Polarised Electroluminescence from Vacuum-Grown Organic Light-Emitting Diodes

Sublimed organic thin films are usually amorphous, and light-emitting diodes that utilise such films as the emissive layer emit isotropically distributed light with random polarisation. However, α-sexithiophene (αT6) forms highly ordered thin films, and we show here that, by controlling the morphology of the emissive layer in a LED based on αT6, we can achieve polarised electroluminescence. Vacuum-sublimed films of αT6 contain highly ordered grains with the long axes of the molecules standing almost perpendicular to the substrate surface. Fluorescence arises from the decay of excited states localised on individual molecules (Frenkel excitons), and is polarised along the long molecular axis. Excitons diffuse within the film, and can migrate into defects such as grain boundaries and be trapped. In films containing small grains we observe isotropic emission from randomly distributed defects, but when we increase the grain size (to reduce the defect density) emission occurs from ordered regions and we measure polarised electroluminescence consistent with the bulk morphology.