Reduced efficiency roll-off in all-phosphorescent white organic light-emitting diodes with an external quantum efficiency of over 20%

Although previously reported all-phosphorescent white organic light-emitting diodes (WOLEDs) exhibit impressive electroluminescence efficiencies, problems remain in terms of the severe efficiency roll-off at high brightness. Here, a smart design of the emissive zone structure is presented to make full use of generated excitons to realize high efficiency all-phosphorescent WOLEDs with reduced efficiency roll-off. The fabricated WOLED shows a maximum power efficiency (PE) of 46.6 lm W^-1, a current efficiency (CE) of 46.4 cd A^-1 and an external quantum efficiency (EQE) of 22.4%. These values remain as high as 41.3 lm W^-1, 46.2 cd A^-1 and 22.0%, respectively, at a brightness of 1000 cd m^-2, exhibiting less pronounced efficiency roll-off. The critical current density, where the EQE declines by half from its peak, reaches 220 mA cm^-2, which should be a high value for all-phosphorescent WOLEDs with an external quantum efficiency of over 20%.