Emerging Oxide and Nitride Semiconductors for Solar Energy Capture and Conversion

Transition metal oxide and nitride semiconductors offer considerable promise for a range of applications, from sustainable (opto)electronics to photocatalytic energy conversion. However, such materials are characterized by complex carrier-lattice couplings, defect properties, and chemical susceptibilities that must be characterized and controlled to enable their implementation in functional systems. Here, recent experimental advances, including mechanistic understanding of photoexcited states, defect engineering strategies, and new materials discoveries, will be presented. For example, while bismuth vanadate (BiVO₄) stands as a model oxide photoanode, its impressive performance characteristics are surprising considering its propensity to form small polarons. Here, we present results of recent time-resolved optical absorption, X-ray diffraction, and X-ray absorption measurements that reveal the nature of strong carrier-phonon coupling in this material, which not only results in long-lived polaronic states but also photo-induced structural phase transformations that have significant implications for solar-to-chemical energy conversion. Moving from oxides to nitrides, such as orthorhombic tantalum nitride (Ta₃N₅), we discuss how the underlying electronic structure and defect tuning strategies can yield improved carrier transport characteristics and excited state lifetimes. Moreover, through broader exploration of Ti-Ta-N and Zr-Ta-N composition spaces, we demonstrate not only doping of existing compounds but also identify promising new compound semiconductors. As a key example of this, we highlight bixbyite-type ZrTaN₃, for which complementary experiment and theory reveals new prospects for tuning electronic structure via control of cation order and site symmetry. Overall, these results highlight the promise of both established and new transition metal-based semiconductors with tunable properties for solar energy harvesting.