Sorting Metrics for Customized Phosphorus Diffusion Gettering

Customized solar cell processing based on input material quality has the potential to increase the performance of contaminated regions of multicrystalline silicon ingots. This provides an opportunity to improve material yield and device efficiency without substantially reducing the overall throughput. Simulations and experiments show that in wafers from the top and border regions of an ingot containing as-grown iron concentrations ≳1014cm-3, a high concentration of interstitial iron point defects, i.e., Fei, remains after standard phosphorus diffusion gettering (PDG), severely limiting electron lifetime and simulated efficiencies of PERC-type solar cells. It is shown that an extended PDG leads to a stronger reduction of Fei point defects, enabling high-efficiency devices, even on wafers from the red zone of the ingot. However, a satisfactory performance improvement after standard PDG is already achieved on wafers that contain as-grown total iron concentrations14cm-3, making the low-throughput extended PDG process unnecessary for a large fraction of the ingot. We propose using the total iron concentration and the corresponding photoluminescence contrast between grain boundaries and intragranular regions in the as-grown wafer as a simple sorting metric to determine when extended phosphorus diffusion is warranted.