Local structure of hybrid metal-halide perovskites

Abstract

This thesis reports on the effects of static and dynamic disorder on the structure and op- toelectronic properties of the hybrid metal halide perovskite family MAPbX3 (MA+ = methylammonium, CH3NH+3 , X = Cl−, Br−). Static disorder was introduced by halide substitution through solid state synthesis, providing better stoichiometric control compared to solution based techniques. Composition and temperature dependent X-ray diffraction and heat capacity measurements of MAPb(ClxBr1−x)3 revealed a suppression of the symmetry- lowering phase transitions of the end members in a wide range of intermediate compositions (0.2 < x < 0.8) down to T = 12 K. The suppression was not observed for the closely related inorganic CsPb(ClxBr1−x)3 system, highlighting the importance of the MA+ cation in direct- ing the crystal structure. Density functional theory calculations confirmed the experimental observations of the hybrid and inorganic mixed-halide systems. The findings are consis- tent with the formation of an orientational glass on the MA+ sublattice. Temperature and composition dependent photoluminescence measurements showed that photo-induced halide segregation occurred only in the orientational glass forming compositions.

The local structure of the hybrid metal halide perovskites in the presence of static disorder was investigated using pair distribution function analysis of composition and temperature dependent X-ray and neutron total scattering of d6-MAPb(ClxBr1−x)3. It was found that both the local structure (r < 10 Å) and the average structure (r > 10 Å) at room temperature were well described by the same space group, in contrast to previous reports. Suppression of the symmetry lowering phase transitions in the local structure of the intermediate compositions was confirmed, corroborating the average structure measurements.

The effect of dynamic disorder was studied by time resolved optical pump - diffuse X- ray probe measurements on single crystals of MAPbBr3. Anomalous time delays in the onset of changes in the scattered diffuse intensity revealed the formation of polaronic strain fields on length scales of several nm and time scales of tens of ps. Correlation with time resolved reflection and photoluminescence measurements under similar conditions showed that polaron formation led to an increase of the carrier effective mass.