Furan‐Substituted Phosphine‐Oxide as an Efficient Interfacial Modifier for Wide‐Bandgap Perovskite Solar Cells

ABSTRACT Achieving high performance in wide‐bandgap (WBG) perovskite solar cells (PSCs) is crucial for tandem architectures beyond the Shockley–Queisser limit, yet interfacial nonradiative recombination—particularly at perovskite/charge transport layer (CTL) interfaces—remains a central bottleneck. We introduce phosphine‐oxide interlayers whose P═O groups coordinate undercoordinated Pb 2+ sites in WBG perovskites. By tuning the aryl and furyl substituents, we modulate the P═O Lewis basicity to enable targeted passivation at the buried NiO x hole‐transport layer (HTL)/perovskite interface in p‐i‐n PSCs. In addition, the dipoles formed by the polar groups reinforce the built‐in electric field, promoting more efficient charge extraction. The interlayers also smooth and render the underlying NiO x HTL surface more hydrophobic, consequently yielding perovskite films with higher crystallinity and reduced defect densities, which suppress nonradiative losses. Excellent solubility supports scalable solution processing. Tris(furan‐2‐yl)phosphine oxide (TFPO) outperforms triphenylphosphine oxide (TPPO), attributed to dual coordination sites (P═O and furan oxygen), greater effective Lewis basicity from electron‐rich furyl groups, more favorable energy‐level alignment with the perovskite valence band, and superior surface coverage from the compact furan motif. Devices incorporating TFPO achieve a power‐conversion efficiency of 21.0%, versus 18.3% for controls without interlayers. These results validate a molecularly engineered interfacial strategy for high‐performance WBG PSCs.