Simulation and Optimization of Nanowire-Based Betavoltaic Generators

Abstract

In order to increase the efficiency of betavoltaic devices, an architecture utilizing nanowires has been developed. In this architecture, a radioisotope is deposited between a nanowire array in order to increase the fraction of beta particles captured by the semiconductor converter and minimize the energy lost to self-shielding. Previous work has prototyped such a design; however, performance was limited to an efficiency of 0.5%. This thesis outlines the design and optimization of the nanowire-based betavoltaic generator. Both the nanowire array geometry and the nanowire p-i-n diode design are optimized for maximum radiation capture and conversion efficiency, respectively. First, a model was developed in the GEANT4 Monte Carlo toolkit in order to investigate the radiation capture of various array geometries. Radioisotope sources of elemental 3H, 63Ni, and 147Pm, as well as compounds of each were examined with gallium phosphide nanowires. Overall, it was found that nanowires should be grown as long as possible to accommodate the most source material while the ratio of the diameter to array pitch can be optimized for maximum power capture. Optimized arrays presented an improvement in energy capture of approximately 6 and 15 times for 63Ni and 3H devices, respectively, while 147Pm devices indicated no improvement. Optimized array geometry was extended to both silicon and gallium arsenide and the radiation capture simulations were coupled to drift-diffusion calculations in COMSOL Multiphysics for axial junction nanowires. Following the junction optimization, devices were predicted to be between 4 and 10% efficient with power outputs ranging from 2 to 6 μW cm^-2. Despite the large improvement compared to experimental results, surface recombination was found to limit the performance of long gallium phosphide nanowires. Therefore, core-shell junctions were then investigated and found to improve upon all axial designs. Overall, it has been determined that the nanowire device design is advantageous over planar betavoltaics due to the mitigation of self-shielding effects. Devices utilizing 10 μm long gallium phosphide core-shell nanowires with a 3H source are predicted to achieve the top performance of 12% effciency and a power density of 7 μW cm^-2. In addition, gallium phosphide and gallium arsenide devices with 63Ni are able to achieve an energy density in excess of 1 Wh cm^-2 due to the long half-life.