Optimization Of Outdoor Performance Of ?-? Concentrated Multi-Junction Solar Cells And The Design Of New Device Structure

Solar energy is a clean renewable energy,and it is expected to become one of the main development directions of new energy in the future.As the third generation solar cells,concentrated multi-junction solar cells have broad application prospects due to their high energy conversion efficiency and low production cost.At present,four-junction GalnP/GaAs/GalnAsP/GalnAs solar cells have been successfully fabricated by Fraunhofer in the United States.The efficiency of the four-junction solar cells is as high as 46.0%at 508 suns under the AM1.5D spectrum,and set a new world record.However,the non-uniform illumination,chromatic aberration and the variation of the solar spectrum caused by the concentrated light become one of the main factors restricting its performance when it was used outdoor.So in this thesis,the optimization of outdoor-performance of concentrated multi-junction solar cells and the design of new device structure are investigated.The main research results and innovations are as follows:1.We innovatively adopt the method of reducing the size of solar cells to reduce the influence of non-uniform illumination on the performance of solar cells.A three-dimensional equivalent circuit model of GalnP/GaAs/Ge three-junction solar cells is established by SPICE software.The distribution of non-uniform light spot relative to the cell remains unchanged when changing the size of lens and cell in equal proportion.The power loss in the three-dimensional equivalent circuit model is quantitatively analyzed,and the mechanism of the influence of non-uniform illumination on the performance of the cell is systematically studied.We find that the effect of non-uniform illumination on GalnP/GaAs/Ge three-junction solar cells with a size of 10 mm ×10 mm in commercial applications is mainly due to the thermal loss of the grid line after the optimization of the grid spacing.When the size of the cell is reduced from 10 mm × 10 mm to 1 mm × 1 mm,the thermal loss rate of the grating line under non-uniform illumination decreases from 2.62%to 0.025%,almost negligible.The conversion efficiency increases from 33.49%to 34.56%,the ratio of efficiency between non-uniform and uniform illumination increases from 96.90%to 98.52%,and the effect of non-uniform illumination on the cell efficiency decreases by 49.8%.2.We innovatively adopt the method of reducing the size of solar cells to reduce the influence of chromatic aberration on the performance of solar cells.By changing the size of lens and solar cell in equal proportion,the distribution of chromatic aberration light spot relative to the cell remains unchanged.Moreover,the effects of chromatic aberration on the performance of solar cells with different sizes are systematically studied by analyzing the distribution of the voltage,dark current and the transverse current distribution in each layer.We find that the mismatched photo-generated carriers are more likely to compensate each other in the form of transverse current when the size of solar cell is decreased,which makes the current in the local area of the solar cell more matched and reduces the influence of chromatic aberration.When the size of solar cell is as large as 20 mm × 20 mm,the chromatic aberration gives rise to a reduction in the short circuit current density(JSC)and the current density at the maximum power point(Jm),and the conversion efficiency is only 94%as no-chromatic aberration.Moreover,when the size of solar cell is reduced to 2 mm× 2 mm,the JSC with and without chromatic aberration is equal.However,most of the mismatched photo-generated carriers are lost in the form of dark current at the maximum power point,which reduces the Jm,and filling factor.Furthermore,the performance of solar cell with and without chromatic aberration is nearly the same when the size of solar cell is further reduced to 0.4 mm × 0.4 mm,and the effects of chromatic aberration can be ignored.3.In view of the outdoor solar spectrum deviation,we propose that varying the current matching between different sub-cells properly is helpful to improve the outdoor annual output of electricity.We have calculated the outdoor annual terrestrial solar spectrum in Golmud area of Qinghai Province by SMARTS software.Moreover,the performance of the triple junction GalnP/GalnAs/Ge solar cell under high concentration conditions is studied.We have calculated the outdoor annual terrestrial solar spectrum in Golmud area of Qinghai Province by SMARTS software.Moreover,the performance of the triple junction GalnP/GaInAs/Ge solar cell under high concentration conditions is studied.Furthermore,the mechanism of the effects of the outdoor solar spectrum deviation on the performance of the solar cells is revealed,and the relationship between the current matching and the outdoor solar is studied.It is indicated that the annual quantity of electricity will reach the maximum when the current of middle GalnAs cell is 2.5%less than top GalnP cell if the effect of the concentrating optical system is taken into account.4.On the basis of GalnP/GalnAs/Ge three-junction solar cell,a new type of Si-based GalnP/GalnAs/GeSi three-junction solar cell was designed,considering cost reduction,process difficulty reduction and efficiency improvement.The open-circuit voltage of Si-based GaInP/GalnAs/Ge0.95Si0.05 triple-junction solar cells is 3.42 V at 1000 suns under the AM1.5D spectrum,which is nearly 0.1 V higher than that of GaInP/GaInAs/Ge triple-junction solar cells,and the conversion efficiency is 41.33%,which is 0.89%higher than that of GalnP/GaInAs/Ge.5.We divide the Ge bottom cell of GalnP/GalnAs/Ge three-junction solar cell into two tandem Ge sub-cells,based on the idea of improving efficiency and reducing process difficulty,and design another kind of GalnP/GaAs/Ge/Ge four-junction solar cells with two-junction Ge sub-cells.The open-circuit voltage is as high as 3.82 V at 1000 suns under the AM 11.5D spectrum,which is 0.5V higher than of GalnP/GalnAs/Ge three-junction solar cells,and the conversion efficiency is 43.15%,which is 2.71%higher than that of GaInP/GaInAs/Ge.