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

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 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 (sic) O in [Bmim](+)[DBP](-) IL and Pb2+ can improve the morphology of perovskite film by passivating the uncoordinated Pb2+ defects. The interaction between -P (sic) 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).