1939 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2013, 25, 1939–1947 Probing the Relation Between Charge Transport and Supramolecular Organization Down to Ångström Resolution in a Benzothiadiazole-Cyclopentadithiophene Copolymer Dorota Niedzialek, Vincent Lemaur, Dmytro Dudenko, Jie Shu, Michael Ryan Hansen, Jens Wenzel Andreasen, Wojciech Pisula, Klaus Müllen, Jérôme Cornil, and David Beljonne* D. Niedzialek, Dr. V. Lemaur, Dr. J. Cornil, Dr. D. Beljonne Laboratory for Chemistry of Novel Materials Center for Innovation and Research in Materials and Polymers (CIRMAP) University of Mons (Umons) 20 Place du Parc, 7000 Mons, Belgium E-mail: david.beljonne@umons.ac.be Dr. D. Dudenko, Dr. J. Shu, Dr. M. R. Hansen, Dr. W. Pisula, Prof. Dr. K. Müllen Max Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz, Germany Dr. J. W. Andreasen DTU Energy Conversion and Storage Technical University of Denmark Frederiksborgvej 399, DK-4000 Roskilde, Denmark DOI: 10.1002/adma.201201058 Conjugated polymers show complex microstructure organi- zations in the solid state, [1–7] as the supramolecular arrange- ment of the polymer chains depends sensitively not only on their primary chemical structure but also on the tech- niques used for their processing into thin films (spin versus dip coating, [8–10] use of good versus bad solvent, [11] thermal annealing, [12,13] surface treatment, [14–16] etc.). In contrast to crystalline inorganic materials where long-range order is present, organic conjugated polymers are soft materials and give rise to various bulk morphologies. Rigid-rod conjugated polymer chains often feature a semi-crystalline behaviour, with ordered domains separated by amorphous regions in a ratio that is very sensitive to, among others, molecular weight and dispersity. [17] This has important consequences on the performance of opto-electronic devices based on conjugated polymers, because charge carrier mobilities, for example, are strongly limited by poor packing of the chains and the lack of macroscopic order. Long-range molecular order can be prompted by chemical design of the side chains (a prominent example is the case of regioregular placement of alkyl chains in polythiophenes) or by the use of proper processing tech- niques (e.g., zone-casting). [18–20] Recently, the synthesis and characterization of copolymers con- sisting of an alternating arrangement of cyclopentadithiophene (CDT) as donor and benzothiadiazole (BTZ) as acceptor units has been reported. [21–24] Field-effect transistor (FET) hole mobil- ities exceeding 3 cm 2 V -1 s -1 have been obtained and these were shown to be strongly sensitive to the molecular weight of the hexadecyl substituted copolymers. Solid-state nuclear mag- netic resonance (NMR) was used to assess the supramolecular organization of the conjugated chains. Strikingly, the NMR data suggest that the CDT-BTZ backbones stack with close spacing between donor (acceptor) moieties of adjacent chains, thus ruling out the hypothesis that electrostatic donor-acceptor inter- actions govern the packing and the unprecedented charge trans- port properties of CDT-BTZ thin films. A deep insight into the packing of polymer chains can also be gained from modeling, as illustrated in many recent theoretical studies focusing on polythiophene derivatives. Nowadays, molecular force fields combined to Monte Carlo or Molecular Dynamics simulations are the techniques of choice as they explicitly account for the weak (mostly van der Waals) intermolecular interactions driving the organization of conju- gated molecules in the solid. Several studies have reported on the structural arrangement of substituted thiophene oligomers and polymers using periodic boundary conditions in crys- tals, [25–27] as well as the corresponding disordered structures in the bulk [28–30] or deposited on a substrate. [31,32] The crystalline packing can also be inferred from quantum-chemical calcula- tions provided that the dispersion forces are properly included, which remains an open challenge when dealing with large sys- tems. In that respect, an attractive solution relies on the use of Density Functional Theory (DFT) methods with dispersion cor- rected functionals. [33] Such modeling studies are very useful as they can grasp at the microscopic scale the relationship between the primary chemical structure of the polymer chains and their supramolecular organization, e.g. the influence of side chain regioregularity in polythiophenes [30] or the impact of alkyl chain interdigitation in polymers incorporating fused thiophene rings, [34,35] to cite just two examples. Here, we use modeling to gain insight into the relationship between the supramolecular arrangement of CDT-BTZ polymer chains in the solid, as explored using combined force-field and ab initio methods, and the electronic coupling mediating charge transport, as obtained from density functional theory (DFT) calculations. Through the