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 (Alq3) 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/Alq3/ZnO/Al/Ag. The thermal deposition of ZnO layer onto Alq3 results in Alq3 anion formation, which is beneficial to electron injection by generating some new energy levels in the forbidden band of Alq3. In addition, a large hole-injection barrier of ∼2 eV at the interface of Alq3/ZnO effectively blocks hole injection into Al/Ag cathode, leading to more carrier recombination in the emissive region.