Molecular iridium (II) complexes and their corresponding electrophosphorescent devices

Phosphorescent transition-metal complexes have attracted extensive attention in recent years due to their ability of harvesting both singlet and triplet exciton in organic light-emitting diodes (OLEDs), theoretically enabling internal quantum efficiency to be 100% , while the up-limitation for fluorescent OLEDs is 25%. Among these materials, iridium complexes are the most promising emitters for OLEDs since they usually have good features of strong luminescence, tunable emission color, good thermal and electrochemical stability, as well as being able to form neutral compounds for vacuum deposition. This article reviews the progress in molecular iridium complexes, focusing on the aspects of material design and synthesis, mechanisms for emission, as well as innovation of materials and their performance in OLED devices. Particularly, after describing the typical device structure for phosphorescent emitter, the dominate emission mechanisms for iridium complexes are discussed in two classes, i.e., main ligand-based metal-ligand charge transfer (MLCT) emission for both homoleptic and heteroleptic complexes, auxiliary ligand intermediated MLCT emission for heteroleptic complexes. For material synthesis, according to the nature of reaction, four commonly used synthetic-routes are summarized. Meanwhile, the facial-and meridional- isomers are discussed. In the material development for OLEDs, categorized into different emission color, materials and their corresponding device performance are summarized and discussed. In addition, the host materials for the above emitters are briefly introduced. Finally, further development for phosphorescent materials is proposed.