Theoretical investigation of Cu-containing materials with different valence structure types: BaCu 2 S 2 , Li 2 CuSb, and LiCuS S. Soliman Zagazig University, Faculty of Science, Department of Physics, Zagazig, Egypt article info Article history: Received 25 July 2013 Received in revised form 26 November 2013 Accepted 7 December 2013 Available online 17 December 2013 Keywords: BaCu 2 S 2 Density functional theory Electronic structure LiCuS Li 2 CuSb abstract Optoelectronics research requires cheap materials with a broad spectrum of optical, electronic, and structural properties. The class of Heusler compounds and ternary structures provide many possibilities for finding alternative group IV and III–V semiconductor compounds. This study introduces wider band gap materials for use in solar cells as an alternative to cadmium sulfide buffer layers. The buffer layer is inserted between the absorber layer (p-type) and the transparent window layer (n-type) to enhance the maximum amount of light transmission. Reasonable calculations are reported for the band gaps of copper-containing materials: LiCuS, BaCu 2 S 2 , and Li 2 CuSb. Previous optical analysis measurements of these films determined that the band gaps were 1.8 and 1.9 eV for BaCu 2 S 2 and LiCuS, respectively. In general, semiconductor compounds have been studied theoretically, but there are major differences between the experimental and theoretically calculated band gaps. A suitable calculation method for semiconductor compounds is described in this study. For the first time, calculations based on the Engel and Vosko method are introduced for these semiconductor compounds. This method yields band gaps that are comparable to the experimental values, which facilitate the development of microscopic analyses of these compounds. Direct band gaps of 1.15 and 1.7eV were obtained for BaCu 2 S 2 and LiCuS, respectively, whereas the indirect band gap was 0.7 eV for Li 2 CuSb. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction High-temperature superconductivity compounds represent an attractive alternative to Cu-containing systems where the copper is bound to different types of atoms with an open p-shell [1,2]. Cu-containing compounds have attracted increasing attention since the discovery of their potential use as p-type materials in transparent photo-electronic applications [3–11]. Previous studies of BaCu 2 S 2 , Li 2 CuSb, and LiCuS have demon- strated the importance of these compounds as potential absorp- tion materials for use in thin-film solar cells. Previously, Valencia and Spies reported detailed experimental studies of BaCu 2 S 2 [12–14]. The electronic structure of BaCu 2 S 2 was studied based on density functional theory and a band gap of 1.62 eV was determined using the hybrid functionals [15]. Different experi- mental radii of 6.21 Å and 6.29 Å were reported previously for Li 2 CuSb [16], as well as 6.27 Å [17]. Li 2 CuSb was predicted to be a conductive material by Dai where the exchange-correlation func- tional was calculated based on the generalized gradient approx- imation (GGA) using the parameterization of Perdew et al. [18]. Similar GGA calculations for Li 2 CuSb were reported by Lin, who demonstrated the semiconductor behavior of this compound [19]. Lin also provided a good explanation of the topological arrange- ment of the different angular momentums around the Fermi level for Li 2 CuSb [19]. Kieven et al. reported a band gap of 0.8 eV and a lattice parameter of 5.602 Å for LiCuS [20]. In previous studies, therefore, there are major differences between the experimental and calculated parameters. Thus, the objectives of the present study were to provide accurate calculations of the band gaps based on an experimental approach and to compare p-type materials that may be suitable for transparent electronic applications. The compounds BaCu 2 S 2 , Li 2 CuSb, and LiCuS were examined to elucidate their electronic structures, thereby achieving a better understanding of their bonding and physical properties. The structures, density of states (DOS), partial density of states (PDOS), and calculated band structures are described in detail. 2. Crystal structure and calculation details Full atomic structure optimizations were performed by redu- cing the forces on the atoms to a sufficiently small value. The atoms were distributed randomly among the permissible standard positions. Calculations were then performed for each structure distribution to determine the lowest energy structure. The elec- tronic structure calculations were performed using the lowest energy distribution for each compound as follows. The crystal Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jpcs Journal of Physics and Chemistry of Solids http://dx.doi.org/10.1016/j.jpcs.2013.12.010 0022-3697/& 2013 Elsevier Ltd. All rights reserved. Journal of Physics and Chemistry of Solids 75 (2014) 927–930