Structural and Optical Properties of Cs₂AgBiBr₆ Double Perovskite

Abstract

We\npresent a comprehensive study of the relationship between the\ncrystal structure and optoelectronic properties of the double perovskite\nCs₂AgBiBr₆, which has emerged as a promising\ncandidate for photovoltaic devices. On the basis of single-crystal/powder\nX-ray diffraction and neutron powder diffraction, we have revealed\nthe presence of a structural phase transition at <i>T</i>_s ≈ 122 K between the room-temperature cubic structure\n(space group <i>Fm</i>3̅<i>m</i>) and a\nnew low-temperature tetragonal structure (<i>I</i>4/<i>m</i>). From reflectivity measurements we found that the peak\nexciton energy <i>E</i>_ex ≈ 2.85 eV near\nthe direct gap shifts proportionally to the tetragonal strain, which\nis consistent with the <i>E</i>_ex being primarily\ncontrolled by a rotational degree of freedom of the crystal structure,\nthus by the angle Bi–Ag–Br. We observed the time-resolved\nphotoluminescence kinetics and we found that, among the relaxation\nchannels, a fast one is mainly present in the tetragonal phase, suggesting\nthat its origin may lie in the formation of tetragonal twin domains.