Controlling Nucleation and Growth of Metal Halide Perovskite Thin Films for High‐Efficiency Perovskite Solar Cells

Metal halide perovskite thin films can be crystallized via a broad range of solution‐based routes. However, the quality of the final films is strongly dependent upon small changes in solution composition and processing parameters. Here, this study demonstrates that a fractional substitution of PbCl 2 with PbI 2 in the 3CH 3 NH 3 I:PbCl 2 mixed‐halide starting solution has a profound influence upon the ensuing thin‐film crystallization. The presence of PbI 2 in the precursor induces a uniform distribution of regular quadrilateral‐shaped CH 3 NH 3 PbI 3 perovskite crystals in as‐cast films, which subsequently grow to form pinhole‐free perovskite films with highly crystalline domains. With this new formulation of 3CH 3 NH 3 I:0.98PbCl 2 :0.02PbI 2 , this study achieves a 19.1% current–voltage measured power conversion efficiency and a 17.2% stabilized power output in regular planar heterojunction solar cells.