Proceedings of Materials for Sustainable Development Conference (MAT-SUS) (NFM22)
DOI: https://doi.org/10.29363/nanoge.nfm.2022.077
Publication date: 11th July 2022
Double perovskites have been developed to alternatively replace the divalent cations Pb2+ in single perovskite with a combination of a monovalent and trivalent cation, forming structure as A2BB’X6. Among them, the compounds Cs2Na1-xAgxIn1-yBiyCl6 can emit warm white light with almost unity quantum efficiency and are thus among the most promising materials for solid-state lighting(1). The emission spectrum is attributed to self-trapped excitons (STEs), the emission efficiency is, however, sensitive to the material composition since it is strictly related to the simultaneous presence of a small percentage of Ag and Bi.
Whether Bi atoms cure extrinsic defects or provide natural recombination centers is still a wide-open question while the addition of Ag has been linked to parity breaking in valence band orbitals, increasing the oscillator strength of the interband optical transitions(2,3).
A comprehensive understanding of the microscopic mechanisms producing the rules for efficient emission is fulfilled by a systematic study of the structural and optical properties as a function of the composition to understand the role of Ag and Bi in the high efficiency emission. Photoluminescence quantum yield (PLQY) measurements are complemented with photoluminescence excitation (PLE) and ultrafast spectroscopy in the form of differential transmission (DT/T) and time-resolved photoluminescence (TRPL). The dynamic of optical excitation is studied to investigate the formation of STEs and their decay times.
Bi and Ag substitutions at 0.1% levels are sufficient to promote the formation of STEs with a nearly radiative-recombination-limited dynamics, while effectively curing STE trapping in long-lived dark states. An Ag-induced self-trapping polaronic behavior is also suggested as a potential mechanism for formation of bright STEs thus achieving emission efficiency near unity with a warm white light spectrum in a wide variety of compositions.
Double perovskites realize therefore an almost ideal platform for solid state lighting and these results provide guidance for rational optimization of such compounds in view of the use as phosphors and active materials for LEDs and displays.
The authors acknowledge access to research infrastructure in CeSAR—Centro Servizi di Ateneo per la Ricerca at Università degli Studi di Cagliari. This work was funded by Regione Autonoma della Sardegna through PO-FSE Sardegna 2007–2013, L.R. 7/2007, ‘‘Progetti di ricerca di base e orientata,’’ Projects No. CRP3-114, CRP-17571, CRP-18353, CRP-18013, and CRP-24978, and through Delibera CIPE n. 31 del 20.02.2015 e deliberazione n. 52/36 del 28.10.2015 ‘‘Piano Strategico Sulcis,’’ through Project No. SULCIS-820889 and SULCIS-820947.
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