Evidences of De-Doped Spiro-OMeTAD Employing Tert-Butyl Pyridine As Additive in Hole-Transporting Layers for n-i-p Perovskite Photovoltaics

Spiro-OMeTAD is the mostly employed molecular hole transporting material (HTM) in n-i-p perovskite solar cells (PSCs). Ease of processing from solution and good filmability on top of the perovskite photo-active layer are characteristics that make this HTM outstanding and incomparable for the role. However, both chemical doping with tert-butylpyridine (tBP) and lithium bis(trifluoromethylsulfonyl)-imide (LiTFSI), coupled to further oxidation steps, are required in order to achieve high hole mobility and conductivity. Previous investigations revealed that tBP is fundamental for addressing the best morphology in the hole-transporting layer during processing. We provide here for the first time a spectroscopic evidence of the detrimental impact on long-term conservation of Spiro-OMeTAD structural and electrical properties when tBP is used as additive. These aspects are crucial for the future design and understanding of new molecular HTMs for PSCs.