Accurate Density-Functional Theory for Solid-State Materials

To explore and develop functional materials is of paramount scientific importance and a steady generator for novel technological innovations. The successful realization of the latter, however, critically depends on a detailed microscopic picture of the processes that dictate the properties of materials. Triggered by this demand, first-principles based quantum-mechanical simulations have already evolved to a key discipline in modern materials science. As such, they support and deepen the understanding of already known materials, and lead to the discovery of hitherto unknown ones. Indeed, the success of materials prediction with first-principles calculations strongly depends on the reliability of the underlying theoretical framework, which underscores the need for accurate and efficient atomistic theories. In the present project, density-functional theory (DFT) will be further developed to establish a reliable theoretical description of rather complex solid-state materials, namely semiconducting transition-metal oxides (TMOs) and organic-inorganic halide perovskites (OIHPs). Both these crystalline materials are highly interesting in the context of several different emerging technologies such as photovoltaics, heterogeneous catalysis, and inorganic-organic devices. Yet, their theoretical description poses a number of serious challenges to modern electronic-structure theory, which is why they also stand out as prototypical test cases for advanced simulation techniques. Most notably, when calculating the electronic structure of these solids in the framework of approximate DFT, a severe underestimation of the band gap, as well as an erroneous description of localized electronic states are typically encountered with common DFT functionals. Within this project, the main goal is to substantially improve the theoretical description of TMOs and OIHPs with novel DFT approaches. To achieve this, I propose further development of optimally-tuned range-separated hybrid (OT-RSH) DFT functionals, which as hybrid DFT functionals include a fraction of Fock-exchange. However, contrary to conventional hybrid functionals, in the OT-RSH scheme parameters in the exchange-correlation part of the DFT functional are tuned per system, such that certain exact first-principles conditions are fulfilled. For finite systems such as molecules, as well as for weakly-interacting molecular crystals, OT-RSH functionals were already shown to be remarkably accurate and efficient in calculating electronic properties. Within this project, I will develop this promising DFT approach one step further to the realm of solid-state materials, in which the cohesion arises from stronger interactions and where electronic states in the valence region suffer from pronounced differences in degree of localization. From the results of this project I expect a profound step forward in reliable and efficient atomistic calculations for complex crystalline materials. Additionally, I strongly believe that our studies will reveal several highly relevant insights regarding the electronic structure of TMOs and OIHPs, which will contribute to the continuously growing microscopic understanding of these fascinating solid-state materials.

The ever increasing demand for energy of modern human societies makes it absolutely necessary to explore new ways to generate and store energy, at best using environmentally friendly technologies. Photovoltaic devices can convert solar energy to electricity, and the main factor for the success of this is the device efficiency and cost. Therefore, it is a major goal of world-wide scientific efforts to find new materials which can turn this vision into a reality. In this project our goal was to understand, describe, and improve new functional materials, particularly in the context of photovoltaic applications. Recent years saw the advent of a new material, so called hybrid organic-inorganic perovskites, which has revolutionized this scientific discipline with outstanding device efficiencies. One of the major advantages of these materials is that they can be manufactured cheaply and easily, therefore possibly enabling much more useful new photovoltaic applications. However, it is not well-understood why these materials perform so well. Our goal was to contribute to the basic understanding of hybrid perovskites, which may help to improve their efficiencies even more and to find new materials that follow similar design principles. To this end, we performed advanced computer simulations that allow for characterizing these systems with high precision. Using these techniques, we have predicted certain phenomena in these materials which were new to the community at that point in time. We also collaborated with a number of experimental groups to explain uncommon physical phenomena in these materials from a joint experimental-theory perspective. Our perhaps most interesting findings concerned the charge transport in hybrid perovskites and how it might be affected by significant ionic motion and structural dynamics. Concomitantly with our efforts in the context of photovoltaic materials, we also concentrated on improving the methodology of our computer simulations. In this area, we focused on interfaces between semiconducting materials and metals, which are an integral part of photovoltaic and many other types of devices. Our goal was to improve one of the most common theoretical approaches for describing such systems on the quantum level, which is called density functional theory. For interfaces, we have developed new efficient models that allow for describing these systems more accurately, and for predicting highly-relevant physical observables entirely from theory.