UC Berkeley

In this dissertation, I discuss the electronic and optical properties of materials for energy-related applications, with a focus on silicon (Si), gallium arsenide (GaAs) and the transition metal dichalcogenides (TMDs) in particular.

Si is the most widely used photovoltaic material today, dominating the photovoltaic industry by more than 90%. This is because Si is non-toxic, abundant and benefits from technologies developed over the years in the microelectronics industry. Despite its widespread usage as a photovoltaic material, Si does not efficiently absorb most of the light in the solar spectrum because it has an indirect bandgap. On the other hand, GaAs has a direct bandgap that is optimal for solar energy conversion, even though it is more expensive than Si and is also toxic due to the presence of arsenide. Nonetheless, in the atomically-thin film limit, both cannot be compared with the TMDs, absorbing significantly less light than the TMDs in the solar spectrum. This optical property of the TMDs makes them a very appealing class of candidate materials for flexible ultra-thin solar cells.

In Chapter 1, I give an overview of the different approaches that we use to address different problems in this dissertation. In Chapter 2, I discuss our work that aims to understand how the structure of an alternative Si phase can lead to an improved calculated absorption relative to diamond-Si. In Chapter 3, I discuss how we can use first-principles to calculate the hot carrier dynamics in GaAs, such as by calculating its electron-phonon relaxation times. In the Chapters 4, 5 and 6, I discuss the electronic and optical properties of mono- to few-layer TMDs. Not only are their quasiparticle bandgap and exciton energy levels affected by dielectric screening due to substrates, the dielectric screening environment can also be modified to engineer an intrinsic lateral heterojunction within a homogeneous TMD monolayer. In Chapter 7, we report the first observation and control of the Berry-phase induced splitting of the 2p-exciton states in monolayer MoSe2, and in Chapter 8, we study that the dynamics of atomically-sharp lateral heterojunctions between differently-stacked TMD domains.