Elsevier

Thin Solid Films

Volume 359, Issue 2, 31 January 2000, Pages 160-164
Thin Solid Films

Microstructure and properties of the CdS thin films prepared by electrostatic spray assisted vapour deposition (ESAVD) method

https://doi.org/10.1016/S0040-6090(99)00733-6Get rights and content

Abstract

CdS thin films were fabricated using an electrostatic spray assisted vapour deposition (ESAVD) method from water/ethanol solutions of cadmium chloride and thiourea with different Cd:S ratios. The microstructure and crystallisation of the films were studied by means of XRD, XPS, AFM and SEM techniques. The effect of deposition conditions and precursor composition on the morphology and phase development of the films was investigated. Films with hexagonal phase and preferred [002] orientation were deposited at temperature below 400°C using a precursor solution with a Cd:S ratio of 1:3, whereas films deposited using a precursor solution with a Cd:S ratio of 1:1 showed no preferred orientation. The relationships between the microstructure of the films and their optical and electrical properties are discussed. The Cd:S ratio has very little influence on the optical property but has a tremendous effect on the conductivity of the CdS films. The results show that the ESAVD technique can produce CdS films with optical and electrical properties which may be suitable for solar cell applications.

Introduction

CdS thin films are regarded as one of the most promising window layer materials for heterojunction thin-film solar cells. Polycrystalline thin-film solar cells with efficiencies of 14–16% have been made with CdS as the window layer and CdTe [1] or CuInSe2 [2] as the absorber layers. The most common method for the production of CdS thin films is chemical bath deposition (CBD) [3]. So far, the chemical bath deposition can produce CdS films with the best device performance for solar cells [2], [4]. However, the mass production of CdS films via this process may present environmental concerns because of toxicity and recycling problems associated with chemical solution based on Cadmium. Spray pyrolysis is another widely used method for producing CdS films [5], [6]. This is a convenient, fast and relatively low-cost process which has been used for the deposition of thin films for many years. However, the deposition efficiency of the method is relatively low because of the loss of the aerosol precursor to the surroundings which tends to reduce the low-cost processing advantage, especially for the use of expensive precursors. We have reported the use a novel method which is called Electrostatic Spray Assisted Vapour Deposition (ESAVD) technique to fabricate CdS thin films [7]. The ESAVD process involves spraying atomised precursor droplets across an electric field where the droplets undergo combustion and chemical reaction in the vapour phase near the vicinity of the heated substrate. This produces a stable solid film with excellent adhesion onto the substrate in a single production run. In addition, the electrostatic field can facilitate maximum deposition of the films by attracting the charged aerosols to the substrate, thus minimising the pollution of the toxic precursors used in the processing to the environment. Therefore, this is a more cost-effective method than above mentioned spray pyrolysis and CBD methods.

This paper describes the microstructure and optical/electrical properties of the CdS thin films deposited by the ESAVD process. In particular, the effects of the Cd/S ratio and substrate temperature on the structure and electrical properties of the films are discussed.

Section snippets

Experimental procedure

Cadmium chloride (CdCl2) and thiourea [(NH2)2CS] were used as the precursors for the deposition of CdS. CdCl2 and (NH2)2CS with molar ratio of 1:1, 1:2, and 1:3 were dissolved in water and ethanol to produce 0.005 to 0.01 M solution. The apparatus used for ESAVD process has been described in reference [8]. The substrates used were either uncoated or ITO coated optical glass slides. The deposition temperature was varied from 200 to 450°C, and deposition time from 10 to 60 min.

A PHILIPS PW1710

Crystallisation and morphology development of the CdS films

The crystallisation behaviour and surface morphology of CdS films deposited using different Cd:S ratios and deposition temperatures are illustrated in Fig. 1, Fig. 2, respectively. The CdS films start to crystallise at about 250°C irrespective of the Cd:S ratio. Early reports [5], [12] showed that a complex Cd(SCN2H4)2Cl2 was formed when CdCl2 and SC(NH2)2 were dissolved in an aqueous solution. The TG-DTA data showed that this complex started to decompose at about 210°C [5], [13]. Hexagonal

Conclusions

CdS films with reasonable optical and electrical properties have been fabricated via a novel electrostatic assisted chemical vapour deposition (ESAVD) technique from water/ethanol solution of cadmium chloride and thiourea. The phase and preferred orientation of the films were influenced by substrate temperature and Cd:S ratio in precursor solution. Near stoichiometric films with a small amount of oxygen and chloride impurities could be formed irrespective of the initial Cd:S ratio in precursor

Acknowledgements

The authors wish to acknowledge the financial support of EPSRC-ROPA (GR/L73562) and Drs. K. Senkew, J. Durrant and S. Wright for their kind assistance in the XPS, optical and electrical properties measurements.

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