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.
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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)
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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
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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
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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 .
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
one
Conversion
WHIGHTON E T A L .
4.
of Visible
77
Light
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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.
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
was
78
SOLID STATE
CHEMISTRY
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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 "
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
80
SOLID STATE
1
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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 .
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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.
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
Conversion
of
83
Visible Light
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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
/ /
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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
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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.
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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-
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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
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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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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)]''
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
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
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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
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
(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.
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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.
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