Solution-Processable Ionic Liquid as an Independent or Modifying Electron Transport Layer for High-Efficiency Perovskite Solar Cells

Inorganic metal oxide, especially TiO2, has been commonly used as an electron transport layer (ETL) in regular structure (n-i-p) planar heterojunction perovskite solar cells (PHJ-PSCs) but generally suffers from high electron recombination rate and incompatibility with low-temperature solution processability. Herein, by applying an ionic liquid (IL, 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM]PF6)) as either a TiO2-modifying interlayer or an independent ETL, we investigated systematically IL interface engineering for PHJ-PSCs. Upon spin-coating [EMIM]PF6-IL onto TiO2 ETL as a modification layer, the average power conversion efficiency (PCE) of CH3NH3PbI3 PHJ-PSC devices reaches 18.42 0.65%, which dramatically surpasses that based on commonly used TiO2 ETL (14.20 +/- 0.43%), and the highest PCE (19.59%) is almost identical to that of the record PCE for planar CH3NH3PbI3 PSCs (19.62%) reported very recently. On the other hand, by applying [EMIM]PF6-IL as an independent ETL, we achieved an average PCE of 13.25 +/- 0.55%, and the highest PCE (14.39%) approaches that obtained for PHJ-PSCs based on independent TiO2 ETL (14.96%). Both IL interface engineering methods reveal the effective electron transport of [EMIM]PF6-IL. The effects of [EMIM]PF6-IL on the surface morphology, crystallinity, and optical absorption of the perovskite film and the interface between the perovskite layer and substrate were investigated and compared with the case of independent TiO2 ETL, revealing the role of [EMIM]PF6-IL in efficient electron transport.