Time Evolution of Photoconductivity in TiO 2 Electrodes Fabricated by a Sol Gel Method
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Time Evolution of Photoconductivity in TiO2 Electrodes Fabricated by a Sol Gel Method Zhibin Xie a 1, Victor M Burlakova, Bernard M. Henrya, Kiril R. Kirova, Christopher R. M. Grovenora, Hazel E. Assendera, G. Andrew D. Briggsa, Mitsuru Kanob and Yusuke Tsukaharab (a): Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK (b):Technical Research Institute, Toppan Printing Co., Ltd., Takanodai-Minami 4-2-3, Sugitomachi, Kitakatsushika-gun, Saitama 345, Japan (1): Corresponding author Email: [email protected] Tel: +44 1865 273699 Fax: +44 1865 273789 ABSTRACT We report here on the time evolution of photoconductivity under continuous illumination in nanocrystalline TiO2 samples prepared by a sol gel method, and also on the conductivity decay once the illumination is switched off. We observe strong dependence of the photoconductivity on the illumination intensity for both processes. It is found that the conductivity decay after highintensity illumination is slower than after low-intensity illumination, and we have attempted to explain these experimental results using a model involving hole trapping-detrapping processes. INTRODUCTION The wide gap semiconductor TiO2 is often used as an electron acceptor in composite solar cells (CSC’s) consisting of a conjugated polymer acting as a photon absorber and hole conductor. Photogenerated excitons in the conjugated polymer dissociate at the interface of the conjugated polymer and titania, injecting electrons into the latter [1]. The overall performance of CSC’s is strongly determined not only by how efficiently the electrons are transferred to the electron accepting TiO2, but also on how they pass through the porous nanocrystalline TiO2 thin film and are collected at an anode. Electron transport is heavily affected by both intrinsic imperfections in the TiO2 crystal structure [2, 3] and by the characteristic thin film morphology [4]. High quantum efficiencies in CSC’s require large organic-inorganic interface areas, which in turn suggest the need for highly porous titania. At the same time, large interface areas mean a high concentration of surface states, which may trap electrons and holes. To understand the interplay between interfacial trapping and the transport properties through the nanocrystalline TiO2 thin films, we have studied the photoconductivity (PC) of this material in the time domain. In this paper we present results on the microstructure of nanocrystalline TiO2 thin films prepared by a sol gel method, and how the PC properties depend on various illumination conditions under vacuum. EXPERIMENTAL DETAILS PC samples with ITO/TiO2/Au configuration were formed on pre-patterned ITO/glass substrates. Before depositing titania, the substrates were thoroughly ultrasonically cleaned in isopropanol and acetone for 10 minutes, and then dried with nitrogen gas.TiO2 layers were produced by spin coating a sol similar to the one used in Grätzel cells[5]. Titanium (IV) isopropoxide was added to a 0.1M solution of nitric acid, and was heat
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