Distinct Functions of Additives during Thermal Aging in Vacuum-Flash-Assisted Perovskite Solar Cells

Abstract

Additive engineering has been widely used to improve the efficiency and stability of perovskite solar cells; however, the roles of additives in film formation and degradation behavior during VASP processing after spin coating under humid ambient conditions remain insufficiently understood. In this study, we aimed to compare, in real time using in-situ grazing-incidence X-ray scattering analysis, the differences in film formation and degradation behavior depending on OAm, NH4Cl, and NH4I additives in perovskite films fabricated by VASP under approximately 60% RH. The comparison showed that OAm exhibited preferred orientation and partial structural retention, but its improvement in device performance was limited. This is presumed to be because the free-amine-based coordination function of OAm was weakened during humid ambient processing, preventing the passivation and orientation-inducing effects of OAm from being fully expressed. NH4Cl promoted initial grain growth but showed the largest performance degradation and PbI2 formation after aging. This is interpreted to be because NH4Cl contributed to crystallization through NH4+/Cl–-related intermediate formation during humid ambient film formation, while also leaving nonuniform Cl-containing intermediate or additive-derived species at or near grain boundaries, resulting in a film structure vulnerable to subsequent degradation. In contrast, NH4I showed the best balance between initial performance and post-aging stability. This is interpreted to be because I–-related lattice interaction and possible I–-vacancy passivation contributed to enhanced PL intensity, relatively preserved photovoltaic performance, and structural retention during aging. These results indicate that additives can act differently from their conventionally known roles under humid ambient processing conditions and that NH4I is a relatively superior additive in terms of maintaining additive functionality.