Halide Mixing and Phase Segregation in Cs2AgBiX6 (X = Cl, Br, and I) Double Perovskites from Cesium-133 Solid-State NMR and Optical Spectroscopy

Dominik J Kubicki; Marcin Saski; Stuart MacPherson; Krzysztof Gal Kowski; Janusz Lewiński; Daniel Prochowicz; Jeremy J Titman; Samuel D Stranks

doi:10.1021/acs.chemmater.0c01255

Halide Mixing and Phase Segregation in Cs₂AgBiX₆ (X = Cl, Br, and I) Double Perovskites from Cesium-133 Solid-State NMR and Optical Spectroscopy

Chem Mater. 2020 Oct 13;32(19):8129-8138. doi: 10.1021/acs.chemmater.0c01255. Epub 2020 Sep 17.

Authors

Dominik J Kubicki^{1

2}, Marcin Saski³, Stuart MacPherson^{1

2}, Krzysztof Gal Kowski^{1

4}, Janusz Lewiński^{3

5}, Daniel Prochowicz³, Jeremy J Titman⁶, Samuel D Stranks^{1

2}

Affiliations

¹ Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, U.K.
² Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge, CB3 0AS, U.K.
³ Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw 01-224, Poland.
⁴ Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Toruń 87-100, Poland.
⁵ Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, Warsaw 00-664, Poland.
⁶ School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K.

Abstract

All-inorganic double perovskites (elpasolites) are a promising potential alternatives to lead halide perovskites in optoelectronic applications. Although halide mixing is a well-established strategy for band gap tuning, little is known about halide mixing and phase segregation phenomena in double perovskites. Here, we synthesize a wide range of single- and mixed-halide Cs₂AgBiX₆ (X = Cl, Br, and I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using ¹³³Cs solid-state MAS NMR. We show that mixed Cl/Br materials form pure phases for any Cl/Br ratio while Cl/I and Br/I mixing is only possible within a narrow range of halide ratios (<3 mol % I) and leads to a complex mixture of products for higher ratios. We characterize the optical properties of the resulting materials and show that halide mixing does not lead to an appreciable tunability of the PL emission. We find that iodide incorporation is particularly pernicious in that it quenches the PL emission intensity and radiative charge carrier lifetimes for iodide ratios as low as 0.3 mol %. Our study shows that solid-state NMR, in conjunction with optical spectroscopies, provides a comprehensive understanding of the structure-activity relationships, halide mixing, and phase segregation phenomena in Cs₂AgBiX₆ (X = Cl, Br, and I) double perovskites.