Effects of bridging atom and π-bridge length on physical and photovoltaic properties of A–π-D–π-A oligomers for solution-processed organic solar cells

•Novel D–A oligomers with alkyldicyanovinyl acceptor groups for OSCs were reported.•The structure–property relationships in these oligomers were systematically studied.•Bridgehead atoms influence optical, thermal and structural properties of the oligomers.•Length of π-bridge influences optical and electrochemical properties of the oligomers.

Synthesis of novel acceptor–donor–acceptor oligomers with electron-withdrawing alkyldicyanovinyl groups linked through an oligothiophene π-bridge with either dithienosilole or cyclopentadithiophene electron donor units is described. Changing the bridgehead atom from carbon to silicon in the central donor unit leads to a significant change in optical, thermal and structural properties of the oligomers. In addition, elongation of the oligothiophene π-bridge in the oligomers increases energies of HOMO and LUMO levels and leads to an unexpected hypsochromic shift of their absorption spectrum, because extension of the conjugation length cannot compensate a decrease of the intramolecular charge transfer between the dithienosilole and dicyanovinyl units. Although these minor changes in the chemical structures have a pronounced impact on the morphologies of their blends with PC70BM, the optimized solution-processed organic solar cells based on these small molecules demonstrate similar power conversion efficiencies.

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