Electrical and photoelectrical measurements on ZnO-Nanowires coated with PEDOT:PSS for Dye-Sensitized Solar Cells
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Electrical and photoelectrical measurements on ZnO-Nanowires coated with PEDOT:PSS for Dye-Sensitized Solar Cells Julia Waltermann1, Kay-Michael Günther1, Stefan Kontermann2, Siegfried R. Waldvogel³, Wolfgang Schade1,2 1 Clausthal University of Technology, EFZN, Energy Campus, Am Stollen 19, 38640 Goslar, Germany 2 Fraunhofer Heinrich-Hertz-Institute, Am Stollen 19, 38640 Goslar, Germany 3 Institute of Organic Chemistry, Johannes Gutenberg-University Mainz, Germany ABSTRACT Dye-sensitized solar cells composed of an n-doped ZnO nanowire array and a p-doped polymer layer appears to be a promising candidate for low-cost production of environment-friendly solar cells. In this work, we investigate hybrid devices consisting of a transparent conducting oxide (TCO) substrate, ZnO-nanowires (ZnO-NW) or a sol-gel prepared ZnO layer, a ruthenium dye (N719) and a PEDOT:PSS or P3HT layer. The dense polycrystalline ZnO layer is able to prevent short circuits, which have a strong effect on the performance of the solar cells. This is demonstrated by the use of only the ZnO layer which improves the open circuit voltage by a factor of 2 and the efficiency by a factor of 1.7 compared to cells with nanowires. That indicates that the system combined with a thin but dense ZnO layer and NW grown on it will show further improvement. Furthermore three different TCO substrates were investigated. Impedance spectroscopy (IS) reveals at least one additional Schottky barrier formed with ZnO:Al substrates. Spectral photovoltage measurements clearly show distinct absorption features correlated with the ZnO and N719 dye. INTRODUCTION During the last decade several new design concepts and materials for photovoltaic devices were investigated [1]. Electrolyte-based dye-sensitized solar cells (DSSCs) using TiO2 nanoparticles have recently exhibited efficiencies of more than Ș = 10% [2]. These devices take advantage of the huge surface area provided by the nanoparticles for dye adsorption, but the electron transport in nanoparticle DSSCs is most likely limited by hoping between trap states on neighbored particles [3]. In this context the wide band gap semiconductor ZnO is often discussed, because of its ability to form not only nanoparticles [4], but also wires or rods of various sizes and on different substrates [5]. Although the surface area of a ZnO nanowire (NW) array is naturally much smaller than thesurface area of TiO2 nanoparticles, it has been shown that the NWs have a high carrier density [6] and a two orders of magnitude slower recombination kinetics for rods compared to a system with ZnO particles in [7]. Therefore, an array of ZnO NW should provide good conductivity through the monocrystalline wires and combined with a relatively large surface-to-volume ratio may prove capable to replace TiO2 nanoparticles in photovoltaic and light emitting devices [8]. All DSSCs containing a liquid electrolyte have to overcome leakage and sealing problems, wherefore great effort is undertaken to find a solid-state alternative [9]. For hybrid
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