Primary and final charge separation in the nano-structured dye-sensitized electrochemical solar cell

A time constant of 13 fs was measured via transient absorption for electron transfer from the excited singlet state of the chromophore perylene into anatase TiO2, when the chromophore was bonded to the surface via a carboxyl group. This result suggests that a similarly short time constant should be valid also for primary charge separation from the bipyridyl ligand of the so-called N3 ruthenium dye into anatase TiO2. The electron transfer time of perylene became much longer, 3.8 ps, at a distance of about 1.3 nm. Laser pulse induced transients of the photocurrent were measured in the Gratzel-cell (G-cell) for illumination firstly through the SnO2/TiO2 interface (SE) and secondly through the opposite SnO2/electrolyte interface (EE). For EE illumination the transient showed two peaks, the first in the mus range and the second in the ms range. For SE illumination only the early peak was observed. The different shapes of EE compared to SE transients were easily modeled employing a time dependent diffusion equation and appropriate boundary conditions. The experimental rise to the early peak occurred faster than corresponds to diffusion controlled final charge separation at the SnO2/TiO2 interface if the same diffusion constant was assumed that was valid in the bulk of the nm-structured sponge-type TiO2/electrolyte layer. The significance of this experimental result for photovoltaic energy conversion of the G-cell is discussed. (C) 2004 Elsevier B.V. All rights reserved.