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

While [4-(3,6-Dimethyl-9H-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 Pb²⁺ 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.