The growth and characterization of alpha-sexithienyl-based light-emitting diodes

We describe the mechanisms of growth of sublimed thin films of alpha-sexithienyl and how the morphology of the films can be controlled by using different growth conditions and substrates. The molecules of alpha-T6 prefer to stand upright on the substrate surface if possible, forming a highly ordered film, and we find that partially polarized photoluminescence and electroluminescence is possible from these films provided that energy migration to randomly oriented defect sites is properly controlled. We have also investigated the effects of a SiO interfacial layer at the interface between alpha-TG and the aluminium negative contact in light-emitting diodes grown in ultra-high vacuum. This is expected to alter the injection properties of electrons into the diodes, which should modify the electroluminescent efficiency. The efficiency is a strong function of the SiO thickness, and the optimum thickness lies at about 2 nm of SiO, which is a reasonable figure for the expected mechanism of electron tunnelling across the barrier. The current-voltage curves are thermally activated and we find that they are also strongly affected by the thickness of this interfacial layer. As the thickness increases from 1-4 nm, the voltage required to drive the light-emitting diode at 0.5 mA drops by about an order of magnitude. from about 15-2 V. The reason for this drop in drive voltage is not yet clear.