Luminescent Properties of Semiconductor ... - ACS Publications

11 Luminescent Properties of Semiconductor Photoelectrodes

1

ARTHUR B. ELLIS and BRADLEY R. KARAS Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch011

Department of Chemistry, University of Wisconsin, Madison, WI 53706

The use of luminescent, n-type 5-1000-ppm CdS:Te and 10 ppm-CdS:Ag polycrystalline photoelectrodes as probes of recombination in photoelectrochemical cells is reported. Except for intensity, the emission spectra (λ 600-700 nm) are insensitive to the presence of S /S electrolyte and to the excitation wavelengths and electrode potentials em ployed. With ultraband gap irradiation (λ ≤500 nm) and aqueous S /S or Te /Te electrolytes, optical energy is converted to electricity at 0.1-6% efficiency and to lumi nescence at 0.01-1.0% efficiency; the effects of surface prep aration and grain boundaries in determining efficiency are discussed. Increasingly negative bias applied to CdS:Te and CdS:Ag photoanodes increases emission intensity by 15-100% while the photocurrent simultaneously declines to zero. Band gap edge 514.5-nm excitation yields smaller photocurrents and larger but much less potential dependent emission intensity. These results are consistent with the band bending model presently used to describe photoelectro chemical phenomena. max,

2-

2-

n

2-

2-

n

The

2-

2-

2

desire to c o n v e r t o p t i c a l e n e r g y d i r e c t l y i n t o fuels o r e l e c t r i c i t y h a s

M e d to t h e r a p i d d e v e l o p m e n t o f p h o t o e l e c t r o c h e m i c a l cells ( P E C s ) . A t y p i c a l P E C consists s i m p l y o f a s e m i c o n d u c t o r e l e c t r o d e , a c o u n t e r electrode, a n d a n electrolyte. T h e semiconductor is t h e k e y element o f t h e P E C , since i t serves i n t h e d u a l c a p a c i t y o f p h o t o r e c e p t o r a n d electrode. L i g h t absorbed b y the semiconductor c a n b e channeled into 1

Author to whom inquiries should be addressed. 0-8412-0474-8/80/33-184-185$07.25/0 © 1980 American Chemical Society Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

186

INTERFACIAL

PHOTOPROCESSES

e l e c t r o c h e m i c a l processes l e a d i n g to t h e a f o r e m e n t i o n e d e n e r g y sions. A l t h o u g h the p h y s i c s g o v e r n i n g p h o t o e l e c t r o c h e m i c a l

conver

phenomena

has b e e n e l e g a n t l y r e v i e w e d ( 1 , 2 ) , a b r i e f d e s c r i p t i o n is i n o r d e r . P h o t o e l e c t r o c h e m i c a l events are i n i t i a t e d b y u l t r a b a n d g a p p h o t o n s that, w h e n absorbed b y the semiconductor, produce a conduction b a n d electron a n d valence b a n d hole.

T h e difference b e t w e e n the d a r k a n d

i l l u m i n a t e d electrodes is r e a l l y t h e difference b e t w e e n g r o u n d a n d e x c i t e d states, r e s p e c t i v e l y . F i g u r e 1 illustrates t h i s d i s t i n c t i o n f o r a n n - t y p e s e m i conductor.

T h e s e m i c o n d u c t o r b a n d s are b e n t i n p a r a l l e l ; t h i s is a conse

q u e n c e of t h e m i s m a t c h i n c h e m i c a l p o t e n t i a l s b e t w e e n t h e e l e c t r o l y t e

Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch011

( r e d o x p o t e n t i a l ) a n d s e m i c o n d u c t o r ( F e r m i l e v e l ) . B a n d b e n d i n g occurs o v e r a short d i s t a n c e ( ~

1 ^) f r o m the 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 e i n t o the s e m i c o n d u c t o r b u l k a n d e q u i l i b r a t e s t h e c h e m i c a l p o t e n t i a l s of the t w o phases.

T h e d i s t a n c e over w h i c h b a n d b e n d i n g occurs

is

t e r m e d the d e p l e t i o n or space-charge r e g i o n . O n c e the semiconductor

e x c i t e d state has b e e n p o p u l a t e d ,

band

b e n d i n g exerts c o n s i d e r a b l e influence o v e r the a t t e n d a n t d e a c t i v a t i o n processes.

I n particular, the potential gradient inhibits the recombination

of e l e c t r o n - h o l e p a i r s a n d p r o m o t e s

their separation. T h e

b a n d e l e c t r o n m i g r a t e s to the c o u n t e r e l e c t r o d e

where

conduction

it r e d u c e s

an

electroactive electrolyte species, a n d the v a l e n c e b a n d h o l e m i g r a t e s to the s e m i c o n d u c t o r - e l e c t r o l y t e interface w h e r e i t accepts a n e l e c t r o n f r o m a n electroactive species, t h e r e b y o x i d i z i n g i t . n - T y p e s e m i c o n d u c t o r s , t h e most c o m m o n l y u s e d photoelectrodes,

are thus p h o t o a n o d e s

and dark

cathodes. A m a j o r obstacle to t h e p r a c t i c a l u t i l i z a t i o n of these concepts is t h e u n d e s i r a b l e o x i d a t i o n of the n - t y p e s e m i c o n d u c t o r electrode itself. T y p i c a l is t h e case of C d S , w h i c h u n d e r g o e s Equation 1 (3).

photoanodic

decomposition

T h e p r o b l e m is m i n i m i z e d b y c h o o s i n g

via

electroactive

hp

CdS

> Cd

+ 2

+ S + 2e"

(1)

e l e c t r o l y t e species w h o s e o x i d a t i o n is k i n e t i c a l l y r a p i d e n o u g h to q u e n c h R e a c t i o n 1. F o r e x a m p l e , sulfide (S ~) or p o l y s u l f i d e ( S " ) electrolytes 2

g r e a t l y i n h i b i t the p h o t o a n o d i c

2

d i s s o l u t i o n of C d S ( 4 - 8 ) .

species c a n b e o x i d i z e d at the p h o t o a n o d e at the c o u n t e r e l e c t r o d e

n

Polysulfide

and simultaneously reduced

to y i e l d a P E C that e x h i b i t s l i t t l e c h a n g e i n

e l e c t r o l y t e o r electrode c o m p o s i t i o n , t h u s p e r m i t t i n g t h e s u s t a i n e d c o n v e r s i o n of o p t i c a l energy t o e l e c t r i c i t y . T h i s c o n c e p t has b e e n u s e d to construct P E C s

e m p l o y i n g a v a r i e t y of p h o t o a n o d e s

and

electrolytes

(9-28).

Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

ELLIS A N D KARAS

11.

Luminescent

Properties

of Photoelectrodes

187

Depletion Region

Conduction Band redox


r (a)

'"BG Valence Band

Semiconductor

Conduction Band

I

Electrolyte

e

t I

redox

hv

I

(b) Valence Band

Figure 1. (a) Representation of the dark semiconductor electrode corre sponding to the ground state; (b) irradiation of the electrode produces the excited state, which is deactivated here by redox reactions. E and E are the Fermi level and semiconductor band gap, respectively. Eredox ™ the electrolyte redox potential. Rand bending characteristic of the depletion region formed at the n-type semiconductor-electrolyte inter face is also shown. F

BO



188

INTERFACIAL

Ed

A.

—SEMICONDUCTOR

PHOTOPROCESSES

ELECTROLYTE—

Figure 2. Excited state deactivation pathways of the semiconductor electrode. Wavy arrows signify nonradiative decay routes: k k , and correspond to electron-hole recombination leading to heat, electron-hole separation leading to photoanodic decomposition, and electron-hole sepa ration leading to electrolyte redox reactions, respectively. The straight arrow and k correspond to radiative recombination, the source of lumi nescence. E is the thermodynamic potential for anodic decomposition. Intraband gap states and defects that might play a role in the various deactivation routes have been omitted for simplicity. l9

d

r

d

A m a j o r t h r u s t o f c u r r e n t P E C r e s e a r c h is the i m p r o v e m e n t o f e n e r g y c o n v e r s i o n efficiency.

C e n t r a l t o this g o a l is a n u n d e r s t a n d i n g o f t h e

s e m i c o n d u c t o r e x c i t e d state, p a r t i c u l a r l y t h e extent to w h i c h its d e a c t i v a t i o n routes m i g h t b e a m e n a b l e t o e x p e r i m e n t a l c o n t r o l . T h e

semicon

d u c t o r e x c i t e d state p a r t i t i o n s i n p u t o p t i c a l energy i n t o s e v e r a l p a t h w a y s , as p i c t u r e d i n F i g u r e 2. A b r o a d d i v i s i o n i n t o n o n r a d i a t i v e a n d r a d i a t i v e r e l a x a t i o n routes is e s p e c i a l l y c o n v e n i e n t . A t least three n o n r a d i a t i v e m e c h a n i s m s f o r d e a c t i v a t i o n are k n o w n : h e a t ( l a t t i c e v i b r a t i o n s ) , electrode d e c o m p o s i t i o n , a n d e l e c t r o l y t e r e d o x reactions w i t h c o r r e s p o n d i n g r a t e constants k\, fc , a n d k , r e s p e c t i v e l y . d

H e a t results f r o m t h e n o n r a d i a t i v e r e c o m b i n a t i o n o f

x

photogenerated

e l e c t r o n - h o l e p a i r s a n d its r o l e i n P E C s has b e e n e x p l o r e d b y p h o t o t h e r m a l spectroscopy

(67).

Electrode decomposition

and

electrolyte

r e d o x reactions are also n o n r a d i a t i v e b u t r e s u l t f r o m s e p a r a t i o n o f elec t r o n - h o l e p a i r s , as d e s c r i b e d a b o v e .

T h e relationship of k

s t r o n g l y i n f l u e n c e d b y t h e c h o i c e o f e l e c t r o l y t e (29-32).

d

t o k is x

T h e s u m (k


d

+

11.

k) x

ELLIS A N D KARAS

Luminescent

Properties

of Photoelectrodes

189

is reflected i n t h e c u r r e n t p a s s e d i n t h e e x t e r n a l c i r c u i t , b u t c h e m i c a l

m e t h o d s o f analysis are r e q u i r e d t o differentiate b e t w e e n t h e t w o c u r r e n t sources. F o r C d S - b a s e d P E C s k d o m i n a t e s k i n O H " e l e c t r o l y t e , w h e r e a s d

the opposite

x

is t r u e i n p o l y s u l f i d e m e d i a .

There are thermodynamic

p o t e n t i a l s ( E a n d E dox) associated w i t h these reactions d

re

(29,33,34);

h o w e v e r , t h e significance o f k i n e t i c s is u n d e r s c o r e d b y t h e o b s e r v a t i o n that diffusion-dependent

e l e c t r o l y t e r e d o x processes c a n c o m p e t e w i t h

electrode localized decomposition. affect k

d

and k

x

O t h e r e x p e r i m e n t a l factors t h a t c a n

are excitation intensity a n d electrode potential

(1,5,6,

33,34). A g a i n s t t h i s b a c k g r o u n d w e i n t r o d u c e k , w h i c h represents r a d i a t i v e Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch011

r

d e a c t i v a t i o n r e s u l t i n g f r o m e l e c t r o n - h o l e p a i r r e c o m b i n a t i o n processes. L u m i n e s c e n c e is a p o w e r f u l t o o l for c h a r a c t e r i z i n g e x c i t e d states, b e t h e y o r g a n i c , o r g a n o m e t a l l i c , o r s o l i d state i n n a t u r e .

Emissive properties

including spectral distribution, lifetime, a n d q u a n t u m y i e l d permit t h e c a l c u l a t i o n of rate constants a n d t h e assessment

of whether a given

r e a c t i o n is p o s s i b l e d u r i n g t h e e x c i t e d state l i f e t i m e . A l t h o u g h a vast l i t e r a t u r e exists f o r l u m i n e s c e n t s e m i c o n d u c t o r s

(36,37),

very little is

k n o w n a b o u t r a d i a t i v e d e c a y i n t h e context o f a P E C . Studies t h a t h a v e b e e n c a r r i e d o u t focus o n e l e c t r o l u m i n e s c e n c e

resulting from injection

processes a t extreme p o t e n t i a l s o r i n s t r o n g l y o x i d i z i n g o r r e d u c i n g m e d i a (38-44).

T h e s e are f r e q u e n t l y t r a n s i e n t effects w h i c h are n o t a p p r o p r i a t e

for s u s t a i n e d o p t i c a l e n e r g y c o n v e r s i o n .

P h o t o l u m i n e s c e n c e studies t h a t

p r e d a t e o u r w o r k use n - a n d p - t y p e G a P (44), n - t y p e Z n O , a n d C u - d o p e d Z n O ( Z n O : C u ) photoelectrodes

(68).

A l t h o u g h only p - G a P was photo

inert, these systems p r o v i d e i m p o r t a n t c o m p a r i s o n s w i t h o u r studies, as w i l l b e discussed later. W h a t w e h a d h o p e d to find are electrodes t h a t e m i t w h i l e m i m i c k i n g t h e essential features o f electrodes u s e d i n o p e r a t i n g P E C s . A s s h o w n i n F i g u r e 3, C d S d o p e d w i t h e i t h e r T e o r A g ( C d S : T e , C d S : A g ) acts as just s u c h a d u a l e l e c t r o d e - e m i t t e r .

W e find that C d S : T e a n d C d S : A g

are s i m i l a r t o u n d o p e d C d S electrodes i n t h e i r a b i l i t y t o o x i d i z e a q u e o u s polysulfide

a n d d i t e l l u r i d e species

as p a r t o f degenerate

electrolysis

schemes l e a d i n g t o s u s t a i n e d c o n v e r s i o n o f o p t i c a l e n e r g y t o e l e c t r i c i t y . E m i s s i o n from C d S : T e a n d C d S : A g involves intraband gap electronic states i n t r o d u c e d b y t h e d o p a n t .

T e l l u r i u m i s t h o u g h t t o substitute at S

sites a n d t o y i e l d states a p p r o x i m a t e l y 0.2 e V a b o v e t h e v a l e n c e

band

( 4 5 - 5 2 , 69). A s a n i s o e l e c t r o n i c d o p a n t , T e is n o t e x p e c t e d t o a l t e r t h e e l e c t r i c a l p r o p e r t i e s o f C d S a p p r e c i a b l y . B e c a u s e i t has a s m a l l e r e l e c t r o n affinity t h a n S, T e serves as a t r a p f o r holes t h a t t h e n c a n b i n d c o u l o m b ically a n electron i n or near the conduction b a n d , thus f o r m i n g a n exciton. T h e exciton's b i n d i n g e n e r g y is a b o u t 0.22 e V ; a p p r e c i a b l e e x c i t o n c o n centrations w i l l exist a t r o o m t e m p e r a t u r e . A t h i g h e r T e d o p i n g l e v e l s


Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980. 2

Aqueous Polysulfide Electrolyte (NaOH/NaS/S)

Photoelectrode n-type CdS:Te or CdS:Ag (site of polysulfide oxidation)

Input Visible Light

Light

Figure 3. A PEC employing n-type CdS:Te or CdS:Ag photoelectrodes luminesces as it converts opti cal energy to electricity via a degenerate electrolysis reaction. Polychalcogenide electrolytes have been chosen to minimize photoanodic decomposition of the electrode.

Dark Counterelectrode (site of poly sulfide reduction)

load


11.

ELLIS AND KARAS

Luminescent

Properties

191

of Photoelectrodes

t h e e x c i t o n is t h o u g h t t o b e l o c a l i z e d o v e r s e v e r a l nearest n e i g h b o r T e atoms w i t h a h i g h e r b i n d i n g e n e r g y (47,48).

R a d i a t i v e c o l l a p s e of t h e

exciton produces the observed luminescence. W e have used melt-grown, p o l y c r y s t a l l i n e m a t e r i a l t h a t is n o m i n a l l y 5 - 1 0 0 0 - p p m T e . T h e m e c h a n i s m b y w h i c h C d S : A g emits is m o r e c o m p l e x a n d d e p e n d s b o t h o n the presence of a d d i t i o n a l i m p u r i t i e s a n d o n w h e t h e r A g s u b s t i tutes at C d sites o r i n t e r s t i t i a l l y (53-59).

Substitution for C d w o u l d m a k e

A g a n acceptor a n d thus p a r t i a l l y compensate

the m a t e r i a l .

We

have

u s e d m e l t - g r o w n , p o l y c r y s t a l l i n e 1 0 - p p m C d S : A g , a n d the r e s i s t i v i t y of approximately 10

3

O - c m c o m p a r e d w i t h a p p r o x i m a t e l y 1 O - c m for u n -


d o p e d C d S ( a n d C d S : T e ) is consistent w i t h this r o l e f o r A g . T o e x p l o i t t h e emissive p r o p e r t i e s of C d S : T e - a n d C d S : A g - b a s e d P E C s , the c e l l is a s s e m b l e d i n the emission c o m p a r t m e n t of a spectrophotofluorometer.

I n c l i n i n g the p h o t o e l e c t r o d e

at a b o u t 45° to b o t h a

laser e x c i t a t i o n source a n d the e m i s s i o n d e t e c t i o n optics p e r m i t s t h e s a m p l i n g of front surface e m i s s i o n d u r i n g the course of

photoelectro

c h e m i c a l events. T h u s , changes i n the e m i s s i o n s p e c t r u m ( 5 - n m r e s o l u t i o n ) a n d i n t e n s i t y c a n be m o n i t o r e d i n s i t u .

T h e e x c i t a t i o n source is

the c o n t i n u o u s o u t p u t of a n A r i o n laser. G e n e r a l l y , t h e i n c i d e n t p o w e r w a s 1-15 m W , w h i c h i n t h e a p p r o x i m a t e l y 3 - m m d i a m e t e r b e a m c o r r e sponds to intensities of 1 4 - 2 1 2 m W / c m . 2

A l l data were obtained w i t h

p o l y c r y s t a l l i n e C d S . T e a n d C d S . A g samples f r o m E a g l e - P i c h e r I n d u s t r i e s , M i a m i , O k l a h o m a . T h e g r a i n sizes i n the p o l y c r y s t a l l i n e samples are e s t i m a t e d to be 3 - 8 m m . P r e p a r a t i o n a n d h a n d l i n g of the p o l y s u l f i d e a n d d i t e l l u r i d e electrolytes has b e e n d e s c r i b e d p r e v i o u s l y a n d differs i n the use of N

2

r a t h e r t h a n A r for p u r g i n g

(6).

A s w i l l b e d e s c r i b e d b e l o w , the k e y finding t h a t w e h a v e m a d e is that the emission f r o m C d S : T e a n d C d S : A g photoelectrodes

is a v e r y

sensitive p r o b e o f r e c o m b i n a t i o n processes w i t h i n the d e p l e t i o n r e g i o n a n d h e n c e of the s e m i c o n d u c t o r e x c i t e d state. O u r results thus f a r are consistent w i t h the a f o r e m e n t i o n e d interpret

photoelectrochemical

b a n d b e n d i n g arguments used

phenomena.

Importantly, we

to

present

e v i d e n c e that e x p e r i m e n t a l p a r a m e t e r s s u c h as electrode p o t e n t i a l , e l e c trolyte,

and excitation wavelength

may

be

used

to

manipulate the

s e m i c o n d u c t o r e x c i t e d state processes a n d thus influence t h e course

of

o p t i c a l energy c o n v e r s i o n . Results

and

Stability.

Discussion A s a first step i n c h a r a c t e r i z i n g C d S : T e - a n d C d S : A g -

b a s e d P E C s , w e w i s h e d t o d e t e r m i n e t h e extent to w h i c h t h e i r e l e c t r o c h e m i s t r y resembles t h a t of u n d o p e d C d S . A l l three e l e c t r o d e m a t e r i a l s undergo photoanodic dissolution i n O H " electrolyte according to E q u a -


192

INTERFACIAL

t i o n 1.

Since C d S photoelectrodes

ditelluride

PHOTOPROCESSES

are stabilized b y polysulfide a n d

electrolytes, w e investigated w h e t h e r o r n o t C d s r T e a n d

C d S : A g a r e r e n d e r e d stable b y these m e d i a u s i n g v a r i a t i o n s i n e l e c t r o d e w e i g h t , surface q u a l i t y , p h o t o c u r r e n t , a n d l u m i n e s c e n c e as c r i t e r i a . I n t h e k i n e t i c s c h e m e o f F i g u r e 2, w e a r e s e e k i n g e v i d e n c e t h a t k is m u c h x

greater t h a n fc . d

T h e first c r i t e r i o n of s t a b i l i t y is m e t i f there is n o a p p r e c i a b l e w e i g h t loss after sufficient c u r r e n t has p a s s e d i n t h e e x t e r n a l c i r c u i t t o d e c o m p o s e p a r t o r a l l of t h e electrode.

D a t a i n T a b l e I i n d i c a t e t h a t this is t h e case

for C d S : T e a n d C d S : A g electrodes i n S V S 2

n

2

" and T e V T e 2

2

2

" electrolytes.


T h e m i n i m a l w e i g h t losses o b s e r v e d r e s u l t p r i m a r i l y f r o m c h i p p i n g o f the e l e c t r o d e w h e n i t is d e m o u n t e d .

A l l of the experiments listed i n

T a b l e I w e r e c o n d u c t e d w i t h t h e electrode a t z e r o o r n e g a t i v e b i a s ; u n d e r these c o n d i t i o n s o p t i c a l e n e r g y is c o n v e r t e d to e l e c t r i c i t y , i f t h e o n l y electrochemistry corresponds

to offsetting o x i d a t i o n a n d r e d u c t i o n of

p o l y c h a l c o g e n i d e species ( F i g u r e 3 ) . A d i s c u s s i o n o f surface q u a l i t y s h o u l d b e p r e f a c e d w i t h a d e s c r i p t i o n of s a m p l e p r e p a r a t i o n . T h e "as r e c e i v e d " p o l y c r y s t a l l i n e samples o f C d S : T e a n d C d S : A g u s e d i n this s t u d y w e r e e t c h e d i n c o n c e n t r a t e d H C 1 p r i o r to b e i n g u s e d as electrodes.

T h i s g e n e r a l l y has t h e effect o f i n c r e a s i n g

emission intensity w h i l e leaving t h e spectral distribution intact. D e p e n d i n g u p o n the P E C conditions a n d the sample employed, w e see v a r i a b l e degrees of surface d a m a g e . A t sufficiently h i g h l i g h t i n t e n s i ties (^ 5 0 m W / c m ) a n d p o s i t i v e voltages (^ — 0.3 V v s . S C E ) i n p o l y 2

sulfide electrolytes, w e o c c a s i o n a l l y e n c o u n t e r d a r k e n i n g o f C d S : A g a n d

Table I.

Stability of **-Type C d S : T e and C d S : A g Electrode

Electrode

Electrolyte

b

CdS:Te CdSrAg CdS:Te CdSrAg CdS:Te

1000 10 100 10 100

0

ppm ppm ppm ppm ppm

s ; S S„ " Te„ Te " n

n

2

2

2

2

t t

2

(mol

XIO )' 4

After

Before 0.847 1.87 136.7 1.27 9.36

0.820 1.70 136.7 1.26 8.75*

° Photoelectrochemical cell with the indicated electrode as photoanode. F o r the experiments in Sn " electrolyte, a power supply was used as the load (cf. Figure 3). For C d S : A g in T e " electrolyte, a third, reference electrode was also used in con junction with a potentiostat. P t foil served as the counterelectrode in all cases. * Electrodes were etched in concentrated HC1 prior to use. They are melt-grown, polycrystalline samples from Eagle-Picher Industries. T h e C d S : T e and C d S : A g specimens have resistivities of ~ 1 and 2000 fl-cm, respectively, and were used in irregular shapes. Electrolyte is 1M O H ' / I M S 7 0 . 2 M S (S»2") or 5 M K O H / 0 . 0 5 M T e 7 0 . 0 1 M T e ~ ( T e « ' ) . T h e polysulfide electrolyte was purged with N and the electrode was held at ~ —0.05 V vs. the counterelectrode; the negative lead of the power supply was connected to the photoelectrode. Several of the experiments in Sn " were peri2

n

c

2

2

2

2

2

2

2

2


11.

ELLIS A N D KARAS

Luminescent

Properties

193

of Photoelectrodes

C d S : T e surfaces. E v e n a v o i d i n g these c o n d i t i o n s , t h o u g h , t h e r e i s l i k e l y some surface r e o r g a n i z a t i o n a n d w e sometimes see d i s c o l o r e d e l e c t r o d e surfaces.

R e c e n t studies o f C d S e electrodes

i n polysulfide electrolyte

i n d i c a t e t h a t s u b s t i t u t i o n o f surface S e sites b y S occurs

(19,60).

A

s i m i l a r exchange i n v o l v i n g T e is p l a u s i b l e as is a n o t h e r surface r e o r g a n i zation mechanism based o n the propensity of C d T e to undergo anodic dissolution i n polysulfide media v i a E q u a t i o n 2 ( 6 , 9 ) .

photo I t is

hw

CdTe

>Cd

+ 2

+ T e + 2e"

(2)


p o s s i b l e t h a t t h e H C 1 e t c h leaves a T e - r i c h surface, w h i c h c o u l d t h e n undergo exchange a n d / o r photoanodic decomposition.

Consistent w i t h

the l a t t e r m e c h a n i s m is t h e r e l a t i v e l a c k o f surface d a m a g e w e observe in ditelluride media. B o t h C d S and C d T e are k n o w n to be stabilized b y Te " and T e / T e 2

2

2

2

- electrolytes ( 6 ) . W e find, h o w e v e r , t h a t C d S : A g a n d

C d S : T e e x h i b i t s i m i l a r p r o p e r t i e s w i t h r e g a r d t o surface s t a b i l i t y so t h a t t h e r o l e of T e is n o t r e s o l v e d . T h e v a r i a t i o n i n surface s t a b i l i t y is r e p e a t e d i n t h e t e m p o r a l c h a r a c teristics of p h o t o c u r r e n t a n d l u m i n e s c e n c e .

I n polysulfide electrolyte

n o m i n a l l y i d e n t i c a l samples o f C d S : T e o r C d S r A g h a v e d i s p l a y e d b o t h stable a n d u n s t a b l e p h o t o c u r r e n t s a n d e m i s s i v e p r o p e r t i e s . D e c l i n i n g photocurrents a n d emission intensity are usually accompanied b y t h e a f o r e m e n t i o n e d surface d e t e r i o r a t i o n . C e r t a i n l y one of t h e d i s a d v a n t a g e s of t h e p o l y c r y s t a l l i n e m a t e r i a l u s e d i n this s t u d y is its n o n u n i f o r m i t y . I n p a r t i c u l a r , g r a i n b o u n d a r i e s a r e n o t o r i o u s sources of d i s c o n t i n u o u s Photoelectrodes i n Aqueous Polychalcogenide Electrolytes Electrons (mol X 10 )'

Average\

4

h

1.73 5.14 5.17 1.31 3.96

(mA)

0.069 0.117 0.066 0.086 0.259

Time

0

(hr)

Source*

67.2 117.8 210.0 40.8 41.0

Hg Hg Hg Xe Xe

odically interrupted to renew the electrolyte, since even with N purging there is slow decomposition from impurities. F o r the C d S : A g experiment in T e n " , the photoelectrode was held at —1.04 V vs. S C E , the value measured for J^do*; the C d S : T e experiment in Te« " was run at 0.00 V vs. the counterelectrode. * H g is a UV-filtered 200-W, super high pressure H g arc lamp; X e is an unfiltered 150-W X e lamp. * Crystal chipped badly upon demounting and not all of it could be recovered. Moles of crystal determined by weight before and after the experiment. Moles of electrons passed during the experiment as determined by integrating photocurrent vs. time plots. * Average current during experiment; current densities in m A / c m are roughly a factor of 1-15 larger. 2

2

2

1

9

2


194

INTERFACIAL

b e h a v i o r (61).

PHOTOPROCESSES

S i n c e w e d o n o t k n o w the surface c o m p o s i t i o n of t h e

e t c h e d C d S : T e a n d C d S : A g electrodes, w e are r e l u c t a n t to m a k e a d e f i n i t i v e statement r e g a r d i n g t h e i r s t a b i l i t y i n p o l y s u l f i d e m e d i a .

It

c e r t a i n l y b e a r g u e d t h a t the i n t r a b a n d g a p states m i g h t i n f l u e n c e t h e

can kx

fc c o m p e t i t i o n . W e suspect t h a t the samples that y i e l d the most stable d

p r o p e r t i e s m a y h a v e surfaces m u c h l i k e u n d o p e d C d S . D o p e d m a t e r i a l s w i t h these surfaces c o u l d s t i l l e m i t , since the b u l k of t h e e x c i t a t i o n b e a m is a b s o r b e d b e n e a t h the surface. T h e studies r e p o r t e d here for p o l y s u l f i d e electrolytes w e r e c o n d u c t e d w i t h samples e x h i b i t i n g stable, r e p r o d u c i b l e , p h o t o c u r r e n t a n d emissive p r o p e r t i e s .


T h e r e is less a m b i g u i t y i n d i t e l l u r i d e electrolyte. W e g e n e r a l l y see stable p h o t o c u r r e n t s , e m i s s i o n , a n d surface p r o p e r t i e s , a l t h o u g h at h i g h e r intensities there are often s l o w m o n o t o n i c declines i n p h o t o c u r r e n t a n d luminescence.

T h e r e is o b v i o u s e v i d e n c e for the c o m p e t i t i v e o x i d a t i o n

of colorless T e " t o p u r p l e T e " o r of e i t h e r T e 2

2

2

2 -

or T e " to T e . A t h i g h 2

2

l i g h t intensities the e m i s s i o n is m a s k e d b y a l a y e r of T e a n d / o r T e " t h a t 2

2

c a n b e s w e p t a w a y b y greater s t i r r i n g rates. T h e v i s u a l e v i d e n c e

for

o x i d a t i o n of S ~ is o b s c u r e d because the e m i t t i n g electrode a n d e l e c t r o n

2

l y t e are of s i m i l a r color.

H o w e v e r , w e h a v e o b s e r v e d y e l l o w i n g of t h e

i n i t i a l l y colorless S " solutions as S ~ forms u n d e r P E C c o n d i t i o n s . T a k e n 2

n

2

as a u n i t , the d a t a a r g u e s t r o n g l y for s t a b i l i t y of C d S : T e a n d C d S r A g i n d i t e l l u r i d e electrolytes; for p o l y s u l f i d e electrolytes s u s t a i n e d p h o t o c u r r e n t s a n d m i n i m a l w e i g h t loss w i t h these electrodes m a y b e o b t a i n e d , b u t t h e r e is s t r o n g e v i d e n c e

that surface r e o r g a n i z a t i o n processes are i n v o l v e d .

E x p e r i m e n t s d e s i g n e d to c l a r i f y the c o m p l i c a t i o n s o b s e r v e d i n p o l y s u l f i d e m e d i a are i n progress. Optical Properties.

A s described i n the introduction, the semicon

d u c t o r e x c i t e d state is r e a c h e d b y a b s o r p t i o n of u l t r a b a n d g a p p h o t o n s . U n d o p e d C d S has a b a n d g a p of a p p r o x i m a t e l y 2.4 e V a n d h e n c e a f u n d a m e n t a l a b s o r p t i o n e d g e at a b o u t 520 n m

(62).

The

absorption

onset is v e r y s h a r p because C d S is a d i r e c t b a n d g a p m a t e r i a l . A b s o r p t i o n s p e c t r a of s i n g l e c r y s t a l C d S : T e samples h a v e t h e i r onset r e d - s h i f t e d b y a n a b s o r p t i o n s h o u l d e r ; t h e effect of i n c r e a s i n g T e c o n c e n t r a t i o n is to e x t e n d t h i s s h o u l d e r d e e p e r i n t o t h e r e d (46,47,48,

62, 69).

Spectra that

w e h a v e o b t a i n e d for 1 0 0 - a n d 1 0 0 0 - p p m C d S : T e a r e p r e s e n t e d i n F i g u r e 4. A l t h o u g h these s p e c t r a w e r e o b t a i n e d f r o m p o l y c r y s t a l l i n e s a m p l e s , t h e y are at least q u a l i t a t i v e l y i n a g r e e m e n t w i t h t h e r e p o r t e d s i n g l e c r y s t a l d a t a i n s h a p e a n d c o l o r ; t h e r e is a n o b v i o u s v i s u a l difference, since t h e y e l l o w u n d o p e d C d S b e c o m e s orange at 5 - 1 0 0 - p p m C d S r T e a n d r e d at 1 0 0 0 - p p m C d S : T e . S i m i l a r l y , 1 0 - p p m C d S : A g is r e d a n d 1 0 0 - p p m is b r o w n - b l a c k . I n effect t h e s h o u l d e r masks t h e b a n d e d g e a n d t h e b a n d gap energy

(E ). BG



11.

ELLIS A N D KARAS

500

Luminescent

600

Properties

700

of Photoelectrodes

WAVELENGTH, nm

800

195

900

Figure 4. Optical density of polycrystalline 100-ppm CdS:Te (D) and 1000-ppm CclS.Te (O). Thicknesses are 2.0 and 2.2 mm, respectively, and samples have been polished with 1-jx alumina. The x is a literature value optical density of a 2-mm thick, undoped, polished CdS single crystal (5).

W h a t p r i m a r i l y distinguishes C d S : T e a n d C d S r A g f r o m C d S is t h e i r a b i l i t y to e m i t w h i l e t h e y serve as electrodes.

undoped

In Figure 5 we

present e m i s s i o n s p e c t r a t a k e n at 293 K f o r 5-, 100-, a n d

1000-ppm

polycrystalline C d S r T e a n d for 10-ppm polycrystalline C d S r A g .

A sys

t e m a t i c s t u d y of single c r y s t a l C d S : T e e m i s s i o n spectra has s h o w n t h a t p e a k p o s i t i o n a n d h a l f w i d t h m a y b e c o r r e l a t e d w i t h d o p i n g levels i n these samples (48).

O u r results, t h o u g h u n c o r r e c t e d for detector response,

are q u a l i t a t i v e l y i n a g r e e m e n t : t h e 5- a n d 1 0 0 - p p m e m i s s i o n s p e c t r a a r e v e r y s i m i l a r , w i t h a p e a k m a x i m u m at a r o u n d 600 n m , b u t there is a definite r e d shift of the e m i s s i o n m a x i m u m to a r o u n d 650 n m f o r 1 0 0 0 - p p m C d S : T e . T h e e m i s s i o n itself varies f r o m y e l l o w - o r a n g e to r e d - o r a n g e i n p a s s i n g f r o m t h e 5-, 1 0 0 - p p m to the 1 0 0 0 - p p m C d S : T e . F o r C d S r A g w e observe r e d d i s h e m i s s i o n a n d the b a n d m a x i m u m appears n e a r 690 n m .


196


INTERFACIAL

J

U

500

I

I

L

i

I

I

600 700 WAVELENGTH, nm

I

PHOTOPROCESSES

U

800

Figure 5. Typical 295 K emission spectra of 5-, 100-, 1000-ppm CdS:Te ana 10-ppm CdS:Ag. The CdS:Te samples were excited at 488.0 nm and the CdS:Ag sample at 514.5 nm. Spectra are uncorrected.


11.

ELLIS A N D KARAS

Luminescent

Properties

of Photoelectrodes

197

W e were particularly interested i n determining i f the P E C environ m e n t p e r t u r b e d the e m i s s i o n spectra of t h e d o p e d C d S samples. T y p i c a l results are g i v e n i n F i g u r e 6, w h i c h shows a s e q u e n c e of e m i s s i o n s p e c t r a f o r 5 - p p m C d S : T e t a k e n w i t h o u t electrolyte, w i t h p o l y s u l f i d e electrolyte ( 1 M O H " / l M S ~ / 1 M S ) b u t o u t - o f - c i r c u i t , a n d i n - c i r c u i t i n t h e same 2

e l e c t r o l y t e at — 0.74 V vs. S C E . E x c e p t f o r t h e c h a n g e i n i n t e n s i t y , t h e e m i s s i o n s p e c t r a are essentially i d e n t i c a l . W e i n t e r p r e t t h i s t o m e a n t h a t t h e i n t r a b a n d g a p states r e s p o n s i b l e f o r l u m i n e s c e n c e are i n f l u e n c e d i n t h e same m a n n e r as the c o n d u c t i o n a n d v a l e n c e b a n d s a n d t h u s w o u l d


undergo parallel band bending.

Figure 6. Uncorrected emission spectra of 5-ppm CdS.Te in various environments but in a fixed geometry relative to the 488.0-nm laser ex citation source and emission detection optics. For Curve A no electrolyte was present; Curves B and C were both taken with the electrode im mersed in I M OH'/IM S ' / I M S polysulfide electrolyte but out-of-circuit and in-circuit at —0.74 V vs. SCE, respectively. The sharp intensity drop from AtoB and C is the result of electrolyte absorption; baseline is not preserved at the high energy end of the emission spectrum due to overlap with the tail of the excitation line. 2


198

INTERFACIAL


EXCITATION (x 1/10)

J_

400

500

PHOTOPROCESSES

CdS'T* 100 PPM A. -0.3 V VS. SCE B. -0.8 V VS. SCE C. -I.O V VS. SCE

600 700 WAVELENGTH, nm

800

Figure 7. Uncorrected emission spectra of a 100-ppm CdS:Te electrode at several potentials in I M 0H~/1M S '/0.2M S electrolyte. Curves A , B, and C correspond to identical experimental conditions except for electrode potentials of —0.3, —0.8, and —1.0 V vs. SCE, respectively. The excita tion wavelength is 496.5 nm. 2

A d d i t i o n a l evidence

supporting this interpretation is p r o v i d e d b y

F i g u r e 7. H e r e t h e e m i s s i o n o f a 1 0 0 - p p m C d S : T e p h o t o e l e c t r o d e i s r e c o r d e d at — 0.3, — 0.8, a n d — 1.0 V vs. S C E i n p o l y s u l f i d e electrolyte. A g a i n , a l t h o u g h the e m i s s i o n i n t e n s i t y changes, t h e s p e c t r u m does not. A p p l i c a t i o n o f n e g a t i v e bias t o a n n - t y p e s e m i c o n d u c t o r d i m i n i s h e s t h e a m o u n t o f b a n d b e n d i n g ; c o n v e r s e l y , p o s i t i v e bias increases b a n d b e n d ing

( 1 ) . Invariance of the emission spectrum under conditions where

b a n d b e n d i n g has b e e n a l t e r e d i s thus consistent w i t h p a r a l l e l b e h a v i o r between

d o p a n t states a n d t h e v a l e n c e

and conduction

bands. T h e

s p e c t r a l d i s t r i b u t i o n o f e m i s s i o n appears t o b e i n d e p e n d e n t o f e x c i t a t i o n w a v e l e n g t h f o r t h e laser lines w e n o r m a l l y e m p l o y : 488.0, 496.5, 501.7, a n d 514.5 n m . W e h a v e u s e d these w a v e l e n g t h s as the basis o f p h o t o a c t i o n excitation spectra ( T a b l e I I ) .

and

There is generally a m a r k e d decline i n

p h o t o c u r r e n t i n p a s s i n g f r o m w h a t are c e r t a i n l y u l t r a b a n d g a p energies (488.0, 496.5, 501.7

n m ) t o 514.5 n m .

T h e data i n Table I I were

o b t a i n e d i n o p t i c a l l y t r a n s p a r e n t sulfide e l e c t r o l y t e a t — 0.4 V vs. S C E at r o u g h l y e q u i v a l e n t intensities for e a c h o f t h e f o u r w a v e l e n g t h s .

Photo

currents f r o m 501.7-nm e x c i t a t i o n are a b o u t 4.5 to 10 times those r e s u l t i n g f r o m 514.5-nm e x c i t a t i o n (see T a b l e I I ) . F o r u n d o p e d single c r y s t a l C d S the corresponding experiment was reported to y i e l d a similar photocur-


11.

ELLIS A N D KARAS

Luminescent

Properties

199

of Photoelectrodes

r e n t r a t i o o f 7 - 1 3 f o r e x c i t a t i o n a t these s a m e t w o w a v e l e n g t h s ( 5 ) . T h e r e f o r e , a l t h o u g h t h e a b s o r p t i o n s p e c t r a of d o p e d a n d u n d o p e d C d S differ, t h e f u n d a m e n t a l a b s o r p t i o n e d g e m a y h a v e t h e same w a v e l e n g t h d e p e n d e n c e ; t h a t is, t h e b a n d gaps m i g h t b e i d e n t i c a l . C o m p l e t e p h o t o a c t i o n a n d e x c i t a t i o n s p e c t r a m u s t b e o b t a i n e d t o c l a r i f y this p o i n t , h o w ever, a n d s u c h studies a r e i n progress. B y m e a s u r i n g t h e e m i s s i o n intensity a n d p h o t o c u r r e n t s i m u l t a n e o u s l y , t h e i r r e l a t i o n s h i p as a f u n c t i o n of w a v e l e n g t h c a n b e e x a m i n e d . A l o n g side t h e p h o t o c u r r e n t d a t a i n T a b l e II a r e t h e c o r r e s p o n d i n g e m i s s i o n intensities m e a s u r e d b o t h i n - a n d o u t - o f - c i r c u i t . T w o g e n e r a l trends a r e


d i s c e r n i b l e . O n e is that t h e e m i s s i o n intensity increases w i t h w a v e l e n g t h a n d t h e o t h e r is t h a t t h e i n - c i r c u i t v a l u e s a r e s u b s t a n t i a l l y l o w e r t h a n o u t - o f - c i r c u i t v a l u e s , except a t 514.5 n m w h e r e t h e difference is s m a l l . Table II.

Wavelength Dependence of Emission and Photocurrent Relative Emission Intensity (arbitrary units)'

Electrode"

\ (nm)

Out of

Circuit

In

Circuit

0

Fnotocurrent w

1. C d S : T e 5 ppm

514.5 501.7 496.5 488.0

235 187 147 116

232 147 106 88

12 85 84 72

2. C d S : T e 1000 p p m

514.5 501.7 496.5 488.0

300 310 236 187

295 281 204 158

5 22 28 34

3. C d S : A g 10 p p m

514.5 501.7 496.5 488.0

1623 355 227 158

1612 305 170 115

38 395 347 287

" T h e experiments were conducted in optically transparent 1A/ O H " / l M S " electrolyte with the electrodes at —0.4 V vs. S C E . A t this potential the photocurrent is saturated with respect to potential. Electrodes were excited by an ~ 3-mm diame ter A r ion laser beam of ~ 3 mW. Electrodes are irregularly shaped. The surface area exposed to the electrolyte is 0.15, 0.075, and 0.18 c m for Electrodes 1, 2, and 3, respectively. T h e fraction of the electrode illuminated can be estimated by dividing these numbers into 0.071 c m , the laser beam area. Relative emission intensity was measured with a flat wavelength response radi ometer that was filtered to eliminate the exciting wavelengths and positioned to sam ple front surface electrode emission. A switch on the P A R potentiostat permitted the P E C to be brought in and out of circuit without disturbing the geometry. F o r a given electrode experimental conditions are identical except for the exciting wave length. T h e emission values have been corrected to the same number of einsteins/sec at each wavelength; however, values between electrodes cannot be compared because different geometries and light intensities were employed. Photocurrents have been corrected for each electrode to equivalent einsteins/ sec. For a given electrode experimental conditions are identical except for the excit ing wavelength. Values between electrodes are not comparable (cf. Footnote c). 2

6

2

2

c

d


200

INTERFACIAL

PHOTOPROCESSES

I n the r e g i o n of w h a t appears to b e the b a n d g a p , t h e n , there is a n inverse relationship between photocurrent a n d luminescence intensity. T h e p e n e t r a t i o n d e p t h of the e x c i t a t i o n b e a m is a significant f a c t o r i n e x p l a i n i n g a l l of the p h e n o m e n a w e observe.

L i t e r a t u r e v a l u e s for u n

d o p e d C d S a n d C d S : T e i n d i c a t e t h a t the 300 K a b s o r p t i v i t y (a) n m is a b o u t 1 0 c m ' 3

(46,47,62,69).

1

vs. 1 0 - 1 0 4

5

at 514

c m " at 502 n m a n d shorter w a v e l e n g t h s 1

Electroabsorption measurements on u n d o p e d C d S sug

gest t h a t there s h o u l d b e l i t t l e v a r i a t i o n of a w i t h electrode p o t e n t i a l f o r most of the e x c i t a t i o n w a v e l e n g t h s u s e d (63).

T h e e x c i t e d state d e a c t i v a

t i o n rate constants p i c t u r e d i n F i g u r e 2 s h o u l d b e v e r y d e p e n d e n t

on


p o s i t i o n , since b a n d b e n d i n g decreases w i t h d i s t a n c e f r o m 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 interface. W e w o u l d a p r i o r i p r e d i c t t h a t e l e c t r o n - h o l e p a i r s p r o d u c e d o u t s i d e the d e p l e t i o n r e g i o n are m o r e l i k e l y to r a d i a t i v e l y r e c o m b i n e t h a n those p r o d u c e d w i t h i n the d e p l e t i o n r e g i o n , w h i c h c a n m o r e r e a d i l y separate to p r o d u c e p h o t o c u r r e n t .

S i n c e a greater f r a c t i o n

of 514.5-nm t h a n u l t r a b a n d g a p l i g h t w i l l b e a b s o r b e d o u t s i d e t h e d e p l e t i o n r e g i o n , the l o w energy p h o t o n s s h o u l d b e m o r e effective at y i e l d i n g e m i s s i o n a n d less effective at p r o d u c i n g p h o t o c u r r e n t t h a n u l t r a b a n d g a p photons. T h e i n - a n d o u t - o f - c i r c u i t trends m a y also b e i n t e r p r e t e d i n t h i s m a n n e r . T a k i n g the electrode o u t of c i r c u i t removes d e a c t i v a t i o n routes c o r r e s p o n d i n g to e l e c t r o n - h o l e s e p a r a t i o n a n d thus increases t h e p r o b a b i l i t y for d e a c t i v a t i o n v i a the r e m a i n i n g p a t h w a y s .

Since photocurrent

is least i m p o r t a n t as a d e c a y route for 514.5-nm e x c i t a t i o n , the difference b e t w e e n i n - a n d o u t - o f - c i r c u i t emission s h o u l d be smallest, as is o b s e r v e d . A l t h o u g h this l o g i c i n t i m a t e s at a d i r e c t trade-off b e t w e e n e m i s s i o n a n d p h o t o c u r r e n t , the d a t a i n T a b l e I I are e v i d e n c e t h a t this n e e d not b e so. T h e declines i n e m i s s i o n i n t e n s i t y w i t h w a v e l e n g t h m a y w e l l b e d u e to surface traps that p r o m o t e n o n r a d i a t i v e e l e c t r o n - h o l e p a i r r e c o m b i n a t i o n (64).

S u r f a c e q u a l i t y c a n b e e x p e c t e d to p l a y a m o r e significant r o l e as

the penetration depth

decreases.

O n e f u r t h e r p o i n t concerns the l o n g w a v e l e n g t h extreme. W e h a v e a c t u a l l y o b s e r v e d e m i s s i o n w a v e l e n g t h s as l o n g as 540 n m b e y o n d w h i c h e m i s s i o n i n t e n s i t y b e c o m e s l i m i t e d b y the o p t i c a l d e n s i t y of the s a m p l e . T h i s is p o s s i b l e e v i d e n c e t h a t the a b s o r p t i o n s h o u l d e r corresponds to the e m i t t i n g e x c i t e d state. I f this w e r e the case, t h e n a n alternate e x p l a n a t i o n for the d e c l i n e of e m i s s i o n i n t e n s i t y w i t h w a v e l e n g t h is s i m p l y t h a t the a b s o r p t i o n b a n d l e a d i n g to e m i s s i o n has p e a k e d a n d f a l l e n . A

complete

e x c i t a t i o n s p e c t r u m c o u l d r e v e a l the shape of this b a n d , since most of i t is b u r i e d b e n e a t h the d i r e c t b a n d g a p t r a n s i t i o n . Current-Voltage-Emission Properties.

U n d o u b t e d l y the m o s t s i g

nificant feature of the l u m i n e s c e n t photoelectrodes is the o p p o r t u n i t y t h e y afford to e x a m i n e the i n t e r p l a y of r a d i a t i v e a n d n o n r a d i a t i v e e x c i t e d state


11.

ELLIS A N D KARAS

d e a c t i v a t i o n routes.

Luminescent

Properties

201

of Photoelectrodes

A s described i n the introduction, the P E C c a n be

a s s e m b l e d i n s i d e the s a m p l e c h a m b e r of a n e m i s s i o n

spectrophotofluo-

r o m e t e r so that front surface e m i s s i o n m a y b e m o n i t o r e d d u r i n g P E C o p e r a t i o n . A s t a n d a r d three-electrode g e o m e t r y w a s u s e d i n c o n j u n c t i o n w i t h a P A R potentiostat t h a t regulates the p h o t o e l e c t r o d e a n S C E . A 1.5 X photoelectrodes

p o t e n t i a l vs.

0.9-cm P t f o i l served as t h e counterelectrode.

The

w e r e i r r e g u l a r l y s h a p e d b u t often s m a l l e n o u g h to

be

c o m p l e t e l y i l l u m i n a t e d b y the 3 - m m d i a m e t e r laser b e a m . E l e c t r o d e s l a r g e r t h a n the e x c i t a t i o n b e a m w e r e also u s e d .

I n this

case e x c i t a t i o n at a spot o n t h e surface results i n e m i s s i o n over t h e e n t i r e Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch011

s a m p l e surface.

T h e r e are at least three explanations for this. T h e first

is t h a t excitons m a y m i g r a t e a n d r a d i a t i v e l y r e c o m b i n e t h r o u g h o u t t h e s a m p l e . W e expect e x c i t o n d i f f u s i o n lengths to b e s m a l l at r o o m t e m p e r a t u r e , b u t to o u r k n o w l e d g e t h e y h a v e not b e e n m e a s u r e d for C d S : T e a n d C d S r A g . W e cannot, therefore, r u l e this out. A second p o s s i b i l i t y i n v o l v e s r e a b s o r p t i o n a n d r e e m i s s i o n of e m i t t e d l i g h t e n o u g h t i m e s to t r a n s p o r t i t t h r o u g h o u t the s a m p l e .

S i n c e most of t h e e m i t t e d l i g h t is not a p p r e

c i a b l y a b s o r b e d b y t h e s a m p l e , w e t h i n k t h a t this, too, is u n l i k e l y b u t s t i l l possible.

T h e most p l a u s i b l e e x p l a n a t i o n , w e f e e l , is s i m p l y t h a t

the e m i t t e d l i g h t is scattered to t h e extent that i t emerges t h r o u g h o u t t h e sample.

S o m e r o l e m a y b e p l a y e d b y g r a i n b o u n d a r i e s i n this process.

I n several samples w h e r e o b v i o u s g r a i n b o u n d a r i e s exist, w e find l u m i nescence o n l y w i t h i n the i r r a d i a t e d g r a i n ; e m i s s i o n ceases a b r u p t l y at the boundary.

I n t h i s sense t h e b o u n d a r y m a y b e a c t i n g as a site of

n o n r a d i a t i v e r e c o m b i n a t i o n i f m i g r a t i o n processes are i n v o l v e d o r as a r e f l e c t i n g or a b s o r b i n g surface i f s c a t t e r i n g is the d o m i n a n t m e c h a n i s m . I n e i t h e r case, g r a i n b o u n d a r i e s are not a r e q u i r e m e n t f o r e m i s s i o n ; w e h a v e r e c e n t l y o b t a i n e d single crystals of 1 0 0 - p p m C d S : T e ( v i d e i n f r a ) t h a t e x h i b i t the same e m i s s i o n s p e c t r u m a n d t h i s same p h e n o m e n o n

of

g l o b a l emission f r o m l o c a l e x c i t a t i o n . T h e c r u c i a l finding w i t h r e g a r d to P E C s is that electrode p o t e n t i a l influences b o t h p h o t o c u r r e n t a n d l u m i n e s c e n c e efficiency.

A t y p i c a l set

of results is p r e s e n t e d i n F i g u r e 8 for a 1 0 0 - p p m C d S : T e

photoelectrode

and

1 M O H V 1 M S / 0 . 2 M S electrolyte. W i t h 496.5-nm u l t r a b a n d g a p 2

e x c i t a t i o n the p h o t o c u r r e n t d e c l i n e s w i t h i n c r e a s i n g l y n e g a t i v e p o t e n t i a l , e v e n t u a l l y r e a c h i n g z e r o a r o u n d —1.1 V vs. S C E . O v e r t h e same p o t e n t i a l r a n g e the e m i s s i o n i n t e n s i t y m o r e t h a n d o u b l e s . W e h a v e c h o s e n t h e e x p e d i e n t of m o n i t o r i n g e m i s s i o n i n t e n s i t y at the b a n d m a x i m u m of 600 n m , since the s p e c t r u m is i n d e p e n d e n t of p o t e n t i a l ( v i d e s u p r a ) . T h e e m i s s i o n d a t a p r e s e n t e d i n F i g u r e 8 are s i m i l a r t o t h e results p r e s e n t e d i n the p r e c e d i n g section.

Out-of-circuit emission values such

as those g i v e n i n T a b l e I I s h o u l d c o r r e s p o n d to the e m i s s i o n i n t e n s i t y at z e r o p h o t o c u r r e n t , the p o i n t w h e r e the c u r r e n t - v o l t a g e c u r v e i n t e r c e p t s



Figure 8. Dependence of photocurrent ( , left hand scale) and rela tive emission intensity ( , right hand scale; monitored at 600 nm) on electrode potential for a 100-ppm CdS:Te-based PEC using I M 0H~/1M S '/0.2M S electrolyte. Both measurements were made simultaneously at a sweep rate of 13 mV/sec starting at —0.3 V vs. SCE. Electrolyte redox potential is —0.70 V vs. SCE. The 3-mm diameter laser beam only filled part of the irregularly shaped 0.54-cm electrode surface. Because of electrolyte absorption, only an upper limit of approximately 4 mW for the incident 496.5-nm excitation can be given. 2

2

t h e v o l t a g e axis. R e c a l l that the o u t - o f - c i r c u i t a n d i n - c i r c u i t values w e r e m o s t s i m i l a r for 514.5-nm e x c i t a t i o n a n d q u i t e d i s p a r a t e for u l t r a b a n d gap irradiation.

C o m p l e t e c u r r e n t - v o l t a g e - e m i s s i o n d a t a s h o u l d reflect

this r e l a t i o n s h i p . T h e d i r e c t c o m p a r i s o n is s h o w n i n F i g u r e 9, w h i c h is b a s e d o n a 10-ppm C d S r A g photoelectrode i n 1 M O H ' / I M

S 70.2M 2

S electrolyte.

C o m p a r a b l e intensities of 496.5- a n d 514.5-nm e x c i t a t i o n result i n d r a m a t i c a l l y different p h o t o c u r r e n t - l u m i n e s c e n c e p r o p e r t i e s . photocurrent,

ultraband gap

496.5-nm

W i t h respect

to

excitation yields substantially

l a r g e r p h o t o c u r r e n t s t h a n b a n d g a p e d g e 514.5-nm l i g h t . T h e m a x i m u m o u t p u t v o l t a g e f o r a g i v e n i n t e n s i t y ( E ) , d e f i n e d as t h e v

difference

b e t w e e n t h e v o l t a g e at w h i c h zero p h o t o c u r r e n t obtains a n d t h e v a l u e o f Eredox, is a b o u t 500 m V f o r 496.5-nm l i g h t a n d 320 m V f o r

514.5-nm

e x c i t a t i o n ( f r o m F i g u r e 9 ) . T h e w a v e l e n g t h d e p e n d e n c e of p h o t o c u r r e n t a n d o u t p u t v o l t a g e has b e e n r e p o r t e d f o r several n - t y p e

semiconductor

p h o t o e l e c t r o d e s ; l a r g e r values f o r b o t h are o b t a i n e d w i t h u l t r a b a n d g a p photons

than w i t h b a n d gap

edge photons

because

f r a c t i o n s of l i g h t a b s o r b e d w i t h i n the d e p l e t i o n r e g i o n

of t h e (5,6,10).


different

11.

ELLIS A N D KARAS

Luminescent

Properties

of Photoelectrodes

203

A s the photocurrent i n F i g u r e 9 declines to zero i n passing t o i n c r e a s i n g l y n e g a t i v e p o t e n t i a l s , the e m i s s i o n i n t e n s i t y increases b y u p to 5 0 %

w i t h 496.5-nm excitation, a n d drops slightly

( ~ 6 % ) with

514.5-nm e x c i t a t i o n . T h e e m i s s i o n intensities at t h e t w o w a v e l e n g t h s o f


F i g u r e 9 are a b o u t t h e same b e c a u s e t h e 4 9 6 . 5 - n m e x c i t a t i o n is s o m e w h a t

POTENTIAL, VOLTS VS. SCE Figure 9. Photocurrent ( , left hand scale) and relative emission intensity ( , right hand scale) as a function of electrode potential for a 10-ppm CdSiAg-based PEC in 1M OH'/IU S ~/0.2U S electrolyte. Experimental conditions in the top and bottom figures are identical except that ~15 mW of 496.5-nm light was used in the former and ~11 mW of 514.5-nm light in the latter. Intensities are upper limits since they are not corrected for electrolyte absorption. Emission intensity in both cases was monitored at 670 nm; the points labelled "100" on the relative emission intensity scales are about the same absolute intensity. Photocurrent and emission intensity measurements were made simultaneously at a sweep rate of 13 mV/sec starting at —0.3 V vs. SCE. The electrolyte redox potential is —0.70 V vs. SCE. Electrode surface area exposed to the elec trolyte is ~0.21 cm and only partially filled by the 3-mm diameter laser beam. 2

2


204

INTERFACIAL

m o r e intense.

PHOTOPROCESSES

F o r i d e n t i c a l e x c i t a t i o n i n t e n s i t i e s w e n o r m a l l y see less

l u m i n e s c e n c e i n t e n s i t y at 496.5 n m t h a n at 514.5 n m ( T a b l e I I ) .

The

d r a m a t i c difference i n l u m i n e s c e n c e i n t e n s i t y w i t h e l e c t r o d e p o t e n t i a l a t these t w o w a v e l e n g t h s has b e e n o b s e r v e d b y us w i t h a l l of t h e e l e c t r o d e m a t e r i a l s u s e d a n d i n b o t h sulfide a n d p o l y s u l f i d e electrolytes. G e n e r a l l y , t h e p e r c e n t a g e increase i n l u m i n e s c e n c e i n p a s s i n g to the n e g a t i v e ex t r e m e of

t h e c u r r e n t - v o l t a g e c u r v e is 1 5 - 1 0 0 %

w i t h ultraband gap

e x c i t a t i o n w a v e l e n g t h s of 488.0, 496.5, a n d 501.7 n m .

Excitation

with

514.5 n m y i e l d s less t h a n 6 % changes a n d these are o f t e n d e c l i n e s . T h i s t r e n d persists e v e n at 514.5-nm l i g h t i n t e n s i t i e s t h a t a r e sufficiently h i g h


to p r o d u c e p h o t o c u r r e n t s c o m p a r a b l e to those a c h i e v e d w i t h l o w e r i n t e n s i t y u l t r a b a n d g a p e x c i t a t i o n . A g a i n , w e a s c r i b e t h i s difference to t h e s m a l l e r f r a c t i o n of 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 . E s s e n t i a l l y , a greater f r a c t i o n of l i g h t is t h e n a b s o r b e d i n a r e g i o n of less b a n d b e n d i n g , a r e g i o n w h e r e t h e e x c i t e d state d e a c t i v a t i o n r a t e c o n stants s h o u l d b e m o r e i n s e n s i t i v e to changes i n e l e c t r o d e p o t e n t i a l . T o f u r t h e r p r o b e the g e n e r a l i t y of t h i s p h e n o m e n o n , a s i m i l a r e x p e r i ment was conducted i n ditelluride electrolyte ( F i g u r e 10). is 5 - p p m

CdS:Te.

A t comparable

T h e electrode

i n c i d e n t intensities of

496.5-

and

5 1 4 . 5 - n m l i g h t , a p p r o x i m a t e l y 2 0 % a n d 3 % increases i n e m i s s i o n i n t e n s i t y o c c u r o v e r the r a n g e of — 0 . 7 - — 1.2 V vs. S C E , r e s p e c t i v e l y . I t w o u l d b e difficult to p r e d i c t a p r i o r i h o w the l u m i n e s c e n c e i n d i t e l l u r i d e w o u l d differ f r o m t h a t o b s e r v e d i n p o l y s u l f i d e . I f a l l of t h e r a t e constants of F i g u r e 2 w e r e i d e n t i c a l f o r the t w o electrolytes, t h e n there s h o u l d b e a n exact c o r r e s p o n d e n c e b e t w e e n the c u r r e n t - v o l t a g e - e m i s s i o n c u r v e s . T h i s assertion assumes t h a t C d S : T e a n d C d S : A g b e h a v e as u n d o p e d C d S , f o r w h i c h t h e c o n d u c t i o n a n d v a l e n c e b a n d energies p e n d e n t of p o l y c h a l c o g e n i d e

electrolyte

(6).

are r e l a t i v e l y i n d e

O f course, t h e a p p r o x i

m a t e l y 0.4 V difference i n E edox energies means t h a t o u t p u t voltages i n r

Te '/Te

2

media.

O u r d a t a t h u s f a r i n d i c a t e s i m i l a r features f o r t h e t w o

2

2

' e l e c t r o l y t e w i l l b e c o n s i d e r a b l y r e d u c e d r e l a t i v e to p o l y s u l f i d e electro

lytes, b u t c o n s i d e r a b l y m o r e d a t a n e e d t o b e c o l l e c t e d b e f o r e c o m p a r i s o n s can be comfortably made. T w o features of t h e r a n g e o f c o n d i t i o n s u s e d are p a r t i c u l a r l y n o t e worthy.

F i r s t , t h e k i n d s of p e r c e n t a g e

increases i n e m i s s i o n t h a t

we

o b s e r v e h a v e b e e n o b t a i n e d at v a r i o u s s w e e p rates a n d w i t h p o i n t - b y p o i n t e q u i l i b r a t i o n . T h e difference i n e m i s s i o n at m a x i m u m a n d n e a r - z e r o p h o t o c u r r e n t s is e a s i l y o b s e r v e d v i s u a l l y b y p u l s i n g t h e electrode b e t w e e n t h e a p p r o p r i a t e p o t e n t i a l s . W e also h a v e confidence

t h a t the effect is

r e a l b e c a u s e i t is r e p r o d u c i b l e e i t h e r i n p o i n t - b y - p o i n t or i n t h e reverse s w e e p i n g of p o t e n t i a l . I n some cases w e h a v e s w e p t t h r o u g h t h e p o t e n t i a l r a n g e m a n y t i m e s i n succession a n d s t i l l see the same v a r i a t i o n i n e m i s s i o n and photocurrent.


11.

ELLIS A N D KARAS

Luminescent

Properties

of Photoelectrodes

IZ0

150

125


205

100

o

100

80

75

60

c/)50

40

d

Q_

H (/)

CdS: T « 5 PPM 496.5 nm EXCITATION

^25 O K 0 6 0

a: 3^0

oo

o

o

g

0 z I20Q if) (/)

J_

J_

2 cr

20

o

o - 100 uJ UJ

i

>

80

Luminescent Properties of Semiconductor ... - ACS Publications

Recommend Documents