Growth and Characterization of Cu2ZnSn SSe4 Thin Films for the Solar Cell Applications

Abstract

Cu2ZnSnS4 (CZTS) has emerged as a leading absorber candidate for thin film solar cells over the last few years due to its highly favorable optoelectronic properties. In addition, it has an earth abundant, non-toxic, and inexpensive constituent that promises development of cost-competitive solar cells. However, the formation of the single phase, which is crucial to the performance to the photovoltaic devices, has been found to be a big challenge in CZTS since a single phase exists within a much smaller chemical potential window. The tendency to form binary phases, selective re-evaporation of elements from the film, reaction at contacts, etc. further make it difficult to control the intended composition and the formation of single phase. Consequently, there is extensive ongoing efforts to design synthesis protocols for the fabrication of single phase CZTS with desired properties. Various physical vapor deposition techniques, especially the sputter deposition one, which is being widely used in the optoelectronics industry, have performed poorly for the CZTS based devices. In this work, efforts have been made to fabricate CZTS thin films by RF magnetron sputtering of a single elementary target. The resulting films were post-selenized at different conditions to obtain Cu2ZnSn(SSe)4 (CZTSSe) thin films. The proposed route of film synthesis greatly simplifies the process steps in fabrication of these films. Based on the results, reaction mechanism of evolution of kesterite phase has been proposed. The films grown from a target containing powders of Cu, Zn, Sn and S taken in stoichiometric proportion were always Cu -poor in spite of a large variation in the sputtering and post-deposition sulfurization parameters. This stoichiometric deviation of the precursor film was primarily due to the poor sputter yield of Cu. The Cu-deficiency led to the formation of spurious secondary phases. Cu deficiency in the precursor film was compensated by growing a Cu interlayer between successively sputtered Cu-Zn-Sn-S precursor layers.