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 PbI₂ complexes and perovskite compositions usually leads to light sensitive PbI₂ 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 PbI₂ and ammonium salts by prohibiting the formation of undesired intermediates. N-methylpyrrolidone (NMP) solvent is introduced into the PbI₂ precursor solution to obtain stable and homogeneous PbI₂-NMP complex films. The strong interaction between NMP and formamidinium iodide (FAI) molecules promotes the conversion from PbI₂-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 (T₉₅ = 1200 h), and operating stability with T₉₆ = 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.