Conjugated Oligothiophene Derivatives Based on Bithiophene with Unsaturated Bonds as Building Blocks for Solution-Processed Bulk Heterojunction Organic Solar Cells

A new building block ATVTA that uses stiff carbon-carbon triple bonds (A) on 1,2-di(2-thienyl)-ethene (TVT) has been developed. Oligothiophene derivatives S-01 with a TVT unit, S-02 with a 5,5-diethynyl-2,2-dithienyl (AT2) unit and S-03 with ATVTA were synthesized to compare their effects in a systematic study. Due to the better -conjugation extension of the TVT unit, S-01 exhibits the most red-shifted absorption profile among them, whereas S-02 possesses the deepest HOMO level. While the HOMO level of S-03 is down-shifted by 0.02eV relative to that of S-01, the alkyne linkages can effectively down-shift the HOMO level. By replacing the terminal units of S-03 with stronger electron acceptors, S-04 and S-05 exhibited broader absorption profiles and lower HOMO levels than those of S-03. Organic solar cells based on these molecules were fabricated and an S-03:PC60BM (1:1, w/w) based device afforded the highest V-oc value of 0.96V and a power conversion efficiency (PCE) of 2.19%.