Electrical and Morphological Characterization of Electrochemically Deposited ZnO/dye Hybrid Films
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Electrical and Morphological Characterization of Electrochemically Deposited ZnO/dye Hybrid Films
J. Reemts, J. Parisi and A. Kittel Energy and Semiconductor Research Laboratory, Department of Physics University of Oldenburg, D-26111 Oldenburg, Germany
ABSTRACT Dye sensitized solar cells of the Graetzel type are usually fabricated with TiO2 covered with dye after its synthesis as one electrode. ZnO is, aside from TiO2, a promising alternative semiconducting material for solar energy conversion due to its enhanced surface area and a strong attachment of non-aggregated sensitizer dyes. We investigate ZnO/dye electrodes where the dye is already added during the electro-deposition process of the ZnO. We examine the influence of the dye species and the dye concentration on the surface morphology by means of atomic force and scanning electron microscopy imaging. The electrical properties of the films are characterized by means of current-voltage measurements. In order to study the integral properties of the charge carrier transport in the lateral direction of the film we characterized ZnO thin films electro-deposited across an insulating gap between two microstructured electrodes made of gold.
INTRODUCTION In dye solar cells, porous inorganic semiconducting oxides are taken to support the light absorbing organic dyes and to serve as a pathway for the charge carriers. One prominent candidate is titanium dioxide, which is used together with dye molecules, e.g. ruthenium complexes [1]. Zinc oxide represents a promising alternative material to TiO2 because of its pronounced porosity, which results in a high surface area advantageous for the dye loading and the charge carrier separation process [2]. One of the main advantages of ZnO electrodes is the one-step synthesis of the electrodes by dissolving the dyes in the electro-deposition bath. This way of film preparation does not need a further sintering process and should therefore be applicable to a large number of different substrates. In the following we present results of electrical and morphological characterizations of electro-deposited porous zinc oxide with and without dye loading. As dye sensitizer we used EosinY.
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EXPERIMENTAL DETAILS The electro-deposition process was carried out on transparent SnO2 electrodes with a setup consisting of a rotating disk electrode. The electrolyte was an aqueous solution containing 5mM zinc chloride, 0.1M potassium chloride to improve the conductivity of the electrolyte and the dye. The dye concentration as well as the deposition potential and time were varied to obtain the different electrodes. The influence of the dye concentration on the film morphology was studied via scanning electron microscopy (SEM) and atomic force microscopy (AFM) using standard silicon cantilevers in the non-contact mode. The integral lateral and transversal electrical conductivity were investigated for different temperatures by means of current-voltage (I-V) measurements. The transversal conductivity was measured by placing a spring loa
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