Rubidium Iodide Reduces Recombination Losses in Methylammonium‐Free Tin‐Lead Perovskite Solar Cells

Abstract Outstanding optoelectronic properties of mixed tin‐lead perovskites are the cornerstone for the development of high‐efficiency all‐perovskite tandems. However, recombination losses in Sn‐Pb perovskites still limit the performance of these perovskites, necessitating more fundamental research. Here, rubidium iodide is employed as an additive for methylammonium‐free Sn‐Pb perovskites. It is first investigated the effect of the RbI additive on the perovskite composition, crystal structure, and element distribution. Quasi‐Fermi level splitting and transient photoluminescence measurements reveal that the RbI additive reduces recombination losses and increases carrier lifetime of the perovskite films. This finding is attributed to an approximately ten‐fold reduction in the defect density following RbI treatment, as probed using constant final state yield photoelectron spectroscopy. Additionally, the concentration of Sn vacancies is also reduced, and the perovskite film becomes less p‐type both in the bulk and at the interface towards the electron contact. Thus, the conductivity for electrons increases, improving carrier extraction. As a result, the open‐circuit voltage of RbI‐containing solar cells improves by 61 mV on average, with the best efficiency >20%. This comprehensive study demonstrates that RbI is effective at reducing recombination losses and carrier trapping, paving way for a new approach to Sn‐Pb perovskite solar cell research.