As highlighted in several chapters of this book, and perhaps most compellingly in the device simulation work, development of a high efficiency solar water splitting device is most directly achieved by employing a tandem light absorber and an electrolyte with pH near either 0 or 14. By virtue of being integrated into a multi-project research program with device-oriented objectives, the high throughput experimentation (HTE) project launched with this device framework as a guiding principle, and consequently any materials exploration is related to either the anode (dioxygen evolving) or cathode (dihydrogen evolving) side of the device. The distinction between PV-electrolysis and photo-electrochemistry is another device-level concept that was critical in defining the functional materials of interest. The most compelling materials challenges specific to solar water splitting involve photoelectrochemical processes in which a semiconductor–liquid junction accelerates charge separation, enabling extraction of appreciable photocurrent from a semiconductor that is not of sufficient quality to be competitive for photovoltaic-based generation of electricity. As a solid state materials research group, the four device sub-components provided by this concept for the device architecture were electrocatalysts and photoelectrocatalysts for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). While discovery of an ideal photoelectrocatalyst would circumvent the need for an electrocatalyst for the corresponding reaction, we established a consensus that optimizing device efficiency would likely involve coupling an electrocatalyst to enhance the activity of a photoelectrocatalyst, particularly since known high-efficiency light absorbers were not efficient electrocatalysts and vice versa, with Fe-based (hydr)oxides being a notable exception to this rule.