Optical and Electrical Characterization of Cu2znsns4 Deposited by Silar Technique

Abstract

Currently, Copper Indium Gallium Sulphide (CIGS) is the most efficient thin film material used as an absorber layer in photovoltaic solar cells. Unfortunately, the material poses a challenge in the future sustainability because some of its constituent elements are toxic and scarce on the earth crust (Indium and Gallium). Therefore, there is dire need to replace Indium and Gallium with Zinc and Tin which are abundant on the earth crust and environmentally benign in order to form Copper Zinc Tin Sulphide (CZTS) as an alternative absorber layer for thin film based photovoltaic device. CZTS also has an ideal direct band gap of 1.5 eV as an absorber layer, high absorption coefficient, and suitable optical and electrical properties for thin films solar cells. In this study, a two-stage combinatorial process that involve deposition of the Cu-Zn-Sn-S thin film using SILAR technique followed by a sulphurisation process in sulfur rich tube furnace is presented. The Copper Zinc Tin Sulphide (CZTS) thin film was deposited onto a Transparent Conducting Oxide (TCO) glass substrate using Successive Ion Layer Adsorption Reaction (SILAR) coating machine with stirrer at room temperature of 27±1oC and the number of cycles was varied between 20 and 70 cycles at an interval of 20, 40, 60 and 70 cycles. The samples were then annealed in a tube furnace at a temperature of 450oC and 550oC for 30 minutes each. The effects of annealing temperature and film thicknesses on the optical, electrical and structural properties of the film samples before and after annealing were investigated. The electrical properties of the CZTS thin film samples were measured using four-point probe, while optical properties UV-VIS- IR spectrophotometer. It was observed that, the resistivity of the film samples decreases with an increase in annealing temperature and also with an increase in the thin films’ thickness. Samples annealed at 550oC had the lowest resistivity with optical band gap ranging from1.49eV to1.54eV. The calculated absorption coefficient of all the samples, both as-deposited and after annealing, was ≥104 cm-1. The phase purity of the film sample was determined using Raman spectroscopy which confirmed the formation of quality CZTS film after annealing at 550OC. Elemental composition of the film sample was carried out using X-ray Fluorescence (XRF) instrument which confirmed that the formed CZTS film was nearly stoichiometric.