RECENT PROGRESS OF THE AMORPHOUS SILICON SOLAR CELLS AND THEIR TECHNOLOGY

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RECENT PROGRESS OF THE AMORPHOUS SILICON SOLAR CELLS AND THEIR

TECHNOLOGY

Y. Hamakawa

To cite this version:

Y. Hamakawa. RECENT PROGRESS OF THE AMORPHOUS SILICON SOLAR CELLS AND THEIR TECHNOLOGY. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-1131-C4-1142.

�10.1051/jphyscol:19814244�. �jpa-00220876�

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JOURNAL DE PHYSIQUE

Colloque C4, suppZ6ment au nOIO, Tome 4 2 , octobre 1981 page C4-1131

RECENT PROGRESS OF THE AMORPHOUS S I L I C O N SOLAR CELLS AND T H E I R TECHNOLOGY

Y . Hamakawa

Faculty o f Engineering Science, Osaka University, Toyonaka, Osaka, Japan 560

A b s t r a c t . - A r e v i e w i s g i v e n o n t h e c u r r e n t s t a t e o f t h e a r t i n t h e f i e l d o f a m o r p h o u s s i l i c o n s o l a r c e l l s a n d k e y t e c h n o l o g y t o i m p r o v e c e l l p e r f o r m a n c e . R e m a r k a b l e a d v a n t a g e s o f t h i s n e w m a t e r i a l f o r l o w cost p h o t o v o l t a i c d e v i c e s a r e p o i n t e d o u t , a n d d i s c u s s e d i n v i e w o f b o t h d e v i c e p h y s i c s and m a n u f a c t u r a b ~ l i t y . A n e w c o n c e p t o f t h e d r i f t t y p e p h o t o v o l t a i c e f f e c t i n t e r m s o f the f i e l d d e p e n d e n t p h o t o c a r r i e r g e n e r a t i o n p r o c e s s and c a r r i e r c o l l e c t i o n e f f i c i e n c y i s i n t r o d u c e d . O p t i m i z a t i o n o f p h o t o v o l t a i c p e r f o r m a n c e i n p - i - n j u n c t i o n and i n v e r t e d p - i - n j u n c t i o n d e v i c e s i s e x a m i n e d w i t h some e x p e r i m e n t a l l y d e t e r m i n e d p h y s i c a l c o n s t a n t s , and r e a l i s t i c l i m i t o f t h e c o n v e r s i o n e f f i c i e n c i e s i n t h e p - i - n h o m o j u n c t i o n a n d h e t e r o j u n c t i o n s o l a r c e l l s a r e a l s o c l a r i f i e d . Some n e w a p p r o a c h e s w i t h h e t e r o j u n c t i o n , s o l a r c e l l s u t i l i - z i n g w i d e band g a p a m o r p h o u s s i l i c o n c a r b i d e a n d m i c r o - c r y s t a l l i n e silicon a r e a l s o d e m o n s t r a t e d . P r o g r e s s i n t h e c o n v e r s i o n e f f i c i e n c i e s a n d cell p e r f o r m a n c e s i n v a r i o u s t y p e s o f a m o r p h o u s s i l i c o n s o l a r c e l l s a r e s u r v e y e d .

Introduction.-

A r e c e n t d i s c o v e r y o f t h e e x i s t e n c e o f v a l e n c y c o n t r o l l a b i l i t y i n t h e glow d i s c h a r g e produced amorphous s i l i c o n ( a - S i : H ) by S p e a r and LeComber[l]

opens up a wide v a r i e t y o f t h e p o t e n t i a l a p p l i c a t i o n s t o t h e f i e l d s o f b o t h e l e c - t r o n i c s and o p t o - e l e c t r o n i c s f o r t h e amorphous s e m i c o n d u c t o r s . P a r t i c u l a r l y , an e x c e l l e n t p h o t o c u n d u c t i v e p r o p e r t y w i t h h i g h o p t i c a l a b s o r p t i o n f o r t h e v i s i b l e l i g h t and a l s o t h i n f i l m - l a r g e a r e a p r o d u c i b i l i t y i n t h i s m a t e r i a l match v e r y t i m e l y w i t h s t r o n g p o t e n t i a l n e e d s f o r t h e development o f low c o s t s o l a r c e l l a s a new e n e r g y t e c h n o l o g y . C a r l s o n and Wronski[2] have r e p o r t e d f i r s t l y a n a-Si:H S c h o t t k y b a r r i e r s o l a r c e l l h a v i n g a c o n v e r s i o n e f f i c i e n c y a s h i g h a s 5.5% i n 1977. T h i s h i g h e f f i c i e n c y e n c o u r a g e s t h e f i e l d v e r y much, more t h a n s e v e r a l g r o u p s h a v e i n i t i a t e d t h i s t y p e s o l a r c e l l . As a second key s t e p , OSAKA u n i v e r s i t y group have d e m o n s t r a t e d a h e t e r o f a c e j u n c t i o n c e l l w i t h t h e c e l l c o n s t r u c t i o n o f ITO/p-i-n/ s t a i n l e s s s t e e l h a v i n g t h e e f f i c i e n c y o f 4 . 5 % [ 3 ] i n s p r i n g o f 1978.

I n t h e y e a r o f 1979, a r e m a r k a b l e p r o g r e s s h a s been s e e n i n b o t h b a s i c p h y s i c s and a p p l i c a t i o n f i e l d . I n t h e f i e l d o f p h o t o v o l t a i c d e v i c e s , f o r example, a group o f Sanyo e l e c t r i c company h a s announced a f l u o r e s c e n t lamp s e n s i i v e p h o t o - c e l l [ 4 ] , and

Z

by F u j i - d e n k i C e n t r a l R e s e a r c h L a b o r a t o r y , a wide a r e a (7x7cm ) s o l a r c e l l h a s been r e p o r t e d [ 5 ] . Another b i g impact i n t h e y e a r was a n anouncement o f t h e f l u o r i n a t e d amorphous s i l i c o n (a-Si:F:H) by Ovshinsky group o f ECD. According t o t h e r e p o r t [ 6 ] , a-Si:F:H h a s lower l o c a l i z e d gap s t a t e s d e n i s t y w i t h h i g h e r d o p i n g e f f i c i e n c y t h a n t h e a-Si:H. S t i m u l a t e d e x p e r i m e n t a l t r a c e s have been i n i t i a t e d by a wide v a r i e t i e s o f groups n o t o n l y m a t e r i a l c h a r a c t e r i z a t i o n s b u t a l s o d e v i c e f a b r i c a t i o n g r o u p s [ 7 ] . A new t y p e o f h i g h v o l t a g e p h o t o v o l t a i c d e v i c e h a v i n g t h e s t r u c t u r e o f h o r i z o n t a l l y m u l t i l a y e r e d p - i - n u n i t c e l l s h a s been d e m o n s t r a t e d by OSAKA U n i v e r s i t y [ 8 , 9 ] . A v e r y s i m i l a r c e l l s t r u c t u r e u s i n g cermet f i l m h a s a l s o been demonstrated by Hanak[lO]. A s e r i e s o f tremendous R E D e f f o r t s i n t h e d e v i c e f i e l d s c o m e s , i n t o bloomin i n t h e y e a r o f 1980, t h a t i s , t h e c o n v e r s i o n e f f i c i e n c y o f t h e a-S1 s o l a r c e l l g a s been improved d a y by d a y , and p r e s e n t t o p d a t a anounced a r e 6.1% by RCA[11] and 6 . 9 % by S a n y o [ l 2 ] f o r t h e i n v e r t e d p - i - n a - S i : H c e l l and 6 . 3 % by E c D [ ~ ~ ] f o r a - S i : F : H MIS

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814244

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C4-1132 JOURNAL DE PHYSIQUE

type Schottky barrier cell, respectively. Figure 1 shows progress of the amorphous silicon solar cell conversion efficiencies and their prospect in the near future for various junction strcuture and active area. The efficiency of the large area cell

UNPUBLISHED: RCA

(ITO/p-l-n .-S1 :H)

(INVERTEO

(INVERTED p - l - n p - l - n a-S! a - S i : H ) :!l)

1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 CALENDAR YEAR

Fig.1 Progress in the amorphous siticon solar ceZZ conversim efficiencies for various types of materials and c e l l structures.

for the practical applications has also been getting better and achieved around 5.6% to 6.8% by RCA[14], Fuji[l5], Sanyo [l6], TIT1171 and OSAKA University[l81 etc..

In a view of material science, new topics in the 1980 were micro-crystalline[19]

silicon and the hydrogenated amorphous silicon carbide[20]. These materials are very useful for the window side junction electrode because their excellent optical trans- parency with a considerably good photoconductivity. Utilizing p-type a-SiC:H as a front electrode, a-SiC:H/a-Si:H p-i-n heterojunction solar cell shows 7.55% conversion efficiency quite recently[21] While, outstanding records at present are this 7.55% for a-SiC:H/a-Si:H heterojunction by OSAKA University, 6.9% for a-Si:H p-i-n by Sanyo electric company[22] and 6.6% for a-Si:F:H solar cell by ECD[23] on the small active area, and 6.7% for a-SiC:H/a-SI:H heterojunction by OSAKA University[24]

and 4.1% for inverted p-i-n junction cell by RCA[25] on both about sensitive area of I cm

.

In this paper, these advances in the field of amorphous silicon solar cell and key technologies to improve cell performance are reviewed. As the first step, piinc- pal advantages of this new material for low cost photovoltaic devices are pointed out, and discussed in view of both device physics and manufacturablity. Then a new concept of the drift type photovoltaic effect in terms of the field dependent photo- carrier generation process and carrier collection efficiency is introduced, As the final stage of this review, realistic limit of the conversion efficiencies in the p- i-n homojunction and heterojuntion solar cells are also discussed togather with a survey of the current state of the RED efforts achieved on the cell efficiency improvements.

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P r i n c i p a l Advantages a s a Low Cost S o l a r Cell M a t e r i a l . - I n g e n e r a l , p h o t o v o l t a i c energy conversion c o n s i s t s o f two ~ r i n c i o a l v. A o r o c e s s e s . namelv t h e o h o t o c a r r i e r A

g e n e r a t i o n by t h e i n t e r b a n d o r l e v e l t o band a b s o r p t i o n o f l i g h t i n semiconductors, and t h e s e p a r a t i o n o f t h e generated c a r r i e r s by an i n t e r n a l e l e c t r i c f i e l d . I n t h e p r a c t i c a l d e v i c e s , t h e i n t e r n a l f i e l d i s provided by an i n t e r f a c e p o t e n t i a l b a r r i e r , such a s p-n j u n c t i o n , p - i - n j u n c t i o n , h e t e r o j u n c t i o n , Schottky b a r r i e r , e t c . . [ 2 6 ]

In a p-n j u n c t i o n t y p e s o l a r c e l l , t h e s h o r t c i r c u i t p h o t o c u r r e n t d e n s i t y J can be expressed by t h e photon f l u x d e n s i t y O ( X ) with a s u r f a c e r e f l e c t i v i t y of ?fie semiconductor R a s t h e form,

h e r e , J~~~ i s t h e p h o t o c a r r i e r d e n s i t y based upon t h e photo-generated holes i n t h e window s l d e of n - l a y e r j u n c t i o n , t h a t o f e l e c t r o n d i f f u s i o n Jdfn i n t h e p-region and t h a t due t o t h e d r i f t c a r r i e r s generated i n t h e t r a n s i t i o n r e g i o n J While,

d r ' t h e r a t e o f photogenerated c a r r i e r d i f f u s i o n i n t h e both s i d e o f j u n c t i o n can be e a s i l y determined by Lampart's l a w f o r t h e o p t i c a l a b s o r p t i o n i n semiconductor, and w r i t t e n a s t h e form;

d d -X d

Jd;=I ayexp [-ox] exp [-]dx = *{exp

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^-^{exp [-ad]}^),

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P P P

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a

a + l / L _~ ~ X P _~ [ -"cl! ₌ P

A band diagram i l l u s t r a t i o n o f t h e photo-generated c a r r i e r s i n s i n g l e c r s t a l p-n j u n c t i o n i s shown i n F i g . i [ a - l ) , and t h e g e n e r a t e d c a r r i e r p r o f i l e s of Jd> and J both o b t a i n e d from t h e s o l u t i o n of d i f f u s i o n e q u a t i o n a t p r o p e r boundary con- dPfions a r e a l s o s c h e m a t i c a l l y drawn i n t h e Fig.Z(a-2). I n t h e c a s e o f amorphous p- i - n j u n c t i o n , due t o a very s h o r t c a r r i e r d u f f s i o n l e n g t h and a l s o a v e r y high a b s o r p t i o n c o e f f i c i e n t , t h e r e e x i s t s a c l e a r d i f f e r e n c e of p h o t o v o l t a i c mechanism between t h e s i n g l e c r y s t a l p-n j u n c t i o n and t h e amorphous p - i - n j u n c t i o n , t h a t i s ,

Fig.2 Comparison o f photovoltaic processes i n s i n g l e c r y s t a l b a s i s p-iz junction f a ) and amorphous p-i-n j m c t i o n ( b ) .

Jdfn + JdfP >> Jdf f o r s i n g l e c r y s t a l p-n j u n c t i o n , J~~~ + JdfP << Jdf f o r a-Si:H p - i - n

j u n c t i o n . ( 4 ) Photogenerated c a r r i e r s and t h e i r p r o f i e l s a r e shown i n F i g . 2 ( b - l ) and (b-2) [27]. A s can be seen from t h e s e f i g u r e s , c l e a r d i f f e r e n c e from F i g . Z ( a - l ) and (a-2) i s an e x i s t e n c e o f high e l e c t r i c f i e l d i n t h e amorphous s i l i c o n . D e t a i l e d d i s c u s s i o n s on t h e f i e l d dependent p h o t o c a r r i e r g e n e r a t i o n and recombination p r o c e s s e s , and a l s o t h e e l e c t r i c f i e l d a s s i s t e d c a r r i e r c o l l e c t i o n e f f i c i e n c y i n t h e amorphous s i l i c o n w i l l be made i n t h e next s e c t i o n .

As i t has been seen elesewhere[28,29,30,31]

t h a t t h e o u t s t a n d i n g advantages o f t h e plasma d e p o s i t e d amorphous s i l i c o n a s a p h o t o v o l t a i c m a t e r i a l a r e i t s high a b s o r p t i o n c o e f f i c i e n t and l a r g e p h o t o c o n d u c t i v i t y i n t h e v i s i b l e r e g i o n . Figure 3 shows a comparison o f t h e a b o r p t i o n s p e c t r a i n t h e doped and undoped a - Si:H with t h a t o f s i n g l e c r y s t a l S i . The s o l a r spectrum of t h e AM-1 ( a i r mass one) is a l s o i n s e r t e d i n t h e f i g u r e .

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JOURNAL DE PHYSIQUE

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cr?js%aZ S i wi tit s o l a r i z ? i z d i a t i o ~ l spizctnm.

a-Si has about 012s order magni lu& Lager, Fig.4 A t t a i n a b l e s o l a r p h o t o v o l t a i c conversion nhsorption c o e f f i c i e n t than c-Si a t the p ~ u i c offi&enri.es as a f u n c t i o n o f a c t i v e Za!rer e n e ~ g y o f the aoZar r a d i a t i o n . thickrzess f o r severw;! s o l a r ceZZ n i a t e r i a l s .

A s c a n b e s e e n i n t h e f i g u r e , a-Si:H h a s a b o u t one o r d e r o f magnitude l a r g e r a b s o r p t i o n c o e f f i c i e n t a t t h e maximum s o l a r e n e r g y photon r e g i o n n e a r 5000A.

On t h e b a s i s o f o p t i c a l a b s o r p t i o n c o e f f i c i e n t and a l s o p h o t o c o n d u c t i v i t y s p e c t r a o f t h e s o l a r c e l l m a t e r i a l , one c a n c a l c u l a t e a n optimum t h i c k n e s s o f t h e a c t i v e r e g i o n . F i g u r e 4 shows t h e r e s u l t s o f c a l c u l a t i o n s on t h e p h o t o v o l t a i c c o n v e r s i o n e f f i c i e n c y a s a f u n c t i o n o f a c t i v e r e g i o n t h i c k n e s s f o r p-n j u n c t i o n s i n g l e c r y s t a l l i n e b a s i s s o l a r c e l l and p - i - n j u n c t i o n amorphous s i l i c o n s o l a r c e l l made from s e v e r a l c a n d i t a t e m a t e r i a l s [ 3 2 ] . I n t h e c a l c u l a t i o n , t h e s u r f a c e r e c o m b i n a t i o n v e l o c i t y u s e d were e x p e r i m e n t a l l y o b t a i n e d one f o r t h e s i n g l e c r y s t a l c e l l s [ 3 3 ] . While, g r a i n s i z e a f f e c t s t h e e f f i c i e n c y o f p o l y c r y s t a l l i n e s o l a r c e l l s . A c c e p t a b l e g r a i n s i z e a r e 30-40ym f o r S i and 2-4um f o r GaAs and InP were u s e d i n t h e c a l c u l a t i o n s [ 3 2 ] . I n t h e f i g u r e , t h e p o i n t s marked withY show a n optimum a c t i v e l a y e r t h i c k n e s s f o r e a c h m a t e r i a l . Apart from maximum a t t a i n a b l e c e l l e f f i c i e n c y , one c a n s e e from t h i s f i g u r e , t h e amorphous s i l i c o n s o l a r c e l l i s t h e t h i n n e s t o n e , and o n l y one micron o r l e s s o f t h e a c t i v e l a y e r t h i c k n e s s .

F i g u r e 5 shows r e s u l t s o f c o s t a n a l y s e s f o r t h r e e forms o f s i l i c o n b a s i s s o l a r c e l l module. The c a l c u l a t i o n s were made by assuming t h e mass p r o d u c t i o n s c a l e o f s o l a r g r a d e s i l i c o n o f 8 0 0 t o n s / y e a r (Y3.8/gr) and a n n u a l c e l l p r o d u c t i o n r a t e o f 10 Mega W a t t s / y e a r [ 3 2 ] . O t h e r m a t e r i a l p r i c e s , e x p e n s e s , l a b o r s and bank i n t e r e s t s were a c c o u n t e d on t h e b a s i s o f 1975 p r i c e l e v e l , w h i l e 1 0 0 M e g a W a t t s / y e a r and p - i - n b a s i s s o l a r c e l l i s assumed f o r t h e amorphous s i l i c o n s o l a r c e l l . A s s e e n i n t h e f i g u r e ( a ) t h e s i n g l e c r y s t a l S i a c c o u n t s f o r a b o u t 60% o f t h e t o t a l module c o s t . T h i s t e n d e n c y g e n e r a l l y more emphasized i n t h e c a s e o f advanced m a t e r i a l b a s i s s o l a r c e l l s s u c h a s GaAs, InP and CdTe c e l l s . T h i s means t h a t c o s t r e d u c t i o n o f mother s e m i c o n d u c t o r s i s t h e most i m p o r t a n t neck f o r t h e low c o s t c e l l s t a r a t e g y .

D r i f t Type P h o t o v o l t a i c E f f e c t i n a-Si

-

F i g u r e 6 shows a s c h e m a t i c i l l u s t r a t i o n o f t h e s p a c e c h a r g e d i s t r i b u t i o n p ( x ) , and t h e i n t e r n a l e l e c t r i c f i e l d d i s t r i b u t i o n E(x) c a l c u l a t e d by Okamoto and Hamakawa[27] w i t h ex r i m n t a l y d e t e r m i n e d p h y s i c a l c o n s t a n t s [ 3 4 ] and minimum gap s t a t e d e n s i t y g . = l 0 "eV-'cm-'. A n o t i c e a b l e d i f f e r - e n c e a g a i n s t t h e s i n g l e c r y s t a l p-n j u n c t i o n ??a h i p e r a b r u p t c h a r g e d i s t r i b u t i o n a t , , t h e i - l a y e r s i d e o f amorphous j u n c t i o n , b e c a u s e o f t h e e x p o n e n t i a l l y d i s t r i b u t e d l o c a l i z e d s t a t e s i n t h e amorphous S i . I n t h e h y d r o g e n a t e d a - S i , t h e minimum l o c a l i z e d

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MODULE SALES PRICE:U416rd l a x

Fig. 5. Cost amzlyses d r e s u i t s o f c f e c s i 5 i Z i r g ccicuiat-lon o f t h z s o l a r c e l l s m l e o f t h m e f o m s o f siIico,r rncrerial b a s i s . Mass prodilction s c c l e cssumed i s : 800 . tons/yeor e 3 . 8 / c r ) . S o l c r c e l i s pm;;u7.cCon scaLe:?O!~d/gecr. ?or o ~ h e r m z e r i a l p.-:ces, e T e n s e s , Zcbcrs a r k bank i n t e m s z s , t k t

o f 1-075 t m e 2 G e m e r ; i i e d .

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s o l o r c e l l aze ass~rmed.

MOULE SALES. PRICE:Y3O-42/W

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( a ) C2 S i i c e d S i S o l a r C e l l s ( b ) Ribbon C r y s t a l S.C. ( c ) Amorphous S i S.C.

( p - i - n 1 s . s . )

l t g . 6 Space charge d i s t r i b u t i o n plxi ( a ) m d Fig. 7 E l e c t r i c f i e l d dependence o f the photo- i ~ t t e m i a l e l a c t r i c f i e l d d i s t r i b u t i o n Elxi ( b i c a r r i e r g e n e m t i o n probability as a p a r a m e t e ~ i n the i-Layer o f a-Si:H p-i-n junction s t r u c t u r e . o f t h e m a l i z a t i o n d i a t m c e ro for a-Si:x.

s t a t e s l i e a t about 0.85eV below t h e condution band with an energy gap of 1.78eV a t room t e m p e r a t u r e . T h e r e f o r e , t h e Fermi l e v e l might be pinned a t t h i s p o i n t i n t h e i n t r i n s i c a-Si:H. By t h e doping of t h e donor i m p u r i t y , t h e Fermi l e v e l s h i f t s toward t o t h e conduction band and unbalanced space c h a r g e appears n e a r t h e j u n c t i o n , then, t h e e l e c t r i c a l d o u b l e t by t h e i o n i z e d donors and t h e n e g a t i v e l y charged l o c a l i z e d

s t a t e s would be formed a s shown i n Fig.6 ( b ) . As can be s e e n from t h e f i g u r e (b)

-

m t h a t t h e r e e x i s t s a high i n t e r n a l e l e c t r i c

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5000A, and a r e g i o n having almost c o n s t a n t e l e c t r i c f i e l d o f more 10 V/cm a p p e a r s i n t h e f i g u r e . V a r i a t i o n t h e e l e c t r i c f i e l d d i s t r i b u t i o n w i t h t h e a c t i v e i - l a y e r t h i c k n e s s were a f s o c a l c u l a t e d with same manner [35]

X E l e c t r i c f i e l d dependence o f photo- g e n e r a t i o n p r o b a b i l i t y P G has been f i r s t l y

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(7)

C4-1136 JOURNAL DE PHYSIQUE

c a l c u l a t e d by Pai and Enck on amorphous Selenium[36]. Recently, t h e c a l c u l a t i o n s f o r a - S i have been done by s e v e r a l groups[37.38.39] s e p e r a t e l y , however, a c o n s i d e r - a b l y good c o i n c i d e n c e i s seen i n both r e s u l t s o b t a i n e d . F i g u r e 7 shows t h e r e s u l t by t h e a u t h o r s group. I n t h e numerical c a l c u l a t i o n s , up t o one hundred E power terms i n t h e summation term o f t h e e q u a t i o n 17 o f r e f e r e n c e [ 3 6 ] .

The i n c i d e n t photon energy dependence and i t s s p a c i a l d i s t r i b u t i o n s i n t h e i - l a y e r o f t h e p h o t o c a r r i e r g e n e r a t i o n p r o b a b i l i t y P (x,%w) and t h e c a r r i e r c o l l e c t i o n p r o b a b i l i t y P (X) were c a l c u l a t e d by u t i l i z i n g t h e i n t e r n a l e l e c t r i c f i e l d d i s t r i - G b u t i o n E(x) a s shown i n F i g . 8 ( a ) and ( b ) . As we have seen i n t h e r e s u l t o f F i g . 8 ( a ) , C t h e f r e e c a r r i e r g e n e r a t i o n p r o b a b i l i t y i s almost constant through t h e most o f i- l a y e r r e g i o n i n a-Si:H p - i - n j u n c t i o n but v e r y s e n s i t i v e t o r The s p a c i a l d i s t r i -

0 '

b u t i o n s of t h e p h o t o c a r r i e r c o l l e c t i o n e f f i c i e n c y P e s c a p i n g from nongeminate p r o c e s s e s h a s been a l s o c a l u c u l a t e d and a r e p l o t t e d C l n F i g . 8 ( b ) . A s it i s evident from t h i s f i g u r e , a s t r o n g f i e l d a s s i s t e c o l l e c t i o n e f f e c t is seen. This proves t h a t t h e i n t e r n a l e l e c t r i c f i e l d exceeding l# "/cm g i v e s r i s e t o a s u f f i c i e n t d r i f t c u r r e n t , and p h o t o v o l t a i c behavior o f a-Si:H can be r e f e r e d t o " d r i f t type" photo- v o l t a i c e f f e c t i n n a t u r e . As t h e q u a n t i t a t i v e evidence, t h e d r i f t component c u r r e n t almost f o u r o r d e r s magnitude l a r g e r t h a n t h e d i f f u s i o n c u r r e n t . More c l e a r evidence on t h i s a s p e c t can be s e e n i n t h e i - l a y e r dependence of t h e p h o t o c u r r e n t c o l l e c t i o n e f f i c i e n c y o n l y a s a parameter o f h o l e d i f f u s i o n l e n g t h L by assuming P =conSt. a s shown i n Fig.10[37,38]. I t should be n o t i c e a b l e h e r e t h a t P i n t h e c a s e o f L =1000A G t h e i - l a y e r t h i c k n e s s dependence o f c o l l e c t i o n e f f i c i e n c y i s i n a good agr&ement with t h e experimental d a t a [ 4 0 ] . From t h e a n a l y s i s , a z e r o - f i e l d h o l e d i f f u s i o n l e n g t h L i s considered t o be n e a r l y equal t o l O O O A f o r t h e a u t h o r ' s group a-Si:H f i l m . P

Figure 10 shows a r e l a t i o n s h i p between t h e c o l l e c t i o n e f f c i e n c y and t h e t h e r m a l i z a t i o n d i s t a n c e of p h o t o e x c i t e d e l e c t r o n - h o l e p a i r s r f o r s e v e r a l i n c i d e n t

photon e n e r g i e s c a l s u l a t e d by assuming L =500A which i s c o n v e n t i o n a l l y o b t a i n e d

-

v g l u e [ 3 8 ] . While, t h e d o t t s p l o t t e d i n t h e

I f i g u r e show e x p e r i m e n t a l l y determined

v a l u e from t h e r e a l c o l l e c t i o n e f f i c i e n c y measurement i n t h e p - i - n a-Si:H s o l a r c e l l . With employing t h e determined ro by t h i s a n a l y s i s , one can draw a photon energy dependence o f t h e g e n e r a t i o n p r o b a b i l i t y PG. F i g u r e 11 r e p r e s n t s an i n c i d e n t photon energy dependence o f a z e r o - f i e l d photo-

4 c a r r i e r g e n e r a t i o n p r o b a b i l i t y P (E=O)

8 ^0.2 G

z d e f i n e d by e x p [ - r c / r o ] . A s i m i l a r curve

W o rC= 45 A

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POSITION X ( h ) _I-LAYER THICKNESS di OJrn)

Fig. 8 Photo-carrier generation probabiLity P Ix,hw)

a s a function o f t h e n a l i z a t i o n distance ro Fa) Fig.9 Photocurrant density JL as a ftorctim m d photo-carrier colzection probability PC(=) as o f the i - l a y e r thic2ness di for various a function of hole d i f f u s i o n Length $ I b ) . vaZues of hole d i f f u s i o n length $.

(8)

PHOTON ENERGY

01 ' ,

0 50 100 150 200 250 300 0.4 I I I

THERMALIZATION DISTANCE re(& 1.8 2.2 2.6 3.0

PHOTON ENERGY h d ( e V )

F i g . 11 Photo-carrier generation probabiZit,u PG(r71 Fi2.10 Relationship betvzen the t h e m t i z a t i o n a t E=O and as a f z z c t i o n of the e s n m t i o n a s t m c e ro mtd c o l l e c t i o n e f f i c i m C : i photon energ";' Z w . Eqerimenzal data P ~ ( 0 i obtained for m a-Si:"i p-i-n j u n c t i o ~ c e i t . by Cmndatt ( * ) cnd Fort e t a t . ( A ) m a t s o p l o t t e d .

f o r g e n e r a t i o n p r o b a b i l i t y i n e x i s t e n c e of t h e minimum e l e c t r i c f i e l d E . w i t h i n t h e i - l a y e r o f 5000A t h i c k i n a-Si:H p - i - n j u n c t i o n s t r u c t u r e i s a l s o drawn i n t h e mln f i g u r e . As can be seen i n t h i s f i g u r e , PG(E=O) i n c r e s e s with e n l a r g i n g e x c i t a t i o n photon energy and f a l l s down g r a d u a l l y wlth i n c r e a s i n g photon energy, due t o t h e s u r f a c e recombination e f f e c t , and a l s o due t o a change i n t h e e f f e c t i v e h o l e d i f - f u s i o n l e n g t h with t h e i n t e r n a l e l e c t r i c f i e l d E ( x ) . Here, we could g e t t h e maximum P of 0.85 around 2.leV. This high v a l u e of P a l s o shows a n o t h e r evidence o f ufiexpectedly small geminate recombination losG. In t h e f i g u r e t h e a s t e r i s k and t r i a n g l e p l o t s show t h e z e r o - f i e l d g e n e r a t i o n p r o b a b i l i t y e s t i m a t e d e x p e r i m e n t a l l y by Crandall[41] and Mort[42]. A s can be seen from t h i s b i g d i f f e r e n c e between t h e p r e s e n t e d e x p e r i m e n t a l l y determined p l o t s i n t h e s o l i d l i n e and a s t e r i s k and t r i a n g l e p l o t s . I t should be noted a s an i m p r ~ t a n t judgement t h a t an e x i s t e n c e o f t h e geminate recombination l o s s i n t h e a - S i p h o t o v o l t a i c p r o c e s s i s s u r e l y confirmed, however, t h i s might be a t most 5 % , t h a t i s very small. E v e n t u a l l y , f a i r l y l a r g e p h o t o c a r r i e r generation p r o b a b i l i t y and c a r r i e r c o l l e c t i o n e f f i c i e n c y e x i s t i n t h e d r i f t type p h o t o v o l t a i c process i n t h e moderately good q u a l i t y amorphous s i l i c o n .

R&D E f f o r t s t o Improve t h e Cell Performance.- Experimental v e r i f i c a t i o n s o f t h e p h o t o v o l t a i c p r o c e s s e s and t h e e f f o r t s t o f i n d t h e optimum c e l l parameters have been r e c e n t l y made by many investigators[43,44,29]. For example, an a n a l y s i s o f t h e e f f e c t i v e i - l a y e r t h i c k n e s s W . with b i a s v o l t a g e h a s been made by Shur e t a1.[45]

f o r t h e Schottky b a r r i e r cell1and f o r p - i - n c e l l Konagai[46] and r e c e n t l y by Uchida e t a 1 [ 4 7 ] . Recently, Swartz e t a l . has shown a d i f f e r n t i a l V-I c h a r a c t e r i s t i c measurement, so c a l l e d , a kind o f t h e modulation s p e c t r o s c o p y a n a l y s i s [ 4 8 ] . In t h e p h o t o v o l t a i c c e l l d e s i g n , one important t h i n g t o be i n mind i s a h o l e recombination l i m i t t e d c a r r i e r t r a n s p o r t i n t h e amorphous s i l i c o n . A c l e a r experimental evidence on t h i s p r o c e s s h a s been demonstrated by S t a e b l e r [ 4 9 ] . Recently more d i r e c t i n f o r - mation on t h i s e f f e c t has been shown by i - l a y e r t h i c k n e s s dependence o f t h e photo- c u r r e n t f o r p - f r o n t and n - f r o n t p - i - n j u n c t i o n s [ 3 8 ] . Figure 12 shows a r e s u l t of t h i s experiment. This experimental evidence shows t h a t p - t y p e f r o n t c o n f i g u r a t i o n i s more e f f i c i e n t t o c o l l e c t t h e photogenerated c a r r i e r s i n t h e p - i - n j u n c t i o n c e l l i n g e n e r a l .

An a t t e m p t has been made t o c a l c u l a t e t h e c o l l e c t i o n e f f i c i e n c i e s f o r both t h e p - i - n and t h e i n v e r t e d p - i - n j u n c t i o n s t r u c t u r e [ 5 0 ] . F i g u r e 13 shows t h e r e s u l t of c a l c u l a t i o n s . In t h e f i g u r e , Ae and A E a r e t h e Fermi energy s h i f t from t h e Fermi l e v e l i n t h e i n t r i n s i c t y p e

:PS~:H

byFfhe doping o f a c c e p t o r s and donors, r e s p e c t i v e l y . I t c o u l d be h a r d l y t o imagine t h a t n o t so much d i f f e r e n c e i n t h e c o l l e c t i o n e f f i c i e n c y s p e c t r a between t h e p - i - n and t h e i n v e r t e d s t r u c t u r e i n t h e

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C4-1138 JOURNAL DE PHYSIQUE

A-4700; LIGHT I N C I D E N T

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A-A-~ A

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L I C A T I N C I D E N T ON n-L?.YER

WAVELENGTH

Fig. 13 Calculated c o Z Z e c t i m e f f i c i e n c y s p e c t r a o f norhal p-i-n and i n v e r t e d p-i-n j u n c t i o n c e l l s as a p a m e t e r o f the junction bLdLt-in p o t e n t i a l (Fermi ZeveZ s h i f t ) i n the p-Layer he+

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l

0 0.5 c a s e o f A E =0.73eV. However, i n t h e

i - L A Y E R T H I C K N E S S d; (urn) c a s e o f A E ~ ~ = O . ~ ~ ~ V , c l e a r d i f f e r n c e

can be seegPin t h e c o l l e c t i o n e f f i c i e n c y . This r e s u l t i m p l i e s t h a t t h e i n v e r t e d fig.lZ The i-Zayer thickness dependence o f the P - ~ - ~ might work if the photocurrent y i e l d e d b p W a - s i : ~ p-i-n junction n - t y p e f r o n t l a y e r makes s u f f i c i e n t s o l a r ceZZ w i t h W iZZwri-nation on t h e p- o r n- c o n t a c t p o t e n t i a l t o i - l a y e r w i t h layer.

$ . ²

.

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lo.j

10 20 30 4.0 50 6.0 7.0 80 9.0 i-Loyei T h i cicness(x10~~)

Fig.14 P h o t o v o I t m e perfo-mrncas as a function o f the i - l a y e r zhicknesa d i i n t h e p-i-n b a s i s a-Si:H s o l z r eaZZ.

Ffleze e x i s t s optirnm p o i n t s b o t h i n *Tsc =r.d conversicn ep3Lciency n.

urnever, YDc i s almost c o n s t c n t i n t h e m x g e o f d i mors than 30934.

1 0 0 - 1 5 0 ~ - t h i c k n e s s . However, poor photo- c u r r e n t i n t h e low i n t e r n a l e l e c t r i c f i e l d y i e l d s l o w e r f i l l f a c t o r i n t h e i n v e r t e d p- i - n s t r u c t u r e . To r e a l i z e t h i s s t r u c t u r e c e l l , t h e phosphorus doped n-type m i c r o c r y s t a l l i n e might be very u s e f u l f o r t h e p r a c t i c a l d e v i c e technology.

An i n t e n s i v e i n v e s t i g a t i o n h a s been proceeded q u i t e r e c e n t l y by many workers[38].

The o t h e r c e l l parameter t o be determined i s t h e t h i c k n e s s of i - l a y e r s i n c e t h e i - l a y e r has t h e most improtant r o l e a s a p h o t o c u r r e n t g e n e r a t o r . The s i z e f a c t o r f o r t h e d e t e r m i n a t i o n o f t h e i - l a y e r t h i c k n e s s would be t h e width o f t h e r e g l o n where a d r i f t f i e l d f o r t h e photogenerated c a r r i e r s i s p r e s e n t and a l s o d i f f u s i o n l e n g t h of h o l e s . The remained r e s t r i c t i o n f o r t h e width of t h e i - l a y e r comes from t h e e f f e c t of s e r i e s r e s i s t a n c e , t h a t i s , t h e t h i c k e r i - l a y e r y i e l d s t h e lowering o f J and f i l l f a c t o r . F o r t u n a t e l y , t h e s e r e s t r s E t i o n s o f t h e i - l a y e r t h i c k n e s s o r i g i n a t e d based upon t h e r e s i d u a l f i n i t e l o c a l i z e d s t a t e s i n t h e gap, s h o r t d i f f u s i o n l e n g t h i n amorphous s i l i c o n could be saved by a v e r y s h o r t photon p e n e t r a t i o n depth owing t o t h e l a r g e a b s o r p t i o n c o e f f i c i e n t coupled with t h e l a r g e p h o t o c o n d u c t i v i t y f o r v i s i b l e l i g h t . A s e r i e s o f experimental i n v e s t i g a t i o n s f o r t h e e f f e c t of i - l a y e r t h i c k n e s s on t h e p h o t o v o l t a i c performances has been c a r r i e d o u t . F i g u r e 14 shows a summarized d a t a o f V

,

J nd conversion e f f i c i e n c y d a t a under

P ~ O ~ W ~ B '

s i m u l a t e d s u n l i g h t a s a f u n c t i o n of i - l a y e r t h i c k n e s s d . . As i s seen i n t h e f i g u r e , t h e l a r g e s t Jsc i s ob$ained by an

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1 5 . 0 ~

SMALL A n i A SOLAR C E L L ( ~ . ~ ~ ~ : ? I - a-SIC:il/a-S1 :tl CELL NO. 5 G . 3 1 6 - 1 ORTE 0 1 . 1 . 1 7 V o c = O . 9 0 9 I Y I 1 8 h r 1 3 . 4 5 ImO/cmZl F F s G 1 . 7 ( % l EFF.7.55 1x1 P l n . l 0 0 : O I m V / c n 2 1 n R E n - 0 . 0 3 3 I s m 2 1

Fig.15 Dark c o n d u c t i v i t y ad, nonnaZized p h o t o c o n d u c t i v i t y ~ U and T a c t i v a t i o n energd &? a s a f m c t i o n o f ohping f r a c z i o n o f a236 md FH3 t o [SiZq(O. 80 +cH4(0. 2) l.

CELL NO. P - I D - Z S ORTE 0 1 . 3. 3 V,,=O.UOI ( V 1 I v h = 1 1 . 0 7 I m I l / ~ n Z l F F = 6 4 . 7 1 % ) EFF.S.71 1 % ) P i n - 1 0 0 . 0 l n V / s m Z 1 RRER.0.033 I c n Z 1

FCg.16 2-V c h a r a c t e r i s t i c s o j a-SiC:'i/

a-Si:.Y h e t e r o j u n c c i o i soZar ce L2 m d ordincry p-i-n a-Si:a h o m o j z n c t l m s o l a r c e l l .

O U T P U T V O L T R G E ( V )

i - l a y e r t h i c k n e s s o f 0 . 5 - 0 . 7 v m . By a f u r t h e r o p t i m i z a t i o n o f doped l a y e r s a con- v e r s i o n e f f i c i e n c y o f 5 . 7 1 % i s o b t a i n e d w i t h t e f i l l f a c t o r o f 0 . 9 5 and a c o r r e - sponding V o f 0.801 V and a n Jsc o f 10 mA/c& f o r M-1 100 mW/cm s ~ m u l a t e d li g h t .

0 C

Approaches Employing New Materials and New Structures.- A s it h a s been shown i n t h e former s e c t i o n , p - i - n s t r u c t u r e i s g e n e r a l l y e f f i c i e n t t o g e t h i g h e r c o n v e r s i o n e f f i c i e n c y . However, a t h i c k and h e a v i l y d o p e d p - l a y e r i s n o t a p p r o p r i a t e f o r a p h o t o v o l t a i c c e l l e l e m e n t b e c a u s e o f i t s l a r g e a b s o r p t i o n c o e f f i c i e n t and s h o r t c a r r i e r l i f e t i m e which d r a s t i c a l l y r e d u c e t h e c o l l e c t i o n e f f i c i e n c y p a r t i c u l a r l y i n t h e s h o r t e r wave l e n g t h r e g i o n . While, w i t h o u t p - l a y e r , open c i r c u i t v o l t a g e V i s a t most 0 . 4 V. Under t h e s e c o n t r a d i c t i o n a l c o n d i t i o n s , t h e d o p i n g f r a c t i o n a% t h i c k n e s s o f t h e p - l a y e r have t o d e t e r m i n e a n optimum t h i c k n e s s w i t h c e r t a i n i m p u r i t y c o n t e n t s . According t o a s y s t e m a t i c e x p e r i m e n t a l v e r i f i c a t i o n s on t h e p - l a y e r thickness dependency o f t h e c e l l performance[29], an optimum t h i c k n e s s o f t h e p - l a y e r might b e i n t h e r a n g e o f 50-120A f o r a-Si:H p - i - n s o l a r c e l l s t r u c t u r e . I n t h i s r e s p e c t , a v e r y u s e f u l amorphous m a t e r i a l h a s been r e c e n t l y d e v e l o p e d . T h a t i s , a n amorphous s i l i c o n c a r b i d e and second c a n d i d a t e i s a m i c r o c r y s t a l l i n e s i l i c o n . R e c e n t l y , Tawada e t a l . [51] h a v e f a b r i c a t e d an amorphous s i l c o n c a r b i d e by t h e plasma d e c o m p o s i t i o n s of [SiH +CH ] mixed g a s system. The f i r s t t r i a l o f t h e plasma d e p o s i t e d a-SiC:H filmi(kiiZ)beeA(iIbne by Anderson and S p e a r [ 5 2 ] .

F i g u r e 1 5 shows a summary o f t h e e f f e c t s o f i m p u r i t y d o p i n g on t h e e l e c t r o n i c p r o p e r t i e s o f a-SiC:H p r e p a r e d hy t h e plasma d e c o m p o s i t i o n o f [SiH

'CH41p

.3)

g a s s y s t e m . One b i g a d v a n t a g e o f u t i l i z a t i o n o f t h i s m a t e r i a l i s tff&!'@iere

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JOURNAL DE PHYSIQUE

Fig.17 Realistic energy conversion efficiency n (real. ⁾vs. i-layer thickness as a parameter of minimum localized state density g ~ n i n a-Si:H mad homo- md heterojmction solar celis.

P

' 0 6 0 8 1 0 1 2

WAVE LENGTH 1 < v d

Fig. 18 Collection efficiency spectra ^(a!, v e r y wide range of t h e energy gap c o n t r o l - pifotocwrent distribution o f each cell

for N ! O m?d AM-l smlight (b! md for- l a b i l i t y from 1.76 to2.2eV with i n c r e a s i n g fluorescent light (c! for an a - s i : ~ p-i-n

X. An e x i s t e n c e o f t h e c o n s i d e r a b l y good ~ m c t = i m cell and single crystal Si hetero- p h o t o c o n d u c t i v i t y w i t h a impurity doping a s face j m c t i m celz.

shown i n Fig.15 i n t h e glow d i s c h a r g e produced a-SiC:H makes u s t h a t t h i s m a t e r i a l might become a v e r y u s e f u l window both f o r

p - i - n and i n v e r t e d p - i - n c e l l c o n f i g u r a t i o n s . F i g u r e 16 shows t y p i c a l J-V charac- t e r i s t i c ? o f t h e a-SIC:H/a-Si:tJ h e t e r o j u n c t i o n s o l a r c e l l having a s e n s i t i v e a r e a o f 3.3mm under AM-1 (100mW/cm ) i l l u m i n a t i o n . U t i l i z i n g a-SiC:H a s w i n d ~ w ~ m a t e r i a l , more t h a n 7.5% conversion e f f i c i e n c y has been o b t a i n e d with J =13.45mA/cm

,

V =0.909V and f i l l f a c t o r FF=0.617. [51]. Comparing t h i s a - ~ i ~ ? f i / a - S ~ : H s o l a r c e l l w%h t h e o r d i n a r y p - i - n a-Si:H c e l l , a c l e a r improvement has been seen i n t h e c e l l performances, t h a t i s , about 22% i n

.j

13.5% i n V and 32% i n t h e conversion e f f i c i e n c y . A s f o r a l a r g e a r e a (Icm ycsolaZ cel1,'Ehe conversion e f f i c i e n c y o f 6.78% has been o b t a i n e d w i t h Jsc=12.95mA/cm

,

V =0.866V and f i l l f a c t o r FF=0.605.

A s e r i e s of RED e f f o r t s by t h e new j u n c t i o n a n d o g e l l s t r u c t u r e have been a l s o i n p r o g r e s s such a s m u l t i j u n c t i o n [8,9] and s t a c k e d c e l l [l01

.

Realistic Conversion Efficiency.- In a view p o i n t of t h e low c o s t p h o t o v o l t a i c RED p r o j e c t , t h e f o r c a s t i n g o f r e a l i s t i c l i m i t o f conversion e f f i c i e n c y i n t h e a-Si:H s o l a r c e l l would be a prime i n t e r e s t f o r t h e b a l a n c e o f system c o n s i d e r a t i o n i n t h e t o t a l system. A s e r i e s o f t h e o r e t i c a l e s t i m a t i o n s o f t h e a-Si:H c e l l e f f i c i e n c i e s have been made on both i d e a l and non-ideal c e l l p a r a m e t e r s , and t h e o b t a i n e d numbers a r e 14-15%, 17-18% by Carlson e t a1.[53,54], 8% by Debney[55] and 12% by Hamakawa e t a 1 . [ 2 7 ] . However, no d e f i n i t e numerial t o be r e a l i z e d by t h e p r e s e n t s t a g e o f f i l m q u a l i t y h a s n o t appeared y e t .

A s it h a s been d i s c u s s e d i n t h e former s e c t i o n , t h e major c o n t r i b u t i o n t o t h e p h o t o v o l t a i c p r o c e s s i n t h e a-Si:H s o l a r c e l l i s due t o t h e c a r r i e r d r i f t i n g within t h e i - l a y e r r a t h e r t h a n d i f f u s i o n . To make a c o n c r e t e f e a t u r e o f t h e geminate recombination r e g i o n LGM, a s e r i e s of experimental and t h e o r e t i c a l i n v e s t i g a t i o n on t h e p - i - n b a s i s a-S1.H s o l a r c e l l s have been conducted by many i n v e s t i g a t o r s [ 4 0 , 41,421.

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C4-1141

Figure 17 shows a summarized result of the theoretical calculations on the realistic limit of cell efficiency n . as the functions of total i-layer thickness with the parameters of localized states densities g . . The calculations have been made on the basis of real cell performnaces of V , J and the curve fill factor experimentally obtained from ITO/p-i-n a-Si:H so?ar cell. It is seen from this result that realistic limit of cell efficiency for our film quality is 10.2% for V =0.8V and J =17.OmA/cm with FF=0.75. As can be seen from this figure, the maximum obtainable efficiency n . 1 .. is strongly dependent on the localised state density. Utilizing the p-type amorphous silicon carbide window, the realistic conversion efficinecy is 12.5% with V =0.91V and J =18.3mA/cm at the FF=0.75 by assuming g . =10 cm eV .

6 6m m

Before closing this review, author wishes to demonstrate a basic difference between c-Si and a-Si photovoltaic performances. The numerical calculations on the solar cell photocurrent collection efficiencies were made by using a series of experimentally determined physical constants with the described drift type photovoltaic effect theory[37,38,41]. Figure 18 shows a comparison of the collection efficiency spectra for the most typical c-Si, p-n junction having the active layer thickness of 300 pm and a-Si:H, p-i-n junction photovoltaic devices with the i- layer thickness of S000A. More direct informations for the realistic performance might be given in the short circuit current spectra as shown in Fig.18 (b) and (c) on the calculated results for AM-0, AM-1 and for the fluorescent lamp light in put.

As can be seen from this result (b), the conversion efficiency of the a-Si p-i-n junction cell should reach up to 60% of the c-Si solar cell for sun light. That is, 26% of the theoretical limit of the c-Si p-n junction teaches us roughly 15.6% of a-Si solar cell conversion efficiency as the theoretical limit. While, under the fluorescent lamp light, about same conversion efficiency would be obtained both a- Si and c-Si photovoltaic cell as shown in the figure (d). To show a comprehensive current state of the arts in the field, some outstanding cell construction and their performances are summerzed in Table.1.

Table 1 Summary of the recent achievement of amorphous Si solar oell performances

MATERIAL

a - S i :II ( s m a l l a r e a )

a - S i : I I ( l a r g e a r e a )

a - S i : I I : F

M i i l t i L a y e r e d S t a c k e d C e l l

JUNCTION STRUCTURE i T O / p - i n / s p . S S / p o l y i m i d e S n 02/ p a - S i C : l ! / i n a - S i : l l P t / t n+C v T J a - S i : H / n c - S i I T 0 / i u p a - S i : l l / S . S t e e l I T O / p w i a - S i . : l V M e C a l G l a s s / S i ' . 0²/ p l n a - S i : l l / A i S n O j / p a - S . U : : l l / l n a - S i : l l

p i n a - S i :ll I T O / n i p / M o t a l I T O / n i p / S . S . G U s s / l T O / p i n / A l I T O / p i n / M e t a l G l a s s / I T O / p i n / A l

I T O / n i p / S . S . S M )2/ p a - 5 i C : H / i n a - S i Mi no/i>;i-Si:'H/in:i-Si:U:f A u / t a - S i ; H : F / i i u - S i : l l A u P d / N b O / i > i a - S i : ll:I!/Ho

I T O / p i n p i n / S . S . C o r / p i u / C u r / p t i i / M c c u l p t a - S j / t a - S i G e / n i i - S i

Voc (niV) 690 845 550 875 815 601 909 800 822 84 5 800 830 827 8S0 8 6 S . (

770 570 880 1350 17()0 715

Js

( n i A / c n / ) 9 . 8 1 0 . 4

1 0 . 0 1 3 . 0 1 1 . 3 1 1 . 0 2 1 3 . 4 5 1 3 . 5

1 2 . 5 1 0 . 3 8 . 2 1 1 . 4 1 1 . 7 6 1 1 . 2 1 2 . 9 5

6 . 2 1 2 . 4 1 3 . 1 5 . 0 3 . 1 7 . 9 .

F . F .

(*)

4 9 70

49 62 52 6 4 . 7 6 1 . 7 60

60 62 61 60 S 6 . 9 6 5 . 9 6 0 . 5 0

52 61 57 49 51 57

T:ff. P i n

(*) (mW/cm2)

3 . S 6 . 1 4

2 . 7 6 . 9 1 S-.O 5 . 7 1 7 . 5 5 6 . 4

6 . 1 5 . 6 4 4 . 0 5 . 7 5 . 5 3 6 . 4 7 6 . 7 8 2 . 5

4 . 3 6 . 6 4 . 1 1 . 9 8 3 . 2

9 2 . 5 AMI AMI AMI AMI AMI AMI AMI AMI AMI AMI AMI AMI AMI AMI AMI AMI AMI 80 AMI AMI

A r e a (cm*) 0 . 0 9 0 . 0 3 3 0 . 0 3 1 0 . 0 4 0 . 0 4 0 . 0 3 3 0 . 0 3 3

1 . 2 1 . 2 1.0 1 . 1 9 1.0 1.2 1.0 0 . 0 4

0 . 7 3 0 . 0 3 3 3 5 . 0 l . S

INSTITUTE PRESENTED T e i j i n C . R . L . O s a k a U n i v .

H i r o s h i m a U n i v . S a n y o

NEC Osaka U n i v . Osaka U n i v . Dundee U n i v .

RCA

F u j i E l e c t r i c S a n y o RCA

Tokyo I n s t . T e c h . F u j i E l e c t r i c Osaka U n i v . .Sanyo

Tokyo I n s t . T e c h ECO

O s a k a U n i v . RCA M i t s u b i s h i

RELEASED DATE O c t . , 1 9 8 0 O c c . , 1 9 8 0 M a r c h , 1 9 8 1 M a r c h , 1 9 8 1 M a r c h , 1 9 8 1 M a r c h , 1981 A p r i l , 1 9 8 1

May, 1081 J u n e , 1 9 8 0

J u n e , 1 9 8 0 J u n e , I 9 6 0 J u l y , 1 9 8 0 M a r c h , 1 9 8 1 M a r c h , 1 9 8 1 A p r i l , 1 9 8 1 J u n e , 1 9 8 0 toy,1981 May,1981 J u l y , 1 9 7 9 J u n e , 1980 D e c , 1980

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C4-1142 JOURNAL DE PHYSIQUE

Acknowleqement.- The author i s indebted to Drs. H. Okamoto, Y. Tawada, S. Hotta and Mr. S. Nonomura for t h e i r helpful discussion and a s s i s t a n c e on the preparation of t h i s review paper. The reference papers noted by * were p a r t l y sponsored by the Special Research Project on "Amorphous Materials and Physics" in the Ministry of Education and also by the Sun Shine Project Photovoltaic Division AIST. MITI.

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