Oxygen-related defect centers in solar-grade, multicrystalline silicon.: A reservoir of lifetime killers

The effective lifetime of charge carriers tau (eff) in multicrystalline silicon (mc-Si) Baysix(R) wafers tin our case: Bridgman-type) cut from areas close above the ingot bottom is reduced (tau (eff) < 1 <mu>s). However, the efficiency of solar cells processed on such bottom-near Baysix(R) wafers is comparable to that one of solar cells processed on BaysixO wafers taken from the middle of the ingot. In order to clarify this unusual behavior, specially casted, low-doped n- and p-type me-Si ingots, respectively, were fabricated by Payer AG and were studied by optical and electrical characterization techniques. It is demonstrated that the decrease of the effective lifetime tau (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 (less than or equal to1.5 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.