First-principles study of intrinsic defects in formamidinium lead triiodide perovskite solar cell absorbers

As an alternative to methylammonium lead triiodide (MAPbI₃), formamidinium lead triiodide (FAPbI₃) perovskites have recently attracted significant attention because of their higher stability and smaller band gaps. Here, based on first-principles calculations, we investigate systematically the intrinsic defects in FAPbI₃. While methylammonium (MA)-related defects MA_I and I_MA in MAPbI₃ have high formation energies, we found that formamidinium (FA)-related defects V_FA, FA_I and I_FA in FAPbI₃ have much lower formation energies. Antisites FA_I and I_FA create deep levels in the band gap, and they can act as recombination centers and result in reduced carrier lifetimes and low open circuit voltages in FAPbI₃-based photovoltaic devices. We further demonstrate that through cation mixing of MA and FA in perovskites the formation of these defects can be substantially suppressed.