A new branched carbazole derivative, 1,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB), was prepared as a hole-transporting material for organic light-emitting devices (OLEDs). TECEB is comparable to 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) in terms of highest-occupied molecular orbital/lowestunoccupied molecular orbial energy levels, hole-drift mobility, as well as device performance (maximum luminence of about 10 000 cd m -2 and current efficiency of 3.27 cd A -1 ) in a standard hole-transporting layer/tris-(8-hydroxyquinoline)aluminum double-layer device, but is superior to NPB in terms of its higher glass-transition temperature (T g , 130 °C) and ease of synthesis. The latter features suggest that TECEB can be a potential alternative material to NPB especially for high-temperature applications in OLEDs and other organic electronic devices.

Highly efficient, orange organic light‐emitting diodes (OLEDs) fabricated from newly synthesized iridium complexes show a maximum luminance efficiency of 76 cd A−1 and a peak power efficiency of 45 lm W−1. The white OLEDs containing the orange iridium and traditional blue iridium phosphors exhibit extraordinarily high efficiencies and a peak external quantum efficiency of 26%.

A group of dendrimers with oligo‐carbazole dendrons appended at 4,4′‐ positions of biphenyl core are synthesized for use as host materials for solution‐processible phosphorescent organic light‐emitting diodes (PHOLEDs). In comparison with the traditional small molecular host 4,4′‐N,N′‐dicarbazolebiphenyl (CBP), the dendritic conformation affords these materials extra merits including amorphous nature with extremely high glass transition temperatures (ca. 376 °C) and solution‐processibility, but inherent the identical triplet energies (2.60–2.62 eV). In comparison with the widely‐used polymeric host polyvinylcarbazole (PVK), these dendrimers possess much higher HOMO levels (–5.61 to –5.42 eV) that facilitate efficient hole injection and are favorable for high power efficiency in OLEDs. The agreeable properties and the solution‐processibility of these dendrimers makes it possible to fabricate highly efficient PHOLEDs by spin coating with the dendimers as phosphorescent hosts. The green PHOLED containing Ir(ppy)3 (Hppy = 2‐phenyl‐pyridine) dopant exhibits high peak efficiencies of 38.71 cd A−1 and 15.69 lm W−1, which far exceed those of the control device with the PVK host (27.70 cd A−1 and 9.6 lm W−1) and are among the best results for solution‐processed green PHOLEDs ever reported. The versatility of these dendrimer hosts can be spread to orange PHOLEDs and high efficiencies of 32.22 cd A−1 and 20.23 lm W−1 are obtained, among the best ever reported for solution‐processed orange PHOLEDs.

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A series of bipolar hosts, namely, 5-(2-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (o-CzDPz), 5-(3-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (m-CzDPz), 5-(9-phenyl-9H-carbazol-3-yl)-1,3-dipyrazolbenzene (3-CzDPz), and 5-(3,5-di(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (mCPDPz), are developed for phosphorescent and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). They are designed by selecting pyrazole as n-type unit and carbazole as p-type one. The triplet energy (E(T)), the frontier molecular orbital level, and charge transporting abilities, are adjusted by varying the molar ratio of pyrazole to carbazole and the linking mode between them. They have high E(T) values of 2.76-3.02 eV. Their electroluminescence performance is evaluated by fabricating both phosphorescent and TADF devices with blue or green emitters. The m-CzDPz hosted blue phosphorescent OLEDs achieves high efficiency of 48.3 cd A(-1) (26.8%), the 3-CzDPz hosted green phosphorescent device exhibits 91.2 cd A(-1) (29.0%). The blue and green TADF devices with 3-CzDPz host also reach high efficiencies of 26.2 cd A(-1) (15.8%) and 41.1 cd A(-1) (13.3%), respectively. The excellent performance of all these OLEDs verifies that these pyrazole-based bipolar compounds are capable of being universal host materials for OLED application. The influence of molar ratio of n-type unit to p-type one and the molecular conformation of these hosts on their device performance is discussed and interpreted.

4,4 0 -biphenyl (CBP) is one of the most successful uni-polar host materials for phosphorescent organic light-emitting diodes (PhOLEDs). We report the synthesis and properties of one novel CBP derivative, CBP-CN, with two cyano groups (CN) at the 3-site of carbazole rings. The strong electron-withdrawing CN group was introduced with the expectation to promote electron-injecting/transporting abilities and to achieve bipolar features for CBP-CN. In comparison with the parent CBP, CBP-CN possesses lowered HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels and dramatically increased T g (glass transition temperature, 162 C), but unaltered HOMO-LUMO band gap and triplet energy (2.69 eV). Green and red PhOLEDs were fabricated with CBP-CN as hosts for traditional iridium phosphors. The maximum luminance efficiency (h L ) of 80.61 cd A À1 (23.13%) was achieved for the green PhOLED, and 10.67 cd A À1 (15.54%) for the red one, which represent efficiency increases of 25-33% compared with those of the best devices with CBP host and are even among the best data for phosphorescent OLEDs reported so far. The theoretical calculation and the carrier-only devices investigation confirmed that the electron-injecting/-transporting character and the bipolar nature of CBP-CN should be responsible for the performance enhancements.

By adjusting the para-, meta-, and ortho-linking styles of p-type and n-type units, the physical parameters of bipolar host materials are regularly tuned to a large extent. The meta- and ortho-linked isomer hosts exhibit excellent performance in blue phosphorescent and thermally activated delayed fluorescence organic light-emitting diodes.

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