New triscyclometalated iridium complexes for applications in phosphorescent light-emitting diodes

A series of new triscyclometalated iridium complexes Ir1–3 containing diphenylamino substituted phenylpyridine derivatives as ligands have been synthesized and characterized. Inter-ligand energy transfer and related emission color tuning were observed in these iridium complexes. The phosphorescence emission peaks red-shifted to longer wavelengths with increasing the conjugation length of the ligands. Heteroleptic Ir2 is highly amorphous and readily dissolved in a wide range of organic solvents. High-efficiency electrophosphorescent light-emitting diodes have been fabricated using iridium complexes Ir1–3 doped into hole-transporting host materials as the emitters. The device structure used in this study is ITO/PEDOT-PSS/Ir complex:host material/F-TBB/Alq3/LiF/Al, where F-TBB is morphologically stable 1,3,5-tris(4′-fluorobiphenyl-4-yl)benzene functioning as a hole- and exciton-blocking layer. It was found that the device performance was strongly influenced by a number of factors, such as the triplet energy and electrochemical properties of host materials, the energy offset between host and guest, and the balance of electron and hole in emitting layers. When doped in PVK host, amorphous Ir2 showed promising EL performance with a turn-on voltage of 5 V, a maximum luminance >15,000 cd/m2 at a driving voltage of 18 V, and a peak current-efficiency of 21 cd/A at a luminance of 1200 cd/m2.