Renewable Energy 23 (2001) 471–481www.elsevier.nl/locate/renene

Preparation and characterization of thin films ofelectrodeposited CdTe semiconductors

M. Solimana,*, A.B. Kashyoutb, M. Shabanac, M. Elgamala

a Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University,PO Box 832, Alexandria, Egypt

b Institute of New Materials and Advanced Technologies, Mubarak City for Scientific Research andTechnological Applications, El-Agami, Alexandria, Egypt

c Department of Electrical Engineering, Faculty of Engineering, Alexandria University, Alexandria,Egypt

Abstract

Thin films of CdTe semiconductors were prepared by electrodeposition technique in aqueoussolutions. The deposition mechanism was investigated by cyclic voltammetry. The potentialregions for the formation of the n-CdTe and p-CdTe films were determined. The structure,composition and morphology characteristics of as-deposited thin films of CdTe grown onSnO2/glass and CdS/SnO2/glass were investigated by XRD, EDAX and SEM techniques. Theoptical properties were measured to determine the absorption coefficient and band gap values.The as-deposited CdTe films grown on SnO2/glass contained free Te while those grown onCdS/SnO2/Glass did not contain this phase. The CdTe has the cubic structure with strong (111)orientation. The EDAX analysis showed a nearly stiochiometric Cd:Te ratio. The band gaphas a value of 1.48 eV, which is in a good accordance with those reported in the literature.The effect of annealing at 350 and 400°C after CdCl2 treatment on the structure and mor-phology was also examined. 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction

A solar cell made of polycrystalline cadmium telluride thin film is one of the mostpromising low cost candidates for terrestrial photovoltaic applications. It is a directenergy gap material with high absorption coefficient [1]. CdTe was prepared by usingdifferent techniques such as electrodeposition [2–4], closed-space sublimation CSS

* Corresponding author.

0960-1481/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.PII: S0960 -1481(00 )00153-1

472 M. Soliman et al. / Renewable Energy 23 (2001) 471–481

[5], spray pyrolesis [6], screen printing [7], atomic layer epitaxy [8] and rf sputter-ing [9].

Electrodeposition of CdTe has become a promising method for producing efficientthin film solar cells. The mechanism of electrodeposition of CdTe thin films wasstudied by cyclic voltammetry technique to determine the appropriate Cd and Teconcentrations as well as the potential region for forming both n-type and p-typesemiconductors. Also, the bath temperature was examined to determine the degreeof crystallinity of the deposited film. The effect of using different substrates of SnO2

covered glass or CdS/SnO2 covered glass on the properties of the film was studied.The structure of the film was measured by X-ray diffraction technique. The mor-phology and chemical composition were determined by using scanning electronmicroscopy (SEM) and energy dispersive X-ray analysis (EDAX) respectively.

2. Experimental work

The mechanism of electrodeposition of CdTe thin films was studied by cyclicvoltammetry method. A three electrode cell was connected to a Wenking MP87potentiostat and Voltage Scanner model MVS 87. The reference electrode was aSaturated Calomel Electrode (SCE), the working electrode was either SnO2 coveredglass or CdS/SnO2 covered glass and the counter electrode was a platinum sheet(2×2 cm2). The aqueous solution had different concentrations of bothCdSO4.(8/3)H2O (Sigma Chemicals 99.97%) and TeO2 (Alderich Chemicals99.995%). The pH was adjusted to 2 by adding H2SO4. The SnO2 had a sheet resist-ance of 10V/h. The CdS thin films were prepared by electrodeposition from aqueoussolution containing 0.2 M CdCl2 and 0.2 M Na2 S2O4.5H2O. The bath temperaturewas kept constant at 90°C and the deposition time was 2 h. The CdTe depositionwas also carried out at 90°C for 2 h. The as-deposited thin films were annealed at350 and 400°C for 10 min using Lindberg furnace. Their X-ray diffraction patternswere obtained from a Philips PW1710 diffractometer using Cu–Kα radiation at 40kV, 30 mA and scanning speed 0.02°/s. The Scanning electron microscopy (SEM)was carried out using SEM model Philips XL 30 attached with EDAX unit for study-ing the morphology, grain size and chemical composition of both CdS and CdTedeposited thin films. Band gap of the resulting materials were monitored by opticalabsorption using a V-570 model NIR–UV visible spectrophotometer.

3. Results and discussion

3.1. Cyclic voltammetry

Fig. 1 shows the cyclic voltammogram obtained from the electrodeposition bathof an aqueous solution at pH 2 containing 1023 M TeO2. The potential was changedbetween+1 and20.6 V(SCE). The bath temperature was kept constant at 30°C andthe scanning rate was 4 mV/s. The first cycle (a) was reversed at20.36 V(SCE)

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Fig. 1. Cyclic voltammogram of electrodeposition bath of an aqueous solution (pH 2) containing 1023

M TeO2 and reversed at different potentials (bath temperature=30°C).

and a cathodic peak was observed at20.35 V(SCE), whereas an anodic peak wasdetected at 0.5 V(SCE). The second and third cycles were reversed at20.46 and20.6 V(SCE) respectively. The two cycles had the same value of the cathodic peakwhile the anoodic peak was shifted positively to+0.55 V(SCE). The reduction peakat 20.35 V(SCE) was assigned for the reduction of tellurium according to the reac-tion:

HTeO+213H+14e−⇔Te012H2O

The standard reduction potential of Te is+0.31 V(SCE) [10]. The value recordedhere is less than this value. The actual value was decided by the activity of HTeO2

+

and Te0 and the pH of the solution.Fig. 2 illustrates the cyclic voltammogram of the electrodeposition bath of an

aqueous solution of pH 2 containing 0.05 M CdSO4.(8/3)H2O carried out between1 and20.7 V(SCE) and reversed at different potentials. The bath temperature waskept at 30°C and the scan rate at 4 mV/s. A reduction peak was monitored at20.65V(SCE) while an anodic peak was observed at20.5 V(SCE). The reduction peakwas assigned for the reduction of Cd.

The cyclic voltammogram of the electrodeposition bath of an aqueous solution (pH2) containing 0.05 M CdSO4.(8/3)H2O and 1023 M TeO2 and carried out between+1and 20.75 V(SCE) is shown in Fig. 3. When the potential was reversed at20.5V(SCE) (cycle a), two cathodic and anodic peaks were monitored at20.3 and+0.5V(SCE) respectively. In the second and third cycles (b and c respectively) whichwere reversed at20.62 and20.75 V(SCE), another cathodic peak was recorded at20.55 V(SCE). The anodic peak was slightly shifted to+0.48 V(SCE). The cathodicpeaks at20.3 and20.55 V(SCE) accounted for the reduction of Te and Cd respect-

474 M. Soliman et al. / Renewable Energy 23 (2001) 471–481

Fig. 2. cyclic voltammogram of electrodeposition bath of an aqueous solution (pH 2) containing 0.05M CdSO4.(8/3)H2O (bath temperature=30°C).

Fig. 3. Cyclic voltammogram of electrodeposition bath of an aqueous solution (pH 2) containing 0.05M CdSO4.(8/3)H2O and 1023 M TeO2 (bath temperature=30°C).

ively. The shift in the anodic peak may be due to the formation of the CdTe semicon-ductor which was explained as follows:

HTeO+213H+14e−⇔Te012H2O

CdSO4⇔Cd2+1SO2−4

Cd2+1Te012e−⇔CdTe

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Here, the region for the formation of CdTe semiconductor lies between20.48 and20.65 V(SCE) where the reduction of Cd occurs. Also, it was observed that thereduction peak of Cd was shifted towards a more positive voltage of20.55 V(SCE).

The effect of the bath temperature on the reduction/oxidation process and thecurrent density is shown in Fig. 4. Increasing the temperature from 30 to 70°C thento 90°C had a great effect on both reduction and oxidation currents. The oxidationcurrent increased from 1.5 mA at 30°C to 9.5 mA at 90°C which is an indicationfor the increase in mass transport mechanism. Also, both the reduction peaks of Teand Cd were shifted positively which may be due to the change in Te solubility.

3.2. X-ray diffraction studies

The X-ray diffraction patterns of as-deposited and annealed CdTe on SnO2/glassand deposited at20.6 V(SCE) for two hours are shown in Fig. 5. The as-depositedfilms [Fig. 5(a)] contained both cubic CdTe structure and free tellurium phase. TheCdTe had a strong preferred orientation of the (111) planes parallel to the substrate,with traces of (220), (311), (422) and (511) planes. After annealing at 350°C [Fig.5(b)], the intensity of both (111) and (511) peaks were decreased accompanying withan increase in (200) and (311) peaks. At 400°C [Fig. 5(c)], the free tellurium wasdisappeared and all other CdTe peaks were comparable with CdTe powder structure.

The XRD spectra of CdS thin films deposited by electrodeposition technique at20.65 V(SCE) are illustrated in Fig. 6. The as-deposited film [Fig. 6(a)], showed astrong peak (002) of the hexagonal structure of. Upon annealing to 350°C [Fig. 6(b)],other peaks, (100) and (101) were detected. At 400°C [Fig. 6(c)], the intensity ofthe peaks was increased.

The XRD patterns of both as-deposited and annealed CdTe on CdS/SnO2/glassand deposited at20.65 V(SCE) are illustrated in Fig. 7. The structure of the as-

Fig. 4. Cyclic voltammogram of electrodeposition bath of an aqueous solution (pH 2) containing 0.05M CdSO4.(8/3)H2O and 1023 M TeO2 and were carried out at different bath temperatures; (a)T=30°C,(b) T=70°C and (d)T=90°C.

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Fig. 5. X-ray diffraction patterns of as-deposited and annealed CdTe thin films grown on SnO2 coveredglass: (a) as-deposited, (b) annealed at 350°C for 15 min and (c) annealed at 400°C for 15 min.

Fig. 6. X-ray diffraction patterns of CdS/SnO2 films grown by electrodeposition on (a) as-deposited and(b) air annealed at 400°C for 10 min.

deposited film [Fig. 7(a)], had a strong preferred orientation of the (111) peak. Theother peaks were very weak. After annealing at 400°C for 10 min [Fig. 7(b)], allother peaks showed intensities comparable to pure CdTe powder specimen.

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Fig. 7. X-ray diffraction patterns of CdTe thin films grown on CdS/SnO2 covered glass: (a) as-deposited,(b) annealed at 350°C for 15 min and (c) annealed at 400°C for 15 min.

3.3. Morphology and composition analyses

The scanning electron micrographs (SEM) of the CdTe of as-deposited filmsannealed at different temperatures are shown in Fig. 8. The as-deposited film [Fig.8(a)] showed some grains of about 1µm size. The number of these grains wereincreased for the sample annealed at 350°C [Fig. 10(b)]. At 400°C, the large increasein the number of grains suggested a process of recrystallization at this temperature.

The energy dispersive X-ray analyses (EDAX) for two samples prepared at differ-ent potentials are shown in Fig. 9. The CdTe film prepared at20.6 V(SCE) onCdS/SnO2/glass is illustrated in Fig. 9(a). The film has an excess of tellurium, whilethe other prepared at20.65 V(SCE) [Fig. 9(b)], exhibited a nearly stiochiometricCd/Te ratio.

3.4. Optical properties studies

The absorption spectra of as-deposited CdTe thin film which was deposited at20.65 V(SCE) is shown in Fig. 10(a). The plots for the direct transition (αhν)2 Vs.hν of the CdTe film is given in Fig. 10(b). The extrapolation on the energy axisgave an energy band gap of 1.48 eV.

4. Conclusions

The cyclic voltammetry study obviously specifies the cathodic region where theCdTe could be deposited. The bath temperature affects the growth rate by increasingthe solubility of the tellurium at high temperatures. The free tellurium phase is affec-

478 M. Soliman et al. / Renewable Energy 23 (2001) 471–481

Fig. 8. Scanning electron microscopy (SEM) of as-deposited and annealed CdTe thin films: (a) as-deposited, (b) annealed at 350°C and (c) 400°C.

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Fig. 9. Energy dispersive X-ray analysis (EDAX) of as-deposited CdTe thin films (a) electrodepositedat 20.6 V(SCE) and (b)20.65 V(SCE).

480 M. Soliman et al. / Renewable Energy 23 (2001) 471–481

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481M. Soliman et al. / Renewable Energy 23 (2001) 471–481

ted by the concentration of TeO2 in the solution. The annealing temperature as wellas the CdCl2 treatment seems to play an important for the recrystallization of thedeposited film.

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