As fossil fuels become increasingly depleted and expensive, effective use of solar energy has become an urgent task at the forefront of materials research. Among many different alternatives, the generation of chemical fuels offers a promising way to capture and store solar energy. [ 1,2 ] To deliver an effi cient, integrated solar fuels device, new light-absorbing materials must be identifi ed that meet stringent criteria with respect to bandgap, band edge energies, and electrochemical stability. For a solar water splitting device with a single light absorber, a bandgap in the range of 1.7–2.2 eV is desirable for capturing a sizeable fraction of solar radiation while providing suffi cient photovoltage to drive water-splitting and other reactions. [ 3 ] For tandem light absorber designs, bandgaps in the range of 0.9– 2.0 eV are of prime interest, as combining light absorbers from the low end and high end of this range can provide effi cient capture of solar radiation and produce suffi cient photovoltage. [ 4 ] For this tandem strategy, several p -type semiconductors such as Si, InP, and WSe 2 have been successfully demonstrated as the low-gap component material, further motivating the discovery of an n -type semiconductor with 1.6–2.0 eV gap. To perform photoelectrolysis, the conduction and valence bands of the single or tandem photoabsorber must also straddle the redox potentials of both the hydrogen evolution reaction (HER) [0 Vvs the normal hydrogen electrode (NHE)] and oxygen evolution reaction (OER) (1.23 V vs NHE). Metal oxide semiconductors have been extensively researched for this application due to their chemical and electrochemical stability under oxidizing conditions, but the demanding set of requirements on band energetics has not been met by known metal oxides.