New insights into upgraded metallurgical grade silicon wafers through lifetime spectroscopy

Upgraded Metallurgical-Grade (UMG) silicon (Si) has raised interest as an alternative material for silicon solar cells due to its low cost, low environmental impact and low CAPEX. Maximum cell efficiencies similar to those obtained with high purity poly-Si have been reported. However, a higher defect density and the compensated doping character result in UMG-based efficiencies varying over wider ranges in frequency distribution charts. Developing methods to precisely identify impurities has been an active field of work for many years. Among others, Injection Dependent Lifetime Spectroscopy (IDLS) is considered a very sensitive technique suitable for an accurate determination of the defect's fingerprint. In this work we present a comprehensive analysis of UMG-Si by IDLS on wafers with different starting carrier lifetimes. After P-diffusion gettering optimization and related lifetime improvements up to two orders of magnitude, we hypothesize Cu as the main impurity that governs the minority carver lifetime and its associated defect parameters, energy levels and symmetry factors, based on IDLS results. Furthermore, we point out the importance of relevant factors affecting IDLS analysis, that could lead to defect misinterpretation, such as the mobility model employed in the lifetime analysis (as UMG-Si is a compensated material) and the impact of the surface recombination velocity. We estimate the Cu concentration to be in the range7-13.10(12) cm(-3), based on the agreement between simulations and lifetime measurements.