On the “True” Structure of Push–Pull‐Type Low‐Bandgap Polymers for Organic Electronics

Abstract

Donor-acceptor type conjugated polymers are a dominating 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 the 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 often also not those expected. In this progress report, we provide an overview of the recent literature observations of structural imperfections in push-pull conjugated polymers, the difficulties to observe 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.

CH Activation as a Shortcut to Conjugated Polymer Synthesis

“…It was assumed that conjugated polymers prepared via these techniques were comprised exclusively of a perfectly alternating backbone due to the monomers' complementary functional groups. However, a number of studies have since revealed that these structures are far from “defect‐free”: homocoupling side‐products are widespread in polymers synthesized by these well‐established coupling techniques and they have a clear detrimental effect on the quality of devices prepared from these polymers . Even in small molecule couplings, such side‐reactions can be readily identified.…”

On the homocoupling of trialkylstannyl monomers in the synthesis of diketopyrrolopyrrole polymers and its effect on the performance of polymer-fullerene photovoltaic cells

“…Homocoupling reactions are known to occur in the Suzuki, 12 Stille, 13 and direct arylation 14 crosscoupling polycondenstation reactions that are commonly used to synthesize donor-acceptor small-molecules and polymers that nd widespread use organic solar cells. The topic of structural defects is nding increasing awareness and was recently reviewed by Maes et al 15 The homocoupling of two bromide monomers in the Suzuki and Stille polymerization reaction in low band gap diketopyrrolopyrrole (DPP) polymers has previously been investigated in our group in some detail. 16 The defect is caused by homocoupling of two brominated dithienyl-diketopyrrolopyrrole (T-DPP-T) units, creating T-DPP-2T-DPP-T defects in the polymer main chain.…”

The Impact of Acceptor–Acceptor Homocoupling on the Optoelectronic Properties and Photovoltaic Performance of PDTSQxff Low Bandgap Polymers

“…Even upon applying “nearly perfect” conditions, e.g., an oxygen‐free atmosphere and a proper Pd‐ligand stoichiometry, homocoupling between either two organotin/boron or two arylhalide units can occur . The importance and possible abundance of these misscoupled structural units for the performance of the organic solar cells has only recently been recognized and a few reports on this have appeared …”