Electron transport and recombination in ZnO-based dye-sensitized solar cells

The photovoltaic performance of ZnO-based dye-sensitized solar cells (DSCs) has been studied for three different configurations involving two dyes and two types of electrolytes with the iodide/iodine as redox mediator: ZnO/N719/organic solvent electrolyte (C1), ZnO/D149/organic solvent electrolyte (C2), and ZnO/N719/ionic liquid electrolyte (C3). The DSCs were characterized by measuring current-voltage curves and photovoltage as a function of light intensity and by electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), intensity-modulated photovoltage spectroscopy (IMVS), and open circuit photovoltage decay (OCVD). The results demonstrate the good light harvesting properties of the D149 dye and highlight the photovoltage limitation of the solvent-free (ionic liquid) electrolyte. The intensity dependence of the photovoltage and the OCVD, EIS, and IMVS results provide evidence of the nonlinear character of the recombination kinetics. It has been found that by combining EIS and IMPS data it is possible to overcome the problem of transport resistance determination for cells where there is no clear transmission line feature in the impedance measurements. The resulting electron diffusion lengths (Ln) indicate good electron collection for all studied cells, pointing to poor injection as the main limitation in DSCs based on the ZnO semiconductor.