Benzylphosphonic Acid‐Engineered Compact Self‐Assembled Monolayers for Bifacial Buried Interface Passivation in High‐Performance Inverted Perovskite Solar Cells

While [4‐(3,6‐Dimethyl‐9 H ‐carbazol‐9‐yl)butyl]phosphonic acid (Me‐4PACz) self‐assembled monolayers (SAMs) enable high‐performance inverted perovskite solar cells (PSCs), their sparse coverage on nickel oxide (NiOx) induces heterogeneous interfacial charge distribution at the buried perovskite interface. This increases non‐radiative recombination, ultimately limiting device performance. Herein, benzylphosphonic acid (BPPA) is added, a small molecule featuring a phosphonic acid group, into Me‐4PACz to construct a co‐assembled monolayer (Co‐SAM) with enhanced molecular ordering on NiOx. The resulting compact Co‐SAM hole transport layer (HTL) simultaneously improves electrical conductivity, hole mobility, and interfacial energy level alignment, facilitating efficient hole injection. Moreover, BPPA's phosphonic acid groups enable bifacial passivation: coordinating NiOx surface defects while chelating uncoordinated Pb 2+ at the perovskite interface, significantly suppressing non‐radiative recombination. Optimized Me‐4PACz/BPPA‐based PSCs achieve a champion power conversion efficiency (PCE) of 26.35%, while retaining 90% of the initial efficiency after 3000 h in a nitrogen atmosphere without encapsulation. This molecular co‐assembly strategy concurrently refines HTL properties and buried interface passivation, providing a generalized approach for high‐efficiency, stable PSCs.