Speaker: Dr. Helen Grüninger, Inorganic Chemistry & BayBatt, University of Bayreuth.

Topic:
In recent years mixed halide perovskites MAPbI3-xBrx have become an aspirant for the application in tandem solar cells due to the tunability of their bandgap.[1] However, halide migration in the perovskite particles and across particle interfaces under working conditions, i.e., illumination and elevated temperatures, results in heterogeneous compositions and thus in a deterioration of optoelectronic performance.[2] Tuning the morphology of perovskite particles, as well as the use of “passivation” additives is believed to control ion migration and thus increase the stability of mixed halide compositions, but exact mechanisms are still unclear.

We use in-situ X-ray diffraction in combination with solid-state NMR and optical spectroscopy to probe halide kinetics in MAPbI3-xBrx perovskite powders in-situ either at elevated temperatures or under illumination. Halide mixing, i.e. the formation of a solid solution starting from physical mixtures (1:1) of MAPbI3 and MAPbBr3 powders, is induced in the dark at moderate temperatures between 50 and 90°C. The opposite process of halide segregation takes place in MAPbI3-xBr perovskite powders under illumination. The halide kinetics extracted from our complementary in-situ experiments are correlated with powder morphologies, and the presence and behaviour of “passivation” additives, such as the ionic liquid BMIMBF4. Our recent results indicate that the stability increase with the additive BMIMBF4,[3] is rather due to a halide-specific increase in Bromide diffusion, than a strong suppression.[4] This highlights the necessity of in detail studies of additive-perovskite interactions or the role of morphology for halide transport and thus to derive structure-property relations and future design principles.

[1] D. P. McMeekin, et al., Science, 2016, 351, 151.; J. Xu et al., Science, 2020, 367, 1097.[2] V. Diez-Cabanes et al., Adv. Opt. Mater., 2021, 9, 2001832.; Y. Yuan et al., Acc. Chem. Res., 2016, 49, 286.
[3] S. Bai, et al., Nature, 2019, 571,245.; H. Kim, et. al., Sol. RRL, 2022, 6, 2200013.
[4] C. Greve, et al., ACS Energy Letters, 2023, 8, 5041.

Series: STOE User Talks 2024/2025

Time: 39 min.

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