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 MAI and IMA in MAPbI₃ have high formation energies, we found that formamidinium (FA)-related defects VFA, FAI and IFA in FAPbI₃ have much lower formation energies. Antisites FAI and IFA 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.