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.有机铅卤钙钛矿太阳能电池（organo-lead halide perovskite solar cells）已受到广泛关注，目前其功率转换效率已达到约20%。然而，此类体系中铅的潜在毒性，为其大规模商业化部署带来了环境顾虑。本文针对溶液生长过程中，无铅三碘合锡甲基铵（CH3NH3SnI3）钙钛矿薄膜的结晶溶剂效应展开研究。通过SnI2·3DMSO过渡中间相，可从二甲基亚砜（DMSO）溶液中制备得到高度均匀、无针孔的钙钛矿薄膜。该高质量钙钛矿薄膜可用于制备基于介孔二氧化钛（TiO2）层的异质结耗尽型太阳能电池，且无需任何空穴传输材料，实现了高达21 mA cm–2的空前光电流。电荷抽取与瞬态光电压衰减测试结果表明，CH3NH3SnI3钙钛矿器件的载流子密度较CH3NH3PbI3基器件高一个数量级，但两者的载流子复合寿命相当。锡基钙钛矿相对较高的背景暗载流子密度，是其光电转换效率低于铅基同类器件的核心原因。本研究结果为实现溶液加工的高效无铅钙钛矿太阳能电池、优化钙钛矿形貌调控提供了重要进展。