Non-noble-metal-based organic emitters for OLED applications

Organic light-emitting diodes (OLEDs) employing purely organic functional materials indicate a low-cost manufacturing route towards the next-generation display and solid-state lighting owing to the avoidance of noble heavy metal complex phosphorescent emitters. In recent years, several mechanisms have been proposed to design high performance purely organic emitters. This new generation of purely organic emitters shed light on the realization of both low-cost and high performances. The main idea of this paper is to review how to use purely organic semiconductors to realize high-efficiency OLEDs. This guides us to pay special attention to two aspects: 1) how to break the efficiency bottleneck resulting from exciton spin-statistics, which is critical to determine internal quantum efficiency; 2) how to enhance out-coupling efficiency by molecular designs, which eventually influences on external quantum efficiency. Several significant material design strategies are thus introduced, and the relevant mechanisms are classified as triplet-triplet annihilation, thermally activated delayed fluorescence, hot excitons, room temperature phosphorescence, and luminescent radicals. Then, device strategies by employing organic heterojunctions as the main luminescent center towards high-performance fluorescent OLEDs were introduced. Finally, we outline the progress of enhancing out-coupling efficiency by tuning the dipole orientation of emitters and the operational stability of OLEDs excluding noble heavy metal complex phosphorescent emitters.