D. Karg

Formation and annihilation of oxygen donors in multicrystalline silicon for solar cells

Abstract The efficiencies of solar cells based on multicrystalline silicon (mc-Si) have reached 17% even employing high-throughput crystallization steps and industrial-relevant solar cell processes. The efficiency of multicrystalline solar cells is governed by crystal defects, impurities and the interaction of both. The number of crystal defects, such as dislocations and grain boundaries, crucially depends on the crystallization conditions, while, with regard to impurities, electrically active transition metals, such as iron, are well-known to seriously reduce the minority carrier lifetime. A similarly important role, however, is played by oxygen. Various oxygen or oxygen-containing defect centers showing strong recombination activity may form in monocrystalline silicon as well as in mc-Si. In mc-Si blocks the formation of so-called thermal donors and nitrogen-oxygen complexes can take place during the relatively slow cooling of the ingots. Thermal donors and nitrogen-oxygen complexes lead to reduced lifetimes especially in the edge regions of the ingot. Whereas this lifetime reduction is hardly efficiency-relevant as long as annealing steps above 600°C for several minutes are implemented in solar cell processing, another species of oxygen donor, the new donor, forms in the temperature range between 600 and 900°C that is frequently used for solar cell fabrication. For silicon with a high oxygen content such as the Bayer RGS (ribbon growth on substrate) material, the new donors seem to be the most efficiency-relevant defects which can only be prevented using well-adjusted temperature profiles during crystallization and solar cell processing. Whereas monocrystalline silicon can benefit from high oxygen content through internal gettering steps in microelectronic device processing, a substantial improvement of mc-Si for solar cells is achievable by lowering the oxygen content. Oxygen contents considerably below those of monocrystalline silicon are therefore state of the art for modern high-throughput production material fabricated by the block-casting technology.

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.

Kinetics of hydrogenation and interaction with oxygen in crystalline silicon

Sufficient passivation of recombination active defects in the bulk of crystalline silicon solar cells using atomic hydrogen is a key feature for reaching high conversion efficiencies. This is of special interest for promising low-cost multi-crystalline (mc) materials, as a substantial cost reduction concerning Watt-peak(Wp)-costs seems to be possible. The effectiveness of this hydrogenation is strongly influenced by the diffusion kinetics of atomic hydrogen in silicon. Oxygen impurities seem to play a major role, as they have the ability to trap hydrogen, slowing down the diffusion of hydrogen atoms. For two crystalline silicon materials the influence of different oxygen concentrations on hydrogen kinetics is discussed. We demonstrate that not only the overall oxygen concentration, but as well the thermal history of the samples has to be taken into account. Precipitation of oxygen alters the diffusion kinetics and has an influence on vacancy concentration. Faster passivation of crystal defects can be reached in low-oxygen samples.