Ultrafast Excited-State Localization in Cs 2 AgBiBr 6 Double Perovskite

• Published in: The journal of physical chemistry letters 2021-04, Vol.12 (13), p.3352-3360
• Main Authors: Wright, Adam D, Buizza, Leonardo R V, Savill, Kimberley J, Longo, Giulia, Snaith, Henry J, Johnston, Michael B, Herz, Laura M
• Format: Article
• Language: English
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.1c00653

Cs AgBiBr is a promising metal halide double perovskite offering the possibility of efficient photovoltaic devices based on lead-free materials. Here, we report on the evolution of photoexcited charge carriers in Cs AgBiBr using a combination of temperature-dependent photoluminescence, absorption and optical pump-terahertz probe spectroscopy. We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm V s at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge whose band gap and associated exciton binding energy shrink with increasing temperature in a correlated manner. Overall, our observations suggest that strong electron-phonon coupling in this material induces rapid charge-carrier localization.