Influence of TiO2 nanoparticle size on electron diffusion and recombination in dye-sensitized TiO2 solar cells

Nanoporous electrode films are prepared from different sized anatase TiO2 nanoparticles, of which the average diameter is 14, 19, and 32 nm. Dye-sensitized solar cells are prepared from the films. Diffusion coefficients of electrons (D) in the solar cells are estimated by photocurrent transient measurements using a small-intensity laser pulse under continuous irradiation of bias light. Electron recombination lifetimes (T) in the solar cells are measured by intensity-modulated photovoltage measurement (IMVS). It is found that the D increases and T decreases with the increase of the particle size up to 32 nm. The increase of the D is interpreted with the decrease of the film surface area, where the charge trap sites are likely to exist, and the condition of grain boundary. The decrease of T is discussed with the change of surface area and D with the particle size.