Dopant-free hole transport materials based on alkyl-substituted indacenodithiophene for planar perovskite solar cells

Two dopant-free hole transporting materials (HTMs) comprising a planar indacenodithiophene (IDT) core with different alkyl chains (either C₄ or C₆) and electron-rich methoxytriphenylamine (TPA) side arms were synthesized (namely IDTC₄-TPA and IDTC₆-TPA, respectively) and successfully employed in CH₃NH₃PbI₃ perovskite solar cells. These HTMs can be obtained from relatively cheap starting materials by adopting a facile preparation procedure that does not use expensive and complicated purification techniques. In the crystal lattice, each of these molecules interacted with either the CH/π or CH/O hydrogen bonds. At the same time, the IDTC₆ backbone enables a tight molecular arrangement based on π-π stacking interactions (3.399 Å); this causes the new material to have a higher hole mobility than the standard IDTC₄-based HTM. Moreover, the photoluminescence quenching in a perovskite/HTM film structure was shown to be more effective at the perovskite/IDTC₆-TPA interface than at the perovskite/IDTC₄-TPA interface. Consequently, devices fabricated using IDTC₆-TPA show superior photovoltaic properties (exhibiting an optimal performance of 15.43%) compared with IDTC₄-TPA-containing devices.