Properiy Evolution And Damage Mechanisms Of GaAs/Ge Solar Cells Under Exposure Of Protons And Electrons With Lower Energies

Using the ground-based environmental simulations for the irradiations of protons, electrons and their synergistic effects, the electric property degradations and damage mechanisms of the new-generation GaAs/Ge solar cells were investigated. In this paper, there were five types of irradiated modes chosen to perform the investigations, namely irradiations of single protons, single electrons, protons or electrons followed sequentially by electrons or protons, and synchronous one of both electrons and protons. The energies of protons and electrons were set in the range from 70keV to 170keV, while the maximum fluences were up to 1×1016cm-2 for electrons and 3×1012cm-2 for protons. Some complementary techniques, such as I-V measurements, spectral responses (SR) analysis, photoluminescence spectra (PL), deep level transition spectra (DLTS), x-ray double-crystal diffraction and Hall effect analysis, were applied to expose the property evolutions and damaged mechanisms of the irradiated cells.The results indicate that, after <200keV proton irradiations, the electric properties such as the short circuit current (Isc),the open circuit voltage (Voc) ,the maximum output power (Pm) and the filling factor (FF) of the GaAs/Ge solar cells decrease with increasing proton fluence. In the meantime, the damage extent of the solar cells increases with increasing proton energy at a given flux and fluence of protons.The analysis of various types of spectra present that proton irradiation induces a great damage and degrades the conversion efficiency of the solar cells. The degradation extent of the spectral responses of the GaAs/Ge solar cells increases with proton energy and fluence in their ranges under investigation. Under the proton irradiation, the degradation occurs in the short wavelength band as the proton energy is lower while the spectral response decreases tend to take place in the longer wavelength bands with increasing the proton energy. The experimental results show also that with increasing the proton energy and fluence, the photoluminescence peak (at around 890 nm) of the GaAs/Ge solar cells reduces, broadens intensively and also presents a slight red-shift in the mean time. It is found from the DLTS results that proton irradiations induce various kinds of deep-level defects in the solar cells. The concentrations and the types of deep level defects in the solar cell change with the proton energy and fluence. The detected defects show deep-level energies at Ec-0.25eV, Ec-0.35eV, Ec-0.54eV, Ec-0.72eV, and Ec-0.75eV, respectively.During the <200keV electron irradiation, the in-situ I-V measurements indicate that the irradiation could reduce the electric parameters of the solar cells. The degradation extent increases with increasing the electron energies and fluences. However, it is interested to be noted that this property degradation induced by <200keV electrons can be recovered gradually during afterwards storage period at room temperature. It was also found that the external quantum efficiency shows a slight decrease but the reflectance of the solar cells rises slightly after the electron irradiations. No deep-level defects were detected in the eellctron-irradiated solar cells.The results on the combined irradiations of electrons and protons indicate that the electric properties of the solar cells present more degradation after irradiations in the order of the synchronous proton/electrons irradiation, the sequential electron irradiation followed by protons and then protons irradiation followed by electrons. During the sequential process, the damage induced by low-energy protons domains the property degradation of the solar cells. These results imply that there is a synergistic damage effect during the synchronous irradiations. It was also found the degradation effects on the PL spectra and spectral responses of the combined irradiations of the solar cells. Under the experimented irradiations, the quantum effeciencies decrease and the decrease extent is in the following order: synchronous irradiation of electrons and protons > sequential irradiations of protons (electrons) followed by electrons (protons) > single proton irradiations >> single electron irradiations.