Dresden 2014 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 54: Computational Materials Modelling VIII - Functional materials
MM 54.2: Talk
Thursday, April 3, 2014, 12:15–12:30, IFW D
Conducting monolayers of Sodium at stacking faults in Silicon — •Benedikt Ziebarth1,2, Matous Mrovec1,2, Christian Elsässer1,2, and Peter Gumbsch1,2 — 1Fraunhofer IWM, Freiburg, Germany — 2Karlsruhe Institute of Technology, IAM-ZBS, Karlsruhe, Germany
Sodium decorated stacking faults (SFs) in Silicon were identified recently as an origin for potential-induced-degradation failure of solar-cell modules based on crystalline silicon [1]. The SFs were observed to be covered by about one monolayer of Na atoms, and this decoration was interpreted to cause local electrical short-circuits of the p-n-junction in the solar cell. In the present study such a SF in Si decorated with Na was investigated by means of density functional theory in order to elucidate its structural, diffusion and electronic properties. The Na atoms lead to a substantial elongation of Si-Si bonds across the SF. Minimum-energy-path calculations for Na atoms diffusing along the SF yield a strong dependence of the activation barrier on the degree of layer coverage of the SF with Na. In a SF with a half monolayer of Na the barrier is higher than for interstitial Na in bulk Si, whereas in a SF with a full monolayer of Na the barrier is lower by an order of magnitude. Electronic-structure calculations reveal that the Na at the SF cause defect levels in the band gap of Si which are partially filled and hence lead to electrical conduction along the SF. [1] V. Naumann et al., phys. stat. sol. (RRL) 6, 331-333 (2012).