Limiting Defects in n‐Type Multicrystalline Silicon Solar Cells

Abstract

Multicrystalline (mc) solar cells made from n‐type silicon feedstock have shown record efficiencies of 22.3% in a tunnel‐oxide passivating contact (TOPCon) cell structure. Still, material‐related carrier recombination limits the attainable efficiency. Herein, the findings of metallic impurity and structural defect concentration present in n‐type mc silicon are summarized, and their limiting properties on carrier lifetime and cell performance are elaborated. Applying a dedicated model for carrier recombination at precipitate–silicon interfaces, it is demonstrated that carrier recombination at metallic precipitates may dominate in the analyzed material. Direct evidence of the recombination activity of iron precipitates in n‐type silicon is given by an analysis of intentionally grown iron precipitates: From a comparison of micro X‐ray fluorescence (μXRF) analyses of differently sized iron precipitates and local carrier recombination from micro‐photoluminescence (μPL), a direct correlation with enhanced carrier recombination is found. From these results, it is concluded that the high‐performance n‐type mc silicon is limited by recombination at (decorated) structural defects, namely, dislocation clusters and grain boundaries, whereas defects in the inner grains are not limiting the efficiency potential. Finally, cell degradation under illumination and elevated temperature for n‐type mc‐silicon solar cells are discussed.