Research On The Performance Of Inverted Metamorphic Four-junction GaAs Solar Cell Under Electron Irradiation And Its Simulation

With higher solar photoelectric conversion efficiency,four-junction solar cells will gradually replace the current commonly used three-junction solar cells.At present,multi-junction solar cells are mainly used in spacecraft power supply system,and in outer space,continuous bombardment of high-energy universal particles will cause that the solar cells' performance will be gradually reduced,so radiation resistance of the solar cells in some measurement determines lifetime of the spacecrafts.The study of radiation damage related to solar cells has been carried out for more than ten years and they are still being studied.However,the related researches on four-junction solar cells are still blank.Multi-junction solar cell radiation related research needs to be carried out both experimentally and theoretically.Experimentally,since decline of cell performance is the main concern of experimental research,change of the short-circuit current density(Jsc)and the open-circuit voltage(Voc)before and after irradiation of the four-junction solar cells are necessary to be measured.Theoretically,both atomic-level and device-level simulation tools are needed.For the atomic-level simulation,the Casino software based on first-principle is applied to calculate energy absorption of high energy particles injected into the multilayer film of multi-junction solar cells.As for the device-level simulation,this thesis applies the Apsys software of Crosslight company,Canada.The work in this thesis is divided into two parts.One is experimental work and another one is theoretical analysis.In the experimental part,four-junction solar cells were manufactured.Then with reasonable energy and dose of high energy electrons,the Jsc and Voc datas were measured before and after irradiation.Taking into account the statistical factors,five sets of results were chosen to calculate the average of the results.Jsc was attenuated from 15.69mA/cm2 to 13.55mA/cm2 after irradiation,and Voc decayed from3276.76 mV to 2902.64 mV.Theoretical analysis is divided into two parts before and after irradiation.Apsys software was applied for simulating the four-junction solar cells before irradiation.Taking into account the defects generated in the MOSFET processing,with a noval "sub-cell analytical method",by analyzing impact trend of lifetime andtrap density of minority carrier,it's found that Jsc of the third cell is most sensitive to the defect,so the third cell is determined to be the current-limiting cell of four-junction solar cell.The minority carrier lifetime is a commonly used to analyze behavior decay of solar cells,while the trap density analysis is the first time to be used as a critical parameter in such analysis.Actually,the trap density is more reliable to reflect the lattice quality.After confirming the current-limiting cell,by adjusting the trap density of the other three sub-cells,simulated J-V curve is fitted to the measured.The errors of simulated Jsc and Voc are only around 0.1%.The theoretical analysis after radiation has two steps.Firstly,high-energy electronic radiation simulation is carried out by the Casino software.Because the Casino software can't simulate all the thin films of the four-cell solar cell,a "energy loss method" is proposed to deal with sub-cells one by one.With the method,under irradiation of electron with 1MeV energy,the absorbed energies of the four sub-cells are 27.3 keV,28.24 keV,27.8 keV and 29.3 keV,respectively.The lattice defects produced by electron irradiation are acting as traps in the transport of carriers.In order to represent ability of each cell transforming absorbed energy to traps,this thesis proposes a "absorption-energy-trap-density characterization method".Through this characterization method,by using the Apsys software,it's found that the third junction cell is still the current-limiting cell after irradiation.By changing the characterization factor of each sub-cell,the simulated J-V curve is very close to the measured one.Study on four-junction solar cell work is still in the early stages,the following work can be carried out in many aspects,such as measuring the electrical properties of each film and index spectrum of the main absorbing materials to get precise simulation results,as the premise of structural optimization.The analysis methods proposed in this paper are not limited to four-junction cells,and they are also effective for the analysis and optimization of five-junction even six-junction solar cells.