Simultaneous Evolution of Uniaxially Oriented Grains and Ultralow-Density Grain-Boundary Network in CH₃NH₃PbI₃ Perovskite Thin Films Mediated by Precursor Phase Metastability

Solution-processed organic–inorganic halide perovskite (OIHP) thin films typically contain fine, randomly oriented grains and a high-density grain-boundary network, which are unfavorable for key film functions including charge transport and environmental stability. Here, we report a new chemical route for achieving CH 3 NH 3 PbI 3 (MAPbI 3 ) OIHP thin films comprising large, uniaxially oriented grains and an ultralow-density grain-boundary network. This route starts with a new metastable liquid-state precursor phase, MAPbI 3 ·MACl· x CH 3 NH 2, which converts to metastable MAPbI 3 ·MACl and then to MAPbI 3 OIHP upon stepwise release of volatile CH 3 NH 2 and MACl. Perovskite solar cells made via this route show high power conversion efficiency of up to 19.4%, with significantly enhanced environmental stability.