Download or read book Photon-enhanced Thermionic Emission for Concentrated Solar Energy Harvesting written by Jared William Schwede and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Conventional conversion of sunlight into electricity usually takes one of two forms: a "quantum" approach using the large energy of solar photons in photovoltaic (PV) cells, or a "thermal" approach using solar radiation as the heat source in a classical heat engine. Quantum processes boast high theoretical efficiencies as the effective solar photon temperature is ~6000 K, yet suffer in practice from a limited spectral energy collection window and thermalization losses. Thermal processes take advantage of energy throughout the entire spectrum, but efficiency is curbed by practical operating temperatures. Combinations of the two are predicted to have efficiencies above 60%, yet are difficult in practice because PV cells rapidly lose efficiency at elevated temperatures, while heat engines rapidly lose efficiency at low temperatures. As a result, these two approaches have remained disjointed. In this work, I describe a novel method of solar energy conversion called Photon-Enhanced Thermionic Emission (PETE), which uses photoexcitation in a hot semiconductor in conjunction with thermionic emission to generate electricity. By combining this quantum and thermal process, devices based on PETE can in principle exceed the Shockley-Queisser limit on single-junction photovoltaics, and furthermore can potentially operate at temperatures compatible with solar thermal conversion systems, enabling a two-stage cycle with very high theoretical efficiency. After explaining the theoretical potential of PETE devices, I describe the experimental demonstration of the PETE effect. However, these proof-of-concept measurements are seen to display very low performance, quite disconnected from our idealized theoretical modeling. I therefore introduce more detailed analytic models for the PETE process which include carrier diffusion and non-ideal recombination. Using these models, I describe a GaAs/AlGaAs heterostructure which improves efficiency by introducing an internal interface, decoupling the basic physics of PETE from the vacuum emission process and protecting photoexcited electrons from the vacuum interface. The heterostructure is shown to dramatically improve performance, and with further work could form a realistic basis for practical PETE devices.