Open Access
Add to BibSonomy
Abstract
We present a backlight module (BLM) employing a photoluminescent quantum-dot microstructure array for flexible/curved liquid crystal displays (LCDs). Differently sized quantum-dot (QD) BLMs were prepared based on the theoretical spectral model and microstructure fabrication process. A 27-inch curved prototype showed a wide color gamut of 122.79% under the National Television Systems Committee standard while achieving high brightness of over 4000 cd/m2 and brightness/color uniformity of 85.21%/9.2 × 10−3. An LCD monitor prototype equipped with the proposed BLM was also assembled and tested, which showed higher visual performance when compared with a common commercial monitor. This method produces QD BLMs without the need of additional optical elements, and has good compatibility with traditional processes.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Liquid crystal displays (LCDs) dominate the display market because of their various advantages such as high resolution, low power consumption, long lifetime, and low cost. Owing to the significant developments in manufacturing technology, it has gradually become the most suitable structure for mass production of both large-sized displays and next-generation 4K- or 8K-resolution displays [1]. Although the LCD technology is more mature, its display principle is different from that of other displays. With the characteristic of passive light emission, LCDs need a backlight module (BLM) to serve as a plane light source, which affects the optical efficiency, brightness uniformity, color gamut, dynamic range, and viewing angle of the LCDs [2–5]. Currently, slow response time and low color gamut are two bottlenecks in LCD technology [6]; the former can be solved effectively by using polymer-stabilized blue-phase liquid crystals [7,8], whereas the latter depends heavily on the improvement of the BLM. The white light emitting diode (LED) chips are usually applied as the source of traditional BLMs, which use InGaN blue LED chip to excite YAG:Ce yellow phosphor [9]. YAG:Ce yellow phosphor emits a broad fluorescent spectral peak that cannot separate the red and green emission peaks. This is the reason why it is difficult to produce saturated red and green lights, and the color gamut can only reach 75% ~80% under the NTSC standard [10]. YAG:Ce yellow phosphor is replaced by two common methods to improve the color purity of the BLM. One method uses independent red and green fluorescent materials (commonly CaAlSiN3:Eu for red and β-Sialon:Eu for green) [11], and the other method applies red (AlInGaP), green (InGaN), and blue (InGaN) LEDs [12]. Higher spatial color separation implies better spectral characteristic and narrower full width at half maximum (FWHM). However, these methods have similar limitations that their color gamut expansion reduces the light efficiency or complicates the BLM [13]. Achieving a better balance between the optical performance and BLM structure has become an urgent need for further development of LCD technology. With this background, the use of quantum dots (QDs) for LCDs is gradually attracting attention [14–16]. They can provide a wide color gamut with high quantum efficiency, narrow full width of emission spectrum, adjustable emission band, and high color purity [17–19].
Recent advances in QD-enhanced LCDs all demonstrate that “the prime time for QD-enhanced LCDs is around the corner” [20]. For the aspect of device configuration, QD electroluminescence (EL) and photoluminescence (PL) are two different luminous modes that improve the LCD [21,22]. Quantum-dot light emitting diode (QLED) is a typical EL mode that includes a QD emission layer to realize electro-optic conversion. Its current external quantum efficiency is only around 10%, which limits its industrial use [23]. By contrast, the PL mode has demonstrated the feasibility of its application in LCD devices with both high brightness and wide color gamut [24,25]. For example, the quantum-dot color filter (QDCF) is used to replace the traditional filter in LCDs [26]. The crosstalk of the blue light is a major problem in QDCF, which can be eliminated by an additional distributed Bragg reflector, or suppressed by a backlight configuration with a functional reflective polarizer and a patterned half-wave plate [27,28]. A PL based QD backlight may be the most suitable candidate for next-generation LCD devices. Presently, QD BLM can be applied using three existing solutions, all of which mix QDs with polymer resin. First, QD is mixed with silica gel and encapsulated on a blue LED chip, which is called the on-chip solution [29–32]. The function of this QD is similar to that of yellow phosphor mentioned above. As the working temperature of the LED chip is over 150 °C, the luminous efficiency and working life are greatly reduced under such high temperature and intense light stimulation [33,34]. Second, a QD tube is inserted between the LED light bar and edge-lit light guide plate (LGP), which is called the on-edge solution [35]. This long and thin cylindrical glass tube is filled with red/green QD mixture, so that white light will be mixed before entering the LGP. However, its optical processing is highly demanding because each component should be strictly aligned and the glass tube may be easily fragmented. The third solution, called the on-surface solution, is the most widely industrialized, in which a thin QD film containing the QDs and scattering material is applied on the LGP exit surface [36–38]. This “barrier–QD layer–barrier” structure consumes the most QD materials, and the light diffusion will be affected by the laminated structure [39,40]. The summary of the above methods shows that QD BLMs still require an additional component to achieve higher performance. There still exists the fundamental issue of achieving the balance between performance and structure. Moreover, flexible/curved LCD displays compatible with the QDs have not been reported so far.
In this paper, we present a novel flexible/curved BLM employing quantum-dots microstructure array (QMA) for LCD displays. QD microstructures, replacing the traditional net dots, are designed and arranged discretely in arrays on the bottom surface of the LGP. The white-balance QD spectral model is theoretically established, and the experimental fabrication process is studied in detail. The feasibility of this method for flexible/curved LCD displays is also demonstrated by means of differently sized experimental prototypes. The rest of this paper is organized as follows. In Sec. 2, the brief introduction of the proposed QD BLM is presented. The PL spectral model and the white-balance achievement are discussed in Sec. 3 and 4, which provide a preliminary guidance for the prototype preparation. The verification of the principle of QD BLM is performed in Sec. 5, and the feasibility of the flexible/curved QD BLM is demonstrated in Sec. 6. Finally, the conclusions are drawn in Sec. 7.
2. Brief introduction of the BLM with QMA
As shown in Fig. 1(a), the proposed LCD display consists of a blue LED light bar, an LGP with QMA, optical films, and an LC cell, in which the LGP is one of the most important functional components. This BLM structure is different from other existing types of QD backlight; it has no additional component compared with traditional BLMs. However, the QMA is located discretely and arranged in arrays on the bottom surface of the LGP. Each QD microstructure unit contains the mixture of red/green QDs and light scattering particles.
Fig. 1 (a) Structure diagram of the LCD backlight with QMA; (b) optical path in the proposed LGP; (c) partial magnification diagram of the optical path within a single QD microstructure unit.
The incident rays, which are emitted from the blue LED light bar, enter the LGP and propagate inside it according to the law of total reflection, as shown in Fig. 1(b). When the rays hit the QD microstructure unit, they cause light scattering and color mixing. Part of the emergent rays will directly transmit through the upper surface of the LGP, whereas another part will be recycled and reflected by the reflector film. The discrete arrangement of the QMA can be designed to minimize the luminance difference between the different areas on the emergent surface of the LGP. Other optical films, such as orthogonal brightness enhancement film (BEF) or dual brightness enhance film (DBEF), are capable of both converging the rays and homogenizing the light distribution. Colorful output images can be displayed through the LCD module illuminated by the QD BLM.
As shown in Fig. 1(c), the red/green QDs and light scattering particles are homogeneously dispersed into each microstructure unit. The emergent white-balance light can be achieved by mixing the light of red/green QDs stimulated by the blue LED light bar. Therefore, the key technical challenge is the appropriate proportion of red/green QDs and blue LEDs.
3. White-balance spectral model for QD BLM
The QD spectrum achieving white balance always has three clearly separate peaks for the three primary colors [41]. The target spectrum from a blue LED exciting red/green CdSe QDs has been demonstrated to have a well matching by Gaussian fitting [20]. For constructing an effective white-balance spectral model, a series of Gaussian characteristic parameters, including the peak intensity, peak position, and FWHM, are defined to feature the essential spectral information. The white-balance spectral function model φ(λ) of the QD BLM can be determined by the following formula:
where α, β, and θ are the peak intensity, the peak position, and the FWHM, respectively. The character R/G/B represents red/green/blue color emitted from the QD BLM. In Eq. (1), the peak position β and the FWHM θ of the QD materials and blue LEDs should be considered as known parameters during the preparation process. Once αB is pre-defined, φ(λ) will be determined by αR and αG.Considering the CIE 1931 XYZ chromaticity system, the color coordinates x and y can be calculated from the spectral function model of the QD BLM and the tristimulus values, which can be expressed as:
In Eqs. (2) and (3), it is obvious that the color coordinates x and y are only related to the spectral function model φ(λ). Equation (1) shows that φ(λ) is only related to the peak intensities αR, αG, and αB, so that the color coordinates of the QD backlight can also be calculated from the peak intensities αR, αG, and αB. If the peak intensity of the blue LEDs is pre-defined, Eqs. (2) and (3) will become a set of two-variable linear equations with only two variables, αR and αG. A unique solution for αR and αG can be numerically calculated, and then the spectral function model φ(λ) can be determined. Therefore, the color coordinates (x, y) and peak intensity values (αR, αG) satisfy a one-to-one mapping relationship.
It is assumed that the peak positions of βR and βG are set as 628 nm and 519 nm, and the FWHMs of θR and θG are 30 nm, respectively. The peak positions of the blue light LEDs, βB, and FWHM θB are set as 450 nm and 20 nm, respectively. The target color coordinates of the QD BLM are defined as (0.30, 0.29), and the corresponding value of the correlated color temperature (CCT) is calculated to be 7923 K. The effect of the LC cell should be taken into account while the target CCT is defined. The emergent light of BLM will pass through the thin film transistor backplane, LC layer, and CF array successively. That will cause the decrease in certain peak intensity of the emission spectrum, and the color coordinates of LCD monitor will also shift from that of BLM. Therefore, the CCT determination of QD BLM should consider the effect of LC cell in advance. The target color coordinates of BLM is always set as (0.30, 0.29), and that of LCD monitor should be approximately (0.313, 0.329) after LC cell. From Eqs. (2) and (3), the peak intensity of the red/green QDs can be calculated numerically by defining the blue intensity as a unit quantity. After calculation, the ratios of the peak intensities of red, green, and blue are 0.666:0.698:1.
4. White-balance experimental achievement for QD BLM
4.1 Optimum concentration of the QD solution
CdSe@CdS/ZnS QDs are applied as red/green original materials for further experiments [17,18]. The peak positions of red/green QDs used in the following experiment are 628 nm and 519 nm, whereas their FWHMs are 31.5 nm and 30.5 nm, respectively.
The best concentration of the red/green QD solution should be first determined. The UV-visible absorption and PL spectrum for different concentrations of the QD solution were scanned within the visible band at room temperature by using UV-3600 (Shimadzu, Ltd) and F-4600 (Hitachi, Ltd) spectrophotometers, respectively. The (F-4600) photometer was used as the PL testing instrument; it provides an adjustable excitation light source that can meet the spectral characteristics of the actual LED light bar.
A wide range of variation in the red/green QD concentration was applied to analyze the change tendency of the peak intensity and peak position. The experimental results are shown in Fig. 2. It is obvious that the peak intensity increases sharply to a threshold maxima with the increase in the QD concentration, and then decreases gradually and continuously. The PL intensity reaches the maxima at the red/green QD concentration of 0.23/1.5 mg/ml.
Fig. 2 Change tendency of peak intensity and peak position for (a) red and (b) green QD concentrations.
4.2 Peak-intensity balance for red/green QDs
In order to achieve the red/green intensity balance, it is necessary to adjust the proportion of the red/green QD mixed solution. Red and green QD solutions with different volumes were mixed homogeneously in a quartz cuvette, and then the quartz cuvette was measured with the photometer under blue light excitation. The spectral curves at different ratios are drawn in Fig. 3, in which the inset shows the photograph of the red/green QD mixed solution in the quartz cuvette. It can be seen that the intensity of the red QD is much higher than that of green when the volumes of both are 100 μL. When the volume ratio of the red/green QD solution is gradually increased to 100 μL:183 μL, their peak intensities are almost equal. The final volume ratio of the red/green QD solutions is approximately VR-QD: VG-QD = 1:1.83 and the central wavelengths of the red/green QDs are 628 nm and 519 nm, respectively.
4.3 Spectral test of QD slurry for white balance
In preparing the proposed QD BLM, the red/green QD mixed solution should be mixed with the light scattering ink; therefore, the mixed QD slurry and the effect of the electrical parameter such as current and voltage should also be analyzed. It is significant and indispensable to discuss whether the white balance of the QD slurry will be broken under the excitation of different light intensities. The emission spectra of the QD slurry were measured while raising the excitation voltage from 350 V to 550 V, as shown in Fig. 4(a). The optical path diagram of the spectral test is drawn in Fig. 4(b). The peak positions of the excited red/green spectra are accurate at 628 nm and 519 nm, respectively. Their peak intensities synchronously change and remain in balance during the voltage changes. Once the appropriate volume ratio is achieved, the balance of the red/green QDs will not need to be re-considered even when the electrical parameters are changed.
Fig. 4 (a) Emission spectra of the QD slurry under different excitation voltages; (b) optical path diagram of the PL test.
5. Verification of the BLM with QMA
5.1 Optical design and simulation
A sample QD BLM with the diagonal length of 5.5-inch was prepared to verify the principle of the QMA. The effective coverage area of the QMA was approximately 120 mm × 70 mm (5.5 inches). Each of the three microstructures was arranged at the vertices of an equilateral triangle to avoid the influence of moiré fringes. The center-to-center distance between the microstructures was set as 1.139 mm. The profile shape of each microstructure was circular, and its radius could be adjusted to improve the uniformity of the QD BLM.
In our research, the radius of a certain microstructure is derived from the received irradiance at a certain coordinate position. According to the inverse-square law, the microstructure far from the blue LEDs receives less light energy than that near the sources. Assume that per unit size of the microstructure has the same light scattering ability. In order to achieve uniform light distribution on the LGP’s top surface, the microstructure far from the sources needs a larger radius, while the near one has a smaller radius. The received irradiance at a certain coordinate position is considered as an integral of the sum irradiance from each LED chip to this point. According to azimuth coefficient estimation, the irradiance E at certain point (x, y) can be expressed as:
where, l0 and l are the initial position and the length of a single blue LED chip, respectively. i represents the number of the LED chips. The LED chips can be considered as a Lambertian radiator, I0 is the central light intensity of them. G(x, y, a) is the function of the azimuth coefficient a, which can be determined by the light distribution curve of the LED chip.We need uniform irradiance distribution on LGP’s top surface, it means that the emitted energy from the microstructure near the sources is equal to that far from the sources. The radius of certain microstructure should be inversely proportional to the received irradiance at that position. By using mathematical recursive method, the radius r0 of a certain microstructure can be calculated by:
where, M is the luminous exitance of the blue LED sources. The above equation can solve the radius of a certain coordinate position according to the received irradiance. Within the 5.5-inch area, the distributions of the received irradiance E and the radius r0, calculated from Eqs. (4) and (5), are shown in Figs. 5(a) and 5(b), respectively. It can be found that the received irradiance near the LED sources (x = 20 mm, along y axis) has a larger value, while the calculated radius of the microstructure at this position will has a lower value.Fig. 5 Numerical calculation results. (a) The distribution of the received irradiance E at the bottom surface of the LGP; (b) the of the radius r0 at the bottom surface of the LGP.
As mentioned above, the radii of the microstructures far from the sources are larger than that near them. The diameter ranges between 0.3 mm and 0.6308 mm, which is easy to be fabricated. The density distribution of the QMA is shown in Fig. 6(a). The optical module of the QD BLM was established in TracePro according to an actual LGP, and the corresponding illuminance distribution was simulated, the results of which are shown in Figs. 6(b), 6(c), and 6(d), respectively. The illuminance curves in the horizontal/vertical lines present low fluctuations, which demonstrate that the designed BLM has high brightness uniformity.
Fig. 6 (a) Density distribution of the QD microstructures for 5.5-inch LCD backlight; (b) illuminance distribution of the simulated QD BLM; (c) illuminance curve corresponding to the black/red dotted lines in (b); (d) the established optical module of the 5.5-inch QD BLM; (e) density distribution of the QD microstructures for the 5.5-inch partition backlight (characters “FZU”); (f), (g) illuminance distribution and the corresponding illuminance curves of the partition illumination.
It was also found that the QMA design is an effective method for partitioning of the backlight. A QD backlight with the characters “FZU” was simulated. The density distribution and the corresponding simulation results are shown in Figs. 6(e), 6(f), and 6(g), respectively. These simulation results reveal a possible application in special lighting in addition to displays.
5.2 QMA preparation on a bare LGP
The designed QMA was applied for preparing a 5.5-inch sample QD BLM to verify the previous study. The material of the LGP substrate was polymethyl methacrylate (PMMA). The red/green QDs were mixed with the quantitative light scattering ink at the determined ratio, and then the QD slurry was transferred onto the LGP surface by screen printing technology. Figure 7 illustrates the QMA preparation processes, which can be separated into three main steps.
Step I: QD slurry preparation: according to the optimal concentration and the volume ratio, 9 mg red QDs, 110 mg green QDs, and 1.5 g light scattering ink were weighed separately on an electronic balance. N-hexane was chosen as the red/green QD solvent. The red/green QD mixture solution was first sonicated for 15 minutes for well dispersion; then, the QD solution was added to 1.5 g light scattering ink in quantities of 50 μl at a time until the entire quantity was added in. Finally, the QD slurry was obtained after stirring continuously for 3.5 h at a constant speed.
Step II: Screen printing plate preparation: the precision composite screen plate with the mesh number 6794 and the thickness 45 μm was prepared from stainless steel and polyester fabric, and then the photosensitive resist was evenly coated onto the screen plate. The density distribution of the QMA was used to fabricate a QMA mask. After UV exposure of the mask, the QMA design pattern was accurately transferred onto the composite screen.
Step III: One-time molding of the QMA: the composite screen plate was fixed on a printing machine, and the QD slurry was spread on one side of the plate. A bare LGP was located under the screen printing plate. First, the QD slurry was lightly scraped from one side to the other on the plate to ensure that every mesh was filled with the QD slurry; this process is called QD slurry coating. Next, the QD slurry was vigorously scraped again on the screen plate, where the application of higher pressure caused the transfer of the QD slurry from the mesh apertures to the LGP surface. This was a one-time molding process wherein the designed QMA pattern was printed onto the LGP in a single application. Finally, the LGP was placed in an oven at 50°C and thermally cured for 15 min.
5.3 Experimental results and analysis
A series of 5.5-inch QD BLMs were prepared for experimental verification. The red QD BLM was first prepared by printing the red QD slurry without the green QDs, and then the green QD BLM was printed without the red QDs. Figure 8(a) shows these 5.5-inch QD BLM prototypes based on monochromatic QMA under the driving current of 50 mA. Effective color conversion can be achieved with the QMA. A white QD BLM could also be obtained, as shown in Fig. 8(b), which provides a white-balance backlight output. Figure 8(b) also shows the white-balance QD BLM with the specific characters “FZU,” which demonstrates the feasibility of the application of the proposed QD BLM with QMA to special lighting. By comparing the proposed QD LGP and a conventional QD sheet in Fig. 8(c), it is obvious that the QD sheet solution needs a complete QD coating layer while the proposed solution can only distribute the QD slurry to the desired position according to the design pattern. The QD sheet consumes more QD material than the proposed one on a same area of LGP. Figure 8(d) shows a partial magnification of the QMA observed under the optical microscope Olympus DP73. The measured dimensions of the microstructure conformed to the designed size. For further analysis, a single QD microstructure was observed under a 3D measuring laser microscope (OLS4100). It can be seen in Fig. 8(e) that the diameter and height of the microstructure are 373.41 μm and 33.639 μm, respectively. The cross section approximates to an aspheric surface with some irregular morphologies rather than a smooth hemispherical shape. This interface increases the opportunity of light scattering and penetration into the QD microstructure, and helps to improve the output uniformity of the LGP.
Fig. 8 (a) 5.5-inch red/green monochromatic QD BLM prototype; (b) 5.5-inch white-balance QD BLM and the specific partition backlight for the characters “FZU” using the QMA; (c) comparison of a QD LGP and a QD sheet; (d) partial magnification of the QMA; (e) 3D image and the corresponding profile of a single QD microstructure.
6. Demonstration of flexible/curved BLM based on QMA
6.1 Feasibility analysis of flexible QD BLM
As shown in Fig. 9, it is assumed that the curvature radius of the flexible LGP is an invariant constant at a certain moment that can be written as r. When the maximal angle of the incident angle is 90°, the refraction angle within the LGP will be the critical angle Im. The ray with regard to Im will hit the LGP’s bottom surface as an incident angle of θ. According to the sine theore of the plane triangle, the following formula can be obtained:
where d is the LGP thickness. Only when , the light path will abide the law of total reflection. That means Im is the smallest angle that can be propagated in LGP. According to this condition, Eq. (7) can be written as:Therefore, the radius of the LGP can be constrained by this formula:
where Im, calculated by the refractive index of PMMA, is 42.0213°. The thickness d is 1.4 mm in our experiment. Under these conditions, . That means if the curvature radius of the LGP is larger than 12.76 mm, all the rays will abide the law of total reflection and propagate in the LGP.The simulation results also demonstrate the calculated results. Different curvature radii of the LGP is simulated in optical software of TracePro. As shown in Fig. 10, the bare LPGs without QMA are established with different radii of r = ∞, 1500 mm, 1000 mm, 500 mm, 250 mm, 125 mm, 50 mm, 15 mm, and 5 mm. One million rays are traced to simulate the light propagation within the LGP. In Figs. 10(a)-10(h), only a few rays, which have a large diffusion angle close to 90°, directly emit from the LED sources without entering the LGP. A large part of the LED emitting rays will spread in the LGP according to the law of total reflection. However, while the curvature radius of the LGP is less than 12.76 mm, part of the rays will not abide the law of total reflection, and directly emit from the LGP, which can be seen in Fig. 10(I). The experimental results are in good agreement with the theoretical and simulated values.
The fabrication processes were verified previously. However, their effectiveness and practicability for preparing a flexible/curved QD BLM need further discussion. The optimum concentrations of the red/green QD solutions and the corresponding processes were continually applied to a bare LGP with an 8-inch-diagonal active area and thickness of 1.4 mm. Generally, the thinner the thickness of the LGP, the better is its flexibility. However, an excessively thin structure would cause the LGP to warp or deform during the experiment. A thickness of 1.4 mm is appropriate. The flexible QD LGP is measured in a naturally dark ambient environment by the spectral color luminance meters BM-7 and SRC-200M. The detector of the color luminance meter is in front of the LGP with the distance of 500 mm, and the measured point is the center of the LGP. Figure 11 shows the brightness fluctuation test data of this 8-inch QD BLM, where it can be seen that the brightness changes slightly when the BLM is curved along different curvatures. The Y axis means the central luminance values, and the X axis is the number of the measurement while the BLM is curved at a certain curvature. The LGP with QMA has both good flexibility and high brightness tolerance, which make it suitable for flexible displays.
6.2 Preparation and testing of a large-scale curved QD BLM
Based on the previous studies, a large-scale curved LCD display monitor prototype employing the proposed LGP with QMA was specially designed and tested extensively. The dimensions of the QD-LGP were enlarged to 601 mm × 350 mm × 1.4 mm, which was equal to a diagonal length of 27 inches. The key process for fabricating the 27-inch QMA was the printing technology on the screen plate. In Fig. 12(a), the 27-inch screen plate was fixed on a printing machine, and the QD slurry was spread on one side of the plate. A bare LGP was located under the screen printing plate. As shown in Fig. 12(b), the QD slurry was first lightly scraped by the coating scraper from one side to the other to ensure that every mesh was filled with the QD slurry. In Fig. 12(c), the QD slurry was then vigorously scraped by the printing scraper, where the application of higher pressure caused the transfer of the QD slurry from the mesh apertures to the LGP surface. This one-time molding effectively ensures the uniformity of brightness and color.
Fig. 12 (a) Printing preparation; (b) QD slurry coating by the coating scraper; (c) QMA printing by the printing scraper.
The fabricated screen printing plate for 27-inch QD LGP can be seen in Fig. 13(a), where the QMA design patterns close to and far from the LED sources are shown in Fig. 13(b) and 13(c), respectively. The pattern on the screen printing plate meets the design output, which can be found that the radii of the patterns far from the sources that have are larger than that of the microstructures near them. The microscopic picture of the screen printing plate is shown in Fig. 13(d), where the mesh grid is obvious. As shown in Figs. 13(e) and 13(f), fabricated different-sized QMAs on the 27-inch QD LGP is observed under a 100 × handheld microscope.
Fig. 13 (a) The fabricated 27-inch screen printing plate for 27-inch QD LGP; the QMA design patterns close to (b) and far from (c) the LED sources; (d) the screen printing plate under a 100 × handheld microscope; (e) and (f) different-sized printed QMAs on the 27-inch QD LGP under a 100 × handheld microscope.
Figure 14(a) shows the 27-inch curved QD BLM prototype and the normalized spectra under different driving currents. The optical parameters, including the PL spectra, brightness, color coordinates, and CCT, can be measured by the spectral color luminance meters BM-7 and SRC-200M. The central wavelength of the red/green QDs remained stable when the driving current was increased. The ratio of the spectral peak intensities of red, green, and blue light was 0.661:0.663:1, close to the theoretical spectral model presented previously. Figure 14(b) is the chromaticity diagram of the proposed QD backlight under the CIE 1931 standard colorimetric system. The measured color coordinates are (0.3031, 0.2854), which are marked with a red star. It also compares the color gamut of the measured QD BLM, the NTSC standard, and Rec. 2020 (ITU-R Recommendation BT.2020). The black solid triangle, the dashed triangle, and the longer dashed triangle represent the measured color gamut spaces of the QD backlight, the NTSC standard, and the Rec. 2020 standard, respectively. Color gamut is defined as the coverage ratio between the RGB triangular area and that of the standard color gamut [42]. After calculation, the proposed BLM based on QMA can reach 122.79% of the NTSC color gamut, and it is equivalent to 92% under the Rec. 2020 standard.
Fig. 14 (a) Normalized spectra and (b) chromaticity diagram of the 27-inch curved QD BLM prototype with QMA.
Uniformity properties such as brightness and color are important criteria for evaluating display devices. The brightness uniformity of the backlight is calculated as the ratio of the minimum and the maximum luminance across the nine test positions, while the color uniformity is achieved by two color coordinates with the largest difference under CIE 1976 color coordinates. Under the operating current of 0.6 A, the minimum and maximum brightness are 3.7314 × 103 cd/m2 and 4.3787 × 103 cd/m2, respectively. The brightness uniformity can reach 85.21%. Under CIE 1976, the two points with the largest color difference are (0.2069, 0.4335) and (0.2072, 0.4424). Therefore, the color uniformity is calculated to be 9.2 × 10−3. It demonstrates that the proposed BLM with the QMA can provide high brightness and color uniformity while providing wider color gamut.
6.3 Demonstration of the curved LCD display monitor prototype
As shown in Fig. 15(a), the 27-inch curved LCD monitor prototype includes the QD BLM and the corresponding LC cell, i. e., a reflector film, a QD LGP, two BEFs, a DBEF, and the LC cell. After measurement, the central brightness and color coordinates under white-balance condition are 380.3 cd/m2 and (0.3134, 0.3326), respectively. The color gamut reaches 109.59% and 81.84% under the NTSC standard and the Rec. 2020 standard. It is found that the color gamut of the LCD monitor prototype is lower than that of the QD BLM. The main reason is the mismatch of the spectrum of the QD BLM and the transmittance of the LC cell. After the filter of LC cell, the emission spectrum is also measured and drawn in Fig. 15(b), which has an obvious peak intensity change from the emission spectrum of QD BLM (Fig. 14(a)).
Fig. 15 (a)The structure and (b) the emission spectrum of the 27-inch LCD display monitor prototype.
Figure 16 shows the images displayed by the 27-inch curved LCD monitor with QMA and traditional LCD monitor without QMA. Compared to a traditional LCD monitor, higher color saturation and richer color information can be obtained by the LCD monitor with QMA. The experimental results demonstrate the potential applicability of the QMA to LCD displays.
Fig. 16 Actual display effect. (a) The 27-inch curved LCD monitor with QMA; (b) the traditional LCD monitor without QMA.
7. Conclusions
High color gamut and flexible structure are two desirable features in LCD displays. In this paper, we presented a novel flexible/curved BLM with the QMA, and realized a corresponding LCD display monitor prototype. As a replacement of conventional LGPs, the QMA was specifically designed to combine the functions of both spatial homogenization of light and color mixing for white balance. During the design process, the theoretical white-balance QD spectral model was first calculated and established by using a Gaussian function, and then the absorption and PL spectra of the determined QD material were analyzed to find the proper red/green QD concentrations. Based on this, the QD slurry formulation, which combined the red/green QDs with light scattering particles, was prepared to satisfy the theoretical spectral model. For preliminary principle verification, a QMA with 5.5-inch active area was designed, simulated, and fabricated. Screen printing technology was utilized to realize effective one-time molding of the QMA on the LGP. Based on the previously determined parameters and the technological process, medium-sized and large-sized QD BLMs were prepared to demonstrate the flexibility and performance of the curved structure using essential optical tests. The 8-inch flexible QD BLM showed high brightness tolerance under different curvatures. A 27-inch curved QD BLM prototype was tested and found to possess a wide color gamut of 122.79%/92% under NTSC/Rec. 2020 standard with a high central brightness of 4378.7 cd/m2 and good luminance/color uniformity of 85.21%/9.2 × 10−3. Finally, an LCD monitor prototype was assembled with the curved QD BLM, which provided a color gamut of 110% NTSC and luminance of 380 cd/m2. Compared to commercial LCD monitors, the proposed prototype can offer better imaging quality, higher color gamut, and higher brightness under the same driving current. The measured spectrum also satisfies the theoretical white-balance spectral model well.
The proposed BLM based on QMA is feasible for application to flexible/curved LCD displays without the need of any additional optical element, and is easy to fabricate and industrialize due to the simple fabrication process. Compared with conventional QD sheets, this BLM has a less QD material consumption, and provides a simpler optical structure. In addition, it has some potential applications such as special QD lighting, which was also demonstrated in the paper by the lighting of the characters “FZU.”
Funding
National Key Research and Development Program (2017YFB0404600); Training Program of Fujian Excellent Talents in University (FETU).
Acknowledgments
The authors would like to extend their sincere gratitude to the colleagues of Top Victory Electronics (Fujian) Co., Ltd. and Guangdong Poly Opto-Electronics Co., Ltd. for their assistance on this thesis.
References and links
1. H. J. Kim, M. H. Shin, J. Y. Lee, J. H. Kim, and Y. J. Kim, “Realization of 95% of the Rec. 2020 color gamut in a highly efficient LCD using a patterned quantum dot film,” Opt. Express 25(10), 10724–10734 (2017). [CrossRef] [PubMed]
2. S. Kobayashi, S. Mikoshiba, and S. Lim, LCD Backlights (John Wiley & Sons, 2009).
3. T. Okumura, A. Tagaya, Y. Koike, M. Horiguchi, and H. Suzuki, “Highly-efficient backlight for liquid crystal display having no optical films,” Appl. Phys. Lett. 83(13), 2515–2517 (2003). [CrossRef]
4. K. Käläntär, “Modified functional light-guide plate for backlighting transmissive LCDs,” J. Soc. Inf. Disp. 11(4), 641–645 (2003). [CrossRef]
5. D. Feng, Y. Yan, X. Yang, G. Jin, and S. Fan, “Novel integrated light-guide plates for liquid crystal display backlight,” J. Opt. A, Pure Appl. Opt. 7(3), 111–117 (2005). [CrossRef]
6. Y. Ukai, “TFT-LCDs as the future leading role in FPD,” SID Symposium Digest of Technical Papers 44(1), 28–31 (2013).
7. H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater. 1(1), 64–68 (2002). [CrossRef] [PubMed]
8. Y. Huang, H. Chen, G. Tan, H. Tobata, S. Yamamoto, E. Okabe, Y. F. Lan, C. Y. Tsai, and S. T. Wu, “Optimized blue-phase liquid crystal for field-sequential-color displays,” Opt. Mater. Express 7(2), 641–650 (2017). [CrossRef]
9. X. Shen, D. F. Zhang, X. W. Fan, G. S. Hu, X. B. Bian, and L. Yang, “Fabrication and characterization of YAG:Ce phosphor films for white LED applications,” J. Mater. Sci. Mater. Electron. 27(1), 1–6 (2015).
10. S. H. Ji, H. C. Lee, J. M. Yoon, J. C. Lim, M. Jun, and E. Yeo, “Adobe RGB LCD monitor with 3 primary colors by deep green color filter technology,” SID Symposium Digest of Technical Papers 44(1), 1332–1334 (2013).
11. R. J. Xie, N. Hirosaki, and T. Takeda, “Wide color gamut backlight for liquid crystal displays using three-band phosphor-converted white light-emitting diodes,” Appl. Phys. Express 2(2), 022401 (2009). [CrossRef]
12. M. Anandan, “Progress of LED backlights for LCDs,” J. Soc. Inf. Disp. 16(2), 287–310 (2008). [CrossRef]
13. Y. Ito, T. Hori, H. Tani, Y. Ueno, T. Kusunoki, H. Nomura, and H. Kondo, “A backlight system with a phosphor sheet providing both wider color gamut and higher efficiency,” SID Symposium Digest of Technical Papers 44(1), 816–819 (2014).
14. T. H. Kim, K. S. Cho, E. K. Lee, S. J. Lee, J. Chae, J. W. Kim, D. H. Kim, J. Y. Kwon, G. Amaratunga, S. Y. Lee, B. L. Choi, Y. Kuk, J. M. Kim, and K. Kim, “Full-colour quantum dot displays fabricated by transfer printing,” Nat. Photonics 5(3), 176–182 (2011). [CrossRef]
15. Y. Shirasaki, G. J. Supran, M. G. Bawendi, and V. Bulovic, “Emergence of colloidal quantum-dot light-emitting technologies,” Nat. Photonics 7(1), 13–23 (2013). [CrossRef]
16. S. Coe-Sullivan, W. Liu, P. Allen, and J. S. Steckel, “Quantum Dots for LED Downconversion in Display Applications,” ECS J. Solid State Sci. Technol. 2(2), R3026–R3030 (2013). [CrossRef]
17. X. Peng, M. C. Schlamp, A. V. Kadavanich, and A. P. Alivisatos, “Epitaxial growth of highly luminescent CdSe/CdS core/shell nanocrystals with photostability and electronic accessibility,” J. Am. Chem. Soc. 119(30), 7019–7029 (1997). [CrossRef]
18. B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, K. F. Jensen, and M. G. Bawendi, “(CdSe) ZnS core-shell quantum dots: synthesis and characterization of a size series of highly luminescent nanocrystallites,” J. Phys. Chem. B 101(46), 9463–9475 (1997). [CrossRef]
19. R. Xie, D. Battaglia, and X. Peng, “Colloidal InP nanocrystals as efficient emitters covering blue to near-infrared,” J. Am. Chem. Soc. 129(50), 15432–15433 (2007). [CrossRef] [PubMed]
20. H. Chen, J. He, and S. T. Wu, “Recent advances on quantum-dot-enhanced liquid crystal displays,” IEEE J. Sel. Top. Quantum Electron. 23(5), 1900611 (2017). [CrossRef]
21. H. Huang, F. Zhao, L. Liu, F. Zhang, X. G. Wu, L. Shi, B. Zou, Q. Pei, and H. Zhong, “Emulsion Synthesis of Size-Tunable CH3NH3PbBr3 Quantum Dots: An Alternative Route toward Efficient Light-Emitting Diodes,” ACS Appl. Mater. Interfaces 7(51), 28128–28133 (2015). [CrossRef] [PubMed]
22. Q. Zhou, Z. Bai, W. G. Lu, Y. Wang, B. Zou, and H. Zhong, “In Situ Fabrication of Halide Perovskite Nanocrystal-Embedded Polymer Composite Films with Enhanced Photoluminescence for Display Backlights,” Adv. Mater. 28(41), 9163–9168 (2016). [CrossRef] [PubMed]
23. X. Y. Yang, K. Dev, J. Wang, E. Mutlugun, C. Dang, Y. Zhao, S. Liu, Y. Tang, S. T. Tan, X. W. Sun, and H. V. Demir, “Light Extraction Efficiency Enhancement of Colloidal Quantum Dot Light-Emitting Diodes Using Large-Scale Nanopillar Arrays,” Adv. Funct. Mater. 24(38), 5977–5984 (2014). [CrossRef]
24. P. Adel, A. Wolf, T. Kodanek, and D. Dorfs, “Segmented CdSe@CdS/ZnS Nanorods Synthesized via a Partial Ion Exchange Sequence,” Chem. Mater. 26(10), 3121–3127 (2014). [CrossRef]
25. S. Jun and E. Jang, “Bright and stable alloy core/multishell quantum dots,” Angew. Chem. Int. Ed. Engl. 52(2), 679–682 (2013). [CrossRef] [PubMed]
26. J. Y. Lien, C. J. Chen, R. K. Chiang, and S. L. Wang, “Patternable Color-conversion Films based on Thick-shell Quantum Dots,” SID Symposium Digest of Technical Papers48(1), 558–561 (2017). [CrossRef]
27. K. J. Chen, H. C. Chen, K. A. Tsai, C. C. Lin, H. H. Tsai, S. H. Chien, B. S. Cheng, Y. J. Hsu, M. J. Shih, C. H. Tsai, H. H. Shih, and H.-C. Kuo, “Resonant-Enhanced Full-Color Emission of Quantum-Dot-Based Display Technology Using a Pulsed Spray Method,” Adv. Funct. Mater. 22(24), 5138–5143 (2012). [CrossRef]
28. H. W. Chen, R. D. Zhu, J. He, W. Duan, W. Hu, Y. Q. Lu, M. C. Li, S. L. Lee, Y. J. Dong, and S. T. Wu, “Going beyond the limit of an LCD’s color gamut,” Light Sci. Appl. 6(9), e17043 (2017). [CrossRef]
29. S. Coe-Sullivan, “Quantum dot developments,” Nat. Photonics 3(6), 315–316 (2009). [CrossRef]
30. E. Jang, S. Jun, H. Jang, J. Lim, B. Kim, and Y. Kim, “White-light-emitting diodes with quantum dot color converters for display backlights,” Adv. Mater. 22(28), 3076–3080 (2010). [CrossRef] [PubMed]
31. S. Kim, S. H. Im, and S. W. Kim, “Performance of light-emitting-diode based on quantum dots,” Nanoscale 5(12), 5205–5214 (2013). [CrossRef] [PubMed]
32. J. H. Oh, H. Kang, M. Ko, and Y. R. Do, “Analysis of wide color gamut of green/red bilayered freestanding phosphor film-capped white LEDs for LCD backlight,” Opt. Express 23(15), A791–A804 (2015). [CrossRef] [PubMed]
33. J. Kurtin, N. Puetz, B. Theobald, N. Stott, and J. Osinski, “Quantum dots for high color gamut LCD displays using an on-chip LED solution,” SID Symposium Digest of Technical Papers 45(1), 146–148 (2014).
34. J. Chen, V. Hardev, J. Hartlove, J. Hofler, and E. Lee, “A high-efficiency wide-color-gamut solid-state backlight system for LCDs using quantum dot enhancement film,” SID Symposium Digest of Technical Papers 43(1), 895–896 (2012).
35. J. S. Steckel, J. Ho, C. Hamilton, J. Xi, C. Breen, W. Liu, P. Allen, and S. Coe-Sullivan, “Quantum dots: The ultimate down-conversion material for LCD displays,” J. Soc. Inf. Disp. 23(7), 294–305 (2015). [CrossRef]
36. J. Chen, V. Hardev, J. Hartlove, J. Hofler, and E. Lee, “A high-efficiency wide-color gamut solid-state backlight system for LCDs using quantum dot enhancement film,” SID Symposium Digest of Technical Papers 43(1), 895–896 (2012).
37. Z. Luo, D. Xu, and S. T. Wu, “Emerging quantum-dots-enhanced LCDs,” J. Disp. Technol. 10(7), 526–539 (2014). [CrossRef]
38. J. Thielen, D. Lamb, A. Lemon, J. Tibbits, J. V. Derlofske, and E. Nelson, “Correlation of accelerated aging to in-device lifetime of quantum dot enhancement film,” SID Symposium Digest of Technical Papers 47(1), 336–339 (2016).
39. J. Chen, J. Hartlove, V. Hardev, J. Yurek, E. Lee, and S. Gensler, “High efficiency LCDs using quantum dot enhancement films,” SID Symposium Digest of Technical Papers 45(1), 1428–1430 (2014).
40. J. S. Steckel, J. Ho, C. Hamilton, J. Xi, C. Breen, W. Liu, P. Allen, and S. Coe-Sullivan, “Quantum dots: the ultimate down-conversion material for LCD displays,” J. Soc. Inf. Disp. 23(7), 130–133 (2015). [CrossRef]
41. S. Nizamoglu, T. Erdem, X. W. Sun, and H. V. Demir, “Warm-white light-emitting diodes integrated with colloidal quantum dots for high luminous efficacy and color rendering,” Opt. Lett. 35(20), 3372–3374 (2010). [CrossRef] [PubMed]
42. R. Zhu, Z. Luo, H. Chen, Y. Dong, and S. T. Wu, “Realizing Rec. 2020 color gamut with quantum dot displays,” Opt. Express 23(18), 23680–23693 (2015). [CrossRef] [PubMed]
