Rubidium Halide Additive Engineering for Efficient and Stable Bifacial Perovskite Solar Cells

Abstract Bifacial perovskite solar cells (PSCs) possess a dual light‐absorbing structure, which enables higher power output at lower additional costs. However, the replacement of metal electrodes with transparent ones leads to serious light loss, necessitating a thicker perovskite layer to compensate for this drawback. In this study, the impact of different rubidium halide (RbCl, RbBr, and RbI) additives is systematically investigated on precursor solubility and the quality of micron‐sized perovskite thick films prepared using a two‐step method. It is observed that RbCl exhibited the strongest interactions with PbI₂, enhancing its solubility and leading to the formation of multihole PbI₂ films that promote subsequent crystallization of thick perovskite film. Additionally, the RbCl‐based perovskite film demonstrates a narrowed bandgap attributed to the formation of a‐FAPbI₃. Consequently, the short‐circuit current density (J_sc) and open‐circuit voltage (V_oc) in the RbCl‐based bifacial PSCs are simultaneously enhanced, and a record efficiency of 20.86% (21.04% certified) is also obtained among n‐i‐p bifacial PSCs, exhibiting an ≈80% bifaciality and a power generation density (PGD) exceeding 24 mW cm⁻² with a reflectivity value of 0.2. More importantly, the optimized device does not show any performance degradation after continuous illumination under 1sun for 1000 h.