Transient Photocurrent Response of Dye-Sensitized Porous Nanocrystalline TiO 2 Electrodes
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ALBERT GOOSSENS, G.K. BOSCHLOO, AND J. SCHOONMAN Delft University of Technology, Laboratory for Applied Inorganic Chemistry, Julianalaan 136, 2628 BL Delft, The Netherlands. [email protected]
ABSTRACT In order to investigate the fundamentals of electron migration in nanostructured metal-oxide semiconductors, the transient photocurrent response of dye-sensitized porous nanocrystalline TiO 2 is studied. The time-resolved photocurrent response at light steps or pulses shows a faster transient upon increasing the light intensity. Intensity-modulated photocurrent spectroscopy (IMPS) reveals that the transient photocurrent is dominated by two time constants, i.e. the geometrical one and a characteristic time related to electron trapping. A theoretical model is derived in which the occupation dynamics of a single electron trap is considered using Shockley-Read-Hall kinetics. The geometrical RC time of the electrode is also included. Excellent agreement between this model and the measured IMPS spectra is obtained.
INTRODUCTION Porous nanocrystalline metal-oxide electrodes are the basis of new types of efficient electrochemical devices, such as photoelectrochemical solar cells, photocatalysts, Li-ion batteries, and electrochromic windows. They are prepared by low-temperature sintering of colloidal particles, resulting in a 3-dimensional porous network with a large internal surface area. The small primary particle size (5-30 nm) and the high surface-to-volume ratio give these electrodes several advantageous properties compared to dense electrodes. Investigations on this type of nanostructured electrodes have been reviewed by several authors to which the interested reader is referred [1]. Nanostructured metal-oxide electrodes are particularly useful in dye-sensitized photoelectrochemical solar cells. In particular, Gratzel and co-workers [2] developed efficient solar cells based on sol-gel derived ('fractal') TiO 2 electrodes sensitized by Ru(II) dyes. At present, efficiencies close to 10% under AM 1.5 conditions can be reached. In porous nanocrystalline electrodes, the injected electrons travel a long distance through the porous structure before they are collected at the back contact. On their way they remain close to the semiconductor / electrolyte interface and recombination with oxidized dye molecules or electron acceptors in the electrolyte occurs to some extend. Furthermore, the probability that the electrons are trapped is high, because there is a high concentration of traps at the surface and on grain boundaries. Trapping sites are usually sources for recombination of charge carriers, leading to reduced quantum efficiencies and lower open-circuit photovoltages. Here, in particular, intensity-modulated photocurrent spectroscopy (IMPS) is employed to study the dynamics of irradiated dye-sensitized porous nanocrystalline TiO2 607
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
electrodes. IMPS is a powerful technique to measure time constants of processes that occur at illuminated semico
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