Influence of properties of sputtered Al cathodes on OLED characteristics

Abstract

The dependence of device performance on the properties of Al cathodes of p-i-n OLEDs (Organic Light Emitting Diode) is investigated in this study. OLEDs were fabricated in vertical in-line equipment, and organic materials were evaporated in vacuum on ITO substrates. The OLED stack comprised doped charge transport layers at both electrode sides and an orange-red emitting layer consisting of alpha-NPD(4,4-bis [N-(1-naphtyl)-N-phenylamino]biphenyl) and Iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) as host and phosphorescence dye dopant, respectively. In order to form the cathode on organic layers DC magnetron sputtering has been applied and the sputter process parameters were varied. For comparison an OLED with evaporated Al cathode was prepared as well. The fabricated OLEDs were characterized by I-V-L measurements. Differences in operating voltages as well as in luminescent efficiencies were observed. Investigations on electrical, optical and morphological properties of Al films on organic layer were performed to explain the origin of the different characteristics of OLEDs. The properties of the Al cathode films affect carrier injection into organic layer as well as optical out-coupling and cause therefore differences in external efficiencies of OLEDs