  Zinc Phosphide ($Zn_3P_2$) could be the basis for cheap and highly efficient
solar cells. Its use in this regard is limited by the difficulty in n-type
doping the material. In an effort to understand the mechanism behind this, the
energetics and electronic structure of intrinsic point defects in zinc
phosphide are studied using generalized Kohn-Sham theory and utilizing the
Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional for exchange and
correlation. Novel 'perturbation extrapolation' is utilized to extend the use
of the computationally expensive HSE functional to this large-scale defect
system. According to calculations, the formation energy of charged phosphorus
interstitial defects are very low in n-type $Zn_3P_2$ and act as 'electron
sinks', nullifying the desired doping and lowering the fermi-level back towards
the p-type regime. This is consistent with experimental observations of both
the tendency of conductivity to rise with phosphorus partial pressure, and with
current partial successes in n-type doping in very zinc-rich growth conditions.
