Analysis of a photon-enhanced thermionic emission (PETE) solar converter with a GaInP based cathode

Conversion of solar radiation to electricity with Photon-Enhanced Thermionic Emission (PETE) uses both photonic and thermal excitation of charge carriers in a semiconductor. This device includes a semiconductor cathode (which can be composed of several semiconductor materials) illuminated and heated by concentrated sunlight, and a cooler anode, separated by a small vacuum gap. First theoretical studies of this device have predicted that under concentrated solar radiation, the conversion efficiency can reach up to 45%, however all those studies did not take into account actual properties of materials, more complicated model geometries and other important effects. In this thesis, a GaInP based cathode is investigated in a scenario of a complete PETE device, where the actual properties of this III-V semiconductor are taken into account. The Negative Space Charge (NSC) effect, is also incorporated in this work, as well as the inherent dependency of the cathode temperature with the energy balance of the entire device. This investigation includes modification of parameters related to the cathode and the anode, such as the electron affinity (cS) on the cathode emission surface, the anode work function (fA), the surface recombination velocity (SRV) on the cathode emission surface and the vacuum spacing between the cathode and the anode. Each of these parameters was investigated, to determine the best, nominal and worst case operation conditions of the PETE converter. In addition, the converter efficiency was investigated with respect to device properties, such as the addition (or lack of) an IR coupling element and the addition (or lack of ) a spacer between the cathode to the anode that induces heat but is electronically isolated. This thesis also includes basic research conducted on GaInP samples that investigates the dependency of minority carriers lifetime with respect to temperature, for the first time above room temperature. Experimental data was compared with simulations to extract this dependency. By using the simulative tools, it has been enabled to explain the mechanism of carriers escape velocity by rigorous mathematical means. In the last section of the performance of a complete converter, a comparison between measurements done on those GaInP based cathodes and the simulation is shown. Results of the complete converter performance analysis show that best conversion efficiency is 22%, obtained at Sun concentration of x1000, where an IR coupling element is introduced in the system, no spacer is included and the cathode temperature is predicted to reach as high as 1500 K, with the vacuum gap is narrowed to 500 nm, in order to eliminate NSC effects. Another