Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells

Abstract

The capture and conversion of sunlight into a useful chemical fuel (H 2 , CH 4 , CH 3 OH, etc.) is a central goal of the field of artificial photosynthesis. Water oxidation to generate O 2 and protons stands as the major bottleneck in these processes. Relatively few stable photosensitizers can generate sufficient oxidizing power to drive water oxidation, and those that do contain rare elements such as ruthenium. In this paper, we show that metal-free organic photosensitizers are capable of driving photoelectrochemical water oxidation. Significantly, these photosensitizers exhibit comparable activity to that of ruthenium-containing photosensitizers under broadband illumination. In addition, we report to our knowledge the first demonstration of a molecular photosensitizer, outside of natural photosynthesis, that can drive water oxidation utilizing only red light. Solar fuel generation requires the efficient capture and conversion of visible light. In both natural and artificial systems, molecular sensitizers can be tuned to capture, convert, and transfer visible light energy. We demonstrate that a series of metal-free porphyrins can drive photoelectrochemical water splitting under broadband and red light (λ > 590 nm) illumination in a dye-sensitized TiO 2 solar cell. We report the synthesis, spectral, and electrochemical properties of the sensitizers. Despite slow recombination of photoinjected electrons with oxidized porphyrins, photocurrents are low because of low injection yields and slow electron self-exchange between oxidized porphyrins. The free-base porphyrins are stable under conditions of water photoelectrolysis and in some cases photovoltages in excess of 1 V are observed.