Saddle-Point Excitons and Their Extraordinary Light Absorption in 2D β-Phase Group-IV Monochalcogenides

In 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, monolayer beta-phase group-IV monochalcogenides (MX, M = Ge or Sn; X = S or Se) are shown to be a new class of 2D materials that possess saddle-points in both the lowest conduction band and the highest valence band as well as in the joint density of states. Due to the existence of saddle-points, a remarkable absorption peak within the fundamental gap is found in these materials when the light polarization is along the armchair (y) direction. 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 bandgap 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.