Research On Intermediate Reflector Layer In High Efficiency Silicon-based Thin Film Tandem Solar Cells

The tandem structure of multi junction solar cells can not only broaden the response of the solar spectrum, but also conducive to improve the stability of the cell, and thus become an important way to further improve the performance of silicon-based thin film solar cell. Because of its short wave reflection ability, intermediate reflector layer inserted between the sub cells can improve the short-circuit current density in top cell and thus improve the conversion efficiency and stability of the overall solar cells, and becomes one of the key research on silicon based thin film tandem cells. However, the research between mechanism and material properties is still in its infancy, new materials used as intermediate reflector layer are desperately needed to break the bottleneck. In this thesis, with plasma enhanced chemical vapor deposition and magnetron sputtering process, silicon based thin film tandem solar cells was researched. In order to further improve the conversion efficiency and stability of the tandem structure, the structure, principle, materials and devices on the intermediate reflector layer were studied and explored. The main research contents and achievements of this thesis are as follows:First, theoretical design, simulation and calculation of tandem structure on silicon based thin film solar cells. 1) Research on advanced light management and optimize design the tandem structure. The advanced light management technology of silicon based thin film tandem cell is researched, and the reasonable distribution and effective utilization of the transmitted light at the interface of the sub cells is studied. Research on the intermediate reflector layer to increase the short-circuit current density of amorphous silicon top cell, reducing the light-induced degradation, optimal design the absorption layer thickness and optical band gap of sub cell to improve the overall cell conversion efficiency and stability; 2) Optical simulation and theoretical calculation of the tandem structure. Simulation research the intermediate reflector layer between the sub cells of tandem cell, respectively design the optical model based on intermediate reflector layer with selective reflection effect and the electrical model based on tunnel junction with tunnel recombination theory. Calculating the visible light reflectivity at the intermediate reflector layer and the recombination rate of photo carriers in the tunnel junction. Calculating the matching relationship between refractive index and thickness of the optimal intermediate reflector layer. The optimal relationship between the refractive index and the thickness of intermediate reflector layer is obtained with the refractive index is about 1.59, the thickness is about 94.3 nm, and the relationship between the thickness and refractive index of d =150/n(nm); 3) Optimizing the preparation process of intermediate reflector layer. Study on the influence of gas flow rate, deposition pressure, RF power and substrate temperature on the thin film photoelectric properties, such as deposition rate, oxygen content, crystalline fraction, optical band gap and refractive index.Second, theoretical study of a new type of tunnel recombination reflector based on the tandem structure. 1) Put forward a new tunnel recombination reflector theory. According to the shortcoming study of tandem structure, put forward tunnel recombination reflector theory with light trapping properties and tunnel recombination properties. This Layer plays the role of the intermediate reflector layer and tunnel recombination layer simultaneously, solving the light trapping problem and photogenerated carriers collection problem in tandem cell; 2) Establishment of the selection principle of the material system of the tunnel recombination reflector. Low refractive index, high conductivity, wide band gap and high density of defect states are required to be the selection principle of the material of the tunnel recombination reflector with excellent performance.Third, Study on preparation and properties of a new type of tunnel recombination reflector. According to the current conditions of the laboratory, preparation of Al doped silicon oxide thin film by magnetron sputtering process. Effect of annealing temperature on the crystalline fraction of silicon oxide thin films was studied, and the mechanism of the effect of Al doping on the refractive index of thin films was also studied. The experiment with adjusting the process parameters of the sputtering pressure in chamber, sputtering power and time of different targets, the doping amount of Al, annealing temperature and annealing time, tunnel recombination reflector material with a low refractive index, high conductivity, wide band gap and high defect density requirements was prepared.Fourth, combination of research results and device level solar cells. With the advanced light management technology and the new tunnel recombination reflector technology, design the structures of a single junction, double and multi junctions tandem silicon film solar cells. With PECVD system, the N type μc-SiO_x:H optimization designed on back reflection layer in single junction microcrystalline silicon solar cell and double function N layers in three-junction tandem silicon based alloy thin film solar cells, the P type μc-SiO_x:H optimization designed on P type window layer in double-junction amorphous/microcrystalline silicon tandem solar cells. The conversion efficiency of 6.8% single junction μc-Si:H solar cell, the conversion efficiency of more than 13% high efficiency a-Si:H/μc-Si:H double-junction tandem solar cells and conversion efficiency of nearly 14% high efficiency a-Si:H/a-SiGe:H/aSiGe:H three-junction tandem solar cells were prepared.