Vapor-based sequential deposition of CsSnI3: analysis of phase evolution with in situ X-ray diffraction

All-inorganic CsSnI 3 is a lead-free halide perovskite with promising applications, which has been successfully synthesized by a variety of methods. Although industrially attractive, its sequential vacuum-based synthesis through deposition and annealing is challenging, because precursor conversion and thermal degradation occur in adjacent temperature ranges. In this work, CsSnI 3 films are prepared by deposition of CsI and SnI 2 precursor layers by sequential evaporation, followed by a thermal annealing process. We monitor the evolution of different crystalline phases and their conversion and degradation by in situ X-ray diffraction (ISXRD) throughout the complete deposition-conversion-degradation sequence. Room-temperature deposition produces a crystalline precursor stack composed of CsI and strongly textured β-SnI 2 , with no apparent initial CsSnI 3 formation. Upon thermal processing with a temperature ramp, we observe phase transitions of the precursor phases, before a cubic α-CsSnI 3 perovskite phase is formed. At even higher temperatures, we track the intermediate phases and the full decomposition of the film. This ramp experiment allows us to identify an apparent process window for suitable processing temperatures for the successful synthesis of photoactive, black α-CsSnI 3 perovskite thin films under the presented conditions. Finally, we demonstrate the successful synthesis of black α-CsSnI 3 thin films upon isothermal annealing at 160 °C. SEM images reveal compact polycrystalline films, and EDX shows an I/Sn ratio close to the nominal value of 3.