Modeling the size distribution of iron silicide precipitates in multicrystalline silicon
Precipitation of iron in multicrystalline (mc) silicon during typical solar cell processing is investigated with an advanced model that has been previously demonstrated to be able to simulate the iron distribution in two dimensions. The improving detection limits of precipitate measurements with x-ray fluorescence microscopy (μ XRF) allow a first comparison of measured and simulated precipitate densities in mc solar cell material. We focused on simulating the size distribution of iron-silicide precipitates at a grain boundary for a better comparison with experimental data. The simulated line densities of detectable iron precipitates as well as the size distribution along a grain boundary are in good agreement with experiments. These results verify the iron kinetics model, independent of the interstitial iron concentration. The dependency of the assumed total iron concentration and the model structure is investigated by varying these input parameters. The model predicts a fast dissolution of small and medium iron precipitates during inline phosphorus diffusion, resulting in an increase of the mean size of precipitates. Low-temperature anneals effectively reduce the interstitial iron concentration, but influence only the density of small precipitates, which are below experimental detection limits. The simulations demonstrate that iron precipitates become more important for the charge-carrier lifetime during solar cell processing.
Experimental and theoretical study of heterogeneous iron precipitation in silicon
Heterogeneous iron precipitation in silicon was studied experimentally by measuring the gettering efficiency of oxide precipitate density of 1×1010cm−3. The wafers were contaminated with varying iron concentrations, and the gettering efficiency was studied using isothermal annealing in the temperature range from 300to780°C. It was found that iron precipitation obeys the so-called s-curve behavior: if iron precipitation occurs, nearly all iron is gettered. For example, after 30min annealing at 700°C, the highest initial iron concentration of 8×1013cm−3 drops to 3×1012cm−3, where as two lower initial iron concentrations of 5×1012 and 2×1013cm−3 remain nearly constant. This means that the level of supersaturation plays a significant role in the final gettering efficiency, and a rather high level of supersaturation is required before iron precipitation occurs at all. In addition, a model is presented for the growth and dissolution of iron precipitates at oxygen-related defects in silicon during thermal proces...
Evaluation of Processing Steps Regarding Lifetime of Iron/Copper Contaminated mc Si Wafers
It is well known that gettering and passivation steps during solar cell processing enhance the minority charge carrier lifetime (here simply referred to as lifetime) in the silicon wafers. The purpose of this work is to study the influence of different solar cell processing steps on lifetime depending on the impurity level. Therefore neighboring wafers of standard multicrystalline as well as differently contaminated ingots were treated with various POCl3-diffusion and/or hydrogenation steps. The sequence of treatments is varied to check the effect of each processing step individually and to investigate if an individual processing step is less or more effective if another step was applied before. Afterwards these wafers were examined by microwave photoconductance decay measurements (μPCD). The gained results might be important for defect engineering and the development of an optimized solar cell process on cost effective and impurity-rich silicon material to reduce the detrimental impact of metal impurities on solar cell parameters.