International Scholarly Research Network ISRN Nanotechnology Volume 2011, Article ID 281329, 5 pages doi:10.5402/2011/281329 Research Article Electron-Density and Electron-Lifetime Profile in Nanocrystalline-TiO 2 Electrode of Dye-Sensitized Solar Cells Analysed by Voltage Decay and Charge Extraction Seigo Ito, 1, 2 Robin Humphry-Baker, 1 Paul Liska, 1 Pascal Comte, 1 Peter P´ echy, 1 Mohammad K. Nazeeruddin, 1 and Michael Gr¨ atzel 1 1 Laboratoire de Photonique et Interfaces, ´ Ecole Polytechnique F´ ed´ erale de Lausanne, 1015, Lausanne, Switzerland 2 Department of Electrical Engineering and Computer Sciences, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Hyogo, Himeji 671-2201, Japan Correspondence should be addressed to Seigo Ito, itou@eng.u-hyogo.ac.jp Received 6 May 2011; Accepted 28 June 2011 Academic Editors: C. S. Casari, B. R. Kimball, and A. Taubert Copyright © 2011 Seigo Ito et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The dependence of the electron density and electron lifetime in nanocrystalline-TiO 2 electrode on the electron back reaction with I − 3 in a dye-sensitized solar cell, based on a mesoporous titania (TiO 2 ) matrix immersed in an iodine-based electrolyte, has been presented by analyzing the results of voltage decay and charge extraction measurements without modelling, without interpretation of mechanism, and without complicating calculations. This new analysis approach utilizes simple equations and basic definition of kinetics, concluding in absolute electron-density and charge-lifetime measurements. The relation of electron lifetime to open- circuit voltage indicates a peak of the long-lifetime charge at 5.5V, which is consistent with the information of middle band suggested by Bisquert et al. (2004) . Mesoscopic dye-sensitized solar cells (DSCs) have been attracting intensive interest for scientific research and indus- trial applications because of their high photon-to-electricity conversion efficiency and low cost compared with traditional photovoltaic cells [1–4]. DSCs are constructed using a high surface area film of nanocrystalline-TiO 2 coated on conducting glass as working electrode. A sensitizing dye is adsorbed on the nanocrystalline-TiO 2 . The sandwich cell is completed by sealing a nonaqueous electrolyte containing the I 3 − /I − couple with a second glass plate coated with a thin layer of platinum as counter electrode. The photoexcited dye injects electrons into the conduction band of the TiO 2 on a femtosecond to picosecond timescale [5–7], and the dye is regenerated on a microsecond timescale from its oxidised state by electron transfer from I − [8](Figure 1). The I 3 − formed in the reduction of dye cation diffuses to the counter electrode where electron transfer regenerates I − , which diffuses back into the porous TiO 2 . At the open circuit condition, the injected electrons recombine with I 3 − . In order to understand the mechanism of DSC and improve the photovoltaic performance, the electron density and the electron lifetime (τ n ) in the TiO 2 conduction band at the open circuit condition have been discussed significantly. The basic definition of τ n is τ n = 1 k , (1) where k, which is called time constant, is presented in an equation: dN dt =−kN , (2) where N is the number of electrons (density). There are two ways to elucidate the electron density and the electron lifetime. One way is “transient LASER spectroscopy,” which can show the density of injected electrons directly [5]. However, complete sealed cells (sandwich type) are not readily amenable for this transient LASER measurement