Improved performances in top-emitting organic light-emitting diodes based on a semiconductor zinc oxide buffer layer

Electroluminescence performances are improved by inserting a semiconductor zinc oxide (ZnO) buffer layer into the emissive tris(8-hydroxyquinoline)aluminum (Alq(3)) layer and the semitransparent Al/Ag cathode in top-emitting organic light-emitting diodes (TEOLEDs) with structures of Si/SiO2/Ag/Ag2O/4,4', 4-tris(3-methylphenylphenylamino)triphenylamine/ 4,4'-bis[N-(1-naphthyl-1-)- N-phenyl- amino]-biphenyl/Alq(3)/ZnO/Al/Ag. The thermal deposition of ZnO layer onto Alq(3) results in Alq(3) anion formation, which is beneficial to electron injection by generating some new energy levels in the forbidden band of Alq(3). In addition, a large hole-injection barrier of similar to 2 eV at the interface of Alq(3)/ZnO effectively blocks hole injection into Al/Ag cathode, leading to more carrier recombination in the emissive region. (c) 2007 Elsevier B.V. All rights reserved.