Photo De-Mixing in Mixed Halide Perovskites—the Role of Dimensionality

Photo induced phase separation observed in mixed halide perovskites(Photo de-mixing) has opened important questions on the thermodynamics of energetically pumped systems. In addition, the improved phase stability observed experimentally in low dimensional perovskites under light[1] compared with classic 3D hybrid perovskites, raises questions on the influence of dimensionality on the defect chemistry and the phase properties of these mixtures.<br/><br/>Here, we investigate the dependence of the phase behavior of mixed halide perovskites with different dimensionality under light. We consider thin films of 3D, 2D and nanocrystalline (size ~10 nm) mixed halide perovskites and we probe the changes in absorption properties and conductivities when the films are exposed to light. We find that nanocrystalline films show no de-mixing in contrast to 2D and 3D under similar illumination intensities, consistent with previous observations[2] and with the role of interfacial energy in the formation of de-mixed phases.[3]We then consider Dion-Jacobson mixtures based on (PDMA)Pb(Br_0.5I_0.5)₄ (PDMA: 1,4-phenylenedimethanammonium spacer) as a model system for evaluating thermodynamic properties of photo de-mixing, given their superior stability compared to 3D films. We demonstrate that 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 and structural properties. We further conduct temperature-dependent absorption measurements under light to extract the photo de-mixed compositions and to map the photo-miscibility-gap with three different methods.[4] We finally compare the kinetics and thermodynamics of photo de-mixing in 2D and 3D systems.<br/><br/>[1] a) X. Xiao, J. Dai, Y. J. Fang, J. J. Zhao, X. P. Zheng, S. Tang, P. N. Rudd, X. C. Zeng, J. S. Huang, ACS Energy Lett. 2018, 3, 684-688; b) J. Cho, J. T. DuBose, A. N. T. Le, P. V. Kamat, ACS Mater. Lett. 2020.<br/>[2] a) A. F. Gualdron-Reyes, S. J. Yoon, E. M. Barea, S. Agouram, V. Munoz-Sanjose, A. M. Melendez, M. E. Nino-Gomez, I. Mora-Sero, ACS Energy Lett. 2019, 4, 54-62; b) L. Hu, X. Guan, W. Chen, Y. Yao, T. Wan, C.-H. Lin, N. D. Pham, L. Yuan, X. Geng, F. Wang, C.-Y. Huang, J. Yuan, S. Cheong, R. D. Tilley, X. Wen, D. Chu, S. Huang, T. Wu, ACS Energy Lett. 2021, 6, 1649-1658.<br/>[3] a) Y.-R. Wang, G. Y. Kim, E. Kotomin, D. Moia, J. Maier, JPhys Energy 2022, 4, 011001; b) G. Y. Kim, A. Senocrate, Y.-R. Wang, D. Moia, J. Maier, Angew. Chem. Int. Ed. 2021, 60, 820-826.<br/>[4] 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.