ELECTRICAL AND LUMINESCENT PROPERTIES OF DOUBLE-LAYER OLIGOMERIC POLYMERIC LIGHT-EMITTING-DIODES

We report the use of alpha-sexithiophene (T6) thin films sublimed onto glass substrates coated with indium-tin oxide in light-emitting diode s. Absolute photoluminescence quantum efficiencies were found to be in the range 10(-2)-10(-3)%, and indicate that T6 should be used as a ho le injector into an emissive layer, rather than as a luminescent layer . We have fabricated double-layer organic light-emitting diodes where a cyano-substituted derivative of poly (p-phenylenevinylene) (PPV), ME H-CN-PPV, was spun on top of the T6 layer prior to evaporation of Ca-A l cathodes. Turn-on voltages for electroluminescence of about 3 V were found in structures with total thickness between 160 and 230 nm, whil e internal quantum efficiencies were up to 0.4%, i.e, at least ten tim es less than those measured in comparative devices where a PPV hole-in jecting layer was used in place of T6. The substantial difference is i nterpreted on the basis of a significant lowering of the barrier to el ectron ejection from the luminescent layer into the hole-injecting lay er when passing from PPV to T6. This allows inefficient recombination to take place in T6, as confirmed by the electroluminescence spectra.