News.hust.edu.cn (Correspondent: Ji Xianxian) On November 8, Nature published the paper "Efficient and stable emission of warm-white light from lead-free halide double perovskites" co-authored by JTang Group of Wuhan National Laboratory for Optoelectronics and Professor Yanfa Yan of the University of Toledo in the USA. Professor Tang Jiang and Professor Yan Yanfa are the co-corresponding authors, whilst Luo Jiajun (doctoral student), Li Shunran (postgraduate) and Liu Jing (postgraduate) of Wuhan National Laboratory for Optoelectronics and Dr. Wang Xiaoming of the University of Toledo are the co-first authors of the paper. Huazhong University of Science and Technology is the signature unit of the first author.

Lighting is ubiquitous in human society and accounts for one-fifth of the global electricity consumption. Compared with traditional lighting, semiconductor lighting technology based on GaN-based LED excited phosphor has many advantages such as energy saving, environmental protection, high efficiency, long operationlifetime and wide application range, and is the mainstream technology in the current lighting market. The disadvantage is that there are too many blue light components in lighting, which can easily cause irreversible damage to human eyes, especially children's retinas, that is, the so-called "blue damage". At the same time, most phosphor materials rely on strategic rare earth materials as raw materials. Therefore, it is necessary to develop a new-generation single-emitter-based white-light-emitting phosphor to avoid "blue damage" and the use of rare earth elements and realize green lighting.

Metal halide perovskites have recently shown good application prospects in solar cells, photodetectors and other fields due to their excellent defect tolerance, low-cost solution processing and easily tunable advantages, which have attracted great attention in the field of scientific research as well as industrial community. In particular, the "soft" lattice property of perovskite material makes it have a strong coupling effect between photons and phonons, and the electron-hole pairs generated after excitation can easily cause lattice distortion and generate self-trapped excitons (STEs). STEs-based luminescence has wide spectral characteristics. If the spectrum can be further extended to white light and high-efficiency luminescence can be realized, it will hopefully build a high-performance single-emitter-based white-light-emitting phosphor, avoid the problems of unstable spectrum and self-absorption of traditional multi-element phosphors, and have unique advantages and certain application prospects in the field of lighting.

Compared with the current mainstream research on lead-based perovskites, Professor Tang Jiang’s Group has been focusing on the research of lead-free perovskite materials and optoelectronic devices based on environmental protection and application considerations, and has developed high-performance antimony-based and bismuth-based lead-free perovskite photoluminescence quantum dots (Angew. Chem. Int. Ed. 2016, 55, 15012; Adv. Funct. Mater. 2017, 1704446; Nano Lett. 2018, 18, 6076; Adv. Funct. Mater. 2018, 1801131.) and cesium silver bismuth bromide (Cs₂AgBiBr₆) single-crystal X-ray direct detector with a low detection limit (Nat. Photonics 2017,11,726). On this basis, JTang Group found that the photoluminescence spectrum of the lead-free double perovskite (Cs₂AgInCl₆) covered the entire visible band of wavelength from 400nm to 800nm, reflecting the STE luminescence characteristics caused by its soft lattices. Theoretical calculations indicate that there is a strong electron-lattice coupling in Cs₂AgInCl₆, and its coupling Huang-Phys constant is as high as 37. After being excited, the free exciton is captured by the lattice after 238 fs and becomes a self-trapped exciton. The self-trapped exciton originates from Jahn-Teller distortion of AgCl₆ octahedron in excited state.

Research on calculation of STEs in Cs₂AgInCl₆: a: GW band structure in Cs₂AgInCl₆, b: Electron and hole wave functions of STE in Cs₂AgInCl₆, c: STEs' position coordinates and corresponding energy states, d: Comparison of calculated spectra with experimental spectra.

Although the lead-free double perovskite (Cs₂AgInCl₆) exhibits a relatively unique white-light-emitting property, its luminescent efficiency limited by the parity forbidden transition property and relatively high electronic dimensionality, which leads to its very low photoluminescence quantum yield (<0.1%) and does not have application value. Professor Tang Jiang’s Group innovatively introduced sodium cations alloying and doped trace Bi to prepare Cs₂(NaAg)InCl₆:Bi³⁺, and obtained a single-emitter-based white-light-emitting phosphor with the highest luminous efficiency of 86% through composition control and process optimization. Experimental characterization combined with theoretical calculation confirmed the STE luminescence mechanism of the phosphor, and revealed that the main reasons for the improvement of its fluorescence efficiency are: i) Cs₂AgInCl₆ and Cs₂NaInCl₆ are cubic structures, and the lattice mismatch rate is only 0.3%. The introduction of Na can randomly occupy the Ag position of Cs₂AgInCl₆ to form an alloy; ii) The alloying of Na breaks the lattice symmetry and parity forbidden transition, increases the probability of radiative recombination; and iii) The introduction of Na forms the NaCl₆ octahedron, which divides the AgCl₆ octahedron, and thus the electronic dimensionality of the system is reduced, the wave function overlap between electrons and holes is increased, and the probability of radiative recombination is increased significantly; iv) The doping of trace Bi³⁺ improves the perfection of lattices, reduces the density of defects, inhibits the non-radiative recombination rate, and thus further improves its photoluminescence quantum yield. Due to its all-inorganic properties and strong exciton binding energy, this white-light-emitting phosphor also exhibits an excellent stability. The luminous efficiency and white light properties of Cs₂(NaAg)InCl₆:Bi3+ phosphor are barely declining after being heated at 150℃ for 1000 hours on hotplate, or working for 1000 hours at 5000 Cd/m² luminous intensity after being excited by ultraviolet LED without encapsulation.

In a word, this research explains the photoluminescence characteristics of self-trapped excitons in the lead-free double perovskite (Cs₂AgInCl₆), and innovatively realizes efficient and stable emission of single-matrix white light through Na⁺ alloying and trace Bi³⁺ doping, which not only points out a way for the research of lead-free perovskite luminescent materials, but also has the advantages of simple preparation, stability and high efficiency and is expected to realize industrial application in green lighting.

Cs₂(NaAg)InCl₆: Bi³⁺ white-light-emitting phosphor luminescence characteristics: a: Fluorescence peaks at different temperatures, b: Fluorescence intensity change after heating at 150℃ on hotplate; c: Intensity change of the emission peak excited by the ultraviolet LED; d: Thin films prepared by thermal evaporation and their XRD characterization results.

This research was done by JTang Group in cooperation with Professor Joanne Etheridge of Monash University, Professor Zhang Lijun of Jilin University, Professor Edward H. Sargent of University of Toronto, Professor Wang Liduo of Tsinghua University, Professor Han Junbo of Wuhan National High Magnetic Field Center, Professor Liang Wenxi of Wuhan National Laboratory for Optoelectronics, and Professor Wang Jianbo of Wuhan University, who provided theoretical calculations and material characterization support respectively. Thanks for the support from the Ministry of Science and Technology's key R&D program, the National Natural Science Foundation of China, and Huazhong University of Science and Technology's interdisciplinary key innovation team project.

Professor Tang Jiang’s Group has been working on the research of new photoelectric conversion materials and devices. Since its establishment in 2012, it has published 1 paper on Nature, two papers on Nature Photonics, 1 paper on Nature Energy, and 2 papers on Nature Communications, with Huazhong University of Science and Technology as the first author. The Group's current research interests include antimony selenide thin-film solar cells, lead-free perovskite luminescent materials and devices, X-ray direct and indirect detection materials and devices, quantum dot infrared detection materials and devices. Students who are interested in optoelectronic research are welcome to join the Group. We also hope to conduct in-depth exchanges and cooperation with experts, scholars and enterprises in related fields inside and outside the university.

Paper link: https://www.nature.com/articles/s41586-018-0691-0

Group link: http://tfsc.wnlo.hust.edu.cn/index.htm