Fluorene-terminated hole transporting materials with a spiro[fluorene-9,9′-xanthene] core for perovskite solar cells

Three asymmetric spiro[fluorene-9,9′-xanthene] (SFX)-based hole transporting materials (HTMs) with N-(p-methoxyphenyl)-N′-(9,9′-dimethylfluoren-2-yl)amino (FPA) substituents, which are termed SFX-F, SFX-FM, and SFX-FP, have been synthesized by a facile process. When modifying the number and position of FPA substituents in the HTMs, it was found that there is no significant variation in the HOMO energy by increasing the number of FPA groups, while the thermal stability is governed by the substitution position. SFX-FM, which contains two meta-substituted FPA groups on the xanthene unit, exhibits a suitable HOMO energy level (−5.24 eV) and a strikingly high Tg (154 °C). In addition, SFX-FM also shows better film-forming properties and excellent hydrophobicity, which is conducive to fabricating high-performance devices. Noticeably, the PSCs based on SFX-FM achieved a power conversion efficiency (PCE) of 17.29% under AM 1.5 G illumination at 100 mW cm−2, unprecedentedly outperforming PSCs based on spiro-OMeTAD (15.14%).