Improving buried interface contact by diphenylphosphine oxide substituent for highly efficient inverted perovskite solar cells

(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) is widely employed as a self-assembled monolayer (SAM) for the hole transport layer (HTL) in inverted perovskite solar cells (PSCs), but its performance is limited by the carbazole core's insufficient defect passivation, weak phosphate anchoring to NiOx, and energy level mismatch with the perovskite layer. We design and synthesize a stable SAM, 4-(3,6-bis(diphenylphosphoryl)-9H-carbazol-9-yl)butylphosphonic acid (DPPO-4PACz), which enhances and stabilizes the interface between NiOₓ and perovskite. DPPO-4PACz is doped with Me-4PACz to form a co-SAM, which enhances the interfacial binding energy of the NiOx surface, suppresses surface defect states and increases conductivity. Furthermore, DPPO-4PACz promotes the growth of perovskite crystals, passivates buried interface defects, relieves residual stress in the perovskite film, and optimizes the energy level alignment at the hole-selective interface, thereby enhancing hole extraction efficiency. As a result, the champion inverted device achieves a high power conversion efficiency (PCE) of 25.67 %. Notably, after continuous operation at the maximum power point for 1200 h, the encapsulated device retains 93 % of its initial PCE. Our work underscores the critical role of head group design in SAMs at the buried interface for achieving high-performance PSCs, paving the way for further advancements in their practical applications.