Electrical properties of organic light-emitting diodes fabricated on HfO_X-treated indium-tin-oxide surfaces by using an impedance spectroscopy analysis

In organic light-emitting diodes (OLEDs), both the electrons and the holes need to be injected efficiently to obtain the best device performance. This means that a small injection barrier height at the indium-tin oxide (ITO)/organic interface is required. The insertion of an insulating layer between the ITO and the organic layers leads to a significant improvement in the charge injection and the electroluminescence output. In this study, the surface of the ITO anode in OLEDs was treated with an HfO_X deposition process by using an atomic-layer chemical- vapor deposition system (ALCVD). The OLEDs fabricated on the HfO_X-treated ITO anode showed a 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 and to a change in the work function of ITO. In this work, we used an impedance spectroscopy analysis to determine the effect of the HfO _X treatment on the surface of ITO and to model the equivalent circuit for OLEDs. Impedance spectroscopy is one of the most powerful tools used to study equivalent circuit models for and the charge carrier dynamics and the dielectric properties of organic devices. Devices with an ITO/organic material/Al structure could be modeled as resistances and capacitances arranged in parallel or in series. The number of elements depend on the composition of the structure, essentially the number of layers and the contacts.