Two Step Phase-Segregation Process Revealed in Mixed Halide MHPs by Simultaneous In-Situ X-ray Diffraction and Photoluminescence Spectroscopy.
Klara Suchan a, Justus Just b, Pascal Becker c, Carolin Rehermann c, Aboma Merdasa c, Roland Mainz c, Ivan G. Scheblykin a, Eva L. Unger a c
a Chemical Physics and Nano Lund, Lund University, P.O. Box 124, Lund 22100, Sweden
b MAX IV laboratory, Lund University
c Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Straße, 15, Berlin, Germany
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Klara Suchan, presentation 178
Publication date: 20th April 2022

The compositional stability of mixed halide perovskite semi-conductors is limited by the light or electrical bias induced phase segregation. To identify the mechanism and cause of the light-induced phase-segregation phenomena (photo-segregation), we investigated the full compositional range of MAPb(BrxI1-x)3 with x = 0…1, by simultaneous in-situ X-ray diffraction and photoluminescence spectroscopy during illumination. This multimodal approach allows us to quantify the halide redistribution, its kinetics as well as the corresponding charge carrier distribution, in-situ during segregation. We apply previously derived thermodynamic models directly to rationalize the experimental data.[1,2] We find that gap-models (by Kuno et al. and Ginsberg et al.), which are based on the bandgap difference between the parent phase and the newly formed I-rich phase as driving force predict the phase-distributions in the segregated state qualitatively well. However, a more refined model is necessary to rationalize our experimental results quantitively.

We propose a modified gap-model, based on a two-step segregation process. The first step corresponds to the nucleation of photo-segregation, which is thermodynamically driven by the energy reduction of charge carriers accumulating in I-rich domains.[3,4] This triggers reactions that lead to more persistent changes in the compositional distribution in the mixed halide materials. Our experimental results highlight the importance of understanding the interplay between photo-induced changes in the ionic defect density, which can also be related to other dynamic effects observed in these ionic-semiconductors, such as self-healing and hysteresis.

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