Molecular Ferroelectric with Directional Polarization Field for Efficient Tin-Based Perovskite Solar Cells

Due to the relatively inferior dielectric constant, Sn-based perovskites exhibit lower defect tolerance and insufficient dielectric shielding effect compared with Pb-perovskites. Upgrading built-in electric field (BEF) in Sn-based perovskite solar cells (PSCs) can be effective to reduce large voltage deficit and improve poor performance caused by the low defect tolerance resulting from the intrinsic inferior dielectric of Sn-based perovskites. Herein, 2-methylbenzimidazole (MBI) molecular ferroelectric with low coercive field and high Curie-temperature is introduced to construct an additional ferroelectric field in FASnI(3)-based PSCs. The ferroelectric effect of MBI can promote exciton dissociation, enhance carrier population, and suppress the adverse effect of the residual defects on carriers, and the directional polarization of MBI in FASnI(3) film can be driven by the BEF in PSCs to broaden the width of depletion region. Additionally, the MBI molecules with amine functional groups effectively regulate perovskite crystallization, passivate Sn-related defects, and enhance the oxidation barrier. Profiting from the above advantages, the MBI-modified device achieves a champion power conversion efficiency (PCE) of 12.91%, keeping over 94% average PCE after 1056 h in N-2 glovebox for the unencapsulated device. This study highlights the significant role of molecular ferroelectrics in perovskite photovoltaics.