Passivating detrimental grain boundaries in perovskite films with strongly interacting polymer for achieving high-efficiency and stable perovskite solar cells

The grain boundaries in polycrystalline metal-halide perovskite films contain numerous defects, which contribute to performance loss and degradation in perovskite solar cells. In this study, we report a cyano-containing conjugated polymer, composed of benzodithiophene and bicyanobenzothiadiazole (PBDT2CNBT), that functionalizes perovskite crystalline domains for defect passivation and charge carrier extraction. We demonstrate that cyano groups can effectively passivate these defects through strong interactions between the conjugated polymer and perovskite components. The PBDT2CNBT polymer not only modifies the perovskite film surface but also infiltrates the grain boundaries during perovskite crystallization. Notably, these conjugated polymers create favorable energy band bending at the boundary interface between the perovskite and conjugated polymer, enhancing charge separation and collection through grain boundaries by forming a cascade energy level structure. Consequently, we show that PBDT2CNBT transforms the intrinsically detrimental grain boundary into a beneficial feature for defect-free properties and improved charge collection. These advantages directly contribute to achieving high power conversion efficiencies of up to 22.9% and enhanced light, humidity, and thermal stability in perovskite solar cells. This study offers a novel strategy for developing multifunctional conjugated polymers for next-generation perovskite optoelectronics.