A Dash of Colour: Investigating Optical and Kinetic Behaviour of Cesium Lead}\\ {\Large Halide Perovskite Nanocrystals during Anion Exchange

Abstract

Nanoscale semiconductor crystals made from cesium lead halide (CsPbX3) perovskite material are very versatile devices. These nanocrystals show a shift in their photoluminescence wavelength when submerged in a solution of foreign anions (like bromides or iodides), replacing their original anions with the species in the solution. The addition of Cl-, Br- and I- precursors to the crystals in controlled ratio's can make their colour span the whole visible part of the spectrum, with changes occurring on the time scale of just seconds. This property makes the perovskite crystals promising for the reaction analysis and imaging of porous networks in catalysts or the development of a new type of solar cell that can harvest a larger part of the solar spectrum. Experiments on these perovskite crystals are being conducted in manifold, yet good numerical models are lacking till this day. In this thesis, a Kinetic Monte Carlo simulation is developed to get a better understanding of the dynamics of the process. This simulation is then extended to a model of the optical properties of the crystal. Using the effective mass model for excitons, together with the variational method, the band gap can be approximated. The results of this model combined with experiments conducted in a spectrometer suggest that there are two distinct regimes of the anion exchange, a homogeneous mixed one and a 'diffusion limited' one, both with their own characteristic properties. A possible explanation for this phenomenon is proposed and investigated.