We report the local UV-Vis absorption behaviour of single crystals of conjugated poly-3-hexylthiophene (P3HT), obtained by crystallization in dilute solutions at elevated temperatures based on a self-seeding approach and characterized by high internal structural order. Conjugated polymers represent promising active systems for a variety of optoelectronic applications, such as solar cells, 1 light emitting diodes, 2 eld-effect transistors, 3-5 etc. However, the performance of these materials in such devices has so far oen remained poor, mostly due to the difficulties in controlling molecular conformations, structural packing, and morphology 6,7 of these polymers. In order to correctly interpret and nally to improve the functional behavior of conjugated polymers, it is thus necessary to unravel the relationship between the light absorption properties and molecular conformation or polymer microstructure. As a suitable model system for a systematic study of such structure-optoelectronic property relations, we have recently identied highly ordered single crystals of conjugated poly-3-hexylthiophene (P3HT) of weight average molecular weight of 26 400 g mol À1 , characterized by a unique molecular conformation on all length-scales. 8 They were obtained via crystallization in solution by a self-seeding technique. These crystals are composed of closely packed, p-p stacked (p-p distance of $0.33 nm) fully extended chains which are oriented perpendicular to the substrate. 8

Depending on processing conditions, ordered microstructures of conjugated oligomers or polymers exhibit variable amounts of grain boundaries, lattice disorder, and amorphous (disordered) regions. These structural details can be determined very precisely. Their correlations with optical or electronic properties, however, are very difficult to establish, because, for example, optical spectra are usually averaged over regions with different degrees of disorder. In an attempt to facilitate the interpretation of optical spectra, we performed systematic studies on thin films and μm-sized single crystals of thiophene-based conjugated molecules, which allowed identifying the relative contributions of ordered and disordered regions in optical emission spectra. A detailed multipeak analysis of the emission spectra showed that the peak positions, the energies of the emitted photons, showed only minor changes, independent if highly ordered or rather disordered samples were examined. However, the relative emission intensity changed significantly between samples. In particular, for highly ordered single crystals the purely electronic 0−0 transition nearly vanished, that is, it was essentially optically forbidden as theoretically predicted. Thus, changes in emission probability are correlated with the degree of structural order in semiconducting conjugated systems and provide a possibility to quantify structural order.

Active optical waveguides based on functional small organic molecules in micro/nano regime have attracted great interest for their potential applications in high speed miniaturized photonic integrations. Here, we report on the active waveguiding properties of millimeter sized single crystals of a newly synthesized thiophene-based oligomer. These large crystals exhibit low optical loss compared to other organic nanostructures, and optical losses depend on the emission energy. Moreover, we find that the coupling of photoluminescence to waveguide modes is very efficient, typically greater than 40%. These features indicate that such perfect single crystals with a low density of defects and extremely smooth surfaces exhibit low propagation loss, which makes them good candidates for the design and the fabrication of novel organic optical fibers and lasers.

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Most organic semiconductor materials dewet on silicon wafers with thermal oxide layers. While Siwafers represent convenient substrates for building a field effect transistor (FET), dewetting largely destroys the possibility for obtaining a compact and continuous crystalline thin organic semiconductor film and thus limits the mobility in these systems. Using oligothiophenes, we present an approach where the initial dewetting process can be turned into an advantage for generating very thin but large crystalline domains of a size up to the millimetres with all molecules sharing a single orientation. Our approach can be easily extended to other molecules, which have strongly differing growth velocities in the various directions of the crystal, for example due to directional π-stacking interactions. FETs devices based on such large crystalline domains showed charge carrier mobilities that were two orders of magnitude higher compared to non-crystallized films.

We employ energy-momentum spectroscopy on isolated organic single crystals with micrometer-sized dimensions. The single crystals are grown from a thiophene-based oligomer and are excellent low-loss active waveguides that support multiple guided modes. Excitation of the crystals with a diffraction-limited laser spot results in emission into guided modes as well as into quasi-discrete radiation modes. These radiation modes are mapped in energy-momentum space and give rise to dispersive interference patterns. On the basis of the known geometry of the crystals, especially the height, the characteristics of the interference maxima allow one to determine the energy dependence of two components of the anisotropic complex refractive index. Moreover, the method is suited to identify the orientation of molecules within (and around) a crystalline structure.