Demonstrating High Performing Thermophotovoltaic Systems Using Novel Cell-Side Architectures

Abstract

Thermophotovoltaic (TPV) generation is the conversion of thermal radiation to electrical power through the photovoltaic effect. This is a promising approach for various energy applications such as cogeneration of heat and power, thermal energy storage, and waste heat recovery. However, current TPV efficiency levels are insufficient for widespread implementation of these systems. This thesis aimed to overcome this gap by gaining a detailed understanding of energy losses in thin-film TPV cells and establishing scalable fabrication strategies to minimize them. The research focuses on an innovative photovoltaic architecture featuring an active semiconductor layer suspended over wide air cavities. The air-bridge cell maximizes refractive index contrast and minimizes the loss of long-wavelength (out-of-band) photons, thereby increasing conversion efficiency. Each chapter addresses a key challenge related to maximizing photon recovery and utilization in air-bridge TPVs: (Ch. 2) understanding the limitations of air-bridge cells based on InGaAs, (Ch. 3) experimentally demonstrating high efficiency air-bridge cells that circumvents these limitations, (Ch. 4) demonstrating transmissive spectral control as an alternative to reflective control, (Ch. 5) developing a novel approach to realizing tandem TPV cells that overcomes the challenges of conventional approaches.Through this research, significant gains were obtained in understanding energy losses in these devices and in their overall performance and reliability. Detailed optical and electronic simulations identified key losses in heterojunction cells, resulting in device architectures capable of high TPV conversion efficiencies throughout a broad range of emitter temperatures (1000 to 1500 ºC). Notably, an air-bridge cell with a 0.9 eV bandgap achieved nearly 45% efficiency at 1400 ºC emitter temperatures by optimally balancing photon and carrier utilization. Furthermore, novel bifacial devices without back surface reflectors demonstrated efficiencies over 30% at significantly lower emitter temperatures compatible with waste heat streams and nuclear power. Both results represent an 8% absolute efficiency gain compared to prior results in their temperature ranges, largely credited to the near-complete recovery of long-wavelength photons in air-bridge cells. In addition, the research also led to the development of designs and fabrication processes which increased reliability of both single junction and tandem devices. Tandem devices featuring two air-bridge subcells were implemented in a range of semiconductors, demonstrating a way to achieve comparable optical properties to single junction air-bridge cells by avoiding the use of tunnel junctions.Overall, these results suggest the applicability of the air-bridge cells to a range of semiconductor systems suitable for electricity generation from thermal sources found in both consumer and industrial applications. The combination of ultra-high efficiency, low emitter temperatures, and device stability could allow widespread adoption of TPV systems, including their use in long-duration thermal batteries. Thermal batteries equipped with such TPV cells can achieve competitive roundtrip efficiencies with electrochemical approaches, while maintaining their lower costs, which are sufficiently low to enable a fully renewable grid.