First Principles Calculation of Optical Properties of Ternary Semiconductors Cu$_3$PSe$_4$ and Cu$_3$PS$_4$

The ternary semiconducting compounds Cu$_3$PSe$_4$ and Cu$_3$PS$_4$ are of interest as potential optoelectronic materials. Of particular interest for solar photovoltaic devices is Cu$_3$PSe$_4$, as its band gap lies in the desired 1.0 to 1.6 eV range for an absorber. We have theoretically calculated the optical properties of these materials using density functional theory with GGA and hybrid exchange-correlation functionals, as well as with the GW$_0$ approximation from many-particle theory. We find that Cu$_3$PSe$_4$ has a direct band gap with relatively strong optical absorption above 2 eV, indicating that this compound is a candidate photovoltaic absorber. Cu$_3$PS$_4$ has a larger band gap of approximately 2.4 eV, placing it outside consideration as a solar absorber.