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Abstract
In this work, a 25 inch (400 × 500 mm) transparency-adjustable mini-LED (TA-MLED) display is constructed of a transparent mini-LED (T-MLED) screen and an electrochromic (EC) shutter. The shutter shows a high transmittance of 86.5% with imperceptible color shift, enabling a perfect vision experience for see-through application. Furthermore, the response speed of the shutter is accelerated by optimal designs in splicing and driving. The coloring time is 55 s, and bleaching time is 36 s. Transmittance of the TA-MLED could be modulated from 3% to 60%. The transparency-adjustable property extends availability of the see-through display screens under strong light irradiations. The T-MLED’s color gamut in CIE 1976 shrinks from 145.1% sRGB to 3.6% sRGB with 5161 cd/m2 of backside illumination, and is significantly enhanced to 83.5% sRGB with the active EC shutter.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the explosion of display technology research, various functions are developed into the next generation display devices. Flexible, foldable, transparent, reflective and near eye displays come out with various structural forms [1–5]. Among those, the see through display receives remarkable attentions which could be widely applied in the area of vehicle’s head up displays, window displays, public boards and military multi-information interactive displays [6,7].The see through display means that the screen could exhibit colorful streaming media and keep transparent at the same time. These display devices with large aperture ratio are designed to minimize driving and light-emitting area and maximize the transparent area [8,9].
At present, transparent mode has been applied into OLEDs, LCDs and laser projection displays, especially the transparent OLEDs (T-OLED) have been pushed into the market and achieved preferable feedback [10–12]. But there still exist some problems which deteriorated performance of the display. The ambient light would pass through the transparent screen and mix into the screen’s emitting light, thus the display would suffer low contrast ratio, distorted color gamut and low picture quality which limit commercial applications of the see through displays [13–15]. To resolve above problems, several light shutter technologies, such as dye-doped liquid crystal, polymer dispersed liquid crystal, electrochromic and electromotive curtain, are introduced as the display accessory to absorb/reflect ambient light [16–18]. Electrochromic (EC) is extensively used in the smart windows because of its high optical-modulation, long term memory effect and low energy consumption. The 30 × 30 mm EC device has been applied to the T-OLED display as the light shutter, which exhibited high transmittance of 85.56% and high contrast ratio of 85.5: 1 at 562 nm [19]. However, the color shift and difficulty of large scale fabrication of the EC shutter results a large gap between experimental devices and outdoor displays in the previous researches. With the larger size, EC devices always suffer serious problems of slow response speed, processing complexity, and voltage drop. Even more, the transparency of EC shutter needs to be further improved, which directly influence the color shift and quality of the see-through display.
In this work, we developed a 25 inch (400 × 500 mm) EC shutter with high transparency and small color shift. We further assembled the EC shutter with a transparent mini led screen to achieve a transparency-adjustable mini-led screen (TA-MLED). The TA-MLED device possessed an improved transmittance modulation range from 3% to 60%, and high luminance of 2005 cd/m2. The EC shutter device was patterned as 4 areas to drive separately for accelerating switching response. And the transparency adjusting function exhibited enhanced switch speed, 36s for switching to transparent state, and 55s for switching to dark state. The CIE 1976 chromaticity gamut area of the TA-MLED was reduced from 145.1% sRGB to 3.6% sRGB when a 5161 cd/m2 light source was placed on the backside of the EC shutter, but recovered to 83.5% sRGB with the active EC shutter.
2. Experiments and characteristics
2.1 Preparation of EC shutter device
The EC shutter device was prepared via all solid physical vapor deposition (PVD), as the Ref. reported [20,21]. The ITO, WO3, SiO2, NiO were deposited by e-beam evaporation and Li layer was prepared by resistive heating evaporation. Li layer was deposited between SiO2 and WO3 as ionic source to dope into WO3. The device was annealed at 450 °C for 10 minutes in the vacuum ambiance of 10−6 Torr, then annealed at 350°C for 1 h in the air. After that, the EC device was pasted with carbon conductive tape as the driving line. Finally, the anti-reflective films were attached on the external surface of the shutter.
2.2 Preparation of the TA-MLED device
The device was assembled by a see-through MLED screen and an all-solid EC device, and the MLED screen was fabricated by 3 steps: Sn solder paste printing, LED die bonding and reflow soldering, which is reported previously [8]. Here, a high aperture ratio of driving substrate was designed for transparency. After LED soldering, the screen was sealed by silica gel self-leveling for barrier water and oxygen. Finally, the silica gel was self-leveled as thickness of 5 mm upon the EC device to seal the cell with the MLED display.
2.3 Characterization and performance tests
Focused Ion beam-scanning electron microscope (FIB-SEM, Zesis GeminiSEM 300), and a digital microscope (Olympus, DSX150) were used to reveal the morphology and uniformity of the films. The electrochemical performance was measured by an electrochemical workstation (Chenhua, CHI660e). The optical performance was measured by spectroradiometer (Knoica Minolta CS 2000) and Otsuka Electronics (MPRT-2000).
3. Result and discussion
As Fig. 1(a) shows, the T-MLED device was assembled by the TFT substrate, mini LED chips, silica gel and top anti-glare glass. The RGB mini LED chips were arrayed with the pixel pitch of 2.25 mm, and the size of the LED chip was about 125 × 225 µm. A 25 inch (405 × 486 mm) screen with a resolution of 180 × 216 was finally prepared. The aperture ratio of the TFT substrate was designed above 76.5% to maintain the transmittance. The transmittance spectrum of the T-MLED was shown in Fig. 1(b) with average transmittance of 69%. The transmittance spectrum shows only a little color shift in visible light spectrum area. To clarify the effect of light source on the display performance, a backlight unit(BLU) of 5161 cd/m2 was placed behind the T-MLED to measure the radiance spectrum and the color gamut. Figure 1(c) and 1(d) indicate the spectral radiation and color gamut of the T-MLED with and without the BLU. The displayed color was dramatically faded due to the enlarging of full width at half maximum (FWHM) of RGB peaks with strong ambient light, which led the dramatic reduction of the CIE 1976 color gamut from 145.1% sRGB to 3.6% sRGB. It was mainly consistent with the simulation results in Fig. S1 and Table S1 (Supplement 1).
Fig. 1. (a) Scheme of the T-MLED structure; (b) transmittance spectrum of the T-MLED; (c) spectral radiance of T-MLED exposed BLU of 5161 cd/m2; (d) color gamut of the T-MLED with a rear BLU in CIE1931 xy diagram.
To resolve the loss of display color gamut, the EC shutter was placed rear of the T-MLED to absorb the backlight and restore the vivid image as Fig. 2(a) shows. The EC shutter device was designed as a typical film structure, where the EC structure is Indium Tin Oxide (ITO)/ Li-WO3 / SiO2/ NiO/ ITO from bottom to top [22–24]. To minimize the color shift between the EC shutter and T-MLED screen, the chromaticity of the EC shutter was simulated by rigorous coupled-wave analysis in the SetFOS software. The reflective index and extinction coefficient of each material were introduced into the setting, and the ITO thickness was controlled as 350 nm to provide low sheet resistance. The CIE1976 chromaticity of u’ and v’ was fluctuated as functions of the thickness of WO3 and NiO as Table S2 shows (Supplement 1). According to other researches, the thickness of WO3 and NiO are deposited at the range of 400-600 nm and 150-300 nm for high optical modulation and good cycling stability [25]. The CIE1976 chromaticity of the EC shutter was obtained as Figs. S2a and S2b show (Supplement 1) with different thickness of WO3 and NiO. The color shift between the simulated EC shutter and T-MLED screen was defined as the following equation:[26]
Fig. 2. (a) Function scheme of the EC shutter; (b) contour map of the simulated CIE 1976 color shift and the thickness of WO3 and NiO; (c-f) morphology and performance of the EC device: c) cross-section FIB-SEM image; d) CV curves of 1st and 1000th cycle; e) transmittance spectrum at colored and bleached state; f) curves of the device response; (g) optical photographs of the T- MLED with and without the EC shutter.
The u’EC, u’MLED, v’EC and v’MLED are CIE 1976 chromaticity coordinates of the EC shutter and T-MLED, respectively. The relationship between color shift and thickness of WO3 and NiO could be found in Fig. 2(b). The Δu’v’ achieves lowest value with 480-500 nm WO3 and 180-200 nm NiO, and the highest transparency can be achieved with 500 nm WO3 and 200 nm NiO at the wavelength of 550 nm as Fig. S2c shows (Supplement 1). Based on the simulated results, the all solid EC shutter was successfully prepared. The cross section morphology of each layer was measured by FIB SEM, and exhibited in Fig. 2(c). The thickness of each layer were about 350 nm, 500 nm, 20 nm, 200 nm and 350 nm, respectively. In contrast with other EC devices in Table S3 (Supplement 1) [27,28], this EC shutter actually revealed a CIE 1976 transmitted chromaticity of u’=0.2007 and v’=0.4755 at transparent state, which were almost equal to the simulated chromaticity, further approximately equal to the T-MLED display. The transmitted chromaticity of Δu’ and Δv’ between T-MLED and EC shutter were as small as 0.0005 and 0.0014, respectively. Displays with color shift would suffer image distortion and gamma inaccuracy, which brought uncomfortable viewing experience. The little color shift provided remarkable potential for the applications of the EC shutter as Fig. 2 g shown. The cyclic voltammetry method from -1.5 V to +3 V with a scanning rate of 10 mV/s was taken on the device to evaluate the electrochemical behaviors of the EC shutter in Fig. 2(d). After 1000 times cycling, no apparent decrement of current intensity was found, which testified the remarkable electrochemical stability. Transmittance spectrums of EC shutter at redox state are exhibited in Fig. 2(f), the transmittance changed from 3.8% to 86.5% for totally bleaching and coloring. However, Fig. 2(e) presents a slow response speed with coloring time and bleaching time of 5 minutes and 2 minutes, respectively. The slow response speed was mainly attributed to the in-plane resistance of ITO of the large size device.
The response time of different size EC shutters of 100 × 100 mm, 200 × 250 mm and 400 × 500 mm were measured and presented in Fig. 3(b), 3(a) and 2(e), respectively. The smaller size indicated short electric transport distance, meanwhile, brought faster response. For accelerating the response behavior and simplifying fabrication, the 25 inch (400 × 500 mm) EC device was then divided into 4 areas. To reduce splicing gap, the whole mask with 4 active areas was used to accurately prepared the patterned 4 EC devices. The design of driving substrate was schematically illustrated in Fig. 3(d), and the gap between each area was less than 2 mm. The response speed was enhanced with coloring time of 55 s and bleaching time of 36 s as Fig. 3(c) shows, and the patterned EC devices retained original transmittance modulation.
Fig. 3. (a-c) Response time curves of EC device with different sizes and type: a) 200 × 250 mm; b) 100 × 100 mm; c) 4× patterned device of 400 × 500 mm with L shape wire; (d) the scheme of various EC devices.
Figure 4(a) presents photos of 2 × 25 inch tile TA-MLEDs with EC working at on and off states in left and right sides, respectively. With same displayed information, the details of a man watching cellphone is obvious in left display zone, but is hardly distinguishable in right side. In addition, the right EC device could quarterly switch with the TA-MLED as Fig. S3 shows (Supplement 1). When the EC shutter turned off, the behind plants were clearly observed through the transparent zone, which is favorable for see-through applications. On the contrary, the dark section showed better image performance and the color saturation was strongly enhanced by the EC light shutter. In Fig. 4(b) and Fig. 4(c), the EC device provided 20 different transparency states with different applied voltages to adjust gray scale of the shutter, which could be integrated with ambient light sensors for accurately control. The transmittance decreased from 60% to 3% with the increasing potentials as Fig. 4(c) shows, and the contrast ratio reached 30: 1 at the wavelength of 680 nm. Figure 4(d) exhibits the spectral radiance of T-MLED and TA-MLED with different shutter states under the strong BLU behind the display. The FWHM and intensity of RGB peak of TA-MLED with working EC shutter were almost equal to those of the original T-MLED. As shown in Fig. 4(e), the color gamut was reverted from 3.6% sRGB to 83.5% sRGB with the active shutter. Furthermore, the video of TA-MLED with different shutter states with an acceptable response speed was presented in Visualization 1.
Fig. 4. Optical performance of TA-MLED: (a) photograph of 2 × 25 inch TA-MLED with different EC states (see Visualization 1); (b) transmission spectrum of TA-MLED with different applied voltages from 0 V to 2.0 V in step of 0.1 V; (c) gray level of transparency with applied voltage; (d, e) spectral radiance and color gamut diagram of T-MLED and TA-MLED in different states with 5161 cd/m2 from the BLU.
4. Conclusion
In this paper, we successfully fabricated a 25 inch transparency-adjustable mini-LED display with a high transparent electrochromic shutter with little color shift. The transmittance of the TA-MLED could be modulated from 3% to 60% in visible range and the contrast ratio reached 30: 1 at the wavelength of 680 nm. The transmittance could be effectively controlled and achieved 20 gray scales by applying voltage from 0 V to 2 V. The bleaching and coloring response of the adjustable shutter are 36 s and 55 s with the optimizations in driving and patterning. The EC shutter could absorb most visible light to prevent the crosstalk of ambient light, further to enhance the display performance thus the color gamut of the transparent mini LED display was significantly enhanced from 3.6% sRGB to 83.5% sRGB. The experimental results and TA-MLED display implied a great potential of the transparency-adjustable EC shutter for various displays.
Funding
Science and Technology Planning Project of Guangdong Province (2021B1212050009).
Acknowledgements
This work was supported by Science and Technology Planning Project of Guangdong Province (2021B1212050009).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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