Numerical Simulation of the Current-Voltage Curve in Dye-Sensitized Solar Cells

A theoretical model based on the numerical integration of the continuity equation for electrons with trap-limited density-dependent diffusion and recombination constants is implemented to describe the functioning of dye-sensitized solar cells (DSSC). The application of the model combines recent theory on charge transport in nanocrystalline materials with parameters extracted from five simple measurements: the UV/vis spectrum of the dye in solution, the steady-state current-voltage curve, the open circuit photovoltage versus light intensity curve, photocurrent transient upon switching on an illumination source, and open-circuit voltage decay upon switching off the illumination source. As a novel feature not previously included in this kind of calculations, the model includes an additional term that accounts for the charge transfer from the transparent conducting oxide (TCO) Substrate to the electrolyte solution. The general applicability of the model is illustrated by applying it to two different types of cell: a TiO2-based solar cell with an organic solvent electrolyte and a ZnO-based solar cell with a solvent-free electrolyte. It is found that the numerical model is capable of adequately fitting all data for both systems, resulting in quantitative estimates for the main parameters controlling solar cell functioning and efficiency. The results show that it is possible to provide a global description of DSSCs based on fundamental theories for trap-limited transport and recombination using simple experimental techniques available to every solar cell laboratory. The present paper tries to help fill the gap between pure theoreticians and experimentalists working ob this kind of system.