Effect of surface damage on strength of silicon wafer for solar cells

Recently, solar power generation using crystalline silicon wafers has been rapidly growing. It is well known that the mechanical properties of silicon wafers are affected by defects caused by slicing processes of the wafers. However the effect of the defects on the mechanical properties, especially, strength of the crystalline silicon wafers has not been clarified yet. In this paper, to clarify the effect of the defects during slicing process, bending tests are conducted using defects specimens. The specimens are given a defect by indentations referred to the slicing process of wafers. The two indentation loads are chosen to reveal the effect of the defects size on the bending strength. The bending strength is evaluated using based on fracture mechanics. The residual stress and the initial crack affect the bending strength; therefore, the residual stress and Raman spectrometry of the specimens are measured before the bending tests. Fracture surfaces are also observed by Transmission Electron Microscope (TEM). The experiments show that the bending strength depends on the defects size and that there are numerous cracks on the wafer surface caused by the slicing process. Finally, it is found that the minimizing of the mechanical surface defects on wafers gives the high reliability for photovoltaic modules and the increase in the wafer strength. Furthermore the residual stress and Raman spectrometry show that the difference of surface damage mode caused by the slicing processing method.