Highly efficient and stable planar CsPbI2Br perovskite solar cell with a new sensitive-dopant-free hole transport layer obtained via an effective surface passivation

Hole transport layers (HTLs) plays critical role in CsPbI2Br perovskite solar cells (PSCs). However, few studies have been done to develop new and sensitive-dopant-free HTL for CsPbI2Br PSCs. Here, a low-cost fluorinated N',N',N '',N ''-tetralcis(4-methoxypheny)spiro[fluorene-9,9'-xanthene]-2,7-diamine (2mF-X59) was synthesized and applied as sensitive-dopant-free HTL into CsPbI2Br PSCs. After modified the 2mF-X59 and CsPbI2Br surface by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4-TCNQ), a maximum power conversion efficiency (PCE) of 14.42% was obtained for CsPbI2Br PSC with an impressive open-circuit voltage (V-oc) of 1.23 V, which is higher than that of the most previously reported CsPbI2Br PSCs with the doped Spiro-OMeTAD. More importantly, the CsPbI2Br PSCs with the newly developed HTLs showed remarkable stability retaining more, than 94% of their initial PCE value after aging in air for 30 days without encapsulation. Therefore, we firmly believe that our new 2mF-X59 HTL created via molecular engineering is a promising candidate for preparing high performance CsPbI2Br PSCs.