OH + H C

3

3

.c

" f

CH

Η Η >CH + C H + C = C + H C — Η Η 3

4

3

3

b.p. 80°C Figure 5.

b.p. 60°C

Gases

Decomposition o f L u p e r s o l 101.


C H , etc. 3


380

POLYMERS

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of m a t e r i a l s w i t h the same o p t i c a l , e l e c t r i c a l , chemical, and some of the same mechanical requirements ( p a r t i c u l a r l y bond strengths) would be needed f o r t h i n f i l m c e l l e n c a p s u l a t i o n . A l l the p r e v i o u s l y mentioned candidates would q u a l i f y but could probably be used i n t h i n n e r l a y e r s . Because l e s s c r i t i c a l m e c h a n i c a l requirements enable the use o f t h i n n e r l a y e r s , the f i e l d opens t o i n c l u d e more expensive p o s s i b i l i t i e s such a s s i l i c o n e s . Liquid systems such as 100? s o l i d s c a s t i n g m a t e r i a l s o r s o l u t i o n a p p l i e d coatings (by spray, d i p , brush, r o l l e r , e t c . ) a l s o become more p r a c t i c a l f o r a r a p i d throughput f a c t o r y . C a s t i n g l i q u i d s a r e a p o s s i b i l i t y even now but are more complicated t o use w i t h vacuum p r o c e s s i n g than a r e sheets. Outer Cover/Insulator The o u t e r c o v e r i s t h e t o u g h , h a r d , s o i l - r e s i s t a n t , i n h e r e n t l y weather r e s i s t a n t outer l a y e r t h a t p r o t e c t s the s o f t e r pottant l a y e r s from the e f f e c t s o f a b r a s i o n , d i r t , and weathering. I t augments the p o t t a n t i n e l e c t r i c a l l y i s o l a t i n g the c i r c u i t and p r e f e r a b l y a c t s a s a UV s c r e e n i f used on t h e f r o n t . It is u s u a l l y a f l e x i b l e o r conforming p l a s t i c and may be a f i l m o r a solution-applied coating. O p t i c a l , C h e m i c a l , and Cost R e q u i r e m e n t s . I f t h e outer c o v e r / i n s u l a t o r i s the f r o n t cover on a r i g i d - b a c k module, i t must have the same o p t i c a l and chemical p r o p e r t i e s as the p o t t a n t ; t h a t i s , high t r a n s m i s s i o n , good o p t i c a l c o u p l i n g , and inherent weather resistance. Being i n h e r e n t l y weather r e s i s t a n t means meeting a l l the c r i t e r i a p r e v i o u s l y d e s c r i b e d under c h e m i c a l stability requirements f o r p o t t a n t s without need o f added s t a b i l i z e r s . The only known i n h e r e n t l y w e a t h e r r e s i s t a n t o r g a n i c m a t e r i a l s a r e a c r y l i c s , s i l i c o n e s , and f l u o r o c a r b o n s , ranging from $3-5/lb f o r a c r y l i c s , t o $8-10/lb f o r s i l i c o n e s , t o $10-20/lb f o r fluorocarbons. However, i f t h e cover m a t e r i a l i s s u f f i c i e n t l y tough and f l e x i b l e , i t can be q u i t e t h i n (1-4 m i l s ) and c a n be made from a more expensive polymer than the p o t t a n t . I t can cost up t o $10-15/lb and s t i l l be economical. I f t h i n p l a s t i c i s used a s a f r o n t c o v e r i n a s u b s t r a t e design module, i t w i l l not provide a hermetic s e a l no matter how o r i e n t e d t h e m i c r o s t r u c t u r e i s . The o x y g e n a n d w a t e r p e r m e a b i l i t i e s may be low but f i n i t e . The main q u e s t i o n , y e t t o be f u l l y answered, i s whether a non-hermetic package can l a s t f o r 20 years. S i n g l e - c r y s t a l s i l i c o n s o l a r c e l l s themselves are known t o be f a i r l y i n e r t t o t h e e f f e c t s o f heat, l i g h t , oxygen, and water. A c c e l e r a t e d c o r r o s i o n t e s t s a r e i n p r o g r e s s on m i n i c i r c u i t s t o determine the s t a b i l i t y o f the c e l l contacts and i n t e r c o n n e c t systems ( c o p p e r r i b b o n s , s o l d e r , e t c . ) . The s t a b i l i t y o f f u t u r e c e l l s t h e m s e l v e s w i l l be t h e r e m a i n i n g q u e s t i o n t o be answered f o r f u t u r e PV modules. The back cover, may, o f course, be opaque. For stand-alone, glass-front arrays i t i s preferably white i n c o l o r . This i s


23.

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because l i g h t s c a t t e r e d by t h a t white surface i s r e f r a c t e d i n the g l a s s and enhances the t o t a l module output by about 5%· There i s l i t t l e or no enhancement from a white background, however, w i t h a t h i n p l a s t i c f r o n t and a r c h i t e c t s have objected to the p o l k a d o t appearance i n r o o f t o p a p p l i c a t i o n s . Thus, the r o o f t o p module design has a b l a c k - c o l o r e d back p l a s t i c l a y e r w h i c h matches the c e l l s w e l l . I t does not i n c r e a s e the module temperature over a white back f i l m by more than a few degrees. The back cover must be i n h e r e n t l y weather r e s i s t a n t as must the f r o n t but i t s u l t r a v i o l e t s t a b i l i t y can be more e f f e c t i v e l y enhanced w i t h l i g h t absorbing pigments as w e l l as transparent UV stabilizers. Thin p l a s t i c a l o n e on the back cannot p r o v i d e a hermetic s e a l , but because i t can be opaque, a metal f o i l such as s t e e l o r aluminum may be used i n the back c o v e r c o m p o s i t e t o e f f e c t a hermetic s e a l except at the edges, when g l a s s i s used as the f r o n t cover. A minimum t h r e e - l a y e r laminate i s r e q u i r e d f o r a back c o v e r c o m p o s i t e . The f i r s t cover l a y e r serves as the nons o f t e n i n g e l e c t r i c a l i n s u l a t o r between the potted c i r c u i t and the f o i l and i s p r o t e c t e d from oxygen. The metal f o i l l a y e r i s the hermetic b a r r i e r and the t h i r d l a y e r i s the t r u e outer cover where weather r e s i s t a n c e i s more s t r i n g e n t l y r e q u i r e d . Mechanical Requirements. The outer cover p l a s t i c f i l m must be tough and f l e x i b l e t o r e s i s t a b r a s i o n and gouging, both i n manufacture and i n the f i e l d . I f i t i s the f r o n t cover, i t must a l s o be r e l a t i v e l y s o i l r e s i s t a n t . E i t h e r f r o n t or back covers must form r e l i a b l e , m o i s t u r e - r e s i s t a n t bonds to the pottant and to the f o i l i n the case of a back cover. Moisture r e s i s t a n t adhesion i s e s p e c i a l l y important a t the edges where moisture can penetrate even a "hermetic" d e s i g n . L a s t l y , the c o v e r l a y e r s must be dimensionally stable (non-shrinking or y i e l d i n g ) t o thermal c y c l i n g s t r e s s e s of manufacture and f i e l d o p e r a t i n g c o n d i t i o n s . E l e c t r i c a l Requirements, The o u t e r c o v e r must be made o f h i g h v o l u m e r e s i s t i v i t y m a t e r i a l t h a t does not s o f t e n a t l a m i n a t i o n temperatures i n o r d e r t o c o n t r o l e l e c t r i c a l c u r r e n t leakage through i t . Outer covers can be a p p l i e d as o r i e n t e d f i l m s by l a m i n a t i o n , a s l i q u i d o r powder c o a t i n g s by e l e c t r o s t a t i c spray, d i p , brush or r o l l e r , or by e x t r u s i o n . Oriented f i l m s have by f a r the b e s t e l e c t r i c a l p r o p e r t i e s i n terms o f d i e l e c t r i c s t r e n g t h ( v o l t a g e s t a n d o f f per u n i t t h i c k n e s s ) . Only a c r y l i c s could r e a l l y be considered economical i n l i q u i d or powder coatings b e c a u s e o f t h e g r e a t e r t h i c k n e s s e s r e q u i r e d w i t h o u t the o r i e n t a t i o n of a blown f i l m . Oriented f i l m s are a l s o mechanically t h e t o u g h e s t f i l m s and have the lowest gas p e r m e a b i l i t y of most p l a s t i c s because o f the i n c r e a s e d d e n s i t y a n d i n d u c e d c r y s t a l l i n i t y of t h e i r s t r u c t u r e . A f a i r l y l a r g e margin i n terms of d i e l e c t r i c s t a n d o f f i s r e q u i r e d between t h e t e s t o r u s e v o l t a g e s and the p a r a l l e l p l a t e d i e l e c t r i c s t r e n g t h values of the i n s u l a t i n g l a y e r ( s ) because, a s p r e v i o u s l y m e n t i o n e d , t h e i r r e g u l a r i t i e s o r geometry o f the c i r c u i t g i v e r i s e t o u n p r e d i c t a b l e f i e l d concentrations f o r leakage or breakdown.


w


w


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Candidates. The only commercially a v a i l a b l e o r i e n t e d f i l m s known a t t h i s time which f i t the weather r e s i s t a n c e r e q u i r e m e n t s are p o l y v i n y l i d e n e fluoride ( P V F 2 ) , polyvinyl fluoride (Tedlar), polymethyl m e t h a c r y l a t e (PMMA), and p o l y 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 copolymer (PBA/MMA). P V F 2 i s c u r r e n t l y expensive. PBA/MMA i s inexpensive but i n c l e a r form does n o t appear t o be s u f f i c i e n t l y o x i d a t i v e l y s t a b l e f o r our purposes. I t i s a l s o too water s e n s i t i v e and t o o e a s i l y s o f t e n e d i n many l a m i n a t i n g processes. PMMA a p p e a r s t o be somewhat more chemically s t a b l e than PBA/MMA and i s a l s o r e l a t i v e l y inexpensive, but has the same d i m e n s i o n a l s t a b i l i t y problems a t 150°C, t h e normal p o t t a n t processing temperature. Both a c r y l i c s maintain e x c e l l e n t o p t i c a l c l a r i t y on heat aging, however. T e d l a r i s m o d e r a t e i n c o s t a n d h a s known l o n g - t e r m performance o u t - o f - d o o r s . I t h a s e x c e l l e n t toughness, good weather r e s i s t a n c e , and m o d e r a t e l y good e l e c t r i c a l and o p t i c a l performance. I t s thermal s t r e s s r e s i s t a n c e i s marginal b u t adequate (2-6$ shrinkage a t 150°C) f o r most needs. I t s cost i s higher than optimum, but can be used as a t h i n f i l m , e s p e c i a l l y when c o u p l e d with l e s s expensive polyethylene t e r e p h t h a l a t e f i l m f o r b e t t e r e l e c t r i c a l p r o p e r t i e s a t a lower c o s t . Thin F i l m Systems, An i d e a l l o w - c o s t system c o u l d be continuously processed i n t o r o l l s of a r r a y s . These r o l l s would consist of a clear, f l e x i b l e , e l e c t r i c a l l y insulating plastic f r o n t cover, a t h i n l a y e r o f pottant or adhesive on e i t h e r s i d e o f t h e f l e x i b l e t h i n f i l m PV c i r c u i t , and an opaque, f l e x i b l e , e l e c t r i c a l l y i n s u l a t i n g p l a s t i c back c o v e r . These r o l l s c o u l d t h e n s i m p l y be u n r o l l e d on t h e r o o f , n a i l e d i n t o p l a c e , and connected t o the household c i r c u i t r y . A l l components would have t o be f l e x i b l e . Figure 6 i l l u s t r a t e s the p o s s i b l e components o f an a l l f l e x i b l e system — both hermetic and non-hermetic depending on f u t u r e c e l l requirements.

Adhesives, Primers, Surface M o d i f i c a t i o n s Good adhesion peel strengths a t an i n t e r f a c e r e s u l t from a c o m b i n a t i o n o f s e v e r a l i n t e r a c t i n g phenomena. The i n t e r f a c i a l f o r c e s are a combination o f d i s p e r s i o n (van der Waals). p o l a r , and acid-base i n t e r a c t i o n f o r c e s across the interface.9-13 They w i l l determine long-term r e l i a b i l i t y . Besides the i n t e r f a c i a l f o r c e s i n v o l v e d , t h e r h e o l o g y o f t h e m a t e r i a l s a t the i n t e r f a c e has a l a r g e i f not dominant i n f l u e n c e i n determining the p e e l s t r e n g t h o r d e l a m i n a t i o n t e n d e n c i e s o f a p a r t i c u l a r bond. This i s p a r t i c u l a r l y true because o f the t h e r m a l c y c l i n g s t r e s s e s a PV array i s subjected t o . The goal i s t o have cohesive r a t h e r than adhesive f a i l u r e a t the i n t e r f a c e . I f the rheology i s such that s t r e s s c o n c e n t r a t i o n s o c c u r , t h e cohesive f a i l u r e can be q u i t e low. This i s p a r t i c u l a r l y true i f a bubble i s t r a p p e d i n a PV module. E x p a n s i o n and c o n t r a c t i o n o f trapped or e v o l v i n g gases during thermal c y c l i n g generate h i g h l y c o n c e n t r a t e d s t r e s s e s a t



Figure 6 .

Thin Film F l e x i b l e Module.


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t h e b u b b l e p e r i m e t e r by n a t u r e o f t h e geometry. At l e a s t one component of any module i n t e r f a c e should be f l e x i b l e , a s i s t h e s o f t p o t t a n t , t o spread these f o r c e s as much as p o s s i b l e . I f p r o p e r l y f u n c t i o n a l adhesives or c o u p l i n g a g e n t s a n d / o r o t h e r s u r f a c e m o d i f i c a t i o n s a r e used where p o s s i b l e (such as changing the a c i d i c or b a s i c character of one or both s u r f a c e s a t an i n t e r f a c e ) , the r e s u l t should always be a s t a b l e bond.


Edge Sealants and Frames An edge s e a l a n t i s needed i f the module i s t o have an added frame. I t i s p a r t i c u l a r l y important i f one i s making a module w i t h two impermeable outside l a y e r s , e.g., g l a s s and metal f o i l , f o r a " h e r m e t i c a l l y * sealed package. The s e a l s h o u l d be as gas and moisture t i g h t as p o s s i b l e a t the module edges i f the pottant l a y e r s being sealed between t h e impermeable l a y e r s a r e a t a l l o x i d a t i v e l y or h y d r o l y t i c a l l y u n s t a b l e , or i f they are hygroscopic as i s PVB. EVA does not r e a l l y need a m o i s t u r e - t i g h t s e a l , j u s t a mechanical cushion. Edge S e a l a n t s . M e c h a n i c a l l y , the edge s e a l i n g m a t e r i a l f o r a module which contains a h y g r o s c o p i c p o t t a n t must have m o i s t u r e r e s i s t a n t adhesion o f Σ 10 l b / i n . or cohesive f a i l u r e t o a l l the s u r f a c e s i t touches: frame, g l a s s , p o t t a n t , and f o i l c o v e r s . I t must be v e r y f l e x i b l e ( 5 x 1 0 p s i ) and r e l a t i v e l y l o w t h e r m a l e x p a n s i o n c o e f f i c i e n t as c l o s e a s p o s s i b l e t o g l a s s so the channel w i l l not have t o be too deep and can m a i n t a i n a maximum p a c k i n g d e n s i t y o f exposed c e l l s . The m a t e r i a l must be weather r e s i s t a n t (20 years) and bondable. I t should be r e l a t i v e l y low cost ( p r e f e r a b l y < 0 . 2 5 / f t o f m o d u l e ) . Geometry, t h i c k n e s s , f a b r i c a t i o n technique, e t c . , which s t r o n g l y e f f e c t the economics, w i l l v a r y w i t h t h e m a t e r i a l s . The b e s t c a n d i d a t e s t o date a r e extruded o r r o l l - f o r m e d metals w i t h p a s s i v a t e d s u r f a c e s such as anodized aluminum or g a l v a n i z e d s t e e l . Highly f i l l e d , molded, or extruded, weatherable p l a s t i c s a r e a l s o a p o s s i b i l i t y b u t an o p t i m u m c a n d i d a t e h a s n o t y e t b e e n identified. 5

B

2


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Accelerated Testing O v e r a l l , t h e g o a l i s chemical s t a b i l i t y o f the pottant and a l l other organic m a t e r i a l s i n the array t o the extent that they w i l l undergo no more than a 20? change i n o p t i c a l , e l e c t r i c a l , or mechanical p r o p e r t i e s ( i n c l u d i n g bond strengths) over 20 years of o u t d o o r w e a t h e r i n g and module o p e r a t i o n . T h i s i s d i f f i c u l t to determine when most of the m a t e r i a l s have n o t even e x i s t e d t h a t l o n g , l e t a l o n e been exposed outdoors i n a p h o t o v o l t a i c module. The near-term goal with each design change i s t o make s o m e t h i n g b e t t e r i n performance as w e l l as lower i n cost than what was being used. Thus, most t e s t s today compare m a t e r i a l s r e l a t i v e t o each o t h e r i n a c c e l e r a t e d c o n d i t i o n s that i n some cases a r e probably e x c e s s i v e l y severe. F o r example, t h e 150°C d r y oven and 100°C/100? RH exposure t e s t s are used mostly as screening t o o l s .

Literature Cited 1. Cuddihy, E. "LSA Progress Report 18 and Proceedings of the 18th Project Integration Meeting," Jet Propulsion Laboratory, in press. 2. Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulants, 13th Quarterly Progress Report for May 12, 1978-August 12, 1979, DOE/JPL/954527-12, Springborn Laboratories, Inc., Jan. 1980. 3. Megerle, C.; Lewis, K. Encapsulant Degradation in Photovoltaic Modules, this symposium. 4. Arnett, J . C . ; Gonzalez, C. C. "Fifteenth IEEE Photovoltaic Specialists Conference -- 1981," p. 1099. 5. Ross, R. G . , J r . "Fifteenth IEEE Photovoltaic Specialists Conference -- 1981," p. 1157. 6. Gonzalez, C . ; Weaver, R. "Fourteenth IEEE Photovoltaic Specialists Conference -- 1980," p. 528. 7. "Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulants, Annual Report," DOE/JPL - #954527-79-10, Springborn Laboratories, Inc., June 1979. 8. Pluddemann 9. Drago, R. S.; Vogel G. C.; Needham, T. E. J. Am. Chem. Soc. 1971, 93, 6014. 10. Drago, R. S.; Parr, L. B.; Chamberlain, C. S. J. Am. Chem. Soc. 1977, 99, 3203. 11. Fowkes, F. M.; "Donor-Acceptor Interactions at Interfaces," J. Adhesion 1972, 4, 155. 12. Fowkes, F. M.; Maruchi, S. Coatings and Plastics Preprints 1977, 37, 605. 13. Fowkes, F. M.; Mostafa, M. A. Ind. Eng. Chem., Prod. R&D 1978, 17, 3. RECEIVED November 22,1982


Encapsulant Material Requirements for Photovoltaic Modules

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