From Solutions to Photovoltaic Devices: Assessing the Impact of Zn Complexation for Bifacial Cu ₂ ZnSn(S,Se) ₄ Thin Films

Solution-based deposition of high-quality inorganic compound semiconductors onto a variety of substrates is a key challenge toward integrating photovoltaic technologies into a wide range of infrastructures. Cu₂ZnSn-(S,Se)₄ (CZTSSe) is one of the most promising solar absorbers processable by solution-based methods, however there is a substantial knowledge gap linking the chemistry of cation complexes and chalcogen precursors, e.g. thiourea (TU), to the microstructure and opto-electronic properties of the thin-films. In this study, we focus our attention on the complexation of zinc chloride (ZnCl₂) and zinc acetate (ZnAc₂) in dimethylformamide (DMF)-based CZTS precursor inks and how this ultimately affects the performance of CZTSSe thin-film devices on F/SnO₂ (FTO) substrates. Acetate coordination not only improves the overall CZTSSe composition and structural uniformity but also lowers the zinc salt decomposition temperature, from 519 to 284 °C, which significantly affects the rate of grain growth during selenization and final microstructure. ZnAc₂-based CZTSSe films show densely packed grain growth due to the fast rate of selenium incorporation during annealing, while ZnCl₂-based CZTSSe displays slower rates due to the high decomposition temperature of ZnCl₂. Upon the incorporation of 25 nm Mo at the CZTSSe/FTO interface, a champion power conversion efficiency of 6.02% was achieved with ZnAc₂ precursor salt, over two times greater than the equivalent device architecture prepared with ZnCl₂, at 2.62%. This investigation illustrates the significant role of the molecular complexes in tuning the grain growth kinetics and final microstructure and therefore improving device performance on semitransparent substrates.