Charge transport in photoanodes constructed with mesoporous TiO2 beads for dye-sensitized solar cells

Mesoporous TiO2 beads exhibit the beneficial properties of enhanced light scattering and fast charge transport in a single nanostructured assembly, which makes them ideal for dye-sensitized solar cell applications. However, their unique geometry gives rise to charge transport behaviors that are particular to the beads themselves. This study examined charge transport in TiO2 beads for dye-sensitized solar cell applications on both plastic and glass substrate devices. Through small perturbation and transient techniques, two effective diffusion rates within the film were observed due to the contrast between the intrabead and interbead connections. The dip in diffusion rates away from their typical exponential behavior at high charge densities could be attributed to the poor electrical contact between the TiO2 beads and the conductive oxide substrate. By the application of a small nanoparticle under-layer, the high contact resistance was overcome while maintaining relatively high diffusion rates. The identification of these charge transport issues and their causes provides an important step toward the optimized deployment of mesoporous TiO2 beads for solar cell applications.