STM-Excited Electroluminescence and Spectroscopy on Conjugated Polymers

The understanding of charge injection, charge-carrier and energy transport is of crucial importance for the modeling and improvement of organic optoelectronic devices. Here an overview will be given on the use of recently developed STM-based techniques to probe charge injection, transport, and radiative recombination in thin films of organic polymers which are of interest for applications in organic electroluminescent devices. Measurements of tip-height and of STM-excited electroluminescence (EL) intensity vs. the tunnel voltage bias allow a direct determination of the barrier height for electron as well as hole injection from an electrode into the organic material. Here spatial variations of the injection barrier can be detected, which depend on the properties of the electrode surface and/or on the way the organic material interact at the interface. In addition, these spectroscopic techniques yield the single particle energy gap, which is the energy difference between negative and positive polaronic states and, in combination with optical absorption measurements, the exciton binding energy of the organic material. In STM-based EL, the tip functions as one of the electrodes of a light-emitting device. This allows us to map the spatial distribution of the EL intensity across thin-film samples with nanometer lateral resolution as well as to probe the influence of thin-film morphology on the local emission characteristics.