Methoxy groups on bifluorenylidene-based hole transporting materials result in highly efficient and stable dopant-free inverted perovskite solar cells

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Three bifluororenylidene-based compounds featuring different numbers of methoxy groups (none for HTM-1, four for HTM-2, and eight for HTM-3) have been synthesized simply and inexpensively and applied as hole transporting materials (HTMs) in inverted perovskite solar cells (PSCs). For comparison, a typical inverted HTM material, PEDOT:PSS, was also applied in a corresponding inverted PSC. The corresponding cell performances were compared with respect to their number of methoxy groups. Of the PSCs containing the bifluororenylidene-based compounds, the HTM-2–based device exhibited the highest power conversion efficiency (PCE) of 12.0%, with a value of Jsc of 19.4 mA cm−2, a value of Voc of 0.94 V, and a fill factor of 65.6%. In addition, the PCE of this HTM-2–based device was higher than that (9.9%) of the PEDOT:PSS–based cell under similar testing conditions. The relatively high PCE of the HTM-2–based cell was due to its suitable energy levels, relatively high hole mobility, and relatively large grains. Furthermore, the HTM series of compounds were also more hydrophobic than PEDOT:PSS and, therefore, they imparted device stability superior to that of the PEDOT:PSS–based cell.