Single crystallites in “planar polycrystalline” oligothiophene films: Determination of orientation and thickness by polarization microscopy

Thin films of evaporated oligothiophenes (α-nT, n=4–8) show a “planar polycrystalline” structure: each of the individual crystallites has a random azimuthal orientation, the (a,b) face of its unit cell is aligned with the surface plane. We introduce a technique to determine the orientation and thickness of such aligned thiophene crystals by optical polarization microscopy. Due to the optical birefringence of the crystal, it appears with different colors in the microscope dependent on its orientation and thickness. To support the method proposed, we solve Maxwell’s equations and obtain quantitative agreement with the observed colors. The organic crystal shows biaxial anisotropy. For unsubstituted quaterthiophene, α-4T, we find effective refractive indices nb=1.84±0.1 and na=1.61±0.1 for waves under normal incidence. Our conclusions are fully confirmed by atomic force microscopy with molecular resolution. Our analyses result in a simple recipe to obtain the directions of the a and b crystal axes from the optical experiment.