Photo-Electron Spectroscopy Study of Energy Levels in Conjugated Oligomers
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PHOTO-ELECTRON SPECTROSCOPY STUDY OF ENERGY LEVELS IN CONJUGATED OLIGOMERS S.C. Veenstra1, A. Heeres2, U. Stalmach1, J. Wildeman1, G. Hadziioannou1, G.A. Sawatzky2, H.T. Jonkman2 1 Department of Polymer Chemistry, 2Laboratory of Solid State and Applied Physics, Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ABSTRACT We report on the valence orbital structure of poly(para-phenylenevinylene) (PPV)-like oligomers. We studied these molecules as isolated oligomers in the gas phase, as well as in thin films deposited on metal substrates. We use a simple model based on a previously reported Hamiltonian that accurately describes the development of the low lying electronic excitations as a function of the number of repeating units. In the study on the thin organic films we report on the energy level alignment at metal / organic and organic / organic interfaces, where the organic layer is either a PPV-like oligomer or C60. The results are important for understanding organic photovoltaic devices. INTRODUCTION Organic semi-conducting materials have attracted a lot of attention in the scientific community, because of their interesting physics and potential applications in light emitting devices (LEDs), field effect transistors (FETs) and photovoltaic devices (PVDs). We have focussed our attention to these devices, especially the PVDs. Bulk heterojunction PVDs contain most often a conjugated polymer as donor (D) and a C60derivative as acceptor (A). After optical excitation, the charge separation occurs at the DA interface [1]. After this exciton dissociation step the separate charges need to be transported towards the electrodes by propagation of the electron through the fullerene network to the cathode and the hole through the intercalated conjugated polymer to the anode. At the electrode / organic interfaces the charges should efficiently transfer from the organic system to the inorganic metallic electrodes. Here we report on a combined experimental and theoretical approach to get a better understanding of the device properties of D-A based organic PVDs. A brief description is given of a theoretical model with which we obtain good estimates of the different low energy electronic excitations in conjugated oligomeric and polymeric systems. In this paper we mainly focus on the energy level alignment at the different interfaces found in bulk heterojunction organic PVDs. EXPERIMENT The method with which we measured the gas phase photoelectron spectra was described in previous work [2]. All spectra were obtained with He-I radiation of 21.22 eV and the binding energies were calibrated using the Xe 2P3/2 line (gas phase measurements, typical resolution 120meV) or by the low kinetic energy onset of the BB2.6.1
spectrum (solid state measurements) which is directly related to the vacuum level. The latter measurements were performed in another UHV system consisting of a preparation chamber (base pressure
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