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[Display omitted] • The stable α-phase pure FAPbI 3 is prepared by imidazolium-based ionic liquid. • The efficiency enhances to 20.76% due to the high properties of pure FAPbI 3. • The unencapsulated pure FAPbI 3 devices show the excellent stability. The stable black α-phase is the key to attaining high-efficiency formamidinium lead triiodide (FAPbI 3) perovskite solar cells (PSCs). There have been some reports using different sizes of ions to stabilize α-phase FAPbI 3 ; 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 FAPbI 3 perovskite without encountering the above adversities. It is found that the thermodynamic temperature of α-FAPbI 3 is significantly reduced from 150 °C to 145 °C by using AMICl, and the absorption edge of FAPbI 3 remains at 850 nm. The efficiency of a FAPbI 3 device with AMICl is increased from 18.47% for the control device to 20.76%, one of the highest values among pure FAPbI 3 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 FAPbI 3 generated by the formation of intermediate products from PbI 2 and AMICl, demonstrating by density functional theory (DFT) calculation. Meanwhile, FAPbI 3 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. [ABSTRACT FROM AUTHOR]