Abstract

In recent years, lead halide perovskites have not only proven to be a promising material for solar cell applications, but they have also shown a huge potential for light-emitting devices and other optoelectronic applications. Most of the initial studies focused on bulk perovskite materials. However, emerging colloidal perovskite nanocrystals have been shown to exhibit superior qualities relating to their efficiency and functionality. On the nanoscale, size-dependent effects can be exploited to tune the nanocrystals’ properties. In the present work, two-dimensional perovskite nanoplatelets and quasi-two-dimensional layered hybrid perovskites are investigated. A new and systematic variation of the ligand concentration during the nanocrystal synthesis is presented. This enables the fabrication of colloidal two-dimensional methylammonium lead bromide perovskite nanoplatelets of varying thicknesses down to only a single unit cell. Photoluminescence and absorption spectra show the appearance of clear excitonic features in the thinnest structures and a blue-shift in the emission wavelength of more than 90 nm in comparison to the bulk counterpart. Additionally, in the thinnest structures the exciton binding energy increases up to several hundreds of meV due to a reduced excitonic screening. For the first time the quantum size effect in these nanoplatelets is quantitatively described through model calculations. Miniband formation in the multilayer structures lowers the emission energy with respect to the isolated nanoplatelet. Thickness-dependent photoluminescence lifetime measurements are performed. The recorded lifetimes decrease with thinner nanoplatelets as the exciton binding energy increases. Temperature-dependent photoluminescence measurements on cesium lead bromide perovskite nanoplatelets are carried out. A theoretical model considering acoustic and optical phonons as the main sources for scattering of excitons, shows that the former dominate at temperatures below 90 K and the latter above this temperature. Interestingly, temperature-dependent time-resolved photoluminescence measurements display contributions of bright and dark excitons to the decay curves below 60 K. At higher temperatures an antiquenching behavior of the luminescence is observed, indicating that ligands play an important role in these structures with a high surface-to-volume ratio. The interlayer distance in layered hybrid perovskites is systematically varied to investigate its impact on structure-function relationships and electronic coupling between the perovskite layers. It is found that the optical bandgap is determined mainly by two parameters. First, the length of the organic ligands separating the inorganic perovskite sheets influences the tilt angle of the perovskite octahedra. An increase of the optical bandgap with larger tilt angles is observed. Secondly, electronic coupling between the perovskite sheets constituting the multilayer structures is found to have an impact on the bandgap, albeit significantly smaller than the effect induced by the change in tilt angles. Importantly, the electronic coupling only occurs for perovskite multilayers with an interlayer distance below 1.5 nm. This thesis contributes to a fundamental understanding of optical properties of two-dimensional perovskites. It focuses on thickness-dependent quantum size effects, exciton-phonon interactions, and structure-function relationships.