- Email: [email protected]
In situ optical characterization of structures grown by metal-organic chemical vapor deposition
456 GaAs-based
ternary
compounds
and solar cell research*
S. M. V E R N O N , S. P. T O B I N a n d R. G. W O L F S O N
,~pire Corporation, Bedford, MA (U.S.A.) T h e overall goal of this research is to establish a t e c h n o l o g y for t h e p r e p a r a t i o n of very high efficiency m u l t i p l e b a n d gap solar cells for use in n o n - c o n c e n t r a t i n g p h o t o voltaic systems. T h e a p p r o a c h t a k e n involves t h e use of GaAs-based t e r n a r y materials for high efficiency t o p cells ( b a n d gap Eg = 1.7 eV) a n d silicon (Eu = 1.1 eV) for b o t t o m cells and/or substrates. Significant progress has b e e n m a d e in each of t h e four p r o g r a m areas, w h i c h are repres e n t e d b y these highlights o f t h e past y e a r ' s work. (1) High efficiency GaAIAs cells have b e e n fabricated. Specific a c h i e v e m e n t s include 21.2% efficiency at Eg = 1.65 eV a n d 17.2 efficiency at E~ = 1.70 eV. (2) A G a A s P - o n - G a A s cell t e c h n o l o g y has b e e n established. P r e l i m i n a r y e x p e r i m e n t s have resulted in a 14.7% cell at Eg = 1.64 eV. (3) A cell t e c h n o l o g y has b e e n e s t a b l i s h e d involving GaAs g r o w n directly on a silicon s u b s t r a t e . Efficiencies up to 7% have b e e n achieved. (4) Studies have b e e n i n i t i a t e d to e x a m i n e t h e r e p r o d u c i b i l i t y of GaAs, GaAIAs a n d GaAsP cell f a b r i c a t i o n . All cell data r e p o r t e d a b o v e are air mass 1.5 global, 100 mW c m 2, t o t a l area efficiencies for cells w h i c h are 0.5 cm × 0.5 cm in size. Also o c c u r r i n g t h r o u g h o u t this research p r o g r a m has b e e n t h e c o n t i n u e d d e v e l o p m e n t of a m e t a l organic c h e m i c a l v a p o r d e p o s i t i o n ( M O C V D ) r e a c t o r t e c h n o l o g y for t h e effic i e n t p r o d u c t i o n o f high q u a l i t y solar cells. Typical p e r f o r m a n c e data o f t h e m o d e r a t e size ( 1 3 5 c m 2) r e a c t o r s used in this p r o g r a m have b e e n carefully m e a s u r e d w i t h results as follows: t h i c k n e s s u n i f o r m i t y per b a t c h is -+4%; d o p i n g u n i f o r m i t y per b a t c h is -+12%; gallium u t i l i z a t i o n efficiency is 32%. T h e data i m p l y t h e p o t e n t i a l for relatively i n e x p e n sive p r o d u c t i o n of solar cells. Also d e v e l o p e d a n d installed r e c e n t l y at Spire ( u n d e r internal f u n d i n g ) is a large scale M O C V D s y s t e m w i t h a g r o w t h area o f 500 c m z per b a t c h . This p r o g r a m is f u n d e d b y t h e Solar E n e r g y Research I n s t i t u t e u n d e r C o n t r a c t XT-404024-1.
In situ o p t i c a l c h a r a c t e r i z a t i o n chemical
of structures
grown
by metal-organic
vapor deposition*
J. M. O L S O N and A. K I B B L E R
Solar Energy Research Institute, Golden, CO 80302 (U.S.A.) A surface m o n i t o r i n g t e c h n i q u e using t h e quasi-elastic light s c a t t e r i n g o f a laser ha,, b e e n used to c h a r a c t e r i z e t h e surfaces o f s t r u c t u r e s g r o w n b y m e t a l - o r g a n i c c h e m i c a l v a p o r d e p o s i t i o n in situ, including s t r a i n e d - l a y e r s u p e r l a t t i c e s (SLSs) o f G a I n A s / G a A s and h e t e r o - e p i l a y e r s o f GaP/Si a n d G a I n A s / G a A s . F o r t h e g r o w t h o f Gal xInxAs/GaAs, o b s e r v a t i o n s i n d i c a t e t h a t t h r e e - d i m e n s i o n a l g r o w t h o c c u r s at a critical t h i c k n e s s of ac * A b s t r a c t o f a p a p e r p r e s e n t e d at t h e 7 t h P h o t o v o l t a i c A d v a n c e d R e s e a r c h a n d Devel o p m e n t P r o j e c t Review Meeting, Denver, CO, U.S.A., May 13, 1 9 8 6 .
457 initial t w o - d i m e n s i o n a l layer. This critical t h i c k n e s s decreases m o n o t o n i c a l l y w i t h increasing i n d i u m c o n c e n t r a t i o n x a n d is essentially zero for x = 1.0. T h e same s i t u a t i o n o c c u r s for SLSs w h e n a s u b l a y e r lattice c o n s t a n t differs significantly f r o m t h a t o f t h e previously g r o w n layer. T h i s implies t h a t a n SLS w i t h good t w o - d i m e n s i o n a l m o r p h o l o g y is l i m i t e d b y t h e i n t r i n s i c t e n d e n c y o f an e p i t a x i a l layer to a s s u m e a t h r e e - d i m e n s i o n a l m o r p h o l o g y .
Highly efficient
silicon solar cells*
M. B. S P I T Z E R
Spire Corporation, Bedford, MA (U.S.A.) R e s e a r c h o n increased e f f i c i e n c y in silicon solar cells is described. During t h e first 2 years of this w o r k , research focused o n r e d u c i n g t h e m i n o r i t y carrier r e c o m b i n a t i o n at t h e f r o n t surface a n d w i t h i n t h e e m i t t e r o f t h e cell. Having successfully o b t a i n e d low levels o f s u c h r e c o m b i n a t i o n , w o r k in t h e t h i r d year c o m p r i s e d t h e r e d u c t i o n o f base a n d b a c k surface r e c o m b i n a t i o n . A l t h o u g h base r e c o m b i n a t i o n can be r e d u c e d b y decreasing t h e w i d t h of t h e base region, o u r e x p e r i m e n t s s h o w t h a t b a c k surface r e c o m b i n a t i o n emerges as a limiting f a c t o r in such cells. A l t e r n a t i v e l y , t h e base r e c o m b i n a t i o n rate can be r e d u c e d b y g e t t e r i n g i m p u r i t i e s , a n d a n efficiency o f a p p r o x i m a t e l y 19% has b e e n o b t a i n e d in this way, w i t h a n o p e n - c i r c u i t voltage as high as 6 6 0 m V , for b o t h n+pp + and p + n n + cells. This w o r k is s u p p o r t e d b y t h e U.S. D e p a r t m e n t o f E n e r g y u n d e r t h e Solar E n e r g y Research Institute Contract ZB-3-02090-3.
Photovoltaic
safety conference*
WERNER LUFT
Solar Energy Research Institute, Golden, CO 80401 (U.S.A.) A P h o t o v o l t a i c s S a f e t y C o n f e r e n c e was held o n 16 - 17 J a n u a r y 1986. While p h o t o voltaics are c o n s i d e r e d very safe, toxic, p y r o p h o r i c , or o t h e r w i s e d a n g e r o u s m a t e r i a l s -m a i n l y gases -- are used in t h e m a n u f a c t u r e of p h o t o v o l t a i c materials, especially in t h e m a n u f a c t u r e o f t h i n film materials. Universities, n a t i o n a l l a b o r a t o r i e s a n d industrial c o m p a n i e s are active in t h e f a b r i c a t i o n o f p h o t o v o l t a i c materials. T h e o b j e c t i v e o f t h e c o n f e r e n c e was to e x c h a n g e i n f o r m a t i o n b e t w e e n r e s e a r c h e r s a n d facility m a n a g e r s a b o u t s a f e t y practices a n d e q u i p m e n t . 118 p e r s o n s a t t e n d e d t h e c o n f e r e n c e : 42 f r o m i n d u s t r y , 11 f r o m o t h e r n a t i o n a l l a b o r a t o r i e s , 8 f r o m universities, 2 f r o m foreign c o u n t r i e s , t h e rest f r o m various d o m e s t i c o r g a n i z a t i o n s a n d f r o m t h e Solar E n e r g y R e s e a r c h I n s t i t u t e . 23 p a p e r s were p r e s e n t e d in five sessions. T w o sessions addressed personal safety, o n e session covered public safety, o n e session covered t r a i n i n g a n d e m e r g e n c y p r e p a r e d n e s s , a n d o n e session covered legal aspects a n d safety o r g a n i z a t i o n s . Most p a p e r s in t h e area o f p e r s o n a l safety c o n c e n t r a t e d o n t h e t o x i c i t y o f gases a n d liquids used for a m o r p h o u s silicon, I I - V I c o m p o u n d a n d I I I - V c o m p o u n d devices, b u t fire a n d e x p l o s i o n h a z a r d s * A b s t r a c t o f a p a p e r p r e s e n t e d at t h e 7 t h P h o t o v o l t a i c A d v a n c e d R e s e a r c h a n d Develo p m e n t Project Review M e e t i n g , Denver, CO, U.S.A., May 13, 1986.
ternary
compounds
and solar cell research*
S. M. V E R N O N , S. P. T O B I N a n d R. G. W O L F S O N
,~pire Corporation, Bedford, MA (U.S.A.) T h e overall goal of this research is to establish a t e c h n o l o g y for t h e p r e p a r a t i o n of very high efficiency m u l t i p l e b a n d gap solar cells for use in n o n - c o n c e n t r a t i n g p h o t o voltaic systems. T h e a p p r o a c h t a k e n involves t h e use of GaAs-based t e r n a r y materials for high efficiency t o p cells ( b a n d gap Eg = 1.7 eV) a n d silicon (Eu = 1.1 eV) for b o t t o m cells and/or substrates. Significant progress has b e e n m a d e in each of t h e four p r o g r a m areas, w h i c h are repres e n t e d b y these highlights o f t h e past y e a r ' s work. (1) High efficiency GaAIAs cells have b e e n fabricated. Specific a c h i e v e m e n t s include 21.2% efficiency at Eg = 1.65 eV a n d 17.2 efficiency at E~ = 1.70 eV. (2) A G a A s P - o n - G a A s cell t e c h n o l o g y has b e e n established. P r e l i m i n a r y e x p e r i m e n t s have resulted in a 14.7% cell at Eg = 1.64 eV. (3) A cell t e c h n o l o g y has b e e n e s t a b l i s h e d involving GaAs g r o w n directly on a silicon s u b s t r a t e . Efficiencies up to 7% have b e e n achieved. (4) Studies have b e e n i n i t i a t e d to e x a m i n e t h e r e p r o d u c i b i l i t y of GaAs, GaAIAs a n d GaAsP cell f a b r i c a t i o n . All cell data r e p o r t e d a b o v e are air mass 1.5 global, 100 mW c m 2, t o t a l area efficiencies for cells w h i c h are 0.5 cm × 0.5 cm in size. Also o c c u r r i n g t h r o u g h o u t this research p r o g r a m has b e e n t h e c o n t i n u e d d e v e l o p m e n t of a m e t a l organic c h e m i c a l v a p o r d e p o s i t i o n ( M O C V D ) r e a c t o r t e c h n o l o g y for t h e effic i e n t p r o d u c t i o n o f high q u a l i t y solar cells. Typical p e r f o r m a n c e data o f t h e m o d e r a t e size ( 1 3 5 c m 2) r e a c t o r s used in this p r o g r a m have b e e n carefully m e a s u r e d w i t h results as follows: t h i c k n e s s u n i f o r m i t y per b a t c h is -+4%; d o p i n g u n i f o r m i t y per b a t c h is -+12%; gallium u t i l i z a t i o n efficiency is 32%. T h e data i m p l y t h e p o t e n t i a l for relatively i n e x p e n sive p r o d u c t i o n of solar cells. Also d e v e l o p e d a n d installed r e c e n t l y at Spire ( u n d e r internal f u n d i n g ) is a large scale M O C V D s y s t e m w i t h a g r o w t h area o f 500 c m z per b a t c h . This p r o g r a m is f u n d e d b y t h e Solar E n e r g y Research I n s t i t u t e u n d e r C o n t r a c t XT-404024-1.
In situ o p t i c a l c h a r a c t e r i z a t i o n chemical
of structures
grown
by metal-organic
vapor deposition*
J. M. O L S O N and A. K I B B L E R
Solar Energy Research Institute, Golden, CO 80302 (U.S.A.) A surface m o n i t o r i n g t e c h n i q u e using t h e quasi-elastic light s c a t t e r i n g o f a laser ha,, b e e n used to c h a r a c t e r i z e t h e surfaces o f s t r u c t u r e s g r o w n b y m e t a l - o r g a n i c c h e m i c a l v a p o r d e p o s i t i o n in situ, including s t r a i n e d - l a y e r s u p e r l a t t i c e s (SLSs) o f G a I n A s / G a A s and h e t e r o - e p i l a y e r s o f GaP/Si a n d G a I n A s / G a A s . F o r t h e g r o w t h o f Gal xInxAs/GaAs, o b s e r v a t i o n s i n d i c a t e t h a t t h r e e - d i m e n s i o n a l g r o w t h o c c u r s at a critical t h i c k n e s s of ac * A b s t r a c t o f a p a p e r p r e s e n t e d at t h e 7 t h P h o t o v o l t a i c A d v a n c e d R e s e a r c h a n d Devel o p m e n t P r o j e c t Review Meeting, Denver, CO, U.S.A., May 13, 1 9 8 6 .
457 initial t w o - d i m e n s i o n a l layer. This critical t h i c k n e s s decreases m o n o t o n i c a l l y w i t h increasing i n d i u m c o n c e n t r a t i o n x a n d is essentially zero for x = 1.0. T h e same s i t u a t i o n o c c u r s for SLSs w h e n a s u b l a y e r lattice c o n s t a n t differs significantly f r o m t h a t o f t h e previously g r o w n layer. T h i s implies t h a t a n SLS w i t h good t w o - d i m e n s i o n a l m o r p h o l o g y is l i m i t e d b y t h e i n t r i n s i c t e n d e n c y o f an e p i t a x i a l layer to a s s u m e a t h r e e - d i m e n s i o n a l m o r p h o l o g y .
Highly efficient
silicon solar cells*
M. B. S P I T Z E R
Spire Corporation, Bedford, MA (U.S.A.) R e s e a r c h o n increased e f f i c i e n c y in silicon solar cells is described. During t h e first 2 years of this w o r k , research focused o n r e d u c i n g t h e m i n o r i t y carrier r e c o m b i n a t i o n at t h e f r o n t surface a n d w i t h i n t h e e m i t t e r o f t h e cell. Having successfully o b t a i n e d low levels o f s u c h r e c o m b i n a t i o n , w o r k in t h e t h i r d year c o m p r i s e d t h e r e d u c t i o n o f base a n d b a c k surface r e c o m b i n a t i o n . A l t h o u g h base r e c o m b i n a t i o n can be r e d u c e d b y decreasing t h e w i d t h of t h e base region, o u r e x p e r i m e n t s s h o w t h a t b a c k surface r e c o m b i n a t i o n emerges as a limiting f a c t o r in such cells. A l t e r n a t i v e l y , t h e base r e c o m b i n a t i o n rate can be r e d u c e d b y g e t t e r i n g i m p u r i t i e s , a n d a n efficiency o f a p p r o x i m a t e l y 19% has b e e n o b t a i n e d in this way, w i t h a n o p e n - c i r c u i t voltage as high as 6 6 0 m V , for b o t h n+pp + and p + n n + cells. This w o r k is s u p p o r t e d b y t h e U.S. D e p a r t m e n t o f E n e r g y u n d e r t h e Solar E n e r g y Research Institute Contract ZB-3-02090-3.
Photovoltaic
safety conference*
WERNER LUFT
Solar Energy Research Institute, Golden, CO 80401 (U.S.A.) A P h o t o v o l t a i c s S a f e t y C o n f e r e n c e was held o n 16 - 17 J a n u a r y 1986. While p h o t o voltaics are c o n s i d e r e d very safe, toxic, p y r o p h o r i c , or o t h e r w i s e d a n g e r o u s m a t e r i a l s -m a i n l y gases -- are used in t h e m a n u f a c t u r e of p h o t o v o l t a i c materials, especially in t h e m a n u f a c t u r e o f t h i n film materials. Universities, n a t i o n a l l a b o r a t o r i e s a n d industrial c o m p a n i e s are active in t h e f a b r i c a t i o n o f p h o t o v o l t a i c materials. T h e o b j e c t i v e o f t h e c o n f e r e n c e was to e x c h a n g e i n f o r m a t i o n b e t w e e n r e s e a r c h e r s a n d facility m a n a g e r s a b o u t s a f e t y practices a n d e q u i p m e n t . 118 p e r s o n s a t t e n d e d t h e c o n f e r e n c e : 42 f r o m i n d u s t r y , 11 f r o m o t h e r n a t i o n a l l a b o r a t o r i e s , 8 f r o m universities, 2 f r o m foreign c o u n t r i e s , t h e rest f r o m various d o m e s t i c o r g a n i z a t i o n s a n d f r o m t h e Solar E n e r g y R e s e a r c h I n s t i t u t e . 23 p a p e r s were p r e s e n t e d in five sessions. T w o sessions addressed personal safety, o n e session covered public safety, o n e session covered t r a i n i n g a n d e m e r g e n c y p r e p a r e d n e s s , a n d o n e session covered legal aspects a n d safety o r g a n i z a t i o n s . Most p a p e r s in t h e area o f p e r s o n a l safety c o n c e n t r a t e d o n t h e t o x i c i t y o f gases a n d liquids used for a m o r p h o u s silicon, I I - V I c o m p o u n d a n d I I I - V c o m p o u n d devices, b u t fire a n d e x p l o s i o n h a z a r d s * A b s t r a c t o f a p a p e r p r e s e n t e d at t h e 7 t h P h o t o v o l t a i c A d v a n c e d R e s e a r c h a n d Develo p m e n t Project Review M e e t i n g , Denver, CO, U.S.A., May 13, 1986.