Fluorine substitution position effects on spiro(fluorene-9,9′-xanthene) cored hole transporting materials for high-performance planar perovskite solar cells

Fluorine substitution in molecular design has become an effective strategy for improving the overall performance of organic photovoltaics. In this study, three low-cost small molecules of spiro-linked hole transporting materials (SFX-o-2F, SFX-m-2F, and SFX-p-2F) endowed with two-armed triphenylamine moieties were synthesized via tuning of the fluorine substitution position, and they were employed for use in highly efficient perovskite solar cells (PSCs). Despite the fluorine substitution position playing a negligible role in the optical and electrochemical properties of the resulting small molecules, the photovoltaic performance thereof was observed to vary significantly. The planar n-i-p PSCs based on SFX-m-2F demonstrated superior performance (18.86%) when compared to that of the corresponding SFX-o-2F (9.7%) and SFX-p-2F (16.33%) under 100 mW cm⁻² AM1.5G solar illumination, which is competitive with the performance of the benchmark spiro-OMeTAD-based device (18.98%). Moreover, the SFX-m-2F-based PSCs were observed to be more stable than the spiro-OMeTAD-based devices under ambient conditions. The improved performance of SFX-m-2F is primarily associated with improved morphology, more efficient hole transport, and extraction characteristics at the perovskite/HTM interface. This work demonstrated the application of fluorination engineering to the tuning of material film morphology and charge transfer properties, showing the promising potential of fluorinated SM-HTMs for the construction of low-cost, high-efficiency PSCs.