Photovoltaic Effect in Multilayer Organic Systems
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nstitut für Festkörperphysik, Technische Universität Graz, A-8010 Graz, Austria Institut für Organische Chemie, Universität Tübingen, D-72076 Tübingen, Germany c Max-Planck-Institut für Polymerforschung, D-55021 Mainz, Germany d Institut für Organische Chemie und Makromolekulare Chemie, Universität Jena, D-07743 Jena, Germany b
ABSTRACT The organic materials presented here, e.g. cyano-ether-PPV, different bisarylamidineperylenes and conventional as well as substituted forms of phthalocyanines exhibit the advantages of low processing costs and the simplicity of tuning their optical properties. Hence they are promising candidates to be used in large area photovoltaic applications. The investigated cells consist of one or two organic layers sandwiched between electrodes of indium tin oxide (ITO) and aluminum. The experimental techniques of electroabsorption(EA)spectroscopy, photocurrent-action spectroscopy and current voltage characterization were used to gain further insight into the process of charge generation, charge separation and transport of the charged species to the electrodes. To enhance the quantum efficiencies of the photovoltaic cells, combinations of organic materials with electron-accepting and electrondonating properties in multilayer devices were investigated. We chose the organic materials copper-phthalocyanine and a bisarylamidineperylene whose HOMO and LUMO level alignment should favor a charge transfer process in order to increase the photocurrent(PC) responsivity by enhanced exciton dissociation. Additionally, one of the two layers of a double layer device was blended with small amounts of the material constituting the second layer to further increase charge carrier generation by extending the dissociation zone.
1. INTRODUCTION Advances in research on conjugated organic materials for optoelectronic device applications like LED's have also led to increased interest in these materials for other applications like photovoltaic devices[1, 2, 3, 4]. Due to the abundance of organic materials with a variety of different absorption bands the class of these materials seems to be suited very well to cover nearly the whole solar spectrum. Because of the high absorption coefficient of conjugated systems only thin films of a few 100 nanometers are necessary to absorb all of the incident radiation. Films of good quality and the necessary dimension can be easily manufactured by using for example the spin coating technique. Since these materials exhibit a much lower degree of order than their inorganic counterparts, the working mechanism of organic photovoltaic cells is harder to understand. The most common device-configuration used for organic photovoltaic cells is a sandwich structure of the active organic material between two electrodes, made in most cases of semitransparent indium-doped tinoxide(ITO) and a metal with low workfunction like aluminum or calcium. Depending on the material used as the active layer in these cells, its electronic structure and the availability of electron donator or acceptor states,
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