Modelling ion migration and charge carrier transport in planar perovskite solar cells

Perovskite solar cells (PSCs) have significant potential for success in the global market,as both a thin-film technology and as part of perovskite-on-silicon tandem devices. The latter technology is currently in the early stages of commercialisation. The record efficiencies of some types of PSC are close to their theoretical maximum; however, their long-term stability remains an area of concern. Rapid degradation caused by the external environment can be avoided via careful encapsulation of the device. However, cells also show an unforeseen internal behaviour, the effects of which have been termed current voltage hysteresis by the research field. It is now widely accepted that ion migration via vacancies in the perovskite is the origin of hysteresis, and hence that this behaviour is intrinsic to every PSC. Modelling ion migration in PSCs, and its effect on both their transient and steady-state performance, is the topic of this project. A combination of numerical techniques and asymptotic analysis is used to investigate a three-layer driftdiffusion model for ion vacancy migration and charge carrier transport within a PSC. For realistic values of the parameters, such a model displays both temporal and spatial stiffness and hence requires tailored numerical methods of solution. In addition, a simplified surface polarisation model is derived using the method of matched asymptotic expansions.The form of this model reveals how ion vacancy migration controls the evolution of the electric field across the three layers of a PSC and that this is the key to explaining its behaviour. The influence of two important properties of the transport layers, which adjoin the perovskite layer of a PSC, on the distribution of the electric field is highlighted and explained for the first time. Simulations are used to reproduce a number of experimentally observed trends, including why PSCs with organic transport layers tend to display reduced hysteresis. Strategies for determining the dominant charge carrier recombination mechanism, and reducing the associated performance loss, are also discussed