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.

High quantum efficiency above 18% and extended lifetime three times longer than that of phosphorescent organic light-emitting diodes (OLEDs) are demonstrated in blue thermally activated delayed fluorescent OLEDs.

The highly sensitive optical detection of oxygen including dissolved oxygen (DO) is of great interest in various applications. We devised a novel room-temperature-phosphorescence (RTP)-based oxygen detection platform by constructing core-shell nanoparticles with water-soluble polymethyloxazoline shells and oxygen-permeable polystyrene cores crosslinked with metal-free purely organic phosphors. The resulting nanoparticles show a very high sensitivity for DO with a limit of detection (LOD) of 60 nm and can be readily used for oxygen quantification in aqueous environments as well as the gaseous phase.

Transistors On page 5875, J. H. Cho and co-workers demonstrate a new device architecture for flexible vertical Schottky barrier (SB) transistors and logic gates based on graphene-organic-semiconductor-metal heterostructures and ion gel gate dielectrics. The devices show well-behaved p-and n-type characteristics under low-voltage operation (<1 V), yielding high current densities (>100 mA cm-2) and on-off current ratios (>10 3). Biosensors On page 6034, P. K. Wong and co-workers demonstrate a nanorod-based biosensor for dynamic single-cell analysis in native tissue microenvironments. The biosensor is capable of monitoring spatiotemporal mRNA expression in primary human cells, capillary networks, and animal tissues, including the skin, retina, and cornea, challenged mechanically and biochemically. Conjugated Polymers M. Xue and co-workers describe an in situ polymerization method for yielding single-crystal-conjugated polymer (SCCP) arrays on page 5923. As-fabricated SCCP micro-arrays exhibit a smooth surface, excellent environmental stability, and enhanced electron sensitivity, which may bring high performances for CP-based devices, such as supercapacitors, organic solar cells, polymer super-conductors, organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), or some flexible electronics. Photocatalysts Well-designed hetero-nanostructural plasmonic photo-catalysts with a multichannel sensitization effect on the charge-carrier dynamics process are developed by B. Dong and co-workers, as described on page 5906. The rational combination of the semiconductor hetero-junction effect and a surface plasmon resonance (SPR) coupling effect of the plasmonic dimers, as well as the nanostructural property of electrospun nanofibers, results in a remarkable enhancement in the efficiency of sol ar to fuels conversion. Carbazole-and triazine-derived thermally activated delayed fl uorescent (TADF) emitters , with three donor units and an even distribution of the highest occupied molecular orbital, achieve high external quantum effi ciencies of above 25% in blue and green TADF devices.

A carbazole derivative substituted with two diphenylphosphine oxide groups at asymmetric positions of carbazole is synthesized and the substitution position is correlated with the photophysical properties and device performances of blue phosphorescent organic light‐emitting diodes. The carbazole type host with substituents at 2‐ and 5‐ positions of carbazole shows the merits of low driving voltage of 2‐position substitution, and high thermal stability and high quantum efficiency of 5‐ position substitution. Therefore, the carbazole type host exhibits excellent thermal and morphological stability up to 140 °C and record high quantum efficiency of 31.4% and power efficiency of 53.1 lm W‐1 without any outcoupling enhancement and p‐ or n‐doped charge transport layer in blue phosphorescent organic light‐emitting diodes.

High efficiency and color tuning of thermally activated delayed fluorescent emitters were achieved at the same time by designing emitters with a twin emitter molecular design. The control of the interconnect position between two emitters could manage the emission spectrum of the thermally activated delayed fluorescent emitters without affecting the quantum efficiency.

Thermally activated delayed fluorescent (TADF) emitters having bicarbazole as a donor unit based on a twin emitting core design were developed by substituting the bicarbazole donor with a diphenyltriazine acceptor. Three bicarbazole derived delayed fluorescent emitters linked via 2,3′-, 3,3′-, and 3,4′-positions of carbazole were synthesized and investigated as blue dopants of the TADF devices. A twin emitting core based TADF emitter derived from a 3,3′-bicarbazole donor exhibited a high quantum efficiency of 25.0% in the greenish blue TADF device. A 3,3′-linkage induced red shift of the emission wavelength, a small singlet−triplet energy gap, and a high quantum efficiency were revealed.

Three bicarbazole based host materials with different interconnect positions between carbazole units, 3,3'-(9,9'-[3,3'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN33BCz), 3,3'-(9,9'-[3,4'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN34BCz), and 3,3'-(9,9'-[4,4'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN44BCz), were developed and their synthesis, material characterization, and device characterization were reported. Two carbazole units were connected via 3,3'-, 3,4'-, and 4,4'-positions to correlate the interconnect positions with photophysical properties and device performances. The linkage via 4,4'-position increased triplet energy and thermal stability of the host materials, while the linkage via 3,3'-position enhanced current density. All bicarbazole host materials showed good device performances and high quantum efficiency above 25% was attained using the biscarbazole derivatives for green and blue phosphorescent organic light-emitting diodes. In particular, the bicarbazole host materials with a linkage via 4-position showed high quantum efficiency above 30% in the green devices.