  The p-type semiconductor Cu3PSe4 has recently been established to have a
direct bandgap of 1.4 eV and an optical absorption spectrum similar to GaAs
[Applied Physics Letters, 99, 181903 (2011)], suggesting a possible application
as a solar photovoltaic absorber. Here we calculate the thermodynamic
stability, defect energies and concentrations, and several material properties
of Cu3PSe4 using a wholly GGA+U method (the generalized gradient approximation
of density functional theory with a Hubbard U term included for the Cu-d
orbitals). We find that two low energy acceptor-type defects, the copper
vacancy V_Cu and the phosphorus-on-selenium antisite P_Se, establish the p-type
behavior and likely prevent any n-type doping near thermal equilibrium. The
GGA+U defect calculation method is shown to yield more accurate results than
the more standard method of applying post-calculation GGA+U-based bandgap
corrections to strictly GGA defect calculations.
