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 host materials for phosphorescent organic light-emitting diodes

Phosphorescent organic light-emitting devices (OLEDs) have attracted increased attention from both academic and industrial communities due to their potential practical application in high-resolution full-color displays and energy-saving solid-state lightings. The performance of phosphorescent OLEDs is mainly limited by the phosphorescent transition metal complexes (such as iridium(III), platinum(II), gold(III), ruthenium(II), copper(I) and osmium(II) complexes, etc.) which can play a crucial role in furnishing efficient energy transfer, balanced charge injection/transporting character and high quantum efficiency in the devices. It has been shown that functionalized main-group element (such as boron, silicon, nitrogen, phosphorus, oxygen, sulfur and fluorine, etc.) moieties can be incorporated into phosphorescent emitters and their host materials to tune their triplet energies, frontier molecular orbital energies, charge injection/transporting behavior, photophysical properties and thermal stability and hence bring about highly efficient phosphorescent OLEDs. So, in this review, the recent advances in the phosphorescent emitters and their host materials functionalized with various main-group moieties will be introduced from the point of view of their structure-property relationship. The main emphasis lies on the important role played by the main-group element groups in addressing the key issues of both phosphorescent emitters and their host materials to fulfill high-performance phosphorescent OLEDs.

Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic light-emitting devices.

Organic light-emitting diodes (OLEDs) based on vacuum deposited small molecules have undergone significant progress since the first efficient double-layered OLEDs were reported in 1987 by Tang and Van Slyke. Recently, solution processed small molecular OLEDs are also drawing more and more research attention, as such a technology combines advantages of the facile synthesis of small molecules and the low-cost solution process like polymers. The performance of OLEDs made by solution process is gradually catching up with their vacuum deposited counterparts. This feature article will review the device structures adopted to achieve high performance solution processed OLEDs, the development of solution processable small molecules, and the comparisons of the different nature of the films and devices fabricated by solution-process or by vacuum deposition. Finally, the prospects and remaining problems will be discussed.

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Solution processable small molecules for organic light-emitting diodes

Incorporation of main group elements into the π-conjugated frameworks is a sophisticated strategy to alter the fundamental nature of the parent conjugated π-systems, giving rise to attractive electronic and photophysical properties that are otherwise inaccessible with classic carbon- or metal-based materials. Out of all π-conjugated heterocycles, those that are structurally constrained by tethered aryl substituents surrounding the main group center deserve a great deal of attention because not only do they commonly possess the maximum efficiency of π-conjugation and intermolecular interaction, but they also enjoy remarkable thermal and morphological stabilities that are especially crucial for solid-state performances. In certain cases, elucidation of the behavior of such compounds may additionally provide sufficient perspective toward graphene materials doped with main group elements, which are widely considered as potential next-generation optoelectronic materials. In this review, we will specifically focus on historical developments of structurally constrained polycyclic π-electron systems particularly of those with boron, nitrogen, silicon, or phosphorus atoms annulated directly into the center of π-conjugated systems.

Structurally Constrained Boron-, Nitrogen-, Silicon-, and Phosphorus-Centered Polycyclic π-Conjugated Systems.

Biodegradable polyphosphazenes for drug delivery applications.

The design of second-order nonlinear optical chromophores exhibiting blue-shifted absorption and large nonlinearities: the role of the combined conjugation bridge

Bridged triphenylamines as novel host materials for highly efficient blue and green phosphorescent OLEDs.

Synthesis and properties of a new ferromagnetic 2,2′-bipyridine-MnPS3 intercalation compound

A new series of polyorganophosphazenes have been designed for potential photorefractive effect and synthesized via a nucleophilic substitution reaction of poly(dichlorophosphazene) with N -(6-Hydroxyhexanyl)carbazole ( HCZ ) as charge-transporting agent, N -(6-Hydroxyhexanyl)-3-(4-Nitrophenylazo)carbazole ( HNACZ ) as electro-optical chromophore. The component concentrations in the polymers can be controlled by different feed ratios between HCZ and HNACZ . The structural characterization for the high polymers was presented by 1 H-NMR, IR and UV–Visible spectra, gel permeation chromatograply (GPC) and differential scanning calorimetry (DSC).

Synthesis of polyphosphazenes as potential photorefractive materials

Two novel functionalized polyorganophosphazenes containing charge-transporting agent and electro-optical chromophore as side chains for the photorefractive application are synthesized. The structure characterization was carried out by 1H-NMR, IR and UV-Visble spectra, gel permeation chromatograply and differential scanning calorimetry. Due to the good flexibility of the backbone and the high degree of functionalization, polyphosphazenes are expected to be a good candidate for photorefractive polymeric materials.

Jingui Qin

Papers

The design of second-order nonlinear optical chromophores exhibiting blue-shifted absorption and large nonlinearities: the role of the combined conjugation bridge

Bridged triphenylamines as novel host materials for highly efficient blue and green phosphorescent OLEDs.

Synthesis and properties of a new ferromagnetic 2,2′-bipyridine-MnPS3 intercalation compound

Synthesis of polyphosphazenes as potential photorefractive materials

Novel polyphosphazenes containing charge-transporting agent and chromophore as pendant groups