Photoelectrochemical study of p-type copper indium diselenide thin films for photovoltaic applications

Solar Energy Materials 20 (1990) 67-79 North-Holland

67

P H O T O E L E C T R O C H E M I C A L S T U D Y OF p-TYPE C O P P E R I N D I U M D I S E L E N I D E T H I N FILMS FOR P H O T O V O L T A I C A P P L I C A T I O N S D. LINCOT, H. G O M E Z MEIER *, J. KESSLER * *, J. VEDEL Laboratoire d'Electroehimie, UnitO associOe au CNRS, Ecole Nationale SupOrieure de Chimie de Paris, 11 Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France

B. D I M M L E R and H.W. SCHOCK lnstitut ftir Physikalische Elektronik, Universitdt Stuttgart, Pfaffenwaldring 47, 7000 Stuttgart 80, Fed. Rep. of Germany Received 20 March 1989; in revised form 19 July 1989 A photoelectrochemical study of the behaviour of p-type thin films of CulnSe 2 (CIS) in aqueous acidic solutions is presented showing that the liquid junction can be efficiently used for the characterization of as-grown layers. Standard films have doping levels in the range of 10 ~6 cm-3 and their spectral responses present high quantum efficiencies ( - 0.9). The films have a poor photoresponse in electrolyte without added redox species, which has been attributed to slow interfacial kinetics for the reduction of protons. The deposition on the surface of metal islands that catalyze the hydrogen evolution (Pd, Pt) greatly enhances the photoresponse allowing the transfer towards the solution of the internal photocurrent via the hydrogen evolution. With the addition of Eu ~+ ions in solution, the transfer of photocurrent is achieved on bare surfaces allowing the complete characterization of the layer without surface treatment. Changes of the band edge position (i.e. barrier height changes) with the illumination were observed. They are associated with losses of about 0.5 V on the predicted open circuit voltage. The similitude with the behaviour of solid state cells is underlined.

1. Introduction

Copper indium diselenide (CIS) is one of the most promising photovoltaic material for thin film solar cells [1,2]. Actual results of more than 14.2% efficiency (11.3% for a 850 c m 2 module) were obtained with CIS-CdS-ZnO cells [3] and long term stability encourages further efforts to improve the cell performances and, in parallel, the understanding of the physico chemistry of the material itself [2]. A particular way to deal with these problems is the use of photoelectrochemical techniques: they provide opportunities to characterize the material before making any device as for instance measurements of diffusion lengths in bulk p-CIS [4], band gap determinations in CuIn(Se,S)2 alloys [5], test of thin films of CIS [6] and

*

* Present address: Instituto de Quimica, Facultad de Ciencias Basicas y Matematicas, Universidad Catolica de Valparaiso, Casilla 4059, Valparaiso, Chile. * Present address: Institut ftir Physikalische Elektronik, Universit~it Stuttgart, Pfaffenwaldring 47, 7000 Stuttgart 80, Fed. Rep. of Germany.

0165-1633/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

6g

D. l.m~'ol ~:t ell

PE(" vtudv ~)Jp-type ( ' u l n S e : thin jlhns

CuGaSe 2 [7]. Due to the ability of liquid junctions for ionic exchanges, they als~ provide the possibility of deliberate changes in surface or bulk chemistry allowing a comprehensive approach of the related processes. Moreover photoelectrochemical cells are a credible objective for optimizing solid state cells since very good efficiencies in the range of 10 to 20% are now obtained with various semiconductor/electrolyte junctions [8]. Photoelectrochemical studies on n-CuInSe 2 are a good example of these aspects. Very stable and efficient photoelectrochemical cells up to 12.5% efficiencies have been realized with this material by using an aqueous polyiodide electrolyte [9-12] and an important effort is currently done to understand the reactions between the material and the electrolytic environment [13-16]. One important finding is the key role played by these reactions and the surface preparation to explain the performances of the junction, for instance, in situ formation of a superficial p-CuISe3/Se layer surface layer [14,15] or influence of a superficial indium oxide layer on cell properties [12,13]. In comparison, only a few photoelectrochemical results have been published with p-type CIS [1,4,6,16,17] which is the material of interest in solid state devices and, with the exception of ref. [6], they often deal with single crystal materials whereas solar cells are made with polycrystalline thin films. The aim of this paper is to present a study of the photoelectrochemical behaviour of as grown p-type CuInSe 2 thin films prepared on Mo covered glass substrates by coevaporation of the elements [18] with which 10% efficiency solar cells have been realized yet [19]. In a previous paper we have presented a study of CuGaSe 2 thin films with various compositions [7] but, in order to focus on the electrochemical aspects, the experiments presented here were obtained with layers giving a well defined p-type behaviour, the study of a series of samples having indicated a great influence of the composition of the films on their properties.

2. Experimental The studied films were p-type CIS bilayers with a total thickness of 2 - 3 / z m . The composition was determined by EDS. Typical composition of most of the photoactive top layers was: 20.7 (Cu), 26 (In), 53.2 (Se) (in at%), with a square resistance of 53 x 106 ~ / D . This corresponds to the following values for the deviation from ideal molecularity, Am = -0.206, and for the deviation from ideal stoichiometry, As = 7.6 x 10 2 [7]. The layers with parameters close to these values were found to give the best photoelectrochemical responses. For the electrochemical studies an ohmic contact was taken with G a - I n eutectic on the Mo substrate and the sample, except the front surface ( 5 - 7 m m 2) was covered by an insulating tape. The sample was introduced in the solution without any treatment in order to keep the as-grown characteristics of the film. The electrochemical set up was a classical PAR 362 three-electrode potentiostatic device combined with a low frequency generator and a Stanford SR 530 lock-in amplifier that enabled us to make impedance measurements. The frequency used in

D. Lincot et al. / PEC study of p-type CulnSe 2 thin films

69

these experiments was 50 kHz. Spectral responses were carried out with a Jobin & Yvon H 25 monochromator coupled with a Stanford SR 540 mechanical chopper (frequency 30 Hz). Continuous white illumination (2-3 mW c m - 2 ) was provided by a halogen lamp. All the set up was monitored by a PC microcomputer. The solutions were prepared with high purity Millipore deionized water and reagent grade chemicals. Before the experiments, dissolved oxygen was removed by argon bubbling. The potential reference was given by a mercury sulfate electrode (MSE) which is at + 0.65 V versus the standard hydrogen electrode (SHE). Chemical deposition of metals (Pd, Pt, Ru, Hg) was performed in weakly acidic solution (pH = 1) containing the corresponding ions at about (0.5-1) x 10 -3 M, generally for 15 s at room temperature.

3. Results and discussion 3.1. Behaviour in darkness

In darkness, the main feature of the electrochemical characteristics is their evolution towards a stationary state, depending on the experimental conditions. In fig. 1 is shown the I - V curve obtained in darkness in a 0.5M sulphuric acid solution. In the direction of positive potentials, a rapid increase of the anodic current is observed at 0.2 V (versus MSE) which has been related to the CIS oxidation CulnSe 2 ---, Cu 2÷ + In 3÷ + 2 Se + 5 e - ,

(1)

leading to a selenium layer upon the sample surface [20]. At less positive potentials from - 0 . 2 to 0.2 V, a small anodic current is observed during the negative to positive potential scan, which disappears after the formation of the selenium layer. This small positive current is only observed if a negative current (corresponding to an electroreduction) has been allowed to cross the interface. Thus, the Se 1,2 ¢q

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f

-0,2 -1,5 -1,3 -1,1 -0,9 -0,7 -0,5 -0,3 -0,1 0,1

0,3 0,5

APPLIED POTENTIAL ( V vs MSE ) Fig. 1. Current versus potential curve for a p-type C u l n S e 2 thin film in darkness. Electrolyte: 0 . 5 M

H2SO4; sweep rate: 20 m V / s .

D. Lim'ot el al, / PE(' study of p-type ('ulnSe: thm,hlm.~

70

le+15

.

a

160

3

~T 140

4

. . . . . . . . . .

120

~

. . . . .

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8O

-1,0

-0,9

el

-0.8

0,7

0,6

4),5

-0,4

,L) 0e+0

i II

APPIJED POTENTIAl. ( V vs MSE )

21M~

.fIX)

TIME ( s )

Fig. 2. (a) C 2 a n d r e s i s t a n c e versus p o t e n t i a l c u r v e s c o r r e s p o n d i n g to fig. 1, a f t e r c y c l i n g a n d s t a b i l i z a t i o n . T h e e q u i v a l e n t series r e p r e s e n t a t i o n is used. E x c i t a t i o n f r e q u e n c y : f = 50 k H z . (b) Evolut i o n o f C - 2 d u r i n g p o t e n t i a l c y c l i n g b e t w e e n - 1.1 a n d - 0 . 4 V a f t e r i n t r o d u c t i o n in solution.

l a y e r - which may be removed from the surface by cathodic reduction - blocks the oxidation reaction occurring between - 0 . 2 and + 0.2 V. On the negative potential side, the current begins to increase from - 1 . 1 V and if the potential is maintained at more negative values (for instance - 1.3 V), a progressive increase of the negative (i.e. the reverse) current was observed, indicating a degradation of the blocking behaviour of the junction probably due to a partial reduction of CIS itself. The small direct and reverse currents in an electrolyte without added redox system corresponds to weak composition modifications at or near the sample surface and in this work, the working potential was kept between - 0.4 and - 1.1 V where the chemical modifications were small, that is where stationary curves were obtained. In fig. 2a are shown the impedance-potential curves with the impedance assumed equivalent to a series RC circuit as justified by the absence of variation of the resistance when the relation 1 / C 2 = ( 2 / q c % N A )[( VFB - V -

kT)/q]

(2)

is verified. An evolution towards a stationary state was observed, as indicated by fig. 2b which presents the variation of 1 / C 2 versus time for several cycles of potential. Consider first the stationary state. The linear variation of 1 / C 2 versus V enables one to derive the acceptor concentration NA using expression (2) and taking the relative dielectric constant of CIS as 10 [21]. Thus fig. 2a corresponds to a NA value c l o s e 1016 and values in the range of (8 + 3) x 1015 c m - 3 w e r e generally obtained. They were calculated by considering the projected area and due to the surface roughness they are probably overestimated. Nevertheless they are of the same order of magnitude than those obtained on solid state devices [22,24] with the difference that in the present case no decrease of NA appears near the surface [23,24]. Consequently, there is probably a modification of the CIS near the interface when

D. Lincot et al. / PEC study of p-type CulnSe 2 thin films

71

the junction is fabricated, due to reactions occurring between the substrate and the deposited CdS. In fig. 2b it is shown that the 1 / C 2 maximum decreases with the number of potential cycles. As the slope remains the same, this denotes a shift of the flat band potential Vva towards more negative values; it starts from about - 0 . 3 V to reach the stationary value of - 0 . 4 7 V. On the other hand, more positive values ( - 0 V) were observed when considering the first scan after an anodic oxidation of the film at + 0.3 V, followed by a decrease along the successive cycles as for non anodically oxidized films. The variation of the flat band potential is the sign of the variation of the band edge position which is shown to depend on the oxidation state of the surface: the more oxidized, the more positive the band edge position. The small negative reverse current leads to a reduction of the surface oxidized species: large differences in band edge positions (Fermi level pinning) are expected depending on experimental conditions and surface preparations. No specific study concerning the band edge positions of p-type CIS has yet been published but measurements performed in acetonitrile on thin films before and after an etching treatment in bromine in methanol solutions lead to the same tendancy [6]. The question is better documented for n-type CIS for which the valence band edge position was found close to 0.2 V / M S E in the iodide/polyiodide redox solution at pH = 6 [13,25]. This can be related to the present results ( - 0 . 4 7 V / M S E ) by considering that the n-CIS was brought in an oxidized state leading to the more anodic position of the band edges. 3.2. Behaviour under illumination

When a p-type CIS is illuminated, the created photoelectrons are driven towards the electrode/electrolyte interface where they have to be transferred into the solution. This may occur by reduction of either the semiconductor itself, the solvent or an ion (the H ÷ ion in acidic solution, with added redox species). 3.2.1. Electrolyte without added redox species

When a native CIS surface in a 0.5M sulphuric acid solution is first studied, a cathodic photocurrent is observed (fig. 3a) which is small if compared to that expected from the illumination conditions. An important hysteresis is observed between the direct and the reverse curve and a degradation of the electrode response occurs, similar to but less pronounced than that observed when the potential was maintained at - 1 . 3 V in the dark. It can be supposed that a reduction of both the semiconductor and the solvent simultaneously occurs in this case. At the same time, the 1 / C 2 versus V curve is dramatically flattened (fig. 3b) indicating the possibility of a ten-fold increase of the acceptor concentration (table 1). As in this electrolyte the transfer of the photocurrent is made by the reduction of H + ions into hydrogen, we tried to enhance the rate of this reaction by depositing on the semiconductor surface metals of the platinum group which are known to catalyse the hydrogen evolution and other redox reactions on p-type semiconductors [17,26]. It is seen in fig. 3 that the treatment with palladium improves greatly the photocurrent, the increase of the illumination and the prolongation of the experi-

D. Lincot ('t al. / P L C study o/ p-tvpe CulnSe: thin fihn~

72

bl

Ic+15 f),2 cq

0,0 -0,2

< E

0,4

z

-0,6

Z <

-0,8 1,0 1,2 -1,2

i

L

i

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-0,8

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-1,0

APPLIED POTENTIAL ( V vs MSE )

,,

,-~'m,

-0,9

-0,8

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-0,6

-0,5

-0,4

APPLIED POTENTIAL (V/MSE)

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0,5

<

0,0

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i

i

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0,7

0,8

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1,1

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1,2

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•

1,4

WAVELENGTH (gin) Fig. 3. Effect of i l l u m i n a t i o n a n d surface m o d i f i c a t i o n s o n the interfacial behaviour. Electrolyte: 0.5M H2SO4; frequency: 50 kHz; sweep rate: 20 m V / s . C o n t i n u o u s lines c o r r e s p o n d to bare surfaces a n d d o t t e d lines to p a l l a d i u m treated surfaces (15 s). D: in d a r k n e s s ; 1: u n d e r white i l l u m i n a t i o n - 2 . 5 m W / c m 2. (a) C u r r e n t versus p o t e n t i a l curves. (b) C -2 versus p o t e n t i a l curves. (c) A b s o l u t e spectral response of the p a l l a d i u m treated surface. A p p l i e d p o t e n t i a l : - 1.1 V.

Table 1 Interracial p a r a m e t e r s of p - C u I n S e 2 thin film electrodes, w i t h b a r e a n d m o d i f i e d surfaces ( d e p o s i t i o n t r e a t m e n t d u r i n g 15 s at r o o m t e m p e r a t u r e ) a) Surface

Val, ( V / M S E )

NA ( c m - 3 )

1 L (rnA/cm

Bare

-0.47 -0.64 - 0.65 - 0.82 -0.63 -0.84

1.2x1016 (9×1015) 10 lvb) ( 9 x 1 0 1 5 ) 1.2 × 1016 1.7 × 1016 8.5 x 1015 1.2x1016

0.2-0.4 - l -1

Pd Pt

Dark Ill, Dark I11, Dark 111,

(-0.38) (-0.9)

(-0.56) (-0.86)

2)

VI/2 ( V / M S E )

(-1)

-0.85

(-0.94)

- 0.87 -0.85

(-0.87)

a) Electrolyte: H 2 S O 4 (0.5M); values in p a r e n t h e s e s are o b t a i n e d w i t h Eu 3+ ions (0.28M) at p H = 2. I l l u m i n a t i o n level: - 2.5 m W / c m 2. b) A p p a r e n t value.

73

D. Lincot et al. / P E C study of p-type CulnSe e thin films a

b -1

~7 i

-0,9

~.-:-:.~-7::;. o

!

-0.r9 _c 1

-0,7 -0,8 -0,6 -0,5

0--

0

--I),4 30

60

90 120 150

T I ~ (s)

T l ~ (s)

Tl~(s)

Fig. 4. Influence of the duration of the deposition treatment on the interfacial parameters for palladium (open circles) and platinum (solid circles) treatments. Electrolyte: 0.5M H2SOa; illumination level - 2.5 mW/cm 2. (a) Current density at - 1.1 V: (-- - - --) photocurrent, ( ) dark current. (b) Half-wave potential of photocurrent. (c) Flat band potential: ( ) in dark, (-- - - --) under illumination.

ment lead to the formation of hydrogen bubbles, indicating that the charge transfer is due to the H + reduction. The effect of treatment also appears on the capacitance-voltage curves. In the dark, the stationary curve presents a negative shift of the flat band potential ( - 0.2 V) with the slope unchanged. This shows that the bulk doping level remains unchanged after the treatment. Under illumination, the shift of VVB is more pronounced but the the slope of the linear part is again the same as for non-treated surfaces in the dark. This means that the flattening observed with bare surfaces under illumination is not due to an increase of the apparent doping level under illumination (due to the ionization of deep centers for instance) but to a very large shift of VVB. Similar results were obtained when platinum was used as sensitizer (table 1). In fig. 4 is shown the influence of the time of treatment (corresponding to an increase of the amount of deposited metal) on the properties of the treated films. In the dark, it is observed that the current increases with the treatment time and that the stationary flat band potential varies towards more negative values. Under illumination, the catalytic effect shows a maximum for short treatments, the photocurrent onset potential (described here by the photocurrent half-wave potential) and the flat band potential remaining approximately constant and close together. These phenomena may be interpreted as a consequence of the fact that the catalytic metals present high work functions: when deposited on the semiconductor surface, they give energy levels close to the valence band and consequently they are easily in equilibrium with its electrons. The dark current increases and simultaneously negative charges accumulate at the interface causing a shift of the energetic position of the bands until the rate of charge transfer towards the solution becomes high enough to stop the accumulation of charges. Under illumination the process is the same but as there are much more electrons to be transferred the shift is more important, a more negative potential being needed for having a higher charge transfer rate [27].

74

D. Lmcot et a L / PE(" study o] p-tvpe CulnSe, thin.films

The intrinsic quality of the films for photovoltaic application is checked by measuring the absolute spectral response in the photocurrent plateau range (fig. 3c) which displays quantum efficiencies of about 0.7 and a good shape of the curve. Further use of these layers in photoelectrochemical cells for H 2 evolution is handicaped by the fact that the photocurrent onset potential approximately corresponds to the H + reduction on Pd or Pt electrodes (_< - 0 . 6 5 V versus MSE). This is to be associated to the too negative flat band potential of CIS in this situation. In table 1 are also reported the values obtained with a bare surface after cycling the potential between - 1 . 2 V and + 0.3 V. This treatment modifies the surface in such a way that the transfer of the photocurrent to the solution also becomes possible. However, the onset potential is more negative than after Pd or Pt deposition. The obtained value is similar to that reported for bulk crystal in I M HCI solution [16].

3.2.2. Electrolyte with added redox species Films with non-treated surfaces are inefficient photocathodes in acidic solutions despite their good bulk properties. Thus other redox species than protons are needed to work with native surfaces and their redox potential has to be situated in a range where no degradation of the semiconductor occurs. This leads to the range from - 1 . 2 V to - 0 . 2 V previously defined from the I - V curves (fig. 1). We selected the Eu 2+/3+ redox couple, already used in semiconductor photoelectrochemistry [28]. Its redox potential is - 1 . 0 8 V versus MSE, not far from the position of the conduction band edge in darkness. This makes the comparison possible of the CIS/electrolyte energetic diagram with that of the C I S / C d S solid state junctions and may lead also to high photopotentials in photoelectrochemical cells. Furthermore, the europium ions have the advantage of being transparent in the visible range as compared with the V 2+/3+ redox couple [E 0 = - 0 . 9 1 V] which has been also used in efficient photoelectrochemical cells [29]. In fig. 5a are shown the 1 - V curves obtained with a non-treated sample in a p H = 2 solution of 0.28M Eu 3+ ions containing perchlorate anions which improve the reversibility of the reactions [30] and a small quantity of sulphate anions. The amplitude of the photocurrent indicates that the transfer to the solution is now achieved, by means of the reaction E u 3+ + e - --~ E u 2~.

The hump appearing during the cathodic to anodic potential scan at about - 0 . 7 V corresponds to the oxydation of the slightly soluble europium(II) sulphate accumulated near the surface during the reduction step. The other curve of fig. 5a shows the effect of a platinum treatment in the same electrolyte. The unvarying amplitude of the photocurrent further confirms that the collection efficiency is now correct for the non-treated surface. The positive shift of about 0.1 V of the photocurrent half-wave potential indicates that metal deposition improves only weakly the rate of charge transfer. Similar results have been obtained by treating the surface with other metals like Pd, Ru and Hg on which the Eu 2+/3+ is known to be reversible. The lack of marked variation with the treatments (table 1) seems to indicate that the effect is

D. Lincot et al. / P E C study of p-type CulnSe 2 thin films 0,5

75

a

b

0,0 le+15

¢-q < b-, Z

II II

-0,5

r.,l

I1

,¢ ¢..)

Z -1,0

r) < rj

-1,5 -1,3

i -1,1

i -0,9

i -0,7

i -0.5

~

,

-1,1

A P P L I E D P O T E N T I A L ( V vs M S E )

1

•

-1,0

i

-0,9

.

1.,

-0,8

t

-0,7

.

l

-0,6

,

1

-0,5

.

-0,4

A P P L I E D P O T E N T I A L ( V vs M S E )

Fig. 5. (a) Current versus potential curves in a solution of 0.28M E u 3 + at pH = 2: ( ) bare surfaces, (-- -- --) after a palladium treatment (15 s). D: in darkness; I: under white illumination - 2.5 mW/cm 2. (b) C -2 versus potential corresponding to the bare surface of (a) The equivalent series representation

is used.

Exitation

frequency:

f = 50 kHz.

related to some characteristics of the layer independent of the redox couple but dependent of the film itself. In fig. 5b are presented the corresponding capacitance-voltage curves. In opposition with what was observed in the indifferent electrolyte, the 1 / C 2 versus V curve under illumination now presents a straight portion nearly parallel to that obtained in darkness. This behaviour is n o w observed with a bare surface. It confirms the ability of the redox species to transfer the generated photocurrent [27]. Under illumination the flat band potential is still shifted by about 0.5 V towards more negative values. Its value is close to that observed after treatment in the previous section in conditions of hydrogen evolution (table 1). Fig. 6 shows the best spectral response obtained with a bare surface in the region of the photocurrent plateau. Quantum efficiencies of about 0.9 are obtained. The calculated photocurrent under AM1 (96 m W / c m 2) is about 40 m A / c m 2. The energy band gap value was calculated using the classical relations for semiconductors having a direct band gap. The value Eg = 0.988 eV was found.

3.2.3. Interfacial energetics In fig. 7 is reported the variation of the quantum efficiency at ~ = 1.1 /~m measured under conditions of weak illumination level as a function of the potential and the stationary 1 - V curve under higher illumination level. In the first case the photocurrent onset potential is at about - 0 . 3 5 V and is close to the flat band potential in darkness (table 1). In the second case the onset is situated at - 0 . 8 5 V and also corresponds to the flat band potential under these conditions of illumination. This indicates that, as the illumination level increases, the effective barrier

76

D. Lincot et al. / PE(" stud~' oj p-O'pe C u l n S e , lhm ]ihn.~

1,0

[L5

e

Z <

O'

o,fl 0,8

0,9

1,0

1,1

!,2

1,3

1,4

WAVELENGTH ( p.m ) Fig. 6. Best spectral responses obtained in the photocurrent plateau range on a bare surface in the c o n d i t i o n s of fig. 5. (1) V = - 1.2 V, (2) V = - 1.1 V.

height at the interface decreases. This leads to an important reduction (by about 0.5 V) of the achievable photocurrent onset potential. It can be mentioned that the spectral responses shown in fig. 6 are similar to those reported for solid state C I S / C d S / Z n O high efficiency ( - 1 0 % ) thin film solars cells [22]. Moreover large shifts of capacitance-voltage curves (up to 0.7 V) under illumination were also observed [24] demonstrating the similarities between both types of junctions. These shifts were attributed to the charging of interface states and it was observed that the most efficient cells (10.2%) are those with the smallest shift under illumination [24]. To our opinion, by comparing with the similar p h e n o m e n o n we observed in semiconductor photoelectochemistry [27b], the shift of band edges under illumination could be due mainly to slow charge transfer reaction leading to the accumulation of minority carriers at the interface, in surface states or in an inversion layer. Sluggish charge transfer kinetics can arise from either the presence of a blocking layer at the interface or a low rate of the charge transfer reaction itself.

1.0

~~¢ r,.)

'

~,.,O.~ ~ ~ ~.Q.

~ V

0,5

~ 0.5 V

O O

-1,2

-1,0

-0,8

-0,6

-0,4

-(I,2

APPLIED POTENTIAL ( V vs MSE ) Fig. 7. Potential d e p e n d e n c e of the photocurrent on the i l l u m i n a t i o n level: ( - - - - - ) low illumination level, deduced from spectral response m e a s u r e m e n t s at 1.1 ~m. ( ) high illumination level c o r r e s p o n d i n g to the stationary current potential curve of fig. 5,

D. Lincot et al. / PEC study of p-type CulnSe 2 thin films

77

-2 CB -1.5 CB

[-.

-1 + EF . . . . . . . . . . VB - - - -

E redox t ~ 0.2_0.3 V

10.7 V

| EF . . . . . . . . VB,,,

-0.5..~..__

0

SEMICONDUCTOR

ELECTROLYTE

Fig. 8. Energetic diagrams of p-CulnSe2 in acidic solutions showing the shift of the bands under illumination. Standard lines: dark and very weak illumination levels,bold lines: high illumination levels ( >/2.5 mW/cm-2). The proposed energetic diagram of the interface is represented in fig. 8. The comparison with solid state devices is made by taking as reference the redox potential of europium in which is equivalent to the V = 0 value in solid state devices. This gives a barrier height of about 0.7 V in darkness comparable to that of solid state cells (0.9 V [22]) which is reduced to 0.25 V under illumination (0.43 V for the cells with the highest efficiency).

4. Conclusions

It has been shown that the liquid junction configuration can be used efficiently to characterize as-grown thin films of p-type CIS. Doping level measurements can be performed in aqueous acidic solutions in darkness but redox species, here E u 3 + ions, have to be added to the solution for accurate optical characterizations. For the layers under study, the doping level was about 1016 cm-3 and the absolute spectral response presented values about 0.7-0.9 leading to a predicted 40 m A / c m 2 photocurrent value under AM1 (96 mW cm-2). This confirms the intrinsic quality of these layers for solar cells applications. The effect of surface modifications by metal deposition has been investigated. In acidic electrolytes with no redox species, the deposition of noble metals (Pt, Pd, Ru) catalyzes the hydrogen evolution reaction, allowing the transfer of the generated photocurrent otherwise very difficult on bare (non-treated) surfaces. However, the photocurrent onset potential remains very negative if compared with the values of other p-type semiconductors such as InP [26]. This appears to be related with the more negative value of the flat band potential of p-CIS as compared with these materials. The addition of E u 3 + ions

78

l). L m c o t et al, / PEC ~tudv qt p-tvpe C u l n S e , thin tilm,~

allows an efficient collection of the generated photocurrent on bare surfaces and ir~ that case an open circuit voltage of 0.2 V versus E redox is already achievable in the corresponding photoelectrochemical cell. Under low illumination levels an important increase of 0.5 V in the photocurrent onset potential is observed which would lead to much higher values of the open circuit voltage. This effect is due to losses under illumination related with band edge shifts at the interface (effective barrier height changes). This behaviour can be associated with several causes as for instance the slow charge transfer kinetics or the effect of surface states a n d / o r bulk recombination centers. The similarities of the observed behaviour with that reported for some solid state devices could indicate common mechanisms of losses in both types of junctions. This implies a further investigation of the effect of surface or bulk treatment of the layers together with solution optimization. Beneficial consequences of these treatments can also be expected for solid state devices.

Acknowledgement This work was supported by the Commission of the European Communities under contract No. EN 35-0070-F.

References [1] R.A. Mickelsen and W.S. Chen, in: Proc. 15th IEEE Photovoltaic Specialists Conf.. 1981, p. 800; and in: Proc. 16th IEEE Photovohaic Specialists Conf., 1982, p. 781. [2] Copper Indium Diselenide for Photovoltaic Applications, Sol. Cells 16 (1/4) (1986). [3] K.W. Mitchell, C. Eberspracher, J. Ermer, D. Pier and P. Milla, in: Proc. 8th E.C. Photovoltaic Solar Energy Conf., 1988, p. 1578. [4] C.L. Johnson, S. Wagner and K.J. Bachmann, J. Electrochem. Soc. 132 (1985) 264. [5] H. Neff, P. Lange, M.L. Fearheiley and K.J. Bachmann, Appl. Phys. Letters 47 (1985) 1089. [6] Y.W. Chen, D. Cahen, R. Noufi and J.A. Turner, Sol. Cells 14 (1985) 109. [7] D. Lincot, J. Kessler, J. Fauconnier, M. Zouad and J. Vedel, in: Proc. 8th Photovoltaic Solar Energy Conf., 1988, p. 1087. [8] M. Froment, Actual. Chim. (1987) 50. [9] S. Menezes, H.J. Lev'erenz and K.H. Bachmann, Nature 305 (1983) 615. [10] D. Cahen and Y.W. Chen, Appl. Phys. Letters 45 (1984) 746. [11] S. Menezes, Appl. Phys. Letters 45 (1984) 148. [12] G. Razzmi, L. Peraldo Bicelli, B. Scrosati and L. Zanotti, J. Electrochem. Soc. 133 (1986) 351. [13] D. Cahen, Y.W. Chen, R. Noufi, R. Ahrenkiel, R. Matson, M. Tomkiewicz and W.M. Chen, Sol. Cells 16 (1986) 529. [14] S. Menezes, Sol. Cells 16 (1986) 255. [15] H.J. Lewerenz and E.R. K~51tz, J. Appl. Phys. 60 (1986) 1430. [16] S. Menezes, J. Electrochem. Soc. 134 (1987) 2771. [17] H.J. Lewerenz, H. Goslowsky and F.A. Thiel, Sol. Energy Mater. 9 (1983) 159. [18] W. Arndt, H. Dittrich, F. Pfisterer and H.W. Schock, in: Proc. 6th E.C. Photovoltaic Solar Energy Conf., 1985, p. 260. [19] B. Dimmler, H. Dittrich, R. Klenk, R.H. Mauch, R. Menner and H.W. Schock, in: Proc. 8th E.C. Photovoltaic Solar Energy Conf.. 1988, p. 1583.

D. Lincot et al. / P E C study of p-type CulnSe 2 thin films

[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

79

M. Zouad, Thesis, University Paris VI (1988). S. Endo, T. Irie and H. Nakamishi, Sol. Cells 16 (1986) 1. R.R. Potter, Sol. Cells 16 (1986) 521. R.E. Hollingsworth and J.R. Sites, Sol. Cells 16 (1986) 457. R.K. Ahrenkiel, Sol. Cells 16 (1986) 549. G. Razzini, L. Peraldo Bicelli, B. Scrosati, M. Pujadas and P. Salvador, J. Electrochem. Soc. 135 (1988) l l l 0 . A. Aharon-Shalom and A. Heller, J. Electrochem. Soc. 129 (1982) 2865. D. Lincot and J. Vedel, J. Electroanal. Chem. 220 (1987) 179; J. Phys. Chem. 92 (1988) 4103. R. Memming, J. Electrochem. Soc. 125 (1978) 117. A. Heller, B. Miller and F.A. Thiel, Appl. Phys. Letters 38 (1981) 282. W.F. Kinard and R.H. Philp, Jr., J. Electroanal. Chem. 25 (1970) 373.