Authors: Naijun Chen†, Zelong Bai‡, Zengmin Wang₸, Honglei Ji†, Ruikuo Liu§, Chaowei Cao§, Hui Wang₸, Feng Jiang‡, and Haizheng Zhong‡*
† TCL Multimedia Technology Holdings Ltd., Shenzhen, China
‡ Beijing Institute of Technology, Beijing, China
₸ Hefei Lucky Science & Technology Industry Co., Ltd., Hefei, China
§ Zhijing Nanotech Co., Ltd., Beijing, China
Abstract
Perovskite quantum dots film exhibits low cost, low heavy metal content, high brightness and narrow emission peak. Here, we introduced features of Perovskite quantum dots film and its beneficial effects in LCD TVs applications. Afterwards, we thor- oughly compared the Perovskite quantum dots film and CdSe based quantum dots enhancement film.
Author Keywords
quantum dots; perovskite; liquid crystal display; wide color gamut; QDEF; PQDF
Quantum dots (QDs) are emerging as the most promising luminescent materials to achieve wide color gamut (WCG) and high brightness liquid crystal display (LCD) technologies. Compared with traditional rare earth (RE) phosphor, quantum dots (QDs) exhibit narrow emission spectra and high photoluminescent quantum yields (PLQYs), which can significantly enhance the color gamut of LCD monitors from 70% NTSC up to 110%
NTSC (figure 1a) [1,2]. Currently, the most commonly used QDs backlighting units (BLU) configuration is “on-surface”structure. As shown in figure 1b, the quantum dots enhancement film (QDEF) is placed between the light guide plate (LGP) and brightness enhancement film (BEF) and converts blue light into WCG white light [3].
So far, cadmium selenide (CdSe) and indium phosphide (InP) QDs with narrow emission spectra are commonly used in most of commercialized QDEF products (e.g., QDEFTM by Nanosys and 3M) to achieve WCG white light. Unfortunately, the complicated manufacturing process of CdSe and InP QDs and QDEF lead to an extremely high price and low brightness of products.
Meanwhile, the high heavy metal cadmium content doesn’t meet European Restriction of Hazardous Substances (EU RoHS) requirement. Therefore, a low cost and RoHS-compliant QDs film product is critical to QDs LCD technology. Recently, perovskite QDs emerged as low-cost alternative materials for efficient and WCG display technology [4-6].
2. Latest Results
Here, we highlight the up-to-date perovskite quantum dots film (PQDF) with low cost, high brightness and low heavy metal content. This technology originated from Beijing Institute of Technology (BIT) and jointly developed by Zhijing Nanotech Co., Ltd. and Hefei Lucky Co., Ltd.. Furthermore, a LCD TV prototype using PQDF based BLU was achieved by TCL. This demo features WCG (101% NTSC) and high brightness (up to 500 nits).
perovskite quantum dots film (PQDF): In 2016, Zhong et al. reported the “in situ fabrication of halide perovskite nanocrystal-embedded polymer composite films” strategy [7]. This strategy greatly simplifies the fabrication of QDs/polymer composite film and significantly improves the dispersibility of QDs in polymer. As shown in figure 2b, the transmittance of PQDF is about 90% for the free-standing film. Moreover, perovskite QDs exhibit the same or narrower emission spectra with a FWHM of 25 nm and high PLQYs up to 95%. Thus, PQDF can be easily combined with other optical films (e.g. diffusion film, BEF) and achieve high-efficient light conversion due to its high transparency and simple coating processability. Recently, by optimizing formulations and processes, the stability of PQDF is approaching to the standard applied in LCD TVs.
PQDF-LCD TV Prototype: The WCG PQDF-LCD TV prototype was achieved by combining the blue light emitting diodes (LED) chip, red K2SiF6:Mn4+ (KSF) phosphor and green PQDF as RGB light sources. The structure diagram of PQDF-LCD TV prototype is shown in figure 3a, containing with a direct-lit BLU and LCD panel. The blue light emitted from LED chip is partly converted into red and green light by red KSF phosphor and green PQDF, respectively. The spectrum of white light is shown in figure 3b. The main color and brightness parameters of PQDF-LCD TV prototype are listed as follow (figure 3c):
1. Color gamut: 101% NTSC
2. Brightness: 500 nits
3. White point in CIE 1931: (0.301, 0.317)
4. Size: 55”
3. Impact
Compared with CdSe QDs based QDEF, PQDF has significant advantages in terms of production costs, brightness and heavy metal content. The comparison of BLU structures and film structures of the two techniques illustrated in figure 4. The pros and cons of QDEF and PQDF are compared as follows:
1. The cadmium element in CdSe QDs is hazard to human and environment. The cadmium content in QDEF is over 100 ppm, which exceeds the limit of RoHS. While, the inductively coupled plasma mass spectrometry (ICP-MS) result shows that PQDF is Cd-free and Pb content is below 40 ppm (EU RoHS standard is 1000 ppm for Pb element).
2. In the standard process of fabricating QDEF, CdSe QDs are pre-synthesized, thoroughly purified and encapsulated in polymer pellets [8]. These steps are complicated and low-yield, which significantly increased the production costs of QDEF. In contrast, the fabrication of PQDF adopts “in situ fabrication” strategy. High-performance perovskite QDs/polymer composite films can be obtained by simple mixing of raw materials and a coating process, which resulting in a substantial decline in manufacturing cost and increase in yield rate. Thus, the prices of PQDF will be very competitive.
3. As shown in figure 4, green and red CdSe QDs are respectively encapsulated in polymer pellets and then mixed in QDEF. These configurations caused serious scattering loss and self-absorption loss in QDEF, which reduce the light efficiency of the monitor. In PQDF, perovskite QDs undergo in-situ nucleation and growth in polymer matrix which can effectively avoid agglomeration and scattering loss. Meanwhile, the self-absorption loss is also avoided because of the high transmittance at 631 nm (emission peak of KSF phosphor) of PQDF. Thus, coupled with the high PLQYs, PQDF based BLU may have a higher brightness than QDEF based BLU.
4. Summary
PQDF exhibit low cost, RoHS-compliant and high brightness, which make it a complement technology for WCG QD-LCD technology. In addition, the perovskite QDs/polymer composite layer is extremely thin in PQDF (< 10 μm w/o a barrier layer). Therefore, PQDF is expected to partially avoid the edge failure issues and adapt to the application of small-size LCD screens. In 2018, we plan to unveil the PQDF products that is available for LCD manufacturers.
5. Acknowledgements
This work is financially supported by the National Key Research and Development Program of China (No. 2017YFB0404603).
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Symp Digest of Tech Papers – 2018 – Chen – P‐119 Low Cost Perovskite Quantum Dots Film Based Wide Color Gamut Backlight