Novel solution-processible small molecules based on benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene for effective organic photovoltaics with high open-circuit voltage

Abstract

Two novel A–D–A small molecules D1 and D2, containing benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene as the central electron-donating unit, 3-ethylrhodanine as end-capped electron-withdrawing units, and two thiophenes or three thiophenes as conjugated π-bridges, were designed and synthesized. The effects of the conjugated π-bridges on the photophysical, electrochemical and photovoltaic properties as well as the aggregation structure, were fully investigated. Compared with D2, D1 shows stronger packing capability, deeper HOMO energy levels, higher hole mobility, and more appropriate microphase separation with the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), which lead to better photovoltaic performance with higher short-circuit current density (J_sc) and open-circuit voltages (V_oc). Bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices were fabricated using a blend of the as-synthesized small molecules and PC₆₁BM in different solvents. The D1-CF device prepared from chloroform solution with a weight ratio of 1 : 0.5 exhibited a power conversion efficiency (PCE) of 2.10% and an amazingly high V_oc of 1.10 V under AM 1.5G (100 mW cm⁻²) illumination.