Revealing Order and Disorder in Films and Single Crystals of a Thiophene-Based Oligomer by Optical Spectroscopy

Abstract

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.

Low loss optical waveguiding in large single crystals of a thiophene-based oligomer

“…As expected for materials of high molecular order, 3TBT crystals were birefringent. Under crossed polarizers, these crystals exhibited a uniform intensity (birefringence) which varied when the crystals were rotated within the plane of observation 1 .…”

“…Yet, to accurately reproduce the spectrum, we had to fit five Gaussian peaks, as detailed recently. 1 The full width at half maximum (FWHM) and the intensity of the Gaussians were treated as adjustable parameters to give the best fit to the experimental data. Out-coupled PL spectrum at the tip of crystal was decomposed into four Gaussian peaks because the intensity of the highest energy peak was reduced so strongly that it was not possible to decompose the spectra into the five peaks.…”

“…Peak G is probably a combination of vibrations, e.g from peak D (ring breathing) and peak E (carbon bond stretch). For details, see ref 1 .…”

Self-Interference of Exciton Emission in Organic Single Crystals Visualized by Energy-Momentum Spectroscopy

“…More details concerning the crystal structure, as well as absorption and emission of 3TBT molecules in solution, thin film, and single crystals can be found in our recent study. 39…”

“…The origin of these out-of-plane transition dipole moments can be traced back to the presence of a thin layer of 3TBT nanofibers at the single-crystal surface, which are aligned along the long ( y ) axis of the single crytal. 39 They appear to be randomly twisted around their y axis so that a substantial fraction of emitters with z -oriented transition dipole moments is present. Although there could be several layers of nanofibers, the resulting surface roughness is small compared to the wavelength of light, 39 and thus no big deviation from flat single-crystal surfaces considered in our simulations is expected.…”