Optical properties of CdS:Pb thin layer deposited on glass substrate

Abstract

The thermal evaporation technique has been used to deposit a polycrystalline cadmium lead sulphide thin film (Cd_1-x Pb_xS with X=0.20) in a vacuum of about 10^-5 Torr at 450 ^°C. This Cd_1-x Pb_xS thin film has been investigated by X-ray diffraction technique. Optical constant like extinction coefficient (K), dielectric constant (€), and refractive index (n) have been measured from transmission spectrum in wavelength range from 700 nm to 1700 nm by manifacier’s envelope method. Crystallite size estimated from the Scherrer method has been found to be 31.25 nm. Coefficient of absorption (α) as well as film thickness (t) has been estimated by means of transmission spectra. The lattice parameter (a), dislocation density (δ), inter planner spacing (d), and micro strain (ε) have been calculated. X-ray study reveals that film stoichiometry is maintained in deposited film. The optical study confirms that optical band gap of Cd_1-x Pb_xS film can be tuned from visible to near infrared region (2.42-1.20 eV) using relatively low cost technique. The red shift in optical band gap has been observed for Cd_1-x Pb_xS thin film. Pb doping increase absorbance in visible region and near infrared region compared to undoped CdS film. The large decrease in band gap has been observed which makes the film much suitable for absorbing layer in solar cells, IR detector and other photovoltaic and optoelectronic applications. In film band, gap of 1.13 eV has been achieved which shows that doping of Pb in CdS makes it suitable material for solar cell absorber and IR detectors. The film shows high transmittance (about 70 %) in near infrared region. The film is polycrystalline and highly stoichiometric.