Simultaneous Buried and Top-Interface Passivation Enabled by Fluorinated Ionic Liquids for Efficient Inverted CsPbI ₂ Br Perovskite Solar Cells

Owing to their outstanding thermal resilience and appropriate bandgap, all-inorganic CsPbI2Br perovskites are increasingly recognized as attractive materials for advanced photovoltaic applications. Nevertheless, severe energy dissipation caused by interfacial nonradiative recombination, specially at the buried HTL/perovskite junction and the perovskite/ETL interface, remains a major bottleneck that constrains device efficiency. Herein, we develop a dual interface engineering strategy employing ionic liquids (ILs), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) and 1-butyl-3-methylimidazolium hexafluoroantimonate (BMIMSbF6), to simultaneously passivate defects at both interfaces in inverted CsPbI2Br perovskite solar cells (PSCs). In this work, we systematically examine the disparate roles of modification at the buried interface versus the top interface. Notably, treatment of the bottom interface with ILs primarily boosts the fill factor (FF) by virtue of enhanced hole extraction and improved perovskite crystallinity. In contrast, passivation at the upper interface significantly elevates the open-circuit voltage (Voc), owing to efficient defect neutralization at the perovskite/PCBM contact. The combined interfacial passivation affords a champion PCE of 15.08% for the BMIMPF6-based dual-surface passivation (DSP) cell and 14.52% for its BMIMSbF6-DSP counterpart, corresponding to a 24.5% relative improvement over the control device (12.11%). Comprehensive characterization reveals that BMIMPF6 outperforms BMIMSbF6 due to its superior defect passivation capability. This work establishes a comprehensive understanding of IL-mediated interface engineering and provides a rational bifacial passivation strategy for high-efficiency inverted PSCs.