Deposition of cadmium chalcogenide thin films by a solution growth technique using triethanolamine as a complexing agent

Solar Energy Materials 7 (1983) 431-438 North-Holland Publishing Company

431

DEPOSITION OF CADMIUM CHALCOGENIDE THIN FILMS BY A SOLUTION GROWTH TECHNIQUE USING TRIETHANOLAMINE AS A COMPLEXING AGENT A. MONDAL, T. K. CHAUDHURI* and P. PRAMANIK Department of Chemistry, *Department of Physics, Indian Institute oJ'Technology, Kharagpur-721302, India Received 26 May 1982; in revised form 17 August 1982 A method has been developed for the chemical deposition of thin films of CdS and CdSe on glass substrate. For CdS deposition, a triethanolamine complex of cadmium ions, ammonia and thiourea solutions were used, while for CdSe films, sodium selenosulfate solution replaced thiourea, other reagents remaining unchanged. Variation of thickness with different bath parameters have been studied to obtain thickest deposition possible. X-ray characterization, optical absorption and photoconductivity measurements have been performed to confirm the deposition of CdS and CdSe. Triethanolamine has been found to be a useful complexing agent for the deposition of chalcogenide thin films.

1. Introduction

Thin films of II-VI compounds are most promising for utilization in solar cells, out of which the cadmium chalcogenides have received intensive attention, since their band gaps lie very close to the range of maximum theoretically attainable energy conversion efficiency [1]. They can also be used as CdS/CulnSez heterojunction IR detectors, light intensity meters, switching devices, etc. Though, single crystals of these materials yield comparatively better efficiencies in photoelectrochemical solar cells [2, 3], they are presently avoided due to their high cost of production. On the other hand, a little sacrifice of power output in the case of polycrystalline thin films, leads to a drastic cut in the cost of production and make them commercially viable. Various methods [4] can be used for the deposition of polycrystalline thin films of cadmium chalcogenides but the cheapest and easiest is the solution growth technique. So far, authors have used the tetra-ammonium complex of the cadmium ion for CdS [5, 6] and CdSe [7] film deposition. It was observed that with clear solutions of the tetra-ammonium complex of cadmium, deposition of CdSe film was not possible at room temperature. Solutions with a suspension of Cd(OH) 2 yielded very thin films ( ~800 ~) of CdSe at room temperature. In the present investigation, we have used the triethanolamine (TEA) complex of cadmium ion for the deposition of the same. This change in the complexing agent helps in uniform, adherent and thicker deposition of cadmium chaicogenide films even at room temperature (30°C). Further advantage in using this complexing agent lies in the fact that, it forms complexes not 0165-1633/83/0000-0000/$03.00f~) 1983 North-Holland

432

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Deposition t!/ ('dS and CdSe on ~tluss substrate

only with many readily soluble metal salts e.g. Pb ~+ [8], Sb 3 + [9], (_'uz ~ [I0] but also with sparingly soluble substances e.g. Bi 3+ [11. 12] so that metal chalcogenides of them can be deposited. This advantage may be utilized also for preparing alloys of these chalcogenides and doped thin films. In this paper we report a method of preparing CdS and CdSe films, effect of bath parameters on the thickness of the deposited films and the characterization of these materials by X-ray and optical techniques.

2. Experimental The deposition of CdS films was based on the reaction between the triethanolamine complex of Cd z+ ions and thiourea in basic media. 10 ml of 1 M cadmium acetate solution was taken in a 100 ml beaker, to which 5 mt of ~ 7.40 M triethanolamine was added. The solution was stirred well so that the viscous liquid formed a homogeneous solution. Then about 10 ml of z. 13.4 M ammonia solution was added, followed by another 10 ml of 1 M thiourea solution. The resulting solution was made up to 100 ml with distilled water. It was stirred for few seconds and then transferred to another beaker containing scrupulously cleaned glass slide, clamped vertically. After about 18 h, the slide covered with an organge yellow deposit, was taken out, washed with distilled water and dried in air. The deposition technique for CdSe films was essentially the same except that 15 ml of ~0.45 M sodium selenosulphate solution was used instead of the thiourea solution. To optimise the conditions for deposition of the films, the dependence of the growth of the films with time, temperature and volume of the complexing agent were studied at an optimum pH ( -~ 10). The compositions of the films deposited were determined by X-ray powder diffraction technique using an I 1.46 cm Debye-Scherrer camera (Philips) and nickel-filtered CuK~ radiation. The materials were scraped off the substrates for this purpose. The forbidden energy gaps of the materials of the films were determined from the absorption spectra measured by a Cary 17 D spectrophotometer. The films were found to be photoconducting. Hence. their spectral response of photocurrent was ascertained by using an Oriel Universal Monochromatic Source.

3. Results and discussion The X-ray lines obtained for CdS films, prepared by the present method, are given in table 1. The tabulation shows the observed inter-planar spacings, d. and their possible identification by comparing them with standard d-values taken from ASTM Diffraction Data File. Table 1 indicates that the material deposited is CdS and it consists of both hexagonal (~) and cubic (fl) phases. Similar results have been observed by Nayak et al. [13] and Kaur et al. [6] for chemically deposited CdS films prepared by tetra-ammonium complex. The X-ray data of CdSe films are shown in table 2. The table suggests that CdSe films also contain both cubic and hexagonal phases, as reported by Kainthla et al. [7].

A. Mondal et al. / Deposition of CdS and CdSe on glass substrate

433

Table 1 X-ray data of CdS film prepared with TEA complex Possible identification with standard d (~) Observed d (,/~t 3.31 2.047 1.884 1.804 1.762 1.676 1.588 1.516 1.337 1.196 1.154

a-CdS

fl-CdS

(0021 3.36 (110) 2.06 (103) 1.898 (200) 1.791 (112) 1.761

(111) 3.36 (220) 2.058 (311) 1.753 (222) 1.680

(202) 1.581 (104) 1.520 (300) 1.194 (213) 1.158

(331) 1.337 (422t 1.186

Table 2 X-ray data of CdSe film prepared with TEA complex Possible identification with standard d (A) Observed d (A) 3.37 2.061 1.773 1.352 1.218

ct-CdSe

fl-CdSe

{101) 3.29 Ill0) 2.151

(1111 3.49 (220) 2.139 (222) 1.764 (4201 1.353 (422) 1.235

(213) 1.2055

The optical absorption spectra of CdS and CdSe films, prepared by the present method are shown in fig. 1. The band gaps were obtained from these curves by extrapolating the absorption edges to zero absorbance. The band gaps were found to be ~2.40 eV and ~ 1.75 eV for CdS and CdSe, respectively. The values are in good agreement with the standard [-14] band gaps of 2.42 eV and 1.74 eV. The photocurrent spectra of CdS and CdSe is shown in fig. 2. The spectra have maxima at ~506 and z.730 nm corresponding to 2.45 and 1.70 eV for CdS and CdSe, respectively. The chemical reactions that lead to the formation of CdS and CdSe are as follows: Cd(TEA) 2 + + (NH2)2CS + 2 O H - --*CdS + TEA + (NH 2)2CO --FHEO,

( 1)

Cd(TEA) 2 + + NazSeSO3 + 2 O H - ~ C d S e + TEA + Na2SO4 + H 2 0 .

(2)

Fig. 3 shows the growth kinetics of CdS films with time of deposition at 30 and 50 ° C.

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A. Mondal et al. / Deposition of CdS and CdSe on glass substrate

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In the initial stages of growth, the thickness increases at a fast rate. Then the rate decreases resulting in a terminal thickness. At 30°C, the rate of growth is slow (37 ,~/ min) and terminal thickness of 3 #m is attained after 18 h. However, the rate of growth at 50°C is higher (80/~/min) and a terminal thickness of 1.7/~m is reached much earlier, after 5 h. The CdSe film thickness as a function of time for two different temperatures is shown in fig. 4. The nature of growth is similar to that of CdS films. A deposition time of 23 h at 30°C yielded a film thickness of about 0.52/zm, while at 50°C a terminal thickness of 0.32/zm was obtained. Thus, it is seen that an increase in temperature increased the rate of growth (at p H ~ 10) of CdS and CdSe films but decreased their terminal thicknesses. Similar observations have been made by Kaur et al. [6] for CdS films and Kainthla et al. [7-] for PbSe films at relatively low pH value (pH z 10).

A. Mondal et at./Deposition ol CdS and CdSe on glass substrate

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They have explained the film formation and kinetics by i o n - i o n condensation and cluster by cluster growth. We suggest a little modification in explaining the decrease in terminal thickness with increase in temperature in the i o n - i o n condensation process. At a particular temperature, CdS will be deposited if the ionic product (IP) of Cd 2+ and S 2 exceeds the solubility product (SP) of CdS. The Cd 2+ released by TEA complex and S 2 released by thiourea combine at the nucleation centres to produce CdS. The quantity of ions utilized for film formation depends on the rate of formation of CdS on the substrate surface which in turn depends not only on the ratio of the nucleation centres available at the surface of the substrate to those in the volume of the solutions [7], but also on the rate of release of Cd 2 + and S 2 - ions. At 30°C the rate of formation of CdS on the substrate is probably more or less comparable to the rate of release of the corresponding ions, thus resulting in m a x i m u m utilization of the ions and greater terminal thickness. At higher temperatures the concentration of

A. Mondal et al. / Deposition of CdS and CdSe on glass substrate Triethanolomine added ( m l )

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deposition. The deposition rate of CdS is also increased due to the kinetic energy of the ions and hence increased interaction between them [6, 7]. However, the rates of release of Cd 2 + and S 2- ions seem to become very much greater than the rate of formation of CdS at the surface nucleation centres. Hence only a fraction of the ions are utilized for film formation, resulting in a lower terminal thickness. The unutilized ions precipitate via the volume nucleation centres. The above is also true for the formation of CdSe films. The relation between the thickness of the CdS and CdSe films and the volume of complexing agent (TEA) at room temperature is shown in fig. 5. The curves show that as the volume of TEA increases the thickness of the films also increase and then decrease, going through maximas. The m a x i m u m thickness occur at 3 and 5 ml of TEA for CdS and CdSe, respectively. The curves can be explained as follows: If the TEA added is less than the o p t i m u m value the complexing is incomplete and hence fast precipitation occurs leading to the formation of thinner films, in accordance to the suggestions given above. When an excess of TEA is added, the n u m b e r of dissociated C d 2 + ions decreases as the volume of TEA is increased, thus less n u m b e r of ions are available for film formation, resulting in thinner films of CdS and CdSe.

438

A. Mondal et al. / Deposition o / ( ' d S and ('dSe on ~llass substrate

4. Conclusion A solution growth technique has been developed for the deposition of CdS and CdSe thin films. Triethanolamine has been found to be a useful complexing agenl for deposition of chalcogenide thin films.

Acknowledgement The authors (A. M. and T. K. C.J are grateful to the authorities of I.SR.O., Department of Space, Government of India for providing financial assistance during this investigation.

References [1] H. Gerischer, in: Semiconductor Liquid-Junction Solar Cells, ed. A. Heller (The Electrochemical Society, Princeton, 1977) p. 1. [2] A. B. Ellis, S. W. Kaiser and M. S. Wrighton. J. Am. Chem. Soc. 98 (1976) 6855. [3] A Heller, K. C. Chang and B. Miller, J. Electrochem. Soc. 124 11977j 697. [4] K. L. Chopra, Thin Film Phenomena tMcGraw-Hill, New York, 1969~. [5] N, R. Pavaskar, C, A. Menezes and A P. B. Sinha, J. Electrochem Soc, 124 !19771 743. [6] 1. Kaur. D. K. Pandya and K. L. Chopra, J. Electrochem Soc. 127 (1980) 943, [7] R C. Kainthla, D. K. Pandya and K. L. Chopra, J. Electrochem Soc. 127 (1980) 277. [8] A. Mondal and P. Pramanik, to be published. [9] R.N. Bhattacharya and P. Pramanik, J. Solid State Chem. 43 i1982) to be published. [10] A. Mondal and P. Pramanik, J. Solid State Chem, to be published. [11] P. Pramanik, R. N. Bhattacharya and A. Mondak J. Electrochem Soc. 127 (1980~ 1857. [12] P. Pramanik and R. N. Bhattacharya, J. Electrochem Soc. 127 (1980) 2087. [13] B. B. Nayak, H. N. Acharya and G. B. Mitra, Bull. Mater. Sci, 3 (1981i 317. [14] CRC Handbook of Chemistry and Physics, 60th ed, ed. R. C. Weast (CRC Press, Cleveland, 1979~ p E-103,