Bipyridine–Thiosumanene Isomeric Lewis Bases for Synergistic Defect Passivation and Hole Extraction Enables Over 26% Efficient Perovskite Solar Cells

Interface defects between the perovskite layer and the hole transport layer (HTL) seriously limit the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, we designed two isomers, 4BT and 5BT, based on 2,2'-bipyridine-thiosumanene-functionalized polycyclic aromatic hydrocarbons as novel Lewis bases with dual capabilities for deep-level defects (Pb²⁺, V_I) passivation and promoting hole extraction. By shifting the pyridine bridging unit from the tetra to the penta positions, these molecules undergo a configurational transformation from an orthogonal (4BT) to a planar structure (5BT). The configuration-functionality relationship was comprehensively investigated through theoretical and experimental analyses. The planar configuration of 5BT enables more Lewis base sites to interact with perovskite, exhibiting more pronounced deep-level defect passivation effect, while improving hole extraction capability. Consequently, the 5BT-modified n-i-p PSCs achieved a champion PCE of 26.15% (certified at 26.12%) and superior operational stability by retaining 94.0% of the initial PCE according to ISOS-L-2 protocol. This work offers a unique molecular designing mechanism to address the interfacial-related issues for high-performance PSCs.