Dr. David Ginley, of the National Renewable Energy Laboratory, presents “Computational Identification of Materials for Solar Energy Conversion Including Semiconductors for Water Splitting” as part of the Andlinger Center’s 2014-2015 Highlight Seminar Series. ABSTRACT Historically, the development of new materials for solar energy conversion devices has been largely empirical and somewhat unsuccessful. In this talk we will look at new computational approaches to coupling the important parameters for solar energy conversion including contacts and absorbers with the identification of new high performance materials. The discovery of new materials has been enabled by recent theoretical developments and the availability of massively parallel computers allowing for the prediction of a range of properties for real materials with quantitative accuracy and at reasonable computational cost. This allows searching large chemical spaces for new material solutions as well as finding ways to improve the performance of existing materials in a wide range of applications. We will extend this approach to the design of new semiconductor materials for photoelectrochemical water splitting to hydrogen and oxygen which in addition requires accurate prediction of the position of the semiconductor band edges relative to the water oxidation and reduction levels. We describe a theoretical/computational approach [1] that allows quantitative predictions ionization potentials (IPs) and electron affinities (EAs) of semiconductors and insulators and, in addition, provides a route to relate IPs and EAs directly to the electrochemically measured positions of the band edges in aqueous environment. Our computational approach employs a combination of the state-of-the-art GW method for the electronic structure of bulk systems, and density functional theory surface calculations resulting in accurate and surface orientation dependent IPs and EAs. The relation between IPs and EAs and electrochemical measurements we derive from the extensive comparison of calculated IPs and EAs with available experimental data, both from photoemission and electrochemical measurements. From this comparison we find that there is on average a nearly constant 0.5 eV difference between the semiconductor band edges, derived from IPs and EAs, and those measured electrochemically at the pH value corresponding to the point of zero charge (PZC). This 0.5 eV upshift of the band edges of semiconductors in aqueous environment relative to those in vacuum is a consequence of the interaction with water molecules at the interface. This result allows direct alignment of the semiconductor band edges with water redox potentials just on the basis of known IPs, EAs, and PZCs, eliminating explicit modeling of semiconductor/water interfaces, which are still challenging for computationally. In this way an efficient, reliable and relatively simple procedure can be constructed and used in searching for new water splitting materials. [1] V. Stevanovic, S. Lany, D. S. Ginley, W. Tumas, and A. Zunger, “Assessing Capability of Semiconductors to Split Water Using Ionization Potentials and Electron Affinities Only”, Phys. Chem. Chem. Phys.16, 3706 (2014) DOI:10.1039/c3cp54589j BIO Dr. David S. Ginley is currently a Research Fellow and Chief Scientist for Materials and Chemistry Science and Technology at the National Renewable Energy Laboratory in the National Center for Photovoltaics. He received his Ph.D. in Inorganic Chemistry from MIT and his B.S. in Chemistry from the Colorado School of Mines. He is the co-director of the Solar Energy Institute for India and the U.S. and chief experimentalist for the Center for Next Generation Materials by Design EFRC. Current work focuses on advancing solar energy conversion and storage, specifically in the areas of development and application of transparent conducting oxides, organic electronics materials, thermoelectrics, nano-materials and the development of process technology for materials and device development including; combinatorial methods, direct write materials, composite materials and non-vacuum processing. A key focus is looking at how to significantly reduce the cost of solar generated renewable energy through novel devices and processing including the incorporation of thermoelectrics, integrated storage and modular scalable concentrated solar power systems.

• published: 23 Jan 2015
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