Functional Spiro-OMeTAD-like dopant for Li-Ion-free hole transport layer to develop stable and efficient n-i-p perovskite solar cells

Perovskite solar cells (pero-SCs) containing lithium bis(trifluoromethane)sulfonimide (LiTFSI)-doped SpiroOMeTAD-based hole transport layers (HTLs) exhibit outstanding power conversion efficiencies (PCEs). However, the dopant LiTFSI likely causes a severe morphology change and induces ion migration, which can also corrode the underlying perovskite film, owing to its hygroscopicity and small ionic radius. In this study, we developed a Spiro-OMeTAD-like organic salt (Spiro-BD-T) with a large-size cation composed of a Spirobifluorene (Spiro) backbone and phenylpyridin-4-amine (BD) as well as an anion composed of TFSI-. The anion in Spiro-BDT can promote the oxidation of Spiro-OMeTAD and effectively form a weakly bound Spiro-OMeTAD center dot+TFSIsystem. Moreover, the Spiro-OMeTAD-like cation in Spiro-BD-T exhibits good miscibility with Spiro-OMeTAD, excellent hydrophobicity and does not easily migrate, which diminishes the ingress of humidity and eliminates the detrimental Li+ migration. Ultimately, the Spiro-BD-T-doped Spiro-OMeTAD HTL showed a homogeneous and pinhole-free morphology with improved composition stability. The resultant pero-SCs exhibited an excellent PCE of 24.2% with significantly promoted long-term stability.