Organic light-emitting diodes (OLEDs) have been extensively investigated due to their potential applications in flat-panel displays and lighting source. OLEDs show excellent properties such as high light-emitting efficiency, high brightness, low driving voltage, fast response and potential for large-area fabrication. Compared with fluorescent OLEDs, phosphorescent OLEDs (PhOLEDs) show very high internal quantum efficiency (up to 100%) because phosphorescent dyes can utilize both singlet exciton and triplet exciton,which overcomes the limitation of 25% efficiency of conventional fluorescent OLEDs with the nature of emission from pure singlet excitons. However, the relatively long lifetime of phosphorescent heavy metal complexes may lead to dominant triplet-triplet (T1-T1) annihilation at high currents, and may also cause a long range of exciton diffusion that could get quenched in the adjacent layers of materials in OLEDs. Therefore, heavy metal complex phosphors have to be widely dispersed into the host matrix to reduce these competitive factors. In this review, we summarized recent progress of blue phosphorescent host materials. Hole transport-type, electron transport-type, and bipolar transport type host materials are presented according to their different functional groups. 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 as well. Contents
1 Introduction
2 Hole-transport-type host materials
2.1 Carbazole derivatives
2.2 Triphenylamine derivatives
2.3 Aryl silanes
3 Electron-transport-type host materials
3.1 Phenylphosphine oxide/sulfide
3.2 1,3,5-Triazine
3.3 1,3,4-Oxadiazole
4 Bipolar transport hosts with different electron-transport group
4.1 Phenylphosphine oxide/sulfide
4.2 Benzimidazole
4.3 1,3,4-Oxadiazole
4.4 1,2,4-Triazole
4.5 1,3,5-Triazine
4.6 Phenanthroline
5 Conclusion and outlook