Comprehensive characterization of efficiency limiting defects in the swirl-shaped region of Czochralski silicon

Abstract

For Czochralski silicon (Cz-Si) solar cells, swirl-shaped regions in silicon wafers could lead to efficiency degradation, usually accompanied by hot spots and thermal breakdown. In this paper, comprehensive characterization methods including electroluminescence (EL), photo-induced current (LBIC), quantum efficiency (QE), cell efficiency, carrier lifetime, scanning electron microscope (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and deep level transient spectroscopy (DLTS) were used to investigate the formation and properties of the Swirl defects. It is found that there exists a strong correlation between the as-grown swirl defects and the degradation in cell performance. The Swirl defects cause an energy level of E v + 0.33 eV in silicon bandgap, with the capture cross section of 2.3 × 10 −15 cm 2 , which is consistent with the feature of silicon/oxide interface states. The swirl defects can further be evolved into stacking faults after annealing at 1050 °C for 16 h, and cause an additional energy level of E v + 0.42 eV with the capture cross section is 1.0 × 10 −14 cm 2 . These results suggest that oxygen precipitates are the vital component or precursor for the formation of swirl-shaped regions in Cz silicon wafers. • A comprehensive study of the nature of Swirl defects in the photovoltaic Czochralski silicon is obtained. • The direct evidence that Swirl defects are related to oxygen precipitates is provided by deep level transient spectroscopy (DLTS) measurements. • The influence of Swirl defects on solar cell performances and material properties is characterized.