Tunable visible-light excitonic absorption and high photoconversion efficiency in two-dimensional group-VI monolayer materials

Recent discovery of tellurene, a two-dimensional (2D) allotrope of tellurium (Te), has effectively extended the realm of 2D materials to group VI. In this paper, we have proceeded to determine the electronic and excitonic optical properties of 2D tellurene and selenene, together with stable monolayer TeSe2 in this group, by means of the GW-Bethe-Salpeter equation approach. We find that these materials stabilized in 1T-MoS2-like structure appear to be indirect semiconductors and possess ultrahigh isotropic carrier mobilities, which outperform their 2D MoS2 counterpart. The exciton is found to be strongly bound and thermally stable at room temperature with a binding energy of 0.37, 0.53, and 0.54 eV, respectively, for the three types of monolayers. Strikingly, we find that the optical gaps of monolayer selenene and TeSe2 lie in the ideal energy window of 1.0-1.5 eV corresponding to the maximum photoconversion efficiency. The number of layers in the stack further provides an effective means to tune the absorption edge across a wide energy range. These characteristics combined with the strong optical absorbance in the whole visible-light region make both 2D materials promising in the applications of efficient, ultrathin, and flexible photovoltaic devices with upper bounds to the conversion efficiency that rivals organic and dye-sensitized devices.