| Inverted epitaxial growth technology has been used to produce IMM4 J solar cell based on solar-spectral matching principle,which can decrease the energy loss due to the photo-current mismatch of the sub-cells in the conventional tri-junction Ga As-system solar cells,thus improving the photo-electric conversion efficiency apparently.Nowadays this becomes a main researched direction of space-applied high efficiency multi-junction solar cells.In this dissertation,some complementary characterization techniques such as the light and dark I-V characteristic,spectral response,photoluminescence spectrum analysis,et al.,were used to investigate the damage behaviors and mechanisms of IMM4 J,IMM3J and their sub-cells under 1MeV electron irradiations.On this basis,annealing effects were further discussed to reveal the stability and mechanism of the irradiation-induced defects in the IMM4 J and their sub-cells after irradiation.These investigations would provide technical and theoretical support to improve the properties of the IMM4 J cells in terms of materials selection,gap modifications,manufacture process optimizing,safe and reliable services on-orbit.The results show that after 1MeV electron irradiation,both of the newly-designed InGaAs?1.0eV?and InGaAs?0.7eV?sub-cells present similar behaviors to that of the traditional single junction?SJ? GaAs cell,namely the electrical properties(Isc,Voc and Pmax)degrading linearly with the electron fluence log?.Wherein,InGaAs?1.0eV?sub-cell shows the most serious degradation to become the current limiting sub-cell in the IMM4 J cell instead of good matching the output current in each other sub-cells as designed.In order to reveal the damaged mechanisms,a new equivalent model was established in theory between the measured PL signal intensity and effective carrier lifetime under the small-injection condition.This implies that the minority carrier lifetime shortening is the most fundamental factor of the Isc degradation of solar cell.According to the minor-carrier collection concept of PN junction,a short-circuit current degradation model was set up based on the deep level defects introduced and minority carrier diffusion path.Then,the damage coefficient of minority carrier diffusion length KL of InGaAs?1.0eV?,InGaAs?0.7eV?and Ga As sub-cells were determined as 4.68× 10-7?1.57×10-7 and 2.67× 10-7,and the introduction rate of irradiation defects K? of InGaAs?1.0eV?,InGaAs?0.7eV?and Ga As cells is 1.85×10-12cm?1.02×10-12 cm and 1.19×10-12 cm,respectively.In this case,one could see the most serious damage-parameters in InGaAs?1.0eV?sub-cell.On the other hand,a degradation model on the open circuit voltage degradation is established related quantitatively to the majority carrier removal near the junction region based on the characteristics of built-in voltage of PN junction.Afterward,the removal rates ?n of the majority carrier in InGaAs?1.0eV?and InGaAs?0.7eV?sub-cells were determined as 4.42×10-15cm2 and 4.35×10-16cm2,respectively.In the meantime,the dark electrical measurements show that 1MeV electron fluence could result in increasing the serial resistance Rs,the diffusion current Is1 and recombination current Is2,while decreasing the parallel resistance Rsh in the tested sub-cells.Moreover,similar to Isc degradation in the solar cells,the changes of Rs and Is1 were determined related to the damage of base and emitter region,while the changes of Rsh and Is2 were proven mainly related to the damage of the space charge region of the cells,reflecting theVoc degradation.For the corresponding spectra-matching-designed IMM3 J and IMM4 J solar cells,the experimental results show that there appear property degradation models following the formula P=1-cplog?1+?/?0?as a function of the electron fluence ? after 1MeV electron irradiation.The Isc degradation rate of IMM3 J was determined the same as that of InGaAs?1.0eV?sub-cell,implying its worst irradiation-damage resistance,thus the InGaAs?1.0eV?sub-cell would be the controlled part for the Isc degradation in the IMM3 J solar cell.However,for the case of IMM4 J solar cells,it was found that its Isc degradation ratio is consistent with that of InGaAs?0.7eV?sub cell,indicating hence the InGaAs?0.7eV?sub-cell being the controlled one instead of the worst-resistant InGaAs?1.0eV?sub-cell.In this study,it was also found from the spectral response spectra analysis,that the current density from InGaAs?0.7eV?is measured the lowest in the IMM4 J solar cell due to the factors of epitaxial procedure and also structural mismatch during the fabrication.Thus some measurements were put forwards to optimize the properties,such as adjusting the gap of the sub-cell,adding back reflecting layer or decreasing the epitaxial mismatching defects.From the points to improve the irradiation-damage resistance of the solar cells,it is necessary to optimal distribute the gapes of the included sub-cells and also decrease the deposited boundary defects during the fabrication processes,thus improving the service behaviors of InGaAs?1.0eV?sub-cell.As theVoc of the multi-junction solar cellsThe research on the annealing effects of 1MeV electron-irradiated InGaAs?1.0eV?and InGaAs?0.7eV?sub-cells shows that the higher the annealing temperature,or the longer the heating time,the more recovery of the electrical properties could be detected in the temperature range of 60180?.According to the newly established equivalent model between Isc and the irradiation defect concentration,the activate energies for annealing the irradiation-induced defects were obtained as 0.58 eV and 0.3eV,for the InGaAs?1.0eV?and InGaAs?0.7eV?sub-cells,respectively.On the other hand,it was found that the Isc recovery is consistent with that of InGaAs?0.7eV?sub-cell due to its current limiting effect in the IMM4 J cells,while after annealing the IMM4 J solar cells,theVoc recovery amplitude shows less than those of the InGaAs?1.0eV?and InGaAs?0.7eV?sub-cells. |
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