High-efficiency red electrophosphorescent devices based on new osmium(II) complexes

In this paper, we report on highly efficient red-phosphorescent light-emitting diodes using a series of osmium complexes 1-3 doped into poly(N-vinyl-carbazole) (PVK) matrix as emitters, thermally stable 1,3,5-tris(4'-fluorobiphenyl-4-yl)benzene (F-TBB) as a hole-blocking layer, and Alq(3) as an electron injection layer. The CIE 1931 chromaticity coordinates of complexes 1-3 are around (0.650, 0.347), (0.681, 0.317), and (0.696, 0.302), respectively, and remain almost unchanged over a wide range of operation voltages. The maximum luminous efficiencies reached 7.0, 3.5, and 1.2 cd/A for devices based on 10 wt.% of osmium complexes 1, 2 and 3, respectively, even with air stable aluminum as the cathode. We systematically studied the dependence of device performance on the osmium doping concentrations. It was found that the best device performance was observed at 10 wt.% doping concentration for all three osmium complexes. Both maximum luminance and luminous efficiency increased with increasing osmium complex concentrations in the beginning and reached maximum at 10 wt.% doping concentration. However, a further increase in the doping level resulted in a reduction in both device brightness and efficiency due to concentration quenching and triplet-triplet annihilation. This is consistent with the absolute photoluminescence quantum yields of PVK thin films doped with different concentration of osmium complexes. (C) 2005 Elsevier B.V. All rights reserved.