Effect of Surface Protonation of TiO2 on Charge Recombination and Conduction Band Edge Movement in Dye-Sensitized Solar Cells

This paper quantitatively describes how HCl-treatment of nanocrystalline TiO2 films prior to dye adsorption influences charge recombination and conduction band edge movement of dye sensitized solar cells based on dye N749 (the so-called black dye), [(C4H9)4N]3[Ru(Htcterpy)(NCS)3] (tcterpy = 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine), using intensity modulated photovoltage spectroscopy (IMVS). The surface protonation of TiO2 films causes a positive shift of the conduction band edge by 28 mV, corresponding to a decrease in open-circuit photovoltage (Voc) by 28 mV, and a slower charge recombination rate constant by a factor of 5, corresponding to an increase in Voc by 50 mV. The experimentally observed Voc enhancement (19 mV) by surface protonation of TiO2 films is in good agreement with the collective effect (50 mV − 28 mV = 22 mV) of both a positive shift of the conduction band edge and a slower charge recombination. These data indicate that surface protonation of TiO2 can simultaneously improve Jsc due to the positive shift of conduction band edge and Voc due to the suppression of charge recombination.