CdS thin films are widely used as the window material [1] in several thin film solar cells and have been regarded as a prime candidate [2] for solar cell fabrication. Its ideal band gap, high optical absorption, and relative ease of deposition have made CdS especially attractive for the preparation of thin film solar cells. CdS is photoconductive material used for different applications, such as thin film FET transistors, X-ray detectors [3], photodiodes for solar-meters, photocatalytical solar energy stocking [4] etc. CdS thin films have been deposited by several techniques including chemical bath deposition [5], thermal evaporation [5, 6], electrochemical deposition [7], spray pyrolysis [8, 9], laser ablation [10], and closed space sublimation [11]. Each technique has its own advantages. One of the main advantages of the CSS technique is the low loss of evaporated material as compared to evaporation methods. The ion exchange method of CdS films in Ag+ solution was studied by Lokhande et al. [12, 13] who reported partial to total conversion of CdS thin films into Ag2S. Ristova and his co-workers [14] prepared the CdS thin films by chemical bath method and studied the effects of silver (Ag) doping on the electrical, structural, and optical properties of the films. They found that silver doping has no effect on the optical band gap of the CdS thin films and they observed only Ag2S peaks in the XRD pattern. The purpose of this paper is to present results related to the studies of the electrical and structural properties as well as optical properties of CdS films prepared by CSS technique. CdS thin films were deposited on a soda lime glass substrate. The CdS material was sublimated from a graphite source. Mica sheet was used as an insulator between the source and the substrate. The distance between the sublimation source and substrate could easily be changed, permitting the determination of the optimal distance (i.e. better uniform films). The source and the substrate were kept at temperature of 660 ◦C and 500 ◦C respectively. CdS powder (Aldrich 99.99% purity) was used as a source for the evaporation of CdS onto a soda lime glass substrate. Two K-type thermocouples were used to monitor the temperature of the source and the substrate during the deposition process. The sublimation of the material was carried out at a pressure of ∼10−2 mb. The CdS source and substrate were separated by a distance of 2 cm and the source was maintained at a higher temperature than the substrate. Transparent orange to yellow colored thin films were obtained at the end of the deposition process. The films, prepared at the above mentioned deposition parameters, were treated chemically by immersing in low concentrated (0.2 g/500 ml) AgNO3-H2O solution for different periods of time as 30, 60, 120, 180, and 240 s. The color of the film changed from orangeyellow to gray due to the diffusion of Ag in the films. The films after immersion were cleaned in distilled water for 1 min, and then dried. The DC electrical resistivity of the films was measured by Vander Pauw technique. These films were heat-treated at 400 ◦C for one hour in a vacuum chamber in which the pressure was maintained at 10−2 mb. X-ray diffraction (XRD) pattern was recorded with X-ray diffractometer using CuKα radiations. The transmission spectra in the range 400–2000 nm have been recorded by Perkin-Elmer, lambda 19, and UV-VISNIR spectrometer. The electrical resistivity of as-deposited CdS films, prepared at a substrate temperature of 500 ◦C, was more than 106 -cm. The resistivity of the CdS film, immersed in AgNO3-H2O solution for 4 min, was reduced to 2.6 -cm. We noted the resistivity was decreased with increasing immersion time (Fig. 1). These Ag doped CdS films were heat treated at 400 ◦C for 1 h. After heat treatment the resistivity of the CdS films, immersed for 4 min, increased up to 29.4 -cm. The
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Sep 25, 2004
K.P Pernstich + 5
Open Access