Homogenization and Rapid Oxidation of Spiro-OMeTAD with Ionic Liquids for Efficient Perovskite Solar Cells

Spiro-OMeTAD is widely recognized as the most effective hole transport layer (HTL) for n-i-p perovskite solar cells (PSCs), which typically requires doping with LiTFSI to overcome its low inherent conductivity. However, the doping takes a prolonged oxidation (≈24 h) in an ambient atmosphere, hindering the commercial development. Moreover, the aggregation of LiTFSI leads to poor conductivity and accelerated degradation of the HTL, which are often ignored. This study introduces the long-chain ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (OMIMTFSI) as a multifunctional additive to mitigate the aggregation of LiTFSI and promote the oxidation of Spiro-OMeTAD simultaneously. The strong electrostatic interactions between OMIM⁺ and LiTFSI, coupled with the dispersion effect of OMIM⁺ in chlorobenzene, effectively hamper the aggregation of LiTFSI, beneficial for uniform doping and enhanced conductivity. The OMIM⁺ also facilitates rapid oxidation of Spiro-OMeTAD by attracting lone pair electrons from the triphenylamine group. As a result, the power conversion efficiency of PSCs processed in air is significantly improved from 21.48% to 24.04% with enhanced stability, maintaining over 80% of initial values after storing in air for 1360 h or under light and heat treatment for 500 h. This strategy provides valuable insights of designing lithium salt-doped Spiro-OMeTAD for efficient and stable PSCs.