Organic light emitting devices based on vapor deposited films of end-capped sexithiophene: Evidence for Schottky barriers and transport limitations

Light emitting devices with a sandwich structure: metal/EC6T/indium-tin-oxide, using end-capped sexithiophene (EC6T) as active organic material were fabricated by vapor deposition. Current and intensity of electroluminescence (EL) of the EC6T layers were measured as a function of voltage for various metals (Ca, Mg, Al, In, Ag) in a wide range of temperatures (4-300 K) and thicknesses of the EC6T layers (40-350 nm). External quantum efficiencies (10(-6)-10(-3)) and rectification ratios significantly depend on the metal contact which is compatible with a Schottky barrier. Electron injection from the metal at higher voltages correlates with the onset of significant EL. Current-voltage (I-V) curves exhibit a strong temperature and thickness dependence, mainly due to the charge transport across the EC6T layer. At low temperatures I-V curves show space charge limited currents. Modeling including double injection and trap states is performed. Results are discussed under the aspect of further device optimization. (C) 1997 American Institute of Physics.