Detailed evaluation of in-operando potentials in OLED devices: A combined experimental and drift-diffusion study

Abstract

We report on a technique to determine in-operando transport properties of Organic Light Emitting Diodes (OLEDs). Two types of OLEDs that solely differ in the emission layer but obviously exhibit a different potential distribution are investigated in this study. If the emission layer consists of the isomer TH-A a large shift in onset voltage can be observed in case of layer thickness variation of the emission layer. In case of the isomer TH-B a thickness variation has no impact on the onset voltage. Therefore the voltage developments per layer are determined with the help of IV measurements on a set of devices with varying layer thickness. From an empirical point of view the voltage behaviour in each layer follows a simple power law. A drift-diffusion model is developed that well describes the current density dependent evolution of coefficient and exponent of the power law. From the model we are able to derive the carrier injection mechanism into the respective layer as well as the injection barrier height. Also the carrier mobility is determined. Finally we are able to show that the existence of a large injection barrier can not explain the observed onset voltage shifts in case of TH-A. Instead an electric field at or close to the interface is necessary to describe the TH-A behaviour.