Bisphosphonic Acid Functionalized Conjugated Polymer Enables Synergistic Co-Assembly for Fabricating Efficient Perovskite Solar Cells

Abstract The practical application of self-assembled monolayers (SAMs) in inverted perovskite solar cells (PSCs) is severely hindered by insufficient substrate binding, inhomogeneous coverage, and limited defect passivation at the buried interface. Herein, we report the synthesis of poly{(2-(4,8-bis(3-(dimethylamino)propoxy)-6-(thiophen-2-yl)benzo [1,2-b:4,5-b′]dithiophen-2-yl)-7-(thiophen-2-yl)-9H-fluorene-9,9-diyl)[bis(hexane-6,1-diyl)]bis(phosphonic acid)}, a bisphosphonic acid–functionalized polymer, which was co-assembled with 4-phenyl-4-(4-(dicyanomethylene)-2-methyl-6-(pyridin-4-yl)-4H-pyran-4-ylidene)cyclohexanone, a conventional small molecule forming SAMs to fabricate a synergistic composite hole-transport layer. Based on a synergistic dual-site anchoring mechanism, the multidentate bisphosphonic acid groups strongly bind with the transparent conductive oxide substrate, while the sulfur atoms in the polymer backbone coordinate with undercoordinated Pb2+, mitigating interfacial ionic defects. The conjugated backbone facilitates rapid hole transport along the polymer chain, while co-assembly suppresses molecular aggregation, yielding a uniform, pinhole-free buried interface with enhanced wettability. Consequently, the optimized inverted PSCs exhibit a remarkable power conversion efficiency (PCE) of 24.23%, with a short-circuit current density of 24.85 mA cm–2 and high fill factor of 84.26%. The unencapsulated device maintains ∼80% of its initial PCE after 600 h of thermal aging at 80°C under a nitrogen atmosphere. This polymer-small-molecule co-assembly strategy offers a versatile route for developing efficient and stable inverted perovskite photovoltaics.