Effect of sodium incorporation into CuInSe2 from first principles

The presence of small amounts of sodium has been shown to improve the electronic performance of Cu(In,Ga)Se-2 (CIGS) solar cells, but the origins of this effect have not yet been fully resolved. In this work, we have addressed the questions involving the role of sodium in CuInSe2 (CIS) using densityfunctional-theory-based calculations. We find no direct way how the creation of Na-related point defects in bulk CIS would enhance p-type conductivity. Instead, we demonstrate that Na reduces copper mass transport due to the capture of copper vacancies by Na-Cu defects. This finding provides an explanation for experimental measurements where the presence of Na has been observed to decrease copper diffusion. The suggested mechanism can also impede V-Cu-related cluster formation and lead to measurable effects on defect distribution within the material. (C) 2013 AIP Publishing LLC. DOI: https://doi.org/10.1063/1.4819105 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-81910 Accepted Version Originally published at: Oikkonen, L E; Ganchenkova, M G; Seitsonen, A P; Nieminen, R M (2013). Effect of sodium incorporation into CuInSe2 from first principles. Journal of Applied Physics, 114(8):083503. DOI: https://doi.org/10.1063/1.4819105 Effect of sodium incorporation into CuInSe2 from first principles Effect of sodium incorporation into CuInSe2 from first principles L.E. Oikkonen, M.G. Ganchenkova, 2 A.P. Seitsonen, and R.M. Nieminen COMP Centre of Excellence, Department of Applied Physics, Aalto University, P.O. Box 11000, FI-00076 Aalto, Espoo, Finland Department of Materials Science, National Research Nuclear University (MEPhI), 31 Kashirskoe Avenue, 115409 Moscow, Russia Physikalisch-Chemisches Institut, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland (Dated: 2 August 2013) The presence of small amounts of sodium has been shown to improve the electronic performance of Cu(In,Ga)Se2 (CIGS) solar cells, but the origins of this effect have not yet been fully resolved. In this work, we have addressed the questions involving the role of sodium in CuInSe2 (CIS) using density-functional-theorybased calculations. We find no direct way how the creation of Na-related point defects in bulk CIS would enhance p-type conductivity. Instead, we demonstrate that Na reduces copper mass transport due to the capture of copper vacancies by NaCu defects. This finding provides an explanation for experimental measurements where the presence of Na has been observed to decrease copper diffusion. The suggested mechanism can also impede VCu-related cluster formation and lead to measurable effects on defect distribution within the material. PACS numbers: 61.72.Bb,66.30.-h,71.15.Mb