A first-principles model of copper-boron interactions in Si for the light-induced degradation of solar Si

• Published in: Journal of physics. Condensed matter Vol. 29; no. 6
• Main Authors: Wright, E., Coutinho, J., Öberg, Sven, Torres, V.J.B.
• Format: Journal Article
• Language: English
• Published: 2017
• Subjects: Applied Physics; Tillämpad fysik
• ISSN: 1361-648X; 0953-8984
• DOI: 10.1088/1361-648X/aa4d78

The recent discovery that Cu contamination of Si combined with light exposure has a significant detrimental impact on carrier life-time has drawn much concern within the solar-Si community. The effect, known as the copper-related light-induced degradation (Cu-LID) of Si solar cells, has been connected to the release of Cu interstitials within the bulk (2016 Sol. Energy Mater. Sol. Cells 147 115-26). In this paper, we describe a comprehensive analysis of the formation/dissociation process of the CuB pair in Si by means of first-principles modelling, as well as the interaction of CuB defects with photo-excited minority carriers. We confirm that the long-range interaction between the Cu-i(+) cation and the B-s(-) anion has a Coulomb-like behaviour, in line with the trapping-limited diffusivity of Cu observed by transient ion drift measurements. On the other hand, the short-range interaction between the d-electrons of Cu and the excess of negative charge on B-s(-) produces a repulsive effect, thereby decreasing the binding energy of the pair when compared to the ideal point-charge Coulomb model. We also find that metastable CuB pairs produce acceptor states just below the conduction band minimum, which arise from the Cu level emptied by the B acceptor. Based on these results, we argue that photo-generated minority carriers trapped by the metastable pairs can switch off the Coulomb interaction that holds the pairs together, enhancing the release of Cu interstitials, and acting as a catalyst for Cu-LID.