Reducing energy barrier of δ-to-α phase transition for printed formamidinium lead iodide photovoltaic devices

Recent progress in perovskite photovoltaics has witnessed a growing interest in formamidinium lead iodide (FAPbI(3)), primarily due to its high efficiency potential and excellent stability. However, the high energy barrier of delta-to-alpha phase transition presents a major hurdle to fabricate phase-pure alpha-FAPbI(3) layers. Here, we report a two-step phase transition process to deposit high-quality photovoltaic alpha-FAPbI(3) films by printing method. This is realized by judicious selection of a Lewis base N-methyl-2-pyrrolidone (NMP) and its counter Lewis acid, which enables the regulation of intermediary phase to reduce the energy barrier. With fine tuning the phase transition pathway, phase-pure and stable alpha-FAPbI(3) perovskite films are obtained, which yield solar devices with a champion efficiency of 21.35%. The printed mini-modules with active areas of 12.32 cm(2) and 55.44 cm(2) are also fabricated, giving efficiencies of 17.07% and 14.17%, respectively. This work provides new insights of alpha-FAPbI(3) crystallization for constructing efficient and stable printed photovoltaic devices.