4Π space radiation of solar cells

A study was conducted to evaluate the performance of various solar cell types exposed on both surfaces to isotropic irradiation by protons and electrons which approximate the effects of space irradiation. In addition to the normal damage mechanism of lifetime degradation in the base region, another important damage mechanism of series resistance increase was produced by the particles incident from the rear. This damage is produced in the contact region by the low-energy, or high-angle-of-incidence particles which penetrate only this far. The series resistance increase is at least as important as the lifetime degradation in causing the maximum power to degrade. An effective hardening technique is to p₊-dope the back surface so that a higher fluence is required to remove enough carriers that the contact resistance increase becomes significant. On the other hand, lithium-doping of the cells hardens those irradiated from the front only by producing defects that are not as effective as the normal ones in degrading the lifetime. Therefore, it is not useful for cells irradiated from the back. This is because the lithium "hardening" mechanism involves additional carrier removal, which adversely affects the series resistance when significant damage is produced near the rear contract. This series resistance increase causes the lithium-doped cells, especially the float-zone ones, to degrade much more rapidly. The other findings of the study are essentially the same as results on cells irradiated from the front surface only. Lower sensitivity cells damage at a faster rate, and the spectral response shows a larger degradation in the red region. This latter point means that cells with an enhanced violet response would not degrade as rapidly as similar cells without the enhanced response.