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