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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 22mA∕cm2 (10000cd∕m2) by adopting the D-EMLs in Ir(ppy)3 based PhOLEDs, resulting in high external quantum efficiency (ηext=13.1%) at high luminance.
Kang et al. (2007) studied this question.