Investigation of Lifetime-Limiting Defects After High-Temperature Phosphorus Diffusion in Silicon Solar Cell Materials

David P Fenning1, Annika S Zuschlag2, Alexander Frey2, Mariana I Bertoni3, Barry Lai4, Giso Hahn2, Tonio Buonassisi1 1Massachusetts Institute of Technology, Cambridge, MA, United States /2University of Konstanz, Department of Physics, Konstanz, Germany /3Department of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe, AZ, United States /4Advanced Photon Source, Argonne National Lab, Argonne, IL, United States

Phosphorus diffusion gettering of contaminated multicrystalline silicon solar cell materials generally fails to produce material with minority carrier lifetimes that approach that of gettered monocrystalline wafers, due in part to the higher levels of contamination with metal impurities. Higher gettering temperatures should speed up dissolution of precipitated metals by increasing their diffusivity and solubility in the bulk, potentially allowing for improved gettering. In this contribution we investigate the impact of gettering at higher temperatures on mono- and multicrystalline samples with a focus on determining the lifetime-limiting defects.  To analyze the gettering response, we measure: the spatially-resolved lifetime and interstitial iron concentration by microwave photoconductance decay and photoluminescence imaging, the reduction and/or removal of precipitated iron by synchrotron-based micro-X-ray fluorescence, and the structural defect density by Sopori etching and large-area automated quantification. Higher-temperature gettering is seen to improve metal-limited multicrystalline materials dramatically, especially in areas of low dislocation density, while monocrystalline materials degrade with higher gettering temperatures.  In areas of high as-grown dislocation density in the multicrystalline materials, it appears that higher-temperature gettering reduces the dislocation density, but leaves higher local concentrations of interstitial iron which degrade lifetime.