Using Soft X-Rays and Electrons to Determine the Chemical and Electronic Structure of Semiconductors for Solar Water Splitting

Abstract

In order to maximize the contribution of solar energy to the future global energy portfolio, it is important to optimize the performance, stability, and cost of the devices involved in solar energy conversion (in particular solar water splitting). For that purpose, novel material classes are being exploited, invariably leading to the investigation and use of complex, often ternary or even multinary compounds. The purpose of this talk is to demonstrate how a tool chest of soft x-ray and electron spectroscopies (in particular using high-brilliance synchrotron radiation) is uniquely suited to unravel the electronic and chemical properties of surfaces and interfaces of such materials, and how an understanding on such fundamental level can lead to increased performance. It will be shown how photoelectron spectroscopy (PES), x-ray-excited Auger electron spectroscopy (XAES), inverse photoemission (IPES), x-ray emission spectroscopy (XES), and x-ray absorption spectroscopy (XAS) can be suitably combined to derive band gaps, study local chemical bonding and electronic level alignment, and derive insights into chemical stability. As examples, WO3- and GaInP2-based water-splitting devices will be discussed, and Cu(In,Ga)(S,Se)2 solar materials will be used to further illustrate pertinent aspects of the experimental tool chest.