Postpressing dependence of the effective electron diffusion coefficient in electrophoretically prepared nanoporous ZnO a
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Th. Dittrich Hahn-Meitner-Institute, 14109 Berlin, Germany
A. Ofir, L. Grinis, and A. Zabana) Department of Chemistry, Nano-Energy Research Center, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel (Received 12 November 2007; accepted 28 December 2007)
The porosity of electrophoretically prepared nanoporous ZnO and TiO2 films was systematically decreased by postpressing at different pressures. The nanoporous structure of the films was fixed by sintering after the postpressing procedure. The postpressing-induced change of the internal surface area of the nanoporous films was monitored using the dye-removal technique. The effective electron diffusion coefficient (Deff) of the unpressed nanoporous films depended on the thickness according to Fick’s second law. When pressed, the diffusion coefficient of the films increases significantly. In nanoporous TiO2, the increase of Deff follows the percolation theory where transport rate depends on the particle-coordination number. In contrast to the TiO2 films, the value of Deff of pressed nanoporous ZnO films changed with the porosity much stronger than one would expect from the percolation theory with hard spheres. This property has been attributed to the strong increase of necking between ZnO nanoparticles with increasing pressure as indicated by a strong decrease of the internal surface area.
I. INTRODUCTION
Two of the main transparent n-type semiconductors materials that are used as electron-conducting medium in dye-sensitized solar cells (DSSCs) are TiO2 and ZnO. Use of their nanostructure provides a large internal surface area, which is necessary for dye molecules adsorption, thereby leading to sufficient high light absorption.1–3 The potentials of the valance band maximum and the conduction band minimum of those materials are close, which explains their ability to function under the energy level restriction of the other components in dyesensitized solar cell system. The main advantage of ZnO over TiO2 is the ability to form preferred morphologies that can enhance the transport properties of the electron.4 However, at least to date, the reported performance of DSSCs composed from TiO2 nanoporous film is superior over those made by ZnO.5,6 a)
Address all correspondence to this author: e-mail: [email protected] DOI: 10.1557/JMR.2008.0116 J. Mater. Res., Vol. 23, No. 4, Apr 2008
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Electron transport throughout a nanoporous film is strongly influenced by the film morphology. 7,8 Nanoporous layers of metal oxide nanoparticles can be prepared by electrophoretic deposition (EPD).9 The implementation of the EPD process allows a systematic modification of the morphology of the films by applying mechanical press before fixing the structure by sintering. Changes of the morphology are given mainly by the decrease of the porosity after pressing. The decrease of the porosity leads to an increase of the mean coordination number of nanoparticles and therefore to an increase of
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