Large-Grain Double Cation Perovskites with 18 μs Lifetime and High Luminescence Yield for Efficient Inverted Perovskite Solar Cells

Recent advancements in perovskite solar cell performance were achieved by stabilizing the α-phase of FAPbI₃ in nip-type architectures. However, these advancements could not be directly translated to pin-type devices. Here, we fabricated a high-quality double cation perovskite (MA_0.07FA_0.93PbI₃) with low bandgap energy (1.54 eV) using a two-step approach on a standard polymer (PTAA). The perovskite films exhibit large grains (∼1 μm), high external photoluminescence quantum yields of 20%, and outstanding Shockley–Read–Hall carrier lifetimes of 18.2 μs without further passivation. The exceptional optoelectronic quality of the neat material was translated into efficient pin-type cells (up to 22.5%) with improved stability under illumination. The low-gap cells stand out by their high fill factor (∼83%) due to reduced charge transport losses and short-circuit currents >24 mA cm^–2. Using intensity-dependent quasi-Fermi level splitting measurements, we quantify an implied efficiency of 28.4% in the neat material, which can be realized by minimizing interfacial recombination and optical losses.