Neutron Scattering Studies of Hybrid Perovskites for Photovoltaic Applications

Hybrid perovskites (ABX$_{\mathrm{3}})$ have attracted a great deal of attention as light absorbers for photovoltaics. Here the A site is occupied by organic cations, for example methyl ammonium, CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$; the B site is occupied by metal cations, for example Pb$^{\mathrm{2+}}$; and the X site is occupied by halogen anions: I$^{\mathrm{-}}$, Br$^{\mathrm{-}}$, or Cl$^{\mathrm{-}}$. Typical of perovskites, these materials exhibit structural phase transitions involving rotations or tilts of the BX$_{\mathrm{6}}$ octahedra, but with the added complexity that the inorganic framework is coupled to order-disorder transitions of the organic cations. We have used neutron scattering techniques to characterize the vibrations and dynamics of several of these compounds as a function of temperature, including samples where the hydrogen atoms are partially or fully substituted by deuterium. Combined with the results of density functional theory calculations, these studies contribute to our understanding of the excellent photovoltaic properties of these materials.