Photo de-mixing and ion transport in two-dimensional Dion-Jacobson mixed halide perovskites

Ya-Ru Wang^a^,^f, Marko Mladenović^b^,^d, Eugene Kotomin^a^,^e, Chao Yang^a, Kersten Hahn^a, Jaehyun Lee^a, Wilfried Sigle^a, Jovana Milić^c, Peter van Aken^a, Ursula Rothlisberger^b, Michael Grätzel^a^,^c, Davide Moia^a, Joachim Maier^a

^aMax Planck Institute for Solid State Research, Stuttgart, Germany.

^bLaboratory of Computational Chemistry and Biochemistry, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland.

^cLaboratory of Photonics and Interfaces, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland.

^dIntegrated Systems Laboratory, Department of Information Technology and Electrical Engineering, ETH Zurich, Zurich, Switzerland.

^eInstitute of solid-state physics, University of Latvia, LV 1063 Riga, Latvia.

^fDTU ENERGY, Danmarks Tekniske Universitet (DTU), Agnes Nielsens Vej, 2800, Kgs. Lyngby.

Proceedings of MATSUS Fall 2026 Conference (MATSUSFall26)

B4 Fundamental Understanding of Halide Perovskite Materials, Interfaces and Devices

Palma, Spain, 2026 October 26th - 30th

Organizers: Krishanu Dey and Sudipta Seth

Oral, Ya-Ru Wang, presentation 295
Publication date: 22nd July 2026

Mixed bromide-iodide perovskites present great potential for optoelectronic devices due to the facile engineering of their optoelectronic properties. However, such mixtures suffer from photo-induced phase segregation when exposed to light (photo de-mixing)¹. While the process is partially reversible (two phases remix back in the dark to the pristine state, dark re-mixing),^{2, 3}this phase instability can potentially lead to unstable optoelectronic properties and device performance, 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.⁴

Here, we focused on thin films of 2D Dion-Jacobson mixed halide perovskites (PDMA)Pb(Br0.5I0.5)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.⁵First, we tracked 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. We furthermore 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^{6, 7}and should therefore be considered in the overall energy balance of the process, together with the electronic effects discussed in the literature.8 These findings will aid compositional engineering related to halide mixtures to 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 and M. D. McGehee, Chem. Sci, 2015, 6, 613–617.

2. Y.-R. Wang, A. Senocrate, M. Mladenović, A. Dučinskas, G. Y. Kim, U. Rothlisberger, J. V. Milić, D. Moia, M. Grätzel and J. Maier, Advanced Energy Materials, 2022, 12, 2200768.

3. P. S. Mathew, J. T. DuBose, J. Cho and P. V. Kamat, ACS Energy Lett., 2021, DOI: 10.1021/acsenergylett.1c01015, 2499–2501.

4. Y.-R. Wang, Dissertation, Stuttgart, Universität Stuttgart, 2023.

5. Y.-R. Wang, M. Mladenović, E. Kotomin, Y. Chao, K. Hahn, J. Lee, W. Sigle, J. V. Milić, P. A. van Aken, U. Rothlisberger, M. Grätzel, D. Moia and M. Joachim, arXiv preprint arXiv:2512.05879, 2025.

6. G. Y. Kim, A. Senocrate, Y.-R. Wang, D. Moia and J. Maier, Angewandte Chemie International Edition, 2021, 60, 820–826.

7. Y.-R. Wang, G. Y. Kim, E. Kotomin, D. Moia and J. Maier, JPhys Energy, 2022, 4, 011001.

8. S. Draguta, O. Sharia, S. J. Yoon, M. C. Brennan, Y. V. Morozov, J. S. Manser, P. V. Kamat, W. F. Schneider and M. Kuno, Nat Commun, 2017, 8, 200.

References:

[1] E. T. Hoke, D. J. Slotcavage, E. R. Dohner, A. R. Bowring, H. I. Karunadasa and M. D. McGehee. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci, 2015, 6, 613–617.

[2] Wang, Y.-R.; Senocrate, A.; Mladenović, M.; Dučinskas, A.; Kim, G. Y.; Rothlisberger, U.; Milić, J. V.; Moia, D.; Grätzel, M.; Maier, J. Photo De-Mixing in Dion–Jacobson 2D Mixed Halide Perovskites. Adv. Energy Mater. 2022, 12, 2200768.

[3] Mathew, P. S.; DuBose, J. T.; Cho, J.; Kamat, P. V. Spacer Cations Dictate Photoinduced Phase Segregation in 2D Mixed Halide Perovskites. ACS Energy Lett. 2021, 6, 2499–2501.

[4] Wang, Y.-R. Defect Chemistry and Photo-Ionic Effects in Bromide and Iodide Perovskites. Dissertation, Universität Stuttgart, Stuttgart, 2023.

[5] Wang, Y.-R.; Mladenović, M.; Kotomin, E.; Chao, Y.; Hahn, K.; Lee, J.; Sigle, W.; Milić, J. V.; van Aken, P. A.; Rothlisberger, U.; Grätzel, M.; Moia, D.; Joachim, M. Relating the Dynamics of Photo De-Mixing in Mixed Bromide-Iodide Perovskites to Ionic and Electronic Transport. arXiv:2512.05879, 2025.

[6] Kim, G. Y.; Senocrate, A.; Wang, Y.-R.; Moia, D.; Maier, J. Photo-Effect on Ion Transport in Mixed Cation and Halide Perovskites and Implications for Photo-Demixing. Angew. Chem. Int. Ed. 2021, 60, 820–826.

[7] Wang, Y.-R.; Kim, G. Y.; Kotomin, E.; Moia, D.; Maier, J. Photo De-Mixing in Mixed Halide Perovskites: The Roles of Ions and Electrons. J. Phys. Energy 2022, 4, 011001.

[8] Draguta, S.; Sharia, O.; Yoon, S. J.; Brennan, M. C.; Morozov, Y. V.; Manser, J. S.; Kamat, P. V.; Schneider, W. F.; Kuno, M. Rationalizing the Light-Induced Phase Separation of Mixed Halide Organic–Inorganic Perovskites. Nat. Commun. 2017, 8, 200.

Acknowledgements:

This work was performed within the framework of the Max Planck-EPFL Center for Molecular Nanoscience and Technology. DM is grateful to the Alexander von Humboldt Foundation for funding. EK’s research was partly performed in the Center of Excellence of the Institute of Solid-State Physics, University of Latvia, supported through European Union’s Horizon 2020 Framework Program H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2, and partly supported by the M ERA NET project HetCat. U.R. gratefully acknowledges funding from the Swiss National Foundation (grant N. 200020_219440) and computational resources from the Swiss National Computing Centre CSCS. M.G. acknowledges financial support from the Günes Perovskite Solar Cell A.S. company, Adana, Turkey and from the Innovation cheque supported by Innosuisse funding application no. 76256.1 INNO-EE.

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