Proton radiation effects on space solar cell structures and materials

Taylor, Paul Alan (1996) Proton radiation effects on space solar cell structures and materials. University of Southampton, Doctoral Thesis.

Abstract

A comprehensive performance study of the effects of proton irradiation on production Si, GaAs and GaAs/Ge space solar cells and the effects of increasing isotropic proton irradiation at various energies was undertaken. Current-voltage techniques were used to record the variation of maximum power, efficiency, short circuit current and open circuit voltage with increasing fluence. In addition, capacitance-voltage profiling was used to observe majority carrier levels, and dark I-V techniques were also investigated. Absolute spectral response measurements were performed on the cells whilst photoconductive minority carrier lifetime measurements were found to be appropriate only for Si cells. The effect of temperature on cell performance was also examined. Extensive graphs of comparisons of performance data with increasing irradiation at several energies and temperatures have been presented; the superiority of GaAs over Si under proton irradiation was demonstrated, and the superiority of ASEC GaAs/Ge overall after a total fluence of 1e12 protons/cm² observed. The greater destructive nature of low energy proton irradiation was also shown.

Further to this, more focused investigations were undertaken to examine the ASEC GaAs/GaAs and GaAs/Ge cell performances more closely. The cells differed only in the thicker base layer of the GaAs/Ge and the nature of the substrate. Measurements of majority carrier levels determined that no change occurred in either cell at 1e12 protons/cm², whilst Electron Beam Induced Current minority carrier lifetime measurement gave direct insight into the performance variations with fluence and confirmed a decrease in diffusion length with increasing fluence. GaAs/Ge was found to retain a longer diffusion length in the base layer than GaAs/GaAs after irradiation and hence higher values of maximum power (a function of diffusion length). Computer modelling of the cells' spectral responses for a uniform damage case using measured diffusion lengths enabled comparison with the measurements under non-uniform damage and observations of the spectral responses from the individual cell layers. The GaAs/Ge cell models were found to match actual results better.

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