A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2024.230210 , discusses self-polarized RGB device realized by semipolar micro-LEDs and perovskite-in-polymer films for backlight applications.

Polarized light is crucial in numerous applications, including liquid crystal display (LCD) backlights, polarization multiplexing in optical communications, ultrasensitive photodetectors, and optical quantum computing. However, the conventional method of generating polarized light through an optical polarizer reduces the intensity by at least 50% because half of the light in the vertical polarization direction is completely filtered. Therefore, significant research has focused on developing light-emitting materials with intrinsically polarized emissions over the past few decades.

Light must be polarized in red, green, and blue (RGB) to be applied in applications such as backlights for LCDs and polarizer-tunable multiplexed color displays. Halide perovskite nanocrystals are promising color conversion materials due to their high photoluminescence quantum yield, high color purity, and controlled photovoltaic properties. Anisotropic perovskite nanostructures with polarized emission properties could be widely used for displays in the next-generation display devices. Furthermore, research on anisotropic perovskite structures has mostly focused on the green-emission band. In particular, monochromatic polarization-based luminescent components are impractical for scalable luminescence applications. The direct synthesis of red-emitting nanostructured anisotropic perovskites is challenging due to the severe morphological instability of NRs containing iodine and the decrease in polarization anisotropy with time. Similarly, the development of blue perovskite color-conversion layers has lagged behind. Developing devices that can emit polarized light of the three primary colors of red, green, and blue (RGB) can effectively improve the energy utilization efficiency of LCD backlight sources. Micro-light-emitting diode (micro-LED) backlight devices have high dynamic range characteristics due to their local dimming capabilities. In addition, semipolar micro-LED devices have intrinsic polarized emission characteristics compared to c-plane devices. Therefore, it is expected to realize RGB full-color polarization devices based on semipolar blue micro-LEDs as excitation light sources combined with perovskite color conversion layers with anisotropic structures.

Associate professor Tingzhu Wu’s team from the Solid-State Lighting Laboratory of Xiamen University proposed a high-performance, stable device designed for RGB polarized emission. The architecture of the device incorporates an array of blue semipolar μLEDs with intrinsic polarization emission that excite stretched composite films of green CsPbBr₃ NRs and red CsPbI₃-Cs₄PbI₆ hybrid nanocrystals (NCs), acting as the color-conversion layers. Besides the inherent polarization emission from CsPbBr₃ NRs encapsulated in the polymer film, the aligned wire (AWs) structure formed by the hybrid NCs exhibited strong anisotropic emission comparable to that of the nanorod structure due to their high dielectric constant. The green NR color-conversion layer was fabricated by mixing NRs with an EVA polymer and spin-coating the mixture onto a glass substrate to form a thin film. The NRs must have the ability to align well to produce the required polarization properties for backlight. Thus, the composite films were stretched to approximately 300% of their original freestanding lengths using a mechanical stretcher. The stretched composite film exhibited significantly polarized PL emission with a DOLP of approximately 0.44.

The susceptibility of CsPbI₃ black polycrystals to environmental degradation poses a significant challenge for obtaining stable iodine-containing perovskite NRs. Notably, the stability of γ-CsPbI₃ NCs encapsulated within a Cs₄PbI₆ crystalline matrix was markedly enhanced. Furthermore, the high dielectric constant of Cs₄PbI₆ is expected to improve the anisotropy of the emission, rendering it a viable candidate for red color conversion in LCD backplane illumination applications. CsPbI₃-Cs₄PbI₆ hybrid NCs displayed red-emission properties closely resembling those of unadulterated CsPbI₃ NCs. Under excitation with 450 nm light, the hybrid NCs achieved a remarkably high photoluminescence quantum yield (PLQY) of 95.31%. Similarly, CsPbI₃-Cs₄PbI₆ composite films were fabricated based on EVA polymer. The stability of the CsPbI₃ QDs was enhanced when they were embedded in Cs₄PbI₆. Under blue irradiation, the hybrid NC composite film exhibited phase stability for several weeks.

As CsPbI₃ was embedded within Cs₄PbI₆ in the hybrid NCs, growing them into nanorods was challenging. EVA polymer has functional groups interacting with perovskites. The interaction between the NCs and polymer chains led to the migration and linear alignment of the hybrid NCs along the EVA molecular chains during stretching, resulting in a model of the NC-AWs embedded in the polymer, and this structure also can exhibit polarized emission. The DOLP of the stretched composite films reached 0.37.

The process for fabricating the RGB polarized system is depicted in Fig. 2a, where a semipolar blue μLED array was used at the base, pumping the green NR film and red hybrid NC film sequentially in the through-plane orientation. A photograph of the device is presented in Figure 2b. The blue μLED efficiently excited the red and green color conversion layers, producing bright white light from the entire device. The emission spectrum exhibited three primary peaks located at 451, 525, and 676 nm (Fig. 2c). Due to the narrow EL spectrum, the RGB polarized devices assembled from the blue semipolar μLEDs array and perovskite color conversion layer exhibited a wide color gamut of 137.2% of the NTSC and 102.5% of Rec. 2020.

The DOLPs of the device were calculated to be 0.26, 0.48, and 0.38 for blue, green, and red, respectively (Figs. 3a, b). Compared to current research focusing on the green band, the device in this study can emit stable polarized light containing RGB trichromatic colors. This solution, which employs a blue chip to excite the red and green conversion layers, requires no filtering of the additional UV component and maximizes energy utilization when used in backlighting applications.

Currently, the industrial film bidirectional synchronous tensile equipment is capable of manufacturing large-scale stretched polymer films. The perovskite-EVA films in this work were malleable and easy to stretch with high stability. Therefore, it is capable of realizing large-scale manufacturing of perovskite quantum dot color enhanced color conversion stretched film for backlighting of LCD TVs or displays.

Article reference: Lu TW, Lin Y, Zhang TQ et al. Self-polarized RGB device realized by semipolar micro-LEDs and perovskite-in-polymer films for backlight applications. Opto-Electron Adv 7, 230210 (2024). doi: 10.29026/oea.2024.230210

Keywords: halide perovskite / light-emitting-diodes / polarized emission / nanocrystals / stability
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The Solid-State Lighting and Display Laboratory (SSLAB) in Xiamen University (XMU), founded in 2006, currently consists of 16 research fellows. Under the supervision of Academician Rong Zhang (Communist Party Secretary of Xiamen University) and Prof. Zhong Chen (Dean of School of Electronic Science and Engineering, Xiamen University), SSLAB keeps exploring new areas such as measuring methodologies and full-colour solution of micro-LEDs, visible light communications, and UVC LED disinfections. Located in the west of the Taiwan Straits, SSLAB enjoys the privilege of the collaboration with colleagues in Taiwan, such as the research team of Prof. Hao-Chung Kuo in NCTU. Moreover, SSLAB has published 3 books, more than 200 journal papers and 40 patents, and hosted many research funds granted by the NSFC, ministry of science and technology, etc.
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Opto-Electronic Advances (OEA) is a rapidly growing high-impact, open access, peer reviewed monthly SCI journal with an impact factor of 14.1 (Journal Citation Reports for IF2022). Since its launch in March 2018, OEA has been indexed in SCI, EI, DOAJ, Scopus, CA and ICI databases over the time, and expanded its Editorial Board to 30 members from 17 countries.
The journal is published by The Institute of Optics and Electronics, Chinese Academy of Sciences, aiming at providing a platform for researchers, academicians, professionals, practitioners, and students to impart and share knowledge in the form of high quality empirical and theoretical research papers covering the topics of optics, photonics and optoelectronics.