Effects of Thickness of Organic and Multilayer Anode on Luminance Efficiency in Top-Emission Organic Light Emitting Diodes

Abstract

The multilayer contact structures in both the anode and organic layers for top-emission organic light emitting diodes (TEOLEDs) were studied. In the anode, different multilayer structures of Al/Au, Al/Ni, or Al/Pt with different layer thicknesses were studied, where Al was used for high reflectivity and Au, Ni or Pt having high work functions were used for matching with the highest occupied molecular orbital (HOMO) of a hole injection/transport layer. It was found that 5 nm Au layer had a high work function and a good adhesive contact to the hole injection layer, 4,4',4''-tris(N-3-methylphenyl-N-phenyl-amino)-triphenylamine (m-MTDATA). This is the first report in the literature on the use of Al/Au contact as the anode in TEOLEDs. The effects of the thicknesses of the organic layer on luminance characteristics were studied. The thicknesses of a hole injection layer (HIL), a hole transporting layer (HTL) and an electron transporting layer (ETL) were also adjusted [m-MTDATA/N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB)/tris(8-hydroxyquinoline) aluminum (Alq3)] to 20/40/60 nm to balance the electron and hole recombination ratio. The highest brightness and best luminance efficiency of 8041 cd/m2 and 3 cd/A were obtained, respectively. Top-emission white OLED was also studied. White emission was achieved by changing the orange doped position to adjust the blue to orange ratio in the spectra. A white TEOLED with Commission International de l'Eclairage (CIE) coordinates of 0.30 and 0.34 was obtained.