Direct photocurrent generation from nitrogen doped TiO2 electrodes in solid-state dye-sensitized solar cells: Towards optically-active metal oxides for photovoltaic applications

Nitrogen-doped titanium dioxide (TiO2) is considered as a promising photocatalytic material due to its optical absorption extended in the visible region compared to pure TiO2. In the field of photovoltaic applications, dye-sensitized solar cells based on N-doped nanocrystalline titania electrodes have demonstrated improved performance due to the beneficial effects of nitrogen on the electronic and optical properties of TiO2. In this context, we report on the influence of nitrogen doping on the performance of solid-state dye-sensitized solar cells, starting from TiO2 and N-TiO2 nanocrystals synthesized by laser pyrolysis. Using an integrated approach based on experimental and theoretical investigations, the relationship between the local electronic features of the starting metal oxide materials and device operation is described. We demonstrate that the short-circuit current density of the solar cells based on an N-doped TiO2 electrode increases by more than 10% compared to that of pure anatase. This improvement is clearly associated with the extended absorption of the doped electrode, suggesting that alternative charge generation mechanisms occur in the cells in addition to the conventional dye absorption. Computer simulations on isolated nanoclusters, as well as electron paramagnetic resonance (EPR) experiments, confirm that nitrogen atoms in the presence of oxygen vacancies can explain the introduction of additional energy states near the valence band of TiO2. Surface states associated with nitroxide radicals are also suggested to act as charge traps under illumination. These aspects confirm the strong potentialities of optically-active metal oxides for photovoltaic applications.