A voltammetric study of the electrodeposition chemistry in the Cu + In + Se system

279

J. Electroanal. Chem., 271 (1989) 279-294 Elsevier Sequoia S.A., Lausarme - Printed

in The Netherlands

A voltammetric study of the electrodeposition in the Cu + In + Se system

chemistry

K.K. Mishra and K. Rajeshwar * Department (Received

of Chemrsoy, 4 November

The Unrversity of Texas at Arlmgton, Arlington,

TX 76019 (U.S.A.)

1988; in revised form 13 April 1989)

ABSTRACT

The mechanism of formation of CuInSe, thin films on a glassy carbon surface during voltammetric scanning was examined in detail for the first time using a combination of Pourbaix analyses, cyclic and hydrodynamic voltammetry of the binary In + Se and Cu + Se systems, along with cyclic photovoltammetry, i.e. cyclic voltammetry combined with periodic white light illumination of the electrode/electrolyte interphase, on the ternary Cu + In + Se system. A sulfuric acid matrix containing SeO, and uncomplexed Cu*+ and In3’ ions was used in all cases. The data on the binary systems were consistent with the facile formation of a Cu,_,Se solid phase in the Cu + Se system and a kmetically sluggish interaction between In and Se in the In+Se case. An internally consistent mechanistic scheme is proposed for the ternary system involving the concurrent formation of the Cu,_,Se phase. its subsequent reduction coupled with the 6 e- reduction of H,SeO, to H,Se. and finally the underpotential assimilation of In into the solid phase leading to the photoactive chalcopyrite semiconductor, CuInSe,. The cathodic decomposition of the initially formed Cu,-,Se is shown to be a key to subsequent assimilation of In and formation of CuInSe,. The photocathodic response observed for this thin-film formation was diagnostic of a Cu-rich ternary composition and consequent p-type behavior for the conditions pertaming to this study.

INTRODUCTION

The ternary Cu + In + Se system comprises several compounds amongst which the chalcopyrite semiconductor, CuInSe,, has been of particular interest for electro-optical, photovoltaic and photoelectrochemical applications [l-3]. Of the techniques that have been used to fabricate thin films of this material including, for example, molecular beam epitaxy [4], flash evaporation [5], multi-source evaporation [6], single-source evaporation [7], RF sputtering [8], spray pyrolysis [9,10] and electrodeposition [ll-161, the last two candidates appear to offer the greatest

l

To whom correspondence

0022-0728/89/$03.50

should

be addressed.

0 1989 Elsevier Sequoia

S.A.

280

potential in terms of low costs and compatibility with applications requiring large photoactive areas. Previous studies of electrodeposition in the Cu + In + Se system [ll-161 have focused mainly on the optimization of deposition parameters for the synthesis of device quality material rather than on the mechanistic aspects. In this article, we will develop a mechanism for the cathodic electrosynthesis of CuInSe, based on the use of a variety of voltammetric probes, Pourbaix analyses, and data on the binary Cu + Se and In + Se systems respectively. EXPERIMENTAL

A standard two-compartment three-electrode cell configuration was used for voltammetry. The working electrode was glassy carbon polished and pre-treated as described elsewhere [17]. A Pt spiral was used as the counterelectrode. The reference electrode was saturated calomel/l M KC1 which was isolated from the solution in the main cell compartment by a glass-tube equipped with a fritted end. All potentials below are quoted with respect to the SCE reference unless otherwise stated. The cell for cyclic photovoltammetry, wherein voltammetric scanning is combined with periodic white-light illumination of the electrode/electrolyte interphase, has been described elsewhere [18]. An EG&G Princeton Applied Research Model 273 system was used for cyclic voltammetry and cyclic photovoltammetry. Additionally. the latter technique utilized a 100 W ELH tungsten-halogen lamp, an IR filter and an EG&G Model 194A the photoresponse of the growing light chopper. In some instances, semiconductor/electrolyte interphase was measured with an EG&G Model 5208 lock-in analyzer. Hydrodynamic voltammetry was performed with a Pine Instruments bipotentiostat and an RRDE assembly. The disk was glassy carbon (radius: 0.256 cm) and the ring was Pt (inner radius: 0.334 cm, outer radius; 0.380 cm) in these experiments with a theoretical collection efficiency of 16.9% [19]. This latter value was checked by measurements with the Fe(CN)i-/4redox couple. Some experiments utilized simply an RDE system. For the In3’ shielding experiment (Fig. 9 below), the Pt ring was coated with a Hg thin film to minimize interference from proton reduction reactions. For this, the ring was potentiostated at - 0.20 V in a 10 mM solution of Hg 2+ in 0.5 M H,SO,. All experiments were performed on carefully de-oxygenated matrices and at room temperature. The chemicals were of commercial origin and had the highest available purity; they were used as received. A variety of electrodeposition baths have been used by previous authors [ll-161. For example, in previous experiments in our laboratory [12], we employed a mixture of InCl,, CuCl, SeO,, triethanolamine and ammonia with pH adjusted to - 1.0 with HCl. Ueno et al. [15] used a mixture of CuSO,, In,(SO,), and SeO, adjusted to pH 1.0 with H,SO,. On the other hand, Pern et al. [14] employed CuSO,, SeO, and indium sulfamate in acidic ethylenediamine dihydrochloride of pH 1.70. Very recently, Pottier and Maurin [16] reported the electrodeposition of CuInSe, from an acidic aqueous solution (1.5 c pH < 4.5) of CuSO,, In,(SO,),, SeO,, K,SO4 and

281

sodium citrate. Hodes et al. [20] have employed a HBr/glycolic acid bath for electrodeposition of Cu + In alloys followed by selenization in a H,Se atmosphere. The role of the additives in the above formulations has not been characterized systematically in the above studies, although in many instances they appear to act as complexation agents (see below). We have employed an electrodeposition bath similar to the one employed by Ueno et al. [15] for this study. To unravel the mechanistic aspects, we avoided intentionally potential complications in the electrodeposition chemistry from the use of complexing agents. RESULTS

AND DISCUSSION

Thermodynamic aspects and background Preliminary experiments, the data of previous authors [ll-161, and an examination of the standard reduction potentials of Cu*+ and In3’ (+O.lO V and -0.58 V respectively) revealed that formation of the Cu + Se alloy phase precedes the subsequent assimilation of In and formation of the ternary CuInSe, compound respectively. It should be noted that while the reduction of SeO, in acidic media has a fairly high (thermodynamic) potential (- 0.50 V), the rather sluggish kinetics coupled with the passivating influence of the incipient Se0 on the electrode surface, shifts the reduction effectively to more negative potentials (see below, also cf. ref. 21). Thus, the stability regimes of the Cu + Se + H,O system at 25 “C were first mapped using Pourbaix analyses [22]. Table 1 lists the species and the thermodynamic data that were used for these analyses. Thermodynamic data were culled from literature sources [22-241. A specially designed computer program (“DIAGRAM “) which generates the E-pH profiles, was used, and has been described elsewhere [25]. Figure 1 illustrates the Pourbaix diagram for the Cu + Se + H,O system at 25 o C for the specific case wherein the concentrations of the Cu and Se species in the electrolyte were each 0.1 M. The diagram predicts the following for the pH range (0.5-1.5) pertinent to this study. The stable Se(lV) species, namely H,SeO,, reduces to Se0 at 0.72 V (eqn. 1). On further reduction of the electrode potential, CuSe should form via either reaction (2a) or (2b): H2Se03+4H++4e-=Se+3HH,0

(1)

Cu2+ + Se0 + 2 e- = CuSe

(2a)

H,SeO,

(2b)

+ 4 H+ + Cu2+ + 6 e- = CuSe + 3 H,O

Note that the formation of CuSe occurs at more noble potentials than the reduction of cuz+ to Cue (0.50 V US. 0.31 V US. SHE). An elegant thermodynamic treatment of this underpotential formation of compound semiconductors has been presented by Krijger [26]. As the electrode potential is decreased further, CuSe is expected to reduce first to Cu,Se, and then Cu,Se. Finally, Cu ,Se should reduce to Cue and H,Se: Cu,Se + 2 H+ + 2 e- = Cue + H,Se

(3)

282 TABLE

1

Thermodynamic

data for selected reactions in the Cu + Se + H ,O system at 25 o C

Reaction

Log K =

Cu2++H,Se0,+4 H++6e3Cu2++2H2SeOs+8H++14eCuSeO, + 2 H+ Cu2+ +Se+2 e3Cu2++2Se+6eCuSe+2 H’ CuSe,+4H++2e2 Cuzc + 2 e- + HSe3Cu2++2H Se+2e3Cu2++2Hie-+2e2Cuzf+SeO:-+6H++8eCu2++SeO+6H++6eCu2++2SeOi-+12H++lOeH,Se HSeSe+2 eH,SeO, +4 H+ +6 eHSeO; +5H++6ea K is the equihbrium

0.676 0.676 2.2 0.560 0.530 - 22.6 0.075 0.244 0.180 0.210 0.697 0.754 0.820 - 18.7 - 15.1

=Se2-+3 H,O =Se’- +3 H,O

- 0.680 0.269 0.292

constant at 25OC.

Generation of H,Se and Se(Iv): H,SeO,

= CuSe+ 3 Ha0 = Cu,Se, +6 H,O = Cu*+ + H 2’ Se0 3 = CuSe = Cu,Se, =Cu”+H Se =Cu2++2& Se = Cu,Se+H? = Cu,Se, +4 H+ = Cu,Se2 + 2 H+ = CuaSe+3 Ha0 = CuSe+3 Ha0 = CuSe2 + 6 H 2O =Se2-+2 H+ =Se2-+H+ = Se2_

E”/V (vs. SHE)

is also facilitated

at negative

potentials

by the reduction

of Se0

+ 6 H+ + 6 e- = H,Se + 3 H,O

Se + 2 H+ + 2 e- = H,Se

(da) (4b)

Rationalization of experimental data on real systems in the light of the above thermodynamic treatment has to include complications arising from electron-transfer kinetics and homogeneous reaction chemistry. For example, at intermediate pH ranges, a net four-electron process has been observed [21,27] because of the reaction of H,Se with Se(IV): H,SeO,

+ 2 H,Se = 3 Se + 3 H,O

This reaction has been invoked to account for the high Se0 concentration electrodeposited CdSe layers [28,29]. Precipitation of copper selenides can occur via the reaction of Cu species with H,Se, for e.g.: Cu2++H2Se=CuSe+2H+

in also

(6)

Skyllas-Kazacos and Miller [21] concluded from their hydrodynamic voltammetry data on Cu disk electrodes that a mechanism similar to reaction (2b) was operative leading to the postulated formation of Cu,Se. Pottier and Maurin [16] have confirmed the formation of Cu,Se via X-ray diffractometry. On the other

283 2

I

sea, ,

sea:

I I

Cu(OH]

I

/

1.

% ;o. > Gi

-1.1

Ot H,Se , HSe I I

-2.1 3

1

0

8

12

16

PH Fig. 1. Pourbaix diagram for the Cu + Se + H,O system at 25 o C. The potentials are shown relative to the standard hydrogen electrode as reference. Candidate reactions and thermodynamic data for assembhng this diagram are shown in Table 1. Refer to the text for other details.

hand, Ueno et al. [15] found no evidence for Cu,Se formation, but instead reported X-ray diffraction evidence for the Cu,Se, (unmangite) phase (cf. Fig. 1). Their samples were electrosynthesized at 0 V. With reference to Fig. 1, CuSe is the thermodynamically stable phase in this potential regime. Obviously, kinetic factors are important here and it may be more appropriate to characterize the various compound members of the Cu + Se family as Cu,_ .Se with x varying from 0 to 1. We will return to further discussion of this system in connection with our voltammetric data which are presented below. Figure 2 presents a partial phase diagram of the Cu + In + Se system. In addition to the Cu + Se compounds mentioned previously, the S analog of CuIn,Se, has been prepared by slurry-painting and electrodeposition and characterized by photoelectrochemical methods [30]. Single crystals of InSe, which is a lamellar semiconductor which occurs both as n- and p-types, has been studied by Tenne et al. [31]

284 Se 100 0

CU

1;

cu,hl

0 100

cu,hl,

Fig. 2. A partial phase diagram that have been electrochemically

Ill

for the ternary Cu + In+ Se system. characterized to date.

The dots represent

the alloy phases

previously. Pottier and Maurin [16] have identified the alloy phases, Cu,In and Cu,In, from the use of their citrate bath. Within the CuInSe, system, Noufi et al. [32] have located a narrow V-shaped region wherein p-type conductivity is observed, and the hole density increases by several orders of magnitude as the sample is made more Cu-rich. Finally, Table 2 presents thermodynamic data for selected reactions in the In + Se + H,O and Cu + In + Se + H,O systems at 25 o C. Indium is seen to form two compounds, namely InSe and In,Se,, with Se. The standard potentials for these two reactions (-0.19 V and -0.01 V respectively) are shifted positive of the metallic In value (-0.58 V) by the free energy of compound formation.

TABLE

2

Thermodynamic 2s0c

data

for selected

reactions

in the In+ Se+ H,O

and

Cu+ In + Se+ Ha0

Reaction

Log K a

= In = InSe = In,Se, = In,Se, t-9 H,O = 2 Id’ + 3 H,Se = CuInSe, + 6 H,O = CuInSe, = CuInSe, + 2 H+ =3CuInSe2+8H+ = CulnSez = 2 CuInSe, = 2 CuInSe, + 2 H+ = 2 CuInSe,

In’+ + 3 eId+ +Se+3 e21n3++3Se+6e21n3i+12H++3H,Se0,+18eIn,Se, +6 H+ In3++C~2~+2HzSeOs+8H++13eIn3+ +Cu*+ +2 Se+5 eln3+ +&Se+ H,Se+e3 In3++Cu,Se2 +4 H,Se+eId++CuSe+Se+3 e2Cu2++In,Se3+Se+4e2CuZ++In,Se3+H2Se+2eCu 2 Se+2 ln3++3 Se+6 ea K is the equilibnum

constant

at 25OC

(7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

systems

Es/V (vs. SHE) - 0.338 0.046 0.230 0.572

- .34.93 0.587 0.350 0.870 0.660 0.210 0.527 0.169 0.226

at

285

Voltammetry of the binary Cu + Se and In + Se systems The complexity of the ternary Cu + In + Se system presages a discussion of the voltammetric behavior of the binary Cu + Se and In + Se cases first. Figure 3a contains a cyclic voltammogram for a glassy carbon electrode in 0.5 M H,SO, containing 5 X lo-* M In,(SO,), and 2 X lo-* M SeO,. On the forward scan, a cathodic wave is seen at - -0.80 V followed by an appreciable flow of current at potentials negative of - - 1.0 V. On the return scan, a nucleation loop is accompanied by a cathodic peak at - -0.80 V followed by an anodic stripping wave at -0.60 V. The amplitude of the latter is attenuated if the scan is arrested on the forward cycle at potentials 6 - -0.80 V. We assign the cathodic features on the forward scan to the composite reduction of In3’, H,SeO, and water. The cathodic peak at -0.80 V on the return (positive-going) scan is diagnostic of the large nucleation overpotential associated with In3’ reduction. The appearance and the scan-time dependence of the In0 stripping wave at -0.60 V are also diagnostic of the slow kinetics of the reaction between In0 and selenium species to form indium selenide. This kinetic barrier further manifests in the negligible amount of current flow in a potential regime wherein “underpotential” formation of either InSe or In,Se, may be expected (cf. Table 2 and the thermodynamic data presented in the preceding section). However, electrodeposition at potentials < -0.80 V yielded a photoconductive film as measured in separate photoelectrochemical experiments using 0.5 M H,SO, as the electrolyte (see below). These experiments show logically

,Y

t

O.lmA

1

0.75

0.35

h-h

-0.05

-0 85

-0.45 POTENTIAL/V&

1. + 3e

-1.25

SCE)

Fig. 3. Cychc voltammograms at a glassy carbon electrode (potential scan rate: 0.02 V/s) for the bmary In+ Se (a) and Cu+Se (b) systems in 0.5 M H,SO,. The In,(SO,), and C&O4 concentrations in a and b were 5 x lo-* M and 2 X 10-j M, respectively. In both cases. the electrolyte contained 2 X lo-* M

SeO,.

286

T

O.OlmA

I

0.5

0.1 POTENTIAL/V(=

-0.3

‘

-0.7

SCE)

Fig. 4. Hydrodynamic voltammograms for a glassy carbon rotating disk (rotation speed: 3,200 rpm) in 0.5 M H,SO, containing 5 x 1O-3 M CuSO, alone (a), 2 X lo-* M SeO, alone (b) and a mixture of 5 x lo-’ M CuSO, and 2 x 10V2 M SeO, (c).

why Cu + In + Se films electrosynthesized from uncomplexed acidic media are usually In-poor [13] (also see below). Figure 3b contains a representative cyclic voltammogram for a glassy carbon working electrode in 0.5 M H,SO, containing 2 X lop3 M CuSO, and 2 X 10e2 M SeO,. The complex voltammogram shows a sharp cathodic wave on the forward scan at - 0 V followed by a broad wave at - -0.25 V. A set of two sharper cathodic features, the second of which is weakly recovered on the return scan, is accompanied by two broad anodic features between 0.15 V and 0.55 V on the return cycle. To unravel the voltammetric features of the Cu + Se system further, hydrodynamic voltammetry was performed at a glassy carbon RDE in 0.5 M H,SO, with Cu2+ alone (Fig. 4a), SeO, alone (Fig. 4b) and a mixture of Cu*+ and SeO, (Fig. and adheres to classical 4~). The reduction of Cu 2+ is mass transport-controlled Levich behavior [19]. The plateau current is reached at - - 0.50 V (Fig. 4a). On the other hand, the reduction of SeO, in acid is kinetically sluggish and the limiting

287

current, i,, does not scale with o *I2 . The passivating influence of the Se0 which is formed has been mentioned in a preceding paragraph. When Cu2+ and SeO, are present simultaneously, the reduction current starts at a more noble potential than in the absence of either component (- 0.10 V in Fig. 4c vs. - 0.30 V in Fig. 4a). As the potential is decreased beyond - 0.40 V, a sharp peak appears followed by a mass-transport controlled plateau (Fig. 4~). On the return scan, a sharp decrease in the plateau current from i,, to i,,, occurs at - -0.30 V. Note that on further scanning in the positive direction, the stripping peak of Cue (cf. Fig. 4a) is not observed. Thus, Cu is present as Cu2_,Se rather than as Cu’. The “underpotential” deposition of CU~_~S~ may be represented by the equation (cf. reaction 2b): H,SeO,

+ 4 H+ + (2 - x) Cu2+ + (7 - x) e- = Cu2_Se

+ 3 H,O

(2c)

That Cu,_,Se rather than the stoichiometric Cu2+ compound, CuSe, is formed (cf. reaction 2b) is indicated by taking the ratio of the limiting currents corresponding to copper selenide formation, namely, i L.1 (cf. Fig. 4c), and that corresponding to cu2+ ---fCue reduction (a 2 e- process, cf. Fig. 4a). If the reasonable assumption is made that the mass-transport coefficients are comparable in the two instances, values in the range from 3.0 to 3.2 are obtained. We conclude therefore that some of the sites in the copper selenide compound exist in the Cuf state. This conclusion is also supported by the lack of evidence in any previous work [1.5,16,21] for CuSe formation which is predicted from thermodynamic considerations (see above). The sharp peak on the forward scan at -0.40 V in Fig. 4c is assigned to the cathodic reduction of Cu,_,Se: Cu,_,Se Support

+ 2 Ht + 2 e- = (2 - x) Cu + H,Se for our assignment

comes

(20)

from the RRDE

data

contained

in Fig. 5. In

i 0.5

0.1

-0.7

-0.3 DISK POTENTIAL/V@

-1.1

SCE)

Fig. 5. Hydrodynarmc voltammograms on a RRDE (CC disk and Pt ring) wtth the ring potentiostated at 0.20 V. The electrolyte was the same as in the experiment in Fig. 4c. The rotation speeds are shown in ‘pm.

288

Fig. 6. Levich plots of the limiting currents, iLIl and i,,, - 0.30 V and - 0.60 V for these data, respectively.

these experiments, the ring was potentiostated H,Se according to reaction (8): H,Se=Se”+2H++2e-

in Fig. 4c. The potentials were selected at

at 0.20 V to cause the oxidation

of

(21)

Note that the anodic ring currents in Fig. 5 are coincident with the sharp cathodic peak onset at the disk at the various rotation speeds. Only a fraction of the current predictable from the collection efficiency of the RRDE system (see Experimental) is recovered at the ring. We attribute this discrepancy to the homogeneous reaction chemistry between H,SeO, and the electrogenerated H,Se as represented by eqn. (5). Figure 6 contains Levich plots of i, vs . al”2 for the two plateau currents in Fig. 4c. Potentials of - 0.30 V and - 0.60 V were selected for i,,, and i,,2 respectively. It is interesting that the kinetics of the reduction of H,SeO, (via reaction 2c) is fast enough to be mass-transport controlled. Yet, in the absence of Cu2 + in the electrolyte, the reduction of H,SeO, is kinetically sluggish and lea& to passivation of the electrode surface. The influence of varying SeOz concentration in the electrolyte on i,,, and zr,* is illustrated by the plots in Fig. 7. It is seen that i,,, scales with cseo2; on the other hand, i,,, increases initially with cseo and then levels off. This is diagnostic of the fact that at higher SeO, levels, the fo’mation of Cu2_,Se (and hence i,,,) is limited by the availability of Cu sites at the electrode surface. On the other hand, i,,, is always controlled by the supply of H,SeO, (or SeO,) via reaction (2~).

289

%edmM 1

Fig. 7. Influence of SeO, concentration on iL,, and i,,, (cf. Fig. 4c). The rotation and the Cu’+ concentration in the 0.5 M H,SO, electrolyte was 1 x 10m3 M.

speed was 1600 rpm

The data contained in Fig. 3 underline clearly the vast differences in the kinetic facility of compound formation that exist in the In + Se and Cu + Se binary systems. For example the current is negligible prior to the free In3’ + In0 regime (Fig. 3a) relative to the corresponding Cu case (Fig. 3b) in spite of the fact that the In3’ concentration exceeds the Cu2+ concentration in solution by approximately an order of magnitude. Cyclic photovoltammetry of the ternary Cu + In -t Se system We have shown previously [18] how coupling of periodic electrode illumination with voltammetric scanning or “cyclic photovoltammetry” is useful in the diagnosis of mechanistic pathways and product formation. Figure 8 contains a cyclic photovoltammogram for a glassy carbon electrode in 0.5 M H,SO, containing 2 X lop3 M CuSO,, 2 X lop2 M SeO, and 5 X 10m2 M In,(SO,),. The negative sweep was initiated at 0.35 V. A sharp cathodic wave “lc” occurs at - 0 V followed by a occurs soon after this broader feature “2~” at - - 0.20 V. Perceptible photoactivity wave is traversed and onto the peak “3~” at - - 0.45 V. The photoresponse (which is cathodic in nature) increases markedly at potentials past this peak and goes through a maximum across the wave “4~” on the forward scan. As the electrode potential is further decreased, the photoactivity diminishes finally culminating in the H+ reduction wave “5~“. On the return scan, the photocathodic response returns - -0.15 V. The onset of an anodic wave “4a” on the and persists until potentials return scan is also evident in Fig. 8 at potentials > 0.15 V. Based on the results from the binary systems as discussed in the preceding of Cu,_,Se, section, we assign the waves “lc”, “2~” and “3~” to the formation reduction of H,SeO, and the reduction of Cu 2_XSe respectively. We propose that the generation of H,Se via the last two reactions (reactions 4 and 20) results in its rapid assimilation with Cu2_Se and In 3+ leading to the formation of CuInSe,: Cu,_,Se

- H,Se

(ad) + In3’ = CuInSe,

+ 2 H+ + (1 - x) Cu

(22)

290

t lOpA

, 0.35

1

1

-0.05

-0.45 POTENTIAL/V(~S

/

-0.85

SCE)

Fig. 8. Cyclic photovoltammogram (potential scan-rate: 0.05 V/s) for a glassy carbon electrode in 0.5 M electrolyte containing 2X10m3 M CuSO,, 2~10~~ M SeO, and 5X10-* M In,(SO,),. The glassy carbon/electrolyte interphase was illuminated with white light ( - 100 mW/cm-2 with inctdent intensity not corrected for electrolyte absorption or cell reflectron losses) which was chopped manually during voltammetnc scanning. The arrow denotes the direction of the photoresponse which was always cathodic in this case (refer to text).

Note that reaction (22) is a more general representation of the analogs, reactions (14)-(16) in Table 2. The thermodynamic driving force for the underpotential assimilation of In3+ as CuInSe, is apparent from the entries for reactions (14)-(16) in Table 2. The concurrent occurrence of reactions 4a and 2c (the latter with x = 0) will consume a total of 13 electrons per mole of CuInSe, formed, the net process being representable as reaction (12) in Table 2. The cathodic feature “4~” in Fig. 8 is assigned to the formation of CuInSe, and the anodic current flow at “4a” is attributed to the composite stripping of CuInSe, and any CU~_~S~ formed on the return scan, from the electrode surface. The decrease of photoactivity at potentials negative of “4~” is attributed to the masking influence of an In0 overlayer atop the CuInSq. Similarly, the loss of photoresponse at potentials positive of - - 0.15 V on the return scan could be rationalized by band-bending considerations; i.e., the minority-carriers (electrons in this case), in order to be driven to the semiconductor-electrolyte interphase, require negative polarization past the flatband point [33]. Thus, although the CuInSe, layer is not stripped off till potentials close to - +0.15 V are accessed (“4a”), the

291

band-bending across the CuInSe,-electrolyte interphase does not favor collection of the photogenerated electrons (and thus a cathodic photoresponse). We attribute the cathodic photoresponse in Fig. 8 to photocathodic corrosion reactions involving the CuInSe, layer [24,34]. The possibility that excess Se, Cu2_.Se or InSe could contribute to the observed photoresponse was an important consideration. We rule this out on several grounds. Separate cyclic photovoltammetric experiments with Se, Cu,_,Se and InSe layers revealed a negligible photoresponse (see above). Second, CU*_~S~ is likely to exist as a degenerate p-type semiconductor [30] which would then be expected to show a negligible photoelectrochemical response. Weak photoconductive manifestations due to these species, however, cannot be ruled out, and the features in Fig. 8 prior to “3~” are possibly due to these effects. The fact that the CuInSe, layers as synthesized in situ in the experiments in Fig. 8 are p-type (rather than n-type) is also consistent with the kinetic sluggishness of the In3’ assimilation and the Cu richness predicated by reaction (22). We believe that these layers as synthesized contain excess Cu as the “dopant” [13,32]. An important feature of our model is that the cathodic reduction of the initial cu 2_XSe (cf. Fig. S), is a pre-requisite for the formation of CuInSe,. Thus this complex electrodeposition comprises reactions (2), (4), (20) and (22) acting in harness. This model also explains in a rational way why Cu,Se instead of CuInSe, was formed in some of the earlier attempts to electrosynthesize this latter material (cf. discussion contained in ref. 13). For example, if the system was potentiostated at some potential positive of the wave 3c, little decomposition of Cu,_,Se, and thus assimilation of In would be expected. This is indeed what was observed by previous authors. To further reiterate the evidence in our own work for the underpotential assimilation of In3’, we point out that the formation of CuInSe, occurs at potentials (- -0.45 V, Fig. 8) wherein the corresponding activity in the binary In + Se system is negligible (cf., Fig. 3). Further conclusive proof that the CU~_~S~ reduction and the consequent rise in the Se activity within the deposit lead to In3’ assimilation comes from shielding experiments such as those illustrated in Fig. 9. Figure 9 contains RRDE data wherein a Cu,_,Se coated disk was scanned from 0 V to negative potentials in a solution containing 0.01 M In3’ and 0.5 M H,SO,. A Hg-coated ring (cf. Experimental) was held at -0.65 V to monitor In3’ -+ In0 -0.45 V. a large cathodic current flow at reduction. As the disk is scanned past the disk is accompanied by shielding of the In3’ current at the ring. This key experiment is direct evidence for the underpotential In’+ assimilation into the The alternative possibility of thin film leading to CuInSe, formation. cu z_$ formation of In + Se compounds can be ruled out on kinetic grounds (cf. Fig. 3). Finally, Fig. 10 contains some data on the influence of the deposition potential on the photoactivity of the CuInSe, thin films. The family of cyclic photovoltammograms (only the return cycle is shown in each case) from (a) to (e) was generated by V. The total charge varying the switching potential from - 0.45 V to -0.75 consumed during the thin film deposition (on the forward, negative-going scan) was

292

VJ)

(b)

(4

0.0

0.2

4.4

DISK POTENTIAL/V(SCE)

Fig. GC The disk

-0.6

005

-0.15

-0.35 POTENTIAL/V(fi

-0.55

-0.75

SCE)

9. RRDE voltammograms (rotation speed: 100 r-pm) in the shielding mode for a Cu,_,Se coated disk which was scanned from 0 V to - -0.45 V (a) in a 0.01 M solution of In’+ in 0.S M H,SO,. shielding of a Hg-coated Pt ring current held at -0.65 V was correspondingly monitored (b). The was coated with Cu,_,Se under conditions similar to those employed in Fig. 3b.

Fig. 10. Family of cyclic photovoltammograms for a glassy carbon electrode with the switching potential beiig systematically varied: (a) - 0.45 V, (b) - 0.55 V, (c) - 0.60 V, (d) - 0.65 V, and (e) - 0.75 V. Only the return (positive-going) scan is shown in each case. All other conditions and solution composition same as in Fig. 8.

kept constant in this series of experiments. Thus, the photocathodic response increases in magnitude as the switching potential is decreased from - 0.45 V to - 60 V. This trend is explicable in terms of a decrease in the content of the CU*_~S~ phase. The photoresponse diminishes when potentials negative of - -0.65 V are accessed on the forward scan, presumably because of the codeposition of In0 as a separate phase. Therefore, there is a narrow window of potentials wherein the deposition of CuInSe, proceeds to yield a high quality of photoresponse. At the positive potential extremes, admixture with Cu 2 _,Se results; on the other hand, the use of too negative deposition potentials results in contamination with In’.

293

Our results on the influence of deposition potential are qualitatively similar to trends reported by Ueno et al. [15]. A major difference between our study and theirs, is the fact that these authors used potentiostatic deposition of films at fixed (discrete) potentials to assess the quality of the photoresponse and thin film composition. On the other hand, we have employed exclusively a (less tedious) potential-scanning procedure for this purpose. Thus, Ueno et al. claim stoichiometric compositions for CuInSe, deposited at - 0.80 V. In the scanning mode, we place the formation of “stoichiometric” CuInSe, thin films at a more positive potential of - -0.60 V. As Figs. 8 and 10 show, substantial In0 admixture (as a separate phase) results when potentials close to -0.80 V are accessed. A final caveat with regard to comparison of our results with previous work is that the photoresponse measured for our films and associated conclusions (see above) pertain to the “us-formed ” condition of CuInSe,. Post-deposition thermal annealing of these films, as employed by us and others in previous work [12,14,15] is expected to result in rather drastic morphological and compositional changes. A discussion of such factors is beyond the scope of this work. In conclusion, we have utilized thermodynamic considerations, voltammetry on the binary Cu + Se and In + Se systems and cyclic photovoltammetry of the ternary Cu + In + Se system to develop a new mechanistic model for the electrosynthesis of CuInSe,. This model pertains to an uncomplexed selenious acid deposition bath thin-film growth conditions. containing Cu’+ and In3’ ions and potential-cycled ACKNOWLEDGEMENTS

This research was supported in part by a grant (MSM-8617850) from the National Science Foundation. We thank one of the referees for constructive criticisms of an earlier version of this manuscript. REFERENCES

1 For example, J. Shay and J. Wemick, 2 3

4 S 6 7 8 9 10

Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applications, Pergamon, Oxford, 1975. For example, L.L. Kazmerski, F.R. White and G.K. Morgan, Appl. Phys. Lett., 29 (1976) 268; R.A. Mickelsen and W.S. Chen, ibid., 36 (1980) 5. S. Menezes, Appl. Phys. Lett. 45 (1984) 148; S. Menezes, H.J. Lewerenz and K.J. Bachmann, Nature (London), 305 (1983) 615; K.J. Bachmann, S. Menezes, R. KBtz, M. Fearheily and H.J. Lewerenz, Surf. Sci., 138 (1984) 475. S.P. Grindle, A.H. Clark, S. Rezaie-Serej, E. Falconer, J. McNeiIy and L.L. Kazmerski, J. Appl. Phys., 51 (1980) 10. W. Horig, H. Neumann, H. Sobotta, B. Schumann and G. Kuhn. Thin Solid Films, 48 (1978) 67. L.L. Kazmerski, Thin Solid Films, 57 (1979) 99. R.A. Mickelsen and W.S. Chen, Proc. 15th IEEE Photovoltaic Speciahsts’ Conference, Orlando, FL, IEEE, New York, 1981, p. 800. M. Gorska, R. BeaIieu, J.J. Loferski and B. Roessler, Sol. Energy Mater., 1 (1979) 313. G. Salviati and D. Seuret, Thin Solid Films, 104 (1983) L75. P. Raja Ram, R. Thangaraj and O.P. Agnihotri, Bull. Mater. Sci., 8 (1986) 279.

294

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

R.N. Bhattacharya, J. Electrochem. Sot., 130 (1983) 2040. R.N. Bhattacharya and K. Rajeshwar, Sol. Cells, 16 (1986) 237. G. Hodes and D. Cahen, Sol. Cells, 16 (1986) 245. F.J. Pem, J. Goral, R.J. Matson, T.A. Gessert and R. Noufi, Sol. Cells, 24 (1988) 81. Y. Ueno, H. Kawai, T. Sugiura and H. Minoura, Thin Solid Films, 157 (1988) 159. D. Pottier and G. Maurin, J. Appl. Electrochem., in press. E. Mori and K. Rajeshwar, J. Electroanal. Chem., 258 (1989) 415; E. Mori, C.K. Baker, J.R. Reynolds and K. Rajeshwar, ibid., 252 (1988) 441. K.K. Mishra and K. RaJeshwar, J. Electroanal. Chem.. in press. A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, Ch. 8, p. 280. G. Hodes, D. Cahen and M. Antelman, Report to Solar Energy Research Institute, Project No. lL-4-04051-l. 1984. M. Skyllas-Kazacos and B. Miller, J. Electrochem. Sot., 127 (1980) 869. M. Pourbaix, Atlas of Electrochemical Equilibria, Pergamon Press, New York, 1966. W.M. Latimer, Oxidation Potentials, Prentice-Hall, Englewood Cliffs, 1952. D. Cahen and Y. Mirovsky, J. Phys. Chem., 89 (1985) 2818 and references therein. K. Osseo-Asare and T.H. Brown, Hydrometallurgy, 4 (1979) 217. F.A. Kriiger, J. Electrochem. Sot., 125 (1978) 2028. J.J. Lingane and L.W. Niedrach, J. Am. Chem. Sot., 71 (1949) 196. M. Skyllas-Kazacos and B. Miller, J. Electrochem. Sot., 127 (1980) 2378. M. Tomkiewicz, I. Lmg and W.S. Parsons, J. Electrochem. Sot., 129 (1982) 2016. G. Hodes, T. Engelhard. J.A. Turner and D. Cahen, Sol. Energy Mater.. 12 (1985) 211. R. Temre, B. Theys, J. Rioux and C. Levy-Clement, J. Appl. Phys.. 57 (1985) 141. R. Noufi, R. Axton, C. Herrington and S.K. Deb, Appl. Phys. Lett.. 45 (1984) 668. H. Gerischer in H. Eyring, D. Henderson and W. Jost (Eds.), Physical Chemistry - An Advanced Treatise, Vol. 9A, Academic Press, New York, 1970, p. 463. D. Cahen, P.J. Ireland, L.L. Kazmerski and F.A. Thiel, J. Appl. Phys., 57 (1985) 4761.