N -Shaped vs Λ-Shaped Bridging Symmetric Molecules: Geometric Compatibility Drives Performance Improvement in Perovskite Solar Cells

Abstract

Abstract Intrinsic defects limit the further improvement of photovoltaic performance and stability of perovskite solar cells (PSCs). Bridging symmetric molecules have been proved to be a kind of promising additive for passivating defects within perovskites. However, there is currently a lack of systematic understanding of how the geometric configuration of bridging symmetric molecules affects the modulation effect of photovoltaic performance of PSCs. Herein, the S─S bridging symmetric molecules with nitro groups were selected as additives, i.e., bis-(2-nitrophenyl)-disulfid (B2) and bis(4-nitrophenyl) disulfide (B4), which exhibit tilted N-shaped (N) and inverted V-shaped (Λ) configurations, respectively. Compared with N-shaped B2 molecules with S─S bridging groups hidden inside the molecular structure, Λ-shaped B4 molecules with exposed S─S bridging groups exhibit excellent defect passivation performance in perovskite films. Both experimental and theoretical results indicate that the exposed S─S bond in B4 molecule acts as a third anchoring point, optimizing the matching degree between the spacing distance of passivating group within molecules and the distance between adjacent lead ions in the perovskite lattice. Such improved geometric compatibility not only helps to improve the crystal quality but also releases the residual stress of perovskite film. Moreover, B4 modification also optimizes the band alignment so that it accelerates the charge transfer process of PSCs. As a result, the B4-based PSCs achieve a maximum power conversion efficiency of 25.43% with improved stability. This study not only lays the experimental and theoretical foundation for the rational design of bridging symmetric molecular additives, but also promotes the application of such molecules in improving the performance of various perovskite-based optoelectronic devices.