Using mixtures of iodide and bromide in 2D halide perovskites has proved to be a powerful tool for tunable optoelectronic material design. However such mixtures suffer from photo-induced phase segregation when exposed to light(photo de-mixing), similarly to their 3D counterparts[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 evolution of different phases involves significant ion transport, clarifying the underlying defect chemical mechanisms involved in the photo de-mixing is critical.

Here, we study thin films of 2D Dion-Jacobson mixed halide perovskites (PDMA)Pb(Br_0.5I_0.5)₄ (PDMA: 1,4-phenylenedimethanammonium spacer) as model material to investigate their phase behavior both under light and in the dark using a wide range of experimental techniques. First, we are able to 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 monitored the conductivity changes during de-mixing, which allows for local probe of the ionic and electronic charge carriers concentration and ion transport through the de-mixed phases. 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] and should therefore be considered in the overall energy balance of the process, together with the electronic effects discussed in the literature.[4] These findings will aid compositional engineering related to halide mixtures, which will enable optimization of optoelectronic devices as well as the development of other emerging systems exploiting photo de-mixing.

[1] E. T. Hoke, D. J. Slotcavage, E. R. Dohner, A. R. Bowring, H. I. Karunadasa, M. D. McGehee, Chem. Sci 2015, 6, 613-617.

[2] a) Y.-R. Wang, A. Senocrate, M. Mladenović, A. Dučinskas, G. Y. Kim, U. Rothlisberger, J. V. Milić, D. Moia, M. Grätzel, J. Maier, Adv. Energy Mater. 2022, 12, 2200768; b) P. S. Mathew, J. T. DuBose, J. Cho, P. V. Kamat, ACS Energy Lett. 2021, 2499-2501.

[3] a) G. Y. Kim, A. Senocrate, Y.-R. Wang, D. Moia, J. Maier, Angew. Chem. Int. Ed. 2021, 60, 820-826; b) Y.-R. Wang, G. Y. Kim, E. Kotomin, D. Moia, J. Maier, JPhys Energy 2022, 4, 011001.

[4] a) S. J. Yoon, S. Draguta, J. S. Manser, O. Sharia, W. F. Schneider, M. Kuno, P. V. Kamat, Acs Energy Letters 2016, 1, 290-296; b) S. Draguta, O. Sharia, S. J. Yoon, M. C. Brennan, Y. V. Morozov, J. S. Manser, P. V. Kamat, W. F. Schneider, M. Kuno, Nat Commun 2017, 8, 200.