On August 3rd, a research paper titled Efficient blue light emitting diodes based on europium halide perovskites was published online in Advanced Materials by Prof. Tang Jiang’s team under Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology. This paper reports a high-efficiency, lead-free perovskite CsEuBr3 with suitable CIE color coordinate and excellent stability, which potentially inspires further research on lanthanide electroluminescence for next-generation LED applications.

Flat panel displays enjoy 100 billion-dollar markets globally with significant penetration into our daily life, which require efficient, color-saturated blue, green, and red light-emitting diodes (LEDs). The recently emerged lead halide perovskites have demonstrated low-cost and outstanding performance for potential LED applications. However, the performance of blue perovskite light emitting diodes (PeLEDs) lags far behind its red and green cousins, particularly for color coordinates approaching (0.131, 0.046) that fulfill the Rec. 2020 specification for blue emitters.

The bivalent lanthanide (Ln: Ce-Lu) ions not only share a similar ionic radius with Pb²⁺, but also possess fascinating luminescence properties, such as abundant emission lines covering deep-blue to infrared, narrow emission linewidth, high energy conversion efficiency, and excellent stability. Substituting Pb²⁺ with low-toxic Ln²⁺ ions in lead halide perovskite potentially combines each other’s advantages.

This work reports a high-efficiency, lead-free perovskite CsEuBr3 that exhibits bright blue exciton emission centered at 448 nm with color coordinates of (0.15, 0.04). CsEuBr3 possesses an orthorhombic perovskite-type structure with luminescent [EuBr6]- tilting octahedra surrounded by inert Cs+ cations, and Eu2+ ionic radius of 117 pm resembles that of Pb2+ (119 pm). The first-principles density functional theory (DFT) using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation function reveals the band structure of CsEuBr3, which has a direct bandgap at Γ point with an optical band gap of 2.85 eV. The CBM is composed mainly of Eu-5d orbitals while the VBM is derived from admixed Eu-4f and a small portion of Br-4p orbitals. The unforbidden Eu-5d→Eu-4f/Br-4p transition permits a short excited-state lifetime of 151 ns, which is considerably faster than that of other lanthanide ions with f-f transitions.

Figure 1. (a) Crystal structure of the lead-free perovskite CsEuBr₃. Yellow, cyan, and red spheres represent Cs, Eu, and Br atoms, respectively. (b) Band structure (left) and density of state (DOS) (right) of CsEuBr₃. The dashed lines denote the Conduction band minimum. (c) The PL spectrum of CsEuBr₃ crystals with 365 nm UV excitation at room temperature, and the inset shows the photograph of CsEuBr₃ under 365 nm UV light. (d) Schematic of dual-source evaporation deposition for CsEuBr₃. (e) Experimental two-dimensional TPLM images of CsEuBr₃ film at different delay times of 0, 20, 40, 60 ns, respectively. (f) The initial time PL intensity (I_PL, t=0) and PL effective lifetime (τ^e) dependence of carrier density. The carrier density is calculated based on the excitation laser power and absorption coefficient towards 405 nm wavelength laser.

Due to the strong coordination effect of Eu²⁺ with the common organic solvent, we fabricate CsEuBr₃ film via a dual-source thermal vacuum evaporation process to exclude the influence of organic solvent. By optimizing the annealing temperature, we successfully fabricate highly crystalline CsEuBr₃ film. Further optical characterizations reveal its slow exciton-exciton annihilation rate, excellent exciton diffusion diffusivity of 0.0227 cm² s^-1, and high quantum yield of ≈70%. Encouraged by these findings, we constructed deep-blue PeLEDs based on all-vacuum processing methods. The devices show a maximum external quantum efficiency of 6.5% with an operating half-lifetime of 50 minutes at an initial brightness of 15.9 cd m^-2. We anticipate this work will inspire further research on lanthanide electroluminescence for next-generation LED applications. Improvement in devices' brightness remains a great challenge, and further composition engineering, better device architecture, and effective hole injection are future directions.

Figure 2. (a) The device structure of our CsEuBr₃ based LEDs. (b) Band energy alignment of our LEDs. (c) Current density-voltage-luminance characteristics of our CsEuBr₃ based LED. (d) The EQE-current density curves of the fabricated LED devices. (e) Electroluminescence spectra of as-fabricated LEDs under different applied voltage. Inset shows the corresponding CIE coordinate. (f) The stability of as-fabricated LEDs under continuous voltage of 5.8 V for 3000 s.

This research was conducted by JTang Group in cooperation with Professor Jin Shengye of Dalian Institute of Chemical Physics and Dr. Xie Weiwei of Karlsruhe Institute of Technology. Professor Tang Jiang is the co-corresponding author, Dr. Luo Jiajun, Mr. Yang Longbo and Dr. Tan Zhifang of Wuhan National Laboratory for Optoelectronics are the co-first authors of the paper. The research received support from the Ministry of Science and Technology’s key R&D program, the National Natural Science Foundation of China, China Postdoctoral Science Foundation, the Post-Doctoral Innovative Talent Support Program, and Huazhong University of Science and Technology's interdisciplinary key innovation team project.

Professor Tang Jiang’s Group has long been working on the research of new photoelectric conversion materials and devices. Since its establishment in 2012, the Group has published 1 paper on Nature, two papers on Nature Photonics, 1 paper on Nature Energy, and 3 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, and 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://onlinelibrary.wiley.com/doi/10.1002/adma.202101903

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

Source: Wuhan National Laboratory for Optoelectronics

Written by: Tan Zhifang, Gou Bingbing

Edited by: Andrew, Peng Yumeng