Synergistic Co-Passivation via Ultrasmall Molecules for Efficient and Stable Perovskite Solar Cells

Defect passivation is one feasible and effective approach to minimize the charge recombination of perovskite solar cells (PSCs). However, the efficacy of conventional defect passivation using Lewis base-functionalized molecules is limited due to weak bonding and large space steric hindrance. Herein, we propose a molecular engineering approach employing ultrasmall-sized potassium diformate and potassium phosphate to synergistically passivate defects at the SnO₂/perovskite buried interface. It is found that the C═O and P═O groups effectively passivate interfacial traps, reducing trap density by ∼50% relative to the control sample. The co-modification also lowers the surface energy of SnO₂ layers, promoting the growth of perovskite grains with an average grain size increased from 780 to 1370 nm. Moreover, thermally diffused K⁺ ions can inhibit ion migration in the perovskite lattice, thereby suppressing the hysteresis effect. According to these advantages, the optimized devices achieve a champion power conversion efficiency of 24.42% (vs 22.64% for the pristine SnO₂-based counterpart). The unencapsulated co-passivated PSCs retain 92% of their initial efficiency after 2100 h under ambient conditions, substantially outperforming the control devices (74% retention).