| High-efficiency solar cells were the main energy providers for spacecraft,such as space stations,space satellites,near space airships,and long-duration solar reconnaissance aircraft.With the determination and development of the national space strategic plan,the GaAs-based space solar cells currently used were far from meeting the power requirements of future aerospace.Therefore,it was urgent to further improve its conversion efficiency.The surface reflectance of GaAs-based space solar cells was an important factor to restrict the conversion efficiency.Reducing the surface reflection can increase the number of photons participating in the photoelectric conversion through the window layer of the cell,which helps to improve the photoelectric conversion efficiency of the cell.At present,the main method to reduce surface reflectivity is to use antireflection films at home and abroad.However,the lifespan of it and the limitation of the applicable wavelength were difficult to overcome.The micro nano structure of the cell surface structure can effectively avoid the above defects.This paper focused on single-junction III-?group GaAs-based solar cells.By using software simulation,femtosecond laser induction,ultraviolet lithography,and ion beam etching,micro-nano structures with anti-reflection effects on the surface of the cell window layer,which were designed and manufactured by ourselves,have been constructed.After studying the surface characteristics,reflectance and other processing methods of the cell systematically,the surface characteristics of the cell and the increase and decrease mechanism of photoelectric conversion efficiency were explored.The major conclusions were as followed:(1)Two GaAs-based single junction heterogeneous solar cells,A and B,were designed and prepared.The thickness of the window layer of battery A is 200 nm,the surface is flat,the reflectivity is 34.94%,and the photoelectric conversion efficiency is 13.72%;The thickness of the window layer of the B battery is 30 nm,the surface reflectance is 31.72%,and the photoelectric conversion efficiency is 19.36%;By comparing A battery and B battery,it is found that the thickness of the Al In P material of the battery window layer can significantly affect the photoelectric conversion efficiency.The greater the thickness,the greater the consumption of photons incident into the battery,and reducing the thickness can effectively improve the photoelectric conversion efficiency of the battery.The comparison of B battery and C battery with the same design parameters shows that the nano-scale micro-nano texture on the surface of the window layer Al In P material can reduce the reflectivity of the battery surface and improve the photoelectric conversion efficiency.(2)Based on scalar,vector diffraction and equivalent medium theory,the basic principle of micro structure antireflection was explained.FDTD simulation was used to analyze the antireflection characteristics of four kinds of common microstructures(cone,cosine section cone,parabola section cone,pyramid)under different structural dimensions.The laws of the reflectivity changing with the structural height corresponding to the four kinds of microstructures and the best aspect ratio changing with the structural height were obtained;the antireflection characteristics of four common microstructures were analyzed by simulation.The simulation results showed that the reflectance obtained by combining two structures was not equal to the average of the reflectance of two single structures under the same parameters;the reflectance obtained by combining the two structures was lower than the average reflectance of each single structure under the same parameters or was between the reflectances of two single structures under the same parameters.The above results showed that selecting the appropriate microstructure composite was beneficial to improve the reflectivity of the material surface.(3)The femtosecond laser was used to induce the surface of A cell to prepare a cell with a surface microstructure.The test results showed that the damage threshold was 0.0113 J/cm~2under 10 pulses and 0.033 J/cm~2 under 100 pulses.The theoretical threshold was 0.047 J/cm~2.The relative error was caused by the uncertainty of laser energy,the measurement error of spot damage area,the error of linear fitting,the simplification of spot model and the homogenization of Al In P material properties.Under the given parameters,the minimum single pulse energy of the Al In P material damage caused by the femtosecond laser on the surface of the cell under the given parameters was 2.982?J.Studies clarify that as the energy of a single pulse increases,the degree of damage to the cell window layer material by the femtosecond laser gradually increases.Reducing the longitudinal displacement of the light spot under the same single pulse energy can increase the damage of the cell.The surface microtexture prepared by femtosecond laser has obvious antireflection effect,and the surface reflectance decreases with the increase of single pulse energy.The mechanism of increasing and decreasing the photoelectric performance of the cell was analyzed.In the case of minimal damage caused by the minimum single-pulse energy obtained from experiments,the photoelectric conversion efficiency of the cell was reduced from 13.72%to 12.46%.When the single-pulse energy was further increased,the photoelectric performance of the cell decreased rapidly,and the cell was severely damaged,further increasing the single pulse energy.The photoelectric performance of the cell droped rapidly,and the cell was severely damaged.(4)A new generation of laser direct writing technology was used to accurately prepare nano-depth groove-shaped arrays and cross-shaped arrays on the surface of the Al In P material of the window layer of A cell.The optimized laser power was 270m W,the pulse width was 490 ns,and the laser wavelength was 405 nm.The depth of the groove structure on the cell surface can be controlled within 5 nm.The microgroove structure prepared by laser direct writing is not flat,the size of the nanoarray obtained between different samples under the same processing parameters has slight fluctuations,and the microstructure consistency between the same samples was better.The nano-array structure can reduce the reflectivity of the cell surface.Under the condition of the approximate line-to-width ratio,the cross-shaped nano-array had lower reflectivity than the horizontal slot-type nano-array.The conversion rate of the cell with the transverse groove-type nano-array structure was reduced from 13.72%to between 13.48%and 13.01%;the cell performance with the cross-type nano-array structure can reach 13.59%.(5)The ion beam etching process was used to texture the Al In P material on the cell surface window layer,and the test obtained the optimal spin-coated photoresist parameters.When the spin-coating time was 30 s,the acceleration was 1000 rpm/s,and the speed was 2000 rpm.A good photoresist film can be obtained on the chip;the optimal photoresist baking temperature was 135?for 15 min;the optimal photoresist(LC100)exposure time was 5 s;the experiment obtained ion beam etching.The etching rate for the Al In P material of the cell window layer under this equipment and related setting parameters was about 1 nm/s.Under this equipment and related setting parameters,the experiment obtained ion beam etching rate for the Al In P material of the cell window layer was about 1 nm/s.Using the QE system to measure the surface of the cell to prepare 5 nm and 30 nm nanostructures can reduce the reflectivity of the cell by 2-4%,while the photoelectric conversion efficiency of the cell was reduced.In this study,the surface micro-nano structure preparation process for single-junction ?-? GaAs-based solar cells was optimized,which had provided theoretical support for obtaining high-efficiency,stable,long-life,and self-cleaning multi-junction ?-? solar cells. |
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