一、研究背景
胶体NPLs只在厚度方向上具有量子限域效应，从而产生了独特的光学特性，如极窄的发射线宽、较短的光致荧光寿命、各向异性跃迁偶极子分布、高增益系数和超低的激光阈值。这些独特的光学特性使NPLs为光电子应用提供了巨大的潜力，包括太阳能聚光器、固态激光器和LEDs。由于NPLs的偶极子分布可控，NPL-LEDs的EQE可以达到约40%的理论极限，是QLEDs理论极限的1.5倍。然而，相比于胶体量子点，胶体NPLs的生长是由侧面和顶面之间不平衡的表面能驱动，合成的NPLs由于侧面能量高而稳定性差，导致配体结合能低和缺陷态增加。此外，NPLs的热力学稳定性较低且高温易熟化，增加了NPLs高温包壳的难度。虽然低温胶体原子层沉积方法适用NPLs生长壳层，可实现多种核/壳结构，且避免了与低热稳定性相关的问题。然而，低温获得的核/壳NPLs通常表现出相对较低的荧光量子产率（PLQY）且纯化步骤复杂。为了进一步提高NPLs的PLQY，研究者通过添加油酸金属络合物来提高CdSe基NPLs的热力学稳定性，最大限度减少了NPLs在高温下的熟化。虽然NPLs合成方法与NPL-LEDs制备工艺发展迅速，但仍落后于最先进的QLEDs。
二、成果简介
针对以上问题，北京理工大学李红博教授课题组通过引入过渡冠（CdSeS）结构以提升CdSe基NPLs侧面的稳定性，抑制高温长壳过程中的熟化作用，并构建合金梯度（CdZnS）壳层结构来提高CdSe基NPLs的量子产率和稳定性。最终获得尺寸均匀（平均长、宽和厚度分别为23.21±1.01，20.01±0.75和4.81±0.19 nm）、分散良好的CdSe/CdSeS/CdZnS核/冠/壳NPLs，显示出了100%的量子产率和高稳定性。结果表明，与传统的CdZnS壳层相比，CdSeS/CdZnS冠/壳可以有效地抑制CdSe基NPLs的非辐射俄歇复合。在飞秒激光激发下，合成的CdSe/CdSeS/CdZnS核/冠/壳NPLs的光增益阈值为2.11 μJ/cm2，低于CdSe/CdZnS核/壳NPLs（2.56 μJ/cm2）以及报道的CdSe/CdZnS核/壳NPLs的最低阈值（2.35 μJ/cm2)。此外，基于设计的核/冠/壳NPLs制备了LEDs，显示出了高亮度（134306 cd/m2）和高效率（EQE为30.1%），并呈现了 饱和纯红发射和优异的工作寿命（T95＞600 h在1000 cd/m2亮度下）。这是迄今为止报道的NPL-LEDs的最高效率值。
三、图文导读
Figure 1. a) Schematics showing the propagation of light emitted from vertical (top) and horizontal (bottom) dipoles. is the critical angle. b) Random dipole orientations of QDs and horizontally aligned dipole orientations of NPLs. c) Schematics showing light propagation and loss in QLEDs (top) and NPL-LEDs (bottom). In QLEDs, most photons are trapped inside the substrate, while in NPL-LEDs, due to the in-plane oriented dipole moments, emission perpendicular to the substrate is enhanced. d) Scheme of the synthesis of CdSe/CdSeS/CdZnS core/crown/shell NPLs. TEM images and EDS mappings of e) CdSe/CdSeS NPLs and f) CdSe/CdSeS/CdZnS NPLs, showing their highly uniform morphology.
Figure 2. Color map of in situ PL spectral evolution of the CdZnS shell growth process on a) CdSe/CdSeS/CdZnS NPLs and b) CdSe/CdZnS NPLs. c) PL FWHM and PL peak position of the CdSe/CdSeS/CdZnS NPLs and CdSe/CdZnS NPLs extracted from (a) and (b). d) Absorption and PL spectra (excited at 400 nm) of the final CdSe/CdSeS/CdZnS and CdSe/CdZnS NPLs. Inset is a photograph of CdSe/CdSeS/CdZnS NPLs in hexene. e) TRPL dynamics of CdSe/CdSeS/CdZnS NPLs and CdSe/CdZnS NPLs. f) The biexciton recombination kinetics of CdSe/CdSeS/CdZnS NPLs and CdSe/CdZnS NPLs extracted from transient absorption measurements can be fitted by single-exponential decay (green line) with lifetimes of 1574 and 1182 ps, respectively. The pump fluence-dependent kinetic curves are shown in Figure S18 (Supporting Information).
Figure 3. PL spectra of a) CdSe/CdSeS/CdZnS NPLs and b) CdSe/CdZnS NPLs at different excitation densities, insets show the integrated PL intensity as a function of excitation density. Temperature-dependent PL spectra (excited at 400 nm) of c) CdSe/CdSeS/CdZnS NPLs and d) CdSe/CdZnS NPLs. e) PLQY and f) TRPL dynamics of CdSe/CdSeS/CdZnS NPLs and CdSe/CdZnS NPLs in solution and film.
Figure4. a) AFM image and b) PL mapping and c) BFP image of CdSe/CdSeS/CdZnS NPLs films spin-coated on HTL. d) Comparison between simulated (70% and 100% horizontal dipoles) and experimental BFP image line cuts along the red line shown in Figure 4c.
Figure 5. a) Schematic of the NPL-LEDs device structure. b) Energy level diagram of NPL-LEDs. c) EL spectra of NPL-LEDs under different driving voltages, inset shows the corresponding CIE coordinates (0.68,0.31). d) Variation of EQE and current efficiency versus current density. e) Variation of current density and luminance versus voltage, The inset shows a photograph of a working NPL-LEDs emitting uniformly over the entire device area. f) Operating lifetime characteristics of NPL-LEDs at a luminance of 10600 cd m−2.
四、作者简介
李红博，北京理工大学材料学院教授，博士生导师，2010年中国科学院理化技术研究所获博士学位，2010年至2017年先后在意大利理工学院和美国洛斯阿拉莫斯国家实验室从事博士后研究。主要从事无机半导体纳米晶的化学合成、光谱调控、以及光电器件的制备和光伏技术应用等研究。在包括Nature Energy、 Nature Photonics、 J. Am. Chem. Soc.、Adv. Mater.、Matter等学术期刊上累计发表学术论文60余篇，引用超过4000次，6篇论文入选ESI高引用论文。任Rare Materials和《中国材料进展》青年编委，Energy Material Advances青年主编。
曾毅成，2024年6月获得北京理工大学博士学位。主要研究领域为量子点发光材料的合成及其电致发光器件的研究，目前以第一作者在Adv. Mater., Adv. Funct. Mater., Nano Lett.等期刊共发表SCI论文11篇。
五、论文信息
Improved Efficiency and Stability in Pure-Red CdSe Nanoplatelet LEDs Enabled by Gradient Alloyed CdSeS/CdZnS Crown/Shell
Yicheng Zeng, Wenke Yu, Yuan Liu, Weiwei Chen, Qingya Wang, Fan Cao, Jing Wei, Fangze Liu, Xuyong Yang, and Hongbo Li*
Advanced Materials
DOI: 10.1002/adma.202415569
