The emission and absorption of Cs2AgBiBr6 are dominated by the strong carriers–phonon coupling.

We present a comprehensive study of the relationship between the crystal structure and optoelectronic properties of the double perovskite Cs 2 AgBiBr 6 , which has emerged as a promising candidate for photovoltaic. Based on single-crystal/powder X-ray diraction and neutron powder diraction, we have revealed the presence of a structural phase transition at T s ∼122K between the room-temperature cubic structure (space group F m3m) and a new low-temperature tetragonal structure (I4/m). From reectivity measurements we found that the peak exciton energy E ex ≈ 2.85 eV near the direct gap shifts proportionally to the tetragonal strain, which is consistent with the E ex being primarily controlled by a rotational degree of freedom of the crystal structure, thus by the angle Bi-BrAg. We observed the time-resolved photoluminescence kinetics and we found that, among the relaxation channels, a fast one is mainly present in the tetragonal phase, suggesting that its origin may lie in the formation of tetragonal twin domains. 7 it has been predicted to have relatively low carrier eective masses 8 and it has shown long carrier recombination lifetimes. 9 Greul et al. 10 and Gao et al. 11 demonstrated the fabrication of Cs 2 AgBiBr 6 lms and incorporated them into working devices for the rst time. However, there is very little know about the crystallography and its impact upon the optoelectronic properties. This information is important in order to both improve the present family of double perovskites, and to design

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