4,8-Bis(thienyl)-benzo[1,2- b :4,5- b ′]dithiophene based A-π-D-π-A typed conjugated small molecules with mono-thiophene as the π-bridge: Synthesis, properties and photovoltaic performance

Two conjugated A-π-D-π-A type small molecules having a 4,8-bis(thienyl)-benzo[1,2-b:4,5-b′]dithiophene (TBDT) unit as the donor part, a mono-thiophene as the π-conjugation bridge, and 2-cyano-3-octyloxy-3-oxo-1-propenyl (COOP) or dicyanovinyl (DCV) as the terminal acceptor unit were synthesized and investigated. Both compounds showed broad absorption band over 400–600 nm and similar HOMO/LUMO energy levels. However, these two compounds showed a big difference in photovoltaic performance. The DCV-1T-TBDT:PC61BM based solar cell exhibited an open circuit voltage (VOC) of 0.93 V, a short circuit current (JSC) of 8.54 mA cm−2, a fill factor (FF) of 0.56, and an overall power conversion efficiency (PCE) of 4.48%, whereas the COOP-1T-TBDT:PC61BM based solar cell showed a VOC of 1.04 V, a JSC of 2.28 mA cm−2, an FF of 0.29, and a PCE of 0.69. Furthermore, DCV-1T-TBDT showed minor thickness-dependent PCE behavior in PV device, which is beneficial for printing processing.