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)(4)N](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 build edge by 28 mV, corresponding to a decrease in open-circuit photovoltage (V-oc) by 28 in mV, and a slower charge recombination rate constant by a factor of 5, corresponding to an increase in V-oc by 50 mV. The experimentally observed V-oc 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 J(sc) due to the positive shift of conduction band edge and V-oc due to the suppression of charge recombination.