Electronic Structure and Defect Physics of Tin Sulfides: SnS, Sn2 S3, and Sn S2

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 photocatalysts. 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 GW0 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.