InP Solar Cells and their Flight Experiments

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We have developed high-efficiency homojunction 1 cm × 2 cm InP space solar cells by diffusing In_2S_3 into p type InP substrates and investigated their fundamental characteristics such as electrical and mechnical characteristics and thermal properties. On the radiation resistant mechanism of InP cells, we have studied InP cells fabricated at NTT Laboratories and found superior properties such as room temperture annealing and minority carrier injection enhanced annealing phenomena for radiation-induced defects in InP crystals with 1 MeV electron irradiation. We have also found that InP cells are superior to Si and GaAs cells in radiation resistance owing to the energy band structure and lower defect introduction rates of InP cells. Various tests were conducted to evaluate the developed InP cells in simulated space environments and to give them the qualification for space applications. In irradiation tests, proton energy was varied from 0.015 to 15 MeV with a fluence ranging from 10^11 to 10^13 protons/cm^2 and electron energy was varied from 0.5 to 3 MeV with a fluence raging from 10^14 to 10^16 electrons/cm^2. As a result, we have confirmed that series resistance in the InP cells increases owing to a decrease in the majority carrier concentration at higher fluences even for proton irradiation. The developed InP cells were mounted on the twelfth scientific satellite "EXOS-D" as a radiation degradation characteristic instrument. The cells were also mounted on a lunar orbiter on board the thirteenth scientific satellite " MUSES-A" as a power source for the orbiter. We succeeded in working those InP cells in orbit. Large decreases in the short circuit current and in the open circuit voltage for the InP cells without coverglass on board the twelfth satellite turned out to be due to lower energy proton irradiation with our new analytical model. However, it has been confirmed that no degradation of the InP cells occurs by using thin coverglass of 50 μm in thickness. In the lunar orbiter, output power of its solar array was constant as predicted till the separation from the thirteenth satellite to place the orbiter in orbit around the moon. We have concluded that the InP solar cells are applicable to space solar cells under severe radiation environments in orbit.
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InP Solar Cells and their Flight Experiments

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