Advanced Thermophotovoltaic Cells Modeling, Optimized for Use in Radioisotope Thermoelectric Generators (RTGs) for Mars and Deep Space Missions

Abstract

Abstract : To accommodate the need for power on deep space missions, satellites must carry a source capable of providing adequate power for the life of the mission. This is currently done using radioisotope thermoelectric generators (RTGs). Current RTGs consist of a heat source, Pu-238, and a thermocouple that inefficiently converts heat into electricity. To improve the overall efficiency of RTGs, a better thermoelectric converter is needed to replace the thermocouple. This thesis examines the possible use of thermophotovoltaic (TPV) cells in RTGs. Two cells were developed and optimized for the spectrum from a 1300K blackbody that simulates the spectrum of an RTG heat source. Current TPV cells are built and tested with different thicknesses and doping levels to the most efficient design, an expensive and time-consuming method. This thesis presents a model that can predict the output of a cell under various spectra. The model can easily be changed to simulate different cell thicknesses and doping levels, and it can be changed to simulate cells of different materials. The models presented in this thesis were built using the Silvaco Virtual Wafer Fabrication software package. To prove the capabilities of this software package in designing TPV models, an initial cell was designed using a well-documented material: gallium arsenide (GaAs). Voltage-current characteristics and frequency response data were recorded from this model and compared with experimental data from a similar cell. Once the model was verified, more exotic materials could be examined: gallium antimonide (GaSb) and indium gallium arsenide (InGaAs). As with the GaAs cell model, the GaSb and InGaAs models were first optimized for the AM0 spectrum and compared to experimental results. Programs were written using Matlab to run iterations of Silvaco to change layer thicknesses and doping levels. Other programs also were used to help determine the most efficient TPV cell design. (41 figures, 48 refs.)