Effects of surface treatment of ITO anode layer patterned with shadow mask technology on characteristics of organic light-emitting diodes

We investigated the effects of various surface treatments of indium tin oxide (ITO) on the electrical and optical characteristics of organic light-emitting diodes (OLEDs). A 150-nm thick ITO anode layer was patterned directly with a shadow mask during the sputtering process without the use of a conventional photolithography patterning method. The sputtered ITO layer was subjected to thermal and oxygen plasma treatments to reduce the sheet resistance and improve surface roughness. The thermal treatment was performed for 1 h at temperatures of 250 and 380 degrees C, which were chosen so that the glass substrates would not deform from thermal damage. The measured sheet resistance decreased from 30.86 Omega/sq for the as-sputtered samples to 8.76 Omega/sq for the samples thermally treated at 380 degrees C for 1 h followed by oxygen plasma treatment. The root-mean-square surface roughness measured by atomic force microscopy considerably decreased to 3.88 nm with oxygen plasma treatment. The thermal treatment considerably decreased the sheet resistance of the ITO anode layer patterned with the shadow mask. The spike-like structures that are often formed and observed in shadow mask-patterned ITO anode layers were almost all removed by the oxygen plasma treatment. Therefore, a smooth surface for shadow mask-patterned ITO layers with low sheet resistance can be obtained by combining thermal and oxygen plasma treatments. A smooth surface and low sheet resistance improves the electrical and optical characteristics of OLEDs. The surface-treated ITO layer was used to fabricate and characterize green phosphorescent OLED devices. The typical characteristics of OLED devices based on surface-treated shadow mask-patterned ITO layers were compared with those fabricated on untreated and photolithography-patterned ITO layers to investigate the surface treatment effects. The OLED devices fabricated by thermal treatment at 380 degrees C for 1 h followed by oxygen plasma treatment for 180 s showed the highest luminance and current density. Furthermore, the leakage current that might be induced by the rough ITO surface was dramatically reduced to 0.112 mA/cm(2). Our study showed that the shadow mask-patterned ITO anode layer treated by heat and plasma and having a low sheet resistance and surface roughness yielded excellent electrical and optical properties for OLEDs compared to those based on an untreated ITO layer. The fabricated OLED devices using the surface-treated shadow mask-patterned ITO layer exhibited comparable characteristics to those obtained from a conventional photolithography-patterned ITO anode. (C) 2013 Elsevier B.V. All rights reserved.