DOI: 10.7274/25541080
title: From Photophysics to Photocatalysis: Insights into Ternary Semiconductor Nanocrystals

description: Reliance on fossil fuels such as oil, gas, and coal for energy, chemical and pharmaceutical applications, contributes to their widespread use worldwide. However, their contribution to climate change due to the emission of greenhouse gases has become an increasing concern. Considering this reality, the search for clean and renewable energy sources as an alternative to unsustainable fossil fuels continues to gain traction across multidisciplinary research fields. Among the various energy sources (solar, wind, geothermal, hydropower, etc.), solar energy stands out as an inexhaustible and clean option that is widely accessible in most parts of the world.
Silicon-based solar cells are, to date, used to harvest sunlight in the quest to generate renewable energy. However, silicon is not only bulky but also an expensive light-harvesting assembly (photovoltaic cell). This downside of silicon-based solar cells has been the basis for research into newer technologies that are cheaper and easier to process. I-III-VI quantum dots and perovskites are some of the new multicomponent semiconductor materials proposed to replace silicon for light harvesting.
I-III-VI semiconductor systems have bandgaps matching well to our solar spectrum. This feature along with composition-based bandgap tunability and high absorption coefficients across the visible-near-IR spectrum gives them a niche in ongoing research. Unlike their II-VI and III-V binary counterparts, the photophysical properties of I-III-VI semiconductors are complex and peculiar. They are characterized by defect-related states within the bandgap that participate in both light absorption and recombination processes. These sub-bandgap states act as sites for trapping charge carriers hence hampering the overall charge conversion efficiencies. We employed ultrafast measurements to study these charge-trapping events and the origin of the peculiar properties associated with this class of materials. When photoexcited, the electrons and holes generated relax to the trap states at a similar timescale to that of electron transfers. Despite this drawback, further studies employing a suitable redox couple (viologen/thionine) demonstrated that we could extract these trapped charges (electrons). We further show that the use of an inorganic thin shell to encapsulate the I-III-VI cores can serve to not only passivate surface-related defects but also stabilize the electron transfer product. 
Similar approaches as discussed above were applied to perovskite material to enhance its stability and photocatalytic abilities. Perovskites present exceptional properties including tunable bandgaps, long charge diffusion length, high defect tolerance, and high carrier mobility. However, the ionic nature of perovskites has limited their use to only a few nonpolar solvents. By employing inorganic and polymer shells we probed the ultrafast electron transfer events in the presence of polar solvents. The perovskite-viologen hybrid assembly demonstrated an impeccable long-lived charge-separated state that is crucial for any light-harvesting assembly used in photovoltaics or photocatalysis. These and other interesting fundamental insights presented in this dissertation position ternary semiconductors as light-harvesting materials for a brighter, more sustainable future.