Development of Cu2ZnSnS4 Thin-Film Absorber on Glass and Mo Coated Glass Substrates by Solution Process

The ever-increasing demand for energy and depletion of conventional energy resources leads to research and development in the area of renewable energy sources. Copper zinc tin sulfide (CZTS) or kesterite based thin film solar cell is an emerging area and is composed of earth abundant, nontoxic and cheaper materials. Absorber layer for the many of the kesterite based solar cells are typically deposited by vacuum-based approaches such as sputtering or evaporation; however, these routes are quite expensive. Solution based deposition techniques are very cheap and can replace expensive vacuum based techniques. Solutions allow intermixing of the constituents at molecular level and the formation of smooth and homogeneous films. In this study, the precursor solution for kesterite absorber was prepared from copper (II) acetate, zinc (II) acetate and tin (II) chloride, 2-methoxyethanol and mono-ethanolamine (MEA). 2-methoxyethanol and MEA were used as solvent and stabilizer respectively. In addition to soda lime glass substrate, Mo coated glass substrate were also used for the coating, since photovoltaic device requires the formation of the absorber layer on Mo coated substrates. The precursor solution was spin coated on to the pre-cleaned substrates at 3000rpm for 20 sec. The films were preheated in air at 300 degrees C for 5min on a hotplate. This procedure was repeated several times to achieve the desired thickness. The precursor films were sulphurised in a mixture gas of N-2 and H2S (5% H2S + 95% N-2) for 25 minutes at 550 degrees C. Phase purity of the films was confirmed by X-ray diffraction and Raman spectroscopy. Surface morphology and film thickness were examined by SEM, optical band gap was estimated from UV-Vis spectra. The results of these studies are discussed in detail.