Research Article Prediction of a New Phase of Cu x S near Stoichiometric Composition Prashant Khatri and Muhammad N. Huda Department of Physics, University of Texas at Arlington, Arlington, TX 76019, USA Correspondence should be addressed to Muhammad N. Huda; huda@uta.edu Received 16 November 2014; Revised 15 January 2015; Accepted 3 March 2015 Academic Editor: Mahmoud M. El-Nahass Copyright © 2015 P. Khatri and M. N. Huda. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Cu 2 S is known to be a promising solar absorber material due to its suitable band gap and the abundance of its constituent elements. Cu 2 S is known to have complex phase structures depending on the concentration of Cu vacancies. Its instability of phases is due to favorable formation of Cu vacancies and the mobility of Cu atoms within the crystal. Understanding its phase structures is of crucial important for its application as solar absorber material. In this paper, we have predicted a new crystal phase of copper sulfde (Cu x S) around chemical composition of  = 1.98 by utilizing crystal database search and density functional theory. We have shown that this new crystal phase of Cu  S is more favorable than low chalcocite structure even at stoichiometric composition of =2. However, Cu vacancy formation probability was found to be higher in this new phase than the low chalcocite structure. 1. Introduction Cu 2 S is a well-known semiconductor which has the potential to play an important role as a solar absorber material. Tin flms of heterojunctions such as Cu 2 S/CdS have shown an efciency of 9-10% in the past [1–4]. However, Cu 2 S shows complex structural behavior. Experimental results so far have shown that stoichiometric Cu 2 S mainly exists in three forms: monoclinic phase (low chalcocite) forms at temperature below 104 ∘ C, hexagonal phase (high chalcocite) forms between 104 and 436 ∘ C, and cubic phase forms at temperature 436 ∘ C or higher [5, 6]. Teoretical calculations on three chalcocites have shown consistency with exper- imental facts revealing low chalcocite is favorable at 0 K [7, 8]. Te low chalcocite has 96 Cu and 48 S atoms and is monoclinic in nature. A major problem in Cu 2 S is the copper atom’s instability towards the formation of copper vacancies; this leads to the formation of diferent crystal structures depending on Cu vacancy concentrations [7–12]. In addition, this formation of high copper vacancies results in very high p-type doping [13] causing the material to behave like a degenerate semiconductor. Moreover, in Cu 2 S, the positions of Cu within the sublattice of S atoms are not well defned [7, 8] as copper atoms are mobile in nature in Cu 2 S [14, 15]. Despite being earth abundant and nontoxic, these problems hindered the further study of Cu 2 S in photovoltaic community for some time. Table 1 provides names of known Cu 2 S phases those that are experimentally identifed with their composition. Experimentally observed band gaps of these Cu x S phases fall in the range of 1.1–1.2eV; however, the nature of these band gaps is not well identifed yet [7, 8]. Various studies have stated that studying and understanding Cu x S have been a long term challenge [4–6, 16–18], especially in computational works, which is mainly due to the complex behavior of copper in Cu 2 S. Experimental and theoretical studies have shown that Cu vacancies are inevitable in Cu 2 S. Hence, for the efective usage as a solar absorber material an understanding and a detail map of Cu 2 S phases based on Cu vacancy con- centrations are essential. To avoid excessive p-type doping, it is desirable to stabilize a phase of Cu x S near  = 2. It can be assumed that at thermodynamic equilibrium, low chalcocite structure may sustain these composition ranges near  = 2. However, this has not been tested and not reported anywhere. To the best of our knowledge, no phases have been so far identifed with the chemical composition 2 >  > 1.97. Hence, it is important to study Cu x S com- pounds with diferent possible structures with a value of  Hindawi Publishing Corporation International Journal of Photoenergy Volume 2015, Article ID 478978, 7 pages http://dx.doi.org/10.1155/2015/478978