Water-splitting dye-sensitized photoelectrochemical cells potentially provide a solution to solar energy conversion from sunlight to solar fuels. The high surface area photoelectrodes can be leveraged to incorporate light-absorbing molecules to cover the solar spectrum, as well as efficient molecular catalysts for water oxidation reaction. However, before the device can be more broadly applied in solar fuel synthesis, the issue of low quantum efficiency have to be addressed.

Earlier studies have identified that the electron injection from photosensitizers to electrodes is one of the major efficiency loss pathways of water-splitting photoanodes. The charge injection efficiency is only about 30% for ruthenium tris-bipyridine sensitizers with phosphonate groups anchored on nanocrystalline anatase TiO₂ electrodes, in a neutral-pH aqueous electrolyte. It is therefore important to understand why the charge injection efficiency is so low and how to improve the electron injection efficiency of the photoanode.

We have studied the impact of solvent systems on the electron injection efficiency. By using ultrafast spectroscopy, it has shown that by introducing non-aqueous solvent components to the electrolyte, the electron injection efficiency can be significantly improved. This suggests that changing the solvent systems could be a simple knob to tune the charge injection efficiency of the system. We propose that by using a mixed organic/aqueous electrolyte, the reorganization energy is lowered for the electron injection of sensitizers in excited states. We are also investigating this phenomenon from a thermodynamic perspective that the flat band potentials of the electrode may vary in different solvent environment. The results of this fundamental study can help us understand the essential electron transfer processes inside of the energy harvesting devices and find out solutions to realize efficient solar fuels production from abundant matters.

Jacob Spies

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