Color purifying optical nano-thin film for three primary colors in

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Color purifying optical nano-thin film for three primary colors in optoelectronics Jun Hee Han, Dohong Kim, Tae-Woo Lee, Yongmin Jeon, Hoseung Lee, and Kyung Cheol Choi ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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Color purifying optical nano-thin film for three primary colors in optoelectronics Jun Hee Han†, Dohong Kim†, Tae-Woo Lee†, Yongmin Jeon†, Ho Seung Lee† and Kyung Cheol Choi†,*

†

School of Electrical Engineering, the Korea Advanced Institute of Science and Technology,

Yuseong-gu, Daejeon 34141, Republic of Korea

KEYWORDS. nano film, optical filter, optoelectronics, nano cavity, color purity, displays

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ABSTRACT.

Numerous optical films have been developed to implement optoelectronics with advanced performance. In this study, we propose a color purifying optical nano-thin film that improves the performance of optoelectronics by filtering the white light to have a spectrum composed of pure three primary colors of red, green, and blue. It was experimentally confirmed that a wider color gamut that covers 176.33 % of the sRGB could be expressed when the suggested optical nano-thin film was applied to a display system consisting of a white back light unit and color filters. Furthermore, a full width half maximum value of 20 nm on average was observed in the three primary colors. When this film was applied to light recording optoelectronics, such as cameras, it acts as a multiband-pass filter that increases the sensitivity of the three primary colors. The principle of this optical nano-thin film is based on multiple light resonance inside the film. A theoretical analysis and simulations were conducted to design the structure of the nano-thin film and optical characteristics were verified by both experiment and simulation. Because it is fabricated by in-situ thermal evaporation it provides advantages in terms of fabrication time and cost, and it also has potential to be fabricated with well-established deposition equipment such as a sputter apparatus. The results of this paper show that nanoscaled thin films sufficiently control the optical phenomenon with a simple structure, implementing advanced optoelectronics.


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INTRODUCTION.

There are technological limitations to express or accept all colors in the world, and even if there were a technique to use all colors, it would not be efficient to express or accept all colors for optoelectronics. Therefore, the primary colors, red, green, and blue, (RGB), are used in optoelectronics because these three colors can represent various colors by being combined in various ratios.1 When representing colors with RGB in light emitting optoelectronics, especially for displays, the purity of RGB determines the color space, which consists of a subset of colors.2–4 That is, as the purity of the RGB is increased, more colors can be represented because the area of a triangle that has RGB color points as three vertices on a CIE 1931 chromaticity diagram determines the number of colors that can be composed.5 Displays such as liquid crystal displays (LCDs) and white emission type organic light emitting diodes emit color via color filters.5–7 However, it is not easy to realize pure RGB with color filters, especially in the case of LCDs that have white back light units (BLU) composed of blue and yellow, because they are based on a pigment or dye that has a large full width half maximum (FWHM) value.8,9 For this reason, various studies have been carried out to realize pure RGB colors. An optical filter consisting of nanostructure is one of the candidates for implementing pure RGB colors.10–18 However, process difficulties impede its potential application industrially because its ability of color representation comes from a precisely fine patterned nanostructure that can be influenced by just a few nanometer scale changes. Another candidate is a multilayered color filter,7,19–24 but its FWHM is still too large to realize pure RGB colors, and also some alternative materials are not sufficient to realize pure colors.25–27 Instead of implementing colors using color filters, displays consisting of self-emitting sub-pixels such as organic light emitting diodes (OLEDs) have been studied. Even though OLED displays cover DCI-P3 color


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space, which is a wide color gamut for ultra-high definition displays, they are insufficient to enhance the sense of reality by expressing various colors.28–32 More recently, studies have been conducted to implement Rec. 2020 color space, which is wider than DCI-P3, and the only display to realize this space thus far is quantum dot light emitting diodes (QLEDs).33,34 However, quantum dots should be made in uniform size to achieve a specific color and should be patterned in sub-pixel size for each primary color, and thus QLEDs have disadvantages in terms of fabrication process. Furthermore, toxic components in quantum dot have been an ongoing issue whereas non-toxic components accompany cost issues. Color filters that have a large FWHM or cyan, yellow, green, and magenta (CYGM) colored are used in front of complementary metal oxide semiconductor (CMOS) image sensors to absorb and detect various colors in the case of light recording optoelectronics such as cameras.35–37 In order to capture images that capture reality, large FWHM color filters or CYGM color filters are useful, but in the cases where RGB should have higher sensing intensity for vivid images, or when red, green, and blue colors are extracted separately, image processing with a computer based on complicated algorithms is required. An optical film used as a multiband-pass filter in special cases to increase the sensing intensity of RGB would be able to record the desired image with vivid color without a complicated process.38 This study presents a color purifying optical nano-thin film that improves the performance of optoelectronics by filtering white light to have a spectrum composed of pure three primary colors of red, green, and blue. This optical film increases the color gamut of displays up to Rec. 2020 covering 176.33 % of the sRGB. Furthermore, a FWHM value of 20 nm on average for RGB colors was experimentally observed in a display system consisting of a white BLU and color filters, showing similar specifications to QLEDs. When this film was applied to light recording optoelectronics, it performed as a multiband-pass filter that increases the sensitivity of the three primary colors and this was verified by comparing images captured with and


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without optical films. An optical theory was used to analyze the principle of this optical nanothin film and the optical characteristics of this film were verified by both experiment and simulation. The suggested optical film can be fabricated by in-situ thermal evaporation and also has potential to be fabricated with well-established deposition equipment such as a sputtering apparatus, and it thus shows advantages in terms of fabrication time and cost compared to previous studies.

RESULTS AND DISCUSSION.

Figure 1. Schematic images of main function of color purifying optical nano-thin film. (a) The graphic shows the basic structure when the color purifying optical nano-thin film is used in a


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display system consisting of a white BLU and color filters. (b) The experimentally obtained spectrum of a white light emitting diode composed of blue and yellow. (c) The spectrum of light after passing through the color purifying optical nano-thin film implementing distinct red, green, and blue divisions with peak values at wavelength of 451 nm, 526 nm, and 638 nm.

When the white BLU is composed of separated pure red, green, and blue spectra, it is possible to obtain purer RGB colors with a color filter, and the display can thereby express more colors to realize vivid scenes. The color purifying optical nano-thin film (CPF) suggested in this paper shows optical filtering property that makes white light to have a spectrum composed of pure red, green, and blue spectrum. Figure 1a graphically shows the basic structure when CPF is used in a display system consisting of white BLU and color filters. The light emitting from BLU is filtered by the CPF, and then it changed as desired via a color filter. In order to verify the performance of the CPF in the display system, an experiment was conducted with a white light emitting diode composed of blue and yellow spectrum, which is a widely used form in the display industry, as Figure 1b shows. When the light spectrum was obtained after it passes through the CPF, the spectrum has more distinct RGB divisions compared with the previous spectrum, and the peak value has wavelengths of 451 nm, 526 nm, and 638 nm, as shown in Figure 1c. Both spectrums were experimentally obtained by a spectroradiometer.


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Figure 2. Optical characteristic analysis of device with color purifying optical nano-thin film. (a), (b), (c) The light spectrum when white back light unit passes through red, green, and blue color filter with and without a color purifying optical nano-thin film, respectively. The open circle shows the spectrum of light with only red, green, and blue color filters, respectively. The solid circle shows the spectrum of light that sequentially passes through the color purifying optical nano-thin film and red, green, and blue color filters, respectively. The solid line is the transmittance data of red, green, and blue color filters, respectively. (d) CIE 1931 with color coordinates. The circle is sRGB and the triangle is color coordinates of the light when the back light unit passes through only color filters. The star is color coordinates of the light when the back light unit sequentially passes through the color purifying optical nano-thin film and color filters. (E) CIE 1931 with color coordinates. The circle is DCI-P3 and the triangle is Rec. 2020. The star is color coordinates of the light when the back light unit sequentially passes through the color purifying optical nano-thin film and color filters. (f) Photographs of red, green, and


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blue color filtered light of white light emitting diode with the color purifying optical nano-thin film, from top to bottom, respectively.

Table 1. Full width half maximum and peak wavelength values regarding to red, green, and blue colors with and without color purifying optical nano-thin film. FWHM

PEAK WAVELENGTH

COLOR w/ CPF

RED

GREEN

BLUE

w/o CPF

29 nm

58 nm

(622 – 651 nm)

(595 – 653 nm)

18 nm

57 nm

(517-535 nm)

(506 – 563 nm)

13 nm

19 nm

(445 – 458 nm)

(442 – 461 nm)

w/ CPF

w/o CPF

638 nm

613 nm

526 nm

527 nm

451 nm

451 nm

For a detailed analysis, the spectrum was demonstrated considering the case where the BLU passed through the color filters. In order to experimentally observe the spectral changes when color filters are applied, the commercially available color filter was used. The transmittance of the red, green, and blue color filter is represented by a solid line in Figures 2a, 2b, and 2c, respectively. The peak value of the spectrum was normalized to easily compare the two spectrums with and without the CPF. Figures 2a, 2b, and 2c show the light spectra when the white BLU passes through the RGB color filters, respectively. The open circle indicates the light spectrum when the white light passes through only RGB color filters, and the solid circle indicates the light spectrum when the white light sequentially passes through the CPF and RGB color filters. Compared with the spectrum obtained by only the color filter, the spectrum with the CPF shows sharper FWHM values, and as a result the BLU has a well-distinguished RGB


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spectrum. In particular, the FWHM value of green is reduced by about 40 nm when the CPF is used, thus enhancing the color purity (Table 1). As the graph shows, the FWHM values are about 20 nm, on average, with the use of the CPF, and these values are comparable to those of a QLED, which has a very sharp color spectrum.39 Detailed information about peak wavelength and FWHM is provided in Table 1. Color coordinates were calculated using the color matching function from the line spectrum represented in Figures 2a, 2b, and 2c, and marked on CIE 1931, as presented in Figures 2d and 2e. When RGB colors are implemented using only color filters, red, green, and blue color have (x, y) values of (0.6707, 0.3291), (0.2484, 0.6916), and (0.1484, 0.0352), respectively. However, it is confirmed that the color space becomes wider with the CPF, presenting (x, y) values of (0.6945, 0.3054), (0.1714, 0.7655), and (0.1523, 0.0269) for red, green, and blue, respectively. These results graphically show that color space with the CPF is 126.07 % wider than that without the CPF. The reason for the wider color space with the CPF is that the FWHM of green is sharpened by the CPF, and the peak wavelength of red color shifts to larger wavelength from 613 nm to 638 nm with a sharp FWHM. Therefore, the points of green and red on CIE 1931 move to purer green and red zone, respectively, making a larger triangle space that has RGB color points as three vertices. The color gamut with the CPF covers 176.3 % of sRGB and 130 % of DCI-P3, which is a wide color gamut for ultra-high definition displays, as shown in Figure 2d. Although many studies have been conducted to realize Rec. 2020, which is a wider color space than DCI-P3, only a few devices satisfied this level.40,41 The color space with CPF has 93.25% absolute area compared with Rec. 2020 covering 91.1 % of Rec. 2020. This reflects the high potential of the CPF to be used in displays for a wider color gamut covering most area of Rec. 2020, implementing a high quality ultra-high definition screen. Figure 2f shows real images of red, green, and blue after the white light of BLU passes through the CPF and red, green, and blue color sequentially, respectively.


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The CPF with this advantage can be used for displays without modification of the display structure or with minor modification. Application examples of the above two cases can be illustrated using LCD displays. First, as an example of a case where the CPF is utilized without structural modification, we can consider a structure in which one side of an ITO electrode to electrically control the liquid crystal is replaced with a CPF that contains metal thin films (Figure S1). Next, as a case where the CPF is additionally inserted into an existing structure, the CPF can be inserted at the front side of a white back light unit, or it can be inserted at the front or back side of the color filters (Figure S2). Both of the above cases have the advantage that they can be fabricated through deposition without a complicated process such as precise patterning.

Figure 3. Basic principle of color purifying optical nano-thin film. (a) Graphical expression of the optical principle of the color purifying optical nano-thin film. The schematic depicts the red, green, and blue light waves that experience constructive light interference between two reflectors. Therefore, the incident white light filtered to have a spectrum composed of pure


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three primary colors of red, green, and blue. (b), (c), (d) Fabry-Perot factor versus wavelength having peak wavelength at red, green, and blue region, respectively, with increased distance between two reflectors by appropriate spacing for each color.

If the distance between the two reflectors has a common multiple of the wavelength corresponding to RGB light, as shown in Figure 3a, only one particular wavelength does not undergo constructive interference, but all three can have constructive interference. The principle of the CPF is based on this phenomenon. When light is incident on a film composed of two layers of reflectors, reflected light from each reflector causes interference, and this interference determines the properties of the light. One of the important factors determining the nature of light is the distance between the reflectors.

m = 2nd cos

Eq. 1.

Assuming an ideal case in which light travels in a perpendicular direction to the reflector and the phase changes by π at the reflector, only the wavelength of light that corresponds to the distance between two reflectors, as delineated in Eq. 1, undergoes constructive interference.42,43 In Eq. 1 , m is the order of interference, n is the real part of the refractive index of the material filling between the reflectors, d is the distance between two reflectors, and  is the angle of the light incident on the reflector. Here,  is assumed to be 0 degrees, which is vertical to the reflector.


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f FP ( ) =

t2

(1 − r  r 1

2

e

)

− k0 2 d 2

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2

− −  + nk0 2d   + 4 R  sin 1 2  2  

2

Eq. 2.

In practice, however, the transmittance coefficient and reflection coefficient of the reflector should all be considered. Therefore, the Fabry-Perot factor of Eq.2 should be used considering all these conditions for the design of the CPF.42,44–46 In Eq. 2, t2 is the transmission coefficient of reflector 2 and r1 and r2 are the reflectance coefficient of reflector 1 and reflector 2, respectively. R is r1  r2  e− k0 2 d . n and  are real and imaginary parts of the refractive index of the material filling between the reflectors, respectively. k0 is the free space wavenumber ( k0 = 2 ) and d is the distance between two reflectors. 1 and 2 are the  phase change of the light at reflector 1 and reflector 2, respectively. Figures 2b, 2c, and 2d show the Fabry-Perot factor versus wavelength obtained from the calculation of Eq. 2 by changing the distance between two reflectors. In the calculation, reflector 1 and reflector 2 were fixed with silver (Ag) having a thickness of 15 nm and 20 nm, respectively, and the material between the reflectors was set as tungsten trioxide (WO3). In order to allow the CPF to transmit pure color, the reflector thickness on the outgoing side was set to be slightly thicker, referring to existing studies on optical films using light interference. 7

In Figure 2b, the graph shows that the 650 nm wavelength corresponding to red is resonated

as the distance between two reflectors is increased by about 160 nm from the initial resonance distance of 110 nm. In Figure 2c, the graph shows that the 525 nm wavelength corresponding to green is resonated as the distance between two reflectors is increased by about 125 nm from the initial resonance distance of 80 nm. In Figure 2d, the graph shows that 450 nm wavelength corresponding to blue is resonated as the distance between two reflectors is increased by about 100 nm from the initial resonance distance of 60 nm. The results show that as the wavelength


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of light becomes longer, an accordingly larger gap between the two reflectors is needed for the first resonance. Furthermore, the increased distance between the two reflectors for the next resonance is larger when longer wavelength is the target for the resonance. Figure 2 shows that the distance between the two reflectors where all blue, green, and red wavelengths resonated is about 580 nm. As the distance between the reflectors increases, other wavelengths, except for the resonance wavelength, experience more destructive interference. The destructive interference reduces the Fabry-Perot factor and this leads to a reduced FWHM value, and thus the CPF can have a sharp transmittance spectrum shape.


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Figure 4. Structure and transmittance of the color purifying optical nano-thin film. (a) Schematic representation of the design of the proposed color purifying optical nano-thin film, and an overview of the transmittance measurement system. (b) Transmittance data of the color purifying optical nano-thin film obtained from both simulation and experiment.

Figure 4a schematically shows the determined final structural design of the CPF, the thickness of each layer, and how to measure the transmittance. The CPF is composed of WO3


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(100 nm) / Ag (20 nm) / WO3 (570 nm) / Ag (15 nm) / WO3 (90 nm). The final design included capping layers, referring to previous studies that use capping layers for better color purities with optical films working based on the optical resonance.

7,47

The fabrication method of the

CPF is efficient in terms of time and cost because it can be fabricated in-situ using thermal evaporation. Figure 4b shows the transmittance data of the CPF. A simulation was performed by calculation of the characteristic matrix, since the transmittance of the CPF, which has a multilayer structure, cannot be calculated only with the Fabry-Perot factor.7,45,48 Transmittance data show that only the wavelengths corresponding to RGB are transmitted, as intended, and the divisions of each color are clear. In the final design structure, the distance between reflectors was adjusted from 580 nm to 570 nm, considering the additional conditions that were not considered in the Fabry-Perot factor. These conditions include the transmittance of reflector 1 for incident light, the phase change that occurs when the light passes through reflector 1, and the capping layer. FabryPerot factor assume that the incident light does not experience any disturbance such as absorption or phase change while it goes though the films. However, in the real case, the light experience absorption and phase shift while it comes into the films. Thereby, those conditions should be considered for the accurate calculations of transmission of CPF. In the other words, multiplying those two conditions to Fabry-Perot is transmittance of CPF. When these conditions were not considered, the peak wavelengths were red shifted, as given by the open circle line in Figure 4b, which presents the transmittance simulation results of the structure composed of two reflectors having a gap of 580 nm filled with WO3. The experimentally measured transmittance data of the structure considering additional conditions are indicated as a red solid line. As the graph shows, the peak values of the transmittance are increased and the FWHM became narrower than the open circled line. Furthermore, the graph shows that the


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peak values correspond to the wavelengths of 448 nm, 521 nm, and 635 nm, which indicate blue, green, and red regions. The shape of the experimental data was similar to the simulation results, which are marked as a solid circle line with peak wavelengths of 453 nm, 527 nm, and 641 nm. The consistency between the measured and simulated data demonstrates that CPF operates as intended and the basic principle of this filter is optical theory.

Figure 5. Scanning microscope images of color purifying optical nano-thin film (a) Image enlarged to 10,000 times. (b) Image enlarged to 65,000 times.

Figure 5a is a cross section image of the manufactured CPF on a glass substrate observed by a scanning electron microscope (SEM). This image was taken after being enlarged to 10,000 times. Figure 5b is a SEM image enlarged 65,000 times. It shows that each layer has similar thicknesses compared with the designed structure, as given in Figure 4a. Thin Ag films could be identified and it is also confirmed that the Ag is deposited in the form of thin films. The


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thickness of each layer was measured by the software of the SEM system and redrawn for clear visibility.

Figure 6. Application of color purifying optical nano-thin film on light recording optoelectronics. (a) The configuration of the camera including application form of the color purifying optical nano-thin film. (b)-(g) Photographs taken by the camera without the color purifying optical nano-thin film. The graphs next to the photos show the number of pixels corresponding to hue angles. The insets show the color spectrum that matches the hue angles. (h)-(m) Photographs taken by the camera with the color purifying optical nano-thin film. The graphs next to the photos show the number of pixels corresponding to hue angles. The insets show the color spectrum that matches the hue angles. The photographs were taken at the same brightness condition of images and there were no further corrections after shooting.


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As the transmittance data of the CPF show in Figure 4b, it performs as a multiband-pass filter. Therefore, when it is applied to optoelectronics that records the light, it helps to increase the sensitivity of the three primary colors in an easy manner. Figure 6a shows the configuration of the camera, which is a representative form of light recording optoelectronics. The main elements of the camera are the lens, color filter, and integrated circuit, as shown in the schematic. Contrary to the display case, a color filter with a large FWHM is used for the camera to record as many colors as possible. Some devices accept a CYGM colored filter to absorb and detect various colors. Even though, these color filters are useful to capture images that accurately reflect the real scene, sometimes there are cases where higher sensing intensity of the three primary colors is required for a specific reason such as to analyze or modify images. Furthermore, increased sensitivity of RGB not only provides a vivid image, but can also be useful to extract R, G, and B separately. However, in order to increase the sensitivity of light recording optoelectronics for RGB, image processing based on complicated algorithms with a computer is required. When the CPF is used in this case, it works as a multiband-pass filter transmitting the primary three colors, and helps optoelectronics to increase the sensing intensity of RGB. In order to confirm the functional change of the camera, images were taken with and without the CPF. Figures 6b, 6d, and 6f are images taken without the CPF, and Figure 6h, 6j, and 6l are images taken with the CPF. As the figures show, the images taken with the CPF had more vivid red, green, and blue colors compared with the images taken without the CPF. For a more detailed analysis, the hue components of the images were observed by MATLAB. Because the hue, saturation, and value (HSV) component of an image can reduce the error of data due to the brightness of the image rather than the RGB component when the colors of pixels are


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analyzed, the hue component was used for confirmation of the colors of images. This is because hue (H) has an orthogonal property related to the brightness (V) component. Figure 6c and Figure 6i show the number of pixels matching the hue angles in figures 6b and 6h, respectively. Considering the inset image of the color spectrum matching the hue angles, it was confirmed that the sensitivity of red color increases. The number of pixels between 35° and 45° indicates the orange color decreased to 16,503 from 49,166, while the number of pixels between 10° and 20° indicates the red color increased to 30,783 from 16,589. Furthermore, overall the pixels were shifted to lower hue angles as the graph indicates when the CPF was used. This phenomenon happened for the CPF that filters the light to consist of pure three primary colors reducing the transmittance of other wavelengths. In the case of Figure 6d and 6j, most pixels were shifted to hue angles of around 150° from around 110°, as shown in Figures 6e and 6k. Considering that the 520 nm wavelength corresponds to a hue value of around 160°, it can be confirmed that this result is derived from the CPF, which has a peak value at 520 nm. For Figures 6f and 6l, the analyzed data of Figure 6g and Figure 6m show that the CPF filtered the light concentrating the number of pixels around 220°. The results from the experiment confirmed that the CPF can be used to increase the sensing intensity of RGB in special cases by filtering the other colors without complicated image processing with a computer. For practical applications, follow-up studies are required. Since the operation principle of the CPF is based on light interference, there might be a possibility of variation of the transmittance according to the angle, and this problem can cause a color change according to the viewing angle in the display. This problem can be solved by using proper materials, optical design adjustment, and polarized light, because the properties of the electromagnetic wave inside the multilayers, such as phase shift and optical admittance, are changed depending on the polarized light. Furthermore, various studies have been conducted to reduce the angular


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dependency in optical films composed of multilayers.6,23,47 We have also experimentally confirmed that the angular property of the CPF can be controlled by polarized light, as in previous studies (Figure S3). Next, it is necessary to increase the transmittance because low transmittance causes a decrease in the overall efficiency of displays. In the CPF structure using light interference, the light absorption in the metal is the main reason for lowered transmittance, because the light is absorbed during the reflection and transmission process at the metal interface. Through simulations we have confirmed several methods to increase the transmittance, and one method is replacing the thin metal film with a material that has a low real value of refractive index without an imaginary part, which is related to the absorption. By this method, the transmittance of blue, green, and red was improved by about 20 % (Figure S4). Therefore, the problems noted above might be solved by adjusting the materials and polarized light. As numerous material studies to obtain a small real refractive index have been carried out, it is expected that the CPF will be further developed in the future.49

CONCLUSIONS. In conclusion, we have developed a color purifying optical nano-thin film that increases the color reproduction ratio and works as a multiband-pass filter for optoelectronics by using a nano-cavity resonance phenomenon inside the film. When the film was fabricated as intended, it works as a color purifier clearly filtering only the red, green, and blue spectrum with a white BLU. Furthermore, it was experimentally confirmed that a wide color gamut similar to Rec. 2020 covering 176.3 % of sRGB could be implemented by the CPF with a spectrum of 20 nm FWHM. Furthermore, high potential to be used for light recording optoelectronics as a multiband-pass filter was confirmed by analyzing images taken with and without the CPF. Since the CPF is fabricated by in-situ thermal evaporation, it has advantages in terms of


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ACS Photonics

processing time and cost. The results from this study provide practical methods that can be used in optoelectronics for pure three primary colors, and demonstrate that the proposed nanoscaled thin film with a simple structure sufficiently enhanced the optical properties for advanced optoelectronics.

MATERIALS AND METHODS Sample Preparation. The CPF was manufactured on a glass substrate cleaned with isopropyl alcohol in an ultrasonic bath. WO3 (1-4 mm pcs 4N, Tasco) and Ag (3-5 mm granule 4 N, Tasco) were deposited alternatively by thermal evaporation with a vacuum pressure of

Color purifying optical nano-thin film for three primary colors in

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