Simultaneous Oxygen Plasma and Thermal Treatments of an ITO Surface to Improve the Electrical Characteristics of Organic Light-Emitting Diodes

Abstract

The electrical characteristics of organic light emitting diodes (OLEDs) were improved by lowpower oxygen plasma processing of an indium tin oxide (ITO) surface at an elevated temperature of 500 ◦C in this work. ITO is widely used as a transparent electrode in light-emitting devices, and OLED device performance is sensitive to the surface properties of ITO. In this research, a simultaneously oxygen-plasma and thermally treated ITO surface showed better device performance than an oxygen-plasma-treated ITO surface in terms of current density, brightness, and power efficiency. OLED devices with the structure of ITO/TPD/Alq3/LiF/Al were fabricated, and the surface properties of ITO were investigated by using various characterization techniques. The work function of the modified ITO, as determined by using ultraviolet photoelectron spectroscopy (UPS), increased from 4.70 to 5.17 eV. An X-ray photoelectron spectroscopy (XPS) analysis showed an oxygen fraction change from 59.9 % to 70.1 % after the surface treatment. Based on the measurements, we conclude that hole injection is improved due to the high electronegativity of the oxygen adsorbed on the ITO surface. The XPS analysis also showed that the surface carbon composition decreased from 15.1 % to 4.67 % after the surface treatment. A reduction in carbon contamination is also attributed to the improvement in the electrical characteristics. Thus, we demonstrated that simultaneous oxygen plasma and thermal treatment is a proper way for improving the electrical characteristics of OLED devices.