Imidazolium-based ionic liquid for stable and highly efficient black-phase formamidinium-based perovskite solar cell

The stable black α-phase is the key to attaining high-efficiency formamidinium lead triiodide (FAPbI₃) perovskite solar cells (PSCs). There have been some reports using different sizes of ions to stabilize α-phase FAPbI₃; unfortunately, they often affect the bandgap, stability and carrier dynamics, in addition to increasing the number of carrier trap sites. Herein, we present 1-allyl-3-methylimidazolium-chloride (AMICl) ionic liquid to stabilize the α-phase of pure FAPbI₃ perovskite without encountering the above adversities. It is found that the thermodynamic temperature of α-FAPbI₃ is significantly reduced from 150 °C to 145 °C by using AMICl, and the absorption edge of FAPbI₃ remains at 850 nm. The efficiency of a FAPbI₃ device with AMICl is increased from 18.47% for the control device to 20.76%, one of the highest values among pure FAPbI₃ solar cells. The high efficiency is attributed to the low trap density, long carrier lifetime and suppressed carrier recombination, thanks to the large grain size and good crystallinity of FAPbI₃ generated by the formation of intermediate products from PbI₂ and AMICl, demonstrating by density functional theory (DFT) calculation. Meanwhile, FAPbI₃ solar cells with AMICl exhibit the excellent environmental stability. The bare device without any encapsulation maintains about 87% of its initial value after being exposed to ambient for 24 days, significantly improved over the control device, which retains about 53% of its initial efficiency under the same conditions.