FA Cs PbI (PF ) 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 Cs PbI formamidinium (FA) perovskite layer is first prepared, then FAPF solution with different concentrations is spin-coated on top of the perovskite film, which leads to a partial substitution of iodide by PF ion. The second phase with nominal composition of FA Cs PbI (PF ) is grown at the grain boundary, which has island morphology and its size depends on the FAPF solution concentration. The lattice is expanded and bandgap is reduced due to inclusion of larger PF 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 ). In addition, current-voltage hysteresis is reduced. Post-treatment with FAPF 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.