Publication date: 3rd July 2020
Lead halide perovskites have gained a lot of popularity in the field of optoelectronics in recent times. Where the toxicity of lead remains a major issue, an emerging non-toxic alternative proposed is lead-free double perovskites with the generic stoichiometric formula A2MIMIIIX6. In double perovskite, the divalent lead is replaced with one monovalent (MI) and one trivalent (MII) metal cation. Cs2AgBiBr6 is one of the stable double perovskite with an indirect bandgap where the optical properties and the charge carrier relaxation processes are not fully understood. We applied time-resolved photoluminescence and differential transmission spectroscopy to explore the photo-excited charge carrier dynamics within the indirect band structure of Cs2AgBiBr6 nanocrystals. We observed a high energetic emission at the direct bandgap, alongside the emission from the indirect bandgap transition. We assign this emission to the radiative recombination of the direct bound excitons originating due to trapping of holes. Due to the electron intervalley scattering process, the emission maximum from this direct bound excitons redshifts over 1 eV within 10 ps, leading to its transfer from direct to indirect bound exciton. We conclude that this direct bound exciton has giant oscillator strength which causes the higher energetic emission to occur at the direct bandgap despite the prevailing intervalley scattering process. These results expand the current understanding of the optical properties and the charge carrier relaxation process in double perovskites family, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites. [1]
This work was supported by the Bavarian State Ministry of Science, Research, and Arts through the grants “Solar Technologies go Hybrid (SolTech)”, the Deutsche Forschungsgemeinschaft (DFG) through the excellence cluster “e-conversion” (EXC 2089/1-390776260). I'd like to thank the Alexander von Humboldt-Stiftung for postdoctoral fellowship.
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