Three solution-processable triphenylamine (TPA)-containing low band gap D-A-D molecules with benzothiadiazole (BT) as acceptor unit and TPA-(4-hexyl)thiophene (TPA-HT), TPA-(4-hexyl)thieno[3,2-b]thiophene (TPA-HTT) and bis(octyloxy)TPA-(4-hexyl)thieno[3,2-b]thiophene (boTPA-HTT) as donor units, TPA-HT-BT, TPA-HTT-BT and boTPA-HTT-BT, have been designed and synthesized for the application in the bulk-heterojunction organic solar cells (OSCs). The molecules show broad absorption band in the visible range and appropriate HOMO energy levels for the donor materials in OSCs. The OSC devices were fabricated with the molecules as donor and PC70BM as acceptor. The power conversion efficiencies of the OSCs based on TPA-HT-BT, TPA-HTT-BT and boTPA-HTT-BT reached 1.44%, 1.44% and 0.75%, respectively, under the illumination of AM.1.5, 100 mW/cm2.Graphical abstractResearch highlights► TPA-containing D-A-D molecules with benzothiadiazole (BT) as acceptor unit show good solubility and broad absorption. ► PCE of the OSCs based on the molecule as donor and PC70BM as acceptor reached 1.44%. ► Thieno[3,2-b]thiophene-vinylene pi-bridge in the molecules broadened the absorption spectra.

Two solution-processable star-shaped D–π–A organic molecules with triphenylamine (TPA) as donor unit, benzothiadiazole (BT) as acceptor unit and 4-hexyl-thienylenevinylene as pi conjugated bridge, S(TPA-TBTT) and S(TPA-TBTT-TPA), have been designed and synthesized for the application as donor materials in bulk-heterojunction organic solar cells (OSCs). The two molecules possess broader absorption from 350 to 700 nm benefitted from the longer pi-bridge in the molecules but weaker absorbance and poorer solubility in comparison with their corresponding organic molecules with shorter vinylene pi-bridge. The OSC based on S(TPA-TBTT): PC70BM (1:3, w/w) exhibited Jsc of 6.41 mA/cm2, Voc of 0.75 V, FF of 39.0% and power conversion efficiency of 1.90%, under the illumination of AM 1.5, 100 mW/cm2.Graphical abstractHighlights► We synthesized two new solution-processable star-shaped D–π–A organic molecules. ► The two molecules S(TPA-TBTT) and S(TPA-TBTT-TPA) possess broader absorption. ► We studied the photovoltaic properties of the molecules. ► The OSC based on S(TPA-TBTT): PC70BM exhibited power conversion efficiency of 1.90%.