Understanding the structure and dynamics of hybrid lead halide perovskites for photovoltaics

The rapid progress in creating high-efficiency photovoltaic devices from hybrid and all-inorganic halide perovskites in the last decade has seen increasing interest in the physical properties of these materials. Major differences are observed with respect to conventional semiconductors in terms of the dynamical lattice properties, with these novel processes thought to underlie some of the remarkable optoelectronic properties reported. Thermal lattice fluctuations, known as phonons, are soft and diverge from the classical harmonic picture, and thus effectively act as a novel source of structural fluctuations, commonly absent in classical semiconductors. The organic cation with finite dipole moment, an integral part of the hybrid perovskite lattice, exhibits stochastic reorientation introducing additional perturbations that change the potential energy landscape experienced by charge carriers. To allow for the engineering of novel halide perovskite materials in the quest for further performance improvements, one of the requirements is a high level understanding of their lattice dynamics. Its complex nature arises from the overlapped contributions from cation reorientations and anharmonic fluctuations which this thesis disentangles. This was achieved by inducing structural phase transitions by lowering the lattice temperature and by varying the composition at both A and X site in the perovskite ABX3 lattice. Thus, this thesis combines X-ray, Brillouin and inelastic neutron scattering techniques to probe static and dynamic lattice properties across a wide dynamical range in energy and reciprocal space. It is demonstrated that while formamidinium based perovskites have the acoustic phonon properties attributed to classical semiconductors, methylammonium based perovskites exhibit strong coupling between the cations and the inorganic lattice, leading to acoustic phonon softening near the zone centre. The combination of methylammonium at the A-site and bromine at X site results in local centro-symmetry breaking, as detected through the observation of the piezoelectric coupling between methylammonium and acoustic phonons near the zone centre. Diffuse scattering rods stretching across the Brillouin zone edges are observed at low temperatures through single crystal X-ray diffraction in the MAPbBr1.5Cl1.5 mixed halide perovskite. The elastic neutron scattering confirmed the presence of diffuse scattering signal along the R-M Brillouin zone edge line in MAPbBr3 and FAPbBr3 perovskites in high temperature cubic and tetragonal phases. This is direct evidence for the presence of short range correlated nano-domains, a manifestation of the dynamically disordered halide perovskite lattice. Based on the experimental findings this thesis discusses how the anion and cation composition influences whether domains may possess ferroelastic or ferroelectric character. The insights gained can be exploited for further improvements in perovskite based solar cells, either through ferroelectric photovoltaics or strain engineering