Ultrafast Excited-State Localization in Cs2AgBiBr6 Double Perovskite

Wright, Adam D., Buizza, Leonardo R. V., Savill, Kimberley J., Longo, Giulia, Snaith, Henry J., Johnston, Michael B. and Herz, Laura M. (2021) Ultrafast Excited-State Localization in Cs2AgBiBr6 Double Perovskite. The Journal of Physical Chemistry Letters, 12 (13). pp. 3352-3360. ISSN 1948-7185

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Abstract

Cs2AgBiBr6 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 Cs2AgBiBr6 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 cm2 V–1 s–1 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.

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