Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes

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Abstract
The authors demonstrate that the reduction of quantum efficiency with increasing current density in phosphorescent light emitting diodes (PhOLEDs) is related to the formation of excitons in hole transporting layer based on the analysis of emission spectra and exciton formation zone. Low roll-off of efficiency in a PhOLED was achieved using dual emitting layers (D-EMLs) by confining the exciton formation near the interface between the emitting layers. The external quantum efficiency was maintained almost constant up to 22 mA ∕ cm 2 ( 10 000 cd ∕ m 2 ) by adopting the D-EMLs in Ir ( ppy ) 3 based PhOLEDs, resulting in high external quantum efficiency ( η ext = 13.1 % ) at high luminance.