Efficient organic light-emitting device from exciplex emission between 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine and 2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole]

A new-type exciplex emission was obtained employing hole-transporting material 4,4,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) doped in the electron-transporting material 2,2,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole] (TPBI) with structures of ITO/m-MTDATA (45 nm)/NPB (5 nm)/m-MTDATA:TPBI ((10 nm, molecular weight ratio were 3:1, 1:1 and 1:3)/TPBI (25 nm)/Alq3 (15 nm)/LiF (0.5 nm)/Al. For comparison, a device with exciplex emission generated at the interface between a hole-transporting material m-MTDATA and an electron-transporting material TPBI with a structure ITO/m-MTDATA (45 nm)/NPB (5 nm)/m-MTDATA (5 nm)/TPBI (30 nm)/Alq3 (15 nm)/LiF (0.5 nm)/Al was also fabricated. The maximum efficiencies of the doped-devices reached 3.8 lm/W (at 4 V with a luminance of 145.7 cd/m2), 3.2 lm/W(at 4 V with a luminance of 132 cd/m2) and 4.6 lm/W (at 4 V with a luminance of 170.5 cd/m2) with different molecular weight ratio (MWR) 3:1, 1:1 and 1:3, respectively, comparing to that of the none-doped device of 1.8 lm/W (at 4 V with a luminance of 49.6 cd/m2). At such a low voltage, exciplex emission was major for all the devices above. Only yellow emission was obtained in none-doped structure and the maximum luminance reached 2555 cd/m2 at 12 V. Commission International de L'Eclairage coordinate (1931, CIE) in this structure changed from (0.397, 0.532) to (0.311, 0.413) with voltage from 4 to 16 V. In doped devices, a yellow emission was obtained at a low bias voltage and then turned into white emission with increasing of voltage for stronger blue emission from m-MTDATA due to the sufficient charge trapping from TPBI to m-MTDATA. The CIE coordinates were (0.406, 0.533), (0.409, 0.533) and (0.427, 0.536) at 4 V for MWR 3:1, 1:1 and 1:3, respectively. At 8, 8, 12 V, white emission were obtained with CIE coordinates (0.320, 0.400), (0.311, 0.384), (0.319, 0.401) for MWR 3:1, 1:1, 1:3 and turned into (0.264, 0.303), (0.246, 0.275), (0.302, 0.363) when voltage increased to 16 V. The maximum luminance was 3810 (at 14 V), 4210 (at 12 V) and 4583 cd/m2 (at 12 V) for MWR 3:1, 1:1 and 1:3, respectively.