Electrical Properties of Organic Light-Emitting Diodes Fabricated on HfOx-Treated Indium-Tin-Oxide Surfaces by Using an Impedance Spectroscopy Analysis

摘要

In organic light-emitting diodes (OLEDs), both the electrons and the holes need to be injectedefficiently to obtain the best device performance. This means that a small injection barrierheight at the indium-tin oxide (ITO)/organic interface is required. The insertion of an insulatinglayer between the ITO and the organic layers leads to a significant improvement in the chargeinjection and the electroluminescence output. In this study, the surface of the ITO anode inOLEDs was treated with an HfO_X deposition process by using an atomic-layer chemical- vapordeposition system (ALCVD). The OLEDs fabricated on the HfOx-treated ITO anode showeda lower impedance and a higher conductance and capacitance. The changes in the capacitance,the conductance and the impedance were attributed to the enhanced carrier tunneling andto a change in the work function of ITO. In this work, we used an impedance spectroscopyanalysis to determine the effect of the HfO_X treatment on the surface of ITO and to modelthe equivalent circuit for OLEDs. Impedance spectroscopy is one of the most powerful toolsused to study equivalent circuit models for and the charge carrier dynamics and the dielectricproperties of organic devices. Devices with an ITO/organic material/Al structure could bemodeled as resistances and capacitances arranged in parallel or in series. The number of ele-ments depend on the composition of the structure, essentially the number of layers and the contacts.