Ionic liquid dopant for hole transporting layer towards efficient LiTFSI-free perovskite solar cells

Regarded as one of the best hole transporting material (HTM) for perovskite solar cells, 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) has suffered from instability induced by Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) hygroscopicity and aggregation. Herein, we replace Li-TFSI with the hydrophobic ionic liquid (IL) 1-Butyl-1-methylpiperidinium bis(trifluoromethyl sulfonyl)imide (BMPIP-TFSI). The BMPIP-TFSI functions as a p -type dopant in the spiro-OMeTAD, increasing the hole mobility, promoting the conductivity and enhancing the moisture-resistance of HTM. The corresponding perovskite solar cells achieve a champion power conversion efficiency exceeding 23%. Furthermore, the un-encapsulated devices show enhanced stability under humid environment, retaining 85% of its initial efficiency after 500 h.