In the search for nano-scale, highly customizable materials for next-generation electronic devices, two-dimensional (2D) materials have generated much interest. 2D materials have complex, layer-dependent optical and electronic properties of which many aspects remain yet to be explored and fully understood. The aim of this thesis was to investigate and explain optoelectronic properties of several 2D materials systems towards device design. This was accomplished using predictive physical modelling at the density functional theory level (DFT) as well as many-body theory (GW+BSE). The optical transitions of bulk ReS2 and ReSe2 were studied using DFT in comparison with experiment. We found that the orbital composition of the band edges determined the sign of the pressure coefficient of the optical gap. Our results provide a step towards understanding the perceived layer-independence of the optical properties of ReS2 and ReSe2. The exciton landscape of MoS2 monolayer was explored in detail using many-body theory (GW+BSE). We found dark excitons very close to bright excitons and even lower in energy. Our results help reverse the common assumption that the lowest-energy exciton in MoS2 is bright. The ideal band offset between recently predicted monolayers of the CaP3 family was predicted using GW theory. We observed chemical trends in the band offsets and explained their origin. Our results serve as indicators for heterojunction design with these novel materials. The effective mass of a test set of eighteen semiconductors including several 2D materials was calculated using DFT with semi-local and non-local hybrid exchange-functionals and compared for accuracy with respect to experimental data. Our analysis details the effect of the nonlocal exchange potential on the accuracy of the effective mass. Our results give guidelines for high-throughput calculations of the effective mass for different material classes, including 2D materials. / Thesis / Doctor of Philosophy (PhD)

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京都大学 / 新制・課程博士 / 博士(工学) / 甲第23918号 / 工博第5005号 / 新制||工||1781(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 佐藤 啓文, 教授 作花 哲夫, 教授 佐藤 徹 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

<p>Approximating the exchange-correlation energy in density functional theory (DFT) is a crucial task. As the only missing element in the Kohn-Sham DFT, the search for better exchange-correlation functionals has been an active field of research for fifty years. Many models and approximations are known and they can be summarized in what is known as the Jacob’s ladder. All the functionals in that ladder are local in the sense that they rely on the information of only one electronic coordinate. That is, even though the exchange-correlation hole, the cornerstone in density functional theory, is a two-electron coordinate quantity, one of the coordinates is averaged over in “Jacob’s ladder functionals.” This makes the calculations considerably more efficient. On the other hand, some of the important constraints on the form of the exchange-correlation functional become inaccessible in the one-point forms. The violation of these constraints leads to functionals plagued by systematic errors, leading to qualitatively incorrect descriptions of some chemical and physical processes.</p> <p>In this thesis the idea of a weighted density approximation (WDA) is explored. More specifically, a symmetric and normalized two-point functional is proposed for the exchange-correlation energy functional. The functional is based entirely on the hole for the uniform electron gas. By construction, these functionals fulfill two of the most important constraints: the normalization of the exchange-correlation hole and the uniform electron gas limit. The findings suggest that we should pursue a whole new generation of “new Jacob’s ladder” functionals.</p> <p>A further step was considered. Given the relevance of the long-range behavior of the exchange-correlation hole, a study of the electronic direct correlation function was performed. The idea was to build up the long-range character of the hole as convoluted pieces of the simple and short-ranged direct correlation function. This direct correlation function provides better results, at least for the correlation energy in the spin-polarized uniform electron gas.</p> <p>The advantage of one-point functionals is their computational efficiency. We therefore attempted to develop new methods that mitigate the relative computational inefficiency of two-point functionals. This led to new methods for evaluating the six-dimensional integrals that are inherent to the exchange-correlation energy.</p> / Doctor of Philosophy (PhD)

It has been 10 years since the first hybrid halide perovskite photovoltaics was fabricated. Power conversion efficiency increases from the initial 3.8% to the current 25.2%. Fabrication method envolves from spin-coating to printable technology, and we deeply experience the drastic development of hybrid halide perovskite photovoltaics. Although hybrid halide photovoltaics render a variety of advantages over traditional photovoltaics, we still cannot find any practical application of these hybrid halide photovoltaics. There exist a few issues which hinder the commercialization of this type of solar cell. Among these issues, the long-term instability of hybrid halide perovskite is the main concern for the next development. This thesis expands on investigating the instability of hybrid halide perovskites from first principles. In Chapter 1, two computational methods employed in the thesis: density functional theory and Ab initio molecular dynamics are introduced. Theoretical investigations of the instability of CH3NH3PbI3 using density functional theory method are mainly conducted at 0 K. The finite temperature effect on this instability of CH3NH3PbI3 is usually neglected. In Chapter 2 of this thesis, we combined density functional calculations and additional thermodynamic data to explain the intrinsic instability of CH3NH3PbI3 under finite temperature conditions. We also analyzed the stability under humid conditions. It is shown that the aqueous solubilities of reactants play an important role in the products’ stabilities. The Born–Haber cycle of NaCl splits the enthalpy change into several components which will give a better understanding of the origin of the corresponding enthalpy change. In Chapter 3, with the extension of the Born–Haber cycle to the hybrid halide perovskites, the reaction enthalpies which govern the intrinsic instability of ionic compounds were analyzed. We proposed a criterion that helps to filter the hybrid halide perovskites with improved stability aimed for photovoltaics. Since the instability of CH3NH3PbI3 is intrinsic. The long-term instability can be settled by discovering alternative perovskite absorber. In Chapter 4, based on literature research, we propose a three-membered ring cation which has a suitable size to fit into the Pb-I framework, leading to optimal band gap for photovoltaics. Besides, the cation has a good ionization energy which will potentially render better stability. Whereas, a comprehensive study of this cyclic ring based perovskite indicates that the instability of the three-membered ring cation will make it impossible to synthesize this theoretical structure. Moisture degradation mechanisms of CH3NH3PbI3 are investigated intensively. More importantly, for practical photovoltaics, we have to imagine different situations the modules will encounter, e.g. after a couple of years, cracks appearing on the modules are inevitable, at this stage, understanding of the degradation mechanism of CH3NH3PbI3 according to liquid water becomes important. Chapter 6 elaborately describes a comprehensive degradation mechanism of CH3NH3PbI3 under liquid water. We investigate the energy barrier for the first dissolution event of CH3NH3PbI3 in water. Furthermore, thermodynamic analyses of CH3NH3PbI3 dissolution in water clearly explain the spontaneity of CH3NH3PbI3 degradation in water. Besides, different mechanisms of CH3NH3PbI3 and CsPbI3 dissolution in water are discussed. / Dissertation / Doctor of Philosophy (PhD)