Comparison of ITO prepared by capacitive RF magnetron sputtering and DC facing target sputtering as an anode on the organic light emitting diode

Indium-tin-oxide (ITO) deposited by conventional capacitive RF magnetron sputtering (rf MS), DC facing target sputtering (dc FTS) and two postannealed samples of these were compared. Comparison of the ITOs aimed at investigating the surface and microstructure effect on the electrical and optical properties of ITOs and how much those facts contribute to enhance the driving voltage and luminescence of build in organic light emitting diodes (OLEDs). Two sputtering methods and postannealing gave ITOs of similar sheet resistivity but those have a different microstructure and surface morphologies. Accordingly, the OLED's performance was fairly different. At as deposited condition by both sputtering methods ITOs were not crystalline and the sheet resistances were ̃40ω/sq., after postannealing those showed a same (400) preferred growth orientation and the sheet resistances were increased to ̃100ω/sq. However, the surface roughness and microstructure were different. The surface roughness of ITO by dc FTS was 0.2 nm in rms at as deposited condition but increased to 1.5 nm after postannealing. While that by rf MS was 0.9 nm in fresh and was 5.4 nm after postannealing. Microstructure by dc FTS was equi-axed, on the while that by capacitive rf MS was almost columnar. Current density-voltage-luminescence (I-V-L) characteristics of OLED showed better performances with ITO by dc FTS after postannealing which has equi-axed grain structure with many fine needles like spikes at surface. For better hole injection at ITO surface that results in better I-V-L characteristics of OLED, therefore, the fine surface with relevant roughness and equi-axed grains was seemed to be preferable.