Solid-State Organization and Ambipolar Field-Effect Transistors of Benzothiadiazole-Cyclopentadithiophene Copolymer with Long Branched Alkyl Side Chains.

The solid-state organization of a benzothiadiazole-cyclopentadithiophene copolymer with long, branched decyl-tetradecyl side chains () is investigated. The substituents are sterically demanding and increase the π-stacking distance to 0.40 nm from 0.37 nm for the same polymer with linear hexadecyls (). Despite the bulkiness, the side chains tend to crystallize, leading to a small chain-to-chain distance between lamellae stacks and to a crystal-like microstructure in the thin film. Interestingly, field-effect transistors based on solution processed layers of show ambipolar behavior in contrast to with linear side chains, for which hole transport was previously observed. Due to the increased π-stacking distance, the mobilities are only 6 × 10 cm²/Vs for electrons and 6 × 10 cm²/Vs for holes, while leads to values up to 5.5 cm²/Vs. The ambipolarity is attributed to a lateral shift between stacked backbones provoked by the bulky side chains. This reorganization is supposed to change the transfer integrals between the and substituted polymers. This work shows that the electronic behavior in devices of one single conjugated polymer (in this case ) can be controlled by the right choice of the substituents to place the backbones in the desired packing.