They have been used in the fabrication of field-effect transistors (FETs), light-emitting diodes (LEDs), and photovoltaic cells, to name a few examples. Among polyconjugated systems, regioregular head-to-tail (HT)-coupled poly(3-alkylthiophene)s [2±4] (P3ATs) seem very promising. They combine the relative facility of their solution processing with very regular chain microstructure which promotes, in turn, the formation of ordered self-organized supramolecular aggregations. As a result, the semiconducting transport properties of these materials are strongly improved. In the case of poly(3-hexylthiophene) (P3HT), field-effect mobilities [5±7] as high as 0.1 cm 2 V ±1 s ±1 have been demonstrated.In solution-processed films, X-ray diffraction (XRD) studies have revealed a lamella structure with two-dimensional (2D) sheets resulting from interchain stacking. The typical size of crystallized domains deduced from XRD [8,9] is of the order of 10 nm (average sizes for P3HT have been reported in the range 8±13 nm). However, despite the occurrence of selforganization in some areas, the global microstructure of P3HT films remains complex, [10] and can not a priori be regarded as simply polycrystalline. Generally, one should rather expect polycrystalline domains embedded in a disordered/amorphous matrix. Such a complex structure strongly impedes the studies of local polymer conformation inside ordered domains by XRD experiments. To get a precise picture of polymer ordering, one can, alternatively, use local probes, [11,12] such as scanning force microscopy (SFM) or scanning tunneling microscopy (STM). This has been done recently, with STM imaging of self-organized P3AT films at the solution±substrate interface. [13,14] In these measurements, local parameters of the conjugated polymer inside ordered domains (i.e., chain-to-chain distance, chain conformations, and folds) were directly visualized and determined.In this study, we present STM measurements of self-assembled P3HT dry films, cast on highly oriented pyrolytic graphite (HOPG) from chloroform solution. On a mesoscopic scale, STM images reveal the organization of polymer polycrystals, the relative orientations of domains, the complex nature of their boundaries, and allow, in particular, the determination of typical sizes of mono-domains with precision. Simultaneously, the resolution of the STM images is sufficient to study local polymer conformations and chain packing on a more local scale. By performing fast Fourier transform (FFT) of topographic images, we analyze both local (i.e., chain-tochain distance) and global (i.e., domain sizes) crystalline parameters in direct and reciprocal spaces. This two scale analysis is discussed in the context of previous XRD [8,9] and STM [13,14] experiments. Figure 1a presents a typical STM image, recorded on a submicrometer scale, showing the structure of the P3HT film. A polycrystalline domain occupying the major part of the image appears in light contrast. Other areas (brown or dark) are probably in part covered by disordered...

Two-dimensional self-organized poly(3-hexylthiophene) films on highly oriented pyrolytic graphite have been probed at the solid/substrate interface by scanning tunneling microscopy (STM). Structural morphology and typical polymer conformations are visualized and discussed from mesoscopic to nanoscopic scales, including mesoscopic assembly of polycrystals, crystalline monodomain orientations and sizes, grain boundaries, chain folds, and other conformational features. STM estimation of the average chain length is in remarkably good agreement with that derived from size-exclusion chromatography. The multiscale analysis supports a picture where heterogeneities exist at different length scales.