Modification of the Optical and Electrical Properties CdS Films by Annealing in Neutral and Reducing Atmospheres
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Modification of the Optical and Electrical Properties CdS Films by Annealing in Neutral and Reducing Atmospheres J. Pantoja Enriquez1, 2, G. Pérez Hernandez3, X. Mathew4, G. Ibáñez Duharte1, J. Moreira1, J. A. Reyes Nava1, J. J. Barrionuevo1, L. A. Hernandez1, R. Castillo2, P. J. Sebastian4 1. Centro de Investigación y Desarrollo Tecnológico en Energías Renovables, Universidad de Ciencias y Artes de Chiapas, Tuxtla Gutiérrez, Chiapas, México. 2. Universidad Politécnica de Chiapas, Tuxtla Gutiérrez, Chiapas, México. 3. Universidad Juárez Autónoma de Tabasco. Villahermosa, Tabasco, México. 4. Centro de Investigación en Energía, UNAM, Temixco, Morelos, México.
ABSTRACT Cadmium sulfide (CdS) films were deposited onto glass substrates by chemical bath deposition (CBD) from a bath containing cadmium acetate, ammonium acetate, thiourea, and ammonium hydroxide. The CdS thin films were annealed in argon (neutral atmosphere) or hydrogen (reducing atmosphere) for 1 h at various temperatures (300, 350, 400, 450 and 500 °C). The changes in optical and electrical properties of annealed treated CdS thin films were analyzed. The results showed that, the band-gap and resistivity depend on the post-deposition annealing atmosphere and temperatures. Thus, it was found that these properties of the films, were found to be affected by various processes with opposite effects, some beneficial and others unfavorable. The energy gap and resistivity for different annealing atmospheres was seen to oscillate by thermal annealing. Recrystallization, oxidation, surface passivation, sublimation and materials evaporation were found the main factors of the heat-treatment process responsible for this oscillating behavior. Annealing over 400 °C was seen to degrade the optical and electrical properties of the film. INTRODUCTION Cadmium sulfide is n-type direct band gap semiconductor and is considered as the bestsuited window material for CdTe and CuIn(Ga)Se2 solar cells [1–4]. Chemical bath deposition technique is the most cost-effective and relatively easy for solar cell fabrication. Cadmium sulfide thin films obtained by this method are uniform, adherent, and reproducible [5,6]. CdS/CdTe thin film solar cells with efficiencies higher than 16% has been reported by using CBD CdS films [7]. Low series resistance, high transmittance and optimum band gap are very important requirements for CdS thin film to be considered a suitable window layer for solar cell. So the CdS should be conductive, thin to allow high transmission and uniform to avoid short circuit effects [8]. However, generally as deposited CdS thin film show high resistivity and optical transmittance, which can be improved. Post deposition heat treatment is one of fundamental steps to improve the electrical and optical properties of CdS thin films. The annealing treatments promotes recrystallization, grain growth and improve the optical and electrical properties of the films and hence the device, having an important role, the temperature and the annealing
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atmosphere. Despite the fact th
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