Efficient Hybrid Mixed-Ion Perovskite Photovoltaics: In Situ Diagnostics of the Roles of Cesium and Potassium Alkali Cation Addition

Perovskite photovoltaics have made extraordinary progress in power conversion efficiency (PCE) and stability due to process and formulation development. Perovskite cell performance benefits from the addition of alkali metal cations, such as cesium (Cs+) and potassium (K+) in mixed-ion systems, but the underlying reasons are not fully understood. Herein, the solidification of perovskite layers is studied, incorporating 5%, 10%, to 20% of Cs+ and K+ using in situ grazing incidence wide-angle X-ray scattering. It is found that K+-doped solutions yield nonperovskite 4H phase rather than the 3C perovskite phase. For Cs+-doped formulations, both 4H and 3C phases are present at 5% Cs+, whereas the 3C perovskite phase is formed in 10% Cs+-doped formulations, with undesirable halide segregation occurring at 20% Cs+. Postdeposition thermal annealing converts the intermediate 4H phase to the desirable 3C perovskite phase. Importantly, perovskite layers containing 5% of Cs+ or K+ exhibit a reduced concentration of trap states and enhanced carrier mobility and lifetime. By carefully adjusting Cs+ or K+ concentration to 5%, perovskite cells are demonstrated with a ≈5% higher-average PCE than cells utilizing higher cation concentrations. Herein, unique insights into the crystallization pathways toward perovskite phase engineering and improved cell performance are provided.