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.