  Germanium Selenide (GeSe) is a van der Waals-bonded layered material with
promising optoelectronic properties, which has been experimentally synthesized
for 2D semiconductor applications. In the monolayer, due to reduced
dimensionality and, thus, screening environment, perturbations such as the
presence of defects have a significant impact on its properties. We apply
density functional theory and many-body perturbation theory to understand the
electronic and optical properties of GeSe containing a single selenium vacancy
in the $-2$ charge state. We predict that the vacancy results in mid-gap "trap
states" that strongly localize the electron and hole density and lead to sharp,
low-energy optical absorption peaks below the predicted pristine optical gap.
Analysis of the exciton wavefunction reveals that the 2D Wannier-Mott exciton
of the pristine monolayer is highly localized around the defect, reducing its
Bohr radius by a factor of four and producing a dipole moment along the
out-of-plane axis due to the defect-induced symmetry breaking. Overall, these
results suggest that the vacancy is a strong perturbation to the system,
demonstrating the importance of considering defects in the context of material
design.
