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 Ts ≈ 122 K between the room-temperature cubic structure (space group Fm3̅m) and a new low-temperature tetragonal structure (I4/m). From reflectivity measurements we found that the peak exciton energy Eex ≈ 2.85 eV near the direct gap shifts proportionally to the tetragonal strain, which is consistent with the Eex 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. Hybrid halide perovskites, with general formula ABX3 (A = organic/inorganic 1+ cation, B = inorganic 2+ cation, and X = halide anion) have gained increasing attention in the scientific community as high-performing semiconductors in solar cell devices. The power conversion efficiency of devices based on these materials has increased to a remarkable 28% in the past few years because of high carrier mobility, a tunable band gap, long diffusion lengths, and strong optical absorption. For the highest thermal stability, an allinorganic perovskite would be preferable. However, to date it has proven highly challenging to stabilize the room-temperature crystalline polymorph of the inorganic lead halide CsPbI3. 6 Furthermore, the realization of the unexpected functionality of the lead halide perovskites has directed the research community to attempt to discover new metal halidebased semiconductors with improved, or complementary, functionality. Recently, double perovskites with general formula A2 MM′X6 have been proposed as all-inorganic alternatives. In particular, Cs2AgBiBr6 is one of the few materials investigated since the discovery of efficient photovoltaic (PV) operation of lead halide perovskites that delivers substantial performance in a PV cell. It is highly stable, it has been predicted to have relatively low carrier effective masses, and it has shown long carrier recombination lifetimes. Greul et al. and Gao et al. demonstrated the fabrication of Cs2AgBiBr6 films and incorporated them into working devices for the first time. However, there is very little known about the crystallography and its impact upon the optoelectronic properties. This information is important both in order to improve the present family of double perovskites and to design new compounds. Although the maximum power conversion efficiency so far achieved in double perovskites (2.23%) is lower than that for hybrid perovskites, these materials are much less mature in terms of device technologies and have significant potential for applications, as demonstrated for example by the discovery of highly efficient near white light emission from Cl double perovskites. Furthermore, they represent a very good model system to study to understand deficiencies and hence routes to improved properties through either design of new compounds or the tuning of the present