Benzo[1,2-b:4,5-b′]dithiophene-fumaronitrile-based D-A type copolymers with different π-bridges: Synthesis, characterization and photovoltaic properties

• Four copolymers composed of donors and acceptors were designed and synthesized.
• Influence of side chain and π-bridge on polymer's properties was studied.
• Broader absorption and narrower band-gap were achieved for the synthesized polymers.
• Deep HOMOs between −5.48 eV and −5.37 eV were confirmed for all the polymers.
• Prospectively high Vocs of the devices based on the four polymers were achieved.

Four new homologous copolymers (POP, POT, PTP and PTT), composed of benzo[1,2-b:4,5-b′]dithiophene (BDT) donors and fumaronitrile (BCNV) acceptors, were designed and synthesized. The effects of different side-chains of BDT units and different π-bridges of BCNV acceptors on polymers’ thermal, optical, electrochemical, chain geometric, hole-transporting properties and photovoltaic performance were systematically investigated. POT and PTT exhibit broader absorption and narrower bandgaps than POP and PTP due to the replacement of phenyl π-bridges with thienyl groups. All polymers show relatively deep highest occupied molecular orbitals (HOMOs) between −5.48 eV and −5.37 eV. Prospectively high open circuit voltages (Vocs) of the devices based on the four polymers were accomplished. The highest Voc 0.975 V was obtained with PTP. However, the obtained PCEs are still relatively low. We consider this is related to the polymers’ poor backbone planarity, relatively low LUMO levels or low polaron photogeneration efficiency resulted from BCNV.

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