STM-excited electroluminescence and spectroscopy on organic materials for display applications

We present an overview of the current status of our work on scanning-tunnelingmicroscope-based (STM) spectroscopy and electroluminescence (EL) excitation to study the physical and electronic structure of organic materials used in organic lightemitting devices (OLEDs). By these means we probe the critical device parameters in chargecarrier injection and transport, i.e., the height of the barrier for charge-carrier injection at interfaces between different materials and the energy gap between positive and negative polaronic states. In combination with optical absorption measurements, we gauge the exciton binding energy, a parameter that determines energy transport and EL efficiency. In STM experiments involving organic EL excitation, the tip functions as an OLED electrode in a highly localized fashion, allowing one to map the spatial distribution of the EL intensity across thin-film samples with nanometer lateral resolution as well as to measure the local EL emission spectra and the influence of thin-film morphology.