Synchrotron-based analysis of chromium distributions in multicrystalline silicon for solar cells

Abstract

Chromium (Cr) can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 10¹⁰cm⁻³. In this contribution, we employ synchrotron-based X-ray fluorescence microscopy to study chromium distributions in multicrystalline silicon in as-grown material and after phosphorous diffusion. We complement quantified precipitate size and spatial distribution with interstitial Cr concentration and minority carrier lifetime measurements to provide insight into chromium gettering kinetics and offer suggestions for minimizing the device impacts of chromium. We observe that Cr-rich precipitates in as-grown material are generally smaller than iron-rich precipitates and that Cr[subscript i] point defects account for only one-half of the total Cr in the as-grown material. This observation is consistent with previous hypotheses that Cr transport and CrSi₂ growth are more strongly diffusion-limited during ingot cooling. We apply two phosphorous diffusion gettering profiles that both increase minority carrier lifetime by two orders of magnitude and reduce [Cr[subscript i]] by three orders of magnitude to 10¹⁰cm⁻³. Some Cr-rich precipitates persist after both processes, and locally high [Cr[subscript i]] after the high-temperature process indicates that further optimization of the chromium gettering profile is possible.