Sn₂P₂S₆ and Sn₂P₂Se₆: Ferroelectric Visible Light Absorbing Semiconductors with High Polarization and Photovoltaic Potential

Ferroelectrics with suitable band gaps, in which the inversion symmetry breaking may promote the separation of photo-excited carriers and allow voltages higher than the band gap, have recently attracted attention as candidate solar absorbing materials for photovoltaics. However, these effects are not fully understood. Here, we report properties of ferroelectric Sn₂P₂S₆ and Sn₂P₂Se₆ using first principles calculations, which may serve as useful model systems for understanding photovoltaic effects in ferroelectric semiconductors. Results are given for the electronic structure, carrier pocket shapes, optical absorption and transport. We find favorable band structures for carrier transport, including both holes and electrons. Strong absorption is found above the direct gaps (2.43 eV and 1.76 eV respectively) .