Realization of red light-emitting diodes with a confinement layer

In order to obtain a light emission with a red color, we have realized light-emitting diodes with an electroactive polymer. the poly(3-octylthiophene) or P3OT. The internal efficiency versus applied bias characteristic is very weak (eta(int) = 0.025%). In order to improve the electron injection, we have inserted a 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4 oxadiazole (or butyl-PBD) layer between the polymer film and the calcium cathode. With this two-layer structure, we obtain a tenfold improvement in efficiency (eta(int) = 0.25%). Lastly, we have used another material in order to obtain a confinement layer. With an 8-tris hydroxyquinoline aluminum (or Alq3) layer, the efficiency is better (0.8%) but we observe a color light change (yellow-orange). We discuss the phenomenon. (C) 1999 Elsevier Science B.V. All rights reserved.