The properties and consequences of mobile ionic defects in metal halide perovskites
Creators: name: Fisher, William Richard
Publisher: name: Imperial College London
Year: 2022

description: Metal halide perovskites are a class of semiconductors that are solution processable and have received much research attention over the last decade for the application of photovoltaics and other electronic devices. They differ from conventional semiconductors since in addition to free electrons and holes (electronic charge carriers) they also contain mobile ionic defects (ionic charge carriers) whose concentration can exceed the electronic charge carriers. Efficiency and other performance metrics have improved greatly yet the devices suffer from instability that hinders their wider application in industry. Further understanding of the mechanisms underlying their characteristics is still needed for the eventual commercial success of these material. Using drift-diffusion simulations, as well as time and frequency resolved optoelectronic measurements, I examine the implications of these ionic charge carriers for the fundamental properties of, interpretation of measurements of, and the stability to humidity of, metal halide perovskite electronic devices. Using drift-diffusion simulations, the mobile ionic charge is found to modify the device’s energetic profile during the attainment of thermodynamic equilibrium, doping the perovskite layer. This doping mechanism is shown to be controllable through the choice of materials and through the application of a long timescale pre-biasing voltage. The existence of this ‘interfacial doping’ mechanism is confirmed experimentally using a pulsed-voltage technique. I show that the relative densities of the ionic and electronic charges in the perovskite define which of the species control the device’s electronic properties, this understanding is critical when characterising the material’s properties. To highlight this, I asses the validity of the commonly applied Mott-Gurney law used to find the electronic mobility from current-voltage measurements of single carrier perovskite devices (space-charge-limited current measurements, SCLC). The electronic charge density needs to be approximately five times higher than the ion density for the law to apply. I show that, even using an experimental protocol that limits ionic redistribution during the measurement, determining the electronic mobility from SCLC measurements is difficult or impossible. Both water, and ionic transport are considered to play important roles in the degradation of metal halide perovskites, understanding their interaction may help to develop strategies to improve stability. Using electrochemical impedance spectroscopy interpreted with an equivalent circuit model I show that the activation energy for ionic charge migration decreases as environmental humidity increases. This observation is consistent with previous ab-initio calculations which calculate that the existence of the monohydrate phase decreases the ion transport activation energy. Using these findings, I suggest the process by which perovskites interact with water and forms new phases on the way to degradation. Finally, I show that the addition of Cs to the perovskite stops this decrease in ion migration activation energy. These findings have significant implications and provide valuable additions to the understanding of the degradation of different metal halide perovskites with exposure to water.