  Ferroelectrics with suitable band gaps have recently attracted attention as
candidate solar absorbing materials for photovoltaics. The inversion symmetry
breaking may promote the separation of photo-excited carriers and allow
voltages higher than the band gap. However, these effects are not fully
understood, in part because of a lack of suitable model systems for studying
these effects in detail. Here, we report properties of ferroelectric
Sn$_2$P$_2$S$_6$ and Sn$_2$P$_2$Se$_6$ using first principles calculations.
Results are given for the electronic structure, carrier pocket shapes, optical
absorption and transport. We find indirect band gaps of 2.20 eV and 1.55 eV,
respectively, and favorable band structures for carrier transport, including
both holes and electrons. Strong absorption is found above the direct gaps of
2.43 eV and 1.76 eV. Thus these compounds may serve as useful model systems for
understanding photovoltaic effects in ferroelectric semiconductors.
