Study On Fabrication And Radiation Effects Of InGaN Solar Cells

InGaN material has excellent absorption coefficients（an order of magnitude higher than the GaN material）,and can almost completely match the visible light by adjusting their band gaps.So the InGaN material has outstanding advantages on fabricating solar cells,combined with their excellent anti-radiation merits.In recent years,with the rapid development of the space technology,InGaN solar cells have drawn significant scientific attention.Although the InGaN solar cell has high photoelectric conversion efficiency in theory,the actual conversion efficiency of InGaN solar cell is still far from satisfactory and cannot meet the industrial needs.This is due to the complicated structure of InGaN solar cell in manufacture,including the unsatisfactory thickness of indium tin oxides（ITO）,the structure of the active region,the growth of p-GaN and n-GaN as well as the controlling of the etching depth and so on.In addition,because of the intrinsic complex structure of the InGaN solar cell,the influence of the irradiation effect on the surface as well as the active region of the device become more complex when the one is irradiated by Co⁶⁰γrays and protons.All of the above bring great challenges in the growth and fabrication of high conversion efficiency InGaN solar cells and the study of radiation effect.In this context,our researches are focused on two aspects including the fabrication and the study on radiation effect of InGaN solar cells.Through in-depth analysis of relationship between the tunneling effect of photon-generated carriers tunneling through the InGaN/GaN multiple quantum wells（MQWs）,the polarization effect and the strain,we fabricated an enhanced InGaN/GaN MQW solar cell with high conversion efficiency through inserting the superlattice structure（SLS）by utilizing its ability of strain release.Then,by studying the radiation effect of the fabricated InGaN/GaN MQW solar cell,we reveals the physical mechanism for the device degradation after irradiation,which can provide first-hand information for the development of radiation reinforcement technologies.The main results and contributions are listed as below:1.The study and fabrication of a novel high efficiency InGaN/GaN MQW solar cell with SLS inseration.By analyzing the starin realease resulted from SLS inseration,and theoretically deriving the relationship between starin realease,piezoelectric polarization intensity and electron tunneling effect,we concluded the mechanism behind the performance enhancement of the fabricated solar cell with SLS.Experiment results reveals that the conversion efficiency of the fabricated device increased effectively.Compared with conventional InGaN/GaN MQW solar cell,the conversion efficiency impr-oved from 0.61%to 1.61%.Raman analysis showed that the the plane strain of GaN was effectively released when the SLS was inserted.The strain relaxation benefited from the inserted SLS can effectively decreased the piezoelectric polarization intensity,and further lead to the enhancement of electron tunneling effect.Then,the number of photo-generated electrons increased,which finally resulted in the conversion efficiency increase.2.The degradation and partial recovery mechanism of InGaN/GaN MQW solar cell under low accumulated dose Co⁶⁰γray irradiation was studied.Firstly,the experiment results showed that both the short circuit current density and photoelectric conversion efficiency decreased after low accumulated dose Co⁶⁰γray irradiation.According to the measurement results of InGaN/GaN MQW material and ITO film,we found that the degradation of ITO layer is the main reason of solar cell degradation.Experiment results showed that the ITO transmittance was decreased after irradiation,and the oxygen vacancies indeed increased in ITO after 0.3 Mrad（Si）Co⁶⁰γ-ray irradiation.Then,the number of free electrons in ITO increased,which recombined with lots of photo-generated holes from active region.As a result,the number of photo-generated holes decreased,which resulted in the degradation of solar cell.In conclusion,the degradation of ITO layer resulted from the increase of oxygen vacancies is the main reason for the degradation of InGaN/GaN MQWs solar cell under low total-doseg-ray irradiation.Secondly,the experiment results showed that the short circuit current density and photoelectric conversion efficiency can be partially recovered several hours after irradiation.The reason comes from the partial recovery of ITO layer,which is verified by the increase of ITO transmittance and decrease of oxygen vacancies.Then,the combined photo-generated holes were released again,the number of photo-generated holes increased again,which finally lead to the partial recovery of solar cell.3.The degradation mechanism of InGaN/GaN MQW solar cell under proton irradiation was studied.When the InGaN/GaN multiple quantum well solar cell was exposed to 3 MeV proton energy with various fluence,the current voltage characteristics and external quantum efficiency（EQE）were measured.Experiments results showed that the open voltage,conversion efficiency and EQE all decreased after irradiation.We found that the ideal factors increased with the increase of fluence,which indicated that defects were introduced in device after proton irradiation.According to measuring the photoluminescence（PL）intensity,full width at half maximum（FWTH）of the active region and ideal factors of the solar cell,we found that the three have the same change trend with the increase of proton fluence.We inferred the defects were generated in the active region.Then,combined with the SRIM simulation and analysis of normalized PL spectra,we found the intensity of yellow band increased,which reveals that the Ga vacancies introduced in active region may be the main reason for device degeneration after proton irradiation.