Ion transport and photo-induced phase instability in mixed halide perovskites

Y.R. Wang, G.Y. Kim, A. Senocrate, D. Moia, J. Maier

The transport of ionic defects in compounds beyond methylammonium lead iodide (MAPI) is relevant to hybrid perovskites used in high-performance solar cells and other optoelectronic devices. A puzzling phenomenon that has been demonstrated for compositions where iodide is partially substituted with bromide ions concerns the phase instability of these mixed halide perovskites upon exposure to light. This process, referred to as photo-demixing, is reversible once samples are left in the dark. Connecting the ionic properties of mixed halide perovskites to their phase behavior is essential to find ways to guarantee thermodynamic stability of these material compositions for devices under operating conditions.

Fig. 1. The difference in the ionic behavior for iodide and bromide perovskites when exposed to light may be the driving force for demixing effects observed for mixed halide compositions. Hole self-trapping is favored by the high polarizability of the iodine environment (the interstitial neutral iodine is further stabilized by ionic rearrangement). For the bromide case, the hole is expected to be delocalized over several unit cells of the bromide sublattice.

In a project led by Dr. Gee Yeong Kim (now at KIST), we investigated the influence of light on the ionic conduction of hybrid perovskites. Striking evidence for an increase in the ionic conductivity in iodide perovskites1 and a relatively unchanged ionic conductivity in bromide compositions are supported by our group’s recent computational study on this question.2 This “light-effect” (see Fig. 1) may provide the driving force for phase segregation under light.3

PhD student Ya-Ru Wang in our group is currently continuing the quest towards a defect chemical description of this process under different conditions and for different perovskite compositions.

1. Kim, G. Y. et al. Large tunable photoeffect on ion conduction in halide perovskites and implications for photodecomposition. Nat. Mater. 17, 445–450 (2018).

2. Evarestov, R. A., Kotomin, E. A., Senocrate, A., Kremer, R. K. & Maier, J. First-principles comparative study of perfect and defective CsPbX3 (X = Br, I) crystals. Phys. Chem. Chem. Phys. 22, 3914–3920 (2020).

3. 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. Chemie - Int. Ed. (2020) doi:10.1002/anie.202005853.

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