Mixtures of bromide and iodide in halide perovskites provide solutions for tunable optoelectronic material design. However, such mixtures suffer from photo-induced phase segregation when exposed to light (photo de-mixing)^[1]. While the process is reversible (two phases remix back in the dark to the pristine state, dark re-mixing),^[2] it can potentially lead to unstable optoelectronic properties and device performance during operation, making its understanding essential to progress the field of halide perovskites. Since the observed light-induced phase separation involves significant ion transport, clarifying the ionic and electronic transport properties and therefore underlying defect chemical mechanisms involved in photo de-mixing is crucial.

Here, mixed bromide-iodide perovskites with different dimensionalities (2D, 3D) are investigated, in terms of phase and charge transport properties using a wide range of experimental techniques as well as theoretical calculations. We focus on 2D Dion-Jacobson mixed bromide-iodide perovskites (PDMA)Pb(Br_0.5I_0.5)₄ (PDMA: 1,4-phenylenedimethanammonium spacer) as a model system to study photo de-mixing, due to its established reversibility.^[2] Firstly, we track the compositional evolution in the films during de-mixing and re-mixing by analyzing their time-dependent in-situ optical absorption properties. We also simultaneously monitor the conductivity changes during de-mixing and re-mixing, which allows for a local probe of the ionic and electronic charge carriers concentration and ion transport through the de-mixed phases. The dependence of the phase behavior and charge transport properties of mixed halide perovskites by varying dimensionality and surface conditions is further revealed using similar techniques. Furthermore, we take advantage of SEM and TEM to investigate the morphological changes and the nature of the iodide-rich and bromide-rich phases resulting from phase segregation. Lastly, we propose a model that considers possible opto-ionic effects, which can contribute to the driving force of de-mixing^{[3, 4]} and should therefore be considered in the overall energy balance of the process, together with the electronic effects discussed in the literature.^[5]Our work sheds light on fundamental questions related to the phase behavior and the role of defects on the driving force of de-mixing. These findings will also aid compositional engineering related to halide mixtures, which will enable the optimization of optoelectronic devices as well as the development of other emerging systems exploiting photo de-mixing or in general photo-ionic effects.