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Renewables > Solar Water SplittingNovel Contacts for Efficient Coupling of Photovoltaics and Catalysts
InvestigatorsPaul McIntyre, Department of Materials Science and Engineering, and Christopher Chidsey, Department of Chemistry, Stanford University
ObjectiveThe goal of this project is to achieve efficient and stable photoelectrolysis of water using crystalline and amorphous, earth abundant semiconductors. The project focuses on the fundamentals of photoexcited carrier dynamics and charge transport across stable metal/oxide/semiconductor (MOS) structures.
Solar radiation represents a potentially vast energy resource, but conversion to useful forms of energy and cost reductions are required for its economical implementation at large scale. Solar fuels, which can store a portion of the incident solar energy in the form of chemical bonds, may provide a solution for the temporal mismatch of solar radiation with the demand for energy.1 Water is the most abundant source of electrons for large-scale solar fuel synthesis reactions, and is a chemical feedstock that can be transformed to fuel and oxidizer in either single-junction or two-junction photosynthetic cells. Sunlight can be used to split water into hydrogen and oxygen, which can be stored in fuel cells to provide power during nighttime hours or when electricity demand is high. The key challenge in using sunlight to drive photoelectrochemical reactions is designing light-absorbing materials and structures that simultaneously achieve efficient absorption of photons, efficient electron-hole separation and transport, and have surfaces with high electrochemical reactivity. It is also important to avoid corrosion or oxidation of the absorbing material in order to prevent the destruction of one or more of these properties.Figure 1: Photoelectrochemical cells based on n-type semiconducting photoanodes. (A) A cell that generates hydrogen, a chemical fuel, through the photo-cleavage of water; and (B) a regenerative cell that produces an electric current from sunlight.2
Photoelectrochemical cells (PECs) can function as photosynthetic cells (Figure 1A) or regenerative photovoltaic cells (Figure 1B). In a regenerative cell, a reduction reaction at the cathode and the reverse (oxidation) reaction at the anode transfer charge across an electrolyte with no overall change in the chemical state of the species in solution. This process produces a net photo-voltage across the cell that can be used to do work in an external electrical load. In a photosynthetic cell, different reactions at the cathode and anode affect a net conversion of species into new chemical forms. Of particular interest is a PEC with stable Schottky tunnel contacts attached to a photoanode and a photocathode that provides two photovoltages in series to drive the device. Stable and catalytic anode materials are required for the oxidation of water, and stable and catalytic cathode materials are required for the production of the fuel. Although there are many viable choices for the cathode material, efficient oxidation of water requires a corrosion-resistant n-type photoanode made of metal oxides (e.g. TiO2, SrTiO3, Fe2O3), which are typically prepared as relatively thick coatings applied to base-metal electrodes or as dispersed nanoparticles.3,4,5
Atomic later deposition (ALD) -grown tunnel oxides hold great promise for electrically coupling high-quality semiconductor absorbers to reduction and oxidation catalysts. ALD also offers the opportunity to locally tailor catalytic properties of surfaces by nanostructuring and control of local composition. The best water oxidation catalysts at present are relatively expensive noble metals and their oxides. The metal oxide semiconductor structures to be investigated in this research will be adaptable to more earth-abundant catalysts as they become available. The research team will study the efficiencies of photovoltaic energy conversion and photosynthetic production of hydrogen and oxygen from water. To achieve efficient and stable photoelectrolysis of water, the team will use crystalline and amorphous, earth-abundant semiconductors with surfaces that remain stable in aqueous electrolytes. In addition, researchers will evaluate the stability and efficiency of various earth-abundant absorbers, such as crystalline and amorphous silicon and gallium phosphate. Atomic layer deposition (ALD) will be used to grow chemically protective insulator layers and overlying metal catalysts in Schottky tunnel contacts on several different semiconductors. An example atomic force microscope image of an islanded ALD-iridium film is shown in Figure 2.Figure 2: Atomic force microscopy image of ALD-Iridium nano-island film.
The density of the islanded structure can be increased significantly by initial pulsing of trimethyl aluminum surfactant just prior to ALD-iridium growth. The local electronic conductivity and iridium composition of the catalyst island coated TiO2 tunnel oxide will also be tested using other established techniques.
These techniques along with appropriate semiconductors should allow efficient absorbance of the solar spectrum. A cell potential sufficient for water splitting should be obtained while avoiding expensive lattice-matched epitaxy of single crystal layers required in state-of-the-art multijunction PV.
References N.S. Lewis and D.G. Nocera, Proc. Natl. Acad. Sci. 103, 15729-35 (2006).  M. Grätzel, Nature 414, 338-44 (2001).  A. Kay, I. Cesar and M. Grätzel, J. Am. Chem. Soc. 128, 15714-21 (2006).  K. Domen, A. Kudo, T. Onishi, N. Kosugi, and H. Kuroda, J. Phys. Chem. 90, 292-95 (1986).  A. Fujishima and K. Honda, Nature 238, 37-38 (1972).
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