A numerical model for charge transport and recombination in dye-sensitized solar cells

We propose a numerical model aimed at obtaining the electrical output of dye-sensitized solar cells from microscopic parameters. The model is based on the solution of the continuity equation as a function of voltage for electron transport with both the diffusion coefficient and the recombination constant dependent on the electron density, i.e., the light intensity and/or voltage. The density dependence of the kinetic parameters can be implemented in analytical form (via a power-law expression) or extracted from experiments or electron transport simulations. We investigate the situation where the recombination rate is limited by the electron transport in the nanostructured film, as has recently been suggested by various authors. It is observed that for a power-law density dependence governed by a single alpha parameter, related to the depth and shape of a trap energy distribution, the solar cell behaves as an ideal diode, where the short-circuit current, open-circuit voltage, and current-voltage characteristics are independent of the a parameter. According to the formal description provided here, where recombination is limited by electron transport, lowering the trap density or changing a by changing the morphology or materials properties, thus improving the conductivity, would not lead to a better performance of the solar cell under steady-state conditions. The numerical results are compared to intensity-dependent current-voltage measurements on chlorophyll-sensitized TiO2 solar cells.