Tailoring the Crystallization Behavior of Mixed Lead-Tin Mixed-Halide Perovskites for Optimal-Bandgap Solar Cells

Incorporating bromide into metal-iodide perovskites is a commonly used approach for widening the bandgap of lead-halide perovskites. Here, mixing of iodide and bromide is explored in narrow-bandgap lead-tin perovskites to create a Cs_0.1FA_0.6MA_0.3Pb_0.5Sn_0.5I_2.5Br_0.5 perovskite composition, achieving the optimal bandgap of 1.34 eV for single-junction solar cells. Introducing bromide into the precursor solution, markedly influenced film formation and resulted in singular 40 µm-sized perovskite crystals. Supported by in situ absorption measurements, it is found that the delay time between starting the spin-coating of the perovskite precursor and depositing the antisolvent is key in controlling the film morphology. By drastically reducing this delay time, homogenous nucleation is induced and smooth closed films are obtained. The Cs_0.1FA_0.6MA_0.3Pb_0.5Sn_0.5I_2.5Br_0.5 perovskite do not show signs of light-induced halide segregation during prolonged illumination. Using ammonium thiocyanate (NH₄SCN) as additive in the precursor solution, the grain size could be further controlled. In solar cells, NH₄SCN improved reproducibility and decreased hysteresis is observed. Applying passivation to reduce non-radiative recombination at the perovskite - electron transport layer interface and optimizing the device configuration results in a power conversion efficiency of 19.0%. This is among the highest for perovskites in the 1.3-1.4 eV bandgap range reported to date.