Spatial location of transport-limiting traps in TiO2 nanoparticle films in dye-sensitized solar cells

The dependence of the electron diffusion coefficient and photoinduced electron density on the internal surface area of TiO2 nanoparticle films in dye-sensitized solar cells was investigated by photocurrent transient measurements. The internal surface area was varied by altering the average particle size of the films. The density of electron traps in the films is found to change in direct proportion with the internal surface area, indicating that the traps are located predominately at the surface of TiO2 particles instead of in the bulk of the particles or at interparticle grain boundaries. The observed scaling of the electron diffusion coefficient with the internal surface area suggests that surface traps limit transport in TiO2 nanoparticle films. These results address a long-standing issue in the understanding of electron transport in dye-sensitized TiO2 solar cells.