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A molecular tandem cell for efficient solar water splitting

Significance A major challenge for artificial photosynthesis is creating a photoelectrode to split water without an added bias. Here, we demonstrate the value of combining of a dye-sensitized photoelectrosynthesis cell and an organic solar cell in a photoanode for water oxidation. With the two component electrodes, and a counter Pt electrode for H2 evolution, the electrode becomes part of a combined electrochemical cell for unassisted water splitting, 2H2O → O2 + 2H2. The results described here offer a major improvement in solar-to-hydrogen conversion efficiency (STH%) for a molecularly based solar fuel electrode. The STH% for water splitting was 1.5% for the tandem cell compared to ∼1% for natural photosynthesis. Artificial photosynthesis provides a way to store solar energy in chemical bonds. Achieving water splitting without an applied external potential bias provides the key to artificial photosynthetic devices. We describe here a tandem photoelectrochemical cell design that combines a dye-sensitized photoelectrosynthesis cell (DSPEC) and an organic solar cell (OSC) in a photoanode for water oxidation. When combined with a Pt electrode for H2 evolution, the electrode becomes part of a combined electrochemical cell for water splitting, 2H2O → O2 + 2H2, by increasing the voltage of the photoanode sufficiently to drive bias-free reduction of H+ to H2. The combined electrode gave a 1.5% solar conversion efficiency for water splitting with no external applied bias, providing a mimic for the tandem cell configuration of PSII in natural photosynthesis. The electrode provided sustained water splitting in the molecular photoelectrode with sustained photocurrent densities of 1.24 mA/cm2 for 1 h under 1-sun illumination with no applied bias.