Halide Segregated Crystallization of Mixed-Halide Perovskites Revealed by In Situ GIWAXS

Abstract

Mixed-halide perovskites of the composition MAPb(Br x I1–x )3, which seem to exhibit a random and uniform distribution of halide ions in the absence of light, segregate into bromide- and iodide-rich phases under illumination. This phenomenon of halide segregation has been widely investigated in the photovoltaics context since it is detrimental for the material properties and ultimately the device performance of these otherwise very attractive materials. A full understanding of the mechanisms and driving forces has remained elusive. In this work, a study of the crystallization pathways and the mixing behavior during deposition of MAPb(Br x I1–x )3 thin films with varying halide ratios is presented. In situ grazing incidence wide-angle scattering (GIWAXS) reveals the distinct crystallization behavior of mixed-halide perovskite compositions during two different fabrication routes: nitrogen gas-quenching and the lead acetate route. The perovskite phase formation of mixed-halide thin films hints toward a segregation tendency since separate crystallization pathways are observed for iodide- and bromide-rich phases within the mixed compositions. Crystallization of the bromide perovskite phase (MAPbBr3) is already observed during spin coating, while the iodide-based fraction of the composition forms solvent complexes as an intermediate phase, only converting into the perovskite phase upon thermal annealing. These parallel crystallization pathways result in mixed-halide perovskites forming from initially halide-segregated phases only under the influence of heating.

Mitigation of Halide Segregation by Cation Composition Management in Wide Bandgap Perovskites

“…In this work, we explored in detail an alternative method to achieve a more stable composition for wide bandgap perovskites suitable for tandem cell application. New perovskite compositions were designed based on a 20% bromide content, which was reported to exhibit minimal halide segregation issues. − These compositions, however, show a bandgap lower than the optimal for double junction cell applications. We focused on tuning the A-cation site in the triple cation composition, Cs x FA y New 1‑ x ‑ y ­Pb­(Br j I 1‑ j ) 3 , where New = another organic cation.…”