High efficiency doping-free warm-white organic light-emitting diodes with strategic-tuning of radiative excitons by combining interfacial exciplex with multi-ultrathin emissive layers

White organic light-emitting diodes (WOLEDs) have widely drawn attention for applications in flat panel display and solid-state lighting. High efficiency and simple device structure are two important factors for large-scale application for WOLEDs. Here, we have presented a relatively reliable method to develop high-performance and doping-free WOLEDs by integrating interfacial exciplex with phosphorescent multi-ultrathin emissive layers. By reasonable design of device architecture, the resulting WOLED with a warm-white light not only realizes a low turn-on voltage of 2.25 V, but also exhibits the high forward-viewing power efficiency and external quantum efficiency of 91.5 lm W-1 and 21.3%, respectively, without light out-coupling enhancement techniques. More importantly, the power efficiency still remain as high as 49.5 lm W-1 at the high luminance of 1000 cd m(-2), which should be one of the best values in reported doping-free WOLEDs. It suggests that multi-ultrathin emissive layers combining with interfacial exciplex can realize the efficient utilization of excitons and improvement of efficiency roll-off.