Small nano-dot incorporated high-efficiency phosphorescent blue organic light-emitting diode

High efficiency phosphorescent blue organic light-emitting diode (OLED) was obtained by incorporating small amino or hydroxyl functional group-modified polymeric nano-dot (APND or HPND) in the hole transporting layer (HTL), poly (ethylenedioxythiophene): poly (styrene sulfonic acid) (PEDOT:PSS). The device comprised a 1250 angstrom anode layer of indium tin oxide, a 350 angstrom HTL of PEDOT:PSS doped with APND or HPND, a 400 A blue emissive layer composed of a molecular host of 4,4'-bis (carbazol-9-yl) biphenyl doped with 14 wt% blue dye of bis (3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) iridium (III), a 320 A electron-transporting layer of 2, 2, 2 ''-(1, 3, 5-benzenetriyl)-tris (1-phenyl-1-H-benzimidazole), a 7 A electron-injection layer of lithium fluoride and a 1500 A cathode layer of aluminum. The resultant power-efficiency at 100 cd/m(2), for example, was increased from 12.0 to 25.91m/W, an increase of 116%, as 7 wt% APND of 8 nm in size was added. By employing 7 wt% HPND, the power-efficiency was 21.7 lm/W. The resultant luminance markedly increased with the incorporation of the PND. Whilst,the corresponding current density continuously decreased. These results indicate that the marked efficiency improvement may be attributed to a better balance of carrier-injection resulted from the hole-blocking-function possessed APND and the hole-trapping-function possessed HPND, which respectively exhibited positive and negative charge on the surface. Moreover, the chromaticity coordinate at 100 cd/m(2), for example, was (0.19, 0.34), barely changed in the presence of the nano-dots. Importantly, since the nano-dot was not employed in the emissive layer, the same concept may be applied to fluorescent blue or other OLEDs.