We report a novel full-color display based on the generation of full-color by a highly efficient blue QD-LED light approach, or so called color-by-blue QD-LED display. This newly proposed color-by-blue QD-LED display combines a blue CdZnS/ZnS QD-LED blue subpixel and excitation source with front-emitting green/red phosphor subpixels. It is carefully estimated that the detailed display characteristics as well as full color-conversion and reasonable device efficiency of blue, green, and red satisfy the minimum requirements for display application. Also, we would like to emphasize that the proposed blue, green, and red device shows maximum luminance of 1570, 12920, and 3120 cd/m2, respectively, luminous efficiency of 1.5, 12.1, and 2.5 cd/A, respectively, and external quantum efficiency of 6.8, 2.8, and 2.0%, respectively. It is expected that full color generation by color-by-blue QD-LED will lead to further technological advancements in the area of efficient and facile display applications.
© 2014 Optical Society of America
The concept of color-by-blue displays was previously proposed for thick-film electroluminescent displays (TFELs) and organic light-emitting diodes (OLEDs) [1, 2]. Neither device type was successfully launched on the display market because of the relatively low luminous efficiency of the blue TFEL or blue OLED compared to counterpart green or red mono-color devices. The most important prerequisite for developing color-by-blue displays is the use of a highly efficient blue excitation source to convert blue light to green and red light in the green/red phosphor layers. With this in mind, if a material preparation method and device fabrication technology for realizing highly efficient and color stable blue colloidal quantum dots (QDs) and QD light-emitting diodes (QD-LEDs) is developed [3–7], blue colloidal QD-LEDs can be a promising candidate as a blue source for color-by-blue displays. Moreover, all-solution-processed, multilayered QD-LEDs are much easier to fabricate compared to the complex vacuum evaporation processes required for fabricating OLED displays. Quite recently, highly efficient and color-stable blue QDs (quantum yield > 97%) and QD-LEDs were realized by the present coauthor’s group by development of an unprecedented synthesis of blue CdZnS/ZnS core/shell QDs. The luminous efficacy and external quantum efficiency of the best blue QD-LED reached 2.2 cd/A, and 7.1%, respectively . Furthermore, the device showed a spectral match between the EL and PL and no change in the EL spectra with increasing voltage, which were realized through chemical modifications such as the addition of a thick shell. Therefore, this kind of highly efficient blue QD-LED can be selected as a candidate for a color-by-blue device because the possibility of highly efficient blue QD-LEDs provides good opportunities for realizing highly efficient color-by-blue displays.
Other concerns in such color-by-blue QD-LED approaches include backward emission due to the omnidirectional emission of the green/red phosphor layers as well as color mixing caused by the transmitted blue light and the downshifted green or red emission from the phosphors. High conversion efficiency and color coordinates of the green and red phosphors that convert the blue light source are needed to obtain high quality color-by-blue displays. For maximized forward emission efficiency and improved green and red subpixel color, a short-wave pass dichroic filter (SPDF) was coated on the rear side of the green and red color-conversion layers to reflect the backward emission of the phosphor layers, and a long-wave pass dichroic filter (LPDF) was coated on the front side of the green and red color-conversion layers to reflect and recycle the otherwise transmitted blue light passing through green/red phosphor layers. Recently, both highly efficient SPDF and LPDFs were developed by our group to enhance the forward efficiency of yellow emission from YAG:Ce phosphor-on-cup blue LEDs and to fabricate monochromatic LEDs by reflecting transmitted blue light from the phosphor layers, respectively [9–12]. The same concept of blue-passing and green/red-reflecting SPDFs with green/red-passing and blue-reflecting LPDFs was employed in color-by-blue QD-LED to enable the recycling of backward emitted green/red light and the removing of unwanted blue light from the green/red phosphor layer, as shown in Fig. 1.
As previously reported, this color-by-blue QD-LED approach has some other desirable features. It reduces the production cost as the QD subpixel does not require color patterning, and provides highly uniform and predictable luminance levels from all red, green, and blue subpixels because each color subpixel will be driven by the same blue QD-LED. Although the current energy efficiency level of blue QD-LEDs is not yet high enough to meet the requirements for commercializing full-color displays, the rapid technological advancements of blue QD-LEDs will drive the challenge and success of color-by-blue displays based on QD-LEDs in the near future. Therefore, this paper proposes a simple concept for an efficient emissive color-by-blue QD-LED display for the first time by combining a blue CdZnS/ZnS based QD-LED device and green/red (SrGa2S4:Eu /(Sr,Ca)AlSiN3:Eu ) color-converting phosphor layers excited by blue, with the green/red phosphor layers sandwiched between a SPDF and LPDF. Furthermore, the optical and electrical properties of the newly designed color-by-blue emissive QD-LEDs were evaluated to check their suitability to meet the requirements for display applications.
2. Experimental methods
Synthesis and fabrication of blue-emitting CdZnS/ZnS QDs and QD LED [8, 15]: Blue-emitting CdZnS/ZnS QDs were synthesized as follows. CdO and Zn acetate were mixed with of oleic acid (OA) in a three-necked reactor. This mixture was heated to 150 °C under a purging Ar environment. Then, octadecene (ODE) was added to the reactor at that temperature and the entire mixture underwent continued heating. Upon reaching 310 °C, the first sulfur (S) stock solution (S dissolved in ODE) was swiftly injected into the above hot mixture, and the reaction was maintained at that temperature for 12 min for the growth of CdZnS core QDs. For the subsequent overcoating of the ZnS shell, a second S stock solution (S dissolved in OA) was added dropwise, and the shelling reaction was maintained for 3 h. The resulting QDs were precipitated with the addition of an excess amount of ethanol and subsequent centrifugation (12,000 rpm, 10min), They were then purified three times by a precipitation/dispersion method with a solvent combination of hexane/ethanol (10/40 in mL) using the same centrifugation conditions, and the purified QDs were re-dispersed into hexane for the spin-coating process the of active emissive layer (EML) of QD LEDs. For the fabrication of the QD LEDs, a hole injection layer (HIL) of poly(ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) (Al 4083) was spin-coated at 3000rpm for 60s on patterned ITO, after which it was baked at 150 °C for 30min in a glove box. For the hole transport layer (HTL), poly(9-vinlycarbazole) (PVK) was dissolved in chlorobenzene and then spin-coated and baked on the top of the HIL under the same conditions used with the HIL. An EML of blue-emitting CdZnS/ZnS QDs dissolved in hexane was spin-coated at 2000rpm for 20s on the HTL and electron transport layer (ETL) of ethanol solution of ZnO NPs was spin-coated at 1500 rpm for 60s on the EML and baked at 60°C for 30 min in a glove box. Finally, Al cathode was thermally evaporated on top of the ETL. Figure 2(a)shows the schematic diagrams of the basic structure of the color-by-blue QD-LED.
Fabrication of the LPDF and SPDF [9–12]: Dielectric LPDFs and SPDFs were fabricated on glass substrates. For the fabrication of the stacks, terminal eighth-wave-thick TiO2 (25 nm) and quarter-wave-thick SiO2 (73 nm) nano-multilayered films ((0.5TiO2/SiO2/0.5TiO2)9, LPDF) and terminal eighth-wave-thick SiO2 (59 nm) and quarter-wave-thick TiO2 (77 nm) nano-multilayered films ((0.5SiO2/TiO2/0.5SiO2)9, SPDF) were coated onto glass substrates by e-beam evaporation at 250°C. For the design of the LPDF and SPDF multilayer films, the characteristic matrix method was used to simulate the reflectance (R), transmittance (T), and absorption (A). Figure 2(b) shows the transmittance spectra of the LPDF and SPDF in the normal direction.
Fabrication of green and red phosphor films: SrGa2S4:Eu  green powder phosphor was synthesized by a solid-state reaction method. (Sr,Ca)AlSiN3:Eu  red powder phosphor purchased from Intematix Corp. The internal quantum efficiency (IQE) of green/red phosphors was 74% and 88%, respectively. The IQE were calculated with the Eq. (1). The efficiency and color gamut are taken into consideration when we select the green/red phosphors.12]. Figure 2(c) shows the normalized EL spectrum of a blue-emitting CdZnS/ZnS QD-LED and normalized PL spectra of green and red phosphor films.
Characterization: The EL spectra, EQEs, and luminance-current density-voltage characteristics of blue QD-LEDs were recorded with a Konica-Minolta CS-1000 spectroradiometer coupled with a Keithley 2400 voltage and current source under ambient conditions. The EQEs were calculated with the Eq. (2)
3. Results and discussion
Figure 2(a) shows the schematic diagram of a basic structure of the color-by-blue QD-LED proposed in this study. The key factors determining the realization of this color-by-blue display are the degree of color-conversion and conversion efficiency of blue to green/red color and the energy efficiency of the blue QD-LED itself.
To look into the feasibility of obtaining pure green and red, based on the color-conversion layers of the QD-LED display, we have prepared devices that use the same QD-LED as excitation source for the red and green subpixels with green SrGa2S4:Eu and red (Sr,Ca)AlSiN3:Eu phosphor films, respectively. As shown in Fig. 1, the modified quarter-wavelength type SPDF (nine pairs of a dielectric multilayers of 0.5SiO2(TiO2)0.5SiO2), and LPDF (nine pairs of a dielectric multilayers of 0.5TiO2(SiO2)0.5TiO2) were placed on the back and front sides of the phosphor layers, respectively. As a result of optimizing the band position of the SPDFs and LPDFs, an SPDF with a central band position of 600 nm and an LPDF with central band position of 470 nm were selected. The normalized EL spectrum of CdZnS/ZnS QD-LED and normalized PL spectra of green (SrGa2S4:Eu) and red ((Sr,Ca)AlSiN3:Eu) phosphor films and the measured transmittance spectra of SPDF and LPDF are shown in Figs. 2(a) and 2(b). The figure shows that the emission band of the blue QD-LED can be overlapped by both the transmission window of the SPDF and the reflection band of the LPDF in the blue wavelength region. The figure also shows that the emission bands of the green and red phosphor layers can be overlapped by the reflection band of SPDF and the transmission window of the LPDF in the green and red wavelength regions. Figures 3(a)-3(d) show that spectral changes and 1931 Commission Internationale d’Eclairage (CIE) color coordinates were observed in the green and red forward emissions when the phosphor layer was sandwiched between the backside SPDF and front-side LPDF. The increases in the luminance of the normal emission of the green and red phosphor layers caused by the sandwiched SPDF and LPDF were 1.58 ± 0.05 and 1.70 ± 0.05, respectively. The luminance of the normal emission of the blue QD-LED also increased by 1.04 times when using the SPDF, as the transmittance of the SPDF exceeded 100% in the blue QD emission wavelength range. As a result, the LPDF clearly suppresses the transmission of any unabsorbed blue light and the SPDF enhances the green and red emissions through recycling of the backward emission. This clearly indicates that the LPDF helps convert the blue QD-LED light to green and red more fully.
For a detailed characterization of the color conversion to green and red in the color-by-blue QD-LED, the normalized brightness and color coordinates of the SPDF/LPDF-assisted green and red cells were considered and compared with conventional, SPDF-only and LPDF-only assisted cells. However, the brightness and color coordinates of color-by-blue QD-LED cells with different optical structures will also vary depending on the phosphor concentration of phosphor films. For further comparison, we need to determine the phosphor concentration in the phosphor layers prepared with both SPDF and LPDF, providing the pure green and red level of color purity and color coordinate.
For full conversion, high concentrations of green (50 wt%) and red (40 wt%) phosphors in the silicon binder matrix were used in cases utilizing a conventional substrate and SPDF-coated glass. Otherwise, low concentrations of green (20 wt%) and red (20 wt%) phosphors in the binder matrix are necessary for obtaining pure green and red colors with the highest brightness when using SPDF/LPDF-assisted phosphor layers as shown in Figs. 4(a) and 4(c). With this in mind, the relative brightness of the green and red phosphors sandwiched between the SPDF and LPDF was respectively improved by 8.2 and 2.0 times that of the blue emission passing though the only SPDF-coated blue QD-LED cell. Hence, the relative brightness ratios of the tri-colors in the SPDF/LPDF-assisted color-by-blue QD-LED displays are comparable to those of commercialized cathode ray tubes (CRTs) or active-matrix liquid crystal displays (AMLCDs). Figures 4(b) and 4(d) show the conversion efficiency of the green and red phosphor layers as a function of phosphor concentration. These figures confirmed that the maximum conversion efficiencies to green and red colors were obtained at the same phosphor concentration for the highest brightness and full conversion. The conversion efficiency of the SPDF/LPDF-assisted green (SrGa2S4:Eu) and red ((Sr,Ca)AlSiN3:Eu) phosphor layers coated on the blue QD-LED cell were 37 and 26%, respectively. These conversion efficiencies from blue to green or red are high enough to be used in the color-by-blue approach.
To demonstrate the efficient blue QD-LED for the application into color-by-blue displays, fabrication of blue CdZnS/ZnS based QD-LED devices was started by sequentially spin-coating solutions of PEDOT:PSS, PVK in chlorobenzene, CdZnS/ZnS QDs in hexane, and ZnO NPs in ethanol, as described in a previous publication. This blue QD-LED cell, which showed a peak with λmax of 452 nm and full width at half-maximum (FWHM) of ~30 nm, formed the excitation source for the green and red subpixel, allowing the evaluation of QD-LED-based green and red cell operation while also serving as a blue subpixel. As previously reported, the PL spectrum of the blue solution-based CdZnS/ZnS QDs and the EL spectrum of blue QD-LEDs are well matched and no changes in the EL spectra or color coordinates were observed with increasing applied voltage (Fig. 5(a)) . Both phenomena were realized by chemical modifications such as the addition of a thick ZnS shell on CdZnS core. By varying the operation voltage, various tones of the blue subpixels were achieved. This clearly demonstrates that careful tuning of the excitation light from the blue QD-LED is possible by controlling the applied voltages to the blue color region. For detailed characterization of the applicability of the color-by-blue QD-LED to full color displays, we measured the EL spectrum of the green and red cells under external voltages ranging from 5.0 to 9.5 V. Figures 5(b) and 5(c) indicate that the EL intensity of the green and red color-by-blue subpixels increases in a stepwise fashion, with the same peak wavelength and peak shape as the applied voltage. Similar to the blue subpixel, almost no changes in the green or red EL spectra or the color coordinates were observed with increased applied voltage. As a result, the blue QD-LED can produce different tones or gray scales for the color-converted green and red subpixels as well as the emitted blue subpixel. As shown in Figs. 5(a)–5(c), at least 8 different steps of blue, green, and red colors were produced by controlling the applied potentials. Therefore, a blue QD-LED provides highly uniform and predictable luminance shades from all red, green and blue subpixels because the electrical operating characteristics of the blue QD-LEDs are the same for all color subpixels.
The color coordinates of possible tri-color cells for color-by-blue emissive QD-LED displays is also important for obtaining a white color and balancing between red, green, and blue colors. Figure 5(d) shows the CIE color coordinates and color gamut measured from the national television standard committee (NTSC) as well as the color coordinates of the SPDF/LPDF-assisted green and red phosphor layers and blue emission through the SPDF-coated QD-LED. The CIE x,y color coordinates (0.16, 0.02) of the emission spectrum of the blue cell was deep enough to comply with the NTSC blue coordinates. Although the chromaticity coordinates of the green SrGa2S4:Eu phosphor layer (0.30, 0.68) and red (Sr,Ca)AlSiN3:Eu phosphor layer (0.63, 0.37) were slightly worse but close to the NTSC green and red coordinates, their colors are comparable to those of the commercially available green ZnS:Cu,Al (0.29, 0.61) and red Y2O2S:Eu (0.64, 0.34) phosphors in CRTs [17, 18]. The figure also suggests that the color gamut of the color-by-blue QD-LED display is 82.6% of that of the NTSC triangle. Although inorganic green/red phosphors show lower NTSC than green/red QDs (NTSC ~100%) , the inorganic green/red phosphors are more stable and have longer life time than green/red QDs. Given that a typical display panel needs to have a color purity of at least 70% of that of the NTSC triangle, the color gamut of the color-by-blue QD-LED display based on CdZnS/ZnS QD-LED, SrGa2S4:Eu, and (Sr,Ca)AlSiN3:Eu phosphor layers would meet these requirements. These color coordinates and color gamut of SPDF/LPDF-assisted color-by-blue QD-LED cells provide many opportunities for achieving suitable color mixing for full color display applications.
Figures 6(a) and 6(b) show voltage-dependent variations of luminance and current density of a blue QD-LED and green/red color conversion cells excited and controlled by blue QD-LEDs. This figure indicates that the measured values of the luminance from the blue QD-LED cell and the SPDF/LPDF-assisted green/red cells at the same voltage (10 V) were 1570, 12920, and 3120 cd/m2, respectively. This means that white luminance above the minimum requirement of brightness (1000 cd/m2) for display panel can be easily obtained from this emissive color-by-blue approach. The same current density-voltage relationship is observed for the green and red cells as well as the blue cell since both the green and red cells use the same CdZnS/ZnS QD-LED as an excitation and control source. Figures 6(c) and 6(d) show luminous efficiency (LE) and external quantum efficiency (EQE) as a function of applied voltages. The maximum LEs from the blue, green, and red cells were 1.5, 12.1, and 2.5 cd/A, respectively, at an applied voltage of 6.5V. As previously reported, the LE of the blue QD-LED was much higher than the maximum LE values of best blue QD-LEDs reported in recent publications. The peak LEs of green and red cells showed comparable values to those of green and red QD-LEDs reported in the literature although they were lower than the highest reported values. Figure 6(c) shows that green and red have still high LEs of about 10.0 and 2.0 cd/A even at a luminance level of 1000 cd/m2. Since LEs are sensitive to the spectral power density (SPD) of the emission spectrum, the EQE value indicates the direct value for evaluating device efficiency. Figure 6(d) shows that the maximum EQEs of the blue QD-LED and green and red color conversion layers excited by blue QD-LED were 6.8, 2.8, and 2.0%, respectively. The EQE of the blue QD-LED is also close to the best value reported in the literature to the best of the authors’ knowledge. Furthermore, the green and red cells showed comparable EQE values to those of other QD-LEDs in previous publications. Therefore, these efficient blue QD-LEDs provide the possibility of realizing color-by-blue QD-LED displays.
This study presents a newly designed device structure for color-by-blue QD-LED displays by combining a blue CdZnS/ZnS QD-LED and green/red (SrGa2S4:Eu/(Sr,Ca)AlSiN3:Eu) color-converting phosphor layers, with the green/red phosphor layers sandwiched between an SPDF and LPDF. The work presented here shows that the color-by-blue emissive QD-LED approach is an outstanding candidate for full color displays. By employing thick-shelled CdZnS/ZnS QDs as the blue QD-LED excitation source and blue sub-pixel, effective blue modulation was achieved. Greatly enhanced color conversion efficiency was achieved through the implementation of an SPDF and LPDF coupled with green/red phosphor layers and a blue QD-LED. Detailed evaluation of the proposed device revealed that its display characteristics such as full color-conversion of green/red cells, reasonable energy efficiency, high color gamut, the realization of gray scale, and reliable blue QD-LED modulations under low operating voltage meet the requirements of the standard display application and that it is ready to compete in the current display market, even at this early stage of development of color-by-blue QD-LED. At the moment, the efficiency and stability of blue QD-LEDs during excitation and blue generation processes seem rather low, as in many QD-LEDs. However, plenty of room for improvement surely exists through the optimized design of the display unit, as we have demonstrated by the relation between the optical characteristics and the blue QD-LED cell structure parameters.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP (Ministry of Science, ICT&Future Planning)) (No. 2011-0017449), (NRF-C1AAA001-2009-0092938), and the MSIP, Korea, under the ITRC (Information Technology Research Center)) support program NIPA-2013-(H0301-13-1004) supervised by the NIPA (National IT Industry Promotion Agency).
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