Novel Nanostructured Photoelectrodes - Electrodeposition of Metal Oxides onto Transparent Conducting Oxide Nanofibers
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Novel Nanostructured Photoelectrodes - Electrodeposition of metal oxides onto transparent conducting oxide nanofibers Rainer Ostermann1, Melanie Rudolph2, Derck Schlettwein2, and Bernd M. Smarsly1 1 Institute of Physical Chemistry, Justus Liebig University, Heinrich-Buff-Ring 58, 35392 Giessen, Germany 2 Institute of Applied Physics, Justus Liebig University, Heinrich-Buff-Ring 16, 35392 Giessen, Germany ABSTRACT Nanostructured metal oxides with high surface areas have been shown to be efficient photoelectrodes for light-to-energy conversion in dye-sensitized solar cells (DSCs). In this work we demonstrate the use of nanofibrous mats of transparent conducting oxides (TCOs) as nanostructured electrodes, especially for DSCs. The nanofibers have been obtained by electrospinning suitable inorganic precursors and polymers, followed by calcination to remove the polymer. Afterwards, TiO2 layers were generated on our 3D-electrodes by electrodeposition. An improved performance as DSC was found compared to flat electrodes of similar thickness, validating our approach. . INTRODUCTION TCO (transparent conducting oxide) materials are widely used in opto-electronic applications as transparent electrodes, commonly as thin, compact films deposited on glass or quartz substrates.1,2 These TCO layers of 200-300 nm thickness possess sheet resistances typically below 20 Ω/□ and transparencies above 85% in the visible range, making them also suitable as two-dimensional electrodes for electrochromic devices3 and solar cells.4 As proposed by Zaban et al., a three-dimensional TCO electrode theoretically offers advantages for electron collection in DSCs,5 due to a higher surface area and a shorter electron diffusion pathway to the back electrode, theoretically limiting recombination losses. The practical realization of such DSCs with TiO2 on ITO nanopowders6 and nanowires generated by laser-ablation7 showed only low conversion efficiency, until recently a more elaborate approach based on a polycarbonate template yielded around 4% efficiency,8 thus validating the concept proposed by Zaban. Here, we present an electrospinning approach to generate a nanofibrous electrode based on TCO nanofibers. EXPERIMENT All chemicals were obtained from Sigma-Aldrich and used without further purification. The preparation of the electrode comprises the following steps: - Electrospinning of TCO nanofibers from a suitable precursor solution - Spray pyrolysis of TCO to enhance mechanical and electrical contact - Anodical electrodeposition
1. Nanofiber preparation In a typical electrospinning experiment for ITO (tin-doped indium oxide) nanofibers, 442 mg of In(acac)3 are dissolved by heating in 800 mg of acetone, followed by the addition of 35 mg of SnCl4 and 600 mg of solution of 12 wt% Poly(vinylpyrrolidone) (PVP, Mw ≈ 1,300,000) in methanol. This solution is fed through a metallic needle by a syringe pump (KDS scientific) at the rate of 0.4 ml/h. The needle is placed at a distance of 6.5 cm from the aluminum foil that serves as collector and a vo
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