Synthesis and characterization of diketopyrrolopyrrole-based conjugated polymer with biphenyl for organic photovoltaic cells

Designing a moderate band gap is sometimes intentionally chosen. When the donor polymer has a deeper HOMO level, a higher Voc can be gained which boosts total PCE in organic photovoltaics (OPVs) (Liao et al., Joule 2020, 4(1), 189–206; Mamba et al., J. Phys. Chem. A 2021, 125(50), 10593–10603; Choi and Jo, Org. Electron. 2013, 14(6), 1621–1628; Jo et al., Org. Electron. 2012, 13(10), 2185–2191). This is crucial in DPP-based copolymer frameworks, which frequently have short band gaps and may experience poor Voc. Since DPP is a low bandgap due to strong-acceptor, thus, modifying it with weak donor units can raise the bandgap and lower the HOMO to make it suitable for multi-junctions or higher Voc. We report the synthesis, properties, and photovoltaic applications of a donor-acceptor (D-A) conjugated copolymer based on diketopyrrolopyrrole (DPP) and biphenyl, namely PDDPPhenyl. This polymer exhibits broad absorption ranging from 400 to 900 nm with a band gap of 1.59 eV. As expected, a low HOMO level of −5.29 was gained by introducing biphenyl as a weaker donor. The optimized weight ratio goes to 1:4 for PDPPPhenyl:PC₇₁BM with 3.8% PCE.