Sulfonate-Anchored Ionic Liquid Enables High-Performance Wide-Bandgap Perovskites with an Ultrahigh Fill Factor

Wide-bandgap (WBG) perovskite solar cells (PSCs) are pivotal for developing high-efficiency tandem solar cells; however, their performance is substantially limited by interfacial defects and nonradiative recombination. Herein, we introduce 1-butyl-3-methylimidazolium tosylate (BDMIMTS), an imidazolium-based ionic liquid, as a multifunctional additive incorporated via precursor doping. This approach simultaneously modulates perovskite crystallization kinetics, suppresses defect states, and optimizes interfacial energy alignment. We elucidate a multisite synergistic passivation mechanism: sulfonate anions form coordination bonds with undercoordinated Pb2+ species, while imidazolium cations constrain the movement of negatively charged halides via hydrogen bonding, thereby enhancing the passivation effect and effectively mitigating the influence of deep-level traps. The optimized devices achieve a power conversion efficiency (PCE) of 22.62%, an open-circuit voltage (VOC) of 1.24 V, and an ultrahigh fill factor (FF) of 87.50%. Furthermore, encapsulated devices retain over 90% of their initial PCE after 1100 h in a nitrogen atmosphere. This work provides a strategic material design paradigm for enhancing the performance and stability of perovskite-silicon tandem solar cells, highlighting a promising pathway toward advanced photovoltaics.