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.