Publication date: 11th March 2026
Halide mixing provides an opportunity to tune the bandgap of perovskite semiconductors seamlessly across the visible range. However, halide segregation in APbX₃ compounds under illumination deteriorates the optoelectronic properties of these materials, placing strong limitations on the practical benefits of bandgap tunability. In our previous work, we showed that the same environmental stressor—light—that induces halide segregation can also be used to remix halides when illumination reaches sufficiently high photon fluxes. We developed a polaron-based model to explain both demixing and remixing. More recently, we have explored photodynamic effects in 2D mixed halide perovskites with the formula BAPbI₄₋ₓBrₓ. We found that the optical properties of these materials change significantly under prolonged illumination, but observed no evidence of classical halide segregation as seen in APbBr₃₋ₓIₓ compounds. Importantly, the lattice of the 2D perovskite analogues contains two distinct halide sites: equatorial (E) sites within the 2D lead halide planes, which are bonded to two neighboring Pb²⁺ ions, and axial (A) sites located above and below the planes, which are bonded to only one Pb²⁺ ion.In this work, we show that light illumination drives a redistribution of Br⁻ and I⁻ between the E and A sites, shifting the system away from thermal equilibrium. These changes are reversible and highly localized. Density functional theory calculations reveal that halide redistribution between E and A sites leads to distinct changes in the electronic properties. The light-induced structural isomerisation observed in BAPbI₄₋ₓBrₓ enables optical patterning of single-crystalline materials without any need for mass transport beyond the unit cell while fully preserving the material’s crystalline integrity. Overall, these findings highlight a fundamentally different light–matter interaction in 2D mixed halide perovskites and point to new opportunities for spatially resolved and reversible control of optoelectronic properties.
This work was financially supported by the Australian Research Council through the Centres of Excellence in Exciton Science (CE170100026) and Future Low-Energy Electronics Technologies (CE170100039), as well as the Australian Renewable Energy Egency through the Australian Centre for Advanced Photovoltaics
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