Towards a Net-zero Carbon Energy System: High Efficiency Photovoltaics and Electrocatalysts

Citation

Omelchenko, Stefan Thomas (2019) Towards a Net-zero Carbon Energy System: High Efficiency Photovoltaics and Electrocatalysts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/B6AS-EA22. https://resolver.caltech.edu/CaltechTHESIS:06022019-111830413

Abstract

Modern society is dependent on energy. Despite increases in energy efficiency, human development and economic goals are expected to increase the global demand for energy by almost 30% in the next 20 years. At the same time, anthropogenic carbon dioxide emissions must approach zero to stabilize global temperatures below the 2°C target set out by international climate agreements. Realizing a net-zero carbon energy system will depend on the development of a highly reliable, sustainable electricity grid to power society and the ability to produce chemicals and fuels in a carbon-free manner. Developing cheap, efficient solar photovoltaics and highly active and selective electrocatalysts is thus pivotal to achieving this goal.

In this work, we address issues limiting photovoltaics and electrocatalysts. Our work on photovoltaics analyzes two effects often neglected in the evaluation of efficiency limits for photovoltaic materials. We show that the shape of the band tail and, in particular, the extent of sub-gap absorption, controls the open-circuit voltage, emission, and ultimately the achievable efficiency of a solar cell. These findings are generalizable to any luminescent material and our analysis suggests that efficiency limits for a material can be determined through simple experimental characterization. In addition, we develop a device physics model which accounts for the presence of excitons, which are the fundamental excitation in a host of emerging photovoltaic materials. A case study in cuprous oxide shows that excitonic effects can play a large role in the device physics of materials with large exciton binding energies and that standard models can drastically underestimate the efficiency limits in these systems. Our work on photovoltaics, culminates in the realization of a novel device architecture for tandem silicon/perovskite solar cells that opens the possibility of achieving efficiencies >30%. Finally, we develop a method to tune the catalytic activity of electrocatalysts for the oxygen-evolution and chlorine-evolution reactions. Our method is based on group electronegativity and is likely generalizable to other reactions and catalysts. The analyses and technologies developed herein are promising steps towards a zero-carbon energy system.

Thesis Files