Unraveling the interfacial homogeneity and bulk crystallization for efficient and stable perovskite solar cells via ionic liquids†

Abstract

Despite advances in the efficiency of inverted perovskite solar cells using self-assembled monolayers (SAMs), challenges persist in both efficiency and stability due to issues at the bottom interface and within the bulk perovskite. The SAM at the bottom interface is prone to being washed away by the overlying perovskite solvent, leading to interface inhomogeneity, which affects the non-radiative recombination. In this study, we introduce ionic liquids (ILs) as a protective layer for the SAM, stabilizing its uniformity and simultaneously passivating the bottom-side perovskite interface and matching the interface energy levels. Additionally, we incorporate the ionic liquid tetramethylguanidine tetrafluoroborate (TMGBF₄) into the perovskite precursor solution to regulate the crystallization of the perovskite. TMGBF₄ provides both electron-withdrawing and electron-donating properties, chemically passivating uncoordinated Pb²⁺ and halide vacancies through coordination and ionic bonds. This passivation reduces the trap defect density and improves the long-term stability of the perovskite film. As a result of the effects of these ILs on both the bulk and interfaces, we achieved a champion power conversion efficiency of 26.18% (certified 25.74%), along with excellent long-term operating stability for 1100 hours under continuous light stress at 65 °C.