Evidence of Spiro-OMeTAD De-doping by tert-Butylpyridine Additive in Hole-Transporting Layers for Perovskite Solar Cells

Spiro-OMeTAD is the most-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, chemical doping with both tert-butylpyridine (tBP) and lithium bis(trifluoromethylsulfonyl)-imide (LiTFSI), coupled with further oxidation steps, is required in order to achieve high hole mobility and conductivity. Previous investigations have revealed that tBP is fundamental for addressing the best morphology in the hole-transporting layer during processing. Here, we provide spectroscopic evidence of the detrimental impact on long-term conservation of Spiro-OMeTAD structural and electrical properties when tBP is used as an additive. These aspects are crucial for the future design and understanding of new molecular HTMs for PSCs.