Crystallization and Resulting Optical Properties of Thiophene-Based Oligomers

Triggered by their tunable, readily-tailored optical and electrical properties and due to their low cost processing, organic semiconducting materials have been widely studied, mainly in view of various optoelectronic applications. Currently, strong efforts are undertaken for improving our understanding of the relation between morphology and electronic structure. Detailed knowledge of this relation may help to improve the performance of devices such as field effect transistors and organic photovoltaics. Prompted by the observation of high charge-carrier mobilities and of long-range energy transport in highly ordered (supramolecular) structures, much effort was invested on the improvement of structural order in organic conjugated materials. However, in most of the studied devices the active layer possesses a complex morphology with a large amount of ordered and disordered regions, including grain boundaries. Depending on processing conditions, ordered microstructures of conjugated oligomers or polymers exhibit variable amounts of grain boundaries, lattice disorder and amorphous (disordered) regions. Their correlations with optical or electronic properties, however, are very difficult to establish, because e.g., optical spectra are usually averaged over regions having different degrees of disorder. The simultaneous existence of highly ordered (crystalline) and structurally more disordered regions make it diffcult to interpret electronic behavior unambiguously and to establish a clear relation between structural features and electronic properties. Micrometer-sized single crystals of organic semiconducting molecules hold the promise to overcome many of the current challenges of device fabrication { due to domain boundary defects { for a wide range of electronic and photonic applications. Such single crystals promote the optical and electrical properties of macromolecules over a large length scale. In order to bridge between structural and optical properties, we have studied the direct formation of oligothiophene single crystals both in solution and in thin film. By precisely controlling the nucleation stage, we have grown large single crystals of newly synthesized thiophene-benzene-thiophene oligomers. pi-pi stacking interactions of these conjugated molecules determined fast growth along the long axis of crystal. The well-defined molecular order over macroscopic dimensions of single crystals results in a high anisotropy in structure as well as in optical properties, with an optical anisotropy value comparable with inorganic crystals. Using different preparation methods (formation of single crystals vs. spin-coated films of a thiophene based conjugated oligomer), we were able to systematically control and quantify the degree of structural order from optical properties by performing spectroscopy measurements. Local spectroscopic measurements allowed to spatially resolve and distinguish disordered and ordered regions and thus to identify the contribution of these regions in emission spectra. In an attempt to facilitate the interpretation of optical spectra, we performed systematic studies on thin films and micrometer-sized single crystals of these thiophene-based molecules, which allowed identifying the relative contributions of ordered and disordered regions in optical emission spectra. A meticulous multi-peak analysis of the emission spectra showed that the peak positions, the energy of the emitted photons, were always the same, independent if highly ordered or rather disordered samples were examined. However, the relative emission intensity changed significantly between samples. In particular, for single crystals, the purely electronic 0-0 transition nearly vanished, i.e. as theoretically predicted, essentially it was optically forbidden. These changes in emission probability provided a possibility to quantify the degree of structural order in semiconducting conjugated systems. Our results showed that these single crystals exhibited a high degree of structural order. Furthermore, without any external stimulation, these well-organized crystals showed an active waveguiding behavior along the long axis of the single crystal over several hundreds of micrometer. Depending on the energy of the emitted photons, the measured optical loss was less than those of organic materials reported in the literature and was comparable to values reported for inorganic materials. Such features indicate that these crystals can be potentially applied in the field of direction-dependent optical device applications like optical waveguides or organic solid-state lasers.