Effect of the acceptor and alkyl length in benzotriazole-based donor acceptor-donor type hole transport materials on the photovoltaic performance of PSCs

Four novel benzotriazole-based donor-acceptor-donor (D-A-D) type hole transport materials (HTMs) BTA1-2 and DT1-2 are synthesized to investigate the effect of the acceptors on the photovoltaic performance of perovskite solar cells (PSCs). The acceptors include nonfused core (thiophene-benzotriazole-thiophene) and fused core (dithienopyrrolobenzotriazole, DTBT). Systematical investigation reveals that the acceptor greatly influences the absorption, energy level, hole transfer and extraction, and photovoltaic performances of the HTMs. The dopant-free HTMs DT1-2 with DTBT core exhibit better hole transfer and extraction performance, and then obtain better photovoltaic performance than that of BTA1-2. Especially, the PSC using DT2 without any dopants obtains a power conversion efficiency of 13.22%, which is outperforming than that of spiro-OMeTAD (10.41%). When doping with Li-TFSI/TBP (lithium bis(trifluoromethylsulphonyl)imide/tert-butylpyridine), DT2-based cell achieves an improved efficiency of 15.77%. In addition, the different alkoxyl chain lengths on triphenylamine has a significant impact on the conductivity, hole mobility, hole extraction, and photovoltaic performance. The stability testing of the PSCs shows that the four HTMs based devices display improved stability than spiro-OMeTAD. These results certify that the fused acceptor core DTBT is a promising core for efficient D-A-D type HTMs towards PSCs.