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Introduction
Due to its suitable optical, electrical and morphological properties, cadmium sulfide (CdS) has been widely recognized as a key component of the thin film-based solar cells technology (Wu 2004; Kazmerski 2006). A semiconducting direct energy bandgap value of 2⋅42 eV and the ease of fabrication in thin film geometry make the CdS an ideal candidate to be used as a buffer layer on the solar cell thin film heterostructures. CdS thin films are commonly used as optical barriers for the solar radiation with the wavelength below 515 nm (Paudel et al2012; Rios-Flores et al2012). High solar cell efficiencies have been recently reported using CdS thin films as a part of the complex solar cell heterostructures (Romeo et al2006; Repins et al2008; Jackson et al2011). Among the different methods for CdS thin films fabrication, the chemical bath deposition (CBD) arises as an attractive option due to its low cost, simple processing and high potential for scalability, the latter highly required for commercial applications. High film homogeneity, low electrical resistivity and high transparency in the visible region are required characteristics for the materials that are intended to be used as window layers for solar cell applications (Kim et al2012). These properties are strongly dependent on film characteristics determined by the deposition process such as the grain size and the surface roughness. For example, low grain size values are commonly associated with low thickness, low roughness and high transparency. For solar cells applications, a large grain size is required in the buffer layer in order to decrease the electrical resistance that as a consequence would increase the cell’s efficiency (Romeo et al2006). Large grain sizes can be achieved from the deposition process or by the application of a thermal treatment after the deposition. Although, CdS is commonly reported to grow as a n-type semiconductor (Moualkia et al2009), some reports have demonstrated p-type growth of CdS (Sebastian 1993). Furthermore, doping CdS films with other chemical elements during CBD process has shown to affect the resulting physicochemical properties of the doped CdS films, such as the electrical resistivity, bandgap energy and crystalline structure. Sebastian (1993) reported a decrease on CdS bandgap energy, as low as 2⋅0 eV, by using...