Synthesis and photovoltaic properties of donor-acceptor type narrow bandgap copolymers based on benzo[def]carbazole

• Benzo[def]carbazole based donor-acceptor type copolymers were synthesized.
• The polymers showed wide absorption bands with narrow bandgaps.
• Their HOMO levels were located in proper range for BHJ polymer solar cells.
• The polymers could work as a donor material in the BHJ solar cells.

Three kinds of narrow bandgap copolymers were synthesized by combination of a donor unit of benzo[def]carbazole with each acceptor unit of benzothiadiazole, thienopyrrole-4,6-dione, and 1,4-diketopyrrolopyrrole. Basic properties of these polymers were investigated for applications in bulk heterojunction polymer solar cells. These copolymers had broad absorption bands in UV and visible regions from 350 to 800 nm that overlapped with the major region of the solar spectrum, and their optical band gaps were in the range of 1.68–2.11 eV. The highest occupied molecular orbitals of these copolymers were situated in the range of −5.22 eV to −5.34 eV, which could provide good air stability and high open circuit voltages in photovoltaic applications. Initial performances of these copolymers in bulk heterojunction photovoltaic devices revealed that the power conversion efficiencies exceeded 1.4% under irradiation of AM 1.5 solar-simulated light (100 mW cm−2).

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