  The performance of perovskite solar cells recently exceeded 15%
solar-to-electricity conversion efficiency for small-area devices. The
fundamental properties of the active absorber layers, hybrid organic-inorganic
perovskites formed from mixing metal and organic halides [\textit{e.g.}
(NH$_4$)PbI$_3$ and (CH$_3$NH$_3$)PbI$_3$], are largely unknown. The materials
are semiconductors with direct band gaps at the boundary of the first Brillouin
zone. The calculated dielectric response and band gaps show an orientation
dependence, with a low barrier for rotation of the organic cations. Due to the
electric dipole of the methylammonium cation, a photoferroic effect may be
accessible, which could enhance carrier collection.
