White organic light-emitting diodes (WOLEDs) are emerged as the core for the next-generation display and illumination devices, and substantial efforts have been devoted to functional materials exploration, exciton manipulation and distribution, configuration optimization and fabrication technique development. To achieve high electroluminescence (EL) efficiencies, all-phosphorescence systems and fluorescence-phosphorescence hybrid systems are widely studied in reported WOLEDs due to the high exciton utilization efficiencies of phosphorescence materials. But the high cost of noble metal-containing phosphorescence materials as well as the lack of robust blue phosphorescence materials impede the widespread commercialization of WOLEDs to a large degree. Purely organic thermally activated delayed fluorescence (TADF) materials can theoretically harness 100% electrogenerated excitons through reverse intersystem crossing (RISC) from the lowest triplet excited (T1) state to the lowest singlet excited (S1) state based on a small energy gap (∆EST) between S1 and T1 states, and thus have been regarded as the successors of phosphorescence materials. The external quantum efficiency (EQE) of monochromatic OLEDs based on TADF emitters have successfully surpassed 30%, and even approached 40% in very recent reports. Therefore, certain high-efficiency TADF emitters with competitive prices hold great potentials to replace phosphorescence materials in OLED industry.