  Two-dimensional (2D) halide perovskites feature a versatile structure, which
not only enables the fine-tuning of their optoelectronic properties but also
makes them appealing as model systems to investigate the fundamental properties
of hybrid perovskites. In this study, we analyzed the changes in the optical
absorption of 2D Dion-Jacobson mixed halide perovskite thin films
(encapsulated) based on (PDMA)Pb(I0.5Br0.5)4 (PDMA:
1,4-phenylenedimethanammonium spacer) exposed to a constant illumination. We
demonstrate that these 2D mixed-halide perovskites undergo photo de-mixing with
direct transformation from the pristine phase to the de-mixed phases. Almost
complete re-mixing of these phases occurs when the sample is left in the dark,
showing that the process is reversible in terms of optical properties. On the
other hand, exposure to light appears to induce structural changes in the thin
film that are not reversible in the dark. We have further investigated
temperature-dependent absorption measurements under light to extract the photo
de-mixed compositions and to map the photo-miscibility-gap. This work thereby
reveals that photo de-mixing occurs in Dion-Jacobson two-dimensional hybrid
perovskites and provides strategies to address the role of light in the
thermodynamic properties of these materials.
