第一原理を用いた太陽電池及び燃料電池に応用する無機ダブルペロブスカイトに関する研究

摘要

Recently, perovskite materials have attracted much attention in the application of renewable energy. For instance, the hybrid of inorganic?organic lead halide perovskite has been regarded as the most promising light?harvesting material for next generation solar cells. On the other hand, the transition metal (TM) oxide perovskites have also been studied extensively as electrodes of fuel cells and lithium?air batteries. Several issues are needed to solve, such as the lead pollution of PSCs and the high working temperature of SOFCs to realize the commercialization of perovskite solar cells (PSCs) and solid oxide fuel cells (SOFCs). Therefore, this thesis focuses on the theoretical studies of a series of inorganic double perovskites as the PSCs absorbers and the SOFCs electrodes. In chapter 1, the structural and electronic properties of perovskite materials are briefly introduced. The working principles of PSCs and SOFCs are described. The previous theoretical studies are also reviewed. In the end of this chapter, the motivation of this thesis is presented. In chapter 2, the theoretical backgrounds and simulation approach are briefly summarized, including the density functional theory (DFT), on?site coulomb potential correction method, optical calculation method, and climbing?image nudged elastic band method. In chapter 3, the electronic and optical properties of the double halide perovskites Cs2NaBX6 (B = Sb, Bi; X = Cl, Br, I) are studied to evaluate the potential application of solar energy conversion. The calculated results revealed that the inorganic double iodide perovskites, Cs2NaSbI6 and Cs2NaBiI6, have suitable bandgaps of 1.65 eV and 1.68 eV, suggesting the potential application as the visible?light absorber of perovskite solar cells. In chapter 4, the electronic and optical properties of Mo?based double oxide perovskites Sr2BMoO6 (B = Mg, Ca, and Zn) are studied by first?principles calculations. The electronic band structures demonstrate that these double perovskites are semiconductor. The alkaline metals (Mg and Ca) doped double perovskites have direct bandgaps, while the Zn?doped double perovskite exhibits the indirect bandgap. B?site substation significantly influence the electronic and optical properties of perovskites, which can be a useful approach to design novel absorbers of perovskite solar cells. 3 In chapter 5, the Mo?based double oxide perovskites Sr2BMoO6 (B = Mg, Cr, Co and Ni) are studied with the primary focus on the mixed ionic and electronic conductivity. The effects of substituted elements on the vacancy formation and migration are analyzed from the calculated ground states energy. Co?substituted double perovskite is predicted to possess the best oxygen ionic conductivity among these Mo?based double perovskites. According to the calculated charge density, the substituted cations (e.g., Mg2+, Cr3+, Co2+, and Ni2+) accommodate the additional electrons released from oxygen vacancy, which plays an important role in the oxygen ionic conductivity in these double oxide perovskites. In chapter 6, the theoretical investigation of transition metal oxide perovskites Sr2TixFe2? xO6?δ (x = 0.5, 1, 1.5) are investigated by first?principles calculations. The calculated formation energy of oxygen vacancy demonstrates the high concentration of oxygen vacancy. The electrons released from the oxygen vacancy tend to reorganize onto the 3d orbital of Fe cations through the weak covalent Fe?O and Ti?O bonds. Given the electronic configuration of Fe cations, this itineracy of leftover electrons facilitates to the oxygen ions conductivity in these perovskites. These results reveal the influence of Fe?O and Ti?O bonds on the oxygen vacancy formation and migration in these perovskites, which provide theoretical information for exploring new electrode materials of solid oxide fuel cells. In the final chapter 7, the general conclusions and future prospect are described. The electronic properties and oxygen ionic conductivity of transition metal oxide double perovskites were studied by the first?principles calculation method. For the future study, the activity energy of oxygen evaluation and reduction reactions should be simulated at the surface of electrode materials. Combined with the ions diffusion properties in the bulk of electrodes, the simulation method will provide a comprehensive theoretical guide for the development of perovskite materials.