Gettering in polycrystalline silicon solar cells

Abstract

Polycrystalline silicon wafers were subjected to extra thermal treatments (0–120 min, 500–800 °C) during the processing of solar cells. It was found that the bulk effective minority carrier diffusion length was enhanced significantly upon annealing, provided the front and back sides of the wafers were doped with phosphorus and aluminium respectively, prior to the anneal. The optimum anneal temperature was 700 °C, yielding an increase of over 10% in diffusion length and a solar cell efficiency improvement of 0.5% absolute, compared with standard cells. Light-beam-induced current measurements have shown that the thermal treatment is most effective in regions with an initially small minority carrier diffusion length. In a separate model experiment with intentionally contaminated wafers, the concentration of metallic impurities such as gold, nickel and copper in the aluminium-doped layer was found to be 1000 times larger than the solid solubility of these metals in silicon at 700 °C. From these observations, it is concluded that gettering of impurities occurs during the annealing and that the aluminium-doped layer (produced by low cost screen-printing process) provides an effective sink for impurities.