Oxygen-Related Defect Centers in Solar-Grade, Multicrystalline Silicon. A Reservoir of Lifetime Killers

The effective lifetime of charge carriers τ eff in multicrystalline silicon (mc-Si) Baysix® wafers (in our case: Bridgman-type) cut from areas close above the ingot bottom is reduced (τ eff < 1 μs). However, the efficiency of solar cells processed on such bottom-near Baysix® wafers is comparable to that one of solar cells processed on Baysix® wafers taken from the middle of the ingot. In order to clarify this unusual behavior, specially casted, low-doped n- and p-type mc-Si ingots, respectively, were fabricated by Bayer AG and were studied by optical and electrical characterization techniques. It is demonstrated that the decrease of the effective lifetime τ eff in as-grown wafers originating from a region directly above the ingot bottom is caused by oxygen-related Thermal Donors (TDs), Shallow Thermal Donors (STDs) and O1-/O2-defects as well as by low concentrations (<1.5 x 10 12 cm -3 ) of vanadium and chromium impurities. Most of the oxygen-related defect centers are thermally dissociated by the solar cell process resulting in comparable efficiencies as obtained for wafers taken from the middle of the ingot.