Substitution Position Effects on Spiro[Fluorene-9,9-Xanthene]-Diphenylamine Hole-Transporting Materials for Perovskite Solar Cells

The molecular structures of hole-transporting materials (HTMs) have a significant effect on the performance of perovskite solar cells (PSCs). In this work, four small-molecular HTMs (SFX-1, SFX-2, SFX-3, and SFX-4) are prepared by regulating the substitution sites of terminal diphenylamine groups on the spiro[fluorene-9,9 '-xanthene] core. As SFX-1 and SFX-2 are well-documented compounds, this article adopts the original publication's acronyms, referring to them as SFX-MeOTAD and HTM-FX ', respectively. It is found that the terminal substitution sites exhibit a noticeable effect on the molecular properties. Among these molecules, SFX-3, whose terminal groups are located at the 3,6-substitution site on the fluorene side of SFX, has high conductivity and hole mobility, and the highest occupied molecular orbital level matches well with the perovskite. SFX-3 also shows better film-forming properties and better hole extraction ability than the molecules with other substitution sites. Higher power conversion efficiency (PCE) in PSCs with SFX-3 is comparable to that of traditional spiro-OMeTAD, but SFX-3's synthesis cost is only about one-third that of spiro-OMeTAD. Furthermore, the device utilizing SFX-3 exhibits remarkable stability, surpassing that of spiro-OMeTAD. Notably, the champion PCE of SFX-3-based PSCs reached 22.42%, marking the highest reported efficiencies among HTMs with SFX as the core.