Bath pH Dependence on the Structural and Optical Properties of Chemical Bath Deposited CdS Thin Films
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Bath pH Dependence on the Structural and Optical Properties of Chemical Bath Deposited CdS Thin Films Udaya S. Ketipearachchi, David W. Lane, Keith D. Rogers, Jonathan D. Painter, Michael A. Cousins Center for Materials and Engineering, Cranfield University, RMCS, Shrivenham, Swindon, SN6 6LA, UK ABSTRACT High quality CdS thin films made with ultrasonic agitation during chemical bath deposition were investigated. The change in band gap and variation of atomic stacking during the film growth was observed as a function of bath pH. The band gap of as-deposited thin films of CdS was found to be between 2.26 eV and 2.50 eV, the highest being observed when the pH was 9.0. The transition from cubic (β-CdS) to hexagonal (α-CdS) was observed with decreasing pH. The lattice parameters of CdCl2 treated CdS were found to be a = 0.414 nm and c = 0.672 nm. Calculations based on the Sherrer formula showed significant grain growth after annealing with CdCl2. No apparent effect of ultrasonication on crystalline structure of CdS was seen in this method, although ultrasonication was noted to produce films with a higher quality optical surface. A maximum bath temperature was fixed at 70oC in order to suppress the rate of homogeneous reaction and minimise the evaporation of ammonia from the chemical bath.
INTRODUCTION The use of polycrystalline thin films in the solar cell industry has become an attractive alternative to the conventional silicon solar cell process due to its low production cost. Absorber layers based on CuInSe2 and CdTe have been reported to have the highest efficiencies of 18.8%[1] and 16.8 %[2] respectively. This has focused more research on this area over past few years. In these devices, an n-type window layer having a good transparency for visible light is used to make a junction with a p-type optical absorber layer. Cadmium sulphide was found to be a promising window layer for the fabrication of high efficiency thin film solar cells with CuInSe2 [1], CdTe [2] and Cu2S [3]. In addition to applications for thin films of CdS in photovoltaic devices, their application for photoconductive sensors and many other optical devices is also documented. There are a number of different approaches for the fabrication of thin films of CdS such as chemical bath deposition (CBD), electrodeposition, closed spaced sublimation (CSS), spray pyrolysis, successive ionic layer adsorption and reaction (SILAR), molecular beam epitaxy, etc. However, CBD CdS is attractive due to its low cost, large area and low temperature fabrication process. The role of pH on the structural and optical properties of CBD CdS thin films is not clear due to its variation with the concentration of species in the solution and also the lack of information in the literature in this regard. Therefore, the work in this communication highlights the effect of bath pH on some important aspects of the film. In comparison to published data [4], it has also been seen that the role of pH on CBD CdS is strongly dependent on solution chemistry and deposition condi
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