Highly stable full-color display device with VLC application potential using semipolar μLEDs and all-inorganic encapsulated perovskite nanocrystal
Fig. 1 Highly stable full-color display device with VLC application potential using semipolar μLED and inorganic encapsulated perovskite nanocrystals
Fig. 2 SiO2 coating improves the stability of PNC under environmental conditions and light radiation conditions.
Fig. 3 SiO2 Frequency response of PNC-μLED.
Fig. 4 Color gamut and color shift of the PNC-μLED under different current densities.
The techniques of high-speed communication and high-resolution display kit are both essential to modern life, as they constitute the foundations of those attractive concepts: Internet of Things, virtual/augmented reality, and metverse. If they are expected to be integrated, being realized simultaneously in one single device in the form of visible light communication (VLC) and micro-LED (μLED) array, it will be thrilling that this new technique would combine the advantages of both, such as avoiding occupying the limited electromagnetic sources, confidentiality, energy saving, and large color gamut, etc.
So far, to combine communication and display remains in a concept, without any primitive prototype yet been invented. There are some drawbacks that hinder the realizations. First, the wavelength shift of μLED chips upon driving current. Second, the μLED display can only produce one color. If extended to the three primary colors that needed for display, it requires either massive transferring or incorporating with color conversion layers.
In current stage, by using fluorescent color conversion layers is a reliable way to realize full-colorization, due to the quick development of quantum dots (QDs). QDs are naturally adapted to the color conversion layer of display technique. The narrow width of emission spectra and low fluorescent lifetime indicate the potentials of large color gamut and high refresh rate, respectively.
Perovskite nanocrystal (PNCs), compared to the II-IV QDs, are superior in defect tolerance, color purity and cost reduction and can be an update to traditional QDs. However, The PNC is prone to the erosion in the ambient environment, in which the oxygen and water steam turn it into non-emissive phases. As consequence, the PNC requires robust shells as protection.
In this article, the Prof. Zhong Chen's group in Xiamen University and Prof Hao-Chung Kuo's group in National Yang Ming Chiao Tung University reports a promising approach for the development of effective full-color displays are to combine blue μLEDs with color conversion layers, as shown in Fig. 1. The stability of PNCs in ambient conditions and under exposure to blue light can be improved using a SiO2 coating, as shown in Fig. 2. Related results were published in Photonics Research Vol. 9, Issue 11, 2021 (Tingzhu Wu, Yue Lin, Yu-Ming Huang, et al. [J]. Photonics Research, 2021, 9(11): 11002132).
This study proposes a device that could be used for both display and VLC applications. The semipolar blue μLED array fabricated in this study shows a negligible wavelength shift, indicating a significant reduction in the quantum confined Stark effect. Owing to its shorter carrier lifetime, the semipolar μLED array exhibits an impressive peak 3 dB bandwidth of 655 MHz and a data transmission rate of 1.2 Gb/s corresponding to an injection current of 200 mA, as shown in Fig. 3.
The PNC–μLED device assembled from a semipolar μLED array with PNCs demonstrates high color stability and wide color-gamut features, achieving 127.23% and 95.00% of the National Television Standards Committee standard and Rec. 2020 on the CIE 1931 color diagram, respectively. These results suggest that the proposed PNC–μLED device is suitable for both display-related and VLC applications, as shown in Fig. 4.
"This work set a milestone for the application of μLED display with VLC", Prof. Chen comments, "In the future, electric devices tend to be highly integrated, as we have already witnessed so far. μLEDs should also follow this trend of being multi-functional, and this work revealed such possibility. Also, it has proven the competence of PNCs in the application end, as long one substantially enhances its reliability."
Currently, Zhong Chen's group and Hao-Chung Kuo's group are working on aspects for further improvement. Investigation on the full duplex VLC system with μLEDs; Exploration of the multiple communication channels on the red and green pixels.
具有可见光通信潜力的全彩化显示器件
图1、一种使用半极性μLED和全无机封装钙钛矿纳米晶体的具有可见光通信应用潜力的高稳定全彩显示设备
图2、SiO2包覆提高了PNC在自然环境条件下和光辐射条件下的稳定性
图3、PNC-μLED的频率响应
图4、不同电流密度下PNC-μLED色域图及色坐标偏移示意图
高速通信与新型显示是构成物联网、AR/VR以及元宇宙等信息系统的重要技术。μLED不仅被誉为下一代新型显示技术，同时也因为能够提供传输高速率数据时所需的超快速闪烁而被用于高速可见光通信。如果μLED器件能够在传输通信信号的同时显示高清视频，那么这项兼顾显示与通信的新技术将是对现有技术的一大改进。
目前，国际上对同时兼顾全彩显示和可见光通信应用的μLED器件的研究较少，要实现该器件还需要应对不少挑战。如μLED芯片在高驱动电流下的发光波长偏移等。目前利用荧光材料制备色转换层是实现全彩化的一个有效途径。与传统量子点相比，钙钛矿纳米晶体(PNC)在缺陷的耐受性、成本、色纯度方面更胜一筹，但PNC在自然环境下稳定性较差，因此还需要进行封装保护以提高使用寿命。
近日，厦门大学陈忠教授课题组和台湾阳明交通大学郭浩中教授课题组合作制备了一种结合半极性蓝色μLED阵列与全无机封装的PNC颜色转换层的全彩显示器件（图1）。相关结果发表于Photonics Research 2021年第11期 (Tingzhu Wu, Yue Lin, Yu-Ming Huang, et al. Highly stable full-color display device with VLC application potential using semipolar μLEDs and all-inorganic encapsulated perovskite nanocrystal[J]. Photonics Research, 2021, 9(11): 11002132)。
使用SiO2包覆可以提高PNC在自然环境和蓝光照射环境下的稳定性（图2）。PNC-μLED器件除了可用于显示之外，还能够实现高速可见光通信。该团队制备的半极性蓝色μLED阵列在不同驱动电流下的色坐标漂移可以忽略不计，表明量子限制的斯塔克效应显著降低。同时由于其较短的载流子寿命，半极性μLED阵列在200 mA的注入电流下具有655 MHz峰值和3 dB带宽，并实现了1.2 Gb/s的高速数据传输速率（图3）。
同时PNC-μLED器件还具有高色彩稳定性和宽色域特性，达到了127.23%的NTSC和95.00%的Rec.2020色域（图4）。这些结果表明，该研究团队所提出的PNC-μLED器件适用于显示和可见光通信（VLC）应用。
“这项工作为结合VLC应用的μLED显示树立了一个里程碑”，陈忠教授表示，“未来，电子设备趋向于高度集成，μLED也应该顺应这种多功能化的趋势，而这项工作揭示了这种可能性。此外，它已经证明了PNC如果能够提升其稳定性，那么其在应用端将具有很大的发展潜力。”
目前，陈忠教授课题组和郭浩中教授课题组正在做进一步改进的工作。包括μLED全双工VLC系统研究，以及对于红绿蓝多通道通信的探索等。