Optoelectronic Physics and Engineering of Atomically Thin Photovoltaics

Citation

Wong, Joeson (2022) Optoelectronic Physics and Engineering of Atomically Thin Photovoltaics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pxk0-3d19. https://resolver.caltech.edu/CaltechTHESIS:12132021-004403821

Abstract

Materials that are atomically thin behave substantially different than those of their bulk counterparts. However, when most materials become thinner, their surface-to-volume ratio increases and the number of unpassivated dangling bonds at the surface approaches the number of internal crystalline bonds, which prevents examining the intrinsic properties of most ultrathin materials. The recent discovery of layered materials, whose crystal structures have naturally passivated basal planes, has enabled the possibility to examine materials’ thicknesses that approach a single atomic layer.

In this thesis, we examine and explore the consequence of this new regime of thickness for active layers in photovoltaic applications. Specifically, we focus on the three aspects that define photovoltaic operation and explore their differences in these ultrathin materials: optical absorption of photons, subsequent carrier generation and transport, and finally, free energy extraction of collected carriers. We first discuss the implications of band-edge abruptness on the maximum efficiency of a solar cell. Then, we show that optical absorption in these ultrathin materials is dominated by cavity wave optics, and design structures that enable near-unity absorption in both ultrathin (~10 nm) and atomically-thin (~7 Å) active layers. Using these optical design rules, we design heterostructures with record incident photon to electron conversion efficiency (>50%). Next, we examine new methods of creating electrical junctions by using thickness to vary the amount of band bending in a material. We spatiotemporally image these 'band-bending junctions' for the first time. Finally, we argue that photoluminescence can be used as a direct readout of the open circuit voltage potential, and motivate examination of monolayer materials which have substantially higher radiative efficiency. We therefore examine the strain tuning of photoluminescence properties of both monolayer TMDC and heterobilayer TMDC systems. This work illustrates that van der Waals materials are an ideal system for examining the novel optoelectronic physics of atomically thin photovoltaics.

Thesis Files