Surface engineering based on ionic liquids for efficient and stable CsPbI₃ perovskite solar cells

All-inorganic perovskite CsPbI3 is widely recognized as a promising material for photovoltaic applications in single-junction or tandem solar cells. However, the spontaneous phase transition of CsPbI3 remains a crucial obstacle to commercialization. In this study, we propose a surface engineering approach using a tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) ionic liquid to prevent the perovskite-to-non-perovskite phase transition induced by moisture. The experimental results demonstrate that by chemically bonding excess lead iodide (PbI2) with a tetrabutylammonium cation (TBA(+)), a low-dimensional stable and hydrophobic material called [TBA]PbI3 was achieved, which effectively enhances the stability of the CsPbI3 perovskite structure. Furthermore, we found that the hexafluorophosphate anion (PF6-) can effectively passivate surface iodine vacancies and interact with uncoordinated Pb2+. Consequently, the CsPbI3-based perovskite solar cells (C-PSCs) employing the TBAPF6 treatment achieved an impressive power conversion efficiency (PCE) of 17.47%. Moreover, the unencapsulated devices retained more than 70% of their initial PCE after being stored in ambient air for 168 hours.