Improvement of cuprous sulphide stoichiometry by electrochemical and chemical methods

Solar Cells, 22 (1987) 187 - 194

187

IMPROVEMENT OF CUPROUS SULPHIDE STOICHIOMETRY BY ELECTROCHEMICAL AND CHEMICAL METHODS MOHAMED DACHRAOUI Laboratoire de Chimie Analytique, Ddpartement de Chirnie, Facultd des Sciences de Tunis, Campus Universitaire, Tunis (Tunisia)

JACQUES VEDEL Laboratoire d'Electrochirnie Analytique et Appliqude, Unitd Associde au CNRS, Ecole Nationale Supdrieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75005 Paris (France)

(Received January 23, 1987; accepted in revised form May 21, 1987)

Summary Two processes are proposed for improving the stoichiometric ratio of cuprous sulphide f o r m e d on either CdS or CdyZnl _yS. The first consists in cathodically depositing copper just after CuxS formation, a thermal treatm e n t following these t w o steps. The second process consists in first thermally treating the j unc t i on and t hen equilibrating the CuxS composition b y means o f the reaction CuxS + 2(2 -- x)Cu I ~ Cu2 + (2 x)Cu n. The first process is well suited to CdS substrates, for which the results are equivalent to those obtained with classical techniques (e.g. vacuum evaporation of copper). Th e second process improves the properties of CdyZnl_yS/Cu2S cells but the results are no better than those obtained using pure CdS. -

-

1. I n t r o d u c t i o n Th e non-stoichiometry of cuprous sulphide is a cause of low efficiency in CuxS/CdS solar cells [1], and addition o f metallic copper has been proposed in order to improve the CuxS composition [2]. Two m et hods were proposed for the copper addition: vacuum deposition [2] and the action of a h y d r o g e n plasma which reduces copper(II) t o copper(I) and leads t o metallic copper b y partial reduction of the cuprous sulphide [3]. The copper addition is followed by a thermal t r e a t m e n t in air to achieve the equilibration reaction CuxS + (2 - - x ) C u

' * Cu2S

(1)

Simultaneously, the excess copper is oxidized to cuprous oxide, decreasing the surface r e c o m b i n a t i o n rate. 0379/6787/87/$3.50

© Elsevier Sequoia/Printed in The Netherlands

188 V a c u u m e v a p o r a t i o n and plasma r e d u c t i o n need a special technological step in the overall f a b r i c a t i o n process. R e c e n t l y , it has been s h o w n t h a t c o p p e r a d d i t i o n can be p e r f o r m e d in an ion exchange solution, just at the end o f the ion exchange process, simply b y partial c a t h o d i c r e d u c t i o n o f the soluble c u p r o u s salt [4]: CuCl32- + e

~ Cu + 3C1

(2)

S o m e results o b t a i n e d using this t e c h n i q u e with CdS and Cd~,Znl_yS substrates are p r e s e n t e d in this paper. G o o d results were o b t a i n e d with CdS b u t with C d y Z n l _ y S a d e g r a d a t i o n o f the cell p r o p e r t i e s was observed. A n o t h e r m e t h o d for c o p p e r e n r i c h m e n t was t h e r e f o r e investigated using a second ion e x c h a n g e step a f t e r an initial t h e r m a l t r e a t m e n t n e e d e d t o f o r m the j u n c t i o n . T h e c o p p e r e n r i c h m e n t r e a c t i o n is CuxS + 2(2 - - x ) C u I

> Cu2S + (2 - - x ) C u n

(3)

leading to an i m p r o v e m e n t in the C d l _ y ZnyS cell properties.

2. E x p e r i m e n t a l details T h e substrates were glass sheets, successively covered, using t h e airless spray t e c h n i q u e , b y a tin o x i d e layer ( 0 . 4 / a m ) , a CdS (or CdZnS) + alumina layer (4 p m ) and finally a CdS (or CdZnS) layer (5 pm). Airless spraying is a t e c h n i q u e in which a liquid is sprayed b y c o m p r e s s i o n in a nozzle, w i t h o u t t h e use o f a gas flow [5]. T h e zinc sulphide p r o p o r t i o n y was varied b e t w e e n 0 and 0.35. S o m e o f t h e p r o p e r t i e s o f t h e films have been given elsewhere [6]. L a y e r properties useful in this w o r k and related to an increase in y are (i) a loss o f the preferential o r i e n t a t i o n observed for pure CdS and (ii) a decrease in the crystallite size associated with a m o r e divided state o f t h e surface. T h e c o m p o s i t i o n o f the ion exchange solution was as follows: NaC1, 200 g 1-1; purified CuC1, 4 g 1-1; s o d i u m t a r t r a t e , 25 g 1-1; h y d r a z i n e c h l o r h y d r a t e , 5 g 1-1. T h e pH was adjusted to 3.7. T h e t e m p e r a t u r e was 90 °C and t h e solution was k e p t u n d e r a c o n t i n u o u s flux o f argon. T h e c a t h o d i c c o p p e r e n r i c h m e n t was p e r f o r m e d b y imposing a c o n s t a n t c u r r e n t b e t w e e n the sample ( c a t h o d e ) and a c o p p e r a n o d e . This required an electrical c o n t a c t to be m a d e o n t h e SnO2 e l e c t r o d e . T h e c u r r e n t density was o p t i m i z e d t o 5 m A cm -2 and the time o f application to 10 s. T h e r m a l t r e a t m e n t s were p e r f o r m e d in a c o n v e n t i o n a l t h e r m o r e g u l a t e d oven u n d e r either an air or a h y d r o g e n a t m o s p h e r e . T h e s t o i c h i o m e t r i c ratio o f c u p r o u s sulphide was d e t e r m i n e d b y solid state c o u l o m e t r y [7] which was also used t o show the presence o f c u p r o u s o x i d e in t h e layer [8]. C u r r e n t - v o l t a g e (I-V) characteristics were o b t a i n e d in a backwall c o n f i g u r a t i o n , with t h e c u r r e n t collected o n the CuxS b y the use o f a piece

189

of graphite felt pressed against the cell. The light source was a halogen lamp standardized to 100 mW cm -2 using a silicon reference cell.

3. Results and discussion In Fig. 1, the I - V characteristics of CdS/CuxS photocells are shown at various steps in their elaboration in a single batch of airless-sprayed CdS: the curve a was obtained just after f or m a t ion by ion exchange at 95 °C, for 7 s; the curve b, after a thermal t r e a t m e n t of the previous cell, in air, at 200 °C for 10 min. The curve c was obtained after form at i on of CuxS (same conditions as above) but followed by a copper addition p e r f o r m e d just at the end o f the ion exchange. The current density was 5 mA cm -2 and the time was adjusted to obtain an equivalent copper thickness of 300 )k (it was observed that the application of the constant current stops the formation reaction, the increase in thickness of the CuxS becoming negligible). Finally, the curve d was obtained after a thermal t r e a t m e n t (air, 200 °C, 10 min) o f the previous cell. In b o th cases (natural and copper-enriched CuxS) the thermal treatm e n t causes an increase in the fill factor but, in the second case, there is also a sharp im pr ove m ent in the Isc and Voc values. This is in agreement with the results given in ref. 4, for which the CdS substrates were obtained by conventional spraying (Table 1). '1

] ,:;"' i -f "

I

/

i

,

... i / ,

~' ~-s

h

Z

~-Io

:

. . . . . . . . . . . ;

-15

........ . . - "

C •.........

.,"

,,"

•"

.,,

-.'--

.......

IT'

;

/

/ .,'

t

:

,

~

I

t

e

s

,'

:

i

: .* : •

,"

-

],

a

i

l

-20 "................................................ " d

- 0.5

i 0

! 0.5

POTENTIAL / V 1. T y p i c a l ] - V c h a r a c t e r i s t i c s o f CdS/CuxS p h o t o c e l l s m a d e o n C d S s u b s t r a t e s : (a) just after formation; (b) after thermal treatment, without cathodic copper addition; (c) after formation and cathodic copper addition; (d) after formation, cathodic copper addition and thermal treatment (air, 200 °C, 10 rain). Fig.

190 TABLE 1 Comparison of the characteristic parameters of treated and non-treated CdS photocells: (a) cathodic charging; (b) vacuum charging Voc (V)

Isc (mA cm-2)

FF

Before thermal treatment

0.410

17

0.48

3.0

this work

After thermal treatment

0.450 0.420 0.430

20.4 22 20

0.66 0.70

6 6.5 5.5

this work(a) ref. 4(a) ref. 4(b)

Efficiency

(%)

0,45

o

C~ ~

o. 0,35 ~ ' , Cl 0.4 ~

bI I-

0.2

,

"l

cl

~. 0,25

~%". b2 °2 "-:'-. 0.5

"-.

I

- : - ....

L

"-

.............. L:

b ':I 1.0

1.5

TIME / mn

Fig. 2. Solid state coulometric analysis of treated and non-treated cuprous sulphide made on CdS substrates: (a) after formation; (b) after formation, copper addition and thermal treatment (200 °C, air) for 10 min; (c) like (b) but after a thermal treatment of 30 min.

The observed i m p r o v e m e n t is related to the decrease in t h e deviation f r o m s t o i c h i o m e t r y , as d e m o n s t r a t e d b y the solid state c o u l o m e t r i c analysis curves (Fig. 2). In this analytical t e c h n i q u e , t h e c u p r o u s sulphide f o r m e d on the p h o t o c e l l substrate is s u b m i t t e d t o a c o n s t a n t c a t h o d i c c u r r e n t (2 m A cm -2) and the variation in its p o t e n t i a l is r e c o r d e d as a f u n c t i o n o f time, t h e electrical c o n t a c t being m a d e o n the back SnO2 electrode. F o r a n e w l y f o r m e d layer {Fig. 2, curve a), there is a sharp decrease in the p o t e n t i a l a few s e c o n d s after the beginning o f the c u r r e n t application, c o r r e s p o n d i n g to t h e r e d u c t i o n o f CuxS t o Cu2S [ 7 ] . T h e elapsed time t I associated with the time n e e d e d f o r a t o t a l r e d u c t i o n o f t h e c u p r o u s sulphide ( n o t s h o w n on the figure) gives a deviation f r o m s t o i c h i o m e t r y equal to 0.037 (Cul.963S). T h e value o f t h e p o t e n t i a l characterizing this r e a c t i o n is b e t w e e n 0.3 and 0.45 V vs. Cu2S. T h e curve c in Fig. 2 was o b t a i n e d b y analysing a copperenriched sample annealed in air for a longer time t h a n for o p t i m i z e d cells

191 (30 m i n instead o f 10 min). T w o waves, c 1 a n d c2, a p p e a r o n t h e t i t r a t i o n curve. T h e w a v e c 2 is a t t r i b u t e d t o t h e r e d u c t i o n o f c u p r o u s o x i d e to c o p p e r a n d t h e w a v e c 1 to t h e r e d u c t i o n o f t h e C u O + CuS m i x t u r e f o r m e d w h e n c u p r o u s sulphide is o x i d i z e d in air Cu2S + ~1O 2

~ C u O + CuS

(4)

This is s h o w n b y solid state c o u l o m e t r i c analysis o f t h e layers a f t e r r e a c t i o n w i t h an a q u e o u s s o l u t i o n o f s o d i u m sulphide: t h e c u p r o u s o x i d e is t r a n s f o r m e d into c u p r o u s sulphide and t h e w a v e d i s a p p e a r s f r o m t h e t i t r a t i o n curve, a n d t h e m i x t u r e C u O + CuS is t r a n s f o r m e d into CuxS leading to a wave similar t o t h a t o f c u r v e a. F o r layers s u b m i t t e d t o t h e o p t i m i z e d t h e r m a l t r e a t m e n t (curve b) t h e r e d u c t i o n o f c u p r o u s o x i d e is a l w a y s o b s e r v e d ( w a v e b2), a n d is p r e c e d e d b y a v e r y n a r r o w w a v e b 1. This w a v e m a y be a t t r i b u t e d either t o t h e r e d u c t i o n o f a residual a m o u n t o f n o n - n e u t r a l i z e d CuxS or to t h e r e d u c t i o n o f a small q u a n t i t y o f CuO + CuS f o r m e d d u r i n g t h e b e g i n n i n g o f t h e c u p r o u s sulphide o x i d a t i o n . T h e value o f t h e c o r r e s p o n d i n g d e v i a t i o n f r o m s t o i c h i o m e t r y (0.09) being at t h e sensibility limit o f t h e a n a l y t i c a l m e t h o d , it is d i f f i c u l t t o d e c i d e to w h i c h species t h e w a v e is d u e . H o w e v e r , b o t h e x p l a n a t i o n s are c o n s i s t e n t with the fact that the added copper neutralizes the non-stoichiometric c u p r o u s sulphide and its excess is t r a n s f o r m e d into c u p r o u s oxide. T h u s , in t h e case o f CdS s u b s t r a t e s , t h e r e is a c o m p e t i t i o n b e t w e e n r e a c t i o n (1) a n d r e a c t i o n (4), t h e e q u i l i b r a t i o n r e a c t i o n being f a s t e r t h a n the degradation reaction. An a t t e m p t was m a d e to i m p r o v e t h e p r o p e r t i e s o f C d y Z n l _ y S / C u x S solar cells b y t h e use o f this t e c h n i q u e . In all cases (various values o f y , various conditions of copper enrichment and/or thermal treatment) a d e g r a d a t i o n o f t h e solar cell c h a r a c t e r i s t i c s was o b s e r v e d . S o m e t y p i c a l results are given in T a b l e 2. T h e s e r e s u l t s m a y b e i n t e r p r e t e d b y c o n s i d e r i n g t h e solid state c o u l o m e t r i c analysis curves o f Fig. 3. T h e c u r v e a was o b t a i n e d w i t h f r e s h l y p r e p a r e d c u p r o u s sulphide. I t s h o w s t h a t t h e initial d e v i a t i o n f r o m stoic h i o m e t r y is 0 . 0 1 9 w h i c h is smaller t h a n t h a t o b s e r v e d using a p u r e CdS s u b s t r a t e . This is a t t r i b u t e d t o a smaller r e a c t i o n r a t e o b s e r v e d as y increases TABLE 2 Action of cathodic copper deposition and thermal treatment on the Cdl_yZnyS/CuxS properties Before thermal treatment Y

0.12 0.25 0.37

After treatment

Voc

Isc

(mA em -2)

7? %

Voc (V)

lsc (mA cm -2)

rl %

0.480 0.530 0.570

17 13 7

4.4 4.0 2.1

0.450 0.470 0.380

11 6.4 4.8

3.5 2.0 1.1

(V)

192

8 >

0.4

Z tu

0.2

I-. 0 Q.

"--.

b

a

~

I 0.5

I 1.0

1.5

TIME / mn

Fig. 3. Solid state coulometric analysis of treated and non-treated cuprous sulphide made on Cd0 ssZn0 t2S: (a) after formation; (b) after cathodic copper addition and thermal treatment (200 °C, air) for 5 rain.

allowing a b e t t e r equilibration o f c u p r o u s sulphide [6]. T h e curve b was o u t a i n e d with a sample annealed in air for a time shorter t h a n for the CdS substrate (5 min instead o f 10 min). In these conditions, the q u a n t i t y o f CuO + CuS is larger t h a n for a 30 min t r e a t m e n t o f a CuxS-on-CdS sample. It is also observed t h a t all the d e p o s i t e d c o p p e r is t r a n s f o r m e d into c u p r o u s oxide. This behaviour was also observed with samples which were t h e r m a l l y t r e a t e d e i t h e r u n d e r v a c u u m or u n d e r a h y d r o g e n a t m o s p h e r e . In a g r e e m e n t with o t h e r w o r k e r s [9, 10], we suppose t h a t o x y g e n is a d s o r b e d on CdyZn~_yS. T h e q u a n t i t y o f a d s o r b e d o x y g e n is greater o n C d y Z n z _ y S t h a n o n CdS, owing to the decrease in the crystallite size. This causes an acceleration o f t h e rate o f the o x i d a t i o n reaction, as indicated b y the increase in t h e d e g r a d a t i o n o f t h e s t o i c h i o m e t r y with annealing t i m e observed as y increases (Fig. 4); in the case o f C d y Z n I yS substrates, the d e g r a d a t i o n r e a c t i o n (reaction (4)) is faster t h a n the equilibration r e a c t i o n (reaction (1)). T h u s the t h e r m a l t r e a t m e n t is the cause o f the d e g r a d a t i o n o f t h e cell properties. As the t h e r m a l t r e a t m e n t is necessary to eliminate t h e CuxS filaments f o r m e d during the CuxS f o r m a t i o n r e a c t i o n [ 11], a low t e m p e r a t u r e charging process, p e r f o r m e d after this t h e r m a l t r e a t m e n t , was investigated. It consists o f equilibrating t h e c u p r o u s sulphide layer b y ion exchange following r e a c t i o n (3). This r e a c t i o n was p e r f o r m e d in the f o r m a t i o n solution, f o r a time short e n o u g h to avoid, the substrate being again transformed into CuxS, and c o n s e q u e n t l y avoiding the f o r m a t i o n o f new filaments. T h e sequence o f o p e r a t i o n s is: (i) f o r m a t i o n o f the CuxS b y ion exchange; (ii) t h e r m a l t r e a t m e n t , to stabilize and f o r m the j u n c t i o n ; (iii) equilibration by ion exchange. T h e c o n d i t i o n s for step (iii) were similar for each substrate c o m p o s i t i o n but, generally, t h e best results were o b t a i n e d with a high r e a c t i o n t e m p e r a t u r e (90 - 95 °C) and a relatively short r e a c t i o n t i m e (4 - 8 s). F o r instance, for Cdo.ssZn0.]2S substrates, the best c o n d i t i o n s were f o u n d to be 95 °C and 5 s. A typical result is given in Fig. 5. T h e e f f i c i e n c y o f t h e cell

193 !

5

_o

J

o

IE n-

_o

1.95

~_

nI-

-5

z uJ n-

o

"-'

0

1.90

-10

It

I--0

1.85

y=0.35

y=0.23

-15

a

y=0.12 -20

1.80 20

40 TIME !mn

60

- 0.5

0

0.5

POTENTIAL / V

Fig. 4. Variation of the stoichiometric ratio of CuxS made on Cdl_yZnyS with the thermal treatment duration. Fig. 5. Typical I - V characteristics of Cdl-yZnyS/CuxS photocells made on Cd0.ssZn0A2S substrates: (a) just after formation; (b) after thermal treatment (air, 200 °C, 20 min) and chemical equilibration (95 °C, 5 s). increased f r o m 4.4% to 5.6%. This was accompanied by the following sequence in th e stoichiometric ratio: after CuxS formation, 1.991; after thermal t r e a t m e n t , 1.958; after chemical equilibration, 1.995. No cuprous oxide was observed. However, t hough the proposed process was f o u n d to increase the efficiency o f the C d l _ y Z n y S solar cell, no efficiency greater than t h a t observed for the CdS/Cu2S solar cell was obtained.

4. Conclusion Th e adjustment o f t he cuprous sulphide stoichiometric ratio in a CuxS/CdS solar cell m a y be carried out following either of t w o methods: (i) cathodic co p p e r deposition at t he end of the CuxS f o r m a t i o n process, followed b y a thermal t r e a t m ent ; (ii) ion exchange in the cuprous chloride solution, but after a thermal t r e a t m e n t needed to form the junction. The first process is easy and leads to results similar to those obtained by use of the previously proposed techniques {vacuum deposition of copper, hydrogen plasma treatment). U n f o r t u n a t e l y , it does not apply to C d y Z n l _ y S cells, for which the lack of success was attributed to t oo fast an oxidation side reaction. Th e second process needs precise control of the equilibration time and is n o t as easy to use as the first one. Nevertheless, it allows an improvem e n t in th e Cdy Znl _y S/Cu~ S cell properties.

194

References 1 W. Palz, J. Besson, T. Nguyen Duy and J. Vedel, Proc. l O t h I E E E P h o t o v o l t a i c Specialists' Conf., Palo A l t o , CA, N o v e m b e r 13 - 15, 1973, IEEE, New York, 1973, pp. 69 - 76. 2 K. Bogus and S. Matis, Proc. 9 t h I E E E P h o t o v o l t a i c Specialists' Conf., Silver Spring, MD, M a y 2 - 4, 1972, IEEE, New York, 1972, p. 106. 3 F. Pfisterer, G. W. Hewig and W. H. Bloss, Proc. 1 1 t h I E E E P h o t o v o l t a i c Specialists' Conf., P h o e n i x , A Z , M a y 6 - 8, 1975, IEEE, New York, 1975, p. 460. 4 J. Vedel, B. Thiebaut, M. Savelli and J. Bougnot, French P a t e n t F R 8 2 . 1 1 4 8 5 . 5 J. Vedel, M. Levart and M. Dachraoui, Bull. Soc. Chim. Fr., 6 (1985) 1205. 6 M. Dachraoui and J. Vedel, Solar Cells, 15 (1985) 319. 7 E. Castel and J. Vedel, Analusis, 3 (1975) 487. 8 J. Vedel and M. Soubeyrand, J. E l e c t r o c h e m . Soc., 1 2 7 (1980) 1730. 9 P. C. Pande, G. J. Russel and J. Woods, J. Phys. D, 16 (1983) 2307 - 1316. 10 L. Hmurcik, J. A p p l . Phys., 53 (12) (1982) 9063. 11 A. Amith, J. A p p l . Phys., 50 (2) (1979) 1160.