Efficient blue light emitting diodes based on europium halide perovskites

The research work titled “Efficient blue light emitting diodes based on europium halide perovskites” from Prof. Jiang Tang's group is online in Advanced Materials. The first authors are Dr. Jiajun Luo, Mr. Longbo Yang and Dr. Zhifang Tan, and the corresponding author is Prof. Jiang Tang. The manuscript is available at https://onlinelibrary.wiley.com/ doi/full / 10.1002/adma.202101903

Introduction

Flat panel displays enjoy 100 billion-dollar markets with significant penetration in 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 red and green cousins, particularly for color coordinates approaching (0.131, 0.046) that fulfill the Rec. 2020 specification for blue emitters.

Two main strategies have been proposed to implement blue PeLEDs. The first strategy relies on mixing halide perovskites with photoluminescence spectra and CIE color coordinates precisely controlled by compositional engineering, but these perovskites suffer from low spectral stability due to ion migration and phase separation under electric field. The second approach involves dimensional engineering, and the hypsochromic shift of emission can be achieved by incorporating the large organic molecules or ligands for dimension reduction. However, the inferior charge transport ability of large organic molecules or ligands leads to the notable efficiency gap between PLQY and electroluminescence EQE. The combined characteristic of high EQE and standard CIE color coordinates has not been realized in blue PeLEDs so far. In addition, previously reported blue PeLEDs rely on toxic lead, which is severely restricted by the regulations in consumer electronics. Therefore, developing efficient blue perovskite emitters with high efficiency, standard color coordinate, good stability, as well as non-toxic composition is an urgent target in this field.

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.

Experiments and results

This work reports report a high-efficiency, lead-free perovskite CsEuBr₃that exhibits bright blue exciton emission centered at 448 nm with a color coordinates of (0.15, 0.04). CsEuBr₃possesses 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 CsEuBr₃crystals with 365 nm UV excitation at room temperature, and the inset shows the photograph of CsEuBr₃under 365 nm UV light. (d)Schematic of dual-source evaporation deposition for CsEuBr₃. (e) Experimental two-dimensional TPLM images of CsEuBr₃film 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 () 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 fabricateCsEuBr₃filmvia a dual-source thermal vacuum evaporation processto exclude the influence of organic solvent. By optimizing the annealing temperature, we successfully fabricate highly crystallineCsEuBr₃film. Further optical characterizations reveal itsslow exciton-exciton annihilation rate, excellent exciton diffusion diffusivity of0.0227cm²s^-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 CsEuBr₃based LEDs. (b) Band energy alignment of our LEDs. (c) Current density-voltage-luminance characteristics of our CsEuBr₃based 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.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (62050039,61725401, 5171101030, 51761145048), the National Key R&D Program of China (2016YFB0700702, 2016YFA0204000, and 2016YFB0201204), the HUST Key Innovation Team for Interdisciplinary Promotion (2016JCTD111),China Postdoctoral Science Foundation (2020M62004075, 2020M 62005089)and the Post-Doctoral Innovative Talent Support Program (BX20200142).The authors from HUST thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices, WNLO. We also thank Prof. XueFei Li from HUST for providing access to ALD systems; thank Prof. Yizheng Jin from Zhejiang University for providing standard QLEDs devices to verify our EL measurement setup; thank Prof. Tianyou Zhai, Jianbing Zhang, and Lin Xu from HUST for providing access to some facilities.