Highly efficient inverted phosphorescent organic light-emitting devices with ZnO nanoparticles electron injection layer

We have developed highly efficient inverted phosphorescent organic light-emitting devices (OLEDs) using ZnO nanoparticles (NPs). The wurtzite structured ZnO NPs having an average particle size of about 5 nm were synthesized by solution-precipitation method. The inverted phosphorescent OLEDs with a tris(2-phenylpyridine) iridium(III) emitter have fabricated using ZnO NPs and 2,2',2"-(1,3,5-benzinetriyl)-tris-(1-phenyl)- 1-H-benzimidazole (TPBi) electron transport layer. ZnO NPs effectively inject electrons from cathode to the electron transport layer. The maximum external quantum efficiency and current efficiency of the phosphorescent inverted OLEDs with ZnO NPs were 15.8% and 55.1 cd/A, respectively.