Surface State Recombination and Passivation in Nanocrystalline TiO₂ Dye-Sensitized Solar Cells

Abstract

The relative role of surface state recombination in dye-sensitized solar cells is not fully understood, yet reductions in the recombination rate are frequently attributed to the passivation of surface states. We have investigated reports of trap state passivation using an Al₂O₃-coated TiO₂ photoanode achieved through atomic layer deposition (ALD). Electrochemical characterization, performed through impedance measurements and intensity modulated photovoltage spectroscopy (IMVS), data showed that the Al₂O₃ deposition successfully blocked electron recombination and that the chemical capacitance of the film was unchanged after the ALD treatment. A theoretical model outlining the recombination kinetics was applied to the experimental data to obtain charge transfer rates from conduction band states, exponentially distributed traps, and monoenergetic traps. The determined electron transfer rates showed that the deposited Al₂O₃ coating did not selectively passivate trap states at the nanoparticle surface but reduced recombination rates equally from both conduction band states and surface states. These results imply that the reduction in the recombination rates reported in core–shell structured photoanodes cannot be attributed to a modification of surface traps, but rather to the weakened electronic coupling between electrons in the film and the electrolyte species