Localized Charge Transfer in a Molecularly Doped Conducting Polymer

Doped conjugated polymers can exhibit exceptionally high conductivity (>10 S cm). Here, doping refers to the formation of charge-transfer complexes (CTCs) or salts by combining appropriate pairs of donors and acceptors. Similar phenomena can be found in crystalline CTCs comprising small molecules. For several decades, the nature of charge transfer (CT) and the dimensionality of charge transport in conducting polymers and small molecule crystals have been at the focus of research. For instance, the degree of CT influences conductivity and determines whether metallic or insulating character prevails, and the strength of interchain interactions determines whether primarily 1D or 3D electronic properties prevail. In addition, disorder—on both a molecular and mesoscopic scale—has a tremendous impact on these properties. In fact, true metallic behavior of a doped conjugated polymer was demonstrated only recently by significantly improving the structural quality of thin films. Apart from the interest in fundamental phenomena occurring in such systems, recent progress in the field of organic electronics has intensified efforts toward improving the understanding of conducting polymers, as they are a key element for the successful realization of printed all-organic (opto-) electronic devices. At present, formulations of poly(ethylenedioxythiophene)/poly(styrenesulfonate) dominate applications; however, it should be interesting to develop alternative routes to conducting polymers that are not based on an aqueous dispersion and thus allow for new processing options and functionality. The most widely studied class of semiconducting polymers that can be rendered conducting upon doping (with inorganic acceptors) is based on polythiophene, which has donor character; tetrafluorotetracyanoquinodimethane (F4TCNQ) is one of the strongest known molecular electron acceptors and has been used for doping molecular organic semiconductors. However, combinations of polythiophenes and strong molecular acceptors have not yet been investigated. Here we show that mixtures of the prototypical soluble polythiophene variant poly(3-hexylthiophene) (P3HT) and F4TCNQ form CTCs with high conductivity in thin films (ca. 1 S cm). Because of the flexibility of the polymer chains and the variety of possible interchain interactions, a large number of different local conformations in P3HT/F4TCNQ CTCs can be expected, leading to large variations in the electronic structure and transport properties within a macroscopic sample. We investigated the local conformation and electronic structure of P3HT/F4TCNQ CTCs in thin films by combining X-ray absorption near edge structure (XANES) measurements with theoretical modeling using density functional theory (DFT). Most notably, we found that only one specific CTC conformation predominated, in which F4TCNQ was strongly bent out of its neutral planar form because of pronounced electron donation from P3HT chain segments. In contrast, in a related F4TCNQ/oligothiophene CT crystal, F4TCNQ remained planar because of dominant intermolecular interactions in an ordered environment. Furthermore, the energy levels of the CTC were clearly shown to be hybrids of the individual levels of the separate donor and acceptor molecules, indicating that the CT in P3HT/F4TCNQ is highly localized and does not involve significant interchain interactions. Thin films of F4TCNQ-doped P3HT prepared from solution typically exhibited a dc conductivity of 1 S cm, which corresponds to an increase in conductivity by five orders of magnitude over pristine P3HT. This increase is lower than the reported value of 30 S cm for ClO4 -doped P3HT. Interestingly, the conductivity of our P3HT/F4TCNQ films was five orders of magnitude higher than the value for dimethylquarterthiophene/F4TCNQ crystals. In these crystals intermolecular interactions are strong, whereas it was proposed that in conducting polythiophenes, conduction happens primarily along single polymer chains because of weak interchain coupling. In this communication, we relate the localized character of the CT between P3HT and F4TCNQ with the observed conductivity and explain the nature of the CT states. The XANES spectrum of neutral F4TCNQ exhibits three main spectral features: peaks A, B, and C (Fig. 1a). Using just C O M M U N IC A IO N