Charge transport and recombination in TiO₂ brookite-based photoelectrodes

The photoactivity of the stable nanosized TiO₂ polymorphs is challenging for many advanced applications. In the present work, brookite TiO₂ nanoparticles with two different shapes have been used as building blocks for the preparation of pure brookite mesoporous layers. The layers have been characterized before and after sensitization. They have been used as photoanodes in dye-sensitized solar cells (DSSCs). The cell functioning coupled processes have been investigated by the impedance spectroscopy (IS) technique at various applied voltages and compared to a reference anatase TiO₂ solar cell. The investigations of the chemical capacitance and of the charge transfer resistance, R_ct, show that, compared to anatase, the brookite surface is less active for the recombination side reaction. The larger R_ct is shown to explain the higher open circuit voltage of the brookite cells. However, the charge transport is much slower in the brookite phase due to a lower electrical conductivity. This parameter has been quantified more than 1 order of magnitude lower in the brookite layers compared to the anatase one. On the whole, the efficiency of brookite DSSCs is mainly limited by two parameters, the dye loading and the charge collection efficiency.