题 目：First-principles Electronic Band Structure Theory for Solar Energy Conversion Materials
Direct utilization of solar energy in the form of solar fuels is currently one of the most actively pursued frontiers in basic energy sciences. In particular, the direct photo-splitting of water to H2 and O2 is regarded as the “holy grail” for solar energy conversion. The most grand challenge for direct solar energy conversion is to find a semiconductor material that meet several crucial requirements simultaneously, including, in particular, 1) a suitable band gap that allows efficient absorption of solar energy, ideally in the visible light regime, and 2) appropriate valence and conduction band positions that match the redox potentials of H+/H2 and H2O/O2. Electronic band structures of photo-catalytic semiconductors therefore play a crucial role in solar-energy conversion. From a theoretical point of view, Kohn-Sham density functional theory (KS-DFT) in the standard local density or generalized gradient approximation (LDA/GGA) has serious difficulty in describing electronic properties of extended systems. In this work, we apply many-body perturbation theory in the GW approximation, currently the most accurate first-principles approach for electronic band structure of extended systems, to investigate electronic properties of several prototypical solar energy conversion materials. In particular we consider several early transition metal dichalcogenides MX2 (M=Zr, Hf, Mo, W, and X=S and Se) and alkali tantalates (ATaO3, A=Li, Na and K). By combining first-principles calculations with phenomenological analysis, key factors that determine the electronic band structures of these materials are discerned. We also calculate absolute band positions with respect to the vacuum level by combining KS-DFT calculations in the slab model and GW quasi-particle corrections. The relevance of our theoretical calculations to visible light photolysis of water is discussed.