Recently, Zeng Qinghui, associate researcher of Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, proposed a high-efficiency fluorescent perovskite nanocrystal induced by the structural transformation of CsPbBr3 and Cs4PbBr6. The fluorescence quantum efficiency of the prepared perovskite nanomaterials can be Up to 99%, and the application of this material with high fluorescence quantum yield to LED devices is very important for the application of perovskite quantum dots in the field of optoelectronic devices. The result was published in the Journal of Materials Chemistry C of the Royal Society of Chemistry (J. Mater. Chem. C, 2019, 7, 7548--7553), and was selected as the front cover article of the magazine. The first author of the article is Su Ying, a PhD student, and the corresponding author is Zeng Qinghui.

All-inorganic perovskite nanomaterials are used in LEDs and lasers because of their excellent optical properties (such as high fluorescence quantum yield, excitation linewidth, narrow fluorescence emission spectrum, adjustable emission spectrum, etc.) And solar cells and other photovoltaic fields have been widely applied research. With the rapid development of all-inorganic perovskite quantum dots, perovskite derivatives such as Cs4PbBr6, Cs2PbBr4 and CsPb2Br5 have attracted more and more attention from researchers. In recent years, research has focused on the chemical conversion of "non-luminous" Cs4PbBr6 nanocrystals and "luminous" CsPbBr3 quantum dots. But so far, the apparent fluorescence enhancement in the conversion process and the underlying mechanism behind it have not been explained.

Zeng Qinghui and other researchers realized the conversion from CsPbBr3 quantum dots to Cs4PbBr6 nanocrystals by adding ZnBr2 as an inducer, and explained the obvious fluorescence enhancement effect existing in this conversion process (the fluorescence quantum yield of CsPbBr3 quantum dots from 58 % To 99%) and the deep mechanism behind it. At the same time, the precise luminescence peak position (336nm) of Cs4PbBr6 nanocrystals was observed, which solved the previous controversial issues.

By adding ZnBr2 as an inducer, the gradual transition from CsPbBr3 quantum dots to Cs4PbBr6 nanocrystals was achieved, and the position of the photoluminescence peak was maintained at 517 nm without much deviation. During the conversion process from CsPbBr3 quantum dots to Cs4PbBr6 nanocrystals, the fluorescence quantum yield of the remaining CsPbBr3 quantum dots remains as high as nearly 99%, which is due to the "survival of the fittest" process and non-radiative transitions that occur in CsPbBr3 quantum dots The process of reduction. Interestingly, when the content of the inducer was increased to 95%, a Cs4PbBr6 nanocrystalline structure was obtained that was almost single-phase. By monitoring its absorption and emission spectra, an emission peak of 336 nm was obtained, which is consistent with previous published theoretical studies, thus indicating the precise luminescence peak position of Cs4PbBr6 nanocrystals. It is further directly clarified that the green emission peak previously seen in the mixed nanocrystalline structure in which CsPbBr3 and Cs4PbBr6 coexist is only from the emission of CsPbBr3 quantum dots and not the Cs4PbBr6 nanocrystals. Through cooperation with researchers from the Chinese University of Hong Kong, green LED devices were prepared based on optimized CsPbBr3 high-fluorescence quantum dots, with a maximum brightness of 1941.6 cd / m2 and a maximum external quantum efficiency (EQE) of about 1.21%. It has opened up a new way for obtaining high-efficiency fluorescent CsPbBr3 perovskite quantum dots and corresponding fruitful optoelectronic devices in the future.

This work was supported by the Jilin Science and Technology Development Plan project.

Schematic diagram of the structure conversion of CsPbBr3 and Cs4PbBr6 perovskite nanocrystals under the action of ZnBr2 inducer

Photographs, fluorescence quantum efficiency (c) and fluorescence lifetime (e) of CsPbBr3 and Cs4PbBr6 perovskite nanocrystals under the action of different proportions of ZnBr2 inducer under natural light (a) and ultraviolet light (b); Cs4PbBr6 perovskite nanocrystals Absorption and fluorescence emission spectrum (d)

(A) Fluorescence emission spectrum and device photo results before and after the perovskite quantum dots are made into LED devices (b) Luminance and external quantum efficiency results of LED devices

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