Authors: Honglei Ji1,2 *, Huaishu Xu2 , Feng Jiang3 , Zelong Bai3 , Haizheng Zhong3
1. University of Chinese Academy of Sciences, Zhejiang, Ningbo,China
2. TCL Electronics Holdings Limited., Shenzhen, China
3. Beijing Institute of Technology, Beijing, China
* Corresponding author, jihl@tcl.com
Abstract
This paper reports a novel backlight system for liquid crystal display by combining blue and red emissive LEDs and perovskite quantum dots embedded composite films as color converters. The backlights can realize 95% BT.2020 color gamut with a brightness of 90% relative to the backlight using blue LEDs and YAG. In comparison to the backlights using blue LEDs, perovskite quantum dots and K2SiF6: Mn4+ phosphors, this backlight system suppresses the tailing problem for dynamic dimming from red emissive K2SiF6: Mn4+ phosphors.
Author keywords
perovskite quantum dots; dual LEDs; hybrid backlight; color gamut; delayed response time
Liquid crystal display (LCD) has undisputedly become the dominant flat panel display technology, [1] which has been widely used in the smartphones, tablets, computers and large-screen elevisions (TVs). Prior shortcomings-including limited viewing angle, poor contrast ratio, high power consumption and slow response time-have been addressed and are now no longer major oncerns. Currently, the image contrast ratio and color gamut are two unresolved technical problems in LCD backlights. Adaptive dynamic dimming control and quantum dots （QDs）display have been proposed to address these challenges[2,3].
QD based TVs with ultra-wide color reproduction have been widely reported since 2013 [4]. The use of QD enhanced films (QDEFs) is a state-of-the-art backlight for QD based TVs. Typically, QDs were encapsulated in the polymeric matrix (Figure 1a). Although CdSe QDs exhibit high backlight system efficiency（80%）and wide color gamut (90% BT.2020), heavy metal element (Cd) limited the potential integration into TV products. On the other hand, InP QDs are environmentally friendly material without any RoHS-restricted heavy metal elements. However, the system efficiency is relatively lower, and the color gamut performance（~80% BT.2020）is inferior to CdSe QDs. Perovskite QDs are emerging as low-cost alternative for display technology with high system efficiency and wide color gamut performance. Very recently, perovskite QDs have been successfully applied to display systems and a color gamut of 95%BT.2020 was achieved [5]. In table 1, we summarize the performance of these three typical QD materials. it is noted that perovskite QDs based composite films only contain a Pb content of 50 ppm. This is much lower than the required value of RoHS (1000 ppm).
In our previous work, we have reported the combination of perovskite QDs based composite films and K2SiF6: Mn4+ (KSF) phosphor for LCD backlight [5]. Although the display backlights show wide color gamut and high brightness, the use of KSF bring the problem of motion tail, which needs to be solved by redesigning the backlight driver. Here, we further replaced the KSF phosphor with red LEDs to solve the tailing problem.
2. Hybrid backlight system based on the dual LEDs and perovskite QDs
In a commercial QDs display solutions, such as CdSe and InP, the red and green QDs were mixed and encapsulated within the sandwich barrier films to obtain the QDEFs [6], and then the QDEFs were integrated to a blue LED based backlight. The blue light emitted by the LEDs excites the red and green QDs in the QDEFs to achieve a white backlight with wide color gamut (Fig. 1a). When the QDEFs with InP material are used, the color gamut is around 80% BT.2020. When the CdSe QDs applied, the color gamut is about 90% BT.2020 (Table 2).
The above backlight structure does not apply to the perovskite QDs due to the poor stability of red emissive perovskite QDs. And the red and green perovskite QDs are not easily formed simultaneously in the polymer matrix through a special in-situ fabrication process [7,8]. Accordingly, the green emissive perovskite QDs were integrated to the backlight by adopting a novel solution, as shown in Figure 1b, the KSF with narrow emission was encapsulated in the blue LEDs to realize the simultaneous blue and red emission. The green emissive perovskite QDs were encapsulated into a film by using in-situ fabrication process, part of the blue light emited by the phosphor-converted LEDs excites the perovskite QDs to generate green light [9]. The white backlight with specific color temperature can be obtained by adjusting the film thickness, QDs concentration and the proportion of KSF phosphor in blue LEDs. The color gamut of perovskite QDs based backlight by using blue LEDs and KSF phosphor reached 95% of BT.2020.Compared with the CdSe and InP QDs, perovskite QDs exhibit higher system efficiency, however the introduction of KSF phosphor brings new problem about response time.
In this paper, the red emissive KSF phosphor was replaced by the red LEDs. In this way, the white balance of the backlight is much easier to adjust. The problem of response time is also solved. As shown in Figure 1c, the dual LEDs (blue LEDs and red LEDs) were orderly arranged in the backlight structure. By using the dual LEDs, combined with the perovskite QD film, the color gamut is up to 95% BT.2020（Table 1）. The emission of green perovskite QDs used in the hybrid backlight is very narrow, which is comparable to the semiconductor LEDs, the full width at half maximum（FWHM）is generally between 15nm and 20 nm. It is concluded that the performance of the hybrid backlight system in terms of luminous efficiency and color gamut can meet the requirements of high-grade LCD applications.
In the above three backlight structure, the wavelength of the blue LEDs is generally selected from 445 nm to 455 nm, the dominant wavelength of the green QDs is generally selected from 520 nm to 535 nm, and the dominant wavelength of the red QDs or LEDs is generally selected from 620 nm to 645 nm.
3. LCDs applications based on the hybrid backlight system
The hybrid backlight is transmitted through the color filter array (CFA) in the LCD panel, which determines the size of the color gamut and therefore the range of colors the display can render. As shown in Figure 2, the EL spectra of hybrid backlight can be well matched to the transmittance spectra of color filter. Furthermore, the emission of dual LEDs and perovskite QDs used in the hybrid backlight system is continuously adjustable, so the color purity of RGB three primary colors can be flexibly and slightly adjusted according to the CFA in the LCD panel. In principle, the BT.2020 color gamut of LCD devices can reach 100%.
The dual LEDs in the hybrid backlight only involves one energy conversion process（electricity to light）, therefore, the red LEDs used in the hybrid backlight have higher system efficiency than that of the red QDs or phosphor (KSF), which related to two energy conversion process: electricity to light and blue light to red light. The photoluminescence quantum yield（PLQYs）of green emissive perovskite QDs used in the hybrid backlight is up to 95%. Therefore, most of the blue light absorbed from the blue LEDs can be converted to red light. So, the backlight system efficiency is higher than other QD based LCD devices.
As shown in Table 3, the delayed response time of the KSF phosphor（Tf~10ms）is three orders of magnitude higher compared to the LEDs （ Tf~30μs ） and perovskite QDs（Tf~50μs）. The existence of KSF phosphor would lead to the motion tail problem in dynamic dimming LCD display, which is not easy to solve. In this paper, the role of KSF phosphor was replaced by red LEDs, therefore the delayed response time was adjusted to the same order of magnitude. In this perovskite QD based display prototype, the two remaining technical fields in LCD displays, image contrast ratio and color gamut, can be well solved by combining the dynamic dimming control.
4. Opportunities and challenges
Perovskite QDs have attracted more and more attention in the display fields due to their excellent luminescent properties including the high PLQYs and narrow emission. At present, the main reason for limiting the commercial application of perovskite QDs is the poor stability under high humidity and heat environment. In order to overcome this reliability problem, many QDs manufacturers and research institutes have invested in researches to improve the stability of perovskite QDs through different solutions, such as surface modification or QDs self-wrapping. Further, the PLQY of red emissive perovskite QDs needs to be improved.
Besides, the hybrid backlight system based on dual LEDs and perovskite QDs requires a power supply to simultaneously control two LEDs with different emission. In this configuration, it is necessary to consider that the voltage drivers and the long-period light decay curves of two kinds of LEDs are different, which require the driving and feedback control capabilities.
5. Summary
The hybrid backlight system with dual LEDs and green emissive perovskite QDs was applied in LCD, the color gamut was up to 95% BT.2020. Due to the delayed response time of the dual LEDs（Tf~30μs）is approximately equal to the perovskite QDs（Tf~50μs）, this novel backlight system will not have the motion tailing problem, when it is applied to the dynamic dimming display device with high image contrast ratio. This prototype will further facilitate the commercialization of perovskite QDs.
References
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Symp Digest of Tech Papers – 2019 – Ji – 37 5 Hybrid Backlight System based on Blue Red LEDs and Perovskite Quantum Dots
