Laser-induced photovoltage transient studies on nanoporous TiO2 electrodes

At open circuit, pulsed-laser-induced photovoltage transients of nanoporous TiO2 electrodes filled with an electrolyte are studied using various pulse intensities and electrode thicknesses. The transients are interpreted using the electron diffusion in the electrodes. This is compared to the studies of electron transport, which was performed using photocurrent transients at short circuit. A similar light intensity dependence of electron diffusion is found from the photovoltage transients. Electron diffusion coefficients are estimated by fitting of the photovoltage transients with the solution obtained by solving a diffusion equation with a simple boundary condition. The fitted values of the electron diffusion coefficients at open circuit are lower than those derived from the photocurrent transients at short circuit. The difference suggests that Fermi level gradient, caused by nonuniform electron distribution in the electrode under short-circuit conditions, has significant influence on the electron transport. Observed photovoltages are plotted with corresponding electron densities in electrodes, showing logarithmic relationship. Difference and advantages of the photovoltage transient measurements in compassion to short-circuit transient measurements are discussed.