Discovery of a New Intermediate Enables One‐Step Deposition of High‐Quality Perovskite Films via Solvent Engineering

Bromide‐based organo‐metal halide perovskites have shown great potential for use in tandem solar cells (SCs), light‐emitting diodes (LEDs), and photodetectors. Herein, a new protocol using a one‐step deposition method for producing formamidinium lead bromide (FAPbBr 3 ) perovskites is reported, which features a solvent‐engineered intermediate phase to achieve superior films. For the first time, a FABr–PbBr 2 –DMSO intermediate is identified and single crystals of the same intermediate compound are synthesized. A systematic investigation of phase evolution in the film formation process reveals that DMSO enables crystallization of the FABr–PbBr 2 –DMSO intermediate and thus modulates the crystallization process of FAPbBr 3 perovskite, achieving uniform, smooth films with Volmer–Weber morphology. To prevent hole leakage arising from the larger bandgap of FAPbBr 3 than FAPbI 3 , an additional layer of Mg‐doped ZnO nanoparticles is added. As a result, inverted SCs using these solvent engineered films achieve power conversion efficiencies (PCEs) of up to 9.06%, the highest reported efficiency for inverted FAPbBr 3 perovskite devices.