Defect diagnostics using scanning photoluminescence in multicrystalline silicon for solar cells

The main goal of the project is to develop a scanning room temperature photoluminescence (PL) mapping system (both hardware and software) and to apply this technique to characterize areas with high and low recombination activity of defects in multicrystalline silicon (mc-Si) wafers for solar cells. The objectives are (1) to analyze the recombination activity of dislocations in as-grown and processed mc-Si wafers, (2) to find a correlation between topology of PL distribution and distributions of minority carrier lifetime, dislocations, and residual elastic stress in full-size mc-Si wafers and solar cells, and (3) develop the algorithm of electronic quality control in silicon wafers using scanning PL spectroscopy.; A positive correlation between the band-to-band PL intensity and the distribution of minority carrier lifetime was found in me-Si produced by Edge-Defined Film-Fed Growth and Block Cast techniques. In the regions with enhanced recombination activity of defects (low minority carrier lifetime) a new “defect” luminescent band with a maximum at 0.78eV was observed. The intensity of this defect band is negligible in wafer regions with low recombination activity. Its distribution measured across the entire mc-Si wafer shows a reverse contrast to the band-to-band PL intensity. The concentration of the 0.78eV defect centers shows only a small reduction between the as-grown wafer and final solar cell, contrary to a strong increase of the lifetime due to gettering and passivation mechanisms. Low-temperature PL spectroscopy gave clear evidence that the defect luminescence has a dislocation origin. This was confirmed by dislocation mapping. The defect PL band has a strong linear polarization up to ∼60% due to the elastic stress field, in contrast to a low polarization degree for the band-to-band luminescence. The internal stress mapping was performed in this project using scanning infrared polariscopy. The defect centers are localized in the vicinity of a high residual stress, which may account for a stress-induced polarization of the defect band. Scanning PL spectroscopy can be applied for monitoring of dislocation activity in commercial mc-Si wafers and solar cells.