Highly efficient electrophosphorescence devices based on iridium complexes with high efficiency over a wide range of current densities

Abstract

Three new luminescent cyclometalated iridium (II I) complexes are successfully synthesized. The cyclometalated ligand used here is 2-(2-fluorophenyl)-benzothiazole (F-BT). The auxiliary ligands are acetylacetone (acac), 1,1,1-trifluoroacetylaceton (3F-acac), 1,1,1,5,5,5-hexafluoroacetylacetone (6F-acac), respectively. All complexes exhibit bright photoluminescence at room temperature. Organic light-emitting diodes are fabricated by doping the iridium (III) complexes in 4, 4′ −N, N′-dicarbazole-biphenyl (CBP), and the device characteristics are investigated. Among these devices, the performances of the optimized devices based on 1 at high current density are among the best reported for devices with iridium (III) complexes as emitters. EL efficiencies show weak dependence on doping concentration and current density. The optimized device exhibits a peak current efficiency of 28.5 cd A−1 and a power efficiency of 11.2 lm W−1, respectively, at 20 mA cm−2, an efficiency of 22.7 cd A−1 at 100 mA cm−2, 80% of the maximum, can be achieved. Short triplet decay time of 1 measured in solid films is supposed to be responsible for the minor loss in EL efficiency, which suggests depressed triplet–triplet annihilation and site saturation of the phosphor. Efficient exciton formation on the molecules of 1 by direct charge trapping and confinement within the emissive layer also make for outstanding electrophosphorescent performances.