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.
μXRF investigations on the influence of solar cell processing steps on iron and copper precipitates in multicrystalline silicon
The material quality of multicrystalline silicon is influenced by crystal defects and contaminations like transition metal precipitates. During solar processing these defects can be restructured and change their electrical activity. The purpose of this work is to study the impact of different solar cell processing steps on the distribution and electric activity of transition metal precipitates like iron and copper. Therefore, neighbouring wafers of a multicrystalline silicon ingot, intentionally contaminated with iron and copper were investigated by μXRF (X-Ray Fluorescence Microscopy) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, to determine the distribution of transition metal precipitates. Afterwards, several solar cell processing steps were applied to these samples. The same sample areas were measured by μXRF again to determine the influence of the applied processing steps on the observed transition metal precipitates. Therefore, a different behaviour of iron and copper precipitates could be observed as expected, due to their different dissolution and diffusion coefficients in silicon. Additionally, the same processing steps were applied to a second set of samples to evaluate the effect of processing steps on the minority charge carrier lifetime and the recombination activity of grain boundaries.
Carbon Dioxide Elimination from Circle Systems
Ten various circle systems were studied in model experiments. All the experiments were performed using a Waters cannister containing 675–685 g of Durasorbr̀. 300 ml of carbon dioxide were supplied to the system per min. The efficacy of these systems was expressed as the time taken to demonstrate a concentration of 0.6% carbon dioxide in the gas mixture supplied to the patient connection during each respiratory cycle. The carbon dioxide elimination was studied at a gas flow of 2, 4 and 6 1 per min. All the systems showed an effective carbon dioxide elimination for 6 h or more at a flow of 2 1. An increasing efficacy was found with increasing flow, but to a varying extent.