Interfacial ?-FAPbI3 phase stabilization by reducing oxygen vacancies in SnO2_x

Reducing nonradiative recombination in SnO2_x has been a critical point for fabricating efficient and stable perovskite solar cells (PSCs). Controlling oxygen vacancies in SnO2_x is an efficient strat-egy, but most research only presents the consequential results without scrutinizing the phenomenological part of the strategy. Here, we deeply examined and revealed a new beneficial effect of controlling oxygen vacancies in SnO2_x. Oxygen atoms of SnO2_x were responsible for retaining a-FAPbI3 at the FAPbI3/SnO2_x inter-face by controlling the formation of iodine interstitials, which are strong initiators of unfavorable perovskite phase transitions. Using crystallographic analysis, we observed suppression of these phase transitions when oxygen vacancies were mitigated in SnO2_x by oxidized black phosphorus quantum dots. Furthermore, formamidi-nium (FA) cation retention was also observed as a beneficial effect of the strategy by introducing hydrogen bonding sources for FA cat-ions at the interface. Our findings suggest the genuine necessity of oxygen vacancy reduction in SnO2_x.