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 C-4 or C-6) 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/pi or CH/O hydrogen bonds. At the same time, the IDTC6 backbone enables a tight molecular arrangement based on pi-pi stacking interactions (3.399 angstrom); 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.