Recently, SUSTech Associate Prof. Kai Wang’s (Department of Electrical and Electronic Engineering) team made substantial progress in the field of perovskite light-emitting diodes, with the results published in high-profile journals of Advanced Materials (IF=27.398), ACS Energy Letters (IF=19.003), Communications Materials (a selective journal from Nature Research), and Optics Express (IF=3.669).
In detail, perovskite light-emitting diodes (PeLEDs) have emerged as attractive optoelectronic devices that have a promising prospect for tunable light emission, high color purity, and high photoluminescence quantum yields (PLQYs). In particular, the qua si-2D perovskites are rising as efficient luminescent materials for highly performed blue PeLEDs due to the cascade energy landscape for efficient exciton transfer and the subsequent radiative recombination.
However, typical quasi-2D perovskites show a wide distribution of low-order phases, especially the low-n phase quasi-2D perovskites which would result in low emission efficiency owing to the inefficient internal energy transfer. Meanwhile, the defects and traps generated during the perovskite crystallization increase nonradiative recombination, further aggravating the external quantum efficiency (EQE). In the work published in ACS Energy Letters, the research team demonstrates a unique quasi-2D perovskite with low-order phase suppression and defect passivation for efficient energy transfer and light emission by incorporating a 2D perovskite and an excess ammonium salt into the quasi-2D perovskite solution. By optimizing the new class of quasi-2D perovskite, the team made blue PeLEDs with EQE of 7.51%, increasing by 117% compared with the control sample of 3.46%. The first author of the work is Dr. Zhenwei Ren (visiting Postdoc. from the University of Hong Kong, HKU), and the co-corresponding authors are Associate Prof. Kai Wang (SUSTech), Prof. Rui Chen (SUSTech), and Prof. Wallace C. H. Choy (HKU). Meanwhile, the work was also supported by Chair Prof. Xiao Wei Sun (SUSTech). (https://doi.org/10.1021/acsenergylett.0c01015)
Figure 1. The PeLED structure and performances, and the time-dependent operational stability measured under constant current.
Besides, it is noted that the quasi-2D perovskite layers are separated from each other by spacer cations. Typical spacer cations, such as PEA+, BA+ can only interact with the perovskite layers at one side and leave a van der Waals gap with other quasi-2D perovskite layers. It is thus a concern that the van der Waals gap will induce a loose space between quasi-2D perovskite layers and subsequently inefficient energy transfer in the perovskite film causing poor PeLED efficiency. In addition, the presence of weak van der Waals gaps also deteriorates the stability of quasi-2D perovskite structure due to easy degradation of perovskite structure upon exposure to different operation conditions, such as continuous heat and light soaking during PeLED operation. In the work published in Advanced Materials, a bifunctional ligand of 4-(2-aminoethyl) benzoic acid (ABA) cation is strategically introduced into the perovskite to diminish the weak van der Waals gap between individual perovskite layers for promoting coupled quasi-2D perovskite layers. In particular, the strengthened interaction between coupled quasi-2D perovskite layers favors an efficient energy transfer in the perovskite films. The introduced ABA can also simultaneously passivate the perovskite defects by reducing metallic Pb for less nonradiative recombination loss. Benefiting from the advanced properties of ABA incorporated perovskites, highly efficient blue PeLEDs with an external quantum efficiency of 10.11% and a very long operational stability of 81.3 min, among the best performing blue quasi-2D PeLEDs, were achieved. The first author of the work is Dr. Zhenwei Ren, and the co-corresponding authors are Associate Prof. Kai Wang, Prof. Rui Chen, and Prof. Wallace C. H. Choy. (https://doi.org/10.1002/adma.202005570)
Figure 2. Schematic illustration of the interaction between neighboring perovskite layers in pristine/ABA perovskite, the CIE coordinate of pristine/ABA PeLEDs, and the corresponding performances.
Besides, excess electron injection and insufficient hole injection are also key issues for efficient PeLEDs. Excess electrons are easy to be accumulated at the interface, leading to exciton quenching and poor device performance. Therefore, it is important to enhance the injection of holes in the device and promote the balance of carrier injection. In the work published in Communications Materials, the team proposes an efficient strategy of introducing an electric dipole layer to enhance hole injection. First, by introducing a hopping model of carrier transport, it is confirmed that the electric dipole layer between the hole injection layer and the hole transport layer could enhance hole injection significantly. Then a thin MoO3 layer is selected as the electric dipole layer. Due to its deep energy level, electrons are easy to be transferred from the adjacent hole injection layer and hole transport layer to MoO3, thereby forming a large number of electric dipoles, which greatly enhances the hole injection rate. Furthermore, the simulation results of the electric field distribution, carrier density distribution, and recombination rate distribution theoretically prove the enhancement of the MoO3 electric dipole layer on hole injection. Moreover, the low-frequency capacitance-voltage measurement and analysis further prove the more efficient hole injection by introducing the electric dipole layer of MoO3. Based on the theoretical analysis and simulation calculations, researchers fabricate the proposed PeLED with an improved EQE by 8.7% up to 16.8%, and improved current efficiency (CE) by 37.2 cd/A up to 72.7 cd/A, which is the highest value for green perovskite NCs based PeLEDs, indicating a feasible approach with the electric dipole layer of MoO3 to achieve a high-performance PeLED. The joint Ph.D. student Xiangtian Xiao and Taikang Ye are co-first authors, and Associate Prof. Kai Wang and Prof. Wallace C. H. Choy are co-corresponding authors. This work is also supported by Chair Prof. Xiao Wei Sun. （https://doi.org/10.1038/s43246-020-00084-0）
Figure 3. (a) Hole transporting assisted by electric dipole layer, (b) distribution of charge density in the device, (c) distribution of electric field in the device, (d) distribution of recombination rate in the device without MoO3, (e) distribution of recombination rate in the device with MoO3, and the device performances: (f) EQE curve, (g) CE curve, and (h) low-frequency capacitance-voltage curves.
Furthermore, the high working temperature will degrade the performance of QLED/PeLED, and Prof. Wang’s team also investigates the factors which will affect the working temperature of QLED/PeLED systematically. In the work published in Optics Express, the team builds a QLED/PeLED thermal model with verification of experiments. Different influence factors, such as the electro-optic conversion efficiency (EOCE), voltage, current density, active area, substrate size, substrate type, and environmental conditions are analyzed in detail on the working temperature of QLED/PeLED. This work clarifies the condition boundaries of safe working temperature and guides the design of stable QLED/PeLED. The joint Ph. D. student Tianqi Zhang is the first author of the work, and Associate Prof. Kai Wang of SUSTech and Associate Prof. Guichuan Xing (UM) is the co-corresponding authors. (https://doi.org/10.1364/OE.410393)
Figure 4. (a) Schematic diagram of converting electric energy into light and heat energy in QLED/PeLED devices; (b) the actual working temperature of QLED/PeLED and (c) the simulated temperature of QLED/PeLED under the same working conditions; (d)-(i) the influence of different factors on the working temperature of QLED/PeLED.
This research was supported by Chair Prof. Xiao Wei Sun (SUSTech), Associate Prof. Fei Wang (SUSTech), Prof. Wanjian Yin of Soochow University, Prof. Xiaobing Luo of Huazhong University of Science and Technology (HUST), Prof. Kam Sing Wong of Hong Kong University of Science and Technology (HKUST), and Prof. Xinhui Lu of Chinese University of Hong Kong (CUHK). It also got supports of SUSTech Analysis and Testing Center, the University Grant Council of the University of Hong Kong, the funds from the National Natural Science Foundation of China, National Key Research and Development Program, Natural Science Foundation of Guangdong Province, and Guangdong University Key Laboratory for Advanced Quantum Dot Displays and Lighting.
Links to the papers:
https://doi.org/10.1021/acsenergylett.0c01015
https://doi.org/10.1002/adma.202005570
