Thesis Title: Efficiency Limit of Far- and Near-field Thermophotovoltaic Energy Conversion

Ever increasing energy demand worldwide coupled with environmental concern forces the current fossil-fuel based energy paradigm to be shifted to alternative or renewable energy sources. Heat energy is such an alternative potential energy source that is abundant in industries as a form of waste heat, which is approximately 20-50% of consumed energy. Utilizing this huge waste energy can lead to a sustainable, efficient, and clean energy production system. For this purpose, solid-state devices are most suitable. Thermophotovoltaic energy conversion (TPV) is a solid-state technology of converting thermal radiation from hot emitter directly into electricity without any moving parts. It has widespread applications in the defense, space, energy, and microelectronics because it is compact, lightweight, and robust. Despite their prospects, TPV devices have not been widely used only due to their poor efficiency. The poor efficiency of TPV device stems from two factors: spectral mismatch between emitter and PV cell, and high dark current in low bandgap PV cells. Additionally, parasitic absorption in the phonon band also reduces efficiency. In other words, the emission spectrum from the emitter holds the key for high-efficiency TPV system. Hence, spectrally selective thermal emission or enhanced near-field radiative heat transfer allows TPV cells to operate at efficiencies much higher than the current record of about 7 %. Here in this thesis, we optimize the design parameters of the thermal emitter together with the PV cell to find out the desired optoelectronic characteristics of a TPV device for any given operating temperature. We also find out the ultimate TPV conversion efficiency possible in real systems operating in the far and near-field configurations. Our analysis shows that sub-bandgap emission suppression and bandgap emission enhancement are the key parameters for high-efficiency operation. High suppression of undesired photons, even with moderate emission in the desired band, is more important for high efficiency. Our study shows that suppression of at least 20 dB and enhancement of at least 100 is necessary for achieving 60 % of Carnot efficiency at 1300 K. Using realistic properties of materials that make emitters, we show that Mo and W are good choices for thermal emitters. Our design framework should serve as a practical design guideline for the development of high-performance TPV system.