Ultrafast Excited-State Localization in Cs₂AgBiBr₆ Double Perovskite

Abstract

Cs 2 AgBiBr 6 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 2 AgBiBr 6 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 2 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.

Lead‐Free Double Perovskite Cs2AgBiBr6: Fundamentals, Applications, and Perspectives

“…A characteristic and strong peak located at around 438 nm is observed and questions remain about the origin of this peak. [50] This sharp peak, which contributes more than 20% of the overall light absorption of Cs 2 AgBiBr 6 perovskite, has been assigned to either excitonic absorption, the chargetransfer-like transition between Ag and Bi/Br orbitals, or localized Bi 6s-6p transitions. Recently, this characteristic absorption peak has been demonstrated to show no strong dependence on Cs 2 AgBiBr 6 nanocrystal sizes, as shown in Figure 2b, reflecting that the peak does not in fact originate from confined excitons.…”

“…[38,52,55] Note that there are reports claiming that the PL peak results from the spatially localized color centers which can be either intrinsic selftrapping of excitons/carriers or defects. [50,56] Instead, we have recently demonstrated that the PL emission of Cs 2 AgBiBr 6 is mainly from the band-to-band emission. To start, the fluencedependent PL intensity versus carrier density is measured and reveals a power-law factor of 1.94 and 1.26 when the excitation density is smaller or larger than 10 16 cm -3 , respectively, which implies both free carrier and exciton emission in Cs 2 AgBiBr 6 samples.…”

Recombination and localization: Unfolding the pathways behind conductivity losses in Cs2AgBiBr6 thin films

“…However, temperature-activated delocalization results in appreciable carrier mobilities at room temperature. 20,21 Moreover, mobile carriers were observed for microseconds after photoexcitation at elevated temperatures, 20 highlighting CABB as a potential alternative for leadhalide analogs.…”

“…1 ps À1 to be intrinsic to CABB. 21 Such fast localization rates would be detrimental to its application in optoelectronic devices. However, temperature-activated delocalization results in appreciable carrier mobilities at room temperature.…”

“…The red-shift is known to be due to strong exciton phonon coupling. 8,11,21,27 Recent work suggests that the charge carrier recombination pathway in CABB proceeds via color centers. 21,24 We observe oscillations in the reflectance (R, the fraction of light reflected) spectrum [green line in Fig.…”