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.

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.

Excellent color purity with a tunable band gap renders organic-inorganic halide perovskite highly capable of performing as light-emitting diodes (LEDs). Perovskite nanocrystals show a photoluminescence quantum yield exceeding 90%, which, however, decreases to lower than 20% upon formation of a thin film. The limited photoluminescence quantum yield of a perovskite thin film has been a formidable obstacle for development of highly efficient perovskite LEDs. Here, we report a method for highly luminescent MAPbBr (MA = CHNH) nanocrystals formed in situ in a thin film based on nonstoichiometric adduct and solvent-vacuum drying approaches. Excess MABr with respect to PbBr in precursor solution plays a critical role in inhibiting crystal growth of MAPbBr, thereby forming nanocrystals and creating type I band alignment with core MAPbBr by embedding MAPbBr nanocrystals in the unreacted wider band gap MABr. A solvent-vacuum drying process was developed to preserve nanocrystals in the film, which realizes a fast photoluminescence lifetime of 3.9 ns along with negligible trapping processes. Based on a highly luminescent nanocrystalline MAPbBr thin film, a highly efficient green LED with a maximum external quantum efficiency of 8.21% and a current efficiency of 34.46 cd/A was demonstrated.

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.

Dual emitting cores for thermally activated delayed fluorescent (TADF) emitters were developed. Relative to the corresponding TADF emitter with a single emitting core the TADF emitter with a dual emitting core, 3,3',5,5'-tetra(carbazol-9-yl)-[1,1'-biphenyl]-2,2',6,6'-tetracarbonitrile, showed enhanced light absorption accompanied by a high photoluminescence quantum yield. The quantum and power efficiencies of the TADF devices were enhanced by the dual emitting cores.

6786 wileyonlinelibrary.com TADF OLEDs. [16][17][18][19][20] The QE of green and blue TADF OLEDs is as high as 20%, which suggests that the TADF OLEDs have a potential as the high effi ciency solution-processed OLEDs. Our group already reported that the QE of green TADF OLEDs can be improved up to 18.3% by increasing the solubility of the TADF emitters using a t -butyl substituent. [ 21 ] The t -butyl substitution has been widely used as an approach to increase the solubility of organic materials and was successful in the design of soluble green TADF emitters. However, the t -butyl modifi cation is not appropriate in the design of blue TADF emitters because of red shift of emission color due to strong electron donating character of the t -butyl modifi ed carbazole unit. Therefore, a different approach to increase the solubility and to induce blue shift of emission color is necessary in order to develop soluble blue TADF emitters.In this work, two blue TADF emitters, 2,4,6-tri(9H-carbazol-9-yl)-3,5-difl uorobezonitrile (3CzFCN) and 2,3,4,6-tetra(9H-carbazol-9-yl)-5-fl uorobenzeonitrile (4CzFCN), were developed as soluble blue TADF emitters with improved solubility and blue emission color. The F atom was introduced as an electron withdrawing unit instead of CN to meet the requirements of the soluble blue TADF emitters through improved solubility by hydrophobic nature of F and blue emission by relatively weak electron withdrawing nature of the F atom. The F atom has never been adopted in the molecular design of soluble TADF emitters as well as vacuum evaporable TADF emitters. The two blue TADF emitters exhibited good solubility and could be easily coated by spin-coating process. A high QE of 20.0% with a blue emission color was achieved using the 4CzFCN blue TADF emitter by solution process. This is the fi rst work reporting high QE of 20% in the solution-processed blue TADF OLEDs. It was also demonstrated that the soluble blue TADF OLEDs are comparable to vacuum evaporable blue TADF OLEDs and solution-processed blue phosphorescent OLEDs in terms of QE. Results and DiscussionThe soluble blue TADF emitters are required to possess good solubility in solvent, small singlet-triplet energy gap for TADF emission, high photoluminescence (PL) quantum yield, and pure blue emission color. Therefore, electron withdrawing moieties which can increase the solubility of the molecule should be included in the molecular design. We reported that

Deep blue thermally activated delayed fluorescent (TADF) emitters with a narrow emission spectrum were developed by managing the molecular structure of the TADF emitters. The deep blue TADF emitters were designed to show large steric hindrance at the central core of the molecule and small singlet− triplet energy gap. The molecular engineering of the deep blue TADF emitters enabled the fabrication of the deep blue TADF device with a full width at half-maximum of only 48 nm and a quantum efficiency of 14.0%. The full width at half-maximum of the deep blue TADF device was similar to that of conventional fluorescent devices, while the quantum efficiency was more than tripled.

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.