Harvesting excited spin-triplet states as light is essential to realize highly efficient electroluminescence (EL) in organic light-emitting devices. In recent years, thermally activated delayed fluorescence (TADF) has attracted much attention as a novel electronic transition process, since it enables harvesting electrically generated triplet energy as EL without the utilization of rare metals such as iridium and platinum. When the energy gap between the excited spin-triplet and spin-singlet states in molecules is small enough to be compared to the environmental thermal energy at room temperature, they exhibit an intense state mixing between them, resulting in highly efficient reverse intersystem crossing from the spin-triplet to the spin-singlet due to the spin allowed transition, and successive light emission as delayed fluorescence from the singlet excited-state. Using molecules exhibiting TADF, internal EL quantum efficiencies of nearly 100%, which is the theoretical limit, have been realized with sophisticated molecular design. Here, we briefly review recent developments of TADF molecules along with the current understanding of spin-flip mechanisms in purely organic molecular systems.
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