Electrical and luminescent properties of double-layer oligomeric/ polymeric light-emitting diodes

We report the use of α-sexithiophene (T6) thin films sublimed onto glass substrates coated with indium-tin oxide in light-emitting diodes. Absolute photoluminescence quantum efficiencies were found to be in the range 10−2–10−3%, and indicate that T6 should be used as a hole 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), MEH-CN-PPV, was spun on top of the T6 layer prior to evaporation of Ca-Al cathodes. Turn-on voltages for electroluminescence of about 3 V were found in structures with total thickness between 160 and 230 nm, while internal quantum efficiencies were up to 0.4%, i.e. at least ten times less than those measured in comparative devices where a PPV hole-injecting layer was used in place of T6. The substantial difference is interpreted on the basis of a significant lowering of the barrier to electron ejection from the luminescent layer into the hole-injecting layer when passing from PPV to T6. This allows inefficient recombination to take place in T6, as confirmed by the electroluminescence spectra.