Over the past decade a new generation of photovoltaic devices has gained a vast amount of attention, not least owing to their rapidly improving efficiencies. In this generation of photovoltaic devices, the component that converts sunlight into electrical energy is composed of a class of materials called ‘hybrid organic-inorganic halide perovskites’ (HOIP). The efficiencies of HOIP devices already rival those of more conventional silicon-based technologies. Furthermore, they are relatively cheap to produce. Thus, in the interest of commercialising HOIP photovoltaic devices, improving our understanding of their photovoltaic properties is essential.

In this thesis, we present a theory and computer simulation study of ‘domains’ and ‘domain walls’ in HOIPs. In a given sample of one of the most widely studied HOIP materials, on a microscopic length scale one often observes stripes of crystal characterised by a regular atomic distortion (domains). The boundaries between these stripes (the domain walls) separate distortions of different orientations. Crucially, at the domain wall itself, atoms are arranged differently to their configuration in the domains themselves. Thus, one expects the wall to possess properties that are not present elsewhere in the material.

Some of our key findings include showing that an electric polarisation necessarily emerges at a commonly observed type of wall in HOIPs. This polarisation is capable of enhancing the flow of electric current along them. Furthermore, although we simulated a material that serves as a model for HOIPs, we argue that this polarisation is also non-zero in the actual HOIP material. We explore the properties of these walls and their constituent domains across a range of chemical compositions and suggest potential pathways for engineering these walls to enhance photovoltaic performance.