FA_0.88Cs_0.12PbI_{3− 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 FA_0.88Cs_0.12PbI₃ 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 FA_0.88Cs_0.12PbI_{3− 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 (V_oc). 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.