Hybrid halide perovskites are currently the most studied optoelectronic materials. They have been successfully employed as the active material in solar cells. Despite the achieved success of these materials, the properties of these hybrid frameworks of an inorganic lattice that includes organic cations are not fully understood. This is because of the multiple complex processes that are operative in these materials and it is very hard to unravel them just on basis of experiments. Therefore, computational studies of these materials are important to gain insight in the material structure, the electronic structure and the processes dictated by these properties. An additional advantage of computational studies is that properties can be predicted without actually making the materials in the lab. Such computational study thus give insights in the functioning of hybrid perovskite materials and gives directions to their further development. Of particular interest in this thesis is the role of the organic cation. In some earlier studies it has been pointed out that he role and presence of the organic cations is just limited to stabilizing the structure of hybrid perovskites without influencing the electronic energy states. In this thesis we examine the role of the organic cation in detail, demonstrating that the organic cation has a distinct effect on the electronic structure of hybrid halide perovskites. ... Read more

This doctoral thesis reports studies on effects of temperature and density on the
mobility and recombination dynamics of charge carriers in low-dimensional van
der Waals materials. ... Read more

In this thesis, we have investigated the optoelectronic properties of metal halide perovskites with a special focus on their application in solar cells. In less than a decade of development, metal halide perovskites have yielded solar cells with efficiencies comparable to commercialized technologies. However, there has been limited knowledge about the fundamental properties of these materials. As mentioned in the introduction, the efficiency of perovskite-based solar cells is still not at its theoretical limit. In order to rationally design solar cells with maximized efficiencies, we need to understand which factors are currently limiting the performance of perovskite-based solar cells. In general, one of the first important processes in a solar cell is the absorption of light. For metal halide perovskites based on lead iodide, a thickness of 0.3 micrometer is already sufficient to absorb a substantial amount of visible (sun-)light, which makes these materials very suitable for solar cells. Furthermore, it is crucial that this absorbed light is converted into a current of moving charges, also known as electricity. Semiconductor materials such as silicon or metal halide perovskites have the ideal properties to generate a current of charges from light. In order to use this current however, the charges need to be collected. The efficiency with which charges are collected in a solar cell is closely related to its power conversion efficiency. ... Read more

In semiconductor quantum dots (QDs), charge carrier cooling is in direct competition with carrier multiplication (CM), a process in which one absorbed photon excites two or more electrons that may improve the light conversion efficiency of photovoltaic devices. CM by an initially hot charge carrier occurs in competition with cooling, with the respective rates determining the CM efficiency. Until now, the factors that determine the onset energy and efficiency of CM have not been convincingly explained. Most research on cooling involves low photoexcitation energies close to the band gap, while the competition between CM and cooling takes place at higher energies where an electron or hole has an excess energy that is at least equal to the band gap. Moreover, CM rates have only been calculated theoretically, while experimental studies of CM have focused mostly on proving its occurrence in various materials. Understanding charge carrier cooling at high excess energy and comparing this to experimental CM rates is therefore of great interest. Chapters 2 and 3 of this thesis are aimed at understanding charge carrier cooling, while Chapters 4 and 5 relate this to the onset energy and efficiency of CM. The presented results are a large step forward in understanding cooling and CM and allow for a screening of materials with an onset of CM close to twice the band gap energy. Such materials are of great interest for development of highly efficient photovoltaic devices. ... Read more