Photoelectrochemical Solar Cells - Advances in Chemistry (ACS

12 Photoelectrochemical Solar Cells

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch012

Chemistry

o f the S e m i c o n d u c t o r - L i q u i d J u n c t i o n

A. HELLER and B. MILLER Bell Laboratories, Murray Hill, NJ 07974

The energy conversion efficiency and stability of semicon ductor-liquid junction solar cells are critically dependent on the chemistry at the photoactive junction. Improvement of the GaAs | selenide-polyselenide | C cell to 12% solar effi ciency and stabilization of the CdS | sulfide-polysulfide | C cells will be discussed in this context. For the GaAs cell, adsorption of ruthenium on the surface has considerably improved the fill factor. Combined with an etching procedure leading to a matte, nonreflective surface, this treat ment is responsible for the output enhancement. In the CdSe cell, stability is materially improved by addition of selenium to the redox electrolyte to suppress deleterious ion exchange processes otherwise occurring under high light intensity photoanodic operation.

J

unctions between semiconductors and liquids form spontaneously; semi c o n d u c t o r - l i q u i d j u n c t i o n solar cells a r e s i m p l e r t o m a k e t h a n other

types o f solar cells.

T h e q u i d p r o q u o is exposure o f t h e j u n c t i o n t o

c h e m i c a l processes, s u c h as c o r r o s i o n , i o n exchange, a n d a d s o r p t i o n o f i m p u r i t i e s , w h i c h m a y alter t h e j u n c t i o n a n d affect t h e l i f e a n d o u t p u t o f the cells.

H o w e v e r , d e l i b e r a t e m o d i f i c a t i o n o f t h e surface to i m p r o v e

p e r f o r m a n c e is possible. W e s u m m a r i z e here studies o f s e m i c o n d u c t o r r e d o x electrolyte c o m b i n a t i o n s i n w h i c h l i f e h a s b e e n e x t e n d e d s u b s t a n tially a n d output

importantly improved

b y analysis o f t h e j u n c t i o n

chemistry. T h e most our laboratory (1,2), which T h i s efficiency

efficient s e m i c o n d u c t o r - l i q u i d j u n c t i o n solar cells m a d e i n n o w c o n v e r t solar t o e l e c t r i c a l p o w e r w i t h a 1 2 % efficiency b r i n g s t h e m i n t o t h e r a n g e o f solid-state j u n c t i o n devices. w a s r e a c h e d b y m o d i f y i n g b o t h t h e surface c h e m i s t r y a n d 0-8412-0474-8/80/33-184-215$05.00/0 © 1980 American Chemical Society

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

216

INTERFACIAL

t h e surface t o p o g r a p h y of g a l l i u m arsenide ( G a A s ) .

PHOTOPROCESSES

B y controlling the

t o p o g r a p h y (3), i t w a s possible t o r e d u c e reflection losses (1,2,8)

at t h e

s e m i c o n d u c t o r - l i q u i d interface. T h e s e aspects of surface m o d i f i c a t i o n w i l l b e d i s c u s s e d i n this c h a p t e r . A n o t h e r c r i t i c a l f a c t o r w e w i s h to discuss i n t h e context of o u r w o r k o n n - c a d m i u m s e l e n i d e - s u l f i d e - p o l y s u l f i d e cells is t h e i o n exchange ( I I 14) process at 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 j u n c t i o n a n d t h e s u p p r e s s i o n Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch012

of l i f e - l i m i t i n g reactions b y c h a n g i n g electrolyte c h e m i s t r y . W e cite some p e r t i n e n t aspects of the p h y s i c a l c h e m i c a l a n d s t r u c t u r a l p r o p e r t i e s of junctions between n-type semiconductors e s t a b l i s h t h e b a c k g r o u n d of these Efficiency and Stability

to

a n d redox c o u p l e solutions t o

developments.

Dissolution

T h e theoretical limiting open-circuit photovoltage achievable i n semi c o n d u c t o r - l i q u i d j u n c t i o n solar cells u n d e r intense i l l u m i n a t i o n is a p p r o x i m a t e l y t h e difference b e t w e e n t h e f e r m i l e v e l of the s e m i c o n d u c t o r a n d the p o t e n t i a l of t h e r e d o x

couple

i n s o l u t i o n (4,5),

as l o n g as this

p o t e n t i a l is w i t h i n t h e s e m i c o n d u c t o r s b a n d g a p . I f t h e redox p o t e n t i a l b e c o m e s o x i d i z i n g w i t h respect to t h e v a l e n c e b a n d , t h e s e m i c o n d u c t o r is i r r e v e r s i b l y o x i d i z e d e v e n i n t h e d a r k . I f t h e p o t e n t i a l is too r e d u c i n g , the r e d o x p o t e n t i a l w i l l a p p r o a c h t h e f e r m i l e v e l of t h e n - t y p e s e m i c o n d u c t o r , w h i c h is l o c a t e d w i t h i n a p p r o x i m a t e l y 0.2 e V of t h e c o n d u c t i o n b a n d i n our materials. T h e photovoltage w i l l then be small a n d the c e l l w i l l b e inefficient. I l l u m i n a t i o n of t h e n - t y p e s e m i c o n d u c t o r

causes holes to m o v e t o

the interface ( F i g u r e 1). T h e holes o x i d i z e t h e s e m i c o n d u c t o r unless s u c h o x i d a t i o n is t h e r m o d y n a m i c a l l y p r o h i b i t e d o r unless the holes s e l e c t i v e l y react w i t h t h e a d s o r b e d , r e d u c i n g m e m b e r semiconductors

of the r e d o x c o u p l e .

For

w i t h b a n d gaps near t h e o p t i m u m f o r one-step solar

energy c o n v e r s i o n (1.4 ± 0.4 e V ) there are n o t h e r m o d y n a m i c a l l y stable systems w i t h sufficiently o x i d i z i n g redox couples that a l l o w photovoltages i n excess of a b o u t h a l f t h e b a n d g a p . A s a result, t h e s i m u l t a n e o u s a c h i e v e m e n t of s o l a r - t o - e l e c t r i c a l c o n v e r s i o n a n d s t a b i l i t y t o o x i d a t i o n r e q u i r e s c o n t r o l of t h e k i n e t i c s to a c h i e v e a s i t u a t i o n f o r w h i c h t h e rate of o x i d a t i o n of t h e a d s o r b e d m e m b e r of a r e d o x c o u p l e g r e a t l y exceeds the o x i d a t i o n of t h e i l l u m i n a t e d s e m i c o n d u c t o r

surface.

A n e x a m p l e of c o n t r o l l e d k i n e t i c s is t h e case of the n - G a A s | K S e 2

K S e - K O H | C c e l l (6). G a A s p h o t o c o r r o d e s 2

2

i n b a s i c aqueous

solutions

u n d e r i l l u m i n a t i o n b y R e a c t i o n 1 ( 7 ) ( w h e r e h* = h o l e ) . I n t h e n - G a A s | G a A s + 6h + 8 0 H " +

Ga(OH) - + As0 - + 4

2

2H 0 2


(1)

12.

HELLER

Photoelectrochemical

AND MILLER

Solar

Cells

217


•WW

2fc +2Se "—Se*" AMOOE 4

2

tt~+(ifP»

2Si " CATHODE 2

Figure 1. Schematic of a semiconductor-liquid junction solar cell. The cell, with the illuminated anode, is shown on the right. Electrode reac tions for the n-GaAs\K Se-K Se -KOH\C cell are shown at the bottom. The diagram on the left shows the bending of the bands at the interface, which results in an electric field near the junction. Absorbed photons produce electron-hole pairs, which are separated in the field. The holes move to the liquid interface and oxidize the redox couple. The electrons move through the semiconductor, the back metal contact, and the external load to the carbon cathode, where they reduce the redox couple. 2

2

2

K S e - K S e - K O H | C cells (13) t h e selenide i o n c o m b i n e s w i t h t h e holes 2

2

2

at a rate that g r e a t l y exceeds t h e p h o t o c o r r o s i o n r e a c t i o n w h e n t h e selenide c o n c e n t r a t i o n is sufficiently h i g h , a n d t h e d e s i r e d a n o d i c r e a c t i o n ( R e a c t i o n 2) d o m i n a t e s . T h e S e " / S e ' c o u p l e is s t i l l sufficiently o x i d i z 2

2

2

2Se " + 2h -> S e ' 2

+

2

(2)

2

i n g r e l a t i v e t o t h e flat b a n d i n this e l e c t r o l y t e to a l l o w a p h o t o v o l t a g e a b o v e 0.7 V . The

kinetics of electrolyte oxidation a n d semiconductor

photocor

r o s i o n are different f o r e a c h s e m i c o n d u c t o r r e d o x c o u p l e system. G e n e r alized kinetic models to predict illuminated semiconductor stability i n different r e d o x c o u p l e s a n d a t different r e d o x c o u p l e c o n c e n t r a t i o n s d o n o t exist as yet. P r e v e n t i o n of reflection losses is essential t o o b t a i n a h i g h o v e r a l l energy

c o n v e r s i o n efficiency.

E t c h i n g of a semiconductor

surface to

p r o d u c e h i l l o c k s o f m i c r o n o r s u b m i c r o n size ( 1 , 2 ) ( F i g u r e 2) increases



218

INTERFACIAL

PHOTOPROCESSES

Figure 2. Scanning electron micrograph of a GaAs(lOO) face etched by a stationary film of a 1:1 30% H 0 -H SO solution. Because of the submicron size structure ("hillocks"), the reflection of light is substantially reduced. Introduction of the hillocks produces an effect similar to the conversion of "shiny" platinum to platinum "black" upon particle size reduction. 2

t h e c u r r e n t efficiency.

2

2

h

T h i s increases t h e o v e r a l l c o n v e r s i o n

efficiency,

since p h o t o n s , w h i c h w o u l d h a v e b e e n reflected because of t h e difference i n i n d e x of r e f r a c t i o n b e t w e e n t h e s e m i c o n d u c t o r a n d t h e s o l u t i o n , are t r a p p e d b e t w e e n the h i l l o c k s . T h e r e s u l t i n g increase i n c u r r e n t efficiency is s h o w n i n F i g u r e 3 (8). Stability

to Ion

Exchange

O p e r a t i o n of a s e m i c o n d u c t o r - l i q u i d j u n c t i o n solar c e l l , o r

even

m e r e i m m e r s i o n of a s e m i c o n d u c t o r i n a r e d o x c o u p l e s o l u t i o n , m a y l e a d to i o n exchange b e t w e e n the s e m i c o n d u c t o r surface a n d the s o l u t i o n . I f t h e e x c h a n g e results i n a n e p i t a x i a l l a y e r of a n e w c o m p o u n d , t h e t h i c k -


12.

HELLER

AND MILLER


Solar

Cells

219


2.0 -

The Electrochemical Society, Inc.

Figure 3. Effects of introduction of surface structure and of Ru(III) treatment in the n-GaAs photoanode on the current-voltage characteristics of the n-GaAs\K Se-K Se -KOH\C cell. Curves 1 and 2, "shiny" smooth electrodes; Curves 3 and 4, "matte" electrodes with hillocks. Curves 1 and 3, untreated electrodes; Curves 2 and 4, Ru(III) treated electrodes. Cur rent (I J is normalized to the maximum value (I J for the "shiny" electrodes (*)• 2

2

2

ness of w h i c h exceeds t h e e l e c t r o n t u n n e l i n g t h i c k n e s s ( 3 0 - 5 0 A ) , a n a d d e d j u n c t i o n w i l l exist i n t h e s e m i c o n d u c t o r .

Such a junction m a y

b l o c k t h e flow of holes to t h e l i q u i d i n t e r f a c e , as s h o w n i n F i g u r e 4 for

the n-CdSe | l M S - - l M S ° - l M N a O H

| C cell

2

(8,9) in which ion

e x c h a n g e results i n a C d S l a y e r o n t h e p h o t o a n o d e

(10-13).

I n the

r e s u l t i n g s t r u c t u r e t h e v a l e n c e b a n d of C d S is a b o u t 0.5 e V b e l o w t h a t of C d S e The

(10). 0.5-eV b a r r i e r to h o l e flow p r o d u c e s a d r o p i n p h o t o c u r r e n t

( F i g u r e 5 ) w h e n a C d S l a y e r e x c e e d i n g t u n n e l i n g thickness results f r o m surface i o n exchange. T h e i o n exchange process is p r o m o t e d b y l i g h t a n d c i r c u m s t a n c e s w h e r e i n a p h o t o c o r r o s i o n step ( R e a c t i o n 3 ) c a n t a k e p l a c e . T h i s step is f o l l o w e d b y S e ° d i s s o l u t i o n ( R e a c t i o n 4 ) a n d C d S r e p r e c i p i tation (Reaction 5 ) . CdSe + 2 h

+

Cd

2 +

+ Se°


(3)

220

INTERFACIAL

PHOTOPROCESSES

CB(CdS) o.2ev

CB(cdse)


CdSe

— - E

F

c d -s — - - E

REDOX F

E

F

- *

2.4 ev

i.7ev

V B (cdse)

o.sev VB(CdS)


Figure 4. Schematic showing a barrier for hole transport from an nCaSe photoanode to a redox couple solution created by an n-CdS surface layer under short-circuit conditions. For simplicity, band bending near the liquid interface is not shown. CB, VB, and E are, respectively, con duction bands, valence bands, and fermi levels (11). F

Se° + Cd

2 +

S * - - > S e S * '2-

(4)

S -->CdS

(5)

2

+

2

A t l o w l i g h t levels a l l holes a r e effectively c a p t u r e d b y t h e r e d o x c o u p l e , o x i d a t i o n of t h e s e m i c o n d u c t o r b y R e a c t i o n 3 does n o t t a k e p l a c e a n d there is little deterioration i n photocurrent. I f the intensity is increased to a p o i n t w h e r e charge transport to t h e redox couple n o longer copes w i t h a l l t h e holes a r r i v i n g a t t h e i n t e r f a c e , R e a c t i o n 3 a n d t h u s i o n


HELLER

12.

AND MILLER


Solar

221

Cells

e x c h a n g e a n d p h o t o c u r r e n t d e t e r i o r a t i o n are r a p i d . R a p i d s t i r r i n g assists i n m a i n t a i n i n g the surface c o n c e n t r a t i o n of a c t i v e species f o r the d e s i r e d r o u t e of s o l u t i o n o x i d a t i o n ; t h u s the rate of d e t e r i o r a t i o n is d i m i n i s h e d . A d d i t i o n of e l e m e n t a l s e l e n i u m to the s o l u t i o n represses t h e C d S e -> C d S c o n v e r s i o n . A p o s s i b l e m o d e of this a c t i o n is t h e f o r m a t i o n of S e S " ions, 2

w h i c h react w i t h dissolving C d

2 +

ions to f o r m a C d S e i . ^ S a . l a y e r .

At

sufficiently l o w v a l u e s of x s u c h a l a y e r has a b a n d g a p close to t h a t of Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch012

C d S e itself a n d does not b l o c k the t r a n s p o r t of holes to the l i q u i d i n t e r face.

A t Se° concentrations

a b o v e 0 . 0 7 M the n - C d S e | l M N a S - l M S ° 2

l M N a O H | C c e l l is stable to d e t e r i o r a t i o n u p o n passage of 20,000 C / c m

100

r

80

1

-

60 -

FACE FACE

40

11

-

I 100

I 200

l 300

l 400

I 500

I 600

I 700

COULOMBS/Cm

2


Figure 5. Formation of a CdS surface layer on CdSe results in a decrease in the short circuit current in the n-CdSe\NagS-S°-NaOH\C cell The decrease is much faster at higher light intensities where the rate of hole transport to (oxidation of) the redox couple no longer matches the rate at which holes arrive at the interface. The curves show short circuit currents as a function of charge passage for light intensities defined by the zero time current level (11).


2

INTERFACIAL

222

STABILIZATION OF THE n - c d s e / i M Na s-IMS°-IM NaOH/c 2

PHOTOPROCESSES

CELL

0 . 0 7 MSe° Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch012

P . 0 2 M se°

FACE

COULOMBS/cm

2

12,000

16,000 A 50

40

20,000 , L_ 60

EQUIVALENT DAYS IN SUNLIGHT Figure 6. Effect of Se addition on the short-circuit output stability of the n-CdSe\Na S-S°-NaOH\C cell In the presence of 0.07M Se, dis solved as SeS ', the blocking CdS layer is no longer formed and the optput is stable. 2

2

15 10

mW/Cm

SUNLIGHT ,64.6

7.4 m A/cm

2

_ _ _ / 6 . 9 %

2

5 CVJ E

! 0 . 6 V \

0.075 M se°

o 0 — < E 15 1 0 --

SUNLIGHT,64.2

mW/Cm

2

.7.1 %

9.1 IT1A/Cm2

'—"—"^i^

NO s e °

1

1

1

0.1

0.2

0.3

| 0.4

Y

0.5

>v

0.5V

0.7

0.6

0.8

VOLTS The Electrochemical Society, Inc. Figure 7. Current-voltage curves, under 64 mW/cm solar irradiance, for the same n-CdSe crystal run in I M Na S-lM S°-1M NaOH with (top) and without (bottom) 0.075M Se present. Maximum efficiency parame ters are indicated (11). 2

2


12.

HELLER AND MILLER


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223

Cells

( F i g u r e 6 ) , the e q u i v a l e n t of 2 m o n t h s of o p e r a t i o n i n s u n l i g h t T h e c u r r e n t - v o l t a g e characteristics of the 7 %

(II).

efficient Se° s t a b i l i z e d a n d

fresh, u n s t a b i l i z e d cells are s h o w n i n F i g u r e 7

(10).

T h e p r o b l e m of i o n exchange c a n b e a v o i d e d b y c h o o s i n g systems i n w h i c h the anions of

the s e m i c o n d u c t o r

lattice a n d the

solution

are

i d e n t i c a l or systems i n w h i c h the c o m p o u n d r e s u l t i n g f r o m t h e r e a c t i o n of the l a t t i c e c a t i o n a n d the s o l u t i o n a n i o n does n o t g r o w e p i t a x i a l l y o n Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch012

t h e s e m i c o n d u c t o r l a t t i c e . A n e x a m p l e of a c e l l w i t h a c o m m o n i o n , i n w h i c h the s e m i c o n d u c t o r is stable to b o t h p h o t o c o r r o s i o n a n d i o n e x c h a n g e is the n - C u I n S | l M N a S - l M S ° - l M K O H | C c e l l (14). 2

N o deterioration

2

i n c e l l p e r f o r m a n c e is o b s e r v e d f o l l o w i n g passage of 2 X 1 0 C / c m . 4

2

The

b a n d g a p of n - C u I n S is 1.53 e V , close to the o p t i m u m f o r solar e n e r g y 2

c o n v e r s i o n , a n d the efficiency of the c e l l at 7 0 ° C is 6%.

B e c a u s e of p o o r

k i n e t i c s , t h e efficiency of this c e l l decreases at l o w e r t e m p e r a t u r e s

(Fig

ure 8 ) . An

e x a m p l e of a c e l l i n w h i c h the p r o d u c t of t h e l a t t i c e c a t i o n a n d

the s o l u t i o n a n i o n does not g r o w o n the s e m i c o n d u c t o r , is t h e n - G a A s | K S e - K O H | C c e l l . H e r e t h e s e l e n i u m - c o n t a i n i n g l a y e r r e m a i n s less t h a n 2

a f e w m o n o l a y e r s t h i c k after passage of 2 X 1 0

4

C/cm . 2


224

INTERFACIAL

Surface States and Stability

to Adsorbed

PHOTOPROCESSES

Impurities

S e m i c o n d u c t o r - b a s e d devices are i n v a r i a b l y sensitive to i m p u r i t i e s a n d are c r i t i c a l l y sensitive to changes i n t h e c h e m i s t r y at t h e i r j u n c t i o n s . 5

T h u s , at first g l a n c e i t appears d o u b t f u l t h a t t h e p o w e r o u t p u t of a n exposed-junction

s e m i c o n d u c t o r - l i q u i d j u n c t i o n solar c e l l , m a d e

with

c o m m o n 8 5 % b o r o s i l i c a t e glass a n d a s t r o n g base a n d o t h e r c h e m i c a l s Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch012

(reagent g r a d e ) , c o u l d b e stable for a reasonable p e r i o d of t i m e . I n d e e d one does note r a p i d d e t e r i o r a t i o n i n p e r f o r m a n c e of most cells i n t h e presence of c e r t a i n i m p u r i t i e s . F o r e x a m p l e , traces of s e l e n i u m i n s o l u t i o n d a m a g e the o u t p u t of the n - C u I n S 1 N a S - S ° - N a O H | C c e l l a n d the 2

performance

of

the

2

n-GaAs | K S e - K S e - K O H | C 2

affected b y cations s u c h as B i

3 +

2

cell

2

is

adversely

or P d \ 2

W e find, h o w e v e r , that i n c o r p o r a t i o n of some i m p u r i t i e s i n t h e surface has a b e n e f i c i a l effect ( 1 , 2 , 8 ) . I n c o r p o r a t i o n of r u t h e n i u m i n t h e surface of n - G a A s p h o t o a n o d e s p r o d u c e s a h i g h e r o p e n c i r c u i t p o t e n t i a l a n d a h i g h e r fill factor, a n d c o n s i d e r a b l y reduces hysteresis. M a x i m u m effect is g a i n e d b y d i p p i n g the n - G a A s electrode that h a d p r e v i o u s l y b e e n i n t h e selenide electrolyte i n t o a R u ( I I I ) s o l u t i o n . A d d i t i o n of R u ( I I I ) at the 1 0 " - " M l e v e l to the c e l l electrolyte p r o d u c e s a s l o w i m p r o v e m e n t 5

6

for a n u n t r e a t e d electrode b y R u a d s o r p t i o n , t h o u g h n o t to the l e v e l o r p e r m a n e n c e of t h e p r e t r e a t m e n t m e t h o d . W e a t t r i b u t e the i m p r o v e m e n t to a n a l t e r a t i o n of the surface states at the G a A s - s o l u t i o n interface b y strongly c h e m i s o r b e d r u t h e n i u m . T h e presence of surface states at adsorbate-free a n d c h e m i c a l l y m o d i f i e d G a A s surfaces is w e l l r e c o g n i z e d

(see

Ref. 16).

R e c e n t l y , surface states at

G a A s i n n o n a q u e o u s electrolytes h a v e b e e n i n v o k e d to e x p l a i n v o l t a m metric behavior w i t h redox couples

T h e surface of

(17,18).

etched

n - G a A s i n o u r cells has at least three types of c h e m i c a l entities w i t h which

surface

states m a y

be

associated:

hydroxides,

n o n s t o i c h i o m e t r i c c o m p o s i t i o n s of g a l l i u m a n d arsenic.

selenides,

and

I f one of

the

r e s u l t i n g surface states o c c u p i e s a p o s i t i o n ( E ) b e l o w t h e e d g e of t h e 8 8

c o n d u c t i o n b a n d , as s h o w n i n F i g u r e 9, a loss i n p h o t o v o l t a g e or fill factor m a y r e s u l t i f electrons t u n n e l f r o m the c o n d u c t i o n b a n d of the i l l u m i n a t e d photoanode

(E *) c b

to the surface state, p r o v i d e d t h e thickness of t h e

b a r r i e r (A) a l l o w s s u c h t u n n e l i n g . I n t h e absence of s u c h a surface state, the p o s i t i o n of t h e c o n d u c t i o n b a n d i n t h e b u l k of t h e moves from E

o b

to E * cb

and a photovoltage V

p h

semiconductor

results. ( A v o l t a m m e t r i c

c u r v e , f o r a w e l l - b e h a v e d p h o t o a n o d e is s h o w n s c h e m a t i c a l l y i n C u r v e a of F i g u r e 1 0 ) .

I f t u n n e l i n g c a n take p l a c e a n d is v e r y r a p i d , a p e r f e c t

s h u n t is c r e a t e d a n d the p h o t o v o l t a g e w i l l n o t e x c e e d E 9 ) , as electrons r a i s e d a b o v e E

8 8

8 8

— E

c

b

(Figure

w i l l s p i l l i n t o the surface state. S u c h a

s i t u a t i o n is r e f e r r e d to i n S c h o t t k y j u n c t i o n cells as " p i n n i n g " b y surface


12.

HELLER AND MILLER


Solar Cells

225


A E

Eredox

DISTANCE FROM THE

INTERFACE The Electrochemical Society, Inc.

Figure 9. Shunting of a liquid junction solar cell by a surface state near the conduction band, cb, vb, ss, and redox refer to the conduction band, valence band, surface state, and solution redox potential, respectively. The positions of the energy levels in an intensely illuminated cell are shown in dashed lines and are marked by an asterisk. is the highest photovoltage that can be reached in the absence of a surface state (S). states a n d leads to t h e v o l t a m m e t r i c b e h a v i o r s h o w n , s c h e m a t i c a l l y , i n F i g u r e 10, C u r v e b . I n o u r e x p e r i m e n t s , w e f o u n d s u c h b e h a v i o r w i t h n - G a A s p h o t o a n o d e s d i p p e d i n P d ( I I ) solutions, a t r e a t m e n t r e s u l t i n g i n m e t a l l i c P d o n t h e surface. If t h e r a t e at w h i c h electrons transfer t o t h e surface state is n o t sufficiently r a p i d , a p h o t o v o l t a g e V

p

n

of £

c b

*

— E

ch

m a y still be a p

p r o a c h e d , b u t c u r r e n t w i l l b e lost a n d t h e fill f a c t o r w i l l b e r e d u c e d . H y s t e r e s i s w i l l also b e o b s e r v e d i n c y c l i c scans, s c h e m a t i c a l l y r e p r e s e n t e d b y F i g u r e 10, C u r v e c, a n d i n p r a c t i c e , w i t h f r e s h l y e t c h e d b u t o t h e r w i s e u n t r e a t e d n - G a A s p h o t o a n o d e s ( F i g u r e 3, C u r v e s 1 a n d 3 ) . A c h e m i s o r b e d i o n m a y c h a n g e t h e p o s i t i o n o f a surface state e i t h e r b y electrostatic i n t e r a c t i o n w i t h t h e surface species o r b y f o r m i n g a b o n d i n w h i c h electrons a r e s h a r e d .


226

INTERFACIAL


t

PHOTOPROCESSES

A

v The Electrochemical Society, Inc.

Figure 10. Current-voltage curves for idealized cells with (a) no sur face states near the conduction band, (b) perfectly shunting surface states, and (c) imperfectly shunting surface states (S). I f t h e electrostatic i n t e r a c t i o n is w e a k , o r i f the s p l i t t i n g that results f r o m the f o r m a t i o n of t h e n e w b o n d is s l i g h t ( F i g u r e 11) the p h o t o v o l t a g e and

t h e fill f a c t o r m a y d e t e r i o r a t e f u r t h e r . T h e r e a s o n f o r this is t h e

i n t r o d u c t i o n of a n e w surface state closer to e t» t h e p o s i t i o n of c

the

c o n d u c t i o n b a n d i n the d a r k i n t h e b u l k of t h e s e m i c o n d u c t o r , w h i c h is t h e p o t e n t i a l o f the c o u n t e r e l e c t r o d e , n e g l e c t i n g iR effects. T h e t u n n e l i n g t h i c k n e s s (A) w i d e n s for t h e n e w l o w e r state a n d t h u s t h e p h o t o v o l t a g e is r e d u c e d a n d hysteresis is i n c r e a s e d . S i n c e t h e i n t e r a c t i o n is w e a k , t h e a d s o r b e d species m a y b e s l o w l y d e s o r b e d .

T h e s e p h e n o m e n a are i n d e e d

o b s e r v e d w i t h ions s u c h as B i ( I I I ) . B i ( I I I ) l o w e r s the

photovoltage,

increases hysteresis ( F i g u r e 1 2 ) , a n d is s l o w l y d e s o r b e d . I f t h e e l e c t r o s t a t i c i n t e r a c t i o n is s t r o n g , o r i f a s t r o n g c h e m i c a l b o n d is f o r m e d b y s h a r i n g of electrons, s t r o n g c h e m i s o r p t i o n is i m p l i e d .

If

t h e i n t e r a c t i o n is electrostatic, t h e surface state w i l l m o v e s u b s t a n t i a l l y closer to t h e v a l e n c e b a n d . I f electrons are s h a r e d , a s p l i t t i n g results t h a t m a y b e a d e q u a t e to r a i s e t h e n e w u p p e r state to a p o s i t i o n a b o v e t h e e d g e of t h e c o n d u c t i o n b a n d a n d to l o w e r t h e l o w e r state to a p o s i t i o n w h e r e A b e c o m e s excessive f o r t u n n e l i n g to t a k e p l a c e ( F i g u r e 1 3 ) .


If

12.

HELLER

AND MILLER


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227

Cells

s u c h a s i t u a t i o n is a c h i e v e d , m a j o r i m p r o v e m e n t s i n c e l l p e r f o r m a n c e are e x p e c t e d as a c o n s e q u e n c e :

the p h o t o v o l t a g e

a n d fill f a c t o r i n c r e a s e ;

hysteresis decreases; a n d t h e s e n s i t i v i t y t o w e a k l y i n t e r a c t i n g i m p u r i t i e s , w h i c h cause p e r f o r m a n c e d e t e r i o r a t i o n , also decreases.

A l l of the above

are i n d e e d o b s e r v e d w h e n R u ( I I I ) i s c h e m i s o r b e d o n G a A s : t h e p h o t o v o l t a g e a n d fill f a c t o r increase; hysteresis decreases

(Figure 3 Curves

2 a n d 4, a n d F i g u r e 1 2 ) ; t h e i m p r o v e m e n t persists; a n d exposure to B i ( I I I ) Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch012

no l o n g e r causes t h e d e t e r i o r a t i o n o f p e r f o r m a n c e as seen i n F i g u r e 12. In summary, ruthenium incorporation i n n - G a A s improves cell per f o r m a n c e b y s t a b i l i z i n g a n e w surface c o m p o s i t i o n that p r o d u c e s a shift of surface state energies so as t o defeat t h e p o w e r loss m e c h a n i s m s . E s s e n t i a l l y , the G a A s surface is m a d e to c l o s e l y a p p r o a c h t h e i d e a l o f n o i n t e r f e r i n g surface states i n the b a n d g a p b y d e l i b e r a t e c h e m i c a l m o d i f i i

E

r

cb

^>E(SS-ION) ^^(SS-ION)

E

redox

Evb

Evb

DISTANCE FROM THE INTERFACE The Electrochemical Society, Inc.

Figure I I . Weak interaction of a surface entity with original state into two new states with energies E' tunneling from the conduction band to E is possible, reduced or the photovoltage is limited to E — E than E — E by about 1 \2 of the splitting (88tion)

88

(88tion)

88

c &

an ion splits the and E . If the fill factor is , a value lower (8). (88fion)

c 6



228

INTERFACIAL

PHOTOPROCESSES

.8

CELL VOLTAGE Applied Physics Letters

Figure 12. Normalized current-voltage curves for the n-GaAs\0.8M K Se-0.1M K Se -lM KOH\C cell after (a) etching, (b) etching, dip in the redox couple solution, followed by dip in 0.01 M Bi(lII)-0.1M HNO , and (c) etching, dip in the redox couple solution, followed by dip in 0.01 M Ru(IU)-0.1MHN0 (2). 2

2

2

s

3

c a t i o n . F i g u r e 14 shows the a c t u a l c u r r e n t - v o l t a g e c u r v e for t h e n - G a A s | 0 . 8 M K S e - 0 . 1 M K S e - l M K O H | C c e l l w i t h the r u t h e n i u m - t r e a t e d p h o 2

2

2

toanode ( 1 , 2 ) .

T h e c e l l c o n v e r t e d s u n l i g h t to e l e c t r i c a l p o w e r w i t h a

12%

a n d m a i n t a i n e d this p e r f o r m a n c e

efficiency

f o r several

months.

U l t i m a t e c e l l f a i l u r e w a s c a u s e d b y l e a k a g e of t h e p l a s t i c e n c a p s u l a n t of the photoanode. T h e 1 2 % e x t e r n a l efficiency w a s r e a l i z e d w h i l e t h e o v e r a l l q u a n t u m (current)

efficiency w a s o n l y 6 5 - 7 0 % .

T h e loss i n q u a n t u m

efficiency

is m a i n l y traceable to a b s o r p t i o n b y t h e electrolyte a n d r e s i d u a l r e f l e c t i o n at t h e a i r - s y s t e m a n d s e m i c o n d u c t o r - l i q u i d

interfaces.

If a l l incident


12.

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AND MILLER


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229

Cells

p h o t o n s w i t h energies e x c e e d i n g t h e b a n d g a p w e r e a c t u a l l y 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 , t h e solar t o e l e c t r i c a l c o n v e r s i o n efficiency w o u l d have exceeded 1 7 % .

W h i l e l o w e r t h a n the efficiency o f s e v e r a l s o l i d -

s o l i d j u n c t i o n solar cells the 1 2 % e x t e r n a l c o n v e r s i o n efficiency r e a l i z e d is t h e h i g h e s t r e p o r t e d f o r p h o t o e l e c t r o c h e m i c a l o r p h o t o c h e m i c a l systems, i n c l u d i n g p h o t o b i o l o g i c a l systems. A l t h o u g h the p o s s i b l e c o m b i n a t i o n s of s e m i c o n d u c t o r s

and

redox


couples seem v e r y l a r g e , the v a r i o u s r e q u i r e m e n t s f o r o u t p u t a n d s t a b i l i t y severely r e d u c e t h e list of c a n d i d a t e s .

T h e r e f o r e , close s c r u t i n y of t h e

latter to o b t a i n the m a x i m u m s t a b i l i t y a n d efficiency a l o n g t h e l i n e s of the experience

reported above w i l l be important i n the future.

We

a n t i c i p a t e t h a t a v e r y close u n d e r s t a n d i n g of t h e i n t e r f a c i a l c h e m i s t r y w i l l b e r e q u i r e d for a c h i e v e m e n t of p r a c t i c a l v i a b i l i t y .

DISTANCE FROM THE

INTERFACE The Electrochemical Society, Inc.

Figure 13. Strong interaction of a surface entity with an ion splits the original state to E' and E . Electrons cannot tunnel to E' because the barrier is too thick. E' is above the conduc tion band edge and cannot capture electrons. The split surface state no longer reduces the photovoltage or the fill factor (8). (88tion)

(88tion)

(88tion)

(88tion)



230

INTERFACIAL

CELL VOLTAGE

PHOTOPROCESSES

(mV)

Figure 14. Current density^ooltage curve under 95 mW/cm sunlight for the cell n-GaAs\0.8M K Se-0.1M KoSe -lM KOH\C with matte-etched, Ru(III) treated n-GaAs(100) face ana 6 X 10 cm' carrier concentration. 2

2

g

16

9


12.

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AND MILLER


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Cells

231

Acknowledgment T h e authors e n j o y e d t h e i r c o l l a b o r a t i o n o n t h e subject o f this a r t i c l e w i t h their colleagues, K l a u s J . B a c h m a n n , K u a n g - c h o u C h a n g ( n o w at Polaroid), Shalini Menezes, Bruce A . Parkinson, M u r r a y Robbins, Bertram Schwartz, G a r y P . Schwartz, a n d Richard G . Vadimsky.


Literature

Cited

1. Parkinson, B.; Heller, A.; Miller, B. Conf. Rec. IEEE Photovoltaic Spec. Conf. 1978, 13, 1253-1254. 2. Parkinson, B. ;Heller, A.; Miller, B. Appl. Phys. Lett. 1978, 33, 521. 3. Hovel, H. J. "Semiconductors and Semimetals Volume II: Solar Cells"; Academic: New York, 1977; pp. 226-227. 4. Gerischer, H. J. Electroanal Chem. 1975, 58, 236. 5. Reiss, H. J. Electrochem. Soc. 1978, 125, 937. 6. Chang, K. C.; Heller, A.; Schwartz, B.; Menenzes, S.; Miller, B. Science 1977, 196, 1097. 7. Harvey, W. W. J. Electrochem. Soc. 1967, 114, 472. 8. Parkinson, B. A.; Heller, A.; Miller, B. J. Electrochem. Soc. 1979, 126, 954. 9. Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. J. Am. Chem. Soc. 1976, 98, 6855. 10. Hodes, G.; Manassen, J.; Cahen, D. Nature 1976, 261, 403. 11. Heller, A.; Schwartz, G. P.; Vadimsky, R. G.; Menezes, S.; Miller, B. J. Electrochem. Soc. 1978, 125, 1156. 12. Cahen, D.; Hodes, G.; Manassen, J. J. Electrochem. Soc. 1978, 125, 1623. 13. Gerischer, H.; Gobrecht, J. Ber. Bunsenges. Phys. Chem. 1978, 82, 520. 14. Noufi, R. N.; Kohl, P. A.; Rogers, J. W., Jr.; White, John M.; Bard, A. J. "Extended Abtracts," Spring Meeting of the Electrochemical Society, Seattle, Washington, May 1978; Abstract No. 417. 15. Robbins, M.; Bachmann, K. J.; Lambrecht, V. G.; Thiel, F. A.; Thomson, J., Jr.; Vadimsky, R. G.; Menezes, S.; Heller, A.; Miller, B.J.Electro chem. Soc. 1978, 125, 831. 16. Morrison, S. R. "The Chemical Physics of Surfaces"; Plenum: New York, 1977; Chapter 8. 17. Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1975, 97, 7429. 18. Bard, A. J.; Kohl, P. A. "Semiconductor Liquid Junction Solar Cells"; Heller, A., Ed.; The Electrochemical Society, Inc.: Princeton, NJ, 1977; pp. 222-230. RECEIVED October 25, 1978.


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