Effects of electrical characteristics of InGaP solar cell on low energy electron beam irradiation

The solar cell used for a satellite has commonly triple-junction, whose top-subcell is made of InGaP. The electrons can recoil the atoms in the crystal lattice of the cells at energies above the threshold energies. Theoretically obtained threshold energies for recoil atoms of the InGaP cell by an electron are about 120 - 320 keV. The research using low energy electron beams is important to investigate the mechanism of the degradation of the cell by the defects. The InGaP solar cell was irradiated with electron beams by the Cockcroft Walton electron accelerator in a vacuum and at an ambient temperature. The energies and the fluences of the electron beams are from 60 to 500 keV and from 3 × 10¹⁴ to 3 × 10¹⁶ cm^-2. Degradation of the open-circuit voltage of solar cell energy at less than 100 keV was observed. This degradation was not predicted on the theory. This result suggests that defects were caused by not only the recoil but also the absorbed dose by electron. At an electron energy of more than 400 keV the RF of the short-circuit current (Isc) has been found to increase at a fluence below about 3 × 10¹⁴. For the higher fluence the RF of Isc has gradually decreased. This phenomenon can be attributed to the increase of carrier concentration due to the irradiation. (author)