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.