Comparison of the performances of dye-sensitized solar cells based on different TiO2 electrode nanostructures

This article reports on the performances of dye-sensitized solar cells based on three different working electrode structures, i.e., (i) sintered TiO2 nanoparticles (20-40 nm diameters), (ii) ordered arrays of TiO2 nanotubules (150 nm external diameters and 80 nm internal diameters), and (iii) ordered arrays of TiO2 nanorods (150 nm diameters). Even though the highest short-circuit current density was achieved with systems based on TiO2 nanotubules, the most efficient cells were those based on ordered arrays of TiO2 nanorods. This is probably due to higher open-circuit photovoltage values attained with TiO2 nanorods than with TiO2 nanotubules. The nanorods are thicker than the nanotubules and therefore the injected electrons, stored in the trap states of the inner TiO2 molecules, are shielded from recombination with holes in the redox electrolyte at open circuit. The high short-circuit photocurrent densities seen in the ordered TiO2 systems can be explained by the fact that, as opposed to the sintered nanoparticles, the parallel and vertical orientation of the ordered nanostructures provide well-defined electrons percolation paths thus significantly reduce the diffusion distance and time constant.