Star-shaped small-molecule hole-transport materials for dopant-free perovskite solar cells

Abstract

Perovskite solar cells hold great potential for efficient photovoltaics, with hole-transporting materials being key to their success. We synthesized three new conjugated small-molecule HTMs—DPAMes-TT, TPA-TT, and PhFF-TT—and evaluated them in n–i–p PSCs. These molecules, featuring triphenylamine or trifluorobenzene cores, were analyzed in terms of their optical, electrochemical, thermal, and charge transport properties. DPAMes-TT and TPA-TT exhibited narrow bandgaps (2.66 eV and 2.61 eV), HOMO levels well-aligned with MAPbI₃'s valence band (−5.28 eV and −5.30 eV vs. −5.4 eV), and high hole mobilities. In contrast, PhFF-TT with trifluorobenzene core had a wider bandgap (2.95 eV), a less favorable HOMO (−5.14 eV), and signs of J-aggregation, impairing charge transport. PSCs with DPAMes-TT delivered the highest efficiency (19.3%), outperforming TPA-TT (18.5%) and the PTAA reference (18.1%). PhFF-TT devices, however, reached only 12.6% limited by recombination and poor interface quality, as revealed by photoluminescence. Charge transport studies confirmed DPAMes-TT's superior hole extraction, while PhFF-TT suffered higher recombination losses. These findings show how molecular design shapes PSC performance, with amine-based HTMs like DPAMes-TT and TPA-TT excelling over trifluorobenzene-based HTMs.