Optimization of Stable Quasi-Cubic FA_<i>x</i>MA_1–<i>x</i>PbI₃ Perovskite Structure for Solar Cells with Efficiency beyond 20%

Complex compositional engineering of mixed halides/mixed cations perovskites has recently fostered a rapid progress in perovskite solar cell technology. Here we demonstrate that when 10% of formamidinium (FA+) is simply added into methylammonium lead iodide (MAPbI3) a highly crystalline and compositionally uniform perovskite is formed, self-organizing into a stable “quasi-cubic” phase at room temperature. We reached power conversion efficiency of over 20.2%, the highest value reported to date for FAxMA1−xPbI3 perovskite.