Supra-Band-Edge Reactions at Semiconductor-Electrolyte Interfaces

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Supra-Band-Edge Reactions at SemiconductorElectrolyte Interfaces Band-Edge Unpinning Produced by the Effects of Inversion 1

J. A . TURNER, J. MANASSEN , and A . J. NOZIK Solar Energy Research Institute, Golden, CO 80401

I.

Introduction

A recent modification (1-3) of the conventional model (4) for photoelectrochemical reactions suggests that photo-generated minority carriers may, under certain conditions, be injected into the electrolyte before they reach thermal equilibrium within the semiconductor space charge layer. This process is called "hot carrier injection. More efficient conversion of optical energy into chemical energy may be possible with hot carrier injection because a greater fraction of the incident photon energy can be deposited in the electrolyte to do chemical work. Experiments designed to test for hot electron injection from p-type semiconductors were conducted by probing reduction reactions having redox potentials more negative than (i.e., above) the conduction band edge. The oxidized components of such redox couples should not be reduced by thermalized photo-generated electrons coming from the conducting band edge. However, hot electrons, having more negative potential, could reduce the higher lying redox couples. The energy level diagram for this process is shown in Figure 1. Analogous results should be obtained for oxidation reactions having redox potentials more positive than the valence band edge of n-type photoanodes. We call such oxidation or reduction reactions lying outside the semiconductor band gap "supra band-edge reactions". Experiments using p-Si in non-aqueous electrolytes (5,6) indicate that the reduction of redox species with redox potentials more negative than the conduction band edge could apparently be achieved. However, further analysis showed that these results are not caused by hot electron injection, but by the unpinning of the semiconductor band edges at the semiconductor-electrolyte interface; this unpinning effect is caused by the creation of an inversion layer at the p-Si surface. This is an important effect, especially for small band gap semiconductors, that has received little attention On Sabbatical leave from the Weizmann Institute of Science, Rehovot, Israel. 1

0097-6156/81/0146-0253$05.00/0 © 1981 American Chemical Society Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

254

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

Fermi Level

Figure 1. Hot electron injection from a p-type semiconductor electrode. The photogenerated electron is injected into the electrolyte to reduce A to A' before it is thermalized in the space-charge layer. A thermalized electron emerging at the conduction band edge (E ) would have insufficient energy to drive the redox couple A/A: c

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

16.

TURNER E T A L .

Supra-Band-Edge Reactions

255

in photoelectrochemistry. Unpinned band edges i n p h o t o e l e c t r o c h e m i c a l s y s t e m s have a l s o r e c e n t l y been p r o p o s e d by o t h e r w o r k e r s . B a r d , e t a l . (7J e x p l a i n t h e e f f e c t as t h e r e s u l t o f a l a r g e d e n s i t y o f s u r f a c e s t a t e s , w h i l e M o r r i s o n (8.) i n v o k e s i n t e r f a c e s t a t e s a r i s i n g from o x i d e l a y e r s . The u n p i n n e d band edges can move w i t h a p p l i e d p o t e n t i a l , maki n g t h e s e m i c o n d u c t o r e l e c t r o d e behave s i m i l a r t o a m e t a l e l e c t r o d e i n t h a t changes i n a p p l i e d p o t e n t i a l o c c u r a c r o s s t h e B e l m h o l t z l a y e r r a t h e r than a c r o s s t h e semiconductor space charge l a y e r . This s i t u a t i o n , o f c o u r s e , d r a s t i c a l l y a l t e r s the energy r e l a t i o n s h i p s between t h e s e m i c o n d u c t o r bands and t h e e l e c t r o l y t e r e d o x p o t e n t i a l s compared t o t h e c o n v e n t i o n a l model ( 4 ) ; i n t h e l a t t e r , t h e s e m i c o n d u c t o r bands a r e p i n n e d a t t h e s u r f a c e , t h e p o t e n t i a l d r o p a c r o s s t h e H e l m h o l t z l a y e r i s c o n s t a n t , and p o t e n t i a l changes i n t h e e l e c t r o d e p r o d u c e c o r r e s p o n d i n g changes i n t h e band b e n d i n g w i t h i n t h e semiconductor space charge l a y e r . The d i f f e r e n c e b e tween p i n n e d and u n p i n n e d band edges i s i l l u s t r a t e d i n F i g u r e 2. In t h e f o r m e r c a s e , t h e p o s i t i o n s o f t h e c o n d u c t i o n and v a l e n c e band edges a t t h e i n t e r f a c e (E£ and Ey) a r e f i x e d w i t h r e s p e c t t o the e l e c t r o l y t e redox l e v e l s , w h i l e i n the l a t t e r case the p o s i t i o n s o f E and E§ w i t h r e s p e c t t o t h e e l e c t r o l y t e r e d o x l e v e l s v a r y w i t h e l e c t r o d e p o t e n t i a l . T h u s , w i t h u n p i n n e d band e d g e s , r e d o x c o u p l e s l y i n g o u t s i d e t h e band gap under f l a t band c o n d i t i o n s can l i e w i t h i n t h e band gap under band b e n d i n g c o n d i t i o n s o r under i l l u m i n a t i o n . II.

Experimental

Results

The e x p e r i m e n t s were p e r f o r m e d w i t h s i n g l e c r y s t a l (111) p - S i e l e c t r o d e s w i t h a r e s i s t i v i t y o f a b o u t 5.5 ohm cm; non-aqueous e l e c t r o l y t e s were used c o n s i s t i n g o f a b s o l u t e m e t h a n o l c o n t a i n i n g t e t r a m e t h y l a m m o n i u m c h l o r i d e (TMAC) o r a c e t o n i t r i l e c o n t a i n i n g t e t r a e t h y l ammonium p e r c h l o r a t e ( T E A P ) . The f l a t - b a n d p o t e n t i a l s o r p - S i i n t h e two e l e c t r o l y t e s were d e t e r m i n e d f r o m M o t t - S c h o t t k y p l o t s ( i n the dark) i n the d e p l e t i o n range o f the p-Si e l e c t r o d e , from o p e n - c i r c u i t p h o t o p o t e n t i a l m e a s u r e m e n t s , and f r o m t h e v a l u e s o f e l e c t r o d e p o t e n t i a l a t which anodic photocurrent i s f i r s t obs e r v e d i n n - t y p e S i e l e c t r o d e s . These t h r e e methods a l l y i e l d e d c o n s i s t e n t f l a t - b a n d p o t e n t i a l v a l u e s f o r p - S i o f + O.05V (vs SCE) ± . 0 5 V i n b o t h m e t h a n o l and a c e t o n i t r i l e . These v a l u e s , combined w i t h t h e d o p i n g d e n s i t y and t h e band gap o f 1.12 eV f o r p - S i p l a c e s t h e c o n d u c t i o n band edge i n m e t h a n o l and a c e t o n i t r i l e a t -O.85V (vs S C E ) . The s u p r a b a n d edqe r e d o x c o u p l e s c h o s e n f o r t h e two e l e c t r o l y t e s were 1,3 d i m e t h o x y - 4 - n i t r o b e n z e n e (&>=-!.0V vs SCE)for m e t h a n o l , and 1 n i t r o n a p h t h a l e n e ( E = - l . 0 8 ) , 1,2 d i c h l o r o 4 m i t r o b e n z e n e ( E = - O . 9 5 ) , and a n t h r a q u i n o n e ( E = - O . 9 5 ) f o r a c e t o n i t r i l e . These r e d o x couples l i e from O.1 V to O.24V above t h e c o n d u c t i o n band edge o f p-Si, and h e n c e , i n t h e c o n v e n t i o n a l model, c o u l d n o t be photoreduced by p - S i . P h o t o c u r r e n t - v o l t a g e d a t a showed t h a t a l l t h e s u p r a - b a n d - e d g e r e d o x s p e c i e s l i s t e d above a r e r e d u c e d upon i l l u m i n a t i o n o f t h e p - S i e l e c t r o d e ; no a p p r e c i a b l e r e d u c t i o n was a c h i e v e d i n t h e d a r k . T y p i c a l r e s u l t s f o r p - S i w i t h methanol and a c e t o n i t r i l e a r e shown 0

o


0

256

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Pinned Band Edges v = u -u B

E

Unpinned Band Edges v * u - u

U'e-

B

E

( b

) b

Ε;

s

Figure 2. Pinned vs. unpinned band edges. In the former case the position of E and Έ remain fixed with respect to the electrolyte redox couples as the electrode potential is varied. In the latter case, E and E change with applied potential. c

8

ν

s

c

s

v


16.

TURNER E T A L .


257

i n F i g u r e 3. The l i g h t i n t e n s i t y dependence o f t h e p h o t o c u r r e n t v o l t a g e b e h a v i o r f o r a n t h r a q u i n o n e i n a c e t o n i t r i l e i s shown i n F i g u r e 4 . A t h i g h l i g h t i n t e n s i t y (442 mW/cm , w h i t e l i g h t f r o m xenon lamp) o n l y a c a t h o d i c r e d u c t i o n p h o t o c u r r e n t i s o b s e r v e d . As t h e l i g h t i n t e n s i t y i s r e d u c e d , an o x i d a t i o n wave grows i n t h a t d e v e l o p s on t h e a n o d i c sweep. T h i s o x i d a t i o n wave i s b e l i e v e d t o r e p r e s e n t t h e r e - o x i d a t i o n o f reduced anthraquinone a t t h e p-Si electrode. I n F i g u r e 5, i t i s seen t h a t t h e o x i d a t i o n wave c a n be generated i f t h e high l i g h t i n t e n s i t y i s suddenly reduced to zero on t h e a n o d i c r e t u r n sweep a f t e r r e d u c t i o n has o c c u r r e d a t t h e high l i g h t i n t e n s i t y . A l t h o u g h t h e r e s u l t s i n F i g u r e s 3 a r e t h o s e t h a t w o u l d be e x pected f o r hot e l e c t r o n i n j e c t i o n e f f e c t s , a d d i t i o n a l experiments and a n a l y s e s i n d i c a t e t h a t h o t e l e c t r o n p r o c e s s e s a r e n o t o c c u r r i n g here. The f i r s t t y p e o f e x p e r i m e n t a l r e s u l t t h a t was i n c o n s i s t e n t w i t h hot e l e c t r o n i n j e c t i o n i n v o l v e d the wavelength depen dence o f t h e r e d u c t i o n c u r r e n t f o r t h e s u p r a - b a n d - e d g e r e d o x s p e c ies. T h i s i s shown i n F i g u r e 6 f o r 1,3 d i m e t h o x y - 4 - n i t r o b e n z e n e i n m e t h a n o l . The r e l a t i v e quantum e f f i c i e n c y o f t h e p h o t o r e d u c t i o n c u r r e n t was f o u n d t o be i n d e p e n d e n t o f w a v e l e n g t h o v e r t h e r a n g e 400 nm t o 800 nm. Above 800 n , t h e quantum e f f i c i e n c y d r o p s and r e a c h e s z e r o a t a b o u t 1100 nm; t h i s d r o p - o f f i s c o n s i s t e n t w i t h t h e 1,12 eV band gap o f S i . A s e c o n d s e t o f e x p e r i m e n t s i n v o l v e d c a p a c i t a n c e measurements as a f u n c t i o n o f e l e c t r o d e p o t e n t i a l , a c s i g n a l f r e q u e n c y , and light intensity. T y p i c a l r e s u l t s f o r p - S i i n a c e t o n i t r i l e and m e t h a n o l a r e shown i n F i g u r e s 7 and 8 , In a c e t o n i t r i l e ( F i g u r e 5), t h e d a r k c a p a c i t a n c e f i r s t d e c r e a s e s w i t h i n c r e a s e d band b e n d i n g and t h e n a t a b o u t - O . 3 V ( v s SCE) r e m a i n s a t a f l a t minimum a t a c s i g n a l f r e q u e n c i e s above 100 H z . Under i l l u m i n a t i o n , t h e c a p a c i tance increases sharply with i n c r e a s i n g l i g h t i n t e n s i t y a t a l l po t e n t i a l s ; t h e c a p a c i t a n c e - v o l t a g e c u r v e shows a minimum a t a b o u t -O.1 V ( v s SCE) and i n c r e a s e s r a p i d l y a t h i g h e r n e g a t i v e p o t e n tials. A t f r e q u e n c i e s b e l o w 100 H z , t h e c a p a c i t a n c e a l s o i n c r e a s e s i n t h e d a r k w i t h i n c r e a s i n g n e g a t i v e p o t e n t i a l beyond - 0 , 3 V ( v s SCE). S i m i l a r r e s u l t s a r e o b t a i n e d w i t h m e t h a n o l ( F i g u r e 6) e x cept t h a t a sharp decrease i n capacitance i s a l s o g e n e r a l l y o b s e r v e d a t a l l f r e q u e n c i e s a t p o t e n t i a l s more n e g a t i v e t h a n - O . 3 t o -O.5V ( v s S C E ) . T h i s d r o p i s a s s o c i a t e d w i t h r e d u c t i o n o f t r a c e amounts o f w a t e r i n t h e e l e c t r o l y t e w h i c h l e a d s t o l a r g e dark c u r r e n t f l o w . 2

Ill.

Discussion

F o r t h e p a r t i e l u a r p - S i e l e c t r o d e used i n t h e s e e x p e r i m e n t s ( d o p i n g d e n s i t y o f 2 χ 10 cm~3) t h e p r e d i c t e d w a v e l e n g t h d e p e n dence o f t h e p h o t o - r e d u c t i o n c u r r e n t f o r h o t c a r r i e r p r o c e s s e s s h o u l d show no p h o t o c u r r e n t f o r w a v e l e n g t h s l o n g e r t h a n 440 nm. T h i s i s because t h e d e p l e t i o n w i d t h o f t h e p-Si e l e c t r o d e w i t h 1 v o l t band b e n d i n g i s 870 nm, and p h o t o g e n e r a t e d e l e c t r o n s a r r i v i n g a t t h e s u r f a c e from t h e f i e l d - f r e e r e g i o n ( a f t e r c r o s s i n g t h e s p a c e - c h a r g e l a y e r ) w o u l d be t h e r m a l i z e d . T h a t i s , t h e d e p l e t i o n


258


INTERFACES

Electrode Potential (vs SCE), Volts

H1.0

Applied Physics Letters Figure 3. Photoreduction on p-Si or redox couples with redox potentials lying above the conduction band edge as determined by dark fiat-band potential measurements, (a) Photoreduction of 1,3 dimethoxy-4-nitrobenzene in methanol (E = —1.0 V vs. SCE); (b) photoreduction of anthraquinone in acetonitrile (E = -O.95 V vs. SCE); E for p-Si in both cases in the dark is -O.85 V vs. SCE; ( ; dark; ( ; light (5). 0

0

c


16.

TURNER E T A L .


259

50

-4 -300

-O.3

O.0

Ε vs. SSCE, Volts

Figure 4. Light intensity dependence of the photoreduction current of anthraquinone in acetonitrile. An anodic wave grows in at about O.0 V (vs. SSCE) with decreasing light intensity. (( ) O.9 mw/cm ; ( ) 50 mw/cm ; ('---) 442 mw/cm ; anthraquinone (ImM); O.5M TEAP, Acetonitrile, 50 mV/s scan) 2

2

2



260

INTERFACES

ρ+O.5

-2.0

-1.5

-1.0

-O.5

+O.5

Ε vs. SSCE, Volts 2

Figure 5. Current-voltage curve for anthraquinone (400 mw/cm ) in acetonitrile at high light intensities: ( ) the current response after the light is shut off during the anodic sweep. An oxidation wave at about O.0 V (vs. SSCE) develops if the light is shut off.


Figure 6. Wavelength dependence of the photoreduction current of 1,3 dimethoxy4-nitrobenzene (20mM) in methanol (1M TMAC)


262


+O.2

O.0 -O.2 -O.4 Electrode Potential (vs.SCE), Volts

INTERFACES

-O.6

Applied Physics Letters Figure 7. Capacitance vs. p-Si electrode potential in acetonitrile as a function of frequency and light intensity (5) (( ) 10 Hz; ( ; 10 Hz; ( ) 5 χ 10 Hz) 2

3


3

TURNER E T A L .

263


2.001

ol +O.2

1 O.0

1 -O.2

1 -O.4

1 -O.6

1— -O.8

Electrode Potential (vs S C E ) , Volts

Applied Physics Letters Figure 8. Dark capacitance vs. p-S/ electrode potential in methanol as a function of frequency (5) (( ; 10 , 5 χ 10 Hz; ( ) 5 X 10 Hz; ( ) 10 Hz) 2

1

3


4

264

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

w i d t h i s w i d e enough t o a l l o w s u f f i c i e n t e l e c t r o n - p h o n o n c o l l i s i o n s t o d i s s i p a t e t h e band b e n d i n g p o t e n t i a l e n e r g y as phonon excitation (heat). In o r d e r t o c r e a t e h o t e l e c t r o n s a t t h e s u r f a c e o f t h e s a m p l e , i t w o u l d be n e c e s s a r y t o i l l u m i n a t e t h e p - S i e l e c t r o d e w i t h p h o t o n s o f e n e r g y g r e a t e r t h a n a b o u t 2.8 eV ( w a v e l e n g t h s l e s s t h a n 440 nm). These p h o t o n s have a b s o r p t i o n c o e f f i c i e n t s g r e a t e r t h a n 2 χ 1 0 c m " and w o u l d be a b s o r b e d w e l l w i t h i n t h e s p a c e c h a r g e l a y e r ; t h e y w o u l d a l s o have e n e r g i e s s u f f i c i e n t l y g r e a t e r t h a n t h e band gap s u c h t h a t p h o t o e l e c t r o n s w o u l d be c r e a t e d a t t h e S i s u r f a c e w i t h e n e r g i e s a t l e a s t O.3 eV above t h e c o n d u c t i o n band e d g e . T h i s e x c e s s e n e r g y w o u l d be s u f f i c i e n t t o d r i v e t h e s u p r a - b a n d - e d g e r e d u c t i o n s u n d e r i n v e s t i g a t i o n ; photons w i t h w a v e l e n g t h s l o n g e r t h a n 440 nm w o u l d be a b s o r b e d d e e p e r i n t o t h e c r y s t a l and c o u l d n o t produce e l e c t r o n s t h a t would a r r i v e a t the s u r f a c e w i t h s u f f i c i e n t energy to d r i v e the r e d u c t i o n r e a c t i o n . Thus, the f a c t t h a t the w a v e l e n g t h dependence o f t h e r e d u c t i o n c u r r e n t i n F i g u r e 6 f o l l o w s t h e a b s o r p t i o n edge o f S i means t h a t h o t c a r r i e r e f f e c t s c a n n o t be occurring here. I t i s i n s t r u c t i v e t o compare t h e d a t a i n F i g u r e s 7 and 8 w i t h t h e i d e a l c u r v e s o b t a i n e d f o r a m e t a l - i n s u l a t o r - s e m i c o n d u c t o r (MIS) device (9J. For MIS d e v i c e s , t h e s p a c e - c h a r g e d e n s i t y and t h e c a p a c i t a n c e d e c r e a s e i n t h e d e p l e t i o n r e g i o n and r e a c h a minimum value at the onset of the i n v e r s i o n r e g i o n . The i n v e r s i o n r e g i o n o c c u r s when t h e band b e n d i n g i s s u f f i c i e n t l y l a r g e s u c h t h a t t h e Fermi l e v e l a t t h e s u r f a c e l i e s c l o s e r t o t h e m i n o r i t y c a r r i e r band r a t h e r t h a n t o t h e m a j o r i t y c a r r i e r band. As t h e i n v e r s i o n region develops, the space-charge d e n s i t y i n the semiconductor surface increases r a p i d l y . The e f f e c t o f t h i s i n c r e a s e d c h a r g e d e n s i t y on t h e measured c a p a c i t a n c e depends upon t h e f r e q u e n c y o f t h e a p p l i e d ac s i g n a l . A t h i g h f r e q u e n c y (>100 Hz f o r S i / S i 0 2 ) t h e e l e c t r o n c o n c e n t r a t i o n c a n n o t f o l l o w t h e ac s i g n a l , and t h e measured c a p a c i t a n c e i s f l a t and m i n i m i z e d i n t h e i n v e r s i o n r e g i o n . However, a t l o w f r e q u e n c y (

Supra-Band-Edge Reactions at Semiconductor-Electrolyte Interfaces

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