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

Abstract

Photovoltaic responses were observed upon irradiating (>420 nm) an ITO electrode spin-coated with a mixture of TiO₂ nanoparticles, polyvinylcarbazole, and carotenoid (canthaxanthin or β-carotene, Car) and immersed in an aqueous KCl solution also containing hydroquinone (QH₂). The anodic photocurrent 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 QH₂. 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 rapid regeneration of Car from Car^˙+ during the electron transfer cycle. In contrast, when the aqueous electrolyte solution was saturated with O₂, a cathodic photocurrent was observed; this reversal of polarity is attributed to electron transfer from Car* to O₂, and subsequent reduction of the resulting Car^˙+ by electrons from the TiO₂ conduction band.