Proceedings of MATSUS Spring 2025 Conference (MATSUSSpring25)
DOI: https://doi.org/10.29363/nanoge.matsusspring.2025.168
Publication date: 16th December 2024
Double perovskite (DP) nanocrystals (NCs) are promising lead-free materials for applications in photovoltaics, light-emitting devices, and plastic scintillators. The state-of-the-art DP NCs, Cs1.92Ag0.2Na0.75K0.07InCl6 doped with 0.5% Bi (CANKBIC), achieve a photoluminescence (PL) quantum yield (QY) of ~70% [1]. However, their excited-state fine structure remains unexplored. In this work, we present temperature- and magnetic field-dependent PL dynamics of CANKBIC NCs, providing evidence for a luminescent state split into three distinct levels with lifetimes ranging from a few ns to ms.
Synthesized CANKBIC NCs exhibit PL centered at 2.0 eV and a multiexponential decay with an average PL lifetime (τavg) of 4.7 µs at room temperature. The PL originates from self-trapped excitons involving BiCl6 and AgCl6 octahedra, though the fine structure of the emissive state remains unexplored. As the temperature drops below 150 K, the τavg triples—a phenomenon previously attributed to the suppression of non-radiative decay, suggesting near-unity QY below this temperature [2]. At temperatures below 15 K, the τavg further increases reaching values of ~ms at 6 K. We interpret these results as a fingerprint of a fine structure in the emissive state, with a lower lying forbidden (dark) and a higher lying allowed (bright) state. Crucially, the analysis of the temperature dependent PL lifetimes and the decay amplitudes, reveals that the dark state is further split into two states exhibiting very different transition probabilities. At 6 K, applying a magnetic field shortens the PL lifetime, consistent with field-induced mixing between the dark states. We attribute the fine structure to a combined effect of electron-hole exchange interaction and spin-oribit coupling in a low symmetry environment, similar to what is observed in e.g. Cu(I) molecular complexes [3].
This study unveils the previously unobserved excited-state fine structure of DP NCs, contributing new insights into their photophysical behavior.
This work was supported by National Science Centre Poland grants no. 2019/35/B/ST3/04235 and 2018/31/G/ST3/03596
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