| Solar energy is a promising renewable energy. More than 80% of the photovoltaic market is dominated by crystalline silicon solar cells. The driving force is the cost which requires the reduction of the silicon wafer thickness and improvement of the cell efficiency. However, with the reduction of wafer thickness, the surface recombination becomes more and more serious, and meanwhile, increasing breakage of wafers during cell process will affect the yield. In addition, the bulk recombination centers, e.g., light-induced degradation, can also degrade the cell efficiency severely.In this work, the recombination behaviors in silicon and solar cells are well demonstrated. Besides, the mechanical properties of thin silicon wafers are investigated. The main results are summarized as follows,(1) The effective ways to reduce silicon surface recombination have been proposed. Using a 0.01mol/l iodine/ethanol solution passivation for p-type<100> Czochralski silicon, the surface recombination velocity can be reduced to 50cm/s. The SiNx film also passivates the silicon surface effectively by depositing charges on the surface, due to a reduction of electron or hole concentration. The surface recombination can be further reduced after a rapid (10-15s) thermal anneal at 400-450℃, which leads to a decrease of the surface state density by the hydrogen diffusion.(2) The influence of surface recombination on the thin cell efficiency has been investigated. With the reduction of wafer thickness, the output parameters of solar cells get worse. The average efficiency of 18.4% is achieved for cells with a thickness of 180μm, while 16.8% for cells with a thickness of 60μm. The efficiency of thin silicon solar cells is mainly limited to the recombination at the rear surface and some loss of light at the longwave length.(3) The effect of germanium (Ge) doping on the light-induced degradation in silicon and solar cells has been demonstrated. The effective segregation coefficient of Ge in silicon is determined to be 0.56. Ge doping with a concentration beyond 1019/cm3 can suppress the formation of the B-O defects in Czochralski silicon, and the reduction percentage of defect concentration increases with the Ge content. The suppression of B-O defects by Ge doping is associated with a reduction of O2i, owing to an increase of the energy barrier for O2i diffusion. It is supposed that Ge doping also increases the capture cross-section of Bs for O2i. After exposure to sunlight, the loss in both the efficiency of Ge-doped silicon solar cells and the power output of corresponding modules is lower. The similar results are also obtained in Ge-doped multicrystalline silicon solar cells. It is found that 15% and 17% of efficiency output is saved by using the Ge-doped Czochralski silicon and multicrystalline silicon solar cells in comparison with the conventional ones.(4) Tin doping in Czochralski silicon can also suppress the generation of light-induced degradation. The B-O defect concentration is significantly reduced by tin-doping (1018cm-3). Compared to conventional Czochralski silicon, higher activation energies for the B-O defect generation/dissociation are both obtained for the tin-doped Czochralski silicon.(5) The mechanical properties of silicon wafers for solar cells have been investigated. There is a smaller roughness on the surfaces of the diamond wire-sawn wafers, which is associated with a presence of phase transformations. The fracture strength of diamond wire-sawn wafers is larger than that of the slurry sawn ones. Ge doping improves the fracture strength of Czochralski silicon, reducing the breakage during cell and module fabrications. The microcracks generated during wire sawing reduce the wafer strength considerably. Grain boundaries might hinder the propogation of cracks in multicrystalline silicon.
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