Optical properties of SnX (X=S,Se): Insight from independent particle approximation and Bethe Salpeter equation

Abstract

First-principles calculations based on density functional theory (DFT) are carried out to study the electronic, structural and optical properties of bulk SnX (X = S, Se). The calculated cohesive energy calculations show that bond strength is smaller in SnSe than SnS that makes SnS energetically more stable than SnSe. Electronic band structures show that, both SnS and SnSe are indirect band gap semiconductors with band gap of 1.09 eV and 0.72 eV, respectively. The BSE method combined with IPA is employed to incorporate excitonic effects, accurately capturing the optical response of the materials. The static dielectric constants of SnS and SnSe exhibit directional anisotropy and show a significant increase when excitonic interactions are considered, highlighting the importance of many-body effects in accurately predicting optical properties. The exciton binding energy and Bohr radius are also calculated to assess the strength of electron–hole interactions. The plasmon frequency is found to be larger for SnS as compared with SnSe due to larger carrier density of SnS. It is shown that SnSe has larger refractive index and extinction coefficient than SnS in both IPA and BSE. Most of the light is absorbed in the visible region for both materials. Both materials have efficient absorption in near infrared (IR) and visible range (VR) and maximum in near ultraviolet (UV) range, but SnSe has a little higher absorption coefficient than SnS in visible spectrum region due to smaller band gap than SnS. Overall, BSE under estimates the optical properties in the UV region as compared with IPA. We, therefore, believe that both materials are efficient for photovoltaic applications.