FA0.88Cs0.12PbI3−x(PF6)x Interlayer Formed by Ion Exchange Reaction between Perovskite and Hole Transporting Layer for Improving Photovoltaic Performance and Stability

Interface engineering to form an interlayer via ion exchange reaction is reported. A FA0.88Cs0.12PbI3 formamidinium (FA) perovskite layer is first prepared, then FAPF6 solution with different concentrations is spin‐coated on top of the perovskite film, which leads to a partial substitution of iodide by PF6− ion. The second phase with nominal composition of FA0.88Cs0.12PbI3−x(PF6)x is grown at the grain boundary, which has island morphology and its size depends on the FAPF6 solution concentration. The lattice is expanded and bandgap is reduced due to inclusion of larger PF6− ions. The power conversion efficiency (PCE) is significantly enhanced from 17.8% to 19.3% as a consequence of improved fill factor and open‐circuit voltage (Voc). In addition, current–voltage hysteresis is reduced. Post‐treatment with FAPF6 reduces defect density and enhances carrier lifetime, which is responsible for the improved photovoltaic performance and reduced hysteresis. The unencapsulated device with post‐treated perovskite film demonstrates better stability than the pristine perovskite, where the initial PCE retains over 80% after 528 h exposure under relative humidity of around 50–70% in the dark and 92% after 360 h under one sun illumination. A FA0.88Cs0.12PbI3−x(PF6)x interlayer is in situ formed at the FA0.88Cs0.12PbI3/spiro‐MeOTAD interface via an ion exchange reaction, which leads to a significant increase in power conversion efficiency from 17.8% to 19.3% and suppression of current–voltage hysteresis at the same time. The ion exchange reaction reduces the defect density of the perovskite film, which is responsible for the improved photovoltaic performance and stability.