Structural and Optical Properties of Cs2AgBiBr6 Double Perovskite

We present a comprehensive study of the relationship between the crystal structure and optoelectronic properties of the double perovskite Cs2AgBiBr6, which has emerged as a promising candidate for photovoltaic devices. On the basis of single-crystal/powder X-ray diffraction and neutron powder diffraction, we have revealed the presence of a structural phase transition at T-s approximate to 122 K between the room-temperature cubic structure (space group Fm (3) over barm) and a new low-temperature tetragonal structure (I4/m). From reflectivity measurements we found that the peak exciton energy E-ex approximate to 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-Ag-Br. 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.