Charge Localization in Cs2AgBiBr6 Double Perovskite: Small Polarons and Self-Trapped Excitons

Halide double perovskites have gained significant interest as promising materials in optoelectronic applications. Their relatively soft structure allows deformations that can potentially trap charge carriers, leading to the formation of small polarons. In this study, we investigate hole and electron polarons in Cs2AgBiBr6, a promising double perovskite, using the PBE0(α) functional. Our results reveal that spin–orbit coupling has a significant effect on the formation energies of small polarons and must be considered for accurate modeling of localized charges in Cs2AgBiBr6. While electron localization on Bi atoms is favorable, the hole polaron localized at the Ag site is very close in energy to the delocalized state. Simultaneous localization of holes and electrons at neighboring Ag and Bi sites leads to the formation of self-trapped excitons due to the significant attraction between the polarons of opposite charges. Our results provide insights into the essential properties of Cs2AgBiBr6 and have potential implications for further research and development of Cs2AgBiBr6-based green devices.