Organic light-emitting diodes (OLEDs) are increasingly used in displays replacing traditional flat panel displays; e.g., liquid crystal displays. Especially, the paradigm shifts in displays from rigid to flexible types accelerated the market change from liquid crystal displays to OLEDs. However, some critical issues must be resolved for expansion of OLED use, of which blue device performance is one of the most important. Therefore, recent OLED material development has focused on the design, synthesis and application of highefficiency and long-life blue emitters. Well-known blue fluorescent emitters have been modified to improve their efficiency and lifetime, and blue phosphorescent emitters are being investigated to overcome the lifetime issue. Recently, thermally activated delayed fluorescent emitters have received attention due to the potential of high-efficiency and long-living emitters. Therefore, it is timely to review the recent progress and future prospects of high-efficiency blue emitters. In this feature article, we summarize recent developments in blue fluorescent, phosphorescent and thermally activated delayed fluorescent emitters, and suggest key issues for each emitter and future development strategies.

Organic emitters have been widely developed since the demonstration of high efficiency in pure organic emitters represented by thermally activated delayed fluorescent (TADF) emitters. Pure organic‐based TADF emitters are found to harvest all singlet and triplet excitons for radiative transition processes, like phosphorescent emitters that utilize all generated excitons for light emission by strong spin–orbit coupling. Red, green, and blue organic emitters are investigated, but major interest is focused on green and blue organic emitters. However, recent studies are being directed to red and even near‐infrared emitters because of their wide applications in bioimaging, sensors, telecommunications, and night vision. Although the external quantum efficiency of the red and near‐infrared TADF organic light‐emitting diodes (OLEDs) is low, the external quantum efficiency of red OLEDs has been advanced to close to 30% and that of near‐infrared OLEDs to the 10% level. Therefore, more attention is being paid to red and near‐infrared TADF emitters. In this work, recent progress of the red and near‐infrared TADF emitters is reviewed to understand the molecular design principle and the molecular structure–photophysical characteristics and molecular structure–device performance relationships. In addition, future prospect of the red and near‐infrared TADF emitters is proposed.

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CN-modified host materials, 9-(2-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile (o-CzCN) and 9-(3-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile (m-CzCN), which can improve the external quantum efficiency and lifetime of both blue phosphorescent and thermally activated delayed fluorescent (TADF) emitters were developed. A molecular design approach to stabilize the molecular structure and reduce the energy gap produced two high triplet energy host materials of o-CzCN and m-CzCN compatible with the phosphorescent and TADF emitters. The new host materials lowered operation voltage, increased quantum efficiency, and elongated lifetime of both phosphorescent and TADF devices.

Yellow thermally activated delayed fluorescence materials were synthesized using two types of acceptors of 2,4,6-triphenylpyrimidine-5-carbonitrile and 4-(3-cyanophenyl)-2,6-diphenylpyrimidine-5-carbonitrile combined with a strong donor.