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

Abstract

AbstractLithium 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 SnO2, 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 Pb2+ can improve the morphology of perovskite film by passivating the uncoordinated Pb2+ 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).