Charge Recombination in Dye-sensitized Solar Cells with Low Adsorbed Concentration of Dye

Abstract

Photovoltaic conversion performances of dye-sensitized solar cells (DSCs) are significantly influenced by the interface charge recombination in DSCs. Lots of factors affecting the charge recombination, such as surface states of TiO2 and components of electrolytes, have been studied and dyes have been always ignored for the charge recombination in DSCs. Although the charge recombination occurring between the injection electrons and triiodide in electrolyte is calculated to take priority kinetically to the one between the injection electrons and oxidized dye molecules, dyes are not independent from the electrolyte related electron recombination. Instead by dye molecules themselves, the chance of injection electrons recaptured by triiodide in electrolyte could rise due to the increase of the adsorbed concentration of dye, which leads to the local concen- tration of triiodide increasing. In this paper, an effect of low charge recombination in DSCs with low adsorbed concentration of dye is observed. The adsorbed concentration of dye is defined as the adsorbed amount of dye in unit specific surface area of TiO2 films and adjusted by adsorbing similar amount of dye on the surface of TiO2 films with different film thickness. The influence of the adsorbed concentration of dye on the charge recombination in DSC is investigated by the electrochemical impedance spectroscopy (EIS) technology. It turns out that with the adsorbed concentration of dye decreasing, the electron lift time within TiO2 film and the interface resistance of TiO2/electrolyte increase significantly, which indicates the charge recombination in DSC decrease. Owing to this effect, with the TiO2 film thickness increasing from about 2 μm to 18 μm, the cells keep the fill factor (ff) as high as 0.72~0.80. And the energy conversion efficiency loss, which resulted from the in- crease of the active area of TiO2 photoanode from 0.25 cm 2 to 1 cm 2 , decreased from 34.7% to 19.6%.