The effect of wafer stress on absorption coefficient and minority carrier diffusion length in cast poly-Si

The surface photovoltage (SPV) technique is an attractive method for measuring the minority carrier diffusion length of solar cells. It has the advantage of not requiring permanent sample contacts. It does require, however, an accurate knowledge of the absorption coefficient-wavelength (α-λ) relationship. This relationship is well known for single-crystal silicon and SPV plots are linear for such materials. However, for polycrystalline silicon, nonlinear SPV plots are frequently found and the authors believe the single-crystal α-λ data not to be applicable. Depending which portion of the plot is used for diffusion length determination, they obtain very different diffusion lengths. By making SPV, short circuit current, and spectral response measurements and comparing theory with experiments, they find a new α-λ relationship that appears to depend on the stress in the sample. They find the low wavelength range of the SPV plots to be most suitable for diffusion length measurements