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4 Conversion of Visible Light to Electrical Energy: Stable Cadmium Selenide

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Photoelectrodes in Aqueous Electrolytes

MARK S. WRIGHTON, ARTHUR B. ELLIS, and STEVEN W. KAISER Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Mass. 02139

Stabilization of n-type CdSe to photoanodic dissolution is reported. The stabilization is accomplished by the competitive oxidation of S or Sn at the CdSe photoanode in an electrochemical cell. Such stabilized cells are shown to sustain the conversion of low energy (≥ 1.7 eV) visible light to electricity with good efficiency and no deterioration of the CdSe photoelectrode or of the electrolyte. The electrolyte undergoes no net chemical change because the oxidation occurring at the photoelectrode is reversed at the cathode. Conversion of monochromatic light at 633 nm to electricity is shown to be up to ~ 9% efficient with output potentials of ~ 0.4V. Conversion of solar energy to electricity is estimated to be ~ 2% efficient. 2-

"photoelectrochemical

2-

cells s u c h as t h a t s c h e m e d i n F i g u r e 1 m a y p r o v e

to b e u s e f u l d e v i c e s f o r c o n v e r t i n g l i g h t t o e l e c t r i c a l e n e r g y o r t o fuels i n t h e f o r m of e l e c t r o l y t i c p r o d u c t s . I t has b e e n k n o w n f o r o v e r a c e n t u r y ( I ) t h a t i r r a d i a t i o n o f a n e l e c t r o d e i n a c e l l c a n r e s u l t i n c u r r e n t flow i n t h e e x t e r n a l c i r c u i t . L i g h t - i n d u c e d c u r r e n t flow results i n p h o t o e l e c t r o l y sis w i t h o x i d a t i o n a t o n e e l e c t r o d e ( a n o d e ) a n d r e d u c t i o n a t t h e o t h e r electrode

(cathode).

I n principle, the current

flow

can be utilized

d i r e c t l y as e l e c t r i c i t y b y m e r e l y i n t r o d u c i n g ( i n series)

a n electrical

l o a d into t h e external circuit. A d d i t i o n a l l y , the electrolytic products m a y represent f u e l ( s ) w h i c h c a n b e u t i l i z e d w i t h e x i s t i n g t e c h n o l o g y . ously, f o r example,

t h e photoelectrolysis

of H

2

0 according

Obvi

to either

E q u a t i o n 1 o r E q u a t i o n 2 represents a l i g h t - t o - c h e m i c a l e n e r g y c o n v e r s i o n 71

In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.


72

SOLID S T A T E

CHEMISTRY

light PHOTOANODE (η-type SC)

CATHODE electrolyte TYPICAL PHOTOELECTROCHEMICAL CELL Figure 1.

Typical photoelectrochemical Η 0->Η 2

2 H 0 -> H 2

+ ι/2θ

2

2

+ H 0 2

of s o m e p o t e n t i a l interest since t h e fuels H

cell (1)

2

(2)

2

2

and H 0 2

2

have, or could

have, considerable utility. W h i l e m e t a l electrodes o f t e n y i e l d p h o t o c u r r e n t s w h e n i r r a d i a t e d , semiconductor photoelectrodes generally give the highest photocurrents. I n m a n y cases e v e r y p h o t o n a b s o r b e d b y t h e s e m i c o n d u c t o r

electrode

yields a n electron i n the external circuit. Semiconductor photoelectrodes h a v e a t least o n e b a s i c p r o p e r t y w h i c h m a k e s t h e m a t t r a c t i v e as t h e p h o t o r e c e p t o r i n t h e c e l l s : a m e c h a n i s m f o r t h e i n h i b i t i o n of r e c o m b i n a t i o n of p h o t o g e n e r a t e d e l e c t r o n - h o l e p a i r s .

F i g u r e 2 shows t h e g e n e r a l

m o d e l associated w i t h t h e s e m i c o n d u c t o r - e l e c t r o l y t e i n t e r f a c i a l r e g i o n , showing that the bands i n the semiconductor c a n b e bent near the surface i n s u c h a w a y as to i n h i b i t r e c o m b i n a t i o n o f e l e c t r o n - h o l e p a i r s .

More

over, t h e b a n d s m a y b e b e n t i n a d i r e c t i o n a p p r o p r i a t e f o r t h e o b s e r v a t i o n of a s u b s t a n t i a l a n o d i c p h o t o c u r r e n t f o r η-type s e m i c o n d u c t o r s a n d a cathodic photocurrent for p-type semiconductors. These b a n d b e n d i n g considerations have been elaborated previously ( 2 ) . T h e separation of t h e p h o t o g e n e r a t e d e l e c t r o n - h o l e p a i r a l l o w s n e t r e d o x c h e m i s t r y to c o m p e t e v e r y effectively w i t h r e c o m b i n a t i o n . T h e i m p o r t a n c e of e l e c t r o n - h o l e s e p a r a t i o n i m m e d i a t e l y after p h o t o generation c a n b e appreciated b y considering a n early proposition for the photoassisted conversion of H

2

0 to H

2

a n d 0 . It was c l a i m e d that 2



2.

hv

Depletion Region

\ Interface

El e c t r o l y t e

Semi c o n d u c t o r -

Depletion Region

of a

Semiconductor Bulk

Valence Band M

Band Gap

photocurrent

p - t y p e Semiconductor

Conduction Band

(b)

(a) Semiconductor-electrolyte interfacial region showing band bending favorable for observation for an retype semiconductor, (b) Same as (a) except for a p-type semiconductor.

Semiconductor Bulk

Valence Band

i

Band Gap

Conduction Band

η-type Semiconductor

Figure

(a)


74

SOLID STATE

one c o u l d c o u p l e t h e o x i d a t i o n a n d r e d u c t i o n of

CHEMISTRY

H 0 to t h e

photo-

2

i n d u c e d o x i d a t i o n a n d r e d u c t i o n of a q u o m e t a l ions s u c h as F e * a n d 2

Fe

3 +

(3).

T h a t i s , i r r a d i a t i o n of F e

a c c o r d i n g to E q u a t i o n s 3 (4-11)

and F e

2 +

3 +

p r o c e e d s , at least i n i t i a l l y ,

a n d 4 (12, 13, 14, 1 5 ) , r e s p e c t i v e l y .

T h e s e s u m to g i v e E q u a t i o n 5, yet the c o n v e r s i o n of H 0 to H 2


Fe

Fe

3 +

2 +

(aq) +

(aq) + H

> Fe water

+

OH"

Fe

2 +

3 +

(aq) +

(aq) +

2

and 0

(3)

ViYL

2

V40 + a

2

V4H

2

Ο

(4)

water 2hv i/ H 0 2

2

2

2 +

cannot be

+

> V2K Fe /Fe water

V40

(5)

2

3 +

sustained using the F e

/Fe

2 +

3 +

photoassistance

u n d e r l y i n g r e a s o n lies i n the f a c t t h a t h a l f a m o l e c u l e of H a t o m . T o f o r m a gaseous H

2

2

agent.

The

is a h y d r o g e n

m o l e c u l e t w o h y d r o g e n atoms m u s t c o m b i n e ,

b u t t h e h y d r o g e n a t o m c a n b a c k react w i t h the p h o t o g e n e r a t e d w h i c h a c c u m u l a t e s w i t h t i m e . T h e p r o b l e m is t h a t H p r o m p t l y g e n e r a t e d w i t h one p h o t o n .

2

Fe

3 +

is n o t i r r e v e r s i b l y ,

T h e h i g h energy hydrogen atom

or p r o t o n a t e d a t o m c a n s i m p l y b a c k react to y i e l d n o n e t c h e m i s t r y . I n h i b i t i n g t h e b a c k r e a c t i o n of t h e h i g h e n e r g y i n t e r m e d i a t e is analogous t o i n h i b i t i n g electron-hole r e c o m b i n a t i o n i n the i r r a d i a t e d s e m i c o n d u c t o r . I t is w i d e l y b e l i e v e d t h a t the d e g r e e of b a n d b e n d i n g i n t h e s e m i c o n d u c t o r is e q u a l t o t h e difference i n the F e r m i levels i n t h e s e m i c o n d u c t o r a n d the e l e c t r o l y t e ( 2 ) .

C o n s e q u e n t l y , the b a n d b e n d i n g m a y

o r m a y n o t b e large e n o u g h to p r e v e n t e l e c t r o n - h o l e r e c o m b i n a t i o n , d e p e n d i n g o n the r e d o x a c t i v e c o m p o n e n t s i n the e l e c t r o l y t e a n d t h e p r o p erties of t h e s e m i c o n d u c t o r itself. M o r e o v e r , t h e m a x i m u m t h e o r e t i c a l 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 is e q u a l to the a m o u n t of b a n d b e n d i n g , a n d thus t h e efficiency of the u t i l i z a t i o n of t h e l i g h t d e p e n d s o n t h e b a n d b e n d i n g for a given semiconductor.

T o affect the b a n d b e n d i n g f a v o r

a b l y a n a p p l i e d b i a s c a n b e s u p p l i e d b y a p o w e r s u p p l y i n series i n t h e e x t e r n a l c i r c u i t . I t is u s u a l l y a s s u m e d t h a t t h e e n t i r e p o t e n t i a l d r o p occurs i n the d e p l e t i o n r e g i o n of t h e s e m i c o n d u c t o r a n d n o t i n t h e elec t r o l y t e as f o r m e t a l electrodes (2).

I f t h e objective is t o p r o d u c e fuels

b y p h o t o e l e c t r o l y s i s , the a p p l i e d p o t e n t i a l u s e d m u s t b e l o w e r t h a n the t h e r m o d y n a m i c r e v e r s i b l e electrolysis p o t e n t i a l . I f t h e a p p l i e d p o t e n t i a l exceeds t h e t h e r m o d y n a m i c p o t e n t i a l , the r o l e of the l i g h t , at best, is to serve as a m e c h a n i s m to r e d u c e o v e r v o l t a g e e n c o u n t e r e d i n c o n v e n t i o n a l


4.

WRIGHTON E T AL.

Conversion

of Visible

75

Light

electrolysis. T h e m a x i m u m t h e o r e t i c a l storage efficiency, i^ax, b y p h o t o electrolysis is g i v e n b y E q u a t i o n 6, w h e r e E

p

is t h e r e v e r s i b l e electrolysis

p o t e n t i a l o f the r e a c t i o n b e i n g d r i v e n (e.g., 1.23 V f o r E q u a t i o n 1 ) ;

E o

is the b a n d g a p e n e r g y of t h e s e m i c o n d u c t o r p h o t o e l e c t r o d e ; a n d V

i is

B

a p P

t h e p o t e n t i a l p r o v i d e d b y the p o w e r s u p p l y i n t h e c i r c u i t . T o o b t a i n m a x i m u m efficiency, t h e n , one m u s t a t t e m p t ( a ) to m a t c h t h e b a n d g a p Ep


ητηΛχ =

^appl ËT Ε BG

/f*\

of t h e s e m i c o n d u c t o r w i t h t h e electrolysis p o t e n t i a l of t h e r e a c t i o n , a n d ( b ) to seek r e d o x c o m p o n e n t s a n d s e m i c o n d u c t o r m a t e r i a l s s u c h t h a t t h e e x t e r n a l p o w e r s u p p l y is n o t n e e d e d . I f the objective is t o p r o d u c e e l e c t r i c a l p o w e r f r o m l i g h t , t h e r e m u s t be no external supply.

O t h e r w i s e , the l i g h t - t o - e l e c t r i c a l energy

s i o n efficiency w o u l d b e negative.

conver

I n m o r e e x p l i c i t terms, i f the p h o t o

e l e c t r o c h e m i c a l c e l l is to p r o d u c e b o t h a f u e l a n d e l e c t r i c i t y , t h e b a n d b e n d i n g m u s t e x c e e d E , a n d the m a x i m u m f r a c t i o n of e n e r g y o u t p u t as p

e l e c t r i c i t y w i l l b e the difference b e t w e e n t h e b a n d b e n d i n g a n d E . T h e p

b a n d b e n d i n g r e q u i r e m e n t is f o r a n o n i l l u m i n a t e d electrode l i b r i u m w i t h the h a l f - c e l l r e d o x r e a c t i o n w h i c h occurs at t h e

in equi photoelec

t r o d e . I t is c o n c e i v a b l e t h a t c o n v e r s i o n of l i g h t to e l e c t r i c a l e n e r g y c a n b e s u s t a i n e d w i t h o u t the p r o d u c t i o n of a f u e l ; i n this case one seeks a c h e m i c a l r e d o x system l i k e t h a t i n d i c a t e d i n E q u a t i o n s 7 a n d 8; i.e., t h e e l e c t r o l y t e contains b o t h A a n d B , a n d t h e i r d i s t r i b u t i o n does n o t c h a n g e . A

>Β

(cathode)

(7)

Β

> A

(anode)

(8)

I n this case the t h e o r e t i c a l m a x i m u m o p t i c a l - t o - e l e c t r i c a l e n e r g y

con

v e r s i o n efficiency is the extent of b a n d b e n d i n g d i v i d e d b y the b a n d g a p energy. Besides c o n t r o l l i n g b a n d b e n d i n g i n the s e m i c o n d u c t o r , t h e components

redox

i n t h e e l e c t r o l y t e c a n also p l a y a k e y r o l e i n w h e t h e r t h e

e l e c t r o n transfer processes to a n d f r o m t h e s e m i c o n d u c t o r are fast e n o u g h to c o m p e t e w i t h e l e c t r o n - h o l e r e c o m b i n a t i o n .

T h e fastest rates of

elec

t r o n transfer c a n b e e x p e c t e d w h e n t h e a p p r o p r i a t e s e m i c o n d u c t o r b a n d o v e r l a p s the p o s i t i o n of the r e d o x levels i n t h e electrolyte. W h i l e m u c h effort has b e e n a p p l i e d i n p r o v i d i n g this u n d e r s t a n d i n g of s e m i c o n d u c t o r photoelectrodes

( 2 , 1 6 , 1 7 , 1 8 , 1 9 ) , s o m e difficulties a r e

e n c o u n t e r e d i n e x p l o i t i n g p h o t o e l e c t r o c h e m i c a l cells. A t least o n e m a j o r p r o b l e m is t h a t t h e s e m i c o n d u c t o r itself is often t h e

electrochemically

r e a c t i v e species, a n d as s u c h i t undergoes i r r e v e r s i b l e p h o t o e l e c t r o l y s i s .


76

SOLID S T A T E

CHEMISTRY

U n t i l r e c e n t l y , i n fact, t h e r e existed n o η-type s e m i c o n d u c t o r

electrode

w h i c h c o u l d s u r v i v e i r r a d i a t i o n i n a n aqueous e l e c t r o l y t e w i t h o u t d e c o m p o s i t i o n . A t y p i c a l s i t u a t i o n is e n c o u n t e r e d w i t h η-type C d S . I r r a d i a t i o n of this m a t e r i a l results i n d i s s o l u t i o n a c c o r d i n g to E q u a t i o n 9

CdS i

Cd

(20,21,22).

(aq) + S (s) + 2e"

2 +

(9)

T h e r e s u l t is t h a t c u r r e n t flows i n the e x t e r n a l c i r c u i t , b u t the c h e m i s t r y Downloaded by UNIV OF NORTH CAROLINA on October 24, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch004

o c c u r r i n g at t h e p h o t o a n o d e results i n the d e c o m p o s i t i o n of the electrode w i t h z e r o v a l e n t s u l f u r p r e c i p i t a t i n g o n the surface a n d C d

ions g o i n g

2 +

i n t o s o l u t i o n . I n i t i a l e x p e r i m e n t s c a r r i e d out b y F u j i s h i m a a n d H o n d a ( 2 3 , 2 4 , 25,26)

o n η-type T i 0

indicated that T i 0

3

t r o d e , a n d s u b s e q u e n t l y others (27-36) t e r i z e m o r e f u l l y the T i 0

photoelectrode.

2

w i t h the c o n c l u s i o n t h a t T i 0

2

A l l findings are consistent

itself is n o t s u s c e p t i b l e t o

d i s s o l u t i o n . I t has n o w b e e n s h o w n t h a t S r T i 0

3

KTa0

(41),

3

(40),

is a n i n e r t p h o t o e l e c -

2

h a v e b e e n s t i m u l a t e d to c h a r a c

a n d KTao.77Nbo.23O3 (40),

W0

3

photoanodic Sn0

(37, 38,39),

and F e 0 2

3

2

( 38),

(42)

are

a l l stable photoelectrodes i n aqueous electrolytes. S t a b i l i t y of t h e m e t a l o x i d e η-type s e m i c o n d u c t o r s a l l o w s t h e i r use as the p h o t o r e c e p t o r i n p h o t o e l e c t r o c h e m i c a l cells f o r the photo-assisted electrolysis of H 0 . I n d e e d a l l of those fisted a b o v e as stable h a v e b e e n 2

s h o w n to assist t h e c o n v e r s i o n of H 0 t o H 2

2

and 0

2

i n cells as s h o w n i n

F i g u r e 3. S i n c e the r e v e r s i b l e electrolysis p o t e n t i a l of H 0 is 1.23 V , t h e 2

a b i l i t y to s u s t a i n the electrolysis at potentials b e l o w this a p p l i e d p o t e n t i a l r e q u i r e s t h e i n p u t of e n e r g y b y s o m e o t h e r m e c h a n i s m .

I r r a d i a t i o n of

t h e stable s e m i c o n d u c t o r electrodes does a l l o w t h e electrolysis to p r o c e e d at a p p l i e d p o t e n t i a l s s u b s t a n t i a l l y l o w e r t h a n 1.23 V , a n d conse q u e n t l y t h e l i g h t c a n b e c o n v e r t e d to c h e m i c a l e n e r g y i n t h e f o r m of the e l e c t r o l y t i c p r o d u c t s H

2

a n d 0 . If energy f r o m H 2

2

and 0

2

is r e c o v e r

a b l e at 56.7 k c a l / m o l H , S r T i 0 - b a s e d cells h a v e ~ 2 5 % efficiency f o r 2

3

the c o n v e r s i o n of 330 n m l i g h t ( 3 7 ) . 3.2 e V ( 4 3 , 44),

Since the b a n d gap i n S r T i 0

3

is

the m a x i m u m efficiency is 1.23/3.2 or ~ 3 8 % . T h u s t h e

closeness of t h e m e a s u r e d efficiency to this t h e o r e t i c a l efficiency i m p l i e s t h a t the q u a n t u m y i e l d is h i g h a n d t h a t t h e a p p l i e d p o t e n t i a l r e q u i r e d is s m a l l , as is t h e case ( 3 7 ) .

T h e h i g h a b s o r p t i v i t y of the S r T i 0

u s e f u l p r o p e r t y i n t h a t i t a l l o w s the p h o t o n s to b e c o m p l e t e l y

3

is also a absorbed

w i t h i n t h e d e p l e t i o n r e g i o n , setting the stage for h i g h o b s e r v e d q u a n t u m efficiency. A

major

hurdle

in

improving

SrTi0

efficiency

3

is

to

lower

t h e b a n d g a p w i t h o u t s a c r i f i c i n g c u r r e n t - v o l t a g e p r o p e r t i e s or s t a b i l i t y . T h e l o w e n e r g y v i s i b l e response of s t a b l e F e 0 , f o r e x a m p l e , is s i g n i f i 2

c a n t l y offset b y the large V H 0. 2

a p p

3

i necessary to d r i v e the p h o t o e l e c t r o l y s i s of

I n this a r t i c l e w e w i s h to s u m m a r i z e o u r i n i t i a l results o n


one

Conversion

WHIGHTON E T A L .

4.

of Visible

77

Light


t

light CATHODE

PHOTOANODE (η-type SC) aqueous electrolyte

Figure 3. Photoelectrochemical cell used to photoelectrolyze H O O . Photoélectrodes of η-type TiO , SnO , SrTiO , KTaO , or have been shown to be effective. g

t

t

t

s

s

to H and KTa Nb . O t

077

0 ts

s

b a s i c a p p r o a c h to d e v e l o p i n g u s e f u l p h o t o e l e c t r o c h e m i c a l cells h a v i n g l o w e n e r g y v i s i b l e response. T h e a p p r o a c h is to use r e d o x a c t i v e e l e c t r o lytes w h i c h c a n b e u s e d c o m p e t i t i v e l y to q u e n c h p h o t o a n o d i c d i s s o l u t i o n by

scavenging photogenerated

occur. (E G = B

holes b e f o r e electrode

dissolution can

Success w i l l b e i l l u s t r a t e d w i t h the s t a b i l i z a t i o n of η-type C d S e 1.7 e V )

(45)

b y u s i n g p o l y s u l f i d e electrolytes.

h a v e b e e n e l a b o r a t e d e l s e w h e r e (46, Results and

T h e f u l l details

47).

Discussion

Stabilization of CdSe.

L i k e η-type C d S , η-type C d S e

trodes u n d e r g o r a p i d p h o t o a n o d i c aqueous e l e c t r o l y t e (48, 49).

photoelec-

dissolution u p o n irradiation i n an

W e h a v e f o u n d (46, 47)

t h a t the p h o t o

a n o d i c d i s s o l u t i o n of C d S e o r C d S c a n b e q u e n c h e d b y a d d i n g p o l y sulfide to t h e aqueous electrolyte.

O x i d a t i o n of the a d d e d p o l y s u l f i d e

occurs at the expense of t h e o x i d a t i o n of t h e selenide of the C d S e as s c h e m e d i n F i g u r e 4. W e choose to define a stable p h o t o e l e c t r o d e

here

as one w h i c h undergoes n o w e i g h t loss as a c o n s e q u e n c e of t h e p h o t o current. T a b l e I summarizes t y p i c a l data w h i c h support the c l a i m that C d S e is " s t a b i l i z e d " b y t h e a d d i t i o n of p o l y s u l f i d e t o a n a q u e o u s e l e c t r o lyte.

I n s e v e r a l instances the n u m b e r of

oxidizing equivalents

e n o u g h to c o n s u m e several times t h e w e i g h t of t h e C d S e crystals.


was

78

SOLID STATE

CHEMISTRY


VISIBLE

DARK

CdS

CATHODE AQUEOUS

(SITE

OF

CdSe

or

POLYSULFIDE

poly-

SULFIDE

LIGHT

(site

REDUCTION)

SULFIDE

of

poly

oxidation)

Figure 4. Schematic of η-type CdSe-based photoelectrochemical cell. Photoanodic dissolution of CdSe does not occur in the aqueous polysulfide electrolyte; rather the polysulfide is oxidized at the photoelectrode.

T h e k i n d of s t a b i l i t y i n d i c a t e d i n T a b l e I f o r C d S e is r e m a r k a b l e . I t has b e e n s h o w n i n s e v e r a l instances t h a t c e r t a i n species c a n b e o x i d i z e d c o m p e t i t i v e l y w i t h t h e e l e c t r o d e d e c o m p o s i t i o n , b u t these d a t a f o r t h e C d S e are t h e first t o s h o w t h a t t h e d e c o m p o s i t i o n a p p a r e n t l y c a n b e t o t a l l y q u e n c h e d . S i m i l a r results w e r e o b t a i n e d f o r C d S (46, 47). stances s u c h as F e ( C N )

Table I.

e

4

Sub

" , Γ , a n d q u i n o n e s c a n b e o x i d i z e d at C d S o r

Stability of η-Type CdSe Photoelectrode in Aqueous Polysulfide Electrolytes 0

Exp. No.'

Crystal Crystal

Face

Before

(mol X 10*) After

Electrons Generated Αν i (mol X 10*) (mA')

Time (h)

1

1

0001

9.39

9.41

4.20

0.64

17.6

2

2

oool

8.61

8.61

3.31

0.41

21.6

3

3

8.78

8.75

4.2

15.4

23.9

P h o t o e l e c t r o c h e m i c a l cell w i t h C d S e p h o t o a n o d e ; see F i g u r e 4. * E x p . 1 : E l e c t r o l y t e is 1.0M N a O H , 1.0M N a S , 1.0M S c o n t i n u o u s l y p u r g e d w i t h A r . I r r a d i a t i o n is at 633 n m (2.8 m W ) at a n a p p l i e d p o t e n t i a l of —0.35 V (negative l e a d to C d S e ) w i t h a P t gauze c a t h o d e . P h o t o e l e c t r o d e e t c h e d to expose the 5 X 5 m m 0001 face. T h e p h o t o e l e c t r o d e is 1 m m t h i c k a n d the resistivity is ~ Exp. 2 : S a m e as E x p . 1 except 0001 surface is e x p o s e d . E x p . 3 : E l e c t r o l y t e is \2bM NaOH, 02M N a S , a n d the C d S e has n o t b e e n e t c h e d . I r r a d i a t i o n is w i t h w a v e l e n g t h s longer t h a n 420 n m f r o m a 200 W super-pressure H g l a m p . A P t wire c a t h o d e a n d a n a p p l i e d p o t e n t i a l of + 2 . 0 V (positive lead to C d S e ) were u s e d . T h e exposed surface o f the 1 m m t h i c k C d S e c r y s t a l was 5 X 5 m m a n d the r e s i s t i v i t y was 14 ficm. M u l t i p l y b y 4.0 c m to o b t a i n m A / c m . β

2

2

9

- 2

2


4.

WRiGHTON E T AL.

Conversion

of Visible

C d S e p h o t o e l e c t r o d e s (48, 49, 50, 51),

79

Light

b u t e v e n at h i g h concentrations

these h a v e not g i v e n g o o d e l e c t r o d e s t a b i l i t y . I t is i n t e r e s t i n g t o s p e c u l a t e o n the r e a s o n f o r t h e r e m a r k a b l e s t a b i l i t y of the C d S e a n d C d S i n the p o l y s u l f i d e system. T h e r e s u l t is v e r y i n t e r e s t i n g since i t is v e r y e v i d e n t t h a t o t h e r a d d i t i v e s are c a p a b l e b e i n g o x i d i z e d to s o m e extent at t h e p h o t o e l e c t r o d e .

The

of

complete

s t a b i l i t y of C d S e o r C d S i n p o l y s u l f i d e electrolytes m u s t b e a c o n s e q u e n c e of t h e v e r y fast rates of p o l y s u l f i d e o x i d a t i o n c o m p a r e d w i t h S e '

> lattice > S° o x i d a t i o n . O t h e r a d d i t i v e s are o n l y c o m p e t i t i v e l y 2

Se° or S "


2

lattice

oxidized. H a v i n g e s t a b l i s h e d the k i n d of s t a b i l i t y of C d S e i n d i c a t e d i n T a b l e I , i t is a p p r o p r i a t e to c o n s i d e r a c r i t e r i o n f o r e l e c t r o d e s t a b i l i t y w h i c h is somewhat more subtle: photocurrent stability. A feeling for photocurrent s t a b i l i t y c a n b e g a i n e d f r o m e x a m i n i n g t h e drop-off i n p h o t o c u r r e n t w i t h t i m e i n 1 . 0 M N a O H c o m p a r e d w i t h the steady p h o t o c u r r e n t

obtained

u s i n g a n e l e c t r o l y t e c o n s i s t i n g of 1 M N a O H , 1 M N a S , a n d 1 M S ( F i g u r e 2

5 ) . W e h a v e f o u n d t h a t , e v e n at v e r y h i g h l i g h t intensities, p h o t o c u r r e n t s i n t h e C d S e - b a s e d c e l l are v e r y stable. A t the v e r y least, t h e p o l y s u l f i d e e l e c t r o l y t e p r o v i d e s a m e c h a n i s m f o r p h o t o c u r r e n t s t a b i l i t y for a p e r i o d w h i c h is m a n y o r d e r s of m a g n i t u d e l o n g e r t h a n for the 1 M N a O H s o l u t i o n . T h e c u r r e n t profile f o r E x p . N o . 2 of T a b l e I is r e p r e s e n t a t i v e : t h e p h o t o c u r r e n t w a s i n i t i a l l y 0.405 m A a n d rose to 0.425 m A w h e r e i t l e v e l l e d for 5.9 h r . O v e r the next 7.3 h r the c u r r e n t f e l l to 0.40 a n d t h e n h e l d c o n stant f o r the r e m a i n i n g 8.3 h r . T h u s the o b s e r v e d p h o t o c u r r e n t is v e r y stable, at least f o r t h e 1 . 0 M N a O H , 1 . 0 M N a S , 1 . 0 M S electrolyte. 2

W e c a n discuss, i n q u a l i t a t i v e terms, t h e s t a b i l i t y of t h e C d S e as a f u n c t i o n of e l e c t r o l y t e c o m p o s i t i o n .

First, w e have generally found that

a n electrolyte c o n s i s t i n g of 1 . 0 M N a O H , l . O A i N a S , a n d l . O A i S p r o v i d e s 2

a v e r y stable system. S i g n i f i c a n t l y d i m i n i s h i n g the a m o u n t of S leads t o c o n s i d e r a b l e changes i n the c u r r e n t - v o l t a g e p r o p e r t i e s ( v i d e i n f r a ) , a n d , f o r e t c h e d C d S e electrodes, t h e r e seems to b e some d e t e r i o r a t i o n of the p h o t o c u r r e n t w i t h t i m e , e s p e c i a l l y at h i g h l i g h t intensities. F o r n o n e t c h e d samples w e h a v e f o u n d t h a t solutions c o n t a i n i n g o n l y l . O A i N a O H a n d 0 . 2 M N a S g i v e satisfactory s t a b i l i t y e v e n at v e r y h i g h l i g h t i n t e n s i 2

ties. A t this p o i n t , w e s i m p l y d o n o t h a v e a g o o d e n o u g h m e a s u r e of e l e c t r o d e s t a b i l i t y to d o a q u a n t i t a t i v e s t u d y at i n t e r m e d i a t e c o n c e n t r a tions of a d d e d sulfide w h e r e there is some o x i d a t i o n of a d d e d sulfide a n d some o x i d a t i o n of C d S e . A t the v e r y least, C d S e has b e e n s t a b i l i z e d to s u c h a n extent t h a t it is n o w p o s s i b l e to s t u d y c u r r e n t - v o l t a g e p r o p e r t i e s , q u a n t u m efficiency, w a v e l e n g t h response, etc. w i t h o u t t h e fear of i r r e v e r s i b l e d e c o m p o s i t i o n


80

SOLID STATE

1


I

I

I

Γ

2

o

0.7'-

I

I

1.0 Μ ΝαΟΗ, 1.0 M N a S , 1.0 M S _|

0.8

0.6

I

CHEMISTRY

o

o

o

o

o

o

< L

0.5

-

0.4

h

Ο

0.3 0.2

-

0.1

-

1.0 M NaOH

-Χ

_J L 4 5 6 Time, minutes

0

ΙΟ

Figure 5. Photocurrent as a function of time in a CdSe-based cell using I M NaOH as an electrolyte (%) or using an electrolyte consisting of I M NaOH, I M N f l . S , and 1M S (Ο). The irradiation source is a heam-expanded He-Ne laser with output at 633 nm. The 0001 face of the crystal is exposed to the elec trolyte.

of t h e electrode.

I n t e r e s t i n g l y , s u c h measurements at h i g h l i g h t i n t e n s i

ties h a v e r e a l l y n e v e r b e e n p o s s i b l e because o f the d e c o m p o s i t i o n p r o b l e m . Wavelength

Response.

G e n e r a l l y , t h e onset o f p h o t o c u r r e n t w i t h

variation i n excitation wavelength w i l l occur near the position of the b a n d g a p t r a n s i t i o n energy.

T h e w a v e l e n g t h response o f t h e C d S e i n

1.0M N a O H , 1 . 0 M N a S is s h o w n i n F i g u r e 6. A s seen i n t h e figure, t h e 2

onset of p h o t o c u r r e n t a n d t h e onset o f t h e b a n d g a p t r a n s i t i o n c o i n c i d e at ~ 7 5 0 n m , consistent w i t h t h e r e p o r t e d £

B

G o f C d S e (45).

With

r e g a r d t o p o t e n t i a l solar energy conversions, w e n o t e t h a t C d S e absorbs c a . 5 0 % of t h e i n c i d e n t solar i n s o l a t i o n (52)

a t t h e earth's surface.

A s i d e f r o m t h e f a c t t h a t t h e onset o f t h e p h o t o c u r r e n t is n e a r t h e b a n d g a p energy, t h e h i g h e n e r g y v i s i b l e response is q u i t e g o o d . I n f a c t , as t h e e x c i t a t i o n e n e r g y is i n c r e a s e d , t h e r e seems to b e a gentle increase i n response.

O n e p o s s i b l e e x p l a n a t i o n f o r t h e i n c r e a s e d response is t h a t

there is a greater p e r c e n t a g e o f t h e i n c i d e n t l i g h t a b s o r b e d w i t h i n t h e d e p l e t i o n r e g i o n a t t h e shorter w a v e l e n g t h s .

T h e absolute

quantum

efficiency f o r e l e c t r o n flow w i l l b e d i s c u s s e d b e l o w .


4.

WRiGHTON E T AL.

Conversion

of Visible

81

Light

T h e open circuit photopotential de

Open C i r c u i t Photopotential.

p e n d s o n t h e l i g h t i n t e n s i t y , a n d o v e r a s i g n i f i c a n t r a n g e of intensities t h e 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 s h o u l d increase l i n e a r l y w i t h t h e l o g of the intensity (2).

N a t u r a l l y , at some p o i n t t h e p h o t o p o t e n t i a l m u s t r e a c h

a v a l u e w h e r e h i g h e r intensities h a v e n o effect.

I n 1.0M N a O H ,

1.0M

N a S , a n d 1.0M S the open circuit photopotential depends o n l i g h t i n t e n 2

s i t y as s h o w n i n F i g u r e 7. T h e s e results c o m p a r e f a v o r a b l y w i t h those r e p o r t e d ( 2 ) f o r C d S e - b a s e d cells i n other electrolytes.


A c c o r d i n g to o u r m o d e l , the l i m i t i n g p h o t o p o t e n t i a l is e q u a l to t h e b a n d b e n d i n g w h i c h c a n b e n o greater t h a n t h e v a l u e of E Q . B

Measure

m e n t of 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 s of t h e o r d e r of 0.5 E Q i n d i c a t e s t h a t B

t h e b a n d b e n d i n g is v e r y s u b s t a n t i a l a n d t h a t w e c a n e x p e c t t h e o r e t i c a l e n e r g y c o n v e r s i o n efficiencies of ~ 5 0 % f o r i r r a d i a t i o n at Ε α · Β

U s i n g a s t a n d a r d three-electrode

C u r r e n t - V o l t a g e Properties.

cell

c o n f i g u r a t i o n a n d a potentiostat, w e h a v e e x a m i n e d t h e c u r r e n t - v o l t a g e

1

1

1

I '

1

' I

1

1

!

1

1

1 1

I ' '

1

I

I ι I ι I ι I I |UI ι ι I ι ι

2.8 Ζ c

l.6_ < >/ ια /> > 1.4 5 υ !c 1.2 Ε Ε

2.4-

O)

3 2.0o ο CL 1.6 α >>

•

·

s

s

•·

•

•

1 0

σ 1.2 ο> cr *ο 0.8 ο> ο -

1

? '

0.8 J σ 0.6 υ Q. Ο

0.41

-0.4

440 480

520

560 600 640

680

-0.2

720 760

Wavelength, nm

Figure 6. Wavelength response curve for a CdSe-based photoelectrochemical cell with a I M NaOH, I M Na S elec trolyte. The filled circles (%) and squares (M) are relative photocurrents as a function of incident irradiation wavelength after correction for variation in intensity with wavelength. The filled circles are values obtained using the excitation optics of an Aminco-Bowman emission spectrophotometer as the irradiation source, and the filled squares are values using a 600-W tungsten source monochromatized using a Bausch & Lomb high intensity monochromator. The open circles (O) are the optical density for a 1-mm thick polished CdSe crystal. t



82

SOLID STATE

10

100 Intensity,

1,000 /iW

10,000

Figure 7. Plot of open circuit photopotential against intensity (514.5 nm) for CdSe-based cell with a I M NaOH, Na S, J M S electrolyte. Open circles (O) represent the face and filled circles (%) represent the 0001 face of the tal exposed to the electrolyte. 2

CHEMISTRY

light IM 0001 crys

p r o p e r t i e s of t h e C d S e - b a s e d cells. T h e r e f e r e n c e electrode w a s a s a t u r a t e d c a l o m e l electrode ( S C E ) , a n d t h e p o t e n t i a l s of b o t h t h e P t elec trode a n d the C d S e potential were monitored; the P t electrode potential d i d n o t v a r y . C u r r e n t - v o l t a g e curves are a v e r y sensitive f u n c t i o n of s u r f a c e t r e a t m e n t ( p o l i s h i n g , e t c h i n g , e t c . ) . A l l d a t a r e f e r r e d to h e r e a r e for e t c h e d s i n g l e crystals. A s e x p e c t e d , t h e c u r r e n t - v o l t a g e p r o p e r t i e s d e p e n d o n l i g h t i n t e n s i t y a n d e l e c t r o l y t e c o m p o s i t i o n as s h o w n i n F i g u r e s 8, 9, a n d 10. F i r s t , i n t h e d a r k t h e r e is o n l y a s m a l l a n o d i c c u r r e n t , c o n sistent w i t h the f a c t t h a t n o m i n o r i t y c h a r g e carriers are a v a i l a b l e to p a r t i c i p a t e i n the c h a r g e transfer. I r r a d i a t i o n , h o w e v e r , p r o d u c e s holes w h i c h g i v e rise to a n a n o d i c p h o t o c u r r e n t as e x p e c t e d for a n η-type s e m i conductor.

T h e onset of t h e a n o d i c p h o t o c u r r e n t s occurs at m o r e n e g a

t i v e p o t e n t i a l s r e l a t i v e t o t h e S C E as t h e l i g h t i n t e n s i t y is i n c r e a s e d . T h i s shift i n a n o d i c p h o t o c u r r e n t onset c a n b e seen c l e a r l y i n F i g u r e 8 w h e r e t h e c u r r e n t - v o l t a g e p r o p e r t i e s f o r C d S e are s h o w n at t h r e e different l i g h t intensities. T h e s h i f t i n a n o d i c p h o t o c u r r e n t onset is consistent w i t h t h e 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 p l o t s g i v e n i n F i g u r e 7.

From Figure 7 we

see a c h a n g e of ~ 0.15 V i n p o t e n t i a l w i t h a n o r d e r of m a g n i t u d e c h a n g e


Conversion

of

83

Visible Light


WRIGHTON E T A L .

Figure 8. Current-volt age properties for an ir radiated (633 nm) n-type CdSe (0.25 cm surface area) electrode with 0001 face exposed to the 1.0M ΝαΟΗ,Ι.ΟΜ Na S,1.0M S electrolyte. The Pt dark electrode potential is constant at —0.71V vs. SCE for any bias. The sweep rate for all curves is 0.2 V per min ute. Note the depend ence of the curves on irradiation power. 2

t

U -1.4

ι

-1.3

ι

L

-1.2 -I.I -1.0 Potential vs. SCE

-0.9

ι

ι


84

SOLID STATE

CHEMISTRY

i n l i g h t i n t e n s i t y . T h e shift i n the a n o d i c p h o t o c u r r e n t onset is a c o m p a r a b l e a m o u n t w i t h the 10-fold c h a n g e i n l i g h t i n t e n s i t y . F i g u r e 8 shows t h a t the l i m i t i n g p h o t o c u r r e n t is a p p r o x i m a t e l y d i r e c t l y p r o p o r t i o n a l t o t h e l i g h t i n t e n s i t y . M o r e o v e r , the shapes of the c u r v e s are f a i r l y s i m i l a r f o r t h e r a n g e of intensities s t u d i e d . A t h i g h e r l i g h t intensities, h o w e v e r , the p h o t o c u r r e n t is n o t d i r e c t l y p r o p o r t i o n a l t o l i g h t i n t e n s i t y . T h e s a t u r a t i o n effect sets i n at different intensities d e p e n d i n g o n t h e e l e c t r o l y t e ( v i d e i n f r a ) a n d s o m e w h a t o n the p a r t i c u Downloaded by UNIV OF NORTH CAROLINA on October 24, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch004

l a r electrode a n d its surface treatment. S a t u r a t i o n effects of t h i s sort are o f t e n e n c o u n t e r e d a n d i n d i c a t e that t h e h o l e c a p t u r e b y S ~ is o n l y n

2

c o m p e t i t i v e w i t h e l e c t r o n - h o l e r e c o m b i n a t i o n , a n d t h e c o m p e t i t i o n is apparently influenced b y light intensity. W e h a v e i n v e s t i g a t e d the c u r r e n t - v o l t a g e p r o p e r t i e s of C d S e as a f u n c t i o n of i r r a d i a t i o n w a v e l e n g t h f o r w a v e l e n g t h s n e a r t h e b a n d (Figure 9). constant.

gap

T h e a c t u a l i n t e n s i t y s t r i k i n g the e l e c t r o d e has b e e n h e l d

H o w e v e r , as seen i n F i g u r e 9, t h e p h o t o c u r r e n t n o t

only

increases as t h e i r r a d i a t i o n w a v e l e n g t h is s h o r t e n e d , b u t t h e c u r r e n t v o l t a g e curves differ i n t h e same w a y t h a t t h e y differ w i t h changes i n 1

I

1

1

1

1

1

1

10.0

8.0

< c Φ

6

600nm ^ ^ ^ ^

0

660 n r n ,

4.0

—

7l0nmX2____

^

^

720nm X2

2.0

0.0

1

-1.2

1

-I.I

1

-1.0

1

1

Dark

X2

1

1

-0.9 -0.8 -0.7 -0.6 Potential vs. S C E

•

-

1

-0.5

Figure 9. Current-Voltage properties of CdSe as a function of inci dent irradiation wavelength. The 0001 face, 0.25 cm surface area, is exposed to the 1.0M NaOH, 1.0M Na S, 1.0M S electrolyte and illuminated at a constant intensity of 7.2 X 10~ einlsec at all four wavelengths. The Ft dark cathode potential was constant at —0.72 V vs. SCE, and the sweep rate was 0.2 V per minute. 2

t

10


WRIGHTON E T A L .

Conversion

of Visible

Light

Τ 1.0 M N a S 0.05 M S 1.0 M NaOH

6h

300

2

S

5h

/

0.045 mW 633nm

250

/ /


4k

200

I 50

I 00

l.65mW 633nm

1.0 M N a S 1.0 M S 1.0 M NaOH

50

2

300 υ

/

0.045 m W 633 nm 250

l.65mW 633 nm

200

150

100

H

50

Dark

-1.4

-1.3

-1.2 -l.l -1.0 Potential vs SCE

0.9

Figure 10. Dependence of CdSe current-voltage proper ties on intensity and electrolyte composition. In each case the sweep rate was 0.2 V per minute, and the 0.25 cm 0001 surface is exposed. In the 0.05M S electrolyte the Pt electrode is at -0.78 V vs. SCE, and in the l.OM S electrolyte the Pt electrode is at -0.71 V vs. SCE. 2


86

SOLID STATE

l i g h t i n t e n s i t y at a constant w a v e l e n g t h . changes

We

therefore

CHEMISTRY

ascribe

the

i n current-voltage behavior w i t h variation i n wavelength

differences i n the a m o u n t of l i g h t a b s o r b e d i n t h e d e p l e t i o n r e g i o n .

to At

t h e p o i n t s w h e r e a b s o r p t i v i t y of C d S e is r e l a t i v e l y s m a l l , the a m o u n t of l i g h t a b s o r b e d i n the d e p l e t i o n r e g i o n is s m a l l , b u t at w a v e l e n g t h s s u b s t a n t i a l l y shorter t h a n b a n d gap, a v e r y s i z a b l e f r a c t i o n of the i n c i d e n t i r r a d i a t i o n is a b s o r b e d i n the d e p l e t i o n r e g i o n . Some

extremes

i n the current-voltage properties

of

CdSe

with


changes i n e l e c t r o l y t e c o m p o s i t i o n are g i v e n i n F i g u r e 10. TTie curves s h o w t h a t t h e s m a l l e r S c o n c e n t r a t i o n leads to deleterious effects a n d t h a t t h e effect is m o s t p r o n o u n c e d at h i g h l i g h t i n t e n s i t y . T h e p h o t o c u r r e n t seems to saturate at a l o w e r l i g h t i n t e n s i t y a n d does n o t i n c r e a s e as s t e e p l y w i t h i n c r e a s i n g a n o d i c bias as f o r t h e e l e c t r o l y t e c o n s i s t i n g of 1 . 0 M N a O H , 1 . 0 M N a S , a n d 1 . 0 M S. 2

Comparison of 0001 and 0001 Faces. I t is p o s s i b l e to expose e i t h e r t h e 0001 or 0 0 0 Î f a c e of C d S e ( 5 3 ) . p r i n c i p a l l y Se exposed.

F o r the 0 0 0 Î face one w o u l d h a v e

D e t e r m i n i n g w h e t h e r there is a difference

t w e e n these t w o is a v e r y s i m p l e e x p e r i m e n t i n p r i n c i p l e , b u t w e

be have

f o u n d t h a t t h e differences are sufficiently s m a l l ( o r o u r r e p r o d u c i b i l i t y so p o o r ) t h a t w e c a n n o t , at this t i m e , r e p o r t differences i n e i t h e r p h o t o p o t e n t i a l or c u r r e n t - v o l t a g e p r o p e r t i e s f o r t h e 0001 a n d t h e 0001 faces. G e n e r a l l y , f o r e x a m p l e , w e h a v e b e e n a b l e to r e p r o d u c e c u r r e n t - v o l t a g e curves for a g i v e n c r y s t a l of C d S e s u c h t h a t t h e a n o d i c

photocurrent

onsets are w i t h i n ~ 100 m V of e a c h other. R e c a l l t h a t the curves d e p e n d s i g n i f i c a n t l y o n e t c h i n g p r o c e d u r e , etc. Power Conversion Efficiency.

T h e sustained conversion

of

light

e n e r g y to e l e c t r i c a l e n e r g y i n t h e present system is possible, since the c u r r e n t - v o l t a g e curves s h o w t h a t the p h o t o c u r r e n t w i l l flow a g a i n s t a n e g a t i v e a p p l i e d p o t e n t i a l . T y p i c a l l y , a n a n o d i c bias w i l l assist the

flow

of c u r r e n t s u c h t h a t o x i d a t i o n s o c c u r at the C d S e . T h e significance of an anodic photocurrent

flowing

w i t h a c a t h o d i c b i a s is t h a t t h e p o w e r

s u p p l y is a n e l e c t r i c a l l o a d , a n d t h e p o w e r o u t p u t of t h e c e l l is just c u r r e n t m u l t i p l i e d b y voltage. A p l o t of p h o t o c u r r e n t vs. a p p l i e d p o t e n t i a l f r o m a p o w e r s u p p l y i n series i n t h e e x t e r n a l c i r c u i t is s h o w n i n F i g u r e 11. T h e c u r v e shows v e r y c l e a r l y t h a t a n a n o d i c p h o t o c u r r e n t w i l l flow

e v e n at v e r y n e g a t i v e a p p l i e d p o t e n t i a l s . H o w e v e r , the m a x i m u m

v a l u e of

c u r r e n t times v o l t a g e occurs

at some i n t e r m e d i a t e n e g a t i v e

a p p l i e d p o t e n t i a l . T h e d a t a f r o m F i g u r e 11 r e v e a l t h a t 9 . 2 %

of

i n p u t o p t i c a l p o w e r at 633 n m is r e c o v e r a b l e as e l e c t r i c a l p o w e r .

the We

h a v e also d e m o n s t r a t e d e q u i v a l e n t s u s t a i n e d p o w e r c o n v e r s i o n efficien cies b y r e p l a c i n g the p o w e r s u p p l y w i t h a resistor i n series i n t h e e x t e r n a l circuit.



2

-0.6

-0.4 Applied

-0.2

Potential, V

0.0

2

f

+Q2

+0.4

Figure 11. Photocurrent against applied potential from a power supply in series in the external circuit. The 0.25 cm 0001 face of the CdSe was exposed to the 1.0M NaOH, 1.0M Na S 1.0M S electrolyte. Maximum power conversion efficiency (9.2%) occurs at —0.35 V applied.

0.040 h-


88

SOLID STATE

Sustained

conversion

of

l i g h t to

electrical energy

CHEMISTRY

demands

a

t o t a l l y n o n - d e t e r i o r a t i n g system. W e h a v e d e m o n s t r a t e d that t h e p h o t o electrode

is stable a n d h a v e a s s u m e d t h a t the P t c a t h o d e

is i n e r t .

T h e e l e c t r o l y t e m u s t b e c a p a b l e of b e i n g o x i d i z e d at C d S e a n d r e d u c e d at P t w i t h n o n e t c h e m i c a l c h a n g e . criterion i n our hands

Aqueous polysulfides meet this

as d e m o n s t r a t e d

by

passing electric

current

t h r o u g h a l . O A i N a O H , l . O A i N a S , 1 M S e l e c t r o l y t e (2.0 m l ) u s i n g t w o 2

P t electrodes.

C u r r e n t w a s passed at ~ 0.2 V a n d ~ 2.0 m A / c m

2

for a


v e r y l o n g p e r i o d w i t h n o o b v i o u s d e t e r i o r a t i o n of t h e system. T w o o t h e r p r a c t i c a l c o n s i d e r a t i o n s s h o u l d b e e m p h a s i z e d . P o l y s u l f i d e s are sensitive t o 0 , a n d electrolytes u s e d h a v e b e e n c o n t i n u o u s l y p u r g e d w i t h A r . 2

S e c o n d l y , aqueous polysulfides ( b u t n o t N a S ) a b s o r b b l u e l i g h t q u i t e 2

strongly. T h e l . O A i N a O H , 1 M N a S , ΙΛί S e l e c t r o l y t e is orange a n d has 2

a n o p t i c a l d e n s i t y of ~ 1.0 i n a 1 m m p a t h l e n g t h at ^ 490 n m . T h u s , v e r y short p a t h lengths a r e r e q u i r e d f o r c o m p l e t e v i s i b l e s p e c t r a l r e sponse i n this electrolyte. T h e 9 . 2 % s u s t a i n e d efficiency s h o w n i n F i g u r e 11 is one of the best values w e have obtained. intensity.

N o t e t h a t this v a l u e is f o r a v e r y l o w l i g h t

I n c r e a s i n g the l i g h t i n t e n s i t y causes s a t u r a t i o n of t h e p h o t o

effect, b u t r e a s o n a b l e s u s t a i n e d efficiencies are f o u n d , T a b l e I I .

Even

h i g h e r absolute p o w e r o u t p u t s f r o m t h e C d S e - b a s e d c e l l a r e p o s s i b l e , b u t , of course, t h e efficiency is less. T h e inefficiency i n the p o w e r c o n v e r s i o n rests w i t h the f a c t t h a t w e d o n o t see a u n i t q u a n t u m y i e l d f o r e l e c t r o n flow at a n e g a t i v e a p p l i e d

Table II.

Power Conversion Efficiency Using CdSe-Based Photoelectrochemical Cells Max Power* Out (mW)

Poten tial

Cur rent

Exp. No.'

Crys tal No.

Face Ex posed

4

1

0001

632.8 [0.10] [2.8]

0.0092 0.168

9.2 6.0

-0.35 -0.35

0.0263 0.480

5

2

000Ï

632.8 [0.10] [2.8]

0.0053 0.117

5.3 4.2

-0.35 -0.35

0.0151 0.333

6

4

not etched

632.8 [2.2]

0.0082

0.4

-0.20

0.041

7

5

0001

514.5 [0.025] [7.30]

0.0012 0.176

4.8 2.4

-0.35 -0.55

0.0034 0.320

Irrdn, λ (nm) [Power (mW)]"

Vfmax

(%)

@ lmax r

(V)

(mA")

' A l l experiments were performed in an electrolyte consisting of 1.0M N a O H , 1.0M N a S , 1.0M S. The circuit is schemed in Figure 4. See also notes in Table I. Multiply by 4.0 c m to obtain m W / c m . Multiply by 4.0 c m to obtain m A / c m . 2

b

- 2

β

- 2

2

2


4.

WRiGHTON E T AL.

Conversion

of Visible

89

Light

Table III. Q u a n t u m Efficiency for Electron Flow for CdSe-Based Photoelectrochemical Cells Exp. No.

Crystal No.

4*

Face Exposed

1

0001

Irrdn,\ [Intensity

(nm) (ein/sec)]''


6

0001

-0.35 +0.40

0.52 0.77

632.8 [148 Χ 10" ]

-0.35 +0.40

0.33 0.55

632.8 [10.6 Χ 10" ] 454.5 [6.76 Χ 1 0 ]

+1.00 +1.00

0.53 0.90

454.5 [6.46 Χ 10" ] 632.8 [10.6 Χ 1 0 ]

+1.00 +1.00

0.67 0.34

632.8 [5.29 Χ

10 ]

-0.35 +0.40

0.30 0.50

632.8 [148 Χ

10 ]

-0.35 +0.40

0.23 0.38

514.5 [1.07 Χ 10" ]

-0.35 0.00

0.33 0.40

514.5

-0.55 0.00

0.11 0.17

10

10

1 0

9*

oooT

7

10

1 0

5"

2

7*

0001

5

0001

16%

632.8 [5.29 Χ 10" ] 10

8*

Φ ±

1 0

1 0

10

[3.4

X10" ] 1 0

• l.OM N a O H , l.OM Na2S, l.OM S electrolyte using same circuit as in Figure 4. • l.OM N a O H , l.OM Na2S electrolyte using same circuit as in Figure 4. • Exposed electrode area is 0.25 c m . 2

p o t e n t i a l e q u a l to t h e b a n d g a p energy. T h i s is because of ( a ) s a t u r a t i o n effects at h i g h l i g h t intensities a n d ( b ) gap energy)

a r e l a t i v e l y ( c o m p a r e d to b a n d

small degree of b a n d bending.

H o w e v e r , the

observed

q u a n t u m y i e l d s f o r e l e c t r o n flow ( n o t c o r r e c t e d f o r reflective losses) a r e r a t h e r h i g h ( b u t c e r t a i n l y n o t u n i t y ) at the a p p l i e d p o t e n t i a l f o r t h e m a x i m u m e n e r g y c o n v e r s i o n a n d r e a c h e v e n h i g h e r v a l u e s at p o s i t i v e , a p p l i e d p o t e n t i a l s ( T a b l e I I I ) . T h e d a t a i n T a b l e I I a n d F i g u r e 11 s u p p o r t t h e c l a i m t h a t t h e C d S e - b a s e d c e l l is one of t h e m o r e efficient r e p o r t e d p h o t o e l e c t r o c h e m i c a l devices for t h e c o n v e r s i o n of o p t i c a l energy. C o n v e r s i o n of s u n l i g h t to e l e c t r i c a l energy w i t h a n efficiency of at least 2%

c o u l d b e e x p e c t e d u s i n g t h e s i n g l e - c r y s t a l C d S e - b a s e d cells.

Summary

and

Perspective

T h e s t a b i l i z a t i o n of C d S e to p h o t o a n o d i c d i s s o l u t i o n b y p o l y s u l f i d e electrolytes has b e e n d e m o n s t r a t e d (46, 47).

O p t i c a l to e l e c t r i c a l e n e r g y

c o n v e r s i o n efficiencies o f ~ 9 % h a v e b e e n o b t a i n e d w i t h n o d e t e r i o r a t i o n of t h e e l e c t r o l y t e o r p h o t o e l e c t r o d e , a n d t h e m a x i m u m p o w e r o u t p u t of t h e C d S e - b a s e d p h o t o e l e c t r o c h e m i c a l c e l l occurs at a p o t e n t i a l of a f e w


90

SOLID S T A T E C H E M I S T R Y

tenths of a v o l t .

T h e s t a b i l i t y has a l l o w e d t h e first measurements

of

c u r r e n t - v o l t a g e p r o p e r t i e s of C d S e e v e n at h i g h l i g h t intensities w i t h o u t p r o b l e m s associated w i t h p h o t o a n o d i c d i s s o l u t i o n . T h e c r u c i a l r e s u l t here is the s t a b i l i z a t i o n . T h i s shows t h a t i t is p o s s i b l e , b y c o m p e t i t i v e r e d o x processes, to q u e n c h c o m p l e t e l y electrolysis of s e m i c o n d u c t o r s .

has b e e n r e p e a t e d successfully i n other laboratories ( 5 4 , 5 5 ) . t i o n to C d S , the η-type B i S 2


(55)

3

B G

(E

B G

=

In addi

1-4 e V ) ( 5 6 , 5 7 ) c a n b e s t a b i l i z e d

u s i n g the p o l y s u l f i d e electrolyte. R e c e n t l y , i n o u r o w n l a b o r a t o r y

w e have stabilized C d T e ( E (E

photo-

T h e essence of t h e results o u t l i n e d h e r e

=

1.35 e V ) , a n d I n P ( E

B G

B G

=

1.4eV), GaP ( E

=

1.25 e V ) u s i n g the p o l y s u l f i d e e l e c t r o

B G

=

2.24eV), GaAs

l y t e o r other c h a l c o g e n i d e - c o n t a i n i n g electrolytes (58, 5 9 ) . T h e s e s e v e r a l examples of s t a b i l i z a t i o n are p r o m i s i n g , b u t there are disadvantages to t h e c o m p e t i t i v e e l e c t r o n transfer a p p r o a c h .

T h e c r u c i a l d i s a d v a n t a g e is

t h a t one c l e a r l y restricts t h e r a n g e of c h e m i c a l reactions t h a t c a n driven photoelectrochemically.

I f t h e objective

be

is to c o n v e r t l i g h t to

e l e c t r i c i t y , this m a y b e n o d r a w b a c k . O t h e r a p p r o a c h e s to the s t a b i l i z a t i o n of s m a l l b a n d g a p

semicon

d u c t o r s exist, a n d t h e y are n o t w i t h o u t d i s a d v a n t a g e s . O n e a p p r o a c h is to coat t h e s m a l l b a n d g a p m a t e r i a l w i t h a t h i n , i n e r t m e t a l film. I t has b e e n c l a i m e d , f o r e x a m p l e , t h a t A u - c o a t e d η-type G a P is stable to p h o t o a n o d i c d i s s o l u t i o n , a n d o n e c a n o x i d i z e H 0 at s u c h a

photoelectrode

2

(60).

T h i s a p p r o a c h is n o different f r o m a S c h o t t k y - b a r r i e r p h o t o c e l l ,

a n d a d i f f i c u l t y e n c o u n t e r e d h e r e w i l l b e i n f a b r i c a t i o n of the s o l i d - s o l i d j u n c t i o n . A n o t h e r t a c t i c has b e e n to a t t e m p t to coat a n u n s t a b l e e l e c t r o d e w i t h a n o t h e r s e m i c o n d u c t o r t h a t is stable. w i t h a t h i n film of T i 0

2

F o r example, coating C d S

has b e e n suggested a n d t r i e d (61,

62).

First,

t h e r e is n o e v i d e n c e t h a t t h e t e c h n i q u e w o r k s at a l l , a n d s h o u l d i t b e successful one junctions.

One

a g a i n faces the difficulties associated w i t h s o l i d - s o l i d final

" c o a t i n g " t e c h n i q u e i n v o l v e s d y e s e n s i t i z a t i o n of

l a r g e b a n d g a p m a t e r i a l s . G e r i s c h e r (2)

states t h a t d y e s e n s i t i z a t i o n has

b e e n k n o w n f o r some t i m e (63, 64, 6 5 ) , b u t t h a t the efficiency is l i m i t e d b y t h e f a c t t h a t o n l y m o n o l a y e r s of d y e c a n b e u s e d .

A n o t h e r factor,

w i t h the d y e s e n s i t i z a t i o n , is t h e s t a b i l i t y of t h e d y e itself. I n s u m m a r y , there are s o m e p r o m i s i n g avenues of r e s e a r c h i n p h o t o assisted r e d o x processes at electrodes.

T h e results o u t l i n e d h e r e are t h e

first of a set s h o w i n g t h a t i t is p o s s i b l e to h a v e i n t e r f a c i a l r e d o x processes w h i c h o c c u r so fast t h a t e l e c t r o d e d e c o m p o s i t i o n c a n n o t c o m p e t e .

The

q u e s t i o n n o w is w h y d o some r e d u c t a n t s w o r k w h i l e others d o n o t ? S t u d y of factors i n f l u e n c i n g the rate of i n t e r f a c i a l e l e c t r o n transfer s h o u l d y i e l d t h e a n s w e r . S i n c e i n t e r f a c i a l e l e c t r o n transfer rates g o v e r n efficiency i n a l l cases, these studies s h o u l d p r o v e u s e f u l i n a l l a p p r o a c h e s to t h e u l t i m a t e u t i l i z a t i o n of p h o t o e l e c t r o c h e m i c a l cells.


4. WRIGHTON ET AL.

Conversion of Visible Light

91

Acknowledgment W e thank the National Aeronautics a n d Space Administration for s u p p o r t of this r e s e a r c h . W e a c k n o w l e d g e t h e c o n s i d e r a b l e c o n t r i b u tions o f P e t e r T . W o l c z a n s k i i n p r e p a r a t i o n o f e l e c t r o d e crystals.


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

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