14 Electrode Band Structure and Interface States in Photoelectrochemical Cells JOHN G. MAVROIDES, JOHN C. F A N , and HERBERT J. ZEIGER
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch014
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, M A 02173
E f f i c i e n t u t i l i z a t i o n of s o l a r energy by means of photoe l e c t r o l y s i s r e q u i r e s an e l e c t r o d e material that i s not only s t a b l e but also has an e l e c t r o n a f f i n i t y that i s small enough to give s u f f i c i e n t band bending. In a d d i t i o n , s a t i s f a c t o r y material must have an energy gap that i s well matched to the s o l a r spectrum. The energy gaps of both TiO and SrTiO , which have values of 3.0 and 3.2 eV, r e s p e c t i v e l y , are considerably too large to s a t i s f y t h i s l a t t e r requirement. Furthermore no b e t t e r s i n g l e chemical compound has been found. This suggests t r y i n g a combination of compounds f o r electrodes. We w i l l d e s c r i b e our program to develop e l e c t r o d e s , with the d e s i r e d electrochemical p r o p e r t i e s , by such an approach. In p a r t i c u l a r , we w i l l discuss composite s t r u c t u r e s and s o l i d s o l u t i o n s . This i s an ongoing program and what w i l l be presented are mainly the concepts with only a few p r e l i m i n a r y results. 2
Composite
3
Electrodes
B e g i n n i n g w i t h c o m p o s i t e e l e c t r o d e s t h e s i m p l e s t scheme h e r e i s t o c o a t t h e s u r f a c e o f a s m a l l gap s e m i c o n d u c t o r t h a t i s w e l l matched t o t h e s o l a r s p e c t r u m b u t w h i c h i s e l e c t r o c h e m i c a l l y u n s t a b l e w i t h a t h i n f i l m o f a wide gap, e l e c t r o c h e m i c a l l y s t a b l e semiconductor. To d e m o n s t r a t e t h e f e a s i b i l i t y o f u s i n g s u c h a c o m p o s i t e e l e c t r o d e , t h e f i l m must be t h i n enough - o f t h e o r d e r o f 5 0 - 1 0 0 Â o r l e s s - so t h a t a t l e a s t some o f t h e p h o t o g e n e r a t e d c a r r i e r s i n t h e s m a l l bandgap m a t e r i a l can tunnel through t o t h e e l e c t r o l y t e . Furthermore t h e f i l m must n o t have any c r a c k s o r p i n h o l e s s i n c e t h e s e w o u l d
0097-6156/81/0146-0217$05.00/0 © 1981 American Chemical Society Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
218
PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE
INTERFACES
a l l o w t h e e l e c t r o l y t e t o p e n e t r a t e t h r o u g h t o t h e s m a l l bandgap m a t e r i a l and c a u s e d i s s o l u t i o n . The f o r m a t i o n o f such a t h i n d e f e c t - f r e e f i l m by c o n v e n t i o n a l t e c h n i q u e s i s v e r y d i f f i c u l t and a l t h o u g h many a t t e m p t s have been r e p o r t e d , s u c c e s s has not been a c h i e v e d {1_ 2J. We a r e i n v e s t i g a t i n g a s l i g h t l y more complex t y p e o f c o m p o s i t e e l e c t r o d e , i n w h i c h t h e s m a l l bandgap s e m i c o n d u c t o r i s p r o t e c t e d by a much t h i c k e r , t w o - p h a s e f i l m s u c h as shown i n F i g u r e 1. Here t h e low bandgap s u b s t r a t e m a t e r i a l i s CdSe, w h i c h has an ες = 1.7 eV, and t h e l a r g e bandgap m a t e r i a l i s SrTi03. In t h i s a r r a n g e m e n t t h e p r o t e c t i v e c o a t i n g , w h i c h i s o f t h e o r d e r o f 50QÂ t h i c k , c o n s i s t s o f s m a l l g r a i n s o f CdSe (~ 5C& in size) i n a matrix of SrTi03. During p h o t o e l e c t r o l y s i s , the h o l e - e l e c t r o n p a i r s a r e g e n e r a t e d i n t h e CdSe and s e p a r a t e d by the depletion f i e l d . The h o l e s move t o t h e s u b s t r a t e - f i l m i n t e r f a c e and t h e n t u n n e l t h r o u g h S r T i 0 3 t o t h e n e a r e s t g r a i n o f CdSe. A f t e r a few jumps between g r a i n s , t h e h o l e s r e a c h t h e electrolyte. T h i s f i l m s h o u l d be e f f e c t i v e i n p r o t e c t i n g t h i s s u b s t r a t e p r o v i d e d t h e g r a i n s are i s o l a t e d from each o t h e r . In t h i s case the g r a i n s t h a t are i n i t i a l l y i n c o n t a c t w i t h the e l e c t r o l y t e w i l l be d i s s o l v e d away u n t i l a c o n t i n u o u s p r o t e c t i v e surface of SrTi03 i s l e f t in contact with the e l e c t r o l y t e . F i g u r e 2 i s a s c h e m a t i c e n e r g y band d i a g r a m o f t h i s c o m p o s i t e e l e c t r o d e i n d i c a t i n g how t h e m i n o r i t y c a r r i e r s f r o m t h e b u l k CdSe v a l e n c e band t u n n e l t h r o u g h t h e S r T i 0 3 t o CdSe g r a i n s v i a s u r f a c e s t a t e s . The e a s y p a s s a g e o f m i n o r i t y c a r r i e r s through the i n t e r f a c e t o the e l e c t r o l y t e r e q u i r e s t h a t t h e v a l e n c e band edge o f CdSe o v e r l a p t h e s u r f a c e s t a t e s i n t h e bandgap o f t h e S r T i 0 3 . For f r e e passage of t h e m a j o r i t y c a r r i e r s between t h e two s e m i c o n d u c t o r s , t h e i r e l e c t r o n a f f i n i t i e s s h o u l d be e q u a l so t h a t t h e p o t e n t i a l b a r r i e r a t t h e i n t e r f a c e s between them i s n e g l i g i b l e . These c o n d i t i o n s a r e f a i r l y w e l l met i n t h e C d S e - S r T i Û 3 s y s t e m . In c o n n e c t i o n w i t h t h i s s y s t e m a number o f q u e s t i o n s a r i s e . F i r s t o f a l l , b e c a u s e o f i n t e r a c t i o n and s c a t t e r i n g e f f e c t s , t h e o p t i c a l a b s o r p t i o n o f t h e t w o - p h a s e f i l m c o u l d be s i g n i f i c a n t l y g r e a t e r than t h a t o f t h e i n d i v i d u a l components. In t h i s c a s e i t w o u l d be n e c e s s a r y t o s h i n e t h e l i g h t o n t o t h e back s u r f a c e o f t h e s u b s t r a t e and t h e s u b s t r a t e w o u l d have t o be v e r y t h i n , o f t h e o r d e r o f t h e d i f f u s i o n l e n g t h ; s e c o n d l y , even i f t h e c a r r i e r s a r e e x c i t e d t h r o u g h t h e p r o t e c t i v e l a y e r and w i t h i n t h e d e p l e t i o n r e g i o n o f t h e s u b s t r a t e , how e f f i c i e n t l y would t h e y t r a n s f e r t o t h e e l e c t r o l y t e ? What w o u l d t h e n a t u r e o f t h e s t a t e s a t t h e i n t e r f a c e s be and how e f f i c i e n t l y w o u l d c h a r g e t r a n s f e r between t h e g r a i n s t o t h e e l e c t o l y t e . What w o u l d t h e t u n n e l i n g e f f i c i e n c y be and w o u l d c a r r i e r s c a t t e r i n g be a p r o b l e m ? T h i r d l y , would the f i l m f a b r i c a t i o n process cause work damage t o t h e s u b s t r a t e ?
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch014
9
Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
MAVROIDES E T A L .
Electrode Band Structure and Interface States
219
«S3» Ο .
1
} Λ* »
ο
Sift,,®.
(A)
0
Û
6
ÎTVBULK SUBSTRATE
Ο ο
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch014
ο
