Abstract: The triplet excitons of the thermally activated delayed fluorescence (TADF) materials can up-convert to the singlet level by the reverse intersystem crossing (RISC) process, emitting delayed fluorescence. TADF materials have attracted widespread attention, because they can reach 100% of theoretical internal quantum efficiency (IQE). Moreover, in contrast to phosphorescent materials, heavy metal-free TADF mate-rials are easier to realize the ultimate EL efficiency. On the base of the principle of molecular design, different types of TADF materials have been reported. The design of small molecule TADF materials requires to separate the HOMO and LUMO energy levels of the molecules, and result in reducing its singlet and triplet state energy gap, by selection of suitable electron donor and acceptor units, or the addition of modified groups to increase steric hindrance. Additionally, in order to improve the fluorescence quantum efficiency, the non-radiative decay needed to be reduced by increasing the rigidity of the small molecule structure. However, small-molecule TADF materials are usually used as dopants for fluorescent devices, and it is difficult to avoid appearing of red-shift or blue-shift of emission that affects color purity of devices. The dendritic and polymeric TADF emitters — macromolecular materials can be dissolved in organic solvents, and highly efficient non-doped organic electroluminescent devices can be fabricated by solution-processing, making up for the shortage of small molecules that can only be va-cuum evaporated. Dendritic TADF materials have been widely used in organic optoelectronic devices due to their high molecular weight, precise molecular structure, adjustable energy gap and color. The dendrites around the dendrimers can prevent concentration quenching and efficiency roll-off caused by intermolecular interactions. However, the dendritic structure conjugated to the core causes decreasing of solubility and changing of luminescent color, so a non-conjugated structure is preferred. In addition, most of bipolar dendritic cores are contributed to balance of electron and hole carriers and high efficiency.Polymers are also attracting attention due to solution processing, flexibility and large-area mass production. However, it is difficult to precisely control the distance and twist angle between donor and acceptor in chain polymer TADF materials. Therefore, it is unable to ensure decreasing of singlet-triplet energy gaps and increasing of photoluminescence quantum efficiency. The polymer can be mainly divided into main chain TADF conjugated polymers and side chain non-conjugated polymers. For conjugated polymers, conjugation of the conjugated unit to the donor-acceptor backbone will well address charge transport balance problems. For non-conjugated polymers, the use of independent monomers with thermally delayed fluorescence properties allows for better inheritance of their thermally delayed fluorescence properties. The recent achievements in research on the design and modification of TADF materials for OLEDs were summarized. Firstly, a brief introduction to its TADF mechanism was given. Then we should focus on characterization and performances of small molecules and polymers TADF materials and its devices. Finally, the prospects and development trends for phosphorescent polymeric host materials were also included.
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2019, Vol. 33 
