Phosphate Functionalized Ionic Liquid: An Efficient Li+ Migration Inhibitor and Hole Extraction Accelerator for Perovskite Solar Cells

Abstract Lithium bis(trifluoromethanesulfonyl)imide (Li‐TFSI) doped 2,2′,7,7′ ‐tetrakis( N , N ‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (Spiro‐OMeTAD) is employed as one of the most common hole transport layer. However, the Li + in Li‐TFSI is easy to migrate from Spiro‐OMeTAD to perovskite and SnO 2 , left the voids/pinholes in perovskite film to accelerate the invasion of oxygen and moisture, and lead to the device degradation. Here, the 1‐butyl‐3‐methyl imidazolium phosphate dibutyl ester ([Bmim] + [DBP] − ) ionic liquid (IL) is prepared and used to modify the perovskite/Spiro‐OMeTAD interface in the perovskite solar cells. The coordination interaction between P  O in [Bmim] + [DBP] − IL and Pb 2+ can improve the morphology of perovskite film by passivating the uncoordinated Pb 2+ defects. The interaction between P  O and Li + can suppress the undesired Li + migration. The regulated energy level assignment and ion nature of [Bmim] + [DBP] − IL can accelerate the carrier extraction at perovskite/Spiro‐OMeTAD interface. The device based on the [Bmim] + [DBP] − IL interface modification yields a highly efficient photoelectric conversion efficiency of 21.16% and improved stability, outperforming that of control (19.31%) devices under the illumination of 100 mW cm −2 (AM 1.5 G).