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Abstract
We propose and fabricate a multi-primary-color (MPC) quantum-dot down-converting film (QDDCF). A four-primary-color QDDCF composed of red (R), yellowish green (YG), bluish green (BG), and blue (B) subpixels was fabricated via totally five rounds of photolithographic processes. A verification platform was built up using a laser projector, and the measured results show that the QD film can expand display color gamut to 118.60% of Rec. 2020 and can cover the entire Pointer’s gamut. The issues of blue light absorption and film thickness are analyzed in detail. The combination of MPC technology and QDDCF is a potential strategy to realize ultra wide color gamut for emerging display technologies.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With rapid development of emerging display technology, the demand for wide color gamut is ever increasing for superior visual effect and accurate color reproduction [1]. In traditional three-primary-color displays, color gamut is usually limited by the well-known “trade-off relation” between color gamut and luminance, in which the area of color gamut would shrink when the output brightness increases [2]. To avoid this problem, certain extra independent primary colors were inserted into traditional three-primary-color subpixels, so-called multi-primary-colors (MPC) display technology [3]. Projection display was the earliest successful application of MPC technology. As early as 1999, a six-primary-color projector prototype was developed by using two conventional RGB projectors and six interference filters [4]. While the major drawback is the system complexity and bulkiness. Later in 2005, a four primary projection optical engine was presented by just inserting an additional dichroic element into illumination module, to improve the brightness and color gamut [5]. Another advantage of MPC projection displays was proven to be able to provide much less color breakup even at lower frame frequencies and achieve accurate color reproduction [6,7].
In addition to projectors, MPC display technology has also been successfully adopted in flat panel displays. In 2005, a six-color (two sets × three colors) light-emitting diode (LED) backlight system was integrated into a liquid crystal display (LCD) panel to boost the color gamut to 170% NTSC standard (National Television System Committee) [8]. Another six-primary-color approach was developed by using color filters composed of additional cyan, magenta, and yellow. Both wide color gamut and high brightness can be achieved by optimizing the subpixel arrangement [9]. The multi-primary color filters have also been reported in [10–13]. Besides LCDs, MPC can also be applied to other displays using striped subpixel arrangement, such as micro-LED displays [14], organic/polymer LEDs [15,16], and other specific scenarios [17]. For further detailed explorations, the design considerations between color gamut and brightness have been fully explored [18], and device performance between different striped pixel arrangements simulated [19]. The above-mentioned studies pave a solid foundation for MPC display technology and illustrate that both higher light utilization rate and more vivid color than conventional three-primary-color ones can be achieved.
Color reproduction approach is indispensable and crucial for MPC displays. To replace traditional absorptive color filters, color down-converting film is a promising candidate. However, their intrinsic mechanism is completely different. Absorptive color filters are usually made up of photoresist (PR) with wide spectra which easily cause overlapping emission spectra and color crosstalk. The desired spectral transmission can be designed by absorbing the unwanted wavelength, but at the cost of high energy loss, low efficiency, and narrow color gamut [20,21]. To overcome this issue, these absorptive materials were replaced by photoluminescence materials, such as phosphor or QDs. As reported in 2014, a spatiotemporal four-primary color LCDs was presented by integrating LEDs with QDs as color down-converting materials, and it exhibited 1.5× higher spatial resolution and 2× higher light efficiency and achieved 130% color gamut under CIE 1931 chromaticity diagram [22]. These down-converting materials absorb high energy (short wavelength) photons and emit lower energy photons. The pixels in QDDCF are isolated so that it could provide wider color gamut and higher color purity without reabsorption. Thus, QDDCF has recently become a hot research topic in LCDs, OLEDs, mini-LED and micro-LED displays [23–25]. The followings are some examples. A patterned QD film can serve as a color conversion layer in a specifically designed LCD system to experimentally realize 95% of Rec. 2020 color gamut (ITU-R Recommendation BT. 2020) [26]. White top-emitting OLEDs were fabricated by combining a blue OLED as pumping source and patterned QD down-converting films [27]. A flexible and uniformly-dispersed QDDCF composed of three primary colors was fabricated and applied to full-color mini-LED displays [28].
From the above discussion, we find that both MPC technology and QDDCF can offer wide color gamut. However, so far there is no related report on the combination of MPC technology and QDDCF fabrication. In this paper, for the first time, we propose the structure design, device fabrication, and confirming experiment of MPC QDDCF. The fine-patterned four-primary-color QDDCF is prepared by five rounds of photolithographic processes, and then the issues of fluorescence decay and blue light absorption are analyzed. After that, a verification platform based on a laser projector is used to demonstrate the color conversion and evaluate key performance metrics. Finally, the optimal film thickness preventing blue light leakage is discussed.
2. Experimental section
2.1 Materials
The materials for fabricating MPC QDDCF are listed as follow. CdSe/ZnS core-shell QDs were purchased from Guangdong Poly OptoElectronics. Negative PR (LZDGK) and TiO2 particles were purchased from Fuyang Sineva Material Technology. The dispersant (BYK-180) was purchased from Sunny Chemicals (Hong Kong). Propylene glycol monomethyl ether acetate (PGMEA, >99.5%) was purchased from Aladdin. Acetone, isopropyl alcohol (IPA), potassium hydroxide solution were purchased from Sinopharm Chemical Reagent. All the above materials were used as received without further purification.
2.2 Fabrication process
Several approaches can be used for fabricating MPC QDDCF, such as micro-contact printing [29], nanoimprinting [30], inkjet printing [31], and photolithography [32]. Machining efficiency of micro-contact printing technology is limited by the size of flexible seals, and high QD concentration and pressure uniformity are both required during the process of transfer printing [33,34]. Nanoimprinting technology relies heavily on imprinting template while patterning, and the processing is relatively complex [35]. For inkjet printing, coffee ring effect and blue light leakage are still two main problems, and it still has a long way to achieve large-area processing [36–38]. Compared to the above approaches, photolithography process has the irreplaceable advantages of ultra-high resolution for patterning and easy expansion for large-area manufacturing [39,40]. Therefore, we chose photolithography in our following experiments.
The fabrication procedure of the MPC QDDCF is briefly described as follows. Step I is the preparation of QDPR solution and the pretreatment of substrate. Firstly, 100 mg of BYK-180, 510 mg of PR, and 90 mg of TiO2 particles, were mixed extensively. Then 300 mg of QD powders were added with magnetically stirring at 50°C for 2 h, the QD powders were fully dispersed and the solution became transparent after stirring. The R-QDPR, YG-QDPR, BG-QDPR solutions were able to stand without any sediment for 60 h with a QD concentration of 30 wt.%. The B subpixel was fabricated by using blank PR with 9 wt.% of TiO2 particles. Glass substrates with the size of 3cm × 3cm were cleaned by ultrasonication successively in acetone, IPA, and deionized water. The clean substrates were dried by blowing nitrogen gas.
Step II is the photolithographic process of black matrix (BM). The black PR was first spin-coated (500 rpm, 30 s) on a glass substrate followed by a soft baking process on hot plate at 100°C for 2 min. The exposure was obtained by using i-line (URE-2000/35, 8 mw/cm2) for 7 s under a BM photomask. After UV irradiation, the film was then developed by potassium hydroxide dilute solution (0.042 wt.%) within 30 s and rinsed by deionized water. The film was dried by blowing nitrogen gas. Finally, a further hard baking process was operated on hot plate at 230°C for 1 h to solidify the pattern on the glass substrate.
Step III is the photolithographic process of four-primary-color QDDCF. Each type of QDPR was spin-coated (500 rpm, 30 s) on the same glass substrate followed by a soft baking process on hot plate at 100°C for 2 min. The exposure was obtained by using i-line (URE-2000/35, 8 mw/cm2) for 15 s under a subpixel photomask. After UV irradiation, the film was then developed by potassium hydroxide dilute solution (0.042 wt.%) within 60 s and rinsed by deionized water. The film was dried by blowing nitrogen gas. Finally, a further hard baking process was operated on hot plate at 150°C for 30 min to solidify the pattern on the glass substrate. The above process was repeated for four times to prepare R, YG, BG and B subpixels.
2.3 Characterization
The morphology of MPC QDDCF was characterized by using a fluorescence microscope (Olympus, BX51M) and a scanning electronic microscope (SEM, Helios G4 CX). The transmission electronic microscopy (TEM) images of QDs and TiO2 particles were recorded using a JEOL JEM-2100F microscope. The thicknesses of films were measured by Dektak-XT (BRUKER). The UV-Vis absorption spectra and the transmittance curves were measured by a UV/Vis/NIR spectrophotometer (Shimadzu, UV-3600). The photoluminescence (PL) spectra were obtained by a Hitachi F-4600 fluorescence spectrophotometer by exciting the samples using a Xe lamp coupled with a monochromator. The PL intensity and luminance were measured by SRC-200 spectral radiance colorimeter. The quantum yield (QY) and external quantum efficiency (EQE) were measured by Transient/steady-state fluorescence spectrophotometer (Edinburgh, FLS920).
3. Evaluation of QDPR and QDDCF
3.1 Characteristics of QD and QDPR
CdSe/ZnS core-shell QDs with nonpolar ligands were used in this study. The solvent of PR was PGMEA. However, QDs were incompatible with PGMEA due to their different polar characteristics. Aggregation and quenching was found when the QD solution was blended with PGMEA [28]. Here, a dispersant (BYK-180) was used to increase the dispersability of QDs in PGMEA [41]. The PL spectra and absorption spectra of various QDs are shown in Figs. 1(a)–1(c). The peak wavelength (λo) of R-QD, YG-QD and BG-QD appear at 621 nm, 531 nm, and 505 nm, and the corresponding full width at half maximum (FWHM) is 24 nm, 24 nm, and 28 nm, respectively. The insets in Figs. 1(a)–1(c) show the PL images and TEM images of uniformly dispersed QDPR solutions under the connection of BYK-180 without aggregation. Subsequently, QD powders were directly dispersed in PR without pre-production of QD/PGMEA solution. Table 1 illustrates the characteristics of QD solution and QDPR. After preparation, the λo of R-QDPR, YG-QDPR, and BG-QDPR exhibits a slight bathochromic shift of 1.4 nm, 1.2 nm, and 1.8 nm, and the FWHM is expanded to 25 nm, 25 nm, and 29 nm, respectively. Thus good dispersion property of QDPR solutions are demonstrated. Compared to QD solutions, the QY of R-QDPR, YG-QDPR, and BG-QDPR decreases from 73.28% to 70.06%, 83.78% to 76.39%, and 84.80% to 74.45%, respectively. The quality of QDDCF was associated with the film thickness, concentration, particle size, and QY of QDs. Especially, QY would decrease dramatically when QD solution was fabricated into QD film because the Förster resonance energy transfer phenomenon occurred [42]. Such phenomenon is difficult to suppress due to the QD particle size. Therefore, it is necessary to keep a high QD concentration level under a limited film thickness within 10 µm [43]. During our experiment, the QDPR was prepared using 30 wt.% QD concentration in PR.
Fig. 1. PL emission spectra and UV-visible absorption spectra of QDs with the emission colors of (a) R, (b) YG and (c) BG. (Insets: TEM images of QDPR solutions under UV excitation.)
Table 1. Characteristic change during the preparation process of QD solution and QDPR.
3.2 Photolithographic process and pixel alignment
Five independent photomasks were prepared for R, YG, BG, B subpixels, and BM, respectively. The photomasks for BM and red subpixel are shown in Figs. 2(a)–2(c). Each photomask was prepared by using alignment signs at four corners. The size of a single pixel is 265 µm × 265 µm and subpixel is 115 µm × 115 µm, respectively, which is intended for large-size TV displays. The distance between adjacent pixels/subpixels is 50 µm / 35 µm, respectively. Each subpixel can be excited and operated independently. The transmittance spectrum of BM with 2-µm film thickness is shown in Fig. 2(d). The BM is used to prevent potential color crosstalk and define the alignment signs. During each round of exposure, the alignment signs on the subpixel photomask should be coincided with the alignment signs on the glass substrate by means of fine adjustment knob under objective lens shown in Fig. 2(e). Based on this, precise alignment and pattern can be ensured.
Fig. 2. Photomasks for (a) BM and (b) red subpixels. (c) One of the alignment signs at upper right corner of this photomask. (d) Transmittance spectrum of BM (Inset: the fabricated BM pattern under microscope). (e) Photolithographic machine (URE-2000/35).
Figure 3 shows the fabrication procedures for the four-primary-color QDDCF. Figures 3(a)–3(h) depict the fine-patterned four-primary-color QDDCF prepared by five rounds of photolithographic processes on a glass substrate. Figures 3(i)–3(l) show the PL images of fine-patterned QDDCF under UV illumination captured by Olympus BX51M microscope. Here, B subpixel was filled with blank PR and TiO2 particles to allow blue light to pass without color conversion. The average film thickness is 7.4 µm. It can be seen that the films show good dispersion and surface smoothness after preparation.
Figure 4 shows the fluorescent and SEM images of the red subpixel pattern corresponding to four rounds of photolithographic processes. The fluorescent images were captured under 50× magnification, and the SEM images were captured under 2,000× magnification. By comparison of Figs. 4(a)–4(d), it can be seen that the difference of the surface morphology is quite limited at different stages. Some tiny spots inside the pattern can be observed under 350,000× magnification from the SEM image in Fig. 4(e). They primarily came from local QD aggregation. Besides, the damage to subpixel pattern was hardly observed even after four rounds of photolithographic processes.
Fig. 4. Surface morphology of the red subpixel pattern at different stages. (a) After 1st round; (b) after 2nd round; (c) after 3rd round; (d) after 4th round of photolithographic process. (e) The red subpixel pattern observed under 350,000× magnification from the SEM.
3.3 Fluorescence decay
QDs can be easily quenched and oxidized due to their sensitivity to water and oxygen [44]. It is necessary to evaluate the fluorescence decay and EQE during each photolithographic round. As shown in Fig. 5(a), the fluorescence intensity of R-QDDCF decreases to 98.2%, 90.6%, 85.1%, and 84.0% during the process of soft baking, exposure, develop and hard baking, respectively. Similar fluorescence decay results are shown in Figs. 5(b) and 5(c) for YG-QDDCF and BG-QDDCF, respectively. Their fluorescence intensities can be kept above 80% after a complete photolithographic process. During the soft and hard baking processes, the fluorescence decay is effectively limited since the QD film was baked in the vacuum dryer under the protection of inert gas. However, the fluorescence decay becomes obvious during the exposure and develop process due to the effect of water and oxygen along with the reduction of film thickness. Noted that our overall fluorescence decay is lower than the data reported recently in [45]. In our experiment, the R-QDDCF, YG-QDDCF, and BG-QDDCF were endured totally four, three, and two rounds of photolithographic processes, respectively. From Figs. 5(a)–4(c), the fluorescence intensity of R-QDDCF was reduced by only 3.7% from the 1st to the 4th photolithographic processes, while the other two QDDCFs achieved even lower fluorescence decay due to simpler fabrication process. It indicates that the hard film remains stable through hard baking process and is much less vulnerable to the photolithographic process. Meanwhile, the EQE is measured and calculated by comparing the number of emitted photons with the number of absorbed photons as:
The measured EQE of R-QDDCF, YG-QDDCF, and BG-QDDCF was 18.2%, 24.3%, and 39.1% after four, three, and two rounds of photolithographic processes, respectively.4. Verification of MPC QDDCF
4.1 Blue light absorption
Increasing QD film thickness and QD concentration are two simple ways to minimize blue light leakage through the color conversion film. However, there still exist practical limitations from QD loading and material cost. In our study, two methods were introduced to enhance blue light absorption. One is the scattering particles added in QDs, and the second is the distributed Bragg reflector (DBR) filters above / below QDDCF.
Scattering particles can help light extraction, increase optical path length, and enhance blue light absorption [46]. TiO2 particles were used in our experiments. The morphology difference of QDs with/without TiO2 particles was observed and analyzed under JEOL JEM-2100F microscope. The TEM images of pure QDs and the mixture of QD/TiO2 are shown in Figs. 6(a) and 6(b). The size of TiO2 particles are approximately 200 nm ∼ 300 nm, which is almost 20× to 30× larger than QDs, as can be seen in the darker areas from Fig. 6(b). TiO2 particles increase the probability of QD excitation, bringing an increase in blue light utilization. The transmittance and absorption spectra with/without TiO2 particles were measured and compared in Fig. 6(c). Under the blue light excitation at 450 nm, the absorbance value of the QDDCF containing 9 wt.% TiO2 particles is 1.1 higher than that without TiO2 particles. The blue light transmittance decreases from 47.86% to 3.81%. The optical density (OD) can be defined as:
After calculation, it is found that the OD value of QDDCF increases from 0.32 to 1.42 with the film thickness of 5.5 µm. To further reduce the remaining 3.81% blue light, a DBR was prepared to reflect unwanted blue light completely.Fig. 6. TEM images of (a) pure QDs and (b) the mixture of QD/TiO2 particles. (c) Transmittance and absorption spectra with/without TiO2 particles. (d) The laminated structure of the patterned QDDCF and DBR structures. (e) Transmittance spectra of SPF and LPF (Insets: SPF and LPF devices). (f) PL intensity of YG-QDDCF with / without a LPF device (Insets: photographs of the YG-QDDCF with / without a LPF device under blue light).
DBR is composed of multiple pairs of two alternating layers with different refractive indices to achieve high reflectivity. It has a wavelength-dependent characteristic that transmits and reflects light selectively by choosing suitable materials and thicknesses for its dielectric layers. Figure 6(d) depicts the laminated structure of the patterned QDDCF and DBR structures including a commercial short pass filter (SPF) device and a prepared long pass filter (LPF) device, which is similar to [24,26]. The SPF device, FES0500, was purchased from Thorlabs. It transmits blue light directly and reflects red and green light to an opposite direction. The LPF device composed of three-layer TiO2 films and two-layer Al2O3 films were deposited alternately with precisely controlled thickness under the temperature of 90 °C by atomic layer deposition (ALD) method (Beneq TFS–200). It transmits red and green lights and reflects most of blue light to prevent blue light leakage. The measured transmission spectra of the used SPF and LPF devices are shown in Fig. 6(e), and the PL intensity of YG-QDDCF with/without the LPF device at the excitation light of 450 nm is shown in Fig. 6(f). The transmittance of blue light decreases dramatically due to the LPF device, but at the same time, the green emission light is also reduced since the LPF transmittance at λ=531 nm is only approximately 40%. It reveals that the DBR application provides a supplement to the addition of TiO2 particles to enhance blue light absorption.
4.2 Verification platform and testing
As shown in Fig. 7, the verification platform was composed of a SONY laser projector (MP-CL1A), the purchased SPF device, and the prepared MPC QDDCF and LPF device. Laser beam scanning is integrated in this projector to generate a focused image at any projection distance. The pixel size of projected image is proportional to the projection distance. A processed blue image, projected from the projector, can serve as a blue surface source and excite the subpixels on MPC QDDCF to realize the color conversion. The main purpose of the prototype is to prove the patterning and pixelation of the proposed QDDCF. Therefore, a realistic image, Lena image, was used as the image source. A nice mixture of detail, flat regions, shading, and texture, contained in Lena image makes it suitable for testing our MPC QDDCF.
Fig. 7. Schematic of the verification platform for the MPC QDDCF and the simplified subpixel mapping and rendering method.
Here, we use a simplified four-primary-color rendering method to prove the functionality of MPC QDDCF, which is illustrated in Fig. 7. Each pixel in the original Lena image was decomposed into four subpixels corresponding to R, YG, BG, and B color channels. The colors can be defined as (rk, gk, bk), k = 1∼4, for the four channels, respectively. Due to the required blue source, the parameters of rk and gk equal to zero. Therefore, the colors of the subpixel in the blue image can be finally defined by (0, 0, bk), k = 1∼4. It means only the blue content of these subpixels were retained. The original green content was divided into two parts for YG and BG channel, respectively. It is a simplified method to align and match between the subpixels of the processed blue image and QDDCF to achieve effective color conversion and color reproduction.
Here, the film thickness of the MPC QDDCF is 7.4 µm, and the OD value is calculated to be 2.19. From Fig. 6(e), the transmittance of the SPF device is higher than 80% in the 400 nm < λ < 500 nm region, but is cut off at λ> 500 nm. That means the downward propagating green and red lights will be reflected back to the top of the glass substrate. On the other hand, the transmittance of the LPF device at λ = 445 nm, 505 nm, 531 nm, and 621 nm is 15%, 25%, 40%, and 92%, respectively. Thus, it can recycle most of the upward propagating blue light. The verification platform and the original / color converted projection images are illustrated in Fig. 8(a). The normalized PL spectrum is shown in Fig. 8(b) where four subpixels are operated independently. In other words, four independent primary colors can be achieved by QD emissions. The excitation wavelength is 445 nm with FWHM = 6 nm and other three peak wavelengths are 510.4 nm (0.22), 538.8 nm (0.23), and 625.8 nm (0.51), respectively. The comparisons of color gamut under CIE 1931 chromaticity system are shown in Fig. 8(c). Here, the color gamut was calculated by comparing the R-YG-BG-B quadrilateral area of MPC QDDCF with the triangular area of the NTSC and Rec. 2020 standard [47]. After measurement, the color gamut of four-primary-color image is 158.93% and 118.60% under NTSC and Rec. 2020 standard, respectively. More importantly, the fabricated QDDCF can cover the entire Pointer’s gamut effectively [48], which means that all the real surface colors can be reproduced. Table 2 compares the performance of the color gamut achieved by existing color-gamut-extension approaches reported in related literatures, including the MPC displays, MPC QD technology, QDDCF technology, and our approach. The results demonstrate that the proposed four-primary-color QDDCF can provide a wider color gamut than other existing color-gamut-extension approaches and realize full color emerging displays.
Fig. 8. (a) The projected four-primary-color image obtained by the verification platform. (b) The corresponding PL spectrum, and (c) color gamut.
Table 2. Comparisons of the color gamut among different approaches.
5. Discussion and analysis
The prepared QDDCF cannot fully absorb blue light without a LPF device. To further explore its characteristics, it is necessary to find the optimal film thickness for preventing blue light leakage. A series of QDDCFs were prepared, with film thickness ranging from 5.1 µm to 15.3 µm. The absorption spectra of YG-QDDCF with different film thicknesses are shown in Fig. 9(a). It is obvious that the absorbance increases with the film thickness. The YG-QDDCF images with different film thicknesses under the same blue light excitation are shown in Fig. 9(b). Here, the excitation wavelength is 450 nm with a luminance of 350 cd/m2. As the film thickness increases, the average transmittance of the blue light decreases, and the color switches from aquamarine blue to pure green gradually. The chromaticity coordinates are shown in Fig. 9(c), where the chromaticity coordinates gradually change from (0.15, 0.03) to (0.26, 0.62) with increased film thickness. The corresponding PL spectra and luminance are shown in Figs. 9(d) and 9(e). Due to unobvious Stoke’s shift, the phenomenon of self-absorption and reabsorption among QDs becomes serious as the film thickness increases, and the bathochromic shift becomes larger. In Fig. 9(e), the luminance of blue light leakage decreases with the increased film thickness. The luminance of converted green light first increases with the film thickness, and then decreases after reaching a maximum. It means that the green light emission becomes saturated, and the film thickness has an optimal value. A thicker film would lead to a decreased transmittance and fluorescence intensity of emitted green light. During the fabrication process, the experimental film thickness is 7.4 µm. After calculation, the EQE of YG-QDDCF is reduced from 28.1% to 20.6% with the increased film thickness. Once the LPF device is inserted, the EQE reduces to 5% ∼ 6%. It reveals that part of the blue light which cannot be converted into desired green light probably decays in other forms, such as scattering, reflection, and absorption. The optimal film thickness can be achieved from mathematical fitting [50]. Table 3 lists the transmittance data of the fabricated QDDCFs. An exponential equation was used to fit the measured transmittance in Fig. 9(f). The fitting parameters of y0, A, and B were calculated as 0.09862, 661.87224, and 1.00019, respectively. Based on this, the film thickness without blue light transmittance can be theoretically achieved, which is around 16 µm. Actually, the blue light leakage can be kept below 0.1% while the film thickness is approximately 10 µm. It is more practical for most application cases. Other important experimental data are listed in Table 3. The transmittance of QDDCFs was reduced from 4.15% to 0.05% with the thickness increased from 5.1 µm to 15.3 µm. The corresponding OD values increase from 1.38 to 3.32. It proves that the experimental data agree well with the fitting results.
Fig. 9. Changing trend of the QDDCF characteristics with the film thickness. (a) Absorption spectra of YG-QDDCF. (b) PL images of the YG-QDDCF under the blue light excitation. (c) Changing trend of chromaticity coordinates. (d) PL spectra of YG-QDDCF. (e) Changing trend of PL intensity and luminance. (f) Fitting curve of the measured transmittances.
Table 3. Experimental data of the fabricated QDDCFs with different film thicknesses.
Compared to absorptive color filters, the combination of MPC technology and QDDCF has obvious advantages, as discussed below. The most prominent one is its ultra-wide color gamut. Such wide color gamut cannot be achieved by absorptive color filters due to the crosstalk issue. Secondly, the low-efficiency issue of absorptive color filters can be well avoided by the color conversion approach based on QDs. Thirdly, the MPC QDDCF can be potentially applied in diverse emerging display systems. Especially for micro LEDs, the processing difficulty can be effectively reduced, since only blue LED sources are needed during mass transfer. From recent researches on these direct-view displays, an absorptive color filter was always inserted and combined with the QDDCF to further cut blue light leakage and suppress ambient light excitation [51,52].
A display system using a MPC QDDCF device does not suffer from the trade-off problem between color gamut and luminance that was unavoidable in traditional tristimulus displays. In traditional tristimulus displays, the white light emitted from backlight was absorbed by absorptive color filter and selectively transmitted through the LCD panel to obtain the desired color. That is to say, in order to achieve purer color and narrower spectrum, more unwanted light energy has to be absorbed [53,54]. However, the MPC QDDCF device is an emitting device rather than an absorptive device. The light source used is blue light. Other primary light is stimulated by blue light source, and each single subpixel can be controlled independently. The QD’s emission wavelength can be optimized according to the needs of the color gamut. Therefore, there is no trade-off problem for the MPC QDDCF application.
6. Conclusion
For the first time, this paper proposes the structure design, fabrication, and verification of MPC QDDCF. A four-primary-color QDDCF composed of R, YG, BG and B subpixels was fabricated by five rounds of photolithographic processes in total. The fluorescence decay was controlled within a reasonable range that the fluorescence intensities of R-QDDCF, YG-QDDCF, and BG-QDDCF can be maintained at 84.0%, 80.5%, and 81.3% after a complete photolithographic process, respectively. Blue light absorption was improved in terms of both material and device structures. The prepared QDDCF containing 30 wt.% of QDs with film thickness of 7.4 µm shows the OD value above 2.0 and the EQE above 30%. A verification platform was built based on a laser projector to demonstrate the color conversion and evaluate key performance metrics. This prototype can provide an ultra-wide color gamut of 158.93%/118.60% under NTSC/Rec. 2020 standard, which is higher than other existing color-gamut-extension approaches. The changing trend of the QDDCF characteristics with the film thickness was further discussed, from which the experimental data agree well with the fitting results that the acceptable film thickness preventing blue light leakage was around 10 µm. We believe that this device can accelerate the development of existing display technologies, such as LCD, OLED, and micro-LED.
Funding
National Key Research and Development Plan (2017YFB0404604); Fujian Science and Technology Key Project (2018H6011); Training Program of Fujian Excellent Talents in University (FETU).
Acknowledgments
The authors would like to express their sincere gratitude to Prof. Shin-Tson Wu and his group members for their assistance in this work.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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