Suppressed Defects and Improved Stability of All-Inorganic CsSnI Films by Solid Additive-Assisted Chemical Vapor Deposition Process.

All-inorganic Sn-based halide perovskite CsSnI is a promising candidate for perovskite solar cells (PSCs) owing to its narrower bandgap (≈1.3 eV) and lower toxicity as compared with traditional organic-inorganic hybrid Pb-based counterparts. However, CsSnI rapidly degrades in ambient air and simultaneously generates intrinsic defect states, thus seriously impairing the optoelectronic property of the film, as well as the corresponding device performance. Herein, a solid additive-assisted chemical vapor deposition (SACVD) method is reported to prepare CsSnI films with high quality. Combining with theoretical calculations, Fourier transform infrared spectroscopy, temperature-dependent photoluminescence, and scanning Kelvin probe techniques prove that the lone electron pairs in the solid additive form coordination interactions with Sn, resulting in the suppression of Sn oxidizing to Sn and thus reducing defect density. This work not only provides a new strategy to prepare eco-friendly and stability-improved tin halide perovskite thin films for PSCs, but also reveals the underlying physical properties of CsSnI film upon solid state modification (i.e., without any organic solvent).