FAPbI3‐Based Perovskite Solar Cells Employing Hexyl‐Based Ionic Liquid with an Efficiency Over 20% and Excellent Long‐Term Stability

Formamidinium lead triiodide (FAPbI₃)‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap, allowing absorption of photons over a broad solar spectrum. However, FAPbI₃‐based materials suffer from a notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This transition dramatically reduces light absorbtion, thus, degrading the photovoltaic performance and stability of ensuring solar cells. In this study, 1‐hexyl‐3‐methylimidazolium iodide (HMII) ionic liquid (IL) is employed as an additive for the first time in FAPbI₃ perovskite to overcome the above‐mentioned issues. HMII incorporation facilitates the grain coarsening of FAPbI₃ crystal owing to its high‐polarity and high‐boiling point, which yields liquid domains between neighboring grains to reduce the activation energy of the grain‐boundary migration. As a result, the FAPbI₃ active layer exhibits micron‐sized grains with substantially suppressed parasitic traps with an Urbach energy reduced by 2 meV. Hence, the resulting perovskite solar cell achieves an efficiency of 20.6% with notable increase in open circuit voltage (V_OC) of 80 mV compared with HMII‐free cells (17.1%). More importantly, the HMII‐doped FAPbI₃‐based cells show a striking enhancement in shelf‐stability under high humidity and thermal stress, retaining >80% of their initial efficiencies at 60 ± 10% relative humidity and ≈95% at 65 °C.