Phosphorescent organic light-emitting diodes (PhOLEDs) unfurl a bright future for the next generation of flat-panel displays and lighting sources due to their merit of high quantum efficiency compared with fluorescent OLEDs. This critical review focuses on small-molecular organic host materials as triplet guest emitters in PhOLEDs. At first, some typical hole and electron transport materials used in OLEDs are briefly introduced. Then the hole transport-type, electron transport-type, bipolar transport host materials and the pure-hydrocarbon compounds are comprehensively presented. The molecular design concept, molecular structures and physical properties such as triplet energy, HOMO/LUMO energy levels, thermal and morphological stabilities, and the applications of host materials in PhOLEDs are reviewed (152 references).

Organic light-emitting diodes (OLEDs) are competitive candidates for the next generation flat-panel displays and solid state lighting sources. Efficient blue-emitting materials have been one of the most important prerequisites to kick off the commercialization of OLEDs. This tutorial review focuses on the design of blue fluorescent emitters and their applications in OLEDs. At first, some typical blue fluorescent materials as dopants are briefly introduced. Then nondoped blue emitters of hydrocarbon compounds are presented. Finally, the nondoped blue emitters endowed with hole-, electron- and bipolar-transporting abilities are comprehensively reviewed. The key issues on suppressing close-packing, achieving pure blue chromaticity, improving thermal and morphological stabilities, manipulating charge transporting abilities, simplifying device structures and the applications in panchromatic OLEDs are discussed.

To date, organic solar cells (OSCs) with the development of photovoltaic materials have realized high power conversion efficiencies (PCEs) through the solution processing strategy with bulk heterojunction (BHJ) structure, but the BHJ morphology is difficult to control in large-scale fabrication of OSCs. Herein, we report an alternative film-forming technology known as layer-bylayer (LbL). As compared to its BHJ counterpart, LbL presents many unique advantages including controllable ''p-i-n'' morphology, good charge transport and extraction properties, and great universality. By using the LbL-bladed coating strategy, a high PCE of 16.35% was achieved in the PM6:Y6 OSCs. Notably, a large-area solar module of 11.52 cm 2 with a geometrical fill factor of over 90% exhibited an outstanding PCE of 11.86%, which represents the highest efficiency of large-area solar modules. The results may pave the way for the fabrication of the photoactive layer in the future industrial production of OSCs.

Figure 1. a) The synthetic routes of IC-Br(in) and IC-Br(out). b) Chemical structures of PY-IT, PY-OT and PY-IOT.

The combination of rigid acridine donor and 1,8-naphthalimide acceptor has afforded two orange-red emitters of NAI-DMAC and NAI-DPAC with high rigidity in molecular structure and strongly pretwisted charge transfer state. Endowed with high photoluminescence quantum yields (Φ ), distinct thermally activated delayed fluorescence (TADF) characteristics, and preferentially horizontal emitting dipole orientations, these emitters afford record-high orange-red TADF organic light-emitting diodes (OLEDs) with external quantum efficiencies of up to 21-29.2%, significantly surpassing all previously reported orange-to-red TADF OLEDs. Notably, the influence of microcavity effect is verified to support the record-high efficiency. This finding relaxes the usually stringent material requirements for effective TADF emitters by comprising smaller radiative transition rates and less than ideal Φ s.

Asymmetric acceptor BTP-2F-ThCl-based devices gave the best PCE of 17.06% due to the optimal energy levels relative to those of the devices based on their symmetrical counterparts, BTP-4F (16.37%) and BTP-2ThCl (14.49%).

Phosphorescent organic light-emitting diodes (PHOLEDs) continue to attract intense interest because they can, in theory, approach a 100 % internal quantum efficiency by utilizing both singlet and triplet excitons.[1] To achieve highly efficient electrophosphorescence by reducing competitive factors such as concentration quenching and triplet-triplet annihilation, phosphorescent emitters of heavy-metal complexes are usually doped into a suitable host material.[2] Thus the synthesis of host materials and dopants are equally important for the formation of efficient PHOLEDs. It is desirable that the host materials have a large enough bandgap for effective energy transfer to the guest, good carrier transport properties for a balanced recombination of carriers in the emitting layer, and energy-level matching with neighboring layers for effective charge injection.Recently, bipolar hosts have aroused considerable interests in the area of organic light-emitting diodes (OLEDs) because they can provide more balance in electron and hole fluxes and simplify device structure.[3] However, a compromise is required between the bipolar transporting property and band gap of the material, because the electron-donating and electron-withdrawing moieties in bipolar molecules unavoidably lower the band gap of the material by intramolecular charge transfer, while the low triplet energy of the host can cause reverse energy transfer from the guest back to the host, which consequently decreases the efficiency of PHOLEDs. To address this issue, most recent molecular designs focus on the interruption of the p conjugation between electron-donating and electron-withdrawing moieties by the incorporation of steric groups [4] and/or meta linkages [2,5] between the two moieties. Efficient blue (46 lm W À1 , 24 %), [2] green (27.3 cd A À1 ) [4b] and orange (22 cd A À1 , 7.8 %) [4a] electrophosphorescence from such small bipolar host molecules has been reported.Carbazole derivatives can be used as host materials because of their high triplet energy and good hole-transporting ability.[ [7] and 5.82 cd A À1 for deep red (a dendritic iridium complex).[8] Unfortunately, the CBP host is prone to crystallization, especially when the dopant concentration is too low. [8,9] Furthermore, red PHOLEDs containing a CBP host usually need high driving voltages because the poor energy match between CBP and adjacent hole-and electron-transporting layers can result in insufficient and/or unbalanced injection of holes and electrons.[10] It is a worthwhile target to develop host materials with good thermal stability and matching energy levels to replace CBP. Oxadiazole derivatives have been proven to be very effective in improving the injection and transport of electrons. For example, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) [11] and 1,3-bis[4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD7) [11b] are usually incorporated in OLEDs as electron-transport materials.Herein we report a novel carbazole/oxadiazole hybrid molecule o-CzOXD linked t...

Owing to the electron spin-orbit coupling (SOC) and fast intersystem crossing (ISC), heavy-metal complexes (such as iridium(III), platinum(II) and osmium(II) complexes, etc.) are phosphorescent emitters at room temperature. Since 1998, heavy-metal complexes as phosphors have received considerable academic and industrial attention in the field of organic light-emitting diodes (OLEDs), because they can harvest both the singlet (25%) and triplet (75%) excitons for emission during the electro-generated processes. Among all the visible colors (blue, green, yellow, orange and red), the yellow/orange heavy-metal complexes play an important role for realizing full-color OLEDs as well as high-efficiency white OLEDs, and thus the development of highly efficient yellow/orange heavy-metal complexes is a pressing concern. In this article, we will review the progress on yellow/orange heavy-metal complexes as phosphors in OLEDs. The general principles and useful tactics for designing the yellow/orange heavy-metal complexes will be systematically summarized. The structure-property relationship and electrophosphorescence performance of the yellow/orange heavy-metal complexes in monochromatic phosphorescent OLEDs (PhOLEDs) and white OLEDs (WOLEDs) will be comprehensively surveyed and discussed.