that satisfy the physical parameters of the OLEDs, and this could be assisted by the use of organic materials. Therefore, organic materials have played a key role in extending the lifetime of OLEDs. In particular, host materials and emitters in the emitting layer have been the major contributors to the improved lifetime of OLEDs, although charge transport materials such as hole and electron transport materials, exciton blocking materials, and carrier injection layers have also supported their extended lifetime. The advance in the lifetime of PhOLEDs has been dominantly assisted by the host materials, while that of TADF OLEDs has been mostly assisted by the emitters. Particularly, among several different types of host materials, the TADF-type hosts were the most effective hosts for long lifetime in the PhOLEDs. Therefore, TADF materials have played a decisive role in the development of highly efficient and long lifetime OLEDs.This progress report discusses the contribution of TADF materials toward the lifetime improvement of PhOLEDs as hosts and TADF OLEDs as emitters. The material design and device performances of the TADF materials are mainly examined based on recent studies that include the long lifetime PhOLEDs and TADF OLEDs assisted by TADF materials. The material designs of the intermolecular-type TADF hosts, the intramolecular-type TADF hosts, and the intramolecular-type TADF emitters for both high efficiency and long lifetime are widely covered in this report. Moreover, future possibilities of the TADF materials as hosts and emitters for long lifetime OLEDs are also proposed. The application of the TADF materials is summarized in Figure 1. Recently, the external quantum efficiency and lifetime of organic light-emitting diodes (OLEDs) have been dramatically upgraded due to development of organic materials and device structure. In particular, an intramolecular or intermolecular complex based on thermally activated delayed fluorescent (TADF) materials has greatly contributed to improving OLED device performance. Although high external quantum efficiency has been the main objective of the development of TADF materials as hosts and emitters, recent interest has been directed towards the lifetime of TADFmaterial-based OLEDs. For the past several years, remarkable advances in the lifetime of phosphorescent and TADF OLEDs have been made using TADF materials as hosts or emitters in the emitting layer. Therefore, since TADF materials are useful as both hosts and emitters for a long lifetime, this work discusses the recent progress made in developing TADF materials for long-lifetime OLEDs.

Violet organic light‐emitting diodes (OLEDs) with a very narrow emission spectrum with a full width at half maximum of 14 nm and y color coordinate of 0.02 are developed using a indolo[3,2,1‐jk]carbazole‐derived pure violet emitter. The violet emitter, 2,5,13,16‐tetra‐tert‐butylindolo[3,2,1‐jk]‐indolo[1′,2′,3′:1,7]indolo[2,3‐b]carbazole (tDIDCz), is designed to have a very rigid molecular structure driven by the multiresonance‐type core structure through the alternating carbon and nitrogen atoms. The violet emitter is decorated with t‐butyl groups to prevent intermolecular aggregation and packing, which allow pure violet emission without excimer emission. The violet OLEDs derived from the tDIDCz emitter show a violet color coordinate of (0.164, 0.018) with a narrow emission spectrum and a full width at half maximum of 14 nm (105 meV). The external quantum efficiency of the pure violet OLEDs is 3.3%. This is the first work reporting pure violet emission without any ultraviolet emission below 380 nm and blue emission above 450 nm by showing a very narrow emission spectrum.

Finding narrow-band, ultrapure blue thermally activated delayed fluorescence (TADF) materials is extremely important for developing highly efficient organic light-emitting diodes (OLEDs). Here, spin-vibronic coupling (SVC)-assisted ultrapure blue emitters obtained by joining two carbazole-derived moieties at a para position of a phenyl unit and performing substitutions using several blocking groups are presented. Despite a relatively large singlet-triplet gap (∆E ST ) of >0.2 eV, efficient triplet-to-singlet crossover can be realized, with assistance from resonant SVC. To enhance the spin crossover, electronic energy levels are fine-tuned, thereby causing ∆E ST to be in resonance with a triplet-triplet gap (∆E TT ). A sizable population transfer between spin multiplicities (>10 3 s −1 ) is achieved, and this result agrees well with theoretical predictions. An OLED fabricated using a multiple-resonance-type SVC-TADF emitter with CIE color coordinates of (0.15, 0.05) exhibits ultrapure blue emissions, with a narrow full-width-at-half-maximum of 21 nm and a high external quantum efficiency of 23.1%.

Achieving narrow‐bandwidth emission and high external quantum efficiency (EQE) simultaneously is a challenge for next‐generation blue‐emitting organic light‐emitting diodes (OLEDs). In this study, novel multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are developed by fusing an indolocarbazole unit with two carbazole skeletons using para‐oriented nitrogen atoms. The resulting rigid and planar π‐system without electron‐accepting atoms exhibits pure blue photoluminescence at 470 nm, reaching a 100% quantum yield with a full‐width‐at‐half‐maximum (FWHM) of 25 nm. Higher‐level quantum chemistry calculations confirm an MR effect within the extended π‐conjugation and an enhanced triplet‐to‐singlet crossover (104 s−1) through a reduced energy gap (ΔEST) coupled with large spin‐vibronic coupling mediated by low‐lying triplet excited states. An OLED fabricated using the MR‐TADF emitter with CIE color coordinates of (0.12, 0.16) exhibits a record high EQE of 30.9% and a small FWHM of 23 nm. With further optimization of the device structure, a high EQE of 33.8% is achieved without additional outcoupling enhancements owing to the near‐perfect horizontal alignment of the emitting dipoles.

High triplet energy exciplex hosts for deep blue phosphorescent organic light-emitting diodes were developed by synthesizing a high triplet energy hole transport type host material designed for exciplex formation with a high triplet energy electron transport type host material derived from a diphenyltriazine.