Saddle-point Excitons and Their Extraordinary Light Absorption in Two-Dimensional beta-phase Group-IV Monochalcogenides

In two-dimensional (2D) materials, saddle-points in the electronic structure give rise to diverging density of states, which leads to intriguing physical phenomena useful for applications, including magnetism, superconductivity, charge density wave, as well as enhanced optical absorption. Using first-principles calculations, we show monolayer beta-phase of group-IV monochalcogenides are a new class of 2D materials that possess saddle-points. Due to the existence of saddle-points, a remarkable absorption peak within the fundamental gap is found in these materials. The properties of saddle-point excitons can be effectively tuned by both the strain and thickness of these materials. Importantly, the strong optical absorbance induced by saddle-point exciton absorptions and the appropriate band gap give ideal power conversion efficiencies as large as 1.11% for monolayer beta-SnSe, significantly higher than reported high-performance ultrathin solar cells using transition metal dichalcogenides. These results not only open new avenues for exploring novel many-body physics, but also suggest beta-phase MXs could be promising candidates for future optoelectronic devices.