Since the ground-breaking article of highly efficient organic light-emitting diodes from delayed fluorescence materials demonstrating nearly 100% internal efficiency of electroluminescence [Nature 492, p. 234-238 (2012)], extensive studies on thermally activated delayed fluorescence (TADF) have been conducted. This has led to the rapid development of new TADF materials and positioned them as the third generation of OLED emitters. Supporting photophysical and theoretical studies have also been published on advanced exciton formation mechanisms under electrical excitation in an effort to comprehensively understand the impact of using these next-generation materials in OLEDs.

This special section of the Journal of Photonics Energy focuses on state-of-the-art TADF science and technology for OLEDs and covers novel TADF materials, interfaces, photophysics, and device physics. The breadth of new TADF materials being developed and studied is on display with several reports discussing new molecular designs with emission ranging from blue to the NIR. An et al. report long-wavelength emission originating from thermally activated delayed fluorescence in chromophores based on fluorescein derivatives including a strong acceptor 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile (TFM), demonstrating NIR TADF with emission maxima ranging from 681 nm to 755 nm and with a 9.02 microsecond delayed lifetimes. Wu et al. report a butterfly-shaped orange-red TADF emitter, ACFO, by integrating electron-donating (D) 9,9-dimethyl-9,10-dihydroacridine units into the $\gamma$-positions of an electron-accepting (A) fluorenone core to form a D-A-D configuration. Gounder et al. describe a new molecular design approach for blue-emitting TADF molecules having a pyridine-functionalized carbazole donor and a benzophenone acceptor, and Komatsu et al. review the recent progress of pyrimidine-based TADF materials with various combinations of donors.

In addition to the development of new materials, a deeper understanding of material and device physics is critical for improving performance and developing new applications, and these areas are also represented by several reports in this special section. Lee et al. demonstrate an extremely high external quantum efficiency of 37% by combining TADF emitters exhibiting a favorable molecular orientation with low-refractive-index electron transport layers. Hasegawa et al. present the self-organization/orientation of TADF molecules on HOPG, indicating promise for application in OLEDs. Hosokai et al. study solvation effects on TADF molecules, and how these influence ${\mathrm{\Delta }E}_{\mathrm{ST}}$ due to the changes in the CT and LE nature of ${\mathrm{S}}_1$ and ${\mathrm{T}}_1$. Kobayashi et al. analyze ISC/RISC processes of TADF materials, demonstrating that a four-level model of the excited states in 4CzIPN derivatives provides the best explanation for the experimental results of the transient PL characteristics. Finally, Tokumaru provides a historical perspective on insights into RISC and ISC in TADF compounds.

The articles in this special section provide a snapshot of the cutting-edge science and technology of TADF, providing insights for the TADF mechanism and advanced TADF materials design. We hope these articles will provide further inspiration into the study of TADF materials and devices.