Solvent-Mediated Crystallization of CH₃NH₃SnI₃ Films for Heterojunction Depleted Perovskite Solar Cells

Organo-lead halide perovskite solar cells have gained enormous significance and have now achieved power conversion efficiencies of ∼20%. However, the potential toxicity of lead in these systems raises environmental concerns for widespread deployment. Here we investigate solvent effects on the crystallization of the lead-free methylammonium tin triiodide (CH₃NH₃SnI₃) perovskite films in a solution growth process. Highly uniform, pinhole-free perovskite films are obtained from a dimethyl sulfoxide (DMSO) solution via a transitional SnI₂·3DMSO intermediate phase. This high-quality perovskite film enables the realization of heterojunction depleted solar cells based on mesoporous TiO₂ layer but in the absence of any hole-transporting material with an unprecedented photocurrent up to 21 mA cm^–2. Charge extraction and transient photovoltage decay measurements reveal high carrier densities in the CH₃NH₃SnI₃ perovskite device which are one order of magnitude larger than CH₃NH₃PbI₃-based devices but with comparable recombination lifetimes in both devices. The relatively high background dark carrier density of the Sn-based perovskite is responsible for the lower photovoltaic efficiency in comparison to the Pb-based analogues. These results provide important progress toward achieving improved perovskite morphology control in realizing solution-processed highly efficient lead-free perovskite solar cells