Ultra-thin solar cells show promise for application in space power systems for particularly harsh radiation environments, due to their high intrinsic radiation tolerance. The work presented in this thesis focuses on such a cell design, incorporating an 80 nm thick GaAs absorber layer, and furthers the understanding of the electrical behaviour of ultra-thin solar cells, as well as the effects of radiation on these devices and the materials comprising them. This was achieved through irradiation of the devices with a variety of radiation types and their characterisation by cathodoluminescence, both continuous wave and time-resolved, and current-voltage measurements. It is demonstrated that the preservation of short-circuit current up to high dose in ultra-thin solar cells can be attributed to the carrier lifetime remaining greater than the time taken for carriers to traverse the absorber layer, even after the lifetime is degraded severely by radiation damage. The eventual collapse in short-circuit current beyond 2 × 10¹³ MeV g⁻¹ is found to likely stem from the rapid increase in traverse time, due to carrier removal in the passivation layers. This study identifies the significant contribution of trap-assisted tunneling to the carrier recombination rate in these devices. This effect accounts for the difference in radiation-induced open-circuit voltage degradation rate between devices with absorber layer doping concentrations of 1 × 10¹⁷ and 1 × 10¹⁸ cm⁻³. Trap-assisted tunneling also partially explains the high ideality factors (> 2) of ultra-thin solar cells. The observed variation in ideality factor with radiation type is correlated with corresponding trends in open-circuit voltage degradation rate, carrier lifetime damage constants and appearance of radiation-induced defects in cathodoluminescence intensity maps. These trends are indicative of the variation in defect type with radiation type and suggest that the presence and size of defect clusters has a significant effect on the electrical behaviour of ultra-thin solar cells.