On the True Structure of Push-Pull-Type Low-Bandgap Polymers for Organic Electronics

Donor-acceptor-type conjugated polymers are a dominant class of active materials in the field of organic electronics. Their adjustable light-harvesting, charge transfer, and charge transport characteristics are beneficially applied in organic photovoltaics, photodetectors, and thin-film transistors. The conventional synthetic approach toward these push-pull polymers is based on Suzuki or Stille cross-coupling of complementary functionalized (hetero)aromatic monomers. In an ideal world, this gives rise to a perfect alternation of the employed building blocks throughout the polymer backbone, and this alternation of electron-rich (donor/push) and electron-deficient (acceptor/pull) moieties leads to a substantial decrease of the bandgap. In recent years, however, it has become increasingly clear that the real structure of the resulting alternating copolymers is often quite different from the projected one. Structural imperfections can, for instance, result from homocoupling of two identical building blocks. Furthermore, the polymer end groups are also often not those as expected. In this progress report, an overview is provided for the recent literature observations of structural imperfections in push-pull conjugated polymers, the difficulties in observing them, and their impact on device performance. The strong effect of homocoupling on material and (organic solar cell) device quality and reproducibility is particularly emphasized.