ABSTRACT
Clean and renewable energy sources are under increasing demand due to the environmental issues and unsustainability of fossil fuels. Solar energy and hydrogen energy are two of the most promising renewable energy sources which have demonstrated great potential to meet the global energy consumption demand. Organic solar cells (OSCs), being used to convert solar energy into electricity, have achieved great success in the past several years. Noncovalent intramolecular interactions have been often used for conformational control to enhance the planarity of polymers or molecules, which may reduce band gaps and promote charge transfer. However, it is not known if noncovalent interactions may alter the electronic properties of conjugated polymers through some mechanism other than the conformational control. Y6-based nonfullerene OSCs have achieved an outstanding power conversion efficiency PCE of over 18% due to the low energy loss and efficient exciton dissociation efficiency with a small energy offset. However, it is still unclear why small energy offset can result in sufficient exciton dissociation in nonfullerene systems but will lead to overwhelming charge recombination in fullerene systems. On the other hand, hydrogen energy has been known as one of the cleanest energy sources due to the fact that the combustion of hydrogen gas only produces water without any carbon emission. Hydrogen gas can be generated through hydrogen evolution reaction (HER) via water splitting, which requires catalysts to facilitate the reaction rate. Recent studies have shown that topological quantum materials can work as efficient electrocatalysts for HER. However, the mechanism underlying the high performance and the role of topological surface states originated from the topological quantum material catalysts remain unclear. In this thesis, we applied first-principles methods, including static DFT methods and non-adiabatic Ehrenfest molecular dynamic simulations, to study the mechanisms in OSC materials and topological quantum materials for renewable energy sources applications. Based on our work, we would like to suggest new mechanisms or promising directions to assist in the design of high-performance OSC materials and cost-effective HER electrocatalysts.