Electron Transport in Porous Nanocrystalline TiO 2 Photoelectrochemical Cells Fei Cao, Gerko Oskam, Gerald J. Meyer, ² and Peter C. Searson* Department of Materials Science and Engineering, The Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed: June 5, 1996 X The photocurrent response of dye-sensitized, porous nanocrystalline TiO 2 cells was studied as a function of light intensity, in both the time domain (photocurrent transient measurements) and the frequency domain (intensity-modulated photocurrent spectroscopy). The photocurrent transients are characterized by a fast and a slow component. The rise time of the transients was in the range of milliseconds to seconds and exhibited a power law dependence on light intensity with an exponent of -0.6 to -0.8. The response to a modulated light intensity is characterized by a depressed semicircle in the complex plane. The time constant obtained from these spectra exhibits the same power law dependence on light intensity. The transient response of these cells is dominated by electron transport in the TiO 2 film, and the results are shown to be consistent with a diffusion model where the diffusion coefficient for electrons in the particle network is a function of the light intensity. Introduction Many recent innovations in photoelectrochemical solar energy conversion have been based on the use of porous nanocrystalline films. 1-5 These films are usually comprised of a three- dimensional network of interconnected nanometer-sized particles and exhibit many unique optical and electrical properties in comparison to planar single-crystal or polycrystalline films. Nanometer-sized particles are generally too small to sustain significant electric fields so that charge separation must be achieved by some other means. In one approach, sub-bandgap illumination may be used to excite dye molecules attached to the surface of the particles. The excited state of the dye molecule injects an electron into the particle, and the dye is regenerated by an electron donor in the solution. Minority carriers are not involved in this process so that electrons may be collected with high efficiency as long as recombination in the form of electron transfer to an electron acceptor in the solution or to the oxidized form of the dye can be minimized. Dye-sensitized, nanoporous TiO 2 photoelectrochemical cells are an example of this approach, and remarkably high-energy conversion efficiencies have been achieved. 1 In another ap- proach, electron-hole pairs generated by direct absorption are separated kinetically. For example, the presence of an efficient hole acceptor in the solution can minimize direct electron- hole recombination in the film. 4 In both cases, only majority carriers are involved in charge transport in the film. Due to the small particle size, electron transport in the network of particles is expected to be dominated by a gradient in the chemical potential of the electrons (diffusion) rather than by an electrical potential gradient (drift). In terms of energetics, transport is dominated by a gradient in the quasi-Fermi level for electrons. Another unique property of porous nanocrystalline films compared to single-crystal materials is the high surface area. This is an important feature for the dye sensitization approach since high dye coverage is critical to obtaining high absorption coefficients for the films and hence high conversion efficien- cies. 1 This feature is also important for the case of direct absorption since the photogenerated holes can be easily removed by hole acceptors in the solution. Previous studies on dye-sensitized TiO 2 photoelectrochemical cells have shown that the photocurrent transient response is relatively slow with time constants on the order of milliseconds to seconds. 6,7 In contrast, the rate of electron injection into the TiO 2 electrode from the excited state of the dye molecule is a very fast process with time constants on the order of 10 -9 s or smaller. 8 As a result, the transient response of devices based on porous nanocrystalline films is expected to be dominated by electron transport through the particle network. Sodergren et al. 9 have proposed a diffusion model for electron transport in these porous films. In this model, the electron diffusion length is assumed to be a constant throughout the film, which is a consequence of the assumptions that the diffusion coefficient and electron lifetime are independent of the electron concentration. The steady-state mass balance equation for electrons in the film was solved, and the calculated photoaction spectra and current-voltage characteristics of the TiO 2 films were shown to be in good agreement with experimental results. Although many features of photoprocesses can be determined from steady state measurements, a complete analysis of carrier transport can only be obtained from non-steady-state measure- ments. In this paper, we report on photocurrent transient measurements and intensity-modulated photocurrent spectros- copy (IMPS) of dye-sensitized porous nanocrystalline TiO 2 photoelectrochemical cells. The transient response is dominated by electron transport in the film and can be explained by a diffusion model where the diffusion coefficient for electrons in the particle network is a function of the light intensity. Experimental Section The TiO 2 photoelectrodes were prepared in the following way. 10 The TiO 2 powder (Degussa P25) was added to a small amount of water and surfactant (Triton X-100), and the colloidal solution was then applied to a conducting tin oxide glass substrate (8 Ω/0). The film was then sintered for about 30 min at 450 °C after air-drying. The dye molecule 4,4′-(dcb) 2 Ru(SCN) 2 5 was attached to the TiO 2 photoelectrodes by immersion in an ethanol solution with a dye concentration of about 0.1 mM for a period of 24 h or more. The polymer gel electrolyte was prepared by refluxing a mixture of 1.4 g of polyacrylonitrile, 10 g of ethylene carbonate, 5 mL of propylene carbonate, 5 mL of acetonitrile, ² Department of Chemistry. X Abstract published in AdVance ACS Abstracts, October 1, 1996. 17021 J. Phys. Chem. 1996, 100, 17021-17027 S0022-3654(96)01657-7 CCC: $12.00 © 1996 American Chemical Society