Surface passivation for efficient and stable perovskite solar cells in ambient air: The structural effect of amine molecules

Amine-based molecules are frequently utilized for defect passivation in perovskite films. However, the passivation effect can vary considerably depending on the structural differences of the amine molecules. In this study, we selected small-sized formamidine iodide (FAI), long-chain octylammonium iodide (OAI), and aromatic nicotinic acid (NA) to passivate surface defects in perovskite films. The charge transport in perovskite solar cells (PSCs) was inhibited by the excessively long alkyl chain of OAI but was enhanced by the π-conjugated structure of NA. Consequently, the power conversion efficiency (PCE) of OAI and NA-passivated PSCs decreased by 0.91% and increased by 0.23%, respectively. Interestingly, FAI was found to react preferentially with the PbI2 at the grain boundary of perovskite films, promoting secondary grain growth and resulting in a 21% increase in grain size. Due to the reduction in grain boundaries and the combined passivation of MA+ and I− vacancies, the PCE of FAI-passivated PSCs increased from 18.63% to 19.35% in ambient air and maintained 91% of their initial values after 76 days of storage under conditions of 20 ± 5 °C and <5% RH. This study introduces a promising strategy for selecting an appropriate amine molecule for surface passivation in efficient and stable fully-ambient air processed PSCs.