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Side-chain engineering is increasingly being utilized as a technique to impact the structural order and enhance the electronic properties of semiconducting polymers. However, the correlations drawn between structural changes and the resulting charge transport properties are typically indirect and qualitative in nature. In the present work, a combination of grazing incidence X-ray diffraction and crystallographic refinement calculations is used to determine the precise molecular packing structure of two thiophene-based semiconducting polymers to study the impact of side-chain modifications. The optimized structures provide high-quality fits to the experimental data and demonstrate that in addition to a large difference in interchain spacing between the two materials, there exists a significant disparity in backbone orientation as well. The calculated structures are utilized in density functional theory calculations to determine the band structure of the two materials and are shown to exhibit a dramatic disparity in interchain dispersion which accounts for the large observed difference in charge carrier mobility. The techniques presented here are meant to be general and are therefore applicable to many other highly diffracting semicrystalline polymers.

An N-Type organic thin-film transistor (OTFT) based on doped 6,13-Bis(triisopropylsilylethynyl)pentacene is presented. A transition from p-type to n-type occurrs with increasing doping concentrations, and the highest performing n-channel OTFTs are obtained with 50 mol% dopant. X-ray diffraction, scanning Auger microscopy, and secondary ionization mass spectrometry are used to characterize the morphology of the blends. The high performance of the obtained transistors is attributed to the highly crystalline and aligned nature of the doped thin films.

Using X-ray diffraction-based pole figures, we pres-ent quantitative analysis of the microstructure of poly(3-hex-ylthiophene) thin films of varying thicknesses, which allows usto determine the crystallinity and microstructure at the semi-conductor-dielectric interface. We find that the interface isapproximately one fourth as crystalline as the bulk of the mate-rial. Furthermore, the use of a self-assembled monolayer(SAM) enhances the density of interface-nucleated crystallitesby a factor of 20. Charge transport measurements as a func-tion of film thickness correlate with interface crystallinity.Hence, we establish the crucial role of SAMs as nucleatingagents for increasing carrier mobility in field-effect devices. V C 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys000: 000–000, 2013 KEYWORDS: charge transport; crystallization; structure-propertyrelations; thin films; X-rayINTRODUCTION Organic semiconductors, comprising smallmolecules and conjugated polymers, are a promising materi-als family for next generation electronic devices such as tran-sistors, light-emitting diodes and solar cells. Processes atinterfaces play a very important role in the operation ofthese devices. Because in organic semiconductors the micro-structure is intimately tied to their electronic properties,understanding the effect of surfaces and interfaces on themorphology of these materials is an important component ofoptimizing their performance. In thin film transistors (TFTs)for instance, mobile charge is transported within a few nano-meters of the semiconductor/dielectric interface, which inthe case of bottom-gated devices is the substrate surface.There is significant experimental evidence demonstratingthat controlling the molecular packing and orientation at thedielectric interface can improve the charge carrier mobilityin thin films of poly(3-hexylthiophene) (P3HT), a modelsemicrystalline, semiconducting polymer, by several ordersof magnitude.

The one-step preparation of highly anisotropic polymer semiconductor thin films directly from solution is demonstrated. The conjugated polymer poly(3-hexylthiophene) (P3HT) as well as P3HT:fullerene bulk–heterojunction blends can be spin-coated from a mixture of the crystallizable solvent 1,3,5-trichlorobenzene (TCB) and a second carrier solvent such as chlorobenzene. Solidification is initiated by growth of macroscopic TCB spherulites followed by epitaxial crystallization of P3HT on TCB crystals. Subsequent sublimation of TCB leaves behind a replica of the original TCB spherulites. Thus, highly ordered thin films are obtained, which feature square-centimeter-sized domains that are composed of one spherulite-like structure each. A combination of optical microscopy and polarized photoluminescence spectroscopy reveals radial alignment of the polymer backbone in case of P3HT, whereas P3HT:fullerene blends display a tangential orientation with respect to the center of spherulite-like structures. Moreover, grazing-incidence wide-angle X-ray scattering reveals an increased relative degree of crystallinity and predominantly flat-on conformation of P3HT crystallites in the blend. The use of other processing methods such as dip-coating is also feasible and offers uniaxial orientation of the macromolecule. Finally, the applicability of this method to a variety of other semi-crystalline conjugated polymer systems is established. Those include other poly(3-alkylthiophene)s, two polyfluorenes, the low band-gap polymer PCPDTBT, a diketopyrrolopyrrole (DPP) small molecule as well as a number of polymer:fullerene and polymer:polymer blends.

The film thickness of one of the most crystalline and highest performing polymer semiconductors, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), is varied in order to determine the effects of interfaces and confinement on the microstructure and performance in organic field effect transistors (OFETs). Crystalline texture and overall film crystallinity are found to depend strongly on film thickness and thermal processing. The angular distribution of crystallites narrows upon both a decrease in film thickness and thermal annealing. These changes in the film microstructure are paired with thin-film transistor characterization and shown to be directly correlated with variations in charge carrier mobility. Charge transport is shown to be governed by film crystallinity in films below 20 nm and by crystalline orientation for thicker films. An optimal thickness is found for PBTTT at which the mobility is maximized in unannealed films and where mobility reaches a plateau at its highest value for annealed films.

Two pairs of indacenodithiophene (IDT) and quinoxaline-based copolymers with meta- or para-hexyl-phenyl side chains on the IDT unit are synthesized. The meta-substituted polymers offer better solubility, higher molecular weight for both fluorinated and non-fluorinated copolymers, and a superior photovoltaic performance with a power conversion efficiency of 7.8%. The side-chain design strategy presented is an efficient way to produce high performance conjugated polymers for organic electronics.