The time varying photocurrent response of dye-sensitized solar cells to switching on illumination and then switching the illumination off after steady state has been reached has been measured and modeled with a multiscale Monte Carlo and continuum approaches within the multiple trapping picture, where electrons in the porous TiO2 electrodes undergo successive trapping and detrapping events. These models are more detailed than the more common formalism employing an effective diffusion coefficient as they take into account the large variation in conduction electron density with distance and time near the extracting electrodes. Agreement between the three sets of results using a trap concentration and trap energy distribution width from a separate set of measurements using charge extraction shows that the multiple trapping formalism holds.

We have examined the combined effects of grain morphology and electron trapping on the transient response of photoelectrons moving through the TiO2 grains in a dye-sensitized nanocrystalline solar cell using a multi-time-scale random walk Monte Carlo model. Our use of a multi-time-scale approach enables us to simulate transport for electrons moving through spherical connected grains in a three-dimensional (3D) voided network and look at the effect of the size of interparticle boundaries on carrier dynamics. We can also address similar times to those over which measurements are taken, namely, 0.1 ms. These times are long because of deep traps in the TiO2 grains. The grains have 2-fold connectivity in one dimension (linear chains) or 4-fold or 6-fold connectivity in three dimensions and traps with an exponential distribution of energies. Photoelectrons are generated by a light pulse of short duration. The spatial distribution of the photogenerated electron density from this pulse either has a uniform profile or is peaked on the electrolyte side. We show that the constrictions at the grain necks slow the electrons, making trapping more likely and hence further delaying their passage to the extracting electrode. By comparing our results for 4-fold and 6-fold coordinated particles on a cubic lattice with 2-fold coordinated particles on linear chains, we show that transport is slowed in the former case due to the additional paths available to the electrons in the 3D network. We also find that the charge and current transients cannot be fit to an analytical solution of the continuum equations with an effective diffusion coefficient even at long times. Therefore, caution must be exercised when attempting to fit experimental transient data with an effective diffusion coefficient.

We present a novel multi-timescale approach to simulating transport for electrons moving through a mesoporous film of TiO2 grains in the dye-sensitized solar cell. This method is unique in its ability to integrate microscopic (intra-grain) with mesoscopic (inter-grain) charge transport processes. Comparisons are made with experimental measurements of transient currents.