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Model Systems for Characterizing the Intrinsic Properties of Polymer Semiconductors: Oligomers and Single Crystals

Polymer semiconductors hold great promise for the realization of inexpensive, flexible electronic devices. One of the greatest strengths of these materials, their low-cost processability from solution, results in a wide range of solid-state structures. The crystallinity and morphology of these materials strongly impacts their performance as charge carriers. Furthermore, the nature of polymerization inherently leads to the production of materials which are disperse in their molecular character, with regard to both coupling and molecular weight. Thus, it is difficult to deconvolute the intrinsic properties of polymer semiconductors from their synthetic and processing conditions. Polythiophenes are particularly susceptible to performance variations due to their dependence on crystallinity for effective charge transport. Moreover, it is difficult to characterize charge transport within their crystallites, which are often tens of nanometers in size. The approaches reported here are intended to provide well-defined model systems, oligomers and single crystals, for characterizing the crystallization and semiconductor physics of conjugated polymers. In Chapter 2, the characterization of monodisperse oligothiophenes is reported. In evaluating the evolution of crystalline, morphological, and optoelectronic properties with increasing chain length, the transition from monomer to polymer is described. The pentamer of poly[bis(3-dodecyl-2-thienyl)-2,2'-dithiophene -5,5'-diyl] (PQT-12) is identified as a potential model system for probing the intrinsic properties of the polymer. In Chapter 3, the interactions between a model oligomer, the trimer of poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), and high molecular weight polymer are characterized. The effects of enhanced crystallinity and connectivity, from the oligomer and polymer, respectively, are reported. In particular, the morphology, crystallinity, and charge transport properties of films of blended oligomer-polymer systems were studied. Although oligomers are often considered impurities in the synthesis of semiconducting polymers for device applications, it was demonstrated that films consisting of up to 83% oligomer exhibited high charge transport mobilities equal to that of pure high molecular weight polymer. In Chapter 4, a novel technique for the fractionation and recrystallization of oligothiophenes in supercritical fluid is reported. Furthermore, the crystal habit, structure, and molecular coupling of the resulting crystals are described. This crystallization method leverages the solubilizing and transport properties of supercritical fluids to create a system that combines the advantages of crystallizations from vapor and solution and could be broadly applied to the crystallization of other polymeric systems.
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