Surface layer investigation of dual ion beam sputtered Cu2ZnSn(S,Se)4 thin film for open circuit voltage improvement

Abstract

We present surface analysis of Cu2ZnSn(S,Se)4 (CZTSSe) thin films deposited on Mo/glass substrates. X-ray photoelectron and energy dispersive x-ray spectroscopy has been performed on CZTSSe thin-film solar cell absorbers for surface and bulk compositional analysis, respectively. It is observed that the surface of the CZTSSe absorber is Cu deficient. For further verification of Cu deficiency, spectroscopic ellipsometry has been used to determine the extinction coefficient of CZTSSe thin films with Cu variation. The surface layer has a higher bandgap of 1.79 eV in reference to the bulk film bandgap of 1.5 eV. This surface bandgap increase is beneficial for solar cell performance. The thin film with a Cu deficient surface has a noticeably higher open circuit voltage of 562 mV using a very thin absorber layer of 300 nm in thickness. The open circuit voltage for the Cu deficient surface is 25% higher than that of the Cu rich surface. This analysis gives an understanding into the significance of surface layer engineering for photovoltaic device.