Interface Modification by Ionic Liquid for Efficient and Stable FAPbI3 Perovskite Solar Cells

Formamidinium lead iodide (FAPbI(3)) perovskite solar cells (PSCs) have attracted significant attention owing to their outstanding optoelectronic properties, but long-term device stability is still a crucial issue related to FAPbI(3) PSCs. FAPbI(3) undergoes phase transition from black perovskite phase to yellow non-perovskite phase at room temperature, and moisture triggers this phase transition. One of the most widely used methods to improve the stability of PSCs is interface engineering. Being green functional solvents, ionic liquids (ILs) have been regarded as potential alternatives to toxic interface modifiers, thereby increasing their commercial viability and accelerating their adoption in the renewable energy market. In this study, an IL, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM[BF4]) was used to modify the interface between the electron transport layer and perovskite layer due to its low volatility, low toxicity, high conductivity, and high thermal stability. The introduction of IL not only reduces interface defects but also improves perovskite film quality. Density functional theory (DFT) calculations show that there is a strong interface interaction between the IL and perovskite surface that is beneficial to decrease the density of defect states of the perovskite surface and stabilize the perovskite lattice. Apart from the defects in the perovskite film, solution processed SnO2 also suffers from surface imperfections. Defects on the SnO2 surface generate defect states, which cause band alignment issues and stability issues. DFT calculations show that the surface gap states with IL are smaller than those without IL. Such weakened surface gap states indicate reduced carrier recombination at the surface region, which improves the device performance. Consequently, we achieved a power conversion efficiency exceeding 22% for the IL-modified FAPbI(3) PSCs (control similar to 21%). After storing for over 1800 h in a dry box (relative humidity (RH) similar to 20%), the champion device retained similar to 90% of its initial efficiency, while the control devices degraded into non-perovskite yellow hexagonal phase (delta-FAPbI(3) ).