Derivation of Critical Parameters of Betavoltaics

Abstract

Betavoltaic cells are nuclear batteries ideal for low-power applications for extended periods of time without maintenance or replacement. Betavoltaics function similarly to photovoltaic (solar) cells where instead of using sunlight, beta particles are used to generate electron-hole pairs within a semiconductor p-n junction to generate current. Even though there have been multiple demonstrations, betavoltaic performance has not been extensively studied. To accurately predict betavoltaic performance, which is important for a device in operation without maintenance for elongated periods, all parameters are required to predict potential fluctuations in cell performance, such as doping densities and resistances for semiconductor variation and absorption coefficients for beta-generated current. However, not all parameters are easily measured, especially when the p-n junction is constantly under irradiation and cannot be separated from the source. Critical parameters were characterized experimentally with the betavoltaic cell by performing capacitance-voltage to determine doping densities and performing current-voltage characterization tests to determine resistances on multiple NanoTritium™ cells, while absorption coefficients were determined from MCNP6 simulations. Experiments indicated that series resistance Rs was 1 × 106 Ω, while shunt resistance Rsh was 2 × 108 Ω from I-V characterization, while doping density ND was determined to be 1 × 1017 cm−3 from C-V characterization. Absorption coefficient α was found to vary with semiconductor material and incoming beta energy and used in conjunction with critical parameters from experimentation to accurately model betavoltaic cell performance similar to experimental results. Both implicit equations and explicit estimations were compared to model betavoltaic cell performance.