Energies of polymer-electrodes junctions

Abstract

The work presents energies of polymer-electrode junctions with simpler polymeric structures. Energy levels and Fermi level positions in the energy gap of F8 and TFB for bottom (polymer spun on electrodes) and top (mostly top evaporated metal) contacts with various substrate electrodes are shown. The transport gap measured with UPS/IPES serves as a reference for hole and electron injection barriers at interferences between the two polymers and the materials studied in this work. The charge distribution in the device and the balance between electron and hole injections and transport are inevitably controlled by a single active layer and its interfaces with the anode and cathode. The energy and the density of the electron and hole transport states and their relative positions across the electrode-organic interfaces are therefore, of paramount importance for the performance of the devices. The rate of change of barrier vs. the electrodes work-function, is described by the slope parameter (S), i.e. a measure of the screening of the semiconductor polymer/electrode interaction at the interface. We note that the electrode work function, in the case of bottom contacts, is measured with UPS after exposing the surface to the solvent to simulate the polymer spin coating process. On the other hand, the work function of the top electrode is assumed to be similar to that a freshly evaporated, atomically clean film of the material. The validity of the assumption has been proven in varies chemically non-interacting metal - small molecular systems, where the injection barrier at the top and bottom contact is the same when both contacts are freshly evaporated in ultra high vacuum.