Interpretation of small-modulation photocurrent transients in dye-sensitized solar cells – A film thickness study
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Electron transport in dye-sensitized solar cells with varying mesoporous TiO2 film thicknesses was investigated using experimental and computational methods. More specifically, photocurrent transients resulting from small-amplitude square-wave modulation of the incident light were recorded for a series of solar cells, whereby the dependence of the wavelength and direction of the illumination was investigated. The responses were compared to simulations using different models for diffusional charge transport and analyzed in detail. The photocurrent transients are composed of two components: an initial fast response in case of illumination from the working electrode side, or an initial apparent delay of photocurrent decay for illumination from the counter electrode side, followed by a single exponential decay at longer times, with a time constant that is identified as the electron transport time. The initial response depends on the thickness and the absorption coefficient of the film. Transport times for different films were compared at equal short-circuit current density, rather than at equal light intensity. Experimentally, the transport time showed a power-law dependence on the film thickness with an exponent of about 1.5. Analysis using the quasi-static multiple trapping (MT) formulation demonstrates that this behavior originates from differences in quasi-Fermi level in the TiO2 films of different thickness when equal photocurrents are generated. The Fokker–Planck relation was used to derive expressions for the electrons flux in porous TiO2 films with a position-dependent diffusion coefficient.