Graded-host phosphorescent light-emitting diodes with high efficiency and reduced roll-off

We demonstrated graded-host phosphorescent organic light-emitting diodes with high efficiency and reduced efficiency roll-off. The emissive layer of the graded host device consists of both electron and hole transport type osts, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) and 4,4',4 ''-tris(N-carbazolyl)triphenylamine, respectively, with graded composition, and the phosphorescent red emitter bis(2-phenylquinoline) (acetylacetonate) iridium(III), which was uniformly doped into the graded host matrix. The graded host device shows improved quantum efficiency and power efficiency with significantly reduced efficiency roll-off as compared to the unipolar-host and double layer heterojunction host devices. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.3701685]