Recent developments in radiation hardening silicon solar cells

Abstract

In the recent past, two items have found particular interest in hardening silicon solar cells against the radiation environment encountered during their mission life. The first is the introduction of lithium as an impurity into the silicon solar cell. This impurity results in the gradual recovery of the solar cell from radiation damage. The second item concerns the behavior of silicon solar cells on a Jupiter flyby mission. On such a mission, the solar power array would be exposed to solar flare radiation, to the effect of the micrometeorite belt, and finally, to the radiation belts of Jupiter. Particularly during this latter phase of the mission, the cells are operating at low temperatures (around 140°K) and at low solar intensities (around 6 mW cm −2. The performance of solar cells after irradiation in such operating conditions had not been explored previously. It has been established that lithium can move about rather freely in silicon, and that it complexes with radiation damage centers in such a way as to neutralize their performance-degrading effect. Since lithium is an n-type impurity, and since its presence is needed in the base region of the solar cell, the ‘ p on n’ structure has to be used for these cells. Significant numbers of experimental lithium containing solar cells have now been prepared with initial efficiencies equal to or better than those obtained on standard production ‘ n on p’ 10 Ω cm solar cells. The lithium containing cells have shown, after bombardment with 1 MeV electrons and recovery, a power output slightly higher than that of the ‘ n on p’ cells. More striking is the advantage of the lithium containing cells under heavy particle bombardment. After recovery from 17 MeV proton or from energetic neutron irradiation, the lithium cells match the power output of standard ‘ n on p’ cells having received an approximately 30 times smaller radiation dose. Lithium containing solar cells can exhibit degradation, both before irradiation and after recovery. A remedy against this instability has been thought to be in the simultaneous introduction of oxygen. The oxygen-rich cells recovery much more slowly from radiation damage than the oxygen free cells: over several months compared to several days. Oxygen-lean lithium containing cells, however, have now been found to be stable after an initial ‘recovery overshoot’, if fabricated by approrpiate methods. Interesting findings have been made relative to solar cell performance in low temperature environments. The solar cell efficiency is approximately 50 per cent higher, and the introduction rate of defects due to particle bombardment is lower than at room temperature. Nevertheless, the radiation damage has been found to be enhanced both by a shift in the absorption curve of the semiconductor and a shift in the Fermi-level. The low solar intensity near Jupiter further enhances the latter effect. On the other hand, while lithium is ineffective at this low temperature, the ‘ p on n’ structure has been found to incur less damage from proton bombardment than the ‘ n on p’ type. And finally, by use of lithium containing cells, solar flare damage—experienced primarily during the early part of the mission—can be eliminated by annealing since the cells are still at high temperatures where the lithium is effective. Finally, the micrometeorite belt is now thought to have negligible effect on the solar cells and on their covers.