In recent years, great headway has been made in the development of efficient polymer donors across the community, with published power conversion efficiencies (PCE) >8% in bulk-heterojunction (BHJ) solar cells with fullerene acceptors (single cells), and PCEs >10% in tandem devices. In most reports, the polymer donor involves elaborate repeat unit and side chain patterns, and deviating from those patterns induces substantial drops in device PCE. While the range of polymer design parameters that impact BHJ solar cell performance remains a matter of some debate, our recent developments indicate that the combination of side-chain substituents and the functional groups appended to the main chain critically impacts polymer performance. These effects are particularly pronounced in poly(benzo[1,2-b:4,5-b’]dithiophene–thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) and wide-bandgap analogs blended with phenyl-C61/71-butyric acid methyl ester (PCBM).[1-11] Furthering our understanding of how the main chain substitution pattern mediates the interplay between donor and acceptor is a critical step as we look to continue improving BHJ solar cell efficiencies.
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