Benzodi(pyridothiophene): a novel acceptor unit for application in A1–A–A1 type photovoltaic small molecules

A series of novel A1–A–A1 type small molecules (SMs) of BDPT-2BT, BDPT-2FBT and BDPT-2DPP were designed and synthesized, in which benzodi(pyridothiophene) (BDPT) was used as a novel weak central acceptor (A) unit, and benzothiadiazole (BT), fluorinated benzothiadiazole (FBT) and diketopyrrolopyrrole (DPP) were used as terminal acceptor (A1) units, respectively. The pentacyclic BDPT aromatic unit can form big conjugated and planar SMs with the A1 unit, resulting in enhanced π–π stacking and crystallinity. The effect of the A1 unit on the optical, electrochemical and photovoltaic properties of three SMs was observed. The broader absorption spectrum, lower HOMO energy level, higher photo-response efficiency and better photovoltaic properties were exhibited for BDPT-2DPP. A maximum PCE of 3.97% with a Voc of 0.84 V, a Jsc of 9.0 mA cm−2 and a FF of 52.37% was obtained in the BDPT-2DPP/PC71BM-based solar cells, which is 1.8 and 1.5 times the values of the BDPT-2BT and BDPT-2FBT-based cells, respectively.