Photovoltaic response of carotenoid-sensitized electrode in aqueous solution: ITO coated with a mixture of TiO2 nanoparticles, carotenoid, and polyvinylcarbazole

Photovoltaic responses were observed upon irradiating (>420 nm) an ITO elec trode spin-coated with a mixture of TiO2 nanoparticles, polyvinylcarbazole, and carotenoid (canthaxanthin or beta-carotene, Car) and immersed in an aq ueous KCl solution also containing hydroquinone (QH(2)). The anodic photocu rrent involves electron transfer from the excited state of the carotenoid ( Car*) to the TiO2 conduction band; the resulting carotenoid radical cation (Car(.+)) is reduced by QH(2). The photocurrent was nearly constant during one hour of continuous irradiation, which indicates that these carotenoids are stable at the electrode/electrolyte interface, presumably because of ra pid regeneration of Car from Car(.+) during the electron transfer cycle. In contrast, when the aqueous electrolyte solution was saturated with O-2, a cathodic photocurrent was observed; this reversal of polarity is attributed to electron transfer from Car* to O-2, and subsequent reduction of the:res ulting Car(.+) by electrons from the TiO2 conduction band.