Chapter 6: Heterojunction Approaches for Stable and Efficient Photoelectrodes

In photoelectrochemical (PEC) solar to chemical energy conversion, photo-generated charges are used to drive reduction and oxidation reactions in an electrolyte solution. In the specific case of solar to hydrogen conversion via PEC water splitting, photo-generated electrons can drive the reduction of protons (or water itself directly) to hydrogen gas and photo-generated holes can oxidize water to oxygen. As the currents in the light absorbers are driven by minority carriers, p-type light absorbing semiconductors are used as the hydrogen-generating photocathodes and n-type semiconductor are used as oxygen-generating photoanodes. Due to the thermodynamic and kinetic constraints of overall-water splitting, typically at least two light absorbing elements are used if the system is to operate without additional electrical bias. Practically implemented PEC devices can have either a photocathode, a photoanode, or both. Devices with a single photo-electrode typically have one or more photovoltaic devices to provide the additional bias required for operation. For realization of this technology for practical energy storage, both high solar to hydrogen conversion efficiency and long operational lifetime are desirable. By analogy with recent trends in photovoltaic energy conversion, it has been more typical to use heterojunctions to affect the needed charge selective contacts for the photo-electrodes. Specific to the PEC application, the heterojunction may also be used to protect the electrode from photo-electrochemical corrosion. While these two desired outcomes are separate, it is possible to find heterojunction designs which perform both functions well. Focusing on work in the Joint Center for Artificial Photosynthesis (JCAP) in the period 2010–2015, this chapter will review the evolving role of heterojunctions in developing high efficiency, stable photocathodes for the generation of hydrogen and photoanodes for the generation of oxygen.