Determination of layer morphology of rough layers in organic light emitting diodes by X-ray reflectivity

X-ray reflectivity (XRR) has been proven to be a useful tool to investigate thin layers as well as buried interfaces in stacks built of very thin layers. Nevertheless, x-ray reflectivity measurements are limited by the roughness of the layers and interfaces as the roughness destroys the interference structure, the so-called Kiessig fringes. As investigations of thin layers in organic light emitting devices (OLEDs) are still subject of research and development, the focus of this paper is the investigation of a layer of indium tin oxide (ITO) which serves as transparent anode material in OLEDs. Due to the fabrication process, ITO shows rough surface structures, so-called spikes, hindering the determination of the ITO layer thickness and roughness in XRR measurements. In this paper, it is theoretically and experimentally proven that a smoothing layer on the ITO enables the determination of the buried ITO layer thickness and roughness as well as the density of the spikes. Furthermore, a sputtered aluminum layer (e.g. cathode material) showing spikes in atomic force microscopy covered with a smoothing layer reveals Kiessig fringes allowing the determination of the density of buried spikes. In general, it is shown that a smoothing layer on a rough surface enhances the sensitivity of x-ray reflectivity measurements.