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.
Distribution of iron in multicrystalline silicon ingots
The distribution of iron in multicrystalline silicon ingots for solar cells has been studied. A p- and a n-type multicrystalline ingot were intentionally contaminated by adding 53ppmwt (μg∕g) of iron to the silicon feedstock and compared to a reference p-type ingot produced from ultrapure silicon feedstock. The vertical total iron distribution was determined by neutron activation analysis and glow discharge mass spectrometry. For the intentionally Fe-contaminated ingots, the distribution can be described by Scheil’s equation with an effective distribution coefficient of 2×10−5. The interstitial iron concentration was measured in the p-type ingots. In the Fe-contaminated ingot, it is almost constant throughout the ingot and constitutes about 50% of the total concentration, which is in conflict with the previous studies. Gettering had a large impact on the interstitial iron levels by reducing the concentration by two orders of magnitude. Considerable trapping was observed at crystal defects on as-cut wafers...
Inhibition of Mild Steel Corrosion Using L-tryptophan and Synergistic Surfactant Additives
The corrosion inhibition characteristics of nitrogen containing amino acid L-tryptophan on mild steel in 0.1 M HCl solution, in the temperature range of 30-50 o C, was studied by weight loss and potentiodynamic polarization measurements. L-tryptophan significantly reduces the corrosion rates of mild steel; the maximum inhibition efficiency being 83% at 50 o C in presence of inhibitor concentration of 500 ppm. The effect of the addition of very small concentrations of anionic surfactant, sodium dodecyl sulfate (SDS), and of cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), on the corrosion inhibition behavior of L-tryptophan was also studied. The inhibition efficiency (IE) of L-tryptophan significantly improved in presence of both surfactants. The effect of SDS and CTAB on the corrosion inhibition behavior of L-tryptophan appears to be synergistic in nature, the values of synergism parameter being greater than unity. The adsorption of inhibitors on mild steel surface obeyed Langmuir’s adsorption isotherm. The calculated thermodynamic parameters for adsorption reveal a strong interaction between the inhibitors and the mild steel surface. The results obtained by electrochemical studies are consistent with the results of the weight loss measurements. L-tryptophan acts more anodic than cathodic inhibitor.