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

Formamidinium lead triiodide (FAPbI(3))-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, FAPbI(3)-based materials suffer from a notorious phase transition from the photoactive black phase (alpha-FAPbI(3)) to nonperovskite yellow phase (delta-FAPbI(3)) 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 FAPbI(3) perovskite to overcome the above-mentioned issues. HMII incorporation facilitates the grain coarsening of FAPbI(3) 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 FAPbI(3) 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 FAPbI(3)-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 approximate to 95% at 65 degrees C.