  Ferroelectric materials possess a spontaneous electric polarization and may
be utilized in various technological applications ranging from non-volatile
memories to solar cells and light polarizers. Recently, hexagonal rare-earth
manganites, h-RMnO$_3$ (R is a rare-earth ion) have attracted considerable
interest due to their intricate multiferroic properties and improper
ferroelectricity characterized by a sizable remnant polarization and high Curie
temperature. Here, we demonstrate that these compounds can serve as very
efficient photovoltaic materials and, in addition, possess remarkable optical
anisotropy properties. Using first-principles methods based on
density-functional theory and considering h-TbMnO$_3$ as a representative
manganite, we predict a strong light absorption of this material in the solar
spectrum range, resulting in the maximum light-to-electricity energy conversion
efficiency up to 33%. We also predict an extraordinary optical linear dichroism
and linear birefringence properties of h-TbMnO$_3$ in a broad range of optical
frequencies. These results uncover the unexplored potential of hexagonal
rare-earth manganites to serve as photovoltaics in solar cells and as
absorptive and birefringent light polarizers.
