Symmetric Polar Molecule Optimized Buried Interface for Stable Flexible Perovskite Solar Cells

Molecular anchoring at the buried interface of flexible perovskite solar cells is an efficient approach to address the low adhesion, easy substrate deformation and defects passivation. Herein, we introduce a polar, symmetric small molecular (DPBCA) between the ETL and perovskite layer as a buffer layer. The strong polar functional groups (-C=O and -CN) in DPBCA molecules exhibit strong adhesion on the fragile ETL/perovskite interface, improving the toughness of the buried interface and reducing residual stress, thereby enhancing the mechanical stability of devices. The results indicate that the symmetric structure of DPBCA provides a sufficient number of ligands to synergistically passivate multisite defects, and the collective interaction strength of the symmetric functional groups on both sides is greater than that of the isolated interactions on one side. The carbonyl and cyano groups in the DPBCA molecules can passivate noncoordinated Pb2+/Sn4+. Additionally, the carbonyl and hydroxyl groups in DPBCA can interact with iodine(I), synergistically passivating I vacancies and suppressing the migration of I, which further enhances the passivation effect. This synergistic passivation greatly lowers defect density and increases the lifespan of charge carriers in the perovskite film. As a result, the efficiency of (0.34 cm2) devices based on FA0.85MA0.15PbI3 is improved by 33% (maximum 23.05%). Under different environmental conditions, after bending test cycles, the target devices show noticeably better stability against exposure to light, water, and temperature while retaining over 81.3% of their initial PCE.