Large enhancement in photocurrent efficiency caused by UV illumination of the dye-sensitized heterojunction TiO2/RuLL′NCS/CuSCN:: Initiation and potential mechanisms

Wide-band-gap, dye-sensitized, heterojunctions of the composition n-TiO2/Ru-dye/p-CuSCN undergo large increases in efficiency when subjected to low-power UV illumination (<40 mW/cm(2)) for periods between 10 and 30 min. Ru-dye refers to any of several ruthenium polypyridyl dyes. The UV illumination increases the incident-photon-to-current-efficiency for light absorbed by the dye by a factor of 5-10 and increases the open circuit voltage by 100-300 mV. This beneficial effect is stable for months after the UV illumination is terminated. Investigation of the effects of varying the UV wavelength and the applied bias show that the reaction(s) responsible for the increase in efficiency begin with the oxidation of some interface species by a photoexcited hole in the TiO2 valence band. Treatment of the TiO2/dye/CuSCN cells with oxidized thiocyanate in solution causes a similar increase in the efficiency of dye sensitization. Results from TiO2 and CuSCN photoelectrochemical cells are also presented. The combined results indicate that the UV illumination creates an interfacial layer of(SCN)(3)(-), and/or its polymerization product (SCN)(x), between the TiO2 and the CuSCN. The data are consistent with the hypothesis that the (SCN)(x)(-) layer increases the rate at which the oxidized dye is regenerated after electron injection into the TiO2.