Site‐Selective Fluorination of Bathocuproine Derivatives for Enhanced Performance and Stability in Perovskite Solar Cells

Interface engineering is vital for optimizing charge transport, stability, and overall efficiency in perovskite solar cells. In this work, two novel fluorinated bathocuproine (BCP) derivatives, BCP-m2F and BCP-m4F, are introduced, featuring site-selective monofluorination at the terminal phenyl rings. Compared to the previously reported BCP-m1, which incorporates aryl substitution for improved planarity and charge transport, these new derivatives leverage fluorination to further tailor the electronic structure and interfacial behavior. The energy-level modulation by fluorination plays only a minor role; however, fluorination significantly enhances device stability through stronger binding with C₆₀ and a pronounced surface passivation effect. Experimentally, BCP-m4F demonstrates superior film uniformity and conductivity compared to BCP-m2F. Time-resolved photoluminescence, J-V analysis, contact angle measurements, and damp heat stability test (ISOS-D3) show improved charge extraction, reduced trap-assisted recombination, increased hydrophobicity, and enhanced thermal and moisture stability, respectively. Notably, a device employing BCP-m4F exhibits minimal open-circuit voltage loss under low-light conditions, highlighting its suitability for indoor or diffuse-light applications. These findings underscore the potential of combining rational backbone design with targeted fluorination to achieve multifunctional interlayers that enhance performance and reliability in next-generation perovskite solar cells.