A study on the electron transport properties of TiO2 electrodes in dye-sensitized solar cells

The influences of annealing temperature and different poly (ethylene glycol) (PEG) contents in nano-crystalline TiO2 electrodes with and without N3 dye on the electron transfer in a dye-sensitized solar cell (DSSC) were investigated. It is found that the power conversion efficiency increases with the increase in annealing temperature and becomes saturated at 400–500 °C, and further increase lowers the performance which is consistent with the enhancement of the crystalline TiO2 particles observed in X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images. Electrochemical impedance spectroscopy (EIS) also confirms this behavior. These results have been further verified by studying the electron lifetimes (τe) and electron diffusion coefficients (De) of a bare TiO2 and a dye-sensitized TiO2 film using a pulsed laser spectrometer. It is noted that both the electron lifetime and the electron diffusion coefficient increase with the increase in annealing temperature. However, the evolution of rutile TiO2 begins beyond 600 °C and this lowers the dye absorbance and the electron diffusion coefficients of TiO2 electrodes. A similar study was made by varying the content of the PEG in the TiO2 films. It is found that with the increase in the PEG content, a decrease in the electron lifetimes and a little hike in the electron diffusion coefficients are noted, where the cell performance remains almost the same. In addition, the dye adsorption decreases the electron lifetime and increases the electron diffusion coefficient of the TiO2 films regardless of the PEG content and the annealing temperature.