Authors: HONGLEI JI, 1,2,4 DAOCHUN YE, 3,4 HUAISHU XU, 2 ENGUO
CHEN, 3,* AND ZIYI GE1
1University of Chinese Academy of Sciences, Beijing 100049, China
2R & D Center, TCL Electronics co., Ltd., Shenzhen 518000, China
3National & Local United Engineering Laboratory of Flat Panel Display Technology, College of Physics and Information Engineering, Fuzhou University, Fuzhou 350108, China
4These authors contributed equally to this work
*ceg@fzu.edu.cn
Abstract: This paper reports an ultra-large laminated quantum dot diffuser plate (QD-DP) prepared by a multi-layer co-extrusion method for television (TV) backlights. The QD-DP has a sandwich-like structure that contains a middle QD functional layer for color conversion and top/bottom polymer layers for both encapsulation and protection. Reliability tests show that the QD-DP exhibits outstanding long-term stability under harsh conditions with continuous blue light excitation, high temperature, and high humidity. A 75-inch TV prototype is assembled by employing an ultra-large QD-DP based backlight module, which can achieve the color gamut of 118.3% NTSC1931, the brightness of 372 nits, and the uniformity of 84%. Compared with conventional QD-film based TV, the proposed TV prototype provides comparable performance with a simpler structure and lower cost. As a promising QD backlight device, this new QD device has a bright application prospect in large-sized displays.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Liquid crystal display (LCD) is the most dominating display technology in current TV market and is still facing challenges in terms of contrast ratio, color gamut, and form factor [1,2]. As one of its core components, high-performance backlight is the key to its display effect enhancement. Quantum dot (QD), as one of the new generation of color conversion materials, are widely applied in backlights due to their excellent properties, such as high quantum yield, wide absorption spectrum, narrow half-peak width, and tunable spectrum [3–9]. Further, the combination of mini-LED and QD in LCD backlights enables them high color performance and high contrast ratio to compete with other self-emitting displays, such as organic light emitting diode (OLED) and Micro- light emitting diode (µLED) [10–15].
Currently, the most commonly used QD types for TV displays are II-VI semiconductor QDs, III-V semiconductor QDs, and perovskite QDs [16–19]. These QDs need to be embedded into different positions in LCD backlights for actual applications. These can be roughly divided into two categories, one is to inserting another device containing QD materials, the other is to combine the QDs with any of the original backlight devices. QD on edge (QDoe) and QD on surface (QDos) are two typical methods that insert certain QD components into the backlight. QDoe structure, using a hybrid chamber containing QDs, is placed between the blue LED light bar and the side edge of the light guide plate (LGP), forming a single-sided or multi-sided light entrance [20–22]. However, this too long glass tube has poor mechanical stability, resulting in low color uniformity. Its industrialization process in TV products is therefore suspended. Currently, the most common and highly commercialized QD backlight application is based on QDos structure, which is a sandwich-like film structure that is generally made by encapsulating red and green QDs between two barrier films [7,23]. This QD film can be placed on top of the LGP in an edge-lit backlight, and also on top of a diffuser plate (DP) in a direct-lit mini-LED backlight. Since the red and green QDs are concentrated inside a limited space (thickness < 100 µm), the agglomeration and self-absorption are inevitable that would reduce the color conversion efficiency.
A better way to incorporate QDs in a backlight is utilizing its original components and keeping the simplest backlight structure unchanged. QD on chip (QDoc) structure is a possible solution that encapsulates the red and green QDs on the surface of the blue LED chip [24]. This is not yet mature due to the QD’s stability. QD on plate (QDop) is an alternative that integrates the QDs into the edge-lit LGP or the direct-lit DP. The QDs can be mixed in the scattering dot array on the LGP’s bottom surface by using screen printing or inkjet printing process [2,25], or even mixed with the LGP’s substrate material and fabricated by injection molding [26]. Unfortunately, the current reported QD on plate structures are not applicable for direct-lit mini-LED backlights. Meanwhile, it remains challenging to develop an ultra-large-sized fabrication process for QD based TV backlights with high uniformity and stability.
In this paper, we propose an ultra-large quantum-dot diffuser plate (QD-DP) fabricated by a multi-layer co-extrusion method for TV backlights. The QD-DP combines the functions of light mixing, homogenization, and color conversion within a relatively low optical distance. A multilayer co-extrusion method is developed to fabricate the three-layered QD-DP. An ultra-large QD-DP is designed for a 75-inch TV, and the performance is evaluated in detail.
2. Materials and method
2.1. Synthesis and granulation of QD masterbatches
At present, II-VI QDs (especially CdSe based QD) are widely used in display backlights because of their high photo luminescence quantum yield (PLQY), narrow full width at half maximum (FWHM), and excellent photo chemical stability [27]. III-V QDs (especially InP based QD) are also of interest to the markets due to their non-toxic and environmentally friendly properties. However, the optical properties of III-V QDs are still inferior to II-VI QDs. The synthesis process of III-V QDs contains complex and harsh conditions, wherein the commonly used precursor tris(trimethyl)phosphine ((TMS)3P) are expensive and hazardous [4,28–30]. In addition, high thermal stability and PLQY are especially important for the fabrication process of the QD-DP. Therefore, the CdSe based II-VI QDs are chosen as the color conversion materials for the QD-DP. The pristine CdSe QDs used in this experiment were provided by Changed New Materials Co., Ltd (Huizhou, China).
Figure 1(a) shows the synthesis and granulation process of the QD masterbatches, which are the raw material for further multi-layer co-extrusion molding. Before that, the pristine CdSe QDs with the ligands of tri-n-octylamine and oleic acid are pretreated with a SiO2 coating layer. The preparation process of SiO2-coated QDs is briefly described as follows. At first, the CdSe is dispersed in acrylic resin. Then, the organosilicon azane and a small amount of water or alcohol are added and reacted at room temperature for 12 ∼ 24 h. After separation and purification, the SiO2-coated QDs can be obtained. The ligands of tri-n-octylamine and oleic acid disappear after SiO2 coating. This SiO2 layer provides an internal barrier to ambient water and oxygen for QDs and reduces the QD’s agglomeration [31].
As shown in Fig. 1(b) and 1(c), the average particle size of raw CdSe QDs is approximately 13.28 nm, which increases to 16.55 nm after encapsulation. The SiO2-coated QDs exhibit a core-shell structure with the CdSe core encapsulated by a SiO2 shell. Before SiO2 coating, the quantum efficiency of the pristine red and green CdSe QDs is 94.01% and 95.12%, which is measured by a fluorescence spectrometer (FluoroMax+) under an excitation spectrum of 450 nm, respectively. The quantum efficiency of the SiO2-coated red and green QDs slightly decreases to 91.84% and 92.06%, respectively. It is clearly seen that the red and green QDs still have high quantum efficiency after SiO2 coating. To achieve white balance, the concentration ratio of red to green QDs used in the experiment was 1 : 3. Actually, the red and green emission intensity can be adjusted by the concentration and mass ratio between red and green QDs for reaching a balance.
In order to obtain the QD masterbatches, the treated QD solution is first evenly mixed with PMMA masterbatches. And then, QD masterbatches are synthesized through the steps of plasticization, water cooling, drying, and cutting within a twin-screw granulator filled with argon and nitrogen under the temperature of 220°C ∼ 240°C. The macroscopic properties of the QD masterbatches directly determine the performance of the QD-DP and its backlight, such as emission wavelength and FWHM.
2.2. Multi-layer co-extrusion method for QD-DP
The QD-DPs will be completely melted and mixed with PMMA masterbatch under ultra-high temperature. Here, the multi-layer QD-DP structure is introduced for stability improvement, which can be fabricated by using a self-developed multi-layer co-extrusion molding equipment. The processing schematic is shown in Fig. 2. This equipment integrates three complete extrusion molding lines with identical configuration and functionality, and the operation parameters, such as the thickness and extrusion materials, can be adjusted individually as required. The upper and lower lines are used for the PMMA diffuser layer molding, and the middle line is for fabricating QD-PMMA functional layer. In Fig. 2, different masterbatches are simultaneously added into their respective feed cavity, and then extruded through three extruders at a melting temperature. After that, the three layers are molded and extruded into a complete QD-DP by means of a roller with a micro-template at the outlet of the extruder. Red and green QDs are evenly distributed in the middle functional layer, which is responsible for color conversion. The upper and lower PMMA diffuser layers are closely packed together with the middle functional layer to isolate the ambient water and oxygen and protect the QDs, which also has the scattering function for light mixing and homogenization.
Actually, the thickness of these three layers can be adjusted separately as required. The molding size of the QD-DP is largely determined by the length of the rollers, and the QD-DP’s thickness of each layer depends on the pre-gap precisely controlled between the rollers. The upper and lower PMMA diffuser layers of the QD-DP affect the brightness and uniformity of the mini-LED backlight. The thicker the layers are, the lower the brightness of the backlight could provide, but the uniformity will be higher. In this work, each layer of the QD-DP has the same thickness of 0.6 mm. Compared with the QD film, the QDs inside a QD-DP can have enough room to disperse, and the QD agglomeration can be effectively reduced. In addition, the QD-DP’s parameters can be adjusted as needed, such as the mixing ratio of QDs and PMMA masterbatches, the thickness, and the extrusion speed. These parameters will determine the performance of the mini-LED backlight.
3. Optical performance and stability
Long-term stability of the QD device is the most troubling factor in backlights, which determines the lifetime and practicability of TV products. In this regard, the optical performance and long term stability of several prepared QD-DPs are tested under the conditions of high temperature (85°C), high humidity (85% relative humidity (RH)), and 450nm blue backlight irradiation. The changing trend of color coordinates x and y are shown in Fig. 3(a) and 3(b), respectively. Results show that the chromaticity deviation of all three QD-DP samples are lower than 0.02, and the fluctuation degree is limited within 6% after 2000 hours. Meanwhile, the brightness fluctuation can be kept within ±10%, and the changes are shown in Fig. 3(c). The result indicates that this multi-layer QD-DP architecture with upper and lower protection layers is effective in solving the key issues of temperature intolerance and the water and oxygen intrusion. Here, the prepared QD-DPs with different PMMA layer thickness were also prepared, and their brightness decay was observed under the condition of 50°C/ 90% RH and over 2000 h of blue light (450nm) irradiation. Figure 3(d) shows the effect of the thickness of the two PMMA layers on stability. The experimental results prove that the brightness decay of the QD-DP sample decreases with the increase of the PMMA layers. By adjusting the thickness of the two PMMA layers, the stability and reliability of the QD-DP can be significantly improved.
The fundamental structure of the QD-DP based TV prototype is shown in Fig. 4(a). It can be divided into two parts, namely the LCD module and the QD-DP backlight. As shown in Fig. 4(b), the QD-DP backlight includes a prism film, a QD-DP, and a mini-LED source array. COB (chip on board) mini-LEDs are used here with the chip size of 228 µm × 331 µm. The pixel pitches between the adjacent mini-LEDs are 13.75 mm in horizontal line and 14.5 mm in vertical line, respectively. The total number of mini-LEDs in the backlight circuit is 7680. All mini-LEDs are integrated on ten circuit boards and divided into 1920 local dimming zones. Each zone contains 4 mini-LEDs. Blue light emitted from the mini-LED array excites the red and green QDs inside the QD-DP, and then mixed together for white light emission. The white light will illuminate the LCD module and finally output a color image. In Fig. 4(c), the backlight illumination picture shows that it exhibits a white balance color point of (0.291, 0.271) in CIE1931 with a color temperature of around 10,000 K. And the front view and side view of the QD-DP shown in Fig. 4(d) are captured under a microscope (BX53M, OLYMPUS) with 5 times magnification. It can be seen from the front view that a rough surface is observed on the outer PMMA layer. It can homogenize the outgoing light emitting from the QD-DP. From the side view, it can be observed clearly that the QD-DP is composed of a three-layered sandwich-like structure, which is consistent with our design.
4. Backlight modules and TV prototypes
In QD-DP based mini-LED backlights, the blue light emitted by the mini-LEDs is down-converted to uniform white light after passing through the QD-DP. That means that the proposed QD-DP combines the QD film and conventional diffuser plates into one component, and the water and oxygen barrier films of the traditional QD film can be eliminated. Herein, the functions of light mixing and color conversion are effectively combined by this single QD-DP, while the prism films and the mini-LED source array are kept the same as traditional backlights. From the market perspective, the proposed QD-DP is estimated to be only $ 10 per square meter. It is beneficial for the QD technology to permeate into middle and low-end TVs and reduce the cost of the final TV product.
For experimental verification and comparison, the QD raw materials are kept the same for preparing the QD-DP and the QD-film sample here. The FWHM of red and green QDs is 25 nm and 21 nm, respectively. The QD-DP has a lower QD concentration compared with QD-film.In our experiment, the QD concentration of the QD-DP is measured to be 0.025%wt, which is lower than 0.05%wt in the QD-film. These data are calculated by the ratio of the weight of the QD materials and the weight of the QD-DP or QD-film. In a QD-DP, the QDs are uniformly dispersed in the middle functional layer of QD-DP. Low concentration may avoid the issues of agglomeration and fluorescence quenching coming from high QD concentration.
The 75-inch TV prototypes based on the ultra-large QD-DP and QD-film backlight are assembled and compared, respectively. As can be seen from the pictures in Fig. 5(a) and 5(b), a simpler backlight architecture can be achieved using a QD-DP, and a same mechanical structure can be perfectly compatible with these two QD devices. This is a good news for TV manufacturers, because the original mechanical structure designed for QD film can be directly used for QD-DP without additional costs.
The overall optical performance comparison is concluded in Table 1, and their display effect and corresponding spectra are shown in Fig. 5(a) and 5(b), respectively. In order to further demonstrate the display effect, the spectra of blue, red, and green displayed from two prototype are shown in Fig. 6, respectively. As can be observed from the figures, the emission spectra of two TV prototypes are almost the same with different QD backlight architectures used. While displaying the same full-color video, the brightness of the QD-DP and the QD-film based TV are respectively 372 nits and 380 nits, which is hardly distinguished from each other by human eyes.
The light diffusion of the upper and lower PMMA layers makes its luminous efficiency slightly lower than QD-film, but this can be compensated by the reflective film. The brightness of QD-DP backlight can be further improved by replacing the PMMA masterbatches with PS masterbatches. The luminance uniformity is tested by the nine point method according to international standard (IEC 62595-2-1) [32]. Measured results show that the QD-DP-based TV has a luminance uniformity of 84%, much higher than the 73.6% of the QD-film backlight-based TV. This is mainly attributed to the stronger scattering property of the upper and lower PMMA layers on a QD-DP than the barrier films on a QD-film. As the color gamut triangles plotted in Fig. 5(c), the QD-DP based TV has a color gamut of 118.3% and a chromatic coordinate of (0.279, 0.281) under the NTSC1931 standard. The QD-DP based TV can provide a satisfactory display effect while having a simpler structure, and a simplified fabrication process. Furthermore, The chromatic aberration is calculated between the QD-DP and QD-film based TVs, which is lower than 0.01 according to the chromatic aberration formula [33]. The above experimental results demonstrate the feasibility of the proposed ultra-large QD-DP backlight in actual TV applications.
5. Conclusion
In this paper, an ultra-large QD-DP is presented and fabricated by using a self-developed multi-layer co-extrusion method. This QD-DP has a typical sandwich-like structure including the upper and lower PMMA diffuser layers and the QD functional layer in between. This QD-DP
can successfully combine the traditional QD-film and a conventional diffuser plate into one component, and the barrier films in a QD-film can be replaced by the low-cost PMMA layers.The QD-DP’s stability is positively correlated with the thickness of its upper and lower PMMA layers. The brightness fluctuation of the QD-DP is kept lower than 10% and the chromaticity deviation is within 0.02 under high temperature/humidity (85℃, 85% RH) and continuous blue backlight irradiation for more than 2000 hours. It can meet the stability requirement of an actual TV product. An ultra-large TV prototype is assembled with a 75-inch QD-DP, which can achieve wide color gamut of 118.3% NTSC1931, high brightness of 372 nits, and high uniformity of 84%. At the same time, its optical performance is comparable to the current QD-film based TV prototype. More importantly, the proposed QD-DP provides a simpler structure, a simplified fabrication process, and lower cost with high performance to mainstream QD backlight, which is approaching mass production in the near future. It is expected that the QD’s stability may be further enhanced to further simplify the QD-DP structure and expand the application scenarios.
Funding. National Natural Science Foundation of China (No. 62175032); Natural Science Foundation of Fujian Province (No. 2021J01579).
Acknowledgments. The authors would like to extend their sincere gratitude to the colleagues of Huizhou Changed New Materials Co., Ltd, (Guangdong, China) for their assistance on this thesis.
Disclosures. The authors declare no conflicts of interest.
Data availability. Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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Multi-layer co-extruded quantum-dot diffuserplate for ultra-large TV backlights
