Recently, organic thermally activated delayed fluorescence (TADF) emitters have attracted a great deal of attention because they can theoretically realize 100% internal quantum efficiency. Many TADF emitters have been developed since the first demonstration of close to 20% external quantum efficiency in the devices. Recently developed TADF emitters demonstrated close to 37% external quantum efficiency in blue, above 30% external quantum efficiency in green, and close to 18% external quantum efficiency in red devices. Therefore, TADF organic light-emitting diodes could potentially be substituted for high-efficiency phosphorescent organic light-emitting diodes. In this work, we reviewed molecular design strategies of organic-based TADF emitters by classifying them into several categories depending on the material parameters required for the TADF emitters. In addition, we proposed a future development direction of TADF emitters to make them competitive with phosphorescent emitters.

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.

Highly efficient green thermally activated delayed fluorescent organic light-emitting diodes with an external quantum efficiency of 31.2% were investigated by using 3-(3-(carbazole-9-yl)phenyl) pyrido[3',2':4,5]furo[2,3-b]pyridine (3CzPFP) derived from carbazole and pyrido[3',2':4,5]furo[2,3-b]pyridine. The host material showed well-matched photoluminescence emission with absorption of the green dopant material, (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) and harvested all excitons of 4CzIPN. The 3CzPFP:4CzIPN film exhibited high photoluminescence quantum yield of 100%, and the green delayed fluorescence device employing the 3CzPFP host showed high maximum quantum efficiency of 31.2 ± 0.5% at 1% doping after optimization of the device structure.

High efficiency green thermally activated delayed fluorescence device with a quantum efficiency above 20% was developed using a bipolar host material, 3-(furo[2,3-b:5,4-b′]dipyridin-3-yl)-N,N-diphenylaniline (3TPAPFP), derived from furodipyridine core structure. The furodipyridine based 3TPAPFP showed a singlet and triplet energy for efficient energy transfer to thermally activated delayed fluorescent dopant, (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), and bipolar charge transport properties for balanced hole and electron density. A high quantum efficiency of 21.2% was achieved in the fluorescent device using the 3TPAPFP host doped with 4CzIPN dopant at a very low doping concentration of 1%.

Three carboline derivatives with nitrogen at different positions of the pyridoindole unit were synthesized and the effect of the position of nitrogen on the photophysical properties and device performances was investigated. β-Carboline was effective in obtaining high triplet energy and stabilizing energy levels, while α-carboline was better than other carbolines in terms of quantum efficiency. However, γ-carboline could not stabilize the LUMO level and low quantum efficiency was obtained.