Bridging‐Solvent Strategy for Quasi‐Single‐Crystal Perovskite Films and Stable Solar Cells

Controllable fabrication of formamidinium (FA)‐based perovskite solar cells (PSCs) with both high efficiency and long‐term stability is the key to their further commercialization. However, the diversity of PbI 2 complexes and perovskite compositions usually leads to light sensitive PbI 2 residues and phase impurities in the film, which can accelerate the device degradation. Here, the crystallization kinetics of FA‐based perovskite films are studied and a bridging‐solvent strategy is proposed to modulate the reaction kinetics between PbI 2 and ammonium salts by prohibiting the formation of undesired intermediates. N‐methylpyrrolidone (NMP) solvent is introduced into the PbI 2 precursor solution to obtain stable and homogeneous PbI 2 ‐NMP complex films. The strong interaction between NMP and formamidinium iodide (FAI) molecules promotes the conversion from PbI 2 ‐NMP into (001)‐oriented quasi‐single‐crystal perovskite films with negligible impurities, long carrier lifetime of 1.5 µs and a large grain size of 3 µm. The optimized PSCs exhibit a high power conversion efficiency of 24.1%, as well as superior shelf stability which maintains 95% initial efficiency after storage in air for 1200 h (T 95 = 1200 h), and operating stability with T 96 = 300 h under continuous working at the maximum power point. This work offers a simple and reproducible method for fabricating phase‐pure and uniaxially oriented perovskite films.