Chemical bath deposition of nanocrystaline CdS and CdPbS layers and investigation of their photoconductivity
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0900-O03-30.1
Chemical bath deposition of CdS and CdPbS nanocrystalline thin films and investigation of their photoconductivity Vilmos Rakovics Optoelectronic Devices Division, Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science, Konkoly-Thege út 29- 33, 1121 Budapest, Hungary ABSTRACT The investigation of photoconductive properties of chemical bath deposited (CBD) cadmium sulphide (CdS) layers was the main objective of this work. For completeness, the CdS layers were characterized using X-ray diffraction, scanning electron microscopy, optical absorption, DC electrical conductivity, photoconductivity measurements. It has been found that the CdS layers grown are hexagonal with (002) preferential orientation. The n-type CdS materials show 100–200 nm clusters consisting of 10–20 nm size crystallites. The optical band gap is 2.42-2.52 eV, which shows a red-shift to 2.34 eV upon heat treatment. The effect of Cu doping and co-deposition of CdS with PbS were also investigated. Dark conductivity was increased after heat treatment of pure CdS layers, and decreased in case of Cu doped samples. Photoconductive response time of Cu doped CdS and Cd(Pb)S samples were smaller than the response time of pure CdS samples. INTRODUCTION CdS thin films are used in commercial photocells and also as window material for solar cells continues as a subject of intense research in order to obtain cells with higher efficiencies. The chemical bath technique (CBD) appears to be a relatively simple, inexpensive method to prepare a homogenous film with controlled composition. In particular, CBD is widely used for achieving good-quality CdS [1-5], and PbS [6-11] films. Extensive research has been done on the deposition and characterization of CdS semiconductor thin films due to their potential applications in the area of electronic device fabrication. Particularly in CdS photoconductivity many fold improvements were reported by Nair et al. [6] and Bhushan et al. [12-16]. In ref. [17] CdxPb1-x (0 ≤ x ≤ 1) thin films were prepared on glass substrates using the chemical bath deposition method. The structural, electrical, and photoelectrical properties of the films were investigated, and all were dependent on the x value. For x > 0.5 (high Cd content), the films show a poor crystallinity and weak photoconductivity. For x < 0.5 (high Pb content), the films exhibit a good crystallinity, with an average size of the crystallites of 100 nm, and good photoconductivity. In this work thin films of CdS, CdS(Cu doped), and Cd(Pb)S (PbS content is less than 20%) were produced in different thicknesses. Their optical properties have been measured in the 4001100 nm wavelength interval. Photoconductive properties are also studied. EXPERIMENTAL DETAILS CdS thin films have been deposited on glass substrate using CBD technique. The substrates used for deposition are commercial glass slides of 76 mm x 25 mm. Baths containing CdSO4, thiourea, NH3, were used. Lead acetate and CuCl2 were also added to the bath for improving the
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