| With the reduction of traditional energy sources and the growing demand for renewable energy in the world,solar energy resources have gained worldwide attention because of its clean,pollution-free and inexhaustible advantages.At present,the main problem of the photovoltaic industry is still how to improve solar cells performance and reduce costs.As the core unit of photovoltaic arrays,improving the efficiency of solar cells and reducing the use of materials are both effective ways to further reduce costs.Solar cells research are focusing on the development of high-efficiency solar cells technology,materials and structure.Multi-junction solar cells are widely used in space facility sources and ground concentrated photovoltaic plants due to its large absorption coefficient,great spectral response and good anti-irradiated qualities.The material characteristics and structural design of the solar cells are the main factors which affecting the spectral response and electrical performance of the solar cell.In this paper,we mainly focus on the current high-efficiency and widely applicated three-junction solar cell,especially the materials qualities,structure optimization.We have explored MATLAB programs to simulate the electrical characteristics of three-junction solar cells and,based on the result we optimize the structure of solar cells.The mismatch and space irradiation damage are calculated.The specific research results were as follows:1.GaInP/GaInAs/Ge triple-junction solar cells structure design and optimization.According to the principle of photovoltaic conversion of multi-junction solar cells,the relationship between the theoretical efficiency and the structural parameters of sub-cells is established.The Lattice matched Ga0.51In0.49P/Ga0.99In0.01As/Ge solar cell structure has the band gap combination of 1.89eV/1.41eV/0.66eV.The best structure is top cell thickness of 1.74 ?m and middle cell of 5.61 ?m,where the efficiency is 42.75%.At this point,the short-circuit current density Jsc is 14.53 mA.cm-2 and the open-circuit voltage Voc is 3.22 V.The experiment has shown that the solar cell efficiency can reach 40.77%under 500 times concentrated light(AM1.5D).For better current matching and higher efficiency,we can achieve greater theoretical conversion efficiency by adjusting the In component to change the band gap of top and middle cells.In the mismatched multi-junction solar cell system,the current is more matched when the band width is 1.77/1.17/0.66 eV,and the best structure is top cell thickness of 2.9 ?m and middle cell of 5.5 ?m,where the efficiency can reach 46.47%.However,in the mismatched system,lattice mismatch will introduce too many defects into the whole cell and the electrical performance will be decreased largely.The top and middle cells lattice matching structure is more practical in this case,and the better lattice combination is 1.66/1.15/0.66 eV(structure B)and 1.85/1.37/0.66 eV(structure C).The best theoretical efficiency is 45.35%and 43.27%,respectively.2.Since the mismatch system introduces dislocation in the cell,we describe the mechanism of the dislocation generation and the relation of dislocation density and the life of carriers.Then the effect of mismatch density on the solar cells is analyzed,and we compare the structure of the lattice match with the mismatched and found that Comparing the theoretical efficiency of lattice matching structure to determine the dislocation density critical value,calculating the dislocation density under different mismatch.We found that B and C structure dislocation density are both below the critical value.Taking the dislocation density into consideration,the theoretical efficiency of B and C is 44.43%and 42.69%,respectively.3.Study on the preparation and performance of triple-junction concentration solar cells.Lattice-matched GaInP/GaInAs/Ge triple-junction solar cells are often used in concentration photovoltaic.An important part of solar cells station Ga0.51In0.49P/Ga0.99In0.01As/Ge chip is made according to the calculated parameters.The subcells are connected by n-AlInP/n-GaInP/p-AlGaAs/p-AlGaInP wide band heterostructure tunneling junction which has a tunneling current up to 443 A/cm2.The AR coating is TiO2/Al2O3 double-layer thin films with reflectivity of 2.17/1.76,The electrodes are made by sputtering with the width of 5?m and the height of 6?m.The solar cell chips and bypass diodes and terminals are attached to the copper-coated ceramic substrate and are welded by soldering paste.then the receiving module of the concentration photovoltaic is obtained and the efficiency is up to 40.77%.In order to ensure the quality of solar cell chips,four batches of samples are tested and the results show that 99.3%of chips have efficiency above 38%and 68.5%above 40%.4.Study of irradiation damage for space solar cells.Solar cells usually degrade rapidly in space when irradiated by high-energy particles.The main factors of irradiation damage are ionization and atomic displacement.The displacement damage caused by the atom displacement when a atom receives excitation and becomes a filled interstitial atom,and then continues to affect other atoms.We study the radiation damage of GaInP/GaInAs/Ge solar cells which are the most commonly used in space equipment as an example.We have studied the effects radiation on short-circuit current density and open-circuit voltage when 1MeV electronics under different doses incident the lattice-matched Ga0.51In0.49P/Ga0.99In0.01As/Ge triple-junction solar cells.And then,with the specific doses 1×1015 cm-2 and 1×1016 cm-2,we simulated the spectral response of the top and middle cells.The decrease of spectral response was found to be significant at 1×1016 cm-2,especially in GaInAs middle cell.We analyzed the characteristics of GaInAs middle cell and concluded that the Ga and As atoms have much higher material damage factor than that of In and P atoms.In addition,because of thick base area having more non-ionization energy loss the long wavelength of light is more difficult to get to the junction.5.Growth of Ge films on silicon substrate.In order to reduce The Ge materials in the Ge-based three-junction solar cell,Ge film can be grown on the Si substrate.In this paper,Ge films were prepared method by magnet sputtering device with graphite buffer layer and graded buffer layer.The Ge films were characterized by XRD and Raman spectra.The experiment results showed that the substrate temperature and annealing parameters had a great influence on Ge films quality.The factors that affecting the quality of thin film crystals and the specific process of thin film crystallization are analyzed.According to the minimum energy principle of Brava,the transformation process of Ge films preferred orientation in the annealing process is explained.6.Design and optimization of GaInP/GaInAs/GeSi triple-junction solar cell structure.The commonly used GaInP/GaInAs/Ge triple-junction solar cell has a bad current matching due to the low bandgap of Ge.To obtain a better current combination and save expensive and rare material,Si can be a good alternative material of Ge.But it's hard to grow ?-?semiconductors materials directly on the silicon substrate due to the large differences in the thermal expansion coefficient and lattice constants mismatch.To solve this problem and to seek a better bandgap combination,Ge2Si1-z can be used as the bottom cell.In lattice-matched structure Ga0.51In0.49P/Ga0.99In0.01As/Ge0.99Si0.01 the best theoretical efficiency is 42.27%when the thickness of the top and the middle cell is 1.29 ?m,5.61 ?m respectively.The short-circuit current density is 14.28 mA.cm-2,and the open-circuit voltage is 3.23V.To further improve theoretical efficiency,the bandgap can be changed by adjusting the material proportion.By calculating program we found that the best bandgap combination is 1.87/1.41/0.78 eV,and the material proportion is x=0.50,y=0.01,z=0.84.The best theoretical efficiency is 42.27%when the thickness of the top and the middle cell is 0.99 5.32 ?m,respectively.The short-circuit current density is 14.30 mA.cm-2,and the open-circuit voltage is 3.12 V.At this point,the mismatch between the top and bottom batteries is 0.6%,the theoretical misalignment density is 3×104 cm-2.The theoretical efficiency of Ga0.51In0.49P/In0.01Ga0.99As/Ge0.84Si0.16 solar cells is higher than that of lattice matched GaInP/GaInAs/Ge solar cells. |
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