Sculpting the electromagnetic spectrum for enhanced renewable energy generation [electronic resource]

In two hours, the sun delivers enough energy to the earth to meet global needs for an entire year. But there's a catch: sunlight is distributed over a broad spectrum of frequencies. This poses a significant challenge for harvesting solar energy because most photovoltaic devices operate with high efficiency only over a narrow band of frequencies. As a result, high-energy sunlight is inefficiently used while low-energy sunlight isn't used at all! Indeed, this 'spectral mismatch' problem is the primary reason single-junction photovoltaic cells are fundamentally limited to efficiencies around 33%. In this thesis, I will describe ways to sculpt the electromagnetic spectrum so that we can make better use of traditionally wasted frequencies of light. First, I'll describe our model of spectral upconverters, materials that absorb unused low-energy photons and re-radiate high-energy photons back to a solar cell. Using a detailed balance analysis, these materials have been shown to increase maximum single-junction solar cell efficiencies from 33% to over 44%. We show that existing upconverters, which are limited by low efficiencies and narrow absorption bandwidths, can yield only marginal (< 0.5%) increases in cell efficiency. However, if the upconversion efficiency of these materials is improved, they could increase cell efficiencies by a few absolute percent, potentially enabling single-junction solar cells with efficiencies exceeding 30%. To this end, I'll describe the first experimental realization of a hot-carrier-mediated upconverter, capable of enabling high-efficiency solar upconversion in a tunable, solid-state device. Then, I'll shift to thermophotovoltaics, a technology that can harvest both excessively low- and high-energy solar photons, as well as capture waste heat from combustion and other existing high-temperature processes. Thermophotovoltaic systems rely on thermal emitter materials that need to retain good optical properties while operating at high temperatures, sometimes in excess of 1200 degrees C. However, the optical properties of most candidate emitter materials have not even been studied in this temperature range. To address this, I'll present work on the scalable fabrication of a promising refractory material: titanium nitride. Using a modified CMOS-industry-standard plasma-enhanced atomic layer deposition process, we demonstrate titanium nitride films and nanoparticles with a strong plasmonic response in the visible and near-infrared. Finally, I'll discuss how we use our custom-built ellipsometry setup to perform in-situ measurements of the temperature-dependent permittivity of titanium nitride, from 21 degrees C up to over 1200 degrees C. I'll describe how the optical permittivity, crystal structure, chemical composition, and surface morphology respond to these extreme conditions, demonstrating a highly robust thermal emitter material that may provide a foundation for next-generation renewable energy technologies.

Keywords: Applied Physics.
Advisor: Brongersma Mark L. primary advisor. Thesis advisor
Dionne Jennifer Anne primary advisor. Thesis advisor
Melosh Nicholas A. advisor. Thesis advisor