Thermally Activated Delayed Fluorescence in Metal-Free Small Organic Materials: Understanding and Applications in OLEDs

Thermally activated delayed fluorescence (TADF) is one of the most attractive photophysical properties due to its potential application as a triplet harvesting mechanism in metal-free OLED emitters. TADF shows a way to achieve 100% theoretical internal quantum efficiency by thermally up-converting the non-radiative triplet excitons. Initially, the mechanism of TADF was proposed to be based on the thermal equilibrium of the lowest singlet (S1) and triplet (T1) states, where the rate constant for reverse intersystem crossing (kRISC) was dependent on the temperature. At the same time, the flipping of spin in TADF is considered to be the result of spin–orbit coupling. Recent findings showed that the TADF and organic room temperature phosphorescence (RTP) property of a purely organic emitter can be tuned by steric hindrance between donor and acceptor groups in D-A-D molecules. The intersystem crossing between triplet charge transfer (3CT) and triplet local exciton (3LE) states depends on the extent of vibronic coupling and the adiabatic energy difference between them. Furthermore, the optimal energy gap of 1CT and 3CT and 3CT and 3LE states is important for efficient TADF. Overall, there are several factors involved in designing the materials for efficient TADF. Herein, we briefly review the fundamental mechanism, photophysical kinetics, and different theories of TADF along with the photophysical characterization of some organic TADF molecules. Also, the applications of some of the TADF materials in OLEDs were summarized.