Modeling and simulating of radiation effects on the performance degradation of GaInP/GaAs/Ge triple-junction solar cells induced by different energy protons
1.Xi’an Research Institute of High-Technology, Xi’an 710025, China 2.State Key Laboratory of Intense Pulsed Irradiation Simulation and Effect, Northwest Institute of Nuclear Technology, Xi’an 710024, China 3.School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, China
Abstract:The GaInP/GaAs/Ge triple-junction solar cells have been widely used for spacecraft energy sources because of their simple manufacturing process, stable structures, high conversion efficiency, and low cost. The performances of the GaInP/GaAs/Ge triple-junction solar cells show a remarkable degradation after space proton irradiation. At present, the experimental researches of proton irradiation of GaInP/GaAs/Ge triple-junction solar cells with different energy and fluence have been carried out. However, the experimental researches can analyze the proton radiation damage only under the specific energy and fluence, but cannot analyze the proton radiation damage under the complete space energy spectrum. The numerical simulation of triple-junction solar cells can be used to accurately analyze the degradation of major parameters under different energy proton irradiations which cannot be achieved experimentally.In this paper, the modeling of degradation for GaInP/GaAs/Ge triple-junction solar cells, induced by proton irradiation with different energy is studied by numerical simulation. The energy values include 0.7 MeV, 1 MeV, 3 MeV, 5 MeV, and 10 MeV. The structure of GaInP/GaAs/Ge model and proton irradiation-induced defect model with different energy and fluence are established. The I-V curves and spectral response curves under different proton irradiation conditions are obtained. The simulation results are in good agreement with the experimental results. The degradation of major parameters of GaInP/GaAs/Ge triple-junction solar cells, caused by different energy and fluence proton irradiations, is studied, these parameters being the short circuit current, open circuit voltage, minority carrier lifetime, electron current density, external quantum efficiency, and maximum power. The degradation curve of the maximum power with displacement damage dose is obtained by fitting the degradation simulation results under different proton irradiation conditions. Displacement damage defects induced by protons are introduced into triple-junction solar cells, which lead the minority carrier diffusion length to degrade. The degradation increases with the proton energy decreasing. In the meanwhile, it will lead the related electrical parameters to degrade, which increases with the proton energy decreasing. The simulation results show that related electrical parameters decrease with the proton irradiation fluence increasing. Under the same proton irradiation condition, the external quantum efficiency degradation of GaAs sub-cell is larger than that of GaInP sub-cell because the irradiation resistance of GaAs is poor. Among the degradations of spectral response of GaAs sub-cell at different wavelengths, the degradation in the long wave is greater than that in the short wave. It is found that the degradations of GaAs sub-cell related electrical parameters are mainly due to the damage to the base region. Keywords:GaInP/GaAs/Ge triple-junction solar cells/ irradiation-induced defects/ proton irradiation model/ numerical simulation
为了验证模拟结果的准确性, 对比三结太阳电池归一化最大输出功率随辐照注量变化的模拟与实验结果[23]. 1和3 MeV质子辐照下, 三结太阳电池归一化最大输出功率随辐照注量变化的模拟与实验结果, 如图2所示. 图中连续曲线表示模拟结果, 离散点表示实验结果. 从图2可见, 随着辐照注量的增加, 三结太阳电池归一化最大功率逐渐减小, 归一化最大输出功率随辐照注量变化模拟结果与实验结果趋势相近, 两者符合性较高, 模拟结果的准确性得到验证. 图 2 最大输出功率随辐照注量变化的模拟与实验结果 Figure2. Normalized maximum power versus fluence at the proton irradiation energy of 1 and 3 MeV (symbols and lines are experimental and simulation results respectively).
为进一步揭示质子辐照对三结太阳电池性能产生的影响, 对不同能量质子辐照后, 三结太阳电池外量子效率的变化情况进行分析. 三结太阳电池外量子效率, 定义为三结太阳电池对外输出电荷数与入射到三结太阳电池表面光子数之比, 表示在一定波长的光照条件下, 三结太阳电池中产生电子-空穴对的效率. 固定辐照注量为3 × 1012 cm–2, 不同能量质子辐照下, 顶电池GaInP和中电池GaAs光谱响应模拟结果如图6所示. 图中未给出底电池Ge的光谱响应, 这是因为底电池Ge在质子辐照后仍有较大的输出电流, 不会影响三结太阳电池输出电流[29]. 从图6可以看出, 随着质子能量的减小, 顶电池GaInP和中电池GaAs光谱响应的退化幅度逐渐增大. 质子辐照后产生的位移损伤缺陷充当少数载流子复合中心, 位移损伤缺陷浓度随质子能量的减小而逐渐增大, 缺陷浓度的增大导致光生载流子非辐射复合的增加, 使三结太阳电池对外输出电荷数减小, 进而诱发三结太阳电池光谱响应的退化. 图 6 辐照注量为3 × 1012 cm–2, 顶电池GaInP和中电池GaAs在不同能量质子辐照下的外量子效率 Figure6. Simulation results of external quantum efficiency of GaInP and GaAs sub-cells before and after different energy proton irradiation with the fluence of 3 × 1012 cm–2.
此外, 从图6可见, 相同辐照条件下, 中电池GaAs光谱响应退化幅度远大于顶电池GaInP退化幅度. 因为质子辐照后, 顶电池GaInP空位缺陷VIn和VP的迁移能分别为0.26和1.2 eV. 中电池GaAs空位缺陷VGa和VAs的迁移能分别为1.79和1.48 eV. 顶电池GaInP空位缺陷的迁移能远小于中电池GaAs空位缺陷的迁移能. 同时在室温下, InP材料的辐照缺陷退火效应更明显[26]. 由此可见, 中电池GaAs的抗辐照性能最差, 中电池抗辐照性能直接决定三结太阳电池的抗辐照性能. 对中电池GaAs单独分析, 相同辐照条件下, 中电池光谱响应在不同波长退化幅度差别较大, 出现明显的短波效应和长波效应. 根据中电池在不同波长下光谱响应结果, 中电池光谱响应在500—700 nm短波范围内退化较小, 而在700—900 nm长波范围内退化较大. 光谱响应在不同波长的退化与电池不同结构的损伤有关. 500—700 nm波长范围内光谱响应退化, 主要因为质子辐照减小了N型发射区少数载流子(空穴)扩散长度, 增加了空穴的非辐射复合. 而700—900 nm波长范围内光谱响应退化, 主要因为质子辐照在P型基区产生了位移损伤, 减小了P型基区少数载流子(电子)扩散长度. 由于中电池基区宽度(2.5 μm)远大于发射区(0.1 μm)宽度, 相比于发射区顶部少数载流子, 基区底部少数载流子更难以扩散到耗尽区. 根据三结太阳电池工作原理, 三结太阳电池对外输出电流主要由两部分组成, 分别是发射区少数载流子(空穴)电流和基区少数载流子(电子)电流. 当受到太阳光照射, 太阳电池将吸收光子, 使发射区少数载流子(空穴)和基区少数载流子(电子)被激发, 发射区空穴和基区电子扩散到耗尽区后, 被耗尽区形成的内建电场所分离, 进而对外产生电流. 因为中电池GaAs基区的厚度远大于发射区厚度, 在中电池GaAs吸收波长范围内, 太阳光谱中的大部分光子被中电池GaAs基区所吸收, 基区被激发的少数载流子数目更多, 所以子电池电流主要由基区少数载流子(电子)的电流组成. 中电池GaAs基区(具体见图1, 中电池P-GaAs基区的厚度为2.5 μm)电子电流随不同能量质子辐照后的退化结果如图7所示. 其中图7(a)为初始状态下, 中电池GaAs基区电子电流密度. 为直观分析基区电子电流密度变化结果, 沿A-A′切线处的基区电子电流密度随中电池GaAs基区厚度的变化结果如图7(b)所示. 利用相同方法, 分别得到辐照注量为3 × 1012 cm–2, 不同能量质子辐照下基区电子电流密度随中电池GaAs基区厚度的变化结果. 当基区厚度大于1 μm时, 基区电子电流密度接近为0, 因此图7(b)中未画出1—2.5 μm的电子电流密度. 其中图7(b)中X轴零点位置, 对应于中电池GaAs基区上表面. 相同辐照注量下, 位移损伤浓度随质子能量的减小而增大, 导致少数载流子非辐射复合随质子能量的减小而增加, 从而诱发中电池GaAs基区电子电流密度随质子能量的减小而减小. 同时可以看到, 电子电流减小到零时所对应的基区厚度随质子能量的减小逐渐减小. 只有基区厚度与耗尽区的距离小于电子扩散长度时, 才能产生电流, 而当基区厚度与耗尽区的距离大于电子扩散长度时, 则不能产生电流. 电子电流减小到零时的基区厚度反映了基区少数载流子的扩散长度. 质子辐照后, 少数载流子扩散长度的退化随质子能量的减小而逐渐增大, 从而导致基区电子电流减小到零时所对应的基区厚度随质子能量的减小而逐渐减小. 图 7 (a)初始中电池GaAs的基区少数载流子(电子)电流(Je)的模拟结果; (b)不同能量质子辐照下, 沿A-A′ 切线的中电池GaAs基区电子电流密度随基区厚度的变化 Figure7. (a) Simulation results of current density (Je) of minority carriers (electron) of GaAs middle cell base region before irradiation; (b) simulation results of current density of minority carriers (electron) versus base thickness for GaAs middle cell base region before and after different energy proton irradiation with the fluence of 3 × 1012 cm–2.
23.6.最大输出功率退化分析 -->
3.6.最大输出功率退化分析
三结太阳电池最大输出功率表示三结太阳电池对外输出功率最大值, 是三结太阳电池最重要的性能参数. 不同辐照能量下, 归一化最大输出功率随辐照注量变化的模拟结果如图8所示. 从图8可见, 对于不同能量的质子辐照, 最大输出功率退化幅度随辐照能量的减小而增大. 因为在三结太阳电池中, 中电池GaAs抗辐照性能最差, 三结太阳电池性能退化主要由中电池性能退化决定, 所以用中电池的位移损伤剂量等效三结太阳电池位移损伤剂量, 三结太阳电池归一化最大输出功率随位移损伤剂量的变化结果如图9所示. 从图9可以看出, 最大输出功率随位移损伤剂量的增加逐渐减小, 利用方程[30]及最大输出功率退化的模拟结果, 拟合得到归一化最大功率随位移损伤剂量的特征方程: 图 8 不同能量质子辐照下, 三结太阳电池最大输出功率随辐照注量的退化结果 Figure8. Simulation results of normalized maximum power versus proton fluence for GaInP/GaAs/Ge triple-junction solar cells irradiated by different energy proton.
图 9 三结太阳电池最大输出功率随位移损伤剂量的退化结果 Figure9. Degradation of normalized maximum power versus displacement damage dose for GaInP/GaAs/Ge triple-junction solar cells.