Tuning Microstructural, Optical, and Electrical Properties of CsSnI ₃ via Sequential Thermal Evaporation for Photovoltaic Application

Tin halide formulations are emerging as the leading sustainable, lead-free options for thin-film perovskite solar cells. In this study, we focused on the fully inorganic CsSnI₃ composition and systematically explored a solvent-free approach to manufacturing device-grade films via sequential thermal evaporation under vacuum. This production technique is compatible with industry standards, offers fewer constraints than coevaporation, and holds great promise for Pb-based perovskite fabrication but has yet to be thoroughly investigated for tin-based formulations. By eliminating solvents, the approach could also prove effective in mitigating the inherent self-p-doping of these materials, a critical requirement for achieving high-efficiency devices. We tested both double- and multilayer fabrication protocols and compared the structural, morphological, optical, and electrical properties of as-deposited and annealed films. This investigation was complemented by integrating the evaporated CsSnI₃ layers into p-i-n solar cells as a diagnostic tool. Our findings provide insights into (i) the impact of the deposition protocol on the material properties and (ii) the potential for fine-tuning them via postdeposition thermal treatments. Both methods yielded highly crystalline and compact films, while self-p-doping persisted in pure stoichiometric CsSnI₃ films, with a free hole density of around 10¹⁹ cm^-3 regardless of the protocol. Notably, a 1-order-of-magnitude reduction in the hole density was achieved by incorporating SnF₂ as a reducing agent. Readily implemented via the deposition of an additional layer, the inclusion of additives emerges as a necessary yet viable route toward device-grade evaporated CsSnI₃.