Influence of incorporation of Na on p-type CuInS 2 thin films

Abstract

We have investigated the electronic structures of Na-incorporated In-rich CuInS 2 based on ab initio electronic band structure calculations and X-ray photoelectron spectroscopy (XPS). From the results of theoretical calculations and experiments, for p-type In-rich CuInS 2 films, we find that interstitial Na species, except for Na at Cu sites, are very unstable and move easily within the crystal. This indicates that the mobile Na will act as a passivator of donor states, such as In at Cu sites and interstitial In, leading to good p-type conductivity of CuInS 2 films. On the other hand, we conclude that the formation of the ionic chemical bonds of Na at Cu sites - S atoms near the surface are energetically favorable due to a decrease in the Madelung energy. This results in the stabilization of ionic charge distributions of CuInS 2:Na Cu with a shift in the energy levels of S 3 p orbitals in the vicinity of the Na atoms towards lower energy regions. As a result, the Na incorporation yielded a surface layer of expressed as (Na,Cu) InS 2 on p-type In-rich CuInS 2 thin films.