Electronic Structure and Defect Physics of Tin Sulfides: SnS, Sn2S3, and SnS2

The tin sulfides SnS, Sn2S3, and SnS2 are investigated for a wide variety of applications such as photovoltaics, thermoelectrics, two-dimensional electronic devices, Li ion battery electrodes, and photo-catalysts. For these applications, native point defects play important roles, but only those of SnS have been investigated theoretically in the literature. In this study, we consider the band structures, band-edge positions, and thermodynamical stability of the tin sulfides using a density functional that accounts for van der Waals corrections and the GW(0) approximation. We revisit the point-defect properties, namely, electronic and atomic structures and energetics of defects, in SnS and newly examine those in SnS2 and Sn2S3 with a comparison to those in SnS. We find that SnS2 shows contrasting defect properties to SnS: Undoped SnS shows p-type behavior, whereas SnS2 shows n type, which are mainly attributed to the tin vacancies and tin interstitials, respectively. We also find that the defect features in Sn2S3 can be described as a combination of those in SnS and SnS2, intrinsically Sn2S3 showing n-type behavior. However, the conversion to p type can be attained by doping with a large monovalent cation, namely, potassium. The ambipolar dopability, coupled with the earth abundance of its constituents, indicates great potential for electronic applications, including photovoltaics.