Highly efficient green phosphorescent organic light-emitting diodes with low efficiency roll-off based on iridium(iii) complexes bearing oxadiazol-substituted amide ligands

Abstract

Two novel iridium(III) complexes [Ir(ppy)₂(PhOXD)] (1, ppy = 2-phenylpyridine, PhOXD = N-(5-phenyl-1,3,4-oxadiazol-2-yl)-benzamide) and [Ir(ppy)₂(POXD)] (2, POXD = N-(5-phenyl-1,3,4-oxadiazol-2-yl)-diphenylphosphinic amide) have been designed and synthesized, and their photoluminescence and electrochemistry properties were investigated. At room temperature, complexes 1 and 2 show green emissions at about 502 and 506 nm with photoluminescence quantum yields (PLQYs) of 0.03 and 0.05 in CH₂Cl₂ solutions, respectively. In the 5 wt% doped poly(methyl methacrylate) (PMMA) film, the PLQYs (0.42 for complex 1, 0.52 for complex 2, respectively) increase significantly. The organic light emitting diodes (OLEDs) with the structure of ITO/HAT-CN (dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile), 10 nm)/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/Ir complexes (10 wt%): mCP (1,3-bis(9H-carbazol-9-yl)benzene, 20 nm)/TPBi (1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) show good performances. In particular, device G2 based on complex 2 shows superior performances with a peak current efficiency (η_c) of 64.7 cd A⁻¹ and a peak power efficiency (η_p) of 42.5 lm W⁻¹ with low electroluminescence efficiency roll-off. The η_c value still remains over 60.6 cd A⁻¹ even at a luminance of 10 000 cd m⁻², which indicates that introducing electron transporting 1,3,4-oxadiazole and diphenyl phosphoryl groups into iridium complexes is an effective means of achieving efficient phosphors in OLEDs.