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We demonstrate high-efficiency white organic light emitting devices (WOLEDs) employing three adjacent phosphorescent emission layers (3-EMLs). The metal-organic dopants for red, green, and blue emissions are each doped in separate hosts, allowing for separate optimization of the three dopant-host material combinations. This structure distributes the exciton generation region across the three hosts to form a stepped progression of highest occupied and lowest unoccupied molecular orbitals. The 3-EML WOLED has a color rendering index of 81 and peak forward-viewing external quantum (EQE) and power efficiencies (PE) of (16.6±0.8)% and 32±1lm∕W, respectively, corresponding to a total EQE=(28±1)% and a total PE=54±3lm∕W. When an n-doped electron transporting layer is used, the total PE peaks at 64±3lm∕W, and rolls off to 34±2lm∕W at 1000cd∕m2.
Sun et al. (2007) studied this question.