Modeling and fabrication of high power density microscale thermophotovoltaic devices

Abstract

There has been renewed interest over the past decade or two in the potential of using thermophotovoltaic (TPV) systems for direct conversion of thermal energy into electricity. The invention of TPV systems can be traced to almost 50 years ago, but systems developed over the following 2-3 decades lacked in efficiency and power density. Later on, the development of III-V materials with low bandgaps promised higher efficiency and power density, and renewed the research interest in TPV. More ideas emerged recently, including the use of two surfaces separated by nanometer scale spacing to exceed the vacuum blackbody limit, potentially realizing TPV systems with greater power density. A new microscale TPV device structure made of fin-shaped interdigitized emitters and photovoltaic cells offers an increase in the surface area available for radiative energy transfer, which could boost the output power density. In addition, if the spacing between the radiator and converter is of the order of a micron, tunneling of energy could occur, which can improve the power density. A periodic structure can recoup sub-bandgap radiation, since such radiation will pass through the converter and will be absorbed in the radiator, thereby improving efficiency of the device. Some of the above-bandgap photons that pass through the converter could also be recycled between emitters and photovoltaic cells. This thesis presents work done on modeling and fabricating an interdigitized microscale TPV device. The advantage in power density that can be obtained from an interdigitized TPV structure is estimated first by a bulk emissivity and view-factor based model, followed by a thin-film emissivity model. Calculations performed to estimate parameters such as power output, power input

(cont.) and conduction heat leakage into the substrate from the radiator, which were crucial to the design ofthe devices, are presented. Crucial steps carried out in the clean room at MIT's Microsystem Technology Labs (MTL) with the objective of microfabricating a set of prototype devices are described. Challenges faced when fabricating the device, and techniques used to address several fabrication and experimental issues are presented. The modeling and fabrication issues addressed in this thesis point towards challenges in the area of (i) modeling nanometer scale radiative heat transfer and (ii) fabricating thermophotovoltaic devices with interdigitized geometry and micron/sub-micron spacing between radiator and converter.