Bulky cation hinders undesired secondary phases in FAPbI3 perovskite solar cells

Interfacial treatments with bulky cations over 3D lead halide perovskites (LHPs) have enabled power conversion efficiencies beyond 25 %. However, the mechanisms behind these improvements still need to be better understood. In this work, we focus on a specific type of perovskite, formamidinium lead iodide (FAPI), without additives, and demonstrate that phenethylammonium iodide (PEAI) can prevent the transformation of the FAPI perovskite into undesirable hexagonal non-perovskite phases when exposed to air. We use various analytical techniques to understand how the organic salt interacts with the 3D perovskite absorber. Through grazing incidence wide-angle X-ray scattering analysis, we study the structure at the surface and bulk, observing that hexagonal FAPI phases dominate the phase composition in FAPI films without surface treatments. Time-resolved photoluminescence (TRPL) and terahertz (THz) spectroscopy show an enhancement in the carrier lifetime and photoconductivity, respectively, when the FAPI films are coated with PEAI. In solar cells, introducing PEAI enables power conversion efficiencies of up to 20.2 % and open circuit voltages (VOC) of up to 1.14 V, which are among the highest reported values for a FAPI solar cells without additional A-site cations.