Poly(p-phenylenevinylene) light-emitting diodes: Enhanced electroluminescent efficiency through charge carrier confinement

We have fabricated light-emitting diodes with poly(p-phenylenevinylene) as the emissive layer, and with an electron-transporting layer formed from a solid state dispersion of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole in poly(methyl methacrylate), placed between this and the negative electrode. These structures show typically a tenfold improvement in efficiency in the low-voltage regime and an eightfold improvement in the high-voltage regime over devices without the electron-transporting layer. Typical efficiencies are about 0.8% photons/electron. We consider that the role of the electron-transport layer is to confine holes to the emissive layer.