Metal halide perovskites have recently emerged as promising alternatives to conventional semiconductors. However, their insufficient stability against external stressors continues to hinder their commercialisation.

Here, we explore an additive-induced self-assembling two-dimensional capping layer strategy in order to prolong the ambient stability of notoriously sensitive mixed lead-tin perovskite thin films, especially interesting for their potential in heterojunctions with modular properties. We found that the additive acts on the two-dimensional component of the perovskite precursor solution, increasing the surface coverage of the capping layer, providing higher ambient stability to the thin films, and effectively creating a heterojunction between an upper two-dimensional layer and the lower three-dimensional bulk. We also carried out microscopy studies correlating their structural and optoelectronic properties, revealing clear links between the heterogeneous domains, opening up these materials to energy-funnelling applications.

We furthermore developed our structural characterisation capabilities at Grazing-Incidence Wide-Angle X-ray Scattering synchrotron facilities via a series of technical developments. We worked our way up from simple static measurements, to in situ lightsoaking and annealing studies, to finally achieve operando full device studies monitoring the structural evolution of perovskite solar cells with an applied electric field under illumination. The developed annealing capabilities ultimately allowed us to observe the pathways by which the aforementioned additive enables the formation of a thoroughly surface-covering two-dimensional capping layer.

Another major obstacle towards the commercialisation of metal halide perovskite solar cells is their instability under photovoltaic operation. Via our operando capabilities in Grazing-Incidence Wide-Angle X-ray Scattering, we successfully observed the real-time structural evolution of state-of-the-art mixed-halide, mixed-cation perovskite solar cells under photovoltaic operation, revealing an electrostrictive effect in the out-of-plane direction determined by the combined action of an electric field and illumination acting along this same direction. This appeared to delay otherwise inevitable phase segregation by instead promoting a remixing of the halide components, and stable electrical operation. Operating the same device in humid air, or operating a more defective starting device, instead favoured their demixing, with an associated loss in electrical performance.

The results in this thesis provide valuable insights into the development of more stable halide perovskite compositions by opening the door to additive-induced deposition of intrinsic heterojunctions, and to the long-term operational stabilisation of perovskite solar cells by the incorporation of native compressive stresses in the perovskite layer. Finally, our strong technical developments for the study of perovskite thin films and devices in Grazing-Incidence Wide-Angle X-ray Scattering provides useful blueprints to the research community for further research.