  The two-dimensional semiconductor phosphorene has attracted extensive
research interests for potential applications in optoelectronics, spintronics,
catalysis, sensors, and energy conversion. To harness phosphorene's potential
requires a better understanding of how intrinsic defects control carrier
concentration, character, and mobility. Using density-functional theory and a
charge correction scheme to account for the appropriate boundary conditions, we
conduct a comprehensive study of the effect of structure on the formation
energy, electronic structure, and charge transition level of the charged
vacancy point defects in phosphorene. We predict that the neutral vacancy
exhibits a 9-5 ring structure with a formation energy of 1.7 eV and transitions
to a negatively charged state at a Fermi level 1.04 eV above the valence band
maximum. The corresponding optical charge transitions display sizeable
Frank-Condon shifts with a large Stokes shift of 0.3 eV. Phosphorene vacancies
should become negatively charged in n-doped phosphorene, which would passivate
the dopants and reduce the charge carrier concentration and mobility.
