Porosity effects on electron transport in TiO2 films and its application to dye-sensitized solar cells

Porosity (P) of TiO2 film in dye-sensitized solar cells affects the light absorption coefficient and electron diffusion coefficient. A theoretical analytical expression of the intensity-modulated photocurrent spectroscopy (IMPS) response involving the light absorption coefficient and the electron diffusion coefficient as a function of the porosity has been proposed to investigate the influence of TiO2 film porosity on the characteristics of electron transport. The incident photon-to-current conversion efficiency (IPCE) and electron transit time depending on the porosity have been analyzed illuminating from both the electrolyte side (IE) and the substrate side (IS). The IPCE derived from the IMPS response reaches its maximum at a porosity of around 30% for IE and 41% for IS, respectively. Electron transit time increases with increasing the porosity for IE, while it declines when P < 0.41 for IS, which is attributable to the influence of the RC time constant. It has also been found that a larger RC time constant will lead to a longer transit time. The electron diffusion coefficient calculated from the transit time for IE corresponds to the results from the porosity reported in previous literature, which indicates that the dependence of the electron transit time tau(d) on the porosity is justifiable. The diffusion coefficient calculated for a larger RC time constant approaches the value from the literature when P > or = 0.41, while it is not practicable when P < 0.41 for IS.