Stability of Cadmium-Chalcogenide-Based Photoelectrochemical Cells

24 Stability of Cadmium-Chalcogenide-Based Photoelectrochemical Cells DAVID CAHEN, G A R Y HODES, JOOST MANASSEN, and RESHEF T E N N E Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch024

The Weizmann Institute of Science, Rehovot, Israel

Cd-chalcogenides (CdS, CdSe, CdTe) a r e among the most s t u d i e d m a t e r i a l s as photoelectrodes i n a photoelectrochemical c e l l (PEC) (jL>.2>3>40 . I n t e r e s t i n such PEC's stems from the f a c t that, i n aqueous p o l y s u l f i d e or p o l y s e l e n i d e s o l u t i o n s , a d r a s t i c decrease i n photocorrosion i s observed, as compared to other aqueous s o l u t i o n s , while reasonable conversion e f f i c i e n c i e s can be a t t a i n e d . An important c o n s i d e r a t i o n , from the p r a c t i c a l p o i n t of view, i s that t h i n f i l m p o l y c r y s t a l l i n e photoelectrodes can be prepared, by various methods, with conversion e f f i c i e n c i e s of more than h a l f of those obtained with s i n g l e c r y s t a l e l e c t r o d e s and with b e t t e r s t a b i l i t y c h a r a c t e r i s t i c s than those obtained with s i n g l e c r y s t a l based PEC s U,4,5). Recently we showed that PEC's using a l l o y s of Cd(Se,Te) as photoelectrodes can be prepared with s t a b i l i t y c h a r a c t e r i s t i c s much b e t t e r than those obtained f o r CdTe based c e l l s and with s i m i l a r conversion e f f i c i e n c e s (6>7) . Those e f f i c i e n c i e s could be improved c o n s i d e r a b l y by s e v e r a l chemical and p h o t o e l e c t r o chemical s u r f a c e treatments. Here we w i l l consider mainly the s h o r t - c i r c u i t current (SCC) output s t a b i l i t y c h a r a c t e r i s t i c s of both small g r a i n , t h i n f i l m CdSe and Cd(Se,Te)-based PEC s as w e l l as those of s i n g l e c r y s t a l CdSe-based c e l l s , where d i f f e r e n t c r y s t a l faces a r e exposed to the s o l u t i o n , a f t e r they have undergone any of a s e r i e s of s u r f a c e treatments. These s t u d i e s show a strong dependence o f the output s t a b i l i t y on s o l u t i o n composition, on r e a l e l e c t r o d e s u r f a c e area, on s u r f a c e treatment, on c r y s t a l face and on c r y s t a l s t r u c t u r e (for the Cd(Se,Te) a l l o y s ) (1,2,7). We suggest a predominantly k i n e t i c explanation f o r these phenomena, i n v o l v i n g the f i n e balance between the r a t e of the photocorrosion r e a c t i o n on the one hand and that of the regenerat i v e redox r e a c t i o n s on the other hand. Because we a r e i n t e r e s t e d , u l t i m a t e l y , i n p r a c t i c a l d e v i c e s , our aim i s to t r y to a t t a i n long term s t a b i l i t y (months-years). 1

?

0097-6156/81/0146-0369$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

370

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Therefore u n s a t i s f a c t o r y s t a b i l i t y by our d e f i n i t i o n does not n e c e s s a r i l y r e f l e c t short term behaviour (minutes-hours). Experimental S i n g l e c r y s t a l s of CdSe were obtained from Cleveland C r y s t a l s Inc., as w e l l as from W. G i r i a t (iVIC-Caracas), and were n-type with t y p i c a l r e s i s t i v i t i e s of 3-15 Ω-cm (VLOm~3 donor concen t r a t i o n ) . C r y s t a l o r i e n t a t i o n was checked by X-ray d i f f r a c t i o n . E l e c t r o d e s were prepared from them as described elsewhere ( 1 ) . P o l y c r y s t a l l i n e CdSe and Cd(Se,Te) e l e c t r o d e s were prepared by p a i n t i n g a s l u r r y of the powder on a T i s u b s t r a t e and subsequent annealing ( 5 ) . The Cd(Se,Te) powders were prepared from CdSe and CdTe by c o s i n t e r i n g (7). S o l u t i o n p r e p a r a t i o n has been described before ( 1 ) . Except f o r the outdoors t e s t s , which used 2-electrode c e l l s with CoS counterelectrode (8), a l l s t a b i l i t y t e s t s were done under p o t e n t i o s t a t i c c o n t r o l at SCC, using Pt counter and r e f e rence e l e c t r o d e s . A s t a b i l i z e d q u a r t z - l 2 lamp provided i l l u m i n a t i o n . Constant temperature was maintained using a thermostatted water bath surrounding the PEC.

Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch024

C

E t c h i n g . The p o l y c r y s t a l l i n e e l e c t r o d e s were etched, before use i n p o l y s u l f i d e s o l u t i o n , i n 3% HNO3 i n cone. HC1 f o r sees. Single c r y s t a l CdSe e l e c t r o d e s were p o l i s h e d , before use, down to O. 3., with alumina p o l i s h . T h e i r v a r i o u s etching treatments were as follows : 1. Chromic a c i d etch only; 10 sees, i n Cr03:HCl:H 0 (6:10:4 w/w). 2. Aqua r e g i a etch; 20 sees, i n aqua r e g i a . 3. Aqua regia/chromic a c i d etch; VL0 sees, i n s o l u t i o n of "1", a f t e r normal aqua r e g i a e t c h . 4. Photoetch; 5 sees, i n HN0 :HC1:H 0 (O.3:9.7:90 ν / ν ) , under AML i l l u m i n a t i o n , as photoelectrode i n PEC with carbon countere l e c t r o d e , a f t e r aqua r e g i a etch (Tenne and Hodes, Appl. Phys. Lett., i n press). 2

3

2

Results During the l a s t few years we have c a r r i e d out s e v e r a l out doors t e s t of PEC's c o n t a i n i n g t h i n f i l m p o l y c r y s t a l l i n e CdSe pho t o e l e c t r o d e s and CoS c o u n t e r e l e c t r o d e s . Figure 1 shows the r e s u l t s of one such t e s t c a r r i e d out during 1979, with an initially high e f f i c i e n c y c e l l . I t i s c l e a r from the f i g u r e that the drop i n e f f i c i e n c y a f t e r 2 months i s due mainly to parameters a f f e c t i n g the SCC. More recent t e s t s , using s e v e r a l of the improvements that can be obtained from the r e s u l t s discussed below, showed CdSe c e l l s of a s i m i l a r type to be s t a b l e w i t h i n 10% f o r 6 months. The s o l u t i o n used i n the c e l l of f i g u r e 1 was chosen i n Dec. 1978 and i t s most important c h a r a c t e r i s t i c i s the [ S ] / [ S ] (added q u a n t i t i e s ) of O.5. Figure 2 shows the considerable importance of t h i s r a t i o on the output s t a b i l i t y of CdSe-based P E C s . We see =

f

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


24.

CAHEN

ET

Cd-Chalcogenide Photoelectrochemical Cells

AL.

371

2

Figure 1. Results of an 8-month outdoors test of PEC containing aO.8-cm thin film, painted CdSe photoelectrode (not photoetched), CoS counter electrode, and 1M KOH, 2M S-, 1M S, ImM Se solution. (OCV) open-circuit voltage; (SCC) short-circuit current; (EFF) solar conversion efficiency (~ A M 1.5). Between mea surements the cell operated on maximum power (68 Ω load). No appreciable change infill-factoroccurred during the test.

Figure 2. Time dependence of short-circuit current of polycrystalline painted CdSe photoelectrodes as a function of solution composition. All solutions contained O.8M KOH and O.8M S . S/S ratio indicated next to plots. Dashed line shows behavior in 3.2M S solution at 50°C All other experiments done at 35°C., potentiostatically. Light intensities adjusted to obtain identical initial current densities. =

=


372

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES


=

that the [ S ] / [ S ] of f i g u r e 1 i s one of the l e a s t favourable ones and that a r a t i o of 2 would y i e l d much b e t t e r r e s u l t s . I f we compare the output s t a b i l i t y of p o l y c r y s t a l l i n e and s i n g l e c r y s t a l CdSe c e l l s we f i n d the p o l y c r y s t a l l i n e CdSe c e l l s to be much more s t a b l e ( f i g u r e 3 ) . Figure 3B furthermore s t r e s s e s the importance of the i n i t i a l current d e n s i t y i n determining the output s t a b i l i t y of such c e l l s . (Note that the i n i t i a l current d e n s i t i e s of f i g u r e s 2 and 3 correspond to an i l l u m i n a t i o n i n t e n s i t y equivalent to 3.5 -5xAKL s o l a r i n t e n s i t y , and as such these experiments are a c c e l e rated l i f e - t i m e t e s t s . Such t e s t s may be much more severe than the f a c t o r s 3.5-5 would i m p l y ) . For comparison, f i g u r e 3B shows r e s u l t s not only f o r the r e l a t i v e l y most s t a b l e face of a s i n g l e c r y s t a l (2) and a p o l y c r y s t a l l i n e t h i n l a y e r prepared by p a s t i n g (5), but a l s o f o r a pressed p e l l e t of CdSe (2). This l a s t e l e c t r o d e shows s t a b i l i t y behaviour intermediate between that of the two other e l e c t r o d e s . This behav i o u r i s evident a l s o i n the decrease i n output s t a b i l i t y , which i s l e s s steep than that of the s i n g l e c r y s t a l e l e c t r o d e . In f i g u r e 4 we show how the output s t a b i l i t y of a c e l l using a s i n g l e c r y s t a l e l e c t r o d e with a s p e c i f i c c r y s t a l face exposed to the s o l u t i o n can vary depending on the surface treatment given to the e l e c t r o d e , before use. For comparison we i n c l u d e the s t a b i l i t y behaviour of the, l e a s t unstable, (1120) face, which was given the most favourable surface treatment (photoetch). Figure 5 provides a c l o s e look at the faces a f t e r the v a r i o u s s u r f a c e treatments. (The completely f e a t u r e l e s s chromic a c i d etched-face i s not shown). The p r o g r e s s i v e l y i n c r e a s i n g s u r f a c e area (from A-C) i s e v i d e n t . The nature of the small C O . l y ) holes obtained a f t e r purposely photocorroding the e l e c t r o d e (termed "photoetching") i s p r e s e n t l y under i n v e s t i g a t i o n . Although f i g u r e s 2-5 i n d i c a t e how we may improve the output s t a b i l i t y of CdSe-based photoelectrochemical c e l l s (and, i n some cases t h e i r conversion e f f i c i e n c y , though never much beyond 4% f o r t h i n - f i l m p o l y c r y s t a l l i n e based ones), a p r a c t i c a l device should have a considerably higher s o l a r conversion e f f i c i e n c y together with an output s t a b i l i t y at l e a s t equal to that obtained f o r the best CdSe-based c e l l s . CdTe with a bandgap of 1.45eV (at RT) as compared to 1.7eV f o r CdSe would be a l o g i c a l choice f o r a t t a i n i n g t h i s goal but, u n f o r t u n a t e l y , c e l l s using n-CdTe are not s t a b l e i n p o l y s u l f i d e s o l u t i o n s under " s o l a r illuminâtion" c o n d i t i o n s . (Such c e l l s are claimed to be s t a b l e i n p o l y s e l e n i d e and p o l y t e l l u r i d e s o l u t i o n s ( 9 ) ) . Also a d d i t i o n of tens of m i l l i m o l a r q u a n t i t i e s of Se to p o l y s u l f i d e s o l u t i o n s was shown to improve the output s t a b i l i t y of CdSe c e l l s (2). Here we are, f o r p r a c t i c a l reasons, i n t e r e s t e d i n s o l u t i o n s c o n t a i n i n g no, or only minimal q u a n t i t i e s of Se and Te because of the much lower t o x i c i t y and lower a i r s e n s i t i v i t y of pure p o l y s u l f i d e s o l u t i o n s ) . Figure 6 shows, however, that a l l o y s of CdSe T e can have o p t i c a l bandgaps comparable to that of CdTe, even i f x>O.5 (10). CdSe and CdTe form homogeneous a l l o y s over the whole composition x

x



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373

Figure 3. (A) Dependence of short-circuit current on total charge passed for painted polycrystalline ( ) and single crystal ( , (1120) face) CdSe photoelectrodes. Solution contained 1M each KOH, S , and S. (B) Same as 3A but at higher initial current density. Behavior of pressed pellet CdSe electrode (2) included as well. Other conditions as for 3A (but 5mM Se added to the solution) and for Figure 2. The behavior of the single crystal from 3A is shown, too, for comparison. (Data obtained at Bell Labs.). =



374

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Figure 4. Logarithmic plot of time dependence of short-circuit current for singlecrystal CdSe photoelectrode, (1010) face exposed to solution: (. . -) after chromic acid etch; (-.--) after aqua regia/chromic acid etch; ( ) after aqua regia etch; ( ) after aqua regia/photoetch; (A) (1010); (B) (1120). Further conditions as in Figure 3A.


CAHEN

ET

AL.


375


24.

Figure 5. Scanning electron microscope pictures of single-crystal CdSe after several surface treatments. (0001) face, Cd-side, shown. (1120) and (1010) faces behave similarly. (A) After aqua regia/chromic acid etch; (B) after aqua regia etch; (C) after aqua regia/photoetch.



376

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

1 0 - 3 0 At. %

50

INTERFACES

70

Se (χ in C d S e T e , ) x

x

Figure 6. Direct optical bandgap of Cd(Se,Te) as a function of Se content and crystal structure. Data for well-annealed samples, at room temperature. (After Ref. 10) (1).

Ο X

>

(nm)

500

(eV) WAVELENGTH (λ) Nature Figure 7. Spectral response of polycrystalline, thin-film, painted CdSe .esTe .35 photoelectrode in 1M KOH, S ,S solution. Corrected for photon density wavelength dependence (6). 0

=


0

24.

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ET AL.

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range (as do CdS and CdSe (10,11) and CdS and CdTe (10)). The a l l o y can have e i t h e r the hexagonal, w u r t z i t e , s t r u c t u r e or the cubic, s p h a l e r i t e , one, depending on i t s composition and the con d i t i o n s o f preparation (10,12). Table I shows that the l a t t i c e parameters of Cd(Se,Te) a l l o y s depend on the a l l o y stoichiometry i n a rather l i n e a r fashion, i . e . Vegard s law i s obeyed (although d e v i a t i o n s of up to one percent are observed). f

Table I Cd S e

x

Te . 1-


ο

ο

ο

X

Hex(%)

a (A)

c(A)

O.00

100

4.31

7.02

0

.70

.30

100

4.38

7.165

0

.65

.35

>95

4.40

7.18

90

4.42

7.20

Stability of Cadmium-Chalcogenide-Based Photoelectrochemical Cells

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