Investigation Of Extreme Radiation Hardness And Drift-Dominated InP Solar Cells

In order to cover the whole Earth's surface, telecommunication satellites are arranged in two ways: one is to put three geosynchronous satellites at the geosynchronous orbit (about 35,700km altitude). Another is to put several tens or several hundreds of satellites at low earth orbit (about 700km altitude). The drawbacks of the former one are that higher power is required to reach a satellite at that distance and a substantial round trip signal delay is inevitable. The drawback of the latter one is that it is too expensive to use so many satellites. It is now considered that 3000km orbit is the optimal selection. It makes the number of use of satellites, the requirement of radio power and the delay of round trip signal reach a balance. However, there exist the so-called van Allen radiation belts at this hight. The solar cells would work there for only several days, even for'radiation hardness'Si solar cells. For this reason, extreme radiation hardness and high power/weight ratio solar cells, which can work properly in van Allen belts, must be developed.At present, the solar cells used on space aircrafts are still mainly Si and GaAs solar cells. However, the radiation resistance ability of these two materials is poor. Since the photoelectric conversion efficiency attenuates rapidly with the radiation, the lifetime of Si and GaAs solar cells is short and their stability is not satisfied for space applications at van Allen belt. As a more and more widely used semiconductor material, InP not only has very good electric properties but also is radiation hardness. InP is the next generation solar cell for space applications.Generally the minority carriers accumulated by the cell are generated either directly from the p-n junction or the distance between the generated minority carriers and the junction is less than the diffusion length of the minority carriers. In this design, two possible working mechanisms, diffusion and drift, are compared and analysed. Since drift-dorminated solar cells are less affected by the radiation, the main work is focused on this design. MBE technology is adoped to realize the drift field. Graded doping is adopted in both sides of the junction (double graded doping). This results in a strong (drift) electric field throughout the whole active layer. This field will accumulate minority carriers effectively and the whole internal quantum efficiency is increased. In this design, the double graded doping solar cell accumulates the minority carriers with the drift field, which is located at the whole graded space. This means that the accumulation of minority carriers doesn't depend on the above conditions.