Microstructure and charge carrier transport in phthalocyanine based semiconductor blends
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1154-B09-12
Microstructure and charge carrier transport in phthalocyanine based semiconductor blends Andreas Opitz1, Julia Wagner1, Bernhard Ecker1, Ulrich Hörmann1, Michael Kraus1, Markus Bronner1, Wolfgang Brütting1, Alexander Hinderhofer2, Frank Schreiber2 1
Institute of Physics, University of Augsburg, Augsburg, Germany.
2
Institute of Applied Physics, University of Tübingen, Tübingen, Germany.
ABSTRACT The continuously growing and wide-spread utilization of blends of organic electron and hole conducting materials comprises ambipolar field-effect transistors as well as organic photovoltaic cells. Structural, optical and electrical properties are investigated in blends and neat films of the electron donor material Cu-phthalocyanine (CuPc) together with fullerene C60 and Cu-hexadecafluorophthalocyanine (F16CuPc) as electron acceptor materials, respectively. The difference in molecular structure of the spherical C60 and the planar molecule CuPc leads to nanophase separation in the blend, causing charge carrier transport which is limited by the successful formation of percolation paths. In contrast, blends of the similar shaped CuPc and F16CuPc molecules entail mixed crystals, as can be clearly seen by X-ray diffraction measurements. We discuss differences of both systems with respect to their microstructure as well as their electrical transport properties. INTRODUCTION From the mid 1990s the concept of “bulk-heterojunction solar cells” revolutionized the field of organic photovoltaics: Yu et al. reported a polymeric solar cell with an interpenetrating donor/acceptor material system which enables a spatially distributed interface accounting for the small exciton diffusion lengths in organic semiconducting materials [1]. Since that time, blends of organic electron and hole conductive materials are widely used for ambipolar charge carrier transport and photovoltaic cells. The application of distributed interfaces in organic solar cells has the advantage that excitons can efficiently dissociate throughout the whole volume of the organic layer yielding higher amounts of free charge carriers as compared to a bilayer system. Nevertheless, for an efficient transport, each material must provide continuous paths to the contacts. Both aspects entail a competition between efficient charge carrier dissociation and preferably undisturbed transport properties inside the blend. In this study we present the analysis of two model systems for donor-acceptor blends. These are (i) Cu-phthalocyanine (CuPc) combined with the Buckminster fullerene C60 and (ii) CuPc in combination with its fluorinated counterpart F16CuPc. While CuPc acts as the donor or p-conductor, C60 and F16CuPc are the n-conducting acceptor materials. In addition to studying the fundamental structural and optical properties, centering on the question of phase separation or formation of mixed crystals, we extend our analysis to electrical charge carrier transport properties. The materials used have been previously investigated in similar configurations partially wi
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