Photoeffects at Semiconductor-Electrolyte Interfaces - American

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Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on November 10, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch025

Photoeffects on Solid-State Photoelectrochemical Cells PETER G. P. A N G and ANTHONY F. SAMMELLS Institute of Gas Technology, IIT Center, 3424 South State Street, Chicago, IL 60616

Photoeffects in solid-state photovoltaic devices have been demonstrated for a variety of interfaces, e.g., at p-n junctions and Schottky barriers. For such devices a series of highly-controlled fabrication steps are necessary since high-purity semiconductor materials are required to reduce electron-electron hole recombination reactions at trap sites. Greater simplicity in solar cell fabrication can, however, be achieved with liquid-junction photoelectrochemical cells (PEC). For such devices a junction is formed merely by introducing a semiconductor electrode into a redox electrolyte. Illumination of, for example an n-type semiconductor, can result in oxidation of a redox species via its reaction with electron holes photogenerated in the semiconductor space charge region. Solar energy conversion efficiencies achieved with such liquid-junction devices have recently been approaching those of conventional solid-state photovoltaic devices. Photoelectrochemical devices have the inherent advantage that either electricity or useful chemical species, potentially available for later electrochemical discharge, can be produced at the interface, whereas only direct electricity generation occurs with solid-state photovoltaic cells. High solar energy conversion efficiencies have been accomplished by narrow band-gap materials which can utilize a large fraction of the solar spectrum. However, in many cases such materials can manifest photoanodic corrosion effects with time, particularly when the redox species equilibrium potential is close to the semiconductor decomposition potential. Approaches to stabilize those semiconductors have included the evaluation of both organic and inorganic nonaqueous electrolytes. Most work is, however, at an early stage. Extending the thought of identifying electrolytes having higher stability and lower nucleophilicity has led us to consider the utility of solid electrolytes for photoelectrochemical devices. No work has been previously reported on the evaluation of solid-state photoelectrochemical systems. It is difficult to conceive a regenerative solid-state PEC because of the ionic specificity of most solid electrolytes. This has led us to consider semiconductor/solid electrolyte interfaces, where an immobile redox species might be present, either in a solid electrolyte lattice 0097-6156/81/0146-0387$05.00/0 © 1981 American Chemical Society Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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p o s i t i o n , o r as a s e p a r a t e p h a s e , i n i n t i m a t e c o n t a c t w i t h t h e s o l i d electrolyte. I l l u m i n a t i o n of a semiconductor/solid e l e c t r o l y t e i n t e r f a c e c o n t a i n i n g such a redox s p e c i e s might r e s u l t i n e i t h e r o x i d a t i o n o r r e d u c t i o n o f t h i s s p e c i e s , d e p e n d i n g upon w h e t h e r t h e s e m i c o n d u c t o r i s a photoanode or a photocathode. Because the redox s p e c i e s i s i m m o b i l e , s u c h a d e v i c e w o u l d s t o r e t h e e n e r g y p r o d u c e d . To some e x t e n t t h e v i a b i l i t y o f t h i s a p p r o a c h w i l l , i n p a r t , be d e p e n d e n t upon t h e a c c o m m o d a t i o n o f a c o n c e n t r a t i o n o f m o b i l e c a t i o n s somewhat g r e a t e r than t h a t r e q u i r e d f o r acceptable i o n i c c o n d u c t i v i t y across such a s o l i d - s t a t e c e l l . This i s necessary to preserve electroneut r a l i t y i n t h e p r o x i m i t y o f t h e i m m o b i l e r e d o x s p e c i e s . From c r y s t a l l o g r a p h i c c o n s i d e r a t i o n s , b e t a - a l u m i n a has a p a r t i c u l a r l y s u i t a b l e l a t t i c e s t r u c t u r e f o r t h i s c o n c e p t . H e r e , t h e a and b a x e s a r e p e r i o d i c a l l y s e p a r a t e d by A1-0-A1 b o n d s , between w h i c h t h e m o b i l e sodium i o n s m i g r a t e . A s i g n i f i c a n t e x c e s s o f s o d i u m i o n s c a n be a c commodated w i t h i n t h i s m a t e r i a l . I n t h e c o u r s e o f t h i s w o r k s o l i d e l e c t r o l y t e s e v a l u a t e d have i n c l u d e d b e t a - a l u m i n a , l i t h i u m i o d i d e , and r u b i d i u m s i l v e r i o d i d e . U n f o r t u n a t e l y , t h e s o d i u m i o n c o n d u c t i v i t y o f b e t a - a l u m i n a a t room t e m p e r a t u r e i s t o o low t o be o f u t i l i t y i n s o l i d - s t a t e p h o t o e l e c t r o c h e m i c a l d e v i c e s . Any p h o t o p o t e n t i a l gene r a t e d w o u l d p r o b a b l y be i n s u f f i c i e n t t o overcome r e s i s t a n c e l o s s e s w i t h i n the s o l i d e l e c t r o l y t e . Because of the h i g h i o n i c c o n d u c t i v i t y of the rubidium s i l v e r i o d i d e a t ambient temperature, e x p e r i m e n t a l e m p h a s i s was p l a c e d on t h i s m a t e r i a l . -1

R u b i d i u m s i l v e r i o d i d e , R b A g I ç , has a c o n d u c t i v i t y o f O.27 ohm cm a t a m b i e n t t e m p e r a t u r e s (1) and has f o u n d a p p l i c a t i o n i n g a l v a n i c c e l l s ( 2 ) . S o l i d e l e c t r o l y t e s have a l s o been s y n t h e s i z e d t h r o u g h compound f o r m a t i o n b e t w e e n A g i and v a r i o u s s u b s t i t u t e d ammonium h a l i d e s (3-50 and a l s o b e t w e e n A g i and s u l f o n i u m i o d i d e s (6). The s i n g l e c r y s t a l s t r u c t u r e s o f c e r t a i n u n i q u e p h a s e s have b e e n i n v e s t i g a t e d (7-11) t o d e t e r m i n e t h e s t r u c t u r a l f e a t u r e s t h a t p e r m i t f a s t ion d i f f u s i o n i n s o l i d s . I t was o b s e r v e d t h a t t h e i o d i d e i o n s f o r m f a c e - s h a r i n g polyhedra ( g e n e r a l l y tetrahedra) w i t h the s i l v e r ions s i t u a t e d w i t h i n these polyhedra. On t h e a v e r a g e t h e r e a r e two o r three vacant s i t e s per s i l v e r i o n , thus p e r m i t t i n g ready d i f f u s i o n through the faces of the polyhedra. P h o t o e f f e c t s a t the s e m i c o n d u c t o r / R b A g I s i n t e r f a c e were i n v e s t i g a t e d w i t h the o b j e c t i v e of i d e n t i f y i n g the u t i l i t y of t h i s s o l i d e l e c t r o l y t e material i n solid-state photoelectrochemical devices with storage. -1

Experimental S o l i d - s t a t e c e l l s w e r e f a b r i c a t e d u s i n g R b A g I s ( R e s e a r c h Org a n i c I n o r g a n i c , Sun V a l l e y , C a l i f o r n i a ) , s i l v e r powder ( C e r a c , I n c . ) and P b l 2 a n d / o r i o d i n e powder. L a y e r s o f t h e a p p r o p r i a t e m i x t u r e s w e r e p r e p a r e d i n s i d e a c o m m e r c i a l KBr d i e and p r e s s e d a t 2000 k g / c m u s i n g a C a r v e r l a b o r a t o r y p r e s s . The r e s u l t i n g c e l l ( a b o u t 2mm t h i c k ) was s a n d w i c h e d b e t w e e n a c o u n t e r e l e c t r o d e and a t r a n s p a r e n t s e m i conductor working e l e c t r o d e . Current c o l l e c t i o n to the working e l e c t r o d e was made by a t r a n s p a r e n t c o n d u c t i n g g l a s s . The conducting 2

Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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g l a s s e l e c t r o d e was t i n o x i d e - c o a t e d C o r n i n g 7059 g l a s s w i t h a t h i c k n e s s o f O.032" and a s t a n d a r d r e s i s t i v i t y o f 100 ± 30 ohms p e r s q u a r e . A s m a l l s i l v e r w i r e r e f e r e n c e e l e c t r o d e was i n s e r t e d i n s i d e the c e l l v i a a s m a l l h o l e . The p h o t o e l e c t r o c h e m i c a l p r o p e r t i e s o f t h e s e m i c o n d u c t o r / s o l i d e l e c t r o l y t e i n t e r f a c e was i n v e s t i g a t e d u s i n g a W e n k i n g ST-72 p o t e n t i o s t a t and a n O r i e l 150-Watt x e n o n l i g h t s o u r c e . Monochromatic l i g h t was o b t a i n e d b y u s i n g an O r i e l M o d e l 7240 monochromator. The s l i t o p e n i n g o f t h e monochromator was s e t a t 1.5mm, t h u s g i v i n g a s p e c t r a l b a n d p a s s o f 10 nm. The e l e c t r o d e p o t e n t i a l was measured u s i n g a Wenking PPT-70 e l e c t r o m e t e r . A T a c u s s e l t y p e GSTP2B p u l s e sweep g e n e r a t o r was u s e d t o c o n t r o l t h e p o t e n t i o s t a t . The c u r r e n t v o l t a g e r e l a t i o n s h i p s o f t h e e l e c t r o d e s w e r e r e c o r d e d on a H e w l e t t P a c k a r d 7046 X-Y r e c o r d e r . An η-type p o l y c r y s t a l l i n e CdSe was made by t h e r m a l vacuum e v a p o r a t i o n o f CdSe powder ( 9 9 . 9 9 9 % , C e r a c , I n c . ) on t h e c o n d u c t i n g g l a s s . T h i s was a c c o m p l i s h e d by u s i n g a n Edwards 306 t h i n - f i l m e v a p o r a t o r a t a p r e s s u r e o f 10*" T o r r . The e l e c t r o d e was a n n e a l e d i n a i r a t 400°C f o r 30 m i n u t e s and c o o l e d down s l o w l y . 6

R e s u l t s and

Discussion

Photoactivity ofRbAg I . Before e v a l u a t i n g photoeffects a t the semiconductor-solid e l e c t r o l y t e i n t e r f a c e , i t i s f i r s t necessary t o c h a r a c t e r i z e any p h o t o a c t i v i t y p r e s e n t i n t h e s o l i d e l e c t r o l y t e itself. R u b i d i u m s i l v e r i o d i d e can be e x p e c t e d t o p o s s e s s some pho t o s e n s i t i v i t y b y analogy t o the p h o t o v o l t a i c e f f e c t demonstrated by t h e b i n a r y s i l v e r h a l i d e s (12» 13.). T h i s was p e r f o r m e d by e v a l u a t i n g a c e l l o f c o n f i g u r a t i o n Cond. g l a s s / P b I + R b A g I s / R b A g i / A g + R b A g i I / C o n d . g l a s s . S o l i d - s t a t e c e l l s b a s e d upon t h i s s o l i d e l e c t r o l y t e u s u a l l y c o n s i s t o f a s i l v e r anode and a c o m p l e x e d i o d i n e c a t h o d e ( 1 4 ) . T h i s c e l l was mounted b e t w e e n two t r a n s p a r e n t c o n d u c t i n g g l a s s e l e c t r o d e s . Upon i l l u m i n a t i o n o f t h e Cond. g l a s s / P b l 2 + RbAgi I i n t e r f a c e , t h i s s i d e developed p o s i t i v e p h o t o p o t e n t i a l s o f b e t w e e n 3 and 50 mV. However, a f t e r i n i t i a l l y p a s s i n g b o t h a n o d i c as w e l l a s c a t h o d i c c u r r e n t s t h r o u g h t h i s i n t e r f a c e , l a r g e r p h o t o p o t e n t i a l s c o u l d be a c h i e v e d . D u r i n g passage o f anodic c u r r e n t a t t h i s i n t e r f a c e , d a r k a r e a s a p p e a r e d on t h e e l e c t r o l y t e s u r f a c e . This might be e x p l a i n e d by the f o r m a t i o n of f r e e i o d i n e at t h i s i n t e r f a c e . L a r g e r p h o t o p o t e n t i a l s were r e a l i z e d a t these d a r k e r a r e a s . F o r e x a m p l e , a p o t e n t i a l o f 450 mV i n t h e d a r k and 850 mV u n d e r i l l u m i n a t i o n was f o u n d v e r s u s t h e s i l v e r r e f e r e n c e e l e c t r o d e . T h i s i s shown i n F i g u r e 1. However, when t h e l i g h t beam was n o t f o c u s e d o n t h e d a r k e n e d s p o t , t h e p h o t o p o t e n t i a l was o n l y a b o u t 5 mV. L a r g e p h o t o p o t e n t i a l s were s t i l l d e t e c t e d a t w a v e l e n g t h s l o n g e r t h a n 4750 A, when a s h a r p - c u t f i l t e r was u s e d i n t h e l i g h t p a t h . The r e s p o n s e o f t h e c e l l u n d e r t h i s c o n d i t i o n i s shown b y t h e s e c o n d d e f l e c t i o n shown i n F i g u r e 1. The a c t i o n s p e c t r a o f t h e c e l l w i l l be d e s c r i b e d l a t e r i n more d e t a i l . u

s

2

t

f

5

5

P h o t o e x c i t a t i o n o f t h e R b A g I s can be e x p e c t e d t o r e s u l t i n t h e formation o f electron-hole p a i r s . The p h o t o e l e c t r o n s w i l l d i f f u s e f r o m t h e s u r f a c e i n t o t h e b u l k b e c a u s e o f t h e i r much g r e a t e r d r i f t m o b i l i t y compared t o t h e p h o t o h o l e s . A net p o s i t i v e space charge



390

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INTERFACES

Figure 1. Photopotential in the cell: cond. glass/Pblt + RbAgJs/RbAgJg/Ag + RbAg l /cond. glass. The cond. glass/PbI + RbAg I interface was illu minated with 200 mW /cm xenon light. k 5

2

!f

5

2

Figure 2. Current-voltage characteristics of the cell: cond. glass/Pbl + RbAg l / RbAg,JrJΆ g + Rb A g,J /cond. glass under chopped illumination (200 mW/cm ) at 25°C (voltage scan rate: 25 mV/s) 2

k 5

2

5



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w i l l p r o b a b l y b e f o r m e d n e a r t h e i l l u m i n a t e d e l e c t r o d e due t o t h i s h i g h e r c o n c e n t r a t i o n o f p h o t o h o l e s . T h e s e p h o t o h o l e s c a n be e x p e c t e d t o o x i d i z e any f r e e s i l v e r p r e s e n t a t t h i s i n t e r f a c e . T h e s e p r o cesses can l e a d t o a p o s i t i v e photovoltage a t the i l l u m i n a t e d e l e c t r o d e , w h i c h c a n b e c o r r e l a t e d t o Dember-type p h o t o v o l t a g e ( 1 3 , 1 5 ) . The c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s o f t h e c e l l w e r e a n a l y z e d by s w e e p i n g t h e c e l l v o l t a g e u s i n g a p o t e n t i o s t a t . The c e l l v o l t a g e was m e a s u r e d a s t h e v o l t a g e b e t w e e n t h e i l l u m i n a t e d s i d e v e r s u s t h e n o n - i l l u m i n a t e d s i l v e r s i d e . U n d e r chopped x e n o n l i g h t i l l u m i n a t i o n , c a t h o d i c a s w e l l a s a n o d i c p h o t o c u r r e n t s w e r e r e c o r d e d , as shown i n F i g u r e 2. T h e s e p h o t o c u r r e n t s , a s w e l l a s t h e p h o t o p o t e n t i a l i n F i g u r e 1, i n d i c a t e t h a t p h o t o e x c i t e d c h a r g e c a r r i e r s a r e p r o d u c e d i n a d d i t i o n t o the s i l v e r ions present i n the dark. This c o r r e l a t e s w e l l w i t h t h e H a l l e f f e c t o f R b A g I s i n t h e d a r k and u n d e r i l l u m i n a t i o n t h a t has been p r e v i o u s l y r e p o r t e d ( 1 6 ) . H a l l s i g n a l s i n t h e d a r k a r e c a u s e d b y t h e Ag+ i o n c a r r i e r s . Under i l l u m i n a t i o n t h e s i g n o f the H a l l v o l t a g e , however, i s r e v e r s e d . From t h i s , and b y v i r t u e o f t h e m a g n i t u d e and t e m p e r a t u r e dependence o f H a l l m o b i l i t y , i t was c o n c l u d e d t h a t t h e H a l l s i g n a l s u n d e r i l l u m i n a t i o n was p r i m a r i l y c o n t r i b u t e d by p h o t o e l e c t r o n s . I f we assume t h a t t h e d a r k a r e a s a r e f o r m e d by p h o t o l y t i c i o d i n e , then the p o s i t i v e p h o t o p o t e n t i a l s i n F i g u r e 1 can be expected t o l e a d t o l a r g e r c a t h o d i c c u r r e n t s . H e r e t h e i o d i n e w i l l be r e d u c e d t o i o d i d e . Under c o n t i n u o u s i l l u m i n a t i o n , i t h a s b e e n shown t h a t c a t h o d i c p h o t o c u r r e n t s c a n b e drawn f o r many h o u r s . When t h e p o t e n t i a l o f t h e i l l u m i n a t e d i n t e r f a c e was made a n o d i c , i o d i d e w i l l b e o x i d i z e d t o i o d i n e on t h e s u r f a c e . Photogenerated charge c a r r i e r s may r e d u c e t h e i n t e r n a l r e s i s t a n c e o f t h e c e l l , t h e r e b y i n c r e a s i n g t h e e f f e c t i v e c u r r e n t d e n s i t y upon i l l u m i n a t i o n , a s shown i n F i g u r e 2. However, a t l a r g e c e l l v o l t a g e s , t h e e l e c t r o l y t e w i l l decompose, a c c o r d i n g t o (14) : 2 RbAgi+Is + 4 A g + 2 R b l + 4 A g i , w i t h t h e c o n c u r r e n t e q u i l i b r a t i o n R b l 3 £ R b l + l£. I o d i n e g e n e r a t e d b y s u c h mechanisms may b e r e s p o n s i b l e f o r t h e d a r k a r e a s on t h e e l e c t r o l y t e s u r f a c e . An i n c r e a s e i n the anodic dark current at v o l t a g e s higher t h a n 900 mV i n F i g u r e 2 c a n b e c o r r e l a t e d w i t h t h i s p r o c e s s . The e x p e r i m e n t , h o w e v e r , was p e r f o r m e d w i t h n o c o m p e n s a t i o n f o r r e s i s t a n c e and p o l a r i z a t i o n l o s s e s w i t h i n t h e s o l i d e l e c t r o l y t e c e l l . Hence, t h e v o l t a g e s f o r t h e i n c e p t i o n o f e l e c t r o c h e m i c a l r e a c t i o n s may d i f f e r f r o m v a l u e s e x p e c t e d f r o m a r e v e r s i b l e e l e c t r o d e . I t i s of i n t e r e s t to i n v e s t i g a t e the p h o t o s e n s i t i v i t y of RbAgIs s o l i d e l e c t r o l y t e doped w i t h f r e e i o d i n e . H e r e t h e s u r f a c e o f t h e p r e s s e d e l e c t r o l y t e was c o a t e d w i t h s o l i d i o d i n e f o r a few s e c o n d s u n t i l i t became l i g h t brown. The c e l l was a s s e m b l e d b e t w e e n t r a n s parent conducting g l a s s current c o l l e c t o r s . The p o t e n t i a l o f t h e i o d i n e - t r e a t e d s u r f a c e was m e a s u r e d a g a i n s t a s i l v e r r e f e r e n c e e l e c t r o d e . Due t o t h e h i g h a c t i v i t y o f t h e i o d i n e , t h e p o t e n t i a l was 661 mV i n t h e d a r k . T h i s s u r f a c e i m m e d i a t e l y d e v e l o p e d a l a r g e p o s i t i v e p h o t o p o t e n t i a l o f a b o u t 100 mV upon i l l u m i n a t i o n w i t h t h e x e n o n light. The c u r r e n t - p o t e n t i a l c h a r a c t e r i s t i c s o f t h i s s u r f a c e was e s s e n t i a l l y t h e same a s t h e one shown i n F i g u r e 2. However, an i n c r e a s e i n t h e d a r k c u r r e n t i n t h e a n o d i c d i r e c t i o n was n o t i c e d . The d e p e n d e n c y o f t h e p h o t o p o t e n t i a l o f t h i s c e l l upon t h e w a v e l e n g t h 3


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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

of the i l l u m i n a t i n g l i g h t i s shown i n Figure 3. Below 300 nm, essen t i a l l y no p h o t o p o t e n t i a l was detected. This was not s u r p r i s i n g s i n c e the g l a s s and the t i n oxide l a y e r on the conducting g l a s s e l e c t r o d e would absorb l i g h t with wavelengths below t h i s value. The band-gap of Sn0 i s w i t h i n t h i s range, i . e . , 3.8 eV. T h i s would correspond to a wavelength of about 326 nm. The conducting g l a s s had no photo a c t i v i t y when i t was i l l u m i n a t e d i n l i q u i d e l e c t r o l y t e s . In Figure 3 the photoresponse peak of t h i s s o l i d e l e c t r o l y t e system occurred a t a wavelength of about 425 nm. The p h o t o p o t e n t i a l decreased at longer wavelength. The peak at 425 nm can be c o r r e l a t e d t o the 425 nm band i n the o p t i c a l absorption spectrum of RbAgi+Is. The spectrum a c t u a l l y showed four bands at 485, 425, 375, and 335 nm (17). The 425 nm band was a t t r i b u t e d t o a forbidden i n t e r n a l t r a n s i t i o n i n the A g which becomes permitted because o f the t e t r a h e d r a l c o o r d i n a t i o n of the Ag i o n i n R b A g I s . The p h o t o p o t e n t i a l , as w e l l as the anodic and cathodic photocurrents were seen up t o wavelengths approaching 700 nm. The p h o t o p o t e n t i a l was a l s o measured as a f u n c t i o n of l i g h t i n t e n s i t y . At 200 mW/cm xenon l i g h t i l l u m i n a t i o n , a p h o t o p o t e n t i a l of about 100 mV was measured. The p h o t o p o t e n t i a l decreased upon a t t e n u a t i o n of the l i g h t i n t e n s i t y with n e u t r a l d e n s i t y f i l t e r s . A p l o t of the p h o t o p o t e n t i a l versus the r e l a t i v e l i g h t i n t e n s i t y i s shown i n F i g u r e 4. The p h o t o p o t e n t i a l i s p r o p o r t i o n a l to the loga rithm of the l i g h t i n t e n s i t y , as observed i n the Dember photovoltage of AgBr (13).


2

+

+

2

CdSe/Solid E l e c t r o l y t e J u n c t i o n . I t has been demonstrated i n the previous s e c t i o n that RbAg l5 i s p h o t o s e n s i t i v e . Our primary goal i n t h i s study was a c t u a l l y t o see p h o t o a c t i v i t y at a semiconductor/ s o l i d e l e c t r o l y t e i n t e r f a c e . An η-type CdSe was chosen f o r t h i s study since a t h i n - f i l m , p a r t i a l l y transparent e l e c t r o d e could be f a b r i c a t e d by thermal vacuum evaporation on a transparent conducting g l a s s . A c e l l of the c o n f i g u r a t i o n CdSe on Cond. glass/12 + R b A g I s / RbAgIs/Cond. g l a s s was f a b r i c a t e d . A small quantity of i o d i n e was introduced onto the s o l i d e l e c t r o l y t e surf ace as redox s p e c i e s . The CdSe e l e c t r o d e was then placed i n contact with t h i s s u r f a c e . The p o t e n t i a l of the CdSe was measured versus a s i l v e r wire reference e l e c t r o d e . The p o t e n t i a l i n the dark was 650 mV and i t moved to about 300 mV under 200 mW/cm xenon l i g h t i l l u m i n a t i o n . However, the ac t u a l l i g h t i n t e n s i t y reaching the CdSe/solid e l e c t r o l y t e i n t e r f a c e was s i g n i f i c a n t l y lower. T h i s r a p i d negative p h o t o p o t e n t i a l i s shown i n Figure 5. T h i s phenomenon i s analogous to the behavior of n-CdSe i n l i q u i d - j u n c t i o n photoelectrochemical c e l l s . The dependency of the CdSe e l e c t r o d e p o t e n t i a l upon the wave length of the l i g h t i s shown i n Figure 6. The spectrum was not cor rected f o r v a r i a t i o n s i n l i g h t i n t e n s i t y by the monochromator. The mean i n t e n s i t y at 625 nm was 70 yW/cm with v a r i a t i o n s at other wave lengths estimated as ±25 uW/cm . No p h o t o p o t e n t i a l was seen below 300 nm. Between 300 and 375 nm, a p o s i t i v e p h o t o p o t e n t i a l was de t e c t e d . This could have the same o r i g i n as the p h o t o p o t e n t i a l of the Cond. glass/RbAg I i n t e r f a c e shown i n Figure 3. A l a r g e negalf

2

2

2

i+

5



ANG

AND

SAMMELLS


393

Figure 3.

Potential of cond. glass/1 + RbAgJs interface vs. Ag as a function of wavelength

Figure 4.

Photoresponse of cond. glass/1 + intensity

2

2

RbAgJJcond.

glass vs. light


394


INTERFACES

DARK Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on November 10, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch025

r

< CO

>

LIGHT 1

1

200 TIME, s

Figure 5.

Photopotential of CdSe/I + RbAgJrJRbAgJJcond. mination with 200 mW/cm xenon light 2

glass, back-illu-

2

Figure 6.

Potential of CdSe in contact with I, + RbAgJs vs. Ag as a function of the wavelength of the light



25.

A N G AND SAMMELLS


395

t i v e p h o t o p o t e n t i a l can be seen i n Figure 6 a t wavelengths longer than 375 nm, up to about 800 nm, which corresponds c l o s e l y to the 1.7 eV band-gap of CdSe. Thus, the negative p h o t o p o t e n t i a l can be assigned to the CdSe/solid e l e c t r o l y t e i n t e r f a c e . The photoresponse between 300 and 500 nm i n F i g u r e 6 might very w e l l be a superposit i o n of the p o s i t i v e p h o t o p o t e n t i a l of the Cond. g l a s s / R b A g I s i n t e r f a c e and the negative p h o t o p o t e n t i a l of the CdSe/RbAgIs i n t e r face. The magnitude of the negative p h o t o p o t e n t i a l was l a r g e s t a t 625 nm. The dependency of t h i s p h o t o p o t e n t i a l upon the i n t e n s i t y of the i n c i d e n t l i g h t was a l s o analyzed. Figure 7 shows the negat i v e p h o t o p o t e n t i a l as a f u n c t i o n of i n c i d e n t o p t i c a l power. The r e l a t i v e s c a l e a t 10° corresponds to 70 yW/cm , where the photopot e n t i a l was —85 mV. With approximately 100 mW/cm white xenon l i g h t , photopotentials up to —300 mV have been detected. At the very low l i g h t i n t e n s i t y of O.7 yW/cm a t 625 nm, photopotentials of —2 mV were s t i l l observed. At moderate l i g h t i n t e n s i t i e s the photopotent i a l i s p r o p o r t i o n a l to the logarithm of the l i g h t i n t e n s i t y . The c u r r e n t - p o t e n t i a l c h a r a c t e r i s t i c s of the CdSe e l e c t r o d e were studied under chopped i l l u m i n a t i o n . The r e s u l t i s shown i n Figure 8. The o p e n - c i r c u i t p o t e n t i a l versus Ag was 350 mV under xenon l i g h t . When the p o t e n t i a l of the CdSe e l e c t r o d e was scanned i n the p o s i t i v e d i r e c t i o n , anodic photocurrents flowed. At high p o t e n t i a l s the photocurrent increased, but the dark current increased as w e l l . When scanned i n the negative d i r e c t i o n , a small cathodic current flowed due t o reduction of i o d i n e . I n i t i a l l y , small anodic photocurrent d e f l e c t i o n s were seen under chopped i l l u m i n a t i o n near the r e s t i n g p o t e n t i a l . However, at higher cathodic p o l a r i z a t i o n the photocurrent became cathodic. A cathodic photocurrent has already been demonstrated by RbAgi+Is as shown i n F i g u r e 2. B r i n g i n g the p o t e n t i a l more cathodic than the e q u i l i b r i u m p o t e n t i a l of s i l v e r r e s u l t e d i n a l a r g e cathodic current corresponding to reduction of s i l v e r ions to s i l v e r . When the p o t e n t i a l was brought back to the anodic d i r e c t i o n , an anodic current flowed due t o r e o x i d a t i o n of the s i l v e r . U s u a l l y , the p h o t o p o t e n t i a l decreased when the e l e c t r o d e i s brought to near 0 mV versus s i l v e r . T h i s could be due t o p l a t i n g of s i l v e r on the e l e c t r o d e surface. However, upon r e o x i d a t i o n of the s i l v e r the p h o t o p o t e n t i a l was r e s t o r e d . The decomposition of RbAgi to e i t h e r s i l v e r or i o d i n e species may create l i m i t a t i o n s to the use of t h i s s o l i d e l e c t r o l y t e f o r photoelectrochemical c e l l s . 2

2

2

Conclusion We have demonstrated p h o t o a c t i v i t y at s o l i d e l e c t r o l y t e / C o n d . g l a s s i n t e r f a c e s . The p o s i t i v e photopotentials could, however, be a Dember-type photovoltage. Cathodic photocurrents passed when the c e l l was discharged. I l l u m i n a t i o n may y i e l d i o d i n e on the i l l u m i nated s i d e with some reduced species (probably s i l v e r ) i n the bulk of the e l e c t r o l y t e . When CdSe was i n contact with the s o l i d e l e c t r o l y t e c o n t a i n i n g i o d i n e redox s p e c i e s , negative photopotentials and anodic photocurrents were detected. This i s analogous to the behavior of n-type semiconductor i n l i q u i d - j u n c t i o n s o l a r c e l l s . In the case of s o l i d


396

INTERFACES



Figure 8.

Current-potential characteristics of CdSe/I + RbAg,I interface under chopped illumination (voltage scan rate: 40 mV/s) 2

5



ANG

AND

SAMMELLS

P-TYPE PHOTOCATHODE


397

ΓΛΝ-ΤΥΡΕ PHOTOANODE X

Figure 9. Photoelectrochemical cell with a solid electrolyte containing reductant and oxidant on each side (the charging circuit contains rectifiers to prevent selfdischarge in the dark)



398

PHOTOEFFECTS AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

e l e c t r o l y t e s , the redox species are immobile. T h i s opens up the pos s i b i l i t y of c o n s t r u c t i n g a s o l i d - e l e c t r o l y t e photoelectrochemical c e l l with a storage c a p a b i l i t y as shown i n Figure 9. The s o l i d s t a t e e l e c t r o l y t e device would c o n s i s t of three phases i n intimate contact where i n the i d e a l case the c r y s t a l l a t t i c e s t r u c t u r e of the reductant, oxidant, and s o l i d e l e c t r o l y t e phases would be s i m i l a r to f a c i l i t a t e r a p i d movement of ions between each phase. The reductant and oxidant p o r t i o n s of the device would contain s e l e c t e d redox species e i t h e r introduced i n t o f i x e d immobile l a t t i c e s i t e s i n the s o l i d e l e c t r o l y t e ( i n the i d e a l case) or dispersed and i n intimate contact with the s o l i d e l e c t r o l y t e i n each of the r e s p e c t i v e compartments. The reductant and oxidant e l e c t r o d e s would possess both i o n i c and e l e c t r o n i c c o n d u c t i v i t y , whereas the separating s o l i d e l e c t r o l y t e would possess only i o n i c c o n d u c t i v i t y . A f t e r s e p a r a t i o n of e l e c t r o n s and holes i n the η-type semiconductor, the f o l l o w i n g general r e a c t i o n s may occur during charge: At oxidant e l e c t r o d e — nC

, +h

+

+ M . l(lattice)

+

-

MÎ!fHÏ l(lattice)

+

(n-DC

+

At reductant e l e c t r o d e — (n-l)C

+ M 2Î ( l a t t„.i c e,) + e-

+

Un'll2 ( l a t t i c e ï) +

+

n

C

+

where: Μ. represents a redox species which i s o x i d i z e d by an e l e c tron hole h ; C represents a mobile i o n conductor; and M2 repre sents a redox species which i s reduced by an e l e c t r o n e". During discharge, the f o l l o w i n g r e a c t i o n may occur: +

+

+ (n+l) 1(lattice)

M+fa- ) 2(lattice) 1

W

M" + 1(lattice) 11

10

M* 2(lattice)

The b a s i c requirement of the redox oxidant i n contact with n-type semiconductors i s that i t has an e q u i l i b r i u m p o t e n t i a l more negative than the decomposition p o t e n t i a l of the semiconductor and more p o s i t i v e than the lower edge of the semiconductor conduction band. The b a s i c requirement of the reductant e l e c t r o l y t e i s that i t s redox e q u i l i b r i u m p o t e n t i a l be negative of the oxidant e l e c t r o l y t e and more p o s i t i v e than the lower edge of the semiconductor conduction band. More work w i l l be necessary at c h a r a c t e r i z i n g s o l i d e l e c t r o lyte/semiconductor i n t e r f a c e s with those s o l i d e l e c t r o l y t e s a v a i l able before s a t i s f a c t o r y s o l i d - s t a t e devices capable of photocharge can be r e a l i z e d . Acknowledgment

for

T h i s work was sponsored by the Solar Energy Research I n s t i t u t e the United States Department of Energy.


25.

A N G AND S A M M E L L S


Literature


399

Cited

1.

Raleigh, D.O. J. Appl. Phys., 1970, 41, 1536.

2.

Owens, B.B.; Sprouse, J.S.; Conference, 1972, 8.

3.

Owens, B.B. J. Electrochem. Soc., 1970, 117, 1536.

4.

Owens, B.B.; Christie, J.H.; Tiedeman, G.T. J. Electrochem. Soc., 1971, 118, 1144.

5.

Owens, B.B., "Advances in Electrochemistry and Electrochemical Engineering," Delahay, P. and Tobias, C.S., Eds., Vol. 8, New York: Wiley Interscience, 1971.

6.

Christie, J.H.; Owens, B.B.; Tiedeman, G.T. The Electrochem. Soc. Extended Abstracts, 1974, No. 20, p. 53, Fall meeting, New York.

7.

Geller,

S. Science, 1967, 157, 310.

8.

Geller,

S.; Lind, M.D. J. Chem. Phys., 1970, 52,5854.

9.

Geller,

S. Science, 1972, 176, 1016.

10.

Geller,

S.; Owens, B.B. J. Phys. Chem. Solids,

11.

Geller, S.; Skarstad, P.M.; Wilber, S.A. J. Electrochem. Soc., 1975, 122, 332.

12.

Masters, J. J. Electrochem. Soc., 1970, 117, 1378.

13.

Tan, Y.T.; Trautweiler,

14.

Owens, B.B.; Oxley, J.E.; Sammells, A.F., "Applications of Halogenide Solid Electrolytes," in Topics in Applied Physics, 1977, Vol. 21, Solid Electrolytes, Geller, S., Ed., Berlin: Springer-Verlag.

15.

Dember, H. Physik. Z., 1932, 33, 207.

16.

Kaneda, T.; Mizuki, E. Phys. Rev. Letters,

17.

Radhakrishna, S.; Hariharan, K.; Jagadeesh, M.S. J. Appl. Phys., 1979, 50, 4883.

Warburton, D.L. 25th Power Sources

1972, 33, 1241.

F. J. Appl. Phys., 1969, 40, 66.

1972, 29, 937.

R E C E I V E D October 3, 1980.


Photoeffects at Semiconductor-Electrolyte Interfaces - American

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