الفهرس 
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Abstract
In this study, the radiation effects on space solar cells in particular for the advanced solar cells such as InGaP and GaAs sub-cells of triple junction InGaP/GaAs/Ge solar cell were investigated. Characterization and modeling for low energy proton irradiation damages to these sub-cells is fundamentally important for the optimizing and predicting the radiation resistant of the multi-junction space solar cells. Such studies are also equally important from the point of view of a thorough and deeper understanding of the properties of these materials. The main results, highlights and conclusion drawn from her work on each of these materials and solar cells are summarized below.
(1) The radiation damages to solar cells can be evaluated by using the fundamental properties of radiative and nonradiative recombination properties in solar cell materials. An analytical model for radiation damages to space solar cells based on the minority-carrier lifetime, damage constant for lifetime, carrier concentration, and carrier removal rate of solar cell materials has been proposed in this study. The damage coefficients as a function of base carrier concentration were investigated using the one dimensional semiconductor device simulator PC1D.
(2) The radiation damages to the InGaP solar cells irradiated by 30KeV protons were investigated. The damage constant for minority carrier diffusion length (KL) was found to increase with initial base carrier concentration, which reflects the effect of the mobility as a functionof initial base carrier concentration. The carrier removal rate (Rc) of InGaP sub-cells was found to slightly increase with base doping concentration. This result is thought to be due to complex defects composed of radiation-induced point defects and impurities in InGaP.
(3) The optimization of the structure of radiation-resistant InGaP solar cells was investigated. Numerical simulation was carried out to optimize radiation-resistant InGaP solar cell structures by using the damage parameters (KL, Rc). As a result of numerical simulation, the n-on-p structure cell was found to be more radiation resistant in a solar cell with shallow junction (0.05μm), thin base thickness (0.4μm) and low base carrier concentration (1x1016 cm-3), while p-on-n structure cell was found to be relatively radiation resistant in deep junction solar cell (0.1μm) with thin base thickness and low base carrier concentration. Numerical simulation has also shown that the EOL (end of life) efficiency in the case of p-on-n structure cell can be improved with formation p-AlInP window layer. On the other hand, no significant improving effect on the EOL efficiency with formation n-AlInP window layer has not been observed in the case of n-on-p structure.
(4) The radiation damages to GaAs solar cell irradiated by 200KeV proton were investigated. The damage constant of minority-carrier diffusion length for GaAs irradiated by 200KeV proton as a function of base carrier concentration was found to increase with increasing initial base carrier concentration, which reflects the effect of the mobility as a function of initial base carrier concentration. However, the carrier removal rate (Rc) was found to be independent
on the initial base carrier concentration for GaAs cells and Rc calculated was found to be 2x104 (cm-1).
(5) Proton energy dependence of radiation damages to GaAs/Ge solar cells was also investigated. The relation between recombination current degradation and displacement damage dose (Dd) was proposed x
d
SCR
SCR
D
D
A
I
I
*
At higher doses ( d D ~<109), the recombination current was found to increases rapidly and carrier removal rate became evident. A significant effect of carrier removal has shown an increase in the width of depletion region. Such an increase in depletion region width is though to cause an increase in the recombination current which in turn affect on the degradation of VOC.
6.2 Conclusion
The radiation damages to InGaP and GaAs sub-cells for InGaP/GaAs based multi-junction solar cells have been studied by 30KeV and 200KeV proton irradiations and using one dimensional semiconductor device simulator PC1D. An analytical model for radiation damages to space solar cells based on the minority-carrier lifetime, damage constant for lifetime, carrier concentration, and carrier removal rate of solar cell materials has been proposed in this study. New results such as effects of base doping concentration upon the carrier removal rate (Rc) of InGaP materials and solar cells,
and effects of base doping concentration upon the damage constant of minority-carrier diffusion length of GaAs materials and solar cells have been found in this study. Some mechanisms such effects of complex defects composed of radiation-induced point defects and impurities in solar cell materials upon radiation damages to InGaP and GaAs cells have also been discussed. Numerical simulation has also shown possibility of optimizing InGaP and GaAs solar cells in order to develop radiation-resistant InGaP and GaAs sub-cells and InGaP/GaAs based multi-junction solar cells.
6.3 Future work
Although high efficiency InGaP/GaAs/InGaAs 3-junction solar cells with 35.8% at 1-sun AM1.5G and InGaP/InGaAs/Ge 3-junction cells with 41.6% under 364-suns AM1.5D have been obtained [3,4], it is necessary to develop a new material with bandgap energy of 1.04eV in order to realize 4- or 5-junction cells in order to develop higher efficiencies of more than 50%. In addition, the new material should be lattice-matched to Ge and GaAs. InGaAsN is one of appropriate materials for 4- or 5-junction solar cell configuration because this material can be lattice-matched to GaAs and Ge. However, present InGaAsN single-junction solar cells have been inefficient because of low minority-carrier lifetime due to N-related recombination centers such as N-H-VGa and (N-N)As and low electron mobility due to alloy scattering and non-homogeneity of N.
To solve above problems scientest try to developing chemical beam epitaxy (CBE) technique. By adapting CBE technique to grow GaAsN thin films, higher mobility of about 200 cm2/Vs and longer minority-carrier lifetime of about 1 ns compared to those grown by the other growth methods have been achieved. According to these electrical properties, more than 15% efficiency is expected in CBE grown homo junction (In)GaAsN solar cells.
In the next study, preliminary results for properties of GaAsN single junction solar cells and numerical analysis for their properties are presented. Effects of minority-carrier lifetime, surface recombination velocity and device structure upon solar cell properties will be discussed. The radiation resist of GaAsN solar cell and structure optimization will be study.