Multifunctional passivation agents for improving efficiency and stability of perovskite solar cells: Synergy of methyl and carbonyl groups

Recently, the incorporation of passivation agents to reduce perovskite recombination has been proven to be one of the best approaches to achieving high-performance perovskite solar cells (PeSCs). Although several passivation compounds have been discovered, the mechanisms of passivation in diverse functional groups remain a mystery. In this work, we design bi-functional passivation agents, including methyl and carboxyl groups, as well as multi-functional passivation agent structures, including both methyl and carboxyl groups, and then we systematically study their passivation- and crystallinity control-abilities for PeSCs. Through electrostatic interactions with perovskite precursors, bi-functional passivation agents combined with piperidine ring and methyl (CH₃) can passivate defects, and the carboxyl group (C = O) can increase the grain size and reduce the grain boundaries. Importantly, different functional groups, such as COCH₃ groups comprising piperidine rings, CH₃, and C = O, further improved the defect passivation effect. As the interaction between Pb²⁺ and COCH₃ increased, the slow nuclear growth process of perovskite films introducing COCH₃ increased the film quality and their charge extraction/collection capability, thus resulting in a PCE of 19.67%. In addition, the COCH₃-based PesC's V_OC and long-term stability are substantially enhanced.