Interfacial energetics of electrodeposited cadmium selenide in acetonitrile

307

J Electroanal. Chem., 239 (1988) 307-319 Elsevier Sequoia S.A., Lausanne - Printed

m The Netherlands

INTERFACIAL ENERGETICS OF ELECTRODEPOSITED SELENIDE IN ACETONITRILE

JEFFREY

P. SZABO

and MICHAEL

CADMIUM

COCIVERA

The Guelph- Waterloo Centre for Graduate Work m Chemtstty, lJnrversr{v of Guelph, Guelph, Ontano NlG 2 WI (Canada) (Received

22nd May 1987: m revtsed form 21st July 1987)

ABSTRACT height of thin film CdSe elecThe open circuit voltage. E,, was used to estimate the barner trodeposited from aqueous selenosulfite solutron in contact wtth various redox couples in acetonitrile. linearly with Films that were heat treated in an or oxygen had E, values that increased approximately the formal redox potenttal, E O, in the range - 520 mV i E o < + 100 mV (SCE) to a maximum of E, - 600 mV. For + 100 < E o c + 1300 mV, Ev is constant due to Fermi level pinning. a behavior present m single crystal CdSe. Cyclic voltammetry of thin film CdSe in the dark and the light gave useful from the difference m the anodic peak positions on quahtative mformation, but E, values determmed rllummated CdSe and Pt were smaller than those determined from direct measurement of the open circuit voltage. For redox potentials more negative than the flatband potential (Era), all films formed ohmtc junctions, which are characterized by reversible electrochemical behavior in the dark and the light. Films that were annealed in the absence of oxygen behaved electrochemically as degenerate senuconductors, since there was no rectification m the dark. even for redox potentials quite positive of Era.

INTRODUCTION

In the ideal model of a semiconductor electrolyte junction, the barrier height, E,, increases linearly with changing redox potential, E O, so long as E o is more positive than the conduction band edge, Ecs: E, = ECB -E”=&,-E”

(1)

In eqn. Cl), E,, is the potential at which there is no band bending or the flatband potential. Single crystal CdSe behaves almost ideally until some maximum value of E FB - E o is reached after which there is no further increase in E, [l]. Pinning of the Fermi level by surface states [2] has been used to explain this behavior. In the present study, we have examined the interface energetics of thin film CdSe electrodeposited from selenosulfite solution [3]. An earlier study of this material in water [4] presented evidence consistent with the presence of sub-bandgap states. The purpose of the present study was to determine whether thin film CdSe had similar 0022-0728/88/$03.50

0 1988 Elsevier Sequoia

S.A.

308

interfacial properties to single crystal CdSe, which exhibited Fermi level pinning in the presence of various redox couples. The presence of oxygen in the post-deposition heat treatment of electrodeposited CdSe has been found to have a dramatic effect on the hole diffusion length, the donor density, quantum efficiency, and power conversion efficiency of films used in photoelectrochemical cells [3,5]. For this reason, we also probed the effect of heat treatment atmosphere on the interfacial properties of the film. EXPERIMENTAL

Thin film CdSe was cathodically electrodeposited from aqueous selenosulfite solution. The electrodeposition conditions and method of deposition solution preparation have been described in detail elsewhere [5]. CdSe films were cathodically electrodeposited on titanium substrates (1 cm X 3 cm X 2 mm) at - 1.100 V vs. SCE for 30 min. giving an estimated thickness of 2.2 pm prior to annealing. A total of nine films were deposited and annealed at 500°C for 10 min. Three films were annealed under vacuum, three were annealed in air, and three were annealed in oxygen. Usually, values given in Tables l-3. are averages of measurements done on 3 different samples. After annealing, the films were dipped in 0.1 M HCl for 5-10 s, rinsed with water, and dipped in 1 M NaCN for 5-10 s. The films were masked with black sealing wax to create exposed spots of CdSe having areas of about 0.04 cm2. The wax was completely insoluble in acetonitrile, but was easily removed with 1,1,2-trichloroethane. Reproducible results were obtained if different spots were exposed for each set of experiments. Platinum wire was prepared for cyclic voltammetry (CV) by cleaning in concentrated nitric acid, followed by rinsing with water and heating in an oxygen/propane flame for 30 s. The wire was quenched in triply distilled water, and rinsed with acetonitrile. Redox couples were obtained from commercial sources and used without further purification. Prior to measurements for each semiconductor spot, CVs on clean platinum wire were obtained for each redox couple to insure that the system was well behaved electrochemically. The following abbreviations are used for chemicals: TBAP = tetrabutylammonium perchlorate; MV2+ = N, N ‘-dimethyl-4,4’-bipyridinBQ = benzoquinone; ium; TMPD = N, N, N ‘, N ‘-tetramethyl-p-phenylenediamine; Neutral HQ = hydroquinone; Ru(bipy):+ 0 = tris(2,2’-bipyridyl)ruthenium(II). anhydrous alumina, TBAP. and 3A molecular sieves were dried under vacuum at 150 o C for 24 h before use. All other chemicals, with the exception of acetonitrile, were not purified or dried before use. Acetonitrile was dried and prepared for voltammetric measurements as described below. All electrolyte solutions contained 0.1 M TBAP in HPLC grade acetonitrile. Electrolyte solutions were dried over 3A molecular sieves for at least 3-4 days, and then passed through a column of activated alumina directly into the electrochemical cell. Nitrogen was bubbled through the solution for 10 min and a positive pressure of N, was maintained during all electrochemical measurements.

309

Cyclic voltammetry was performed with a BAS cyclic generator, using either a Pt or a CdSe working electrode. The data were collected with a Tecmar A/D board interfaced with an IBM PC. In addition to the working electrode, the cell contained a platinum counter electrode and a Ag/O.l M AgNO, + 0.1 M TBAP + CH,CN reference electrode, which was separated from the main cell compartment by a salt bridge containing electrolyte solution. Ionic contact was maintained by the use of leaky platinum/glass joints. The potential of the reference electrode was initially + 350 mV vs. SCE, but drifted slowly over the period of a few days. All potentials are reported relative to SCE. A sweep rate of 100 mV/s was used in all cases and the redox concentration was usually l-2 mM. CVs using CdSe were measured in the dark and in the light. A 300 W ELH tungsten halogen lamp provided > 500 mW/cm’ of white light for illumination of the CdSe electrode. The light intensity was measured with a YSI-Kettering Model 65A radiometer. The open circuit photovoltage was measured in the presence of various redox couples by connecting a digital voltmeter between the CdSe electrode and the platinum counter electrode. No external bias was used. The difference between the cell voltage in the light and dark was taken to be the open circuit voltage. Usually either the reduced (R) or oxidized (Ox) form of the redox couple was available, so electrolysis of one form was done to create a 1 : 1 mixture for the open circuit voltage measurements. Electrolysis was carried out in a two compartment cell at constant potential using large area vitreous carbon electrodes. The ratio of Ox : R was estimated from the decrease in the current for the rapidly stirred solution. If the measurements were done without prior electrolysis, the values for E, were only about 5-10s higher, indicating the presence of a small amount of the oxidised form. The photoelectrochemical cell used for power measurements consisted of a Cu ZS coated brass mesh cathode connected through a variable resistor to the CdSe photoanode. The voltage drop across the load was measured using a digital voltmeter. No external bias was used. The redox solution used for power measurements was 1 M Na,S, 1 M S, and 1 M NaOH. Solar simulated radiation was provided by an ELH tungsten iodide lamp along with an Optikon KG-2 infrared filter. The light intensity, measured with a YSI-Kettering Model 65A radiometer. was 76 mW/cm*. RESULTS

Two methods were used to measure the open circuit photovoltage, E,, of CdSe in various redox solutions. The first method has been used extensively to study a variety of single crystal semiconductors [2,6-91 including CdSe [l]. In this method, which will be called Method I, E, is given by Ev = %,,cdsr

- &A,P~

(2)

in which EpA,Pt and E,, CdSe are the anodic peak potentials for the redox couple on platinum and the illuminated CdSe electrode, as observed in the CVs. Strong light intensity and low concentration of redox couple were used to insure that the

310

photocurrent was controlled by diffusion of the electroactive species to the electrode, and not excitation rate. The second method, Method II, consisted of measuring the open circuit photovoltage directly: a digital voltmeter was connected between the CdSe electrode, and the platinum counter electrode. The difference between the value measured in the dark and the light was taken to be the open circuit photovoltage. The CV measurements indicate three types of behavior depending on the nature of the film heat treatment and the E o of the redox couple. The first is a behavior indicative of a Schottky barrier and is found for air or oxygen annealed films in the presence of redox couples having sufficiently positive E o values. The second is a behavior indicative of the absence of a Schottky barrier (“ohmic contact”) found for these films in the presence of redox couples having very negative E o values. The third is the behavior of vacuum annealed films indicating the absence of a barrier in the presence of any of the redox couples studied, spanning E o values from - 1700 to + 100 mV (SCE). The first type is illustrated in Fig. 1, which shows an example of the voltammetric behavior of the air annealed CdSe in the presence of TMPD. In the dark (Fig. lb), the film displays rectifying behavior, that is, there is no oxidation of the TMPD. In the light (Fig. lc), both oxidation and reduction occur, at potentials several hundred millivolts more negative than on platinum (Fig. la). Note that only the more negative set of peaks (E o = + 100 mV on Pt) could be observed because photocor-

Fig. 1. Cyclic voltammetry of TMPD in CH,CN+O.l dark. and (c) an annealed CdSe in the light.

M TBAP on (a) Pt. (b) air annealed CdSe in the

311

M TBAP on (a) air annealed CdSe in the Fig. 2. Cychc voltammetry of Ru(bipy)3 *+ in CH,CN+O.l dark, and (b) air annealed CdSe in the light. The extra peak in Fig. 2b at - 1000 mV is believed to be due to reduction of photocorrosion products.

I

Fig. 3. Cyclic voltammetry the dark, and (c) vacuum

of TMPD in CH,CN + 0.1 M TBAP on (a) Pt. (b) vacuum annealed Cd.% m the light.

annealed

CdSe m

312 TABLE E,

1

as determined

Redox couple

Ru(bipy):‘Ru(bipy)T” Ru(btpy): +‘+ MV ? +,‘+ TCQN “Fe( n5-C,Me,)~‘0 TMPD +‘”

by voltammetnc

measurements

E O/mV a

-1700 - 1490 -1300 - 450 -300 -120 + 100

(E PA.CdS.z-

EPA.p, )/mv

b

vacuum

air

oxygen

4 18 23 94 17 - 16 55

0 9 1 86 - 357 - 370 -394

-5 10 NA 107 -319 -344 - 398

A E” values are with respect to SCE and are taken from ref. 1, except for TCQN”-. measured with respect to MV’+‘+. b Values are averages of n = 3 trials. The standard errors (s/n”‘) for the average photopotentials were approximately 5% of the mean Eys.

which open

was

ctrcuit

rosion of the semiconductor, which becomes significant at 0 mV vs. SCE, obscures the TMPD’+‘+ peak. The second type is illustrated in Fig. 2, which shows the CV of air annealed CdSe in the presence of Ru(bipy):+. The three sets of waves correspond to Ru(bipy):/-, Ru(bipy):“, and Ru(bipy):+‘+ at potentials of - 1730, - 1490, and - 1300 mV vs. SCE on Pt. There is similar behavior in the dark (Fig. 2a) and the light (Fig. 2b) for all three sets of waves. The third type of behavior is in contrast to that for air-annealed and oxygen-annealed films. As illustrated in Fig. 3b, vacuum-annealed films show no rectifying behavior in the dark since both the oxidation and reduction waves for the TMPD+“’ couple are observed and occur very close to those of the corresponding waves at the Pt electrode. Furthermore, when this film is illuminated, these waves shift only very slightly. Similar results are obtained for all redox couples studied (E o potential range: - 1700 mV to + 100 mV). Table 1 summarizes E, as determined by voltammetric measurements for a number of redox couples. The standard error (s/n”2) was usually 5% of the mean E,. The most positive redox couple that could be used was the TMPD+” couple that has an E o = + 100 mV vs. SCE. Any couple more positive than + 100 mV was obscured by severe photocorrosion. Table 2 gives the E, as determined by direct measurement of the open circuit photovoltage (Method II). This method tended to give values that were larger than those determined by Method I. For instance, air annealed films in contact with TMPD produced E, magnitudes of 394 mV and 574 mV according to Method I and Method II. respectively. For air and oxygen annealed films, the open circuit photovoltage changes approximately linearly with E” from - 520 mV to + 100 mV, then remains relatively constant at E, = 600 mV from + 100 mV to + 1300 mV vs. SCE (Fig. 4a). There was no significant difference between the open circuit voltages of air and oxygen annealed samples. The standard error (.s/~‘/~) was usually less annealed films had a small open circuit than 5% of the mean E,. Vacuum

313 TABLE E,

2

as determined

by direct measurement E “/mV

Redox couple

BQ/HQ

of the open circuit

a

Open circuit voltage/mV

- 520

MV2 +/+ TCQN “Fe( q5-C,Me5)~/o TMPD+” Fe(q’-C,H,):” Ru(bipy):+/2+

photovoltage

- 450 -300 -120 +100 + 430 +1300

b

vacuum

air

oxygen

- 103 -61 -112 -51 -120 -219 - 369

- 171 - 359 - 279 -421 - 574 - 563 -635

-186 - 365 - 282 - 320 - 590 - 598 - 587

a E” values are with respect to SCE and are taken from refs. 1 and 8, except for TCQN’/which was measured with respect to MV’+‘+. b Values are averages of n = 3 trials. The standard errors (s/n ‘12) for the average open circuit voltages were approximately lo-20 mV.

photovoltage

of about

SCE. This increases (Fig. 4b).

- 100 mV in the potential range - 520 mV to + 100 mV vs. slowly to E, = 369 mV at E o = + 1300 mV for Ru(bipy):“*+

.

-*&lo

-500

0

Redox

500

potential/mV

Fig. 4. Absolute value of the open circuit potential for air annealed CdSe.

1000

I

(vs. SCE)

photovoltage

determined

by Method

II as a function

of redox

314 DISCUSSION

The voltammetric behavior of an n-type semiconductor depends strongly on the electrochemical potential of the solution in contact with it, and has been classified according to E o by Wrighton and co-workers [l,lO] as follows: band edge. Reversible Class I: E o is more negative than Z-n, the conduction behavior is observed at the semiconductor in the dark and the light, i.e. it behaves as a metal. Class II: E o is slightly positive of ECB. There is a small anodic peak in the dark, which increases and moves negative in the light. Class III: E o is positive of E,, and the open circuit voltage, Ev, is proportional of the reduced to the barrier height, E, = E o - Ew. There is no dark oxidation species, but there is oxidation in the light. Class IV: E o is positive of EC, and E, is independent of E O. This situation is known as band edge unpinning. There is no dark oxidation of the reduced species, but there is oxidation in the light. Class V: E o is so positive (or negative) that decomposition of the semiconductor becomes kinetically important. In order to distinguish between Class III and Class IV behavior, it is necessary to examine redox couples having a wide range of E O. Ideally, the range of redox potentials should be wider than the bandgap (which is 1.7 V for CdSe). This is not always possible as explained below. The most positive redox couple that is useful in determining barrier heights from voltammetric measurements depends on the relative rates of hole capture by the reduced form of the redox couple (eqn. 3) and photoanodic decomposition of the CdSe (eqn. 4): R+h++Ox+

(3)

CdSe + 2 h+ + Cd’+ + Se

(4)

Decomposition interferes with voltammetric measurements in two ways. First, the decomposition current obscures the position of the photoanodic peak, which must be known to calculate E, (eqn. 2). Second, an insulating layer of Se grows on the surface, which may decrease the measured value of E,. This was suspected to be the case for single crystal CdSe in acetonitrile, in which Ev apparently begins to decrease as E” becomes more positive than + 280 mV vs. SCE [l]. Redox couples more positive than +700 mV could not be studied due to severe photocorrosion of the single crystal semiconductor. For the polycrystalline thin film CdSe used in this study, the photocorrosion current obscured the faradaic current for couples with E o more positive than + 100 mV. There is some evidence to suggest that photocorrosion occurred during cyclic voltammetry in the light to a significant extent even if there was no apparent distortion in the voltammetric peaks. Freshly exposed spots on the film surface yielded reproducible results, so long as the first cycle of the CV was taken. However, the second and third cycles often gave peak positions significantly more positive

315

Fig. 5. Cyclic voltammetry of MV ‘+ in CH,CN +O.l M TBAP on (a) air annealed (b) air annealed CdSe in the light. The average onset potential for photooxidation - 600 mV.

CdSe in the dark, and based on 3 trials IS

than that for the first CV cycle. Also, when the insulating sealing wax was removed to reveal both the exposed and unexposed portions of the CdSe film, there was often a difference in texture or color between exposed and unexposed areas. Due to the possibility of photocorrosion, only previously unexposed portions of the CdSe surface were used and a single cycle of a CV in the light was recorded. Despite these precautions, it appears that the voltammetric data give much smaller values of E, than the open circuit voltage data measured directly (Tables 1 and 2). There are probably two reasons why Method I does not give E, values as large as those measured directly by method II. First, for many of the redox couples, Method I involves scans to potentials that are sufficiently positive to cause photocorrosion if the interfacial electron transfer process is not fast enough. Second, slow interfacial electron transfer at CdSe would move the peak potential more positive and reduce E,. Method II does not have these limitations because current measurement is not required. Other conceivable processes such as direct chemical reaction would be expected to occur in both methods. As a result, Method II could be employed for a substantially more positive range of redox potentials (from -520 mV to + 1300 mV) than was possible with Method I ( - 1700 mV to + 100 mV). The onset of the photoanodic current for the redox couple with the most negative irreversible reduction has been suggested as a good measure of E,, [8]. The average onset of the photocurrent of the MV2+/+ couple occurs at -600 mV (Fig. 5). However, the MV+” peak, which is well behaved on platinum (E o = - 850 mv),

316

shows no appreciable peaks in the light or dark. As a consequence, the onset potential may not be a good measure of the flatband potential. Another estimate of the flatband potential of thin film CdSe in acetonitrile can be made by extrapolating the open circuit photovoltage to zero in Fig. 4a. The x-intercept gives E o = - 920 mV vs. SCE, and the slope of the linear region is 0.5. It is not useful to extrapolate the data for vacuum annealed films. Previous studies in 1 M S2-, 1 M OH- solution found no significant difference for the flatband potentials of vacuum and air annealed thin film CdSe [5]. If we combine the voltammetric data with the open circuit photovoltage voltage data (Table 2 and Fig. 4) then we may classify the voltammetric behavior of the thin film CdSe according to the scheme outlined above. Class I behavior is exhibited for those couples lying negative of E,,. Ru(bipy)i’-, Ru(bipy)l’O, and Ru(bipy):+/+ all behave identically at Pt and in the dark and light at CdSe, regardless of post-deposition treatment (i.e. air, oxygen, or vacuum annealing atmosphere). Furthermore, waves are not shifted when the film is illuminated. This behavior for CdSe is indicative of zero barrier height. Air and oxygen annealed films in contact with redox couples that lie between -600 mV and + 100 mV give E, values (Method II) that change linearly with redox potential. In addition, the films exhibit rectification in the presence of these couples (no anodic wave) in the dark, a characteristic of Class III. For couples more positive than + 100 mV, E, levels off at - 600 mV for air and oxygen annealed films. Similar behavior for single crystal CdSe has been attributed to pinning of the Fermi level by interface states, Class IV [l]. Also, for single crystal CdSe, E, levels off at - 800 mV for redox potentials > - 200 mV vs. SCE [l]. The origin of the interface states in single crystal CdSe is not clear because the voltammetric response was independent of surface pretreatment, even though the type of etch (oxidizing or reducing) greatly affected the surface composition as measured by Auger spectroscopy [l]. The value of E, can also be independent of E o when the depletion layer of the semiconductor is replaced by an inversion layer [ll] which usually occurs when the band bending is greater than l/2 Eg. For the polycrystalline CdSe in the present study. Eg is 1.73 eV [5]. Consequently, it would appear that inversion may not account for the behavior of this material. A similar conclusion was made for the single crystal material [l]. The vacuum annealed films exhibit substantially lower E, values than those for air or oxygen annealed films. In addition, the voltammetric response of vacuum annealed films seems to conform to Class I behavior for all redox couples listed in Table 1. Any explanation for the differences in the voltammetric behavior between vacuum annealed and air (or oxygen) annealed CdSe films must be consistent with differences in hole diffusion length (Lr) and donor density (No). Vacuum-annealed films have a much shorter diffusion length and larger donor density [5] than have air-annealed films (Table 3). It seems likely that the vacuum annealed films have a higher density of intrabandgap states in the bulk, at the surface, and/or at grain boundaries than air annealed films. These states could act as traps and recombination centers, which would be consistent with a low value of L,, and high value of

317 TABLE

3

Hole diffusion

lengths

and donor

densities

Treatment

lo6 L,/cm

N,/cm-’

Nitrogen Vacuum Air Oxygen

0.241 0.205 5.18 7.72

6.97 1.47 2.57 2.07

x x x x

10” 10’8 10” 10”

Nn [12]. Annealing in air or 0, reduces the density of these states, thereby increasing L, and quantum efficiency and reducing No. The non-rectifying behavior of the vacuum annealed films can also be explained by the higher density of states. One effect of the bulk donor states would be to decrease the depletion layer width. This effect, in combination with a high density of deep level states in the depletion layer, would increase the probability of charge transfer by tunneling through the barrier at the semiconductor-solution interface. Another contributing mechanism for this quasi-ohmic behavior may be generationrecombination in the depletion layer. The nature of the excess bulk donor states for vacuum annealed CdSe is not certain, but an explanation consistent with the data is that they are selenium vacancies (cadmium interstitials). Since oxygen is isoelectronic with selenium, it is possible that annealing in air or oxygen fills some of the selenium vacancies with oxygen atoms, thus decreasing the donor density and the conductivity. Other studies have found a large decrease in the dark conductivity of CdSe after high temperature air annealing, and this has been attributed to oxygen doping [13]. The power conversion efficiencies of thin film CdSe in sulfide + polysulfide solution showed some interesting results (Table 4). Air annealed films had power conversion efficiencies that averaged 120 times higher than those of vacuum annealed films. As indicated by Table 4, this is partly due to larger open circuit voltages, and partly due to larger short circuit currents and fill factors. Although there is a large improvement in the efficiency in going from a vacuum to an air atmosphere (20% oxygen), there is no further improvement in going to a 100% oxygen annealing atmosphere. A comparison of the data in Tables 1 and 2 indicates that Method II is the method of choice in the study of the interface energetics of polycrystalline CdSe and

TABLE

4

Power conversion

efficiencies

of thm film CdSe in sulfide + polysulfide

Treatment

Power/%

&/mV

I,,/mA

Vacuum Air Oxygen

0.0521 6.26 4.47

189 571 472

0.588 19.3 19.7

solution cm-*

FF/% 35.6 43.3 36.6

318

perhaps other polycrystalline semiconductors. The E, values obtained by Method I are substantially lower than those by Method II. Furthermore, data obtained by Method I lead to the erroneous conclusion that Fermi level pinning occurs for E o > - 300 mV (SCE), which is 400 mV more negative than the value found by Method II. The poor results obtained by Method I may be a reflection of the slow electron transfer kinetics for the redox reaction at the polycrystalline surface. The possibility that slow interfacial kinetics can be a factor has been considered by others [14]. Another factor is photocorrosion, which becomes important as the electron transfer rate decreases. Furthermore, photocorrosion reduces the range of redox potentials over which Method I can be applied, in comparison to Method II. Thus, while Method I is useful for E" values no greater than + 100 mV, Method II extends to + 1300 mV. In contrast to our polycrystalline material, single crystal CdSe permits the use of Method I up to E o = +700 mV [l], indicating a greater resistance to photocorrosion for this material. The increased susceptibility to photocorrosion of the polycrystalline material may be the result of the large number of grain boundaries. At any rate, despite the greater resistance of the single crystal to photocorrosion, Method I still cannot be extended to a redox potential range as positive as that used for the polycrystalline material in Method II. Furthermore, Method I for single crystal CdSe indicates that Fermi level pinning starts at E o = - 120 mV [l], but Method II applied to thin film CdSe indicates a more positive value of + 100 mV. Although this difference may be due to differences in the two materials, a comparison of the two methods using single crystal CdSe may be instructive. The maximum open circuit photovoltage for single crystal CdSe (ca. 800 mV) is larger than that found for the polycrystalline material (Table 2). Although this difference may be the result of a difference in the degree of band bending in the two materials, one cannot rule out the alternate possibility that these materials have the same barrier. In this description, the smaller maximum E, value for the polycrystalline material is due to a larger recombination rate in the depletion layer, caused by bulk sub-bandgap states. CONCLUSION

Polycrystalline CdSe prepared by cathodic electrodeposition from selenosulfite solution and annealed in either air or oxygen has similar voltammetric properties to single crystal CdSe. Both experience an increasing barrier height as the redox couple becomes more positive until some point is reached beyond which no further increase in E, occurs. Fermi level pinning due to surface states has been used to explain this effect. Thin film CdSe that has been annealed in the absence of oxygen behaves electrochemically as a degenerate semiconductor, or metal. We have suggested that a large density of sub-bandgap states, in the bulk and at the surface. are responsible for this behavior. These states are partly removed by air annealing, possibly by oxygen doping of the lattice. Finally, we have shown that direct measurement of the open circuit voltage is a good method for studying the interface energetics of semiconductors that are susceptible to photocorrosion, such as polycrystalline CdSe.

319 ACKNOWLEDGEMENTS

This work was supported in part by a grant to M.C. from the Natural Sciences and Engineering Research Council of Canada. REFERENCES 1 A. Aruchamy, J.A. Bruce,,S. Tanaka and M.S. Wrighton, J. Electrochem. Sot.. 130 (1983) 359. 2 A.J. Bard, A.B. Bocarsly, F.-R. Fan, E.G. Walton and M.S. Wrighton, J. Am. Chem. Sot.. 102 (1980) 3611. 3 J.P. Szabo and M Cocwera, J. Electrochem. Sot., 133 (1986) 1247. 4 M. Co&era, W.M. Sears and S.R. Morrison, J. Electroanal. Chem., 216 (1987) 41. 5 J.P. Szabo and M. Cocivera, J. Appl. Phys., 61 (1987) 4820. 6 F.-R. Fan and A.J. Bard, J. Electrochem. kc., 128 (1981) 945. 7 S.N. Frank and A.J. Bard, J. Am. Chem. Sot., 97 (1975) 7427. 8 P.A. Kohl and A.J. Bard, J. Am. Chem. Sot.. 99 (1977) 7531. 9 F. Di Quart0 and A.J. Bard, J. Electroanal. Chem., 127 (1981) 43. 10 L.F. Schneemeyer and M.S. Wrighton, J. Am. Chem. Sot., 102 (1980) 6964. 11 J.A. Turner, J. Manassen and A.J. Nozik, Appl. Phys. Lett., 37 (1980) 488. 12 J. Gautron and P. Lemasson. J. Cryst. Growth, 59 (1982) 332. 13 P.E. McQuaid, Vide Couches Minces (Suppl. Proc. 8th Int. Vacuum Congr.), 201 (1980) 707. 14 N. Chandra, J.K. Leland and A.J. Bard, J. Electrochem. Sot.. 134 (1987) 76.