Carbazole/phenylpyridine hybrid compound as dual role of efficient host and ligand of iridium complex: Well matching of host-dopant for solution-processed green phosphorescent OLEDs

Abstract

A new bipolar transport material of 9,9'-(4-(pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (2,4-dCzPPy) is designed and synthesized through a simple catalyst free C-N coupling reaction. The carbazole/phenylpyridine hybrid 2,4-dCzPPy is functionalized as dual role of efficient host material and cyclometalated ligands for green emissive Iridium (III) complex of bis(9,9'-(4-(pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole))(acetylacetonato) Ir(dCzPPy)2(acac). Their thermal, photophysical, electrochemical, and electrolumiescent (EL) properties are fully investigated. The D-π-A structure of bipolar compound of 2,4-dCzPPy suppresses the direct intramolecular interaction between the donor and acceptor subunits to give high triplet energy of 2.70 eV. Besides, solution processable electrophosphorescent devices by using 2,4-dCzPPy/Ir(dCzPPy)2(acac) as host/dopant to form emissive layers at a dopant concentration of 8 wt% show a maximum luminescence of 28622 cd/m2 at 10.3 V and maximum current efficiency of 31.8 cd/A, which is significantly higher than other host/dopant combinations, such as 21 cd/A of 4,4′-bis(9-carbazolyl)biphenyl (CBP):Ir(dCzPPy)2(acac) and 15 cd/A of 2,4-dCzPPy:tris(2-4-tolyl phenylpyridine) iridium (Ir(mppy)3), suggesting the well matching of the new host/dopant system which involves 2,4-dCzPPy both as the host matrix and the ligand of phosphorescent emitter.