Thermally activated delayed fluorescence materials in OLEDs devices: Design, synthesis and applications

Thermally activated delayed fluorescence (TADF) materials possess many advantages, such as the controllable molecular structures, stable performance and low cost. In recent years, it has attracted wide attention in organic light-emitting diodes (OLEDs) field. TADF materials are promising for the next generation of organic light emitters. TADF molecules offer unique optical and electronic properties. Because of small energy gap (ΔEST) between the lowest singlet (S1) and triplet (T1) excited states, TADF moleculers have an efficient way to harvest triplet excitons for fluorescence through facilitated reverse inter system crossing (RISC) (T1→S1). This approach harvests light from both triplet and singlet excitons, allowing the internal quantum efficiency of the device to reach nearly 100%, which clearly breaks 25% efficiency limitation of fluorescent emitters and is comparable to the rare metal-complex phosphorescence emitters. In this review, the mechanisms and fundamental principles on design strategies of TADF have been presented. The main progresses on the recent development of TADF materials and their applications are briefly reviewed. In particular, we will address the synthsis and applications for OLEDs of the differnet kinds of color TADF materials. We are convinced that, with rationale design, precision preparation, and systematic research and development, various highly efficient TADF materials will soon play an indispensable role in illumination and display device.