A Matter of Mixing: Nanoscale Heterogeneity and Stability in Metal Halide Perovskite Solar Cells

To date, halide perovskite solar cells (PSC) yield the highest efficiency and stability when absorbers comprised of a mixture of ions are utilized. Mixed compositions allow for a fine tuning of the geometric conditions and enables the formation of photoactive perovskite phases. Nevertheless, chemical heterogeneity in the film can initiate phase segregation of the material into pure phases, which prefer their non-perovskite δ-phases, and consequently lead to degradation of the device performance. Spatially resolved analyses have revealed microscopic heterogeneity in PSCs, including compositional heterogeneity. However, characterizing the chemical heterogeneity on small length scales has proven challenging. Here, we use X-ray scattering methods to probe nanoscale heterogeneity in PSC absorber layers and couple these results to device level stability studies to understand the role heterogeneity plays in device performance. We prepare a set of perovskite thin films, choosing a variation of annealing conditions to ultimately vary the heterogeneity in the film. In this talk, we will provide a summary of the approach used to characterize the heterogeneity in these films and comment on the loss in device performance and subsequent phase segregation induced by nanoscale heterogeneity.