Defect properties of Na and K in Cu_2ZnSnS_4 from hybrid functional calculation

摘要

The in-growth or the post-deposition treatment of the Cu2ZnSnS4 (CZTS) absorber layer has led to improved photovoltaic efficiency; however, the underlying physical mechanisms of such improvements are less studied. In this study, the thermodynamics of Na and K related defects in CZTS are investigated by a first principles approach using a hybrid functional, with the chemical potential of Na and K established from various phases of their polysulfides. Both Na and K predominantly substitute on Cu sites similar to their behavior in Cu(In,Ga)Se-2, in contrast to previous results using the generalized gradient approximation. All substitutional and interstitial defects are shown to be either shallow levels or highly energetically unfavorable. Defect complexing between Na and abundant intrinsic defects did not show the possibility of significant incorporation enhancement or introducing deep n-type levels. The possible benefit of Na incorporation on enhancing photovoltaic efficiency is discussed. The negligible defect solubility of K in CZTS also suggests this to be a possible surfactant. Published by AIP Publishing.