Influence of partially-oxidized silver back electrodes on the electrical properties and stability of organic semiconductor diodes

Abstract

Abstract Silver (Ag) is one of the most suitable metals for electrodes in high-performance organic optoelectronic devices due to its high conductivity and reflectivity, and low parasitic absorption loss at visible wavelengths. However, the electronic work function of Ag is not ideal for use as either the cathode or anode in many organic optoelectronic devices. In this report, we investigate the formation of an ultrathin surface oxide layer on a Ag electrode and its impact on hole injection into an organic conjugated polymer semiconductor. The surface oxide is formed by exposing the Ag electrode to a low-power O2/Ar plasma and which changes the electrical properties of the pure Ag electrode. We study the morphology and the chemical composition of the Ag surfaces after different plasma treatment times through X-ray photoelectron spectroscopy, scanning electron microscopy and dark-field optical microscopy. After plasma exposure the surface oxide is composed of both AgOx and Ag2CO3. As plasma exposure time increases from 1 s to 7 s, the fraction of AgOx increases while Ag2CO3 decreases gradually. Both the turn-on voltages and barrier heights of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) hole-only devices decrease with plasma treated Ag surfaces indicating that the work function of the Ag surface is increased by the surface oxide. F8BT hole-only devices with a thin MoO3 layer and a thicker AgOx layer on the electrode are found to be stable compared to thinner surface oxides and un-treated Ag surfaces.