We report a novel full-color display based on the generation of full-color by blue light approach, so called color-by-blue display. This newly proposed color-by-blue light-valve display combines a blue backlight excitation source, a blue light-valve shutter, and front-emitting phosphor pixels. Careful evaluation shows that the detailed display characteristics as well as excellent cycling durability under a low operation voltage of 3 V easily satisfy the requirements for the current display application. Also, we would like to emphasize that the proposed method shows a conversion efficiency of 20%, surpassing the value (≈5%) seen in the typical liquid crystal displays. Although the switching response reported here is slower than in a commercial display module due to the solution-phase electrochromic nature of the shutter used, a response time close to that of a liquid crystal display is highly feasible, as we suggest.
©2011 Optical Society of America
Recently, full-color active matrix liquid crystal displays (AMLCDs) have earned a significant portion of the flat display market share due to the higher resolution, lower power consumption, and longer lifetime than those of conventional emissive displays. However, full-color AMLCDs, which use white backlighting through red, green, and blue (RGB) sub-pixels with a liquid crystal (LC) switching array [1,2], have their own demerits owing to the energy loss originating from their operation as passive emitting display devices. The significant energy loss occurs during the LC switching process due to the presence of a set of polarizers employed in the structure, and there is an additional loss at sub-pixels since only one of three RGB emission lines from the backlight is picked up by each color filter for the color generation, leading to reduced transmission efficiency and relatively low contrast ratio, especially under a bright environment. Typically, only about 5% of backlight luminance can be achieved in current AMLCDs .
Since the energy loss in AMLCDs is quite severe, the modulation of the backlight based on micromechanical shutters has been suggested to improve the efficiency of AMLCDs [4,5]. In particular, the telescope-pixel-based display technique proposed by Pyayt et al. has been experimentally demonstrated with a high backlight transmission efficiency of up to 36%, a rapid response time of less than 1 ms, and high image resolution with 100-μm pixel size, which make this technique quite appealing as an possible alternative to existing LCDs . However, this approach requires an operation voltage of 120 V, much higher than those of commercial display units, and the fabrication of a large area display incorporating a high density mechanical shutter array, not readily adaptable to the existing production lines.
Here, we propose a novel full-color display based on the color-by-blue approach as another way of overcoming the inherent limitations in current AMLCDs. This proposed color-by-blue light-valve display combines a blue backlight excitation source, a blue light-valve shutter, and front-emitting phosphor pixels, as shown in Fig. 1(a) . The color-by-blue approach has been already suggested as an alternative technique for color organic light-emitting diode (OLED) and color inorganic thick-film electroluminescence (TFEL) displays [7,8]. As reported previously, this color-by-blue approach has some desirable features. It reduces the production cost as the shutter cell does not require color patterning, and provides highly uniform and predictable luminance levels from all RGB pixels because each color pixel will be driven by the same blue shutter. Also, appropriate color conversion layers are only necessary for the RG pixels since the blue color will be directly derived from the backlight. However, neither OLED nor TFEL displays based on the color-by-blue scheme have been launched successfully in the display market, mostly because of the poor performance of the blue excitation source to achieve the required balance between RGB pixels.
In order for this novel color-by-blue display to be fully functional, there are a few prerequisites that must be considered. One of them is the development of a blue light-valve system with highly efficient on-off characteristics in the short blue wavelength range. Among the light-valve systems developed so far, organic yellow electrochromic (EC) light-valve is most promising as a blue shutter with a high contrast ratio, low power consumption, high coloration and transmission efficiency, and memory effect as well as the desired color change and fast switching time between the different redox states, compared to metal-oxide based EC devices . In this work, recently reported biphenyl dicarboxylic acid diethyl ester (DEB)  will be considered among several organic electrochromic materials such as viologen [11,12] and phenothiazine . Especially, this phthalate derivative exhibits the exceptional quality of electrochemically induced yellow color, required for the shutter application in this emissive color-by-blue device, as well as stable electrochemical and spectroscopic characteristics. When coupled with an InGaN-based blue light-emitting diode (LED), having a typical quantum efficiency over 60% [14,15], it is expected to allow the effective modulation of the blue excitation with high on-off contrast as well as improved transmission efficiency without the use of polarizers or color filters.
Some other concerns in such emissive display include backward emission due to the presence of the RG phosphor layers and color mixing, caused by the unabsorbed blue light as well as the down-converted green or red emission from the phosphors. These will introduce additional loss and limit the efficiency as seen in conventional displays. The necessary minimization of the phosphor conversion loss and proper elimination of any unabsorbed blue transmission can be achieved with a short wave pass filter (SWPF) and a long wave pass filter (LWPF), as shown in Fig. 1(b) and (c) [16–18]. Since the SWPF has a high reflectance band at green and red wavelengths, the SWPF, placed between the phosphor layer and the blue backlight, will allow the recycling of the backward green and red emissions. Also, the highly-reflective LWPF at a blue wavelength, located on the front sides of the green and red phosphor layers, will block the unnecessary transmission of blue light and redirect the blue light back into the green and red phosphors. Through these selective transmission and reflection processes, we expect to further improve the required efficiency in the proposed color-by-blue emissive EC display.
2. Experimental methods
2.1. Fabrications of short wave pass filter (SWPF) and long wave pass filter (LWPF)
The optical SWPF and LWPF were prepared by fabricating multilayered stacks of SiO2 and TiO2 films. The refractive indices (n) and extinction coefficients (k) of individual SiO2 and TiO2 films were measured by a spectroscopic ellipsometer (Sentech, SE800). We have also reported previously the detailed wavelength dispersion characteristics of n and k of the as-grown SiO2 and TiO2 films . With the measured values of n and k, the characteristic matrix method has been used to design the SWPF and LWPF, and simulate the reflectance, transmittance, and absorption of the optical structure. In the simulation model, the thicknesses of the high-index (TiO2) and low-index (SiO2) films were varied to tune the spectral position of the reflectance band. Based on the simulation results, the modified quarter-wave type SWPF, a dielectric multilayer of terminal eighth-wave thick 0.5SiO2 (56 nm) and quarter-wave thick TiO2/SiO2 (73 nm/112 nm), and LWPF of terminal eighth-wave thick 0.5TiO2 (25 nm) and quarter-wave thick SiO2/0.5TiO2 (73 nm/50 nm) structures were coated onto a glass substrate by e-beam evaporation at 250 °C. The deposition was performed at an acceleration voltage of 7 kV with an oxygen partial pressure of 1.9 x 10−4 Torr.
2.2. Preparations of green and red phosphor layers
SrGa2S4:Eu green powder phosphor was synthesized by the solid state reaction method . CaAlSiN3:Eu red powder phosphor was purchased from Mitsubishi Chemical Corporation. Various amounts of green and red powder phosphors between 30 and 80 wt% were dispersed in a silicon binder (Shin-Etsu Chemical Co., Ltd. KER-2500A, KER-2500B) and hydrophobic silica (Degussa, hydrophobic aerosol R202) to form green and red phosphor pastes. After printing these phosphor pastes on the SWPF with a 50 µm-thick spacer, they were dried and hardened by heating at 150 °C for 1 hour.
2.3. Optical Characterization
The corresponding transmission spectra of the DEB-based EC cell, SWPF, and LWPF were measured with a UV/Visible/Near IR spectrophotometer (Varian model Carry 5000) at normal incidence. The forward emission spectra and color coordinates of the RGB pixels were also measured by a photoluminescence spectrometer (PSI Co. Ltd., Darsar) with an integrated sphere. We have further calculated the conversion efficiency of the EC display module based on the ratio between the spectral areas under the intensity spectrum curves of red and green EC cells coated with phosphor layers, and a blue EC cell for the correlated color temperature of 12,000 K. The brightness of each of blue, green, and red cell was determined by an electroluminescence spectrometer (Minolta CS-1000A).
3. Results and discussion
3.1. Fabrication of DEB-based electrochromic cell
To demonstrate the emissive color-by-blue EC shutter display, as described in Fig. 1(d), we have prepared tri-color DEB-based EC pixels. The EC pixel fabrication started with the preparation of 0.36 M of tetra-n-butylammonium perchlorate (TBAP) electrolyte solution containing both 0.24 M of DEB as a yellow dye and 0.24 M of ferrocene (Fc) as a counter redox species with water-removed N-methyl pyrrolidione (NMP) as a solvent [10,20]. The electrolyte was sealed between two ITO-coated glass substrates, separated by a 25-μm thick Surlyn tape. This EC cell, placed on top of an InGaN LED chip with λmax of 450 nm, formed a phosphor-on-cup blue LED structure, allowing the evaluation of DEB-based light valve operation and also serving as a blue pixel. To look into the feasibility of the full-color generation based on the EC shutter display, we have prepared additional EC cell structures for red and green pixels with SrGa2S4:Eu [19,21] green and CaAlSiN3:Eu  red phosphor films. The modified quarter-wave type SWPF, a dielectric multilayer of 0.5SiO2(TiO2)0.5SiO2, and LWPF of 0.5TiO2(SiO2)0.5TiO2 structures were prepared on the back and front sides of the phosphor layers, with a central band position of 600 nm for the SWPF and 470 nm for the LWPF.
The measured transmittance spectra of SWPF and LWPF are shown in Fig. 2(a) as well as the normalized electroluminescence (EL) spectrum of InGaN blue LED and normalized photoluminescence (PL) spectra of green (SrGa2S4:Eu) and red (CaAlSiN3:Eu) phosphor films. The emission band of the blue LED overlaps with both the transmission window of the SWPF and reflection band of the LWPF at the blue wavelength region. Also, the emission bands of the green and red phosphor layers match the reflection band of SWPF and the transmission window of the LWPF at green and red wavelength regions. For a comparison study, we have also prepared SWPF-only assisted cells and conventional cells without both SWPF and LWPF. Their optical characteristics are compared in Fig. 2(b) and (c). As shown, LWPF clearly suppresses the transmission of any unabsorbed blue light and SWPF enhances the green and red emissions through recycling process of the backward emission.
3.2. Operation of color-by-blue display coupled with DEB light-valve
For a detailed characterization of the EC cell with the DEB-based shutter, we measured the transmission under the external voltage ranging from 0 to 3.5 V (Fig. 3(a) ). With no external voltage, no absorption spectra were found in the visible wavelength range. This corresponds to the transparent cell display, allowing the blue excitation light to reach the front of the cell. On the contrary, with the external voltage, the absorption spectra appeared with the maximum absorbance at 475 nm, and further increase in voltage led to the strong suppression of blue light due to the electrochemical reduction of DEB molecules, turning the EC cell off. Between 0 and 3 V, the change in transmittance (Δ%T) of the EC cell under blue LED backlight at 450 nm is about 75.9%, which is large enough for the required backlight modulation.
Figures 3(b), (c) and (d) show the actual color generation from RGB pixels with DEB-based light valve as well as the intensity spectrum of each color as a function of voltage. By varying the operation voltage, various tones of color from RGB pixels were achieved. This clearly demonstrates that the careful tuning of the excitation light with the DEB-based EC shutter is possible by controlling the absorption in the DEB light valve at the blue color region. We would like to also point out that above 3 V, the optical changes are extremely small to be distinguished by a human eye, even under a spectrophotometer. This sets the maximum and minimum levels of the operation voltage. The EC cell is ON with 0 V and OFF with 3 V, which is comparable to the level required in typical battery-operated display modules. From the transmission and photoluminescence spectra of RGB pixels, the contrast ratio of each pixel has been determined as the ratio between the spectrum densities measured at the ON and OFF states. The values were 200:1 for blue, 150:1 for green and 170:1 for red, adequate for the purpose of full-color display.
3.3. Brightness and color gamut of RGB cells
For the construction of a full color image, the relative brightness and color coordinates of tri-color cells are also important to control the level of white color and the balance between RGB colors. The normalized brightness and color coordinates of the SWPF/LWPF-assisted RGB cells were considered and compared with conventional and SWPF-only assisted cells. However, the responses of these cells with different optical structures will also vary with the given phosphor concentration. Therefore, for the detailed comparison study, we need to determine the phosphor concentration in the phosphor layers prepared with both SWPF and LWPF, providing the same level of color purity and color coordinate as those of the conventional and SWPF-only assisted cells. To evaluate the effect of phosphor concentration in the green (Fig. 4(a) ) and red phosphor layers (Fig. 4(b)), various amounts of green and red powder phosphors between 30 and 80 wt% were dispersed in a silicon binder, and the resulting phosphor pastes were printed on the SWPF structure with 50 µm thickness. For the full down-conversion, the low concentrations of green (50 wt%) and red (60 wt%) phosphors in the silicon binder matrix were enough for the SWPF/LWPF-assisted RG phosphor layers to obtain the matching chromaticity coordinates to those of the conventional and SWPF-only assisted cells, containing a high concentration (80 wt%) of RG phosphors  (Fig. 4(c), (d)). At these concentration levels, each cell exhibits the Commission International del’Eclairage (CIE) color coordinates near (0.30, 0.65) for green and (0.63, 0.32) for red.
The corresponding relative brightness of the InGaN blue, SrGa2S4:Eu green, and CaAlSiN3:Eu red cells in the conventional structure were only 1.0, 4.0, and 0.6, respectively. These are too low to be balanced with blue for the proper white and full-color generation. With only SWPF, due to the enhanced forward green/red emission, the relative brightness of blue, green and red cell were 1.0, 8.5, and 1.3, respectively. The addition of the LWPF layer has further increased the ratio to 1.0, 10.9 and 2.1, demonstrating that the relative brightness ratio of the tri colors obtained from the SWPF/LWPF-assisted cell structure is comparable to those of commercialized CRTs or AMLCDs (Table 1 ). In fact, the measured values of the luminance from each of SWPF/LWPF-assisted RGB cells at 0 V were 310, 3380, and 650 cd/m2 (Fig. 5(a) ), indicating that white luminance above 500 cd/m2 can be easily obtainable from this emissive color-by-blue approach.
The inset of Fig. 5(a) shows the CIE color coordinates of the SWPF/LWPF-assisted RGB EC cells compared with the CIE color coordinates from the National Television Standard Committee (NTSC). The CIE x,y color coordinates (0.15, 0.02) of the emission spectrum from the blue cell was deep enough to comply with the NTSC blue coordinates. The chromaticity coordinates (0.30, 0.65) of green SrGa2S4:Eu and (0.63, 0.32) of red CaAlSiN3:Eu phosphor layers, chosen as above, were also close to the NTSC green/red coordinates and 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 . In addition, the color gamut of the color-by-blue DEB-based EC display corresponds to 77% of that of the NTSC triangle. Given that a typical display panel needs to have a color purity of at least 70% of the NTSC color gamut, the color gamut of the proposed method would certainly meet these requirements.
3.4. Cycling durability and switching response of EC cell
The cycling durability and switching speed of the DEB-based EC cell were tested with 0.1 Hz square wave varying between 0 and 3 V. During the continuous operation over 1000 cycles, it did not show any significant drop in the emission intensity, as shown in the inset of Fig. 5(b). This reversible electrochromic performance of the EC cell observed here confirms the stable contemporary redox reaction of ferrocene in TBAP solution [10,19]. The small change in the reflectance, approximately less than 10% after 1000 cycles, results mostly from the improper sealing of the cell and the inadequate deaeration of the electrolyte. Figure 5(b) shows the detailed transmittance response of the DEB-based EC cell as driven between ON and OFF states. The rise time during the coloration process, TON was defined as the time required for the reflectance to reach from 10% to 90% of its full reflectivity and the fall time, TOFF is determined from the time interval between from 90% to 10% of its full reflectivity during the bleaching process. The switching on and off of blue transmission in the DEB-based EC cell with an active area of 0.5 x 0.5 cm2 and the cell gap of 25 µm occurred with TON of 274 ms and TOFF of 87 ms during each of the coloration and bleaching processes.
Since, in most of organic-based EC devices, the switching time depends on the active area and the cell gap of the device, we varied the active area and the cell gap, and observed the changes in TON and TOFF. Figure 5(c) shows the switching times, TON and TOFF for the different cell gap thickness. When the cell gap thickness was varied with a constant active area of 1 x 1 cm2, TON decreased almost linearly with the gap thickness, reaching a value of 212 ms for the gap thickness of 25 µm, while TOFF showed only very small variation between 265 ms and 367 ms. Since the reduced form of DEB molecules, obtained during the electron migration between the electrodes, is responsible for the blue absorbance in the cell, the smaller value of TON, corresponding to the faster coloration process, is possible in the smaller gap due to the much shorter electron migration distance as well as the higher concentration of DEB species. On the contrary, since the most of the reduced DEB molecules aggregate near the electrode surface, the size of the gap does not affect the bleaching process significantly, resulting in almost constant TOFF regardless of the gap size. Even though the smaller gap size is expected to further improve the response of the coloration, the size of the cell gap demonstrated here has been limited to 25 µm only because of the saturated solubility of DEB in an NNP solution. Also, the switching characteristics of the DEB-based EC cells with various active areas ranging from 0.25 to 1.5 cm2 were measured at a constant cell gap of 25 µm (Fig. 5(d)). While the size of the active area did not make much difference for TON, TOFF decreased from 430 ms to 87 ms with the active area.
3.5. Conversion efficiency of color-by-blue emissive EC display
Finally, the total transmission from the emissive color-by-blue DEB-based EC display has been estimated, which can tell us the conversion efficiency of the proposed method. The total amount of backlight transmitting through the EC cell can be determined from the pixel fill factor, the overall transmission loss through the blue sub-pixel, and the additional color conversion efficiencies of red and green sub-pixels. Since the ITO-sandwiched EC cell blocks 25% of the blue backlight, even for a modest value of the fill factor of 70%, 52.5% of the initial blue backlight can transmit through the blue sub-pixel and reach the color conversion layer on the green/red sub-pixel. Since the experimentally determined conversion efficiencies of the SWPF/LWPF-assisted SrGa2S4:Eu and CaAlSiN3:Eu phosphor layers under the blue excitation are 52 and 38%, respectively, and the conversion efficiency of the blue sub-pixel in this color-by-blue approach can be taken as 100%, the average conversion efficiency of a single pixel, regardless of its color, would be 63%. However, to achieve the color temperature of 10,000 ~12,000 K as in the typical LC display, one needs to consider the balance between three primary colors. From separate measurements of InGaN as well as green (SrGa2S4:Eu) and red (CaAlSiN3:Eu) phosphor converted monochromatic LEDs at 12,000 K, we have found that the relative ratio between the spectral areas under the intensity distribution curves of individual RGB cells across the visible range needs to be 1: 1.15: 1.04 (Fig. 6(a) ). When the intensity spectra were measured from each of the fabricated RGB EC cells, due to the low conversion efficiency (38%), the spectral area under the intensity spectrum of the red cell is relatively smaller than those of green and blue cells (Fig. 6(b)). This implies that to maintain the appropriate spectral area ratio for 12,000 K, it is necessary to adjust and reduce the blue transmission and green emission according to the intensity output from the red EC cell, which leads to the lower conversion efficiencies of blue (37%) and green (42%). Therefore, the total transmission efficiency of the color-by-blue EC display is 52.5% × 39% = 20.5%. The total conversion efficiency estimated here is still about four times higher than that of the conventional AMLCD technology for the same backlight intensity.
The work presented here shows that the color-by-blue emissive EC display is an outstanding potential candidate for full color displays. By employing DEB for the electrochromic shutter, the effective backlight modulation was achieved as well as greatly enhanced conversion efficiency due to the implementation of SWPF and LWPF coupled with an InGaN-based blue LED, which has been the main drawback in conventional emissive displays. Further evaluation of the proposed method has revealed that its display characteristics such as high contrast ratio, high color gamut, and reliable switching under low operation 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. At the moment, the responses during the coloration and bleaching processes seem rather slow, as in many organic based EC devices. However, plenty of room for improvement surely exists through the optimized design of the display unit, as we have demonstrated the relation between the switching characteristics and the EC cell structure parameters.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea (No. 2011-0017449 and ERC-R11-2005-048-00000-0).
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