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Abstract
A color wheel (CW) is one of the most essential devices for contemporary projection displays because it provides the color initialization definition and determines the color performance of the whole system. However, conventional color wheels remain limited in terms of color performance and efficiency because of the light-absorbing material and time sequential color generation. Quantum dots, found in 1981 and known as a kind of quasi-zero-dimensional nanomaterial, exhibit excellent features for displays due to their quantum confinement effect, which won the 2023 Nobel Prize in Chemistry. Inspired by this, the paper systematically demonstrates a quantum-dot color wheel (QD-CW) device through theoretical derivation, simulation analysis, and experimental verification. The theoretical model to define the duty circle ratio is presented for the QD-CW and verified by Monte Carlo ray-tracing simulation. In terms of experimental verification, the QD-CW device is realized by multiple rounds of a photolithography process, and then assembled into a blue laser pumped projection prototype for full-color display. The chromaticity coordinates of white-balanced output are finally located at (0.317,0.338), which matches well with a standard D65 source. The color gamut area of the QD-CW device reaches 116.6% NTSC, and the average light conversion efficiency (LCE) of the prepared QD-CW is 57.0%. The proposed QD-CW device has ${\sim}{40}\%$ higher color gamut area and ${1.2} \times$ higher LCE than a conventional CW device. These exciting findings show a groundbreaking approach to color generation in projection displays, which are expected to shed light on other high-quality display applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. INTRODUCTION
The optical engines of current mainstream projectors are commonly composed of a light source, an illumination system, a single micro display chip, and an imaging system. The light source can be a metal mercury lamp or a white LED to produce white light. A color wheel (CW), consisting of multiple light-absorbing color channels, is used to decompose the incident white light into multiple primary colors within the time sequential domain and provide the color initialization definition for projection displays [1–5]. The conventional color wheel is inefficient with the light-absorbing color filter (CF). The light with the required spectral waveband will pass through the CF while the other light is blocked and absorbed, thus enabling the separation and filtering of the incident white light. The color gamut and color purity of the projection display are limited by both the broad spectral distribution of the incident white light and the wide bandwidth of the transmission spectrum of the CW. The other color generation strategy is to use a blue excitation source combined with a phosphor-based CW. For example, Hu and Li developed a portable projector based on a blue laser and phosphorescent wheel, which improved the color gamut by 30% with Rec. 709 standards when operating at full power [6]. However, an additional filter is required to filter out unwanted light due to the broad emission spectrum of the phosphor, which limits the efficiency and the color gamut.
As a rising star, quantum-dot (QD) material enables efficient fluorescent conversion with a wider color gamut for emerging display devices [7–16], which have exhibited their superiority in QD photoluminescent organic light-emitting diodes (QD-OLEDs) [17–19], QD electroluminescent light-emitting diodes (QLEDs) [20–22], and micro-LEDs [23–27]. For example, in 2016, a patterned QD film was first prepared by Kim et al. for the color conversion of an OLED backlight [28]. In 2020, Yin et al. developed a high-efficiency full-color micro-LED display by combining a QD film [29]. In 2015, Han et al. used aerosol jet technology to combine red, green, and blue colloidal QDs with micro-LED arrays to realize independent addressing of full-color micro-LED arrays [30]. In 2020, Chen et al. realized a high-color-gamut micro-LED display combined with QD photoresist by photolithographic preparation [31]. This kind of QD converted micro-LED display can also be realized by inkjet printing technology [32]. The aforementioned researches demonstrate that introducing the superiority of QDs brings certain interesting findings for emerging display devices. Inspired by this, a performance revolution of traditional CW devices is just around the corner, but it has not been explicitly explored before.
Fig. 1. Quantum-dot characterization for the design of a three primary color QD-CW device. Abs and PL spectra of: (a) red and (b) green QDs. XRD of: (c) red and (d) green QDs. TEM images of: (e) red and (f) green QDs, where the inserted graphs are the average particle size distribution of red and green QDs.
For the first time, to the best of our knowledge, this paper presents a QD-color-converted CW device by using three-primary-color QD color conversion layers (QDCCL) instead of an absorptive metal coating, which brings superior optical and color performance compared to conventional projection displays. The detailed theoretical derivation is disclosed and verified via both simulation and experiment. The fundamental theory for the white balance of the QD-CW device is first presented; then, the Monte Carlo ray-tracing simulation is used to explore the light output characteristics of the QD-CW device. Finally, a blue laser pumped laser projector prototype equipped with the proposed QD-CW device is experimentally assembled and discussed.
2. THEORETICAL MODEL
In a traditional CW device, the absorptive metal coating layers usually include three primary color channels or specific additional channels, such as white channels to increase the brightness, and yellow and cyan channels to widen the color gamut. As for the QD-CW device, the backlight should be replaced by a blue excitation source. Considering the three primary color QD channels, the definition of the duty cycle is critical for the output white light within the spatial domain.
White balance is a key display indicator that describes the accuracy of the white color generated by mixing the three primary colors. It can be achieved when the intensity of the red (R), green (G), and blue (B) light matches each other [33–37]. In a standard room, the D65 light source with chromaticity coordinates of (0.313,0.329) is usually defined as the white balance [38–41].
The white balance can be achieved by adjusting the transmission area of the three primary color channels on the QD-CW device. The principle of white balance adjustment for a QD-CW device can be expressed by
where ${P_R}$, ${P_G}$, and ${P_B}$ are the luminous power; ${S_R}$, ${S_G}$, and ${S_B}$ are the areas; and ${\eta _R}$, ${\eta _G}$, and ${\eta _B}$ are the light conversion efficiencies (LCE) of the RGB color channels, respectively. The blue channel is kept blank to allow the blue backlight to pass through so that ${\eta _B}$ can be ideally considered as 100%. Based on Eq. (1), the white-balanced area ratio of RGB color channels can be determined by3. PARAMETERIZATION
A. Quantum-Dot Characterization
The approach to design a QD-CW device is described, and the key parameters come from both the experimental characterization of the QD material and the duty cycle simulation for white balance.
The visible absorption spectra (Abs) and photoluminescence (PL) spectra of red and green core-shell CdSe@ZnS QDs in a toluene solution are measured and shown in Figs. 1(a) and 1(b), respectively. The red and green peak wavelengths are 626 nm and 530 nm, and the full width at half maximum (FWHM) is 26 nm and 21 nm, respectively. The quantum yields (QY) of red and green QD are above 80%, in which the QY of green QD reaches 97%. The PLQY of CdSe QDs is enhanced after ZnS shell coating because the coating layer can eliminate many nonradiative composite centers on the QD’s surface. The existence of the shell will also provide a stable and reliable working environment for water and oxygen resistance [42–45].
Fig. 2. Schematic of the QD-CW device with: (a) the initial duty cycle of ${\theta _R}:{\theta _G}:{\theta _B} = {1}:{1}:{1}$ and (c) the optimized duty cycle of ${\theta _R}:{\theta _G}:{\theta _B} = {3.286}:{4.286}:{1}$. Simulated emission spectra of the QD-CW device with: (b) ${\theta _R}:{\theta _G}:{\theta _B} = {1}:{1}:{1}$ and (d) ${\theta _{R:}}\;{\theta _{G:}}\;{\theta _B} = {3.286}:{4.286}:{1}$.
X-ray diffraction spectra (XRD) of red and green QDs are shown in Figs. 1(c) and 1(d), respectively. Compared to the standard PDF spectra of CdSe and ZnS, the spectral diffraction peaks of red and green QDs are in between the CdSe and ZnS spectral diffraction peaks, which confirms the core shell CdSe@ZnS structure. The interaction between the nanoparticles and the solvent, as well as the volatilization of the solvent molecules, would bring about the aggregation of the QD particles. The agglomeration can seriously affect the LCE and stability of the QD-CW device. To evaluate the dispersibility of QD particles in the toluene solution, the structural morphology of CdSe@ZnS QDs is further characterized by using transmission electron microscopy (TEM). Figures 1(e) and 1(f) both show that the red and green QDs have been well dispersed. The average particle sizes of red and green CdSe@ZnS QDs are 8.16 ± 0.95 nm and ${10.32} \pm {0.84}\;{\rm nm}$, respectively.
B. Duty Cycle for White Balance
Based on the QD materials above, a ray-tracing simulation is then used to determine the duty cycle of the QD-CW device. The initial duty cycle of the QD-CW device is set to ${\theta _R}:{\theta _G}:{\theta _{B}} = {1}:{1}:{1}$. Figures 2(a) and 2(b) show the schematic and the corresponding emission spectrum, where the blue light intensity is significantly larger than red and green. The output CIE chromaticity coordinate point is (0.225, 0.123), obviously far from the target white balance of the standard D65 light source. Keeping the simulation parameters unchanged, the duty cycle is scanned for searching the target white balance. When ${\theta _R}$: ${\theta _G}$: ${\theta _B}$ reaches 3.286: 4.286: 1, the standard D65 white balance can be finally achieved. This result is very close to that calculated in Eq. (3). The detailed derivation of the theoretical validation can be found in Supplement 1. Figures 2(c) and 2(d) show the schematic diagram of the optimized QD-CW device and the corresponding emission spectrum. It can be seen that the blue light is significantly reduced, while the red and green spectra are enhanced accordingly. The output CIE white balance has been achieved with the chromaticity coordinate point of (0.313, 0.329), matching perfectly with the standard D65 source.
4. EXPERIMENTS
A. Preparation and Characterization
As shown in Fig. 3(a), a QD-CW device sample with the duty cycle above can be divided into four segments, and the QDCCLs are fabricated by multiple rounds of a photolithography process. The fabrication details are described in Supplement 1. The left side of Fig. 3(b) shows the QD-CW under natural light, and the right side shows the QD-CW under UV light. It can be seen that the red and green segments are bright and uniform on the glass substrate. Figures 3(c) and 3(d) show the 2D and 3D morphology of the surface roughness of the red and green QDCCL measured by atomic force microscopy (AFM, Bruker, multimode 8). The root mean square height (Sq) and arithmetic means the height (Sa) of the red QDCCLs are, respectively, measured to be 0.329 nm and 0.257 nm, while those of the green QDCCLs are 0.436 nm and 0.337 nm, respectively. This surface roughness proves that the surface of the red and green QDCCLs is smooth and flat [46–48].
Fig. 3. Experimental results of the QD-CW device sample. (a) Four segments of the fabricated QD-CW device sample. (b) Sample under the natural light and UV light. AFM images of: (c) the red and (d) green QDCCLs on the QD-CW device sample.
B. Color Performance
A mini-LED backlight is used here to evaluate the color performance of the fabricated QD-CW device sample. The normalized spectrum of the mini-LED backlight is indicated by A0 in Fig. 4(a). After passing through the green segment the QD-CW sample, the spectrum is split into two independent parts: ${{\rm A1}_{\rm G}}$ and ${{\rm A2}_{\rm G}}$. The red light conversion has similar two output parts. Obviously, ${{\rm A2}_{\rm G}}$ is the intensity of the converted green light, and ${{\rm A1}_{\rm G}}$ represents the intensity of the remaining blue light without color conversion and directly passing through the green QDCCL. This will reduce the color purity; therefore, a long-pass distributed Bragg reflector (DBR) is placed on top of the QDCCL to reduce the blue light transmission. The DBR is made by stacking the multiple periodic films with high and low refractive indices; the thickness of each film is 1/4 of the wavelength of its central reflection [49–51]. The strong wavelength selective transmittance of the DBR can help to reflect the blue light and let the converted light pass through [52,53]. As shown in Fig. 4(b), the transmission spectrum of this DBR has a dramatic change at the wavelength of 500 nm. The transmittance approaches 0 for the wavelength lower than 500 nm and up to 95.5% for the wavelength higher than 500 nm. It means the DBR can provide high reflectivity for blue light and high transmission for converted light from the QDCCL. The parameters of the DBR are also used in the simulation to determine the duty cycle.
Fig. 4. Color performance of the QD-CW device sample. Emission spectrum and the performance of the red and green QDCCLs (a) without the DBR and (c) with the DBR. (b) Transmission spectrum of the DBR. (d) Working mechanism of the DBR.
In Fig. 4(c), the green curve shows the emission spectrum of the green QDCCL by using the DBR. The blue waveband has been eliminated well, with the green intensity growing by 2%. The blue light absorption (BLA) and light conversion efficiency (LCE, $\eta$) are defined, respectively, by
By combining the equations above, the overall LCE of the QD-CW device, ${\eta _{{\rm overall}}}$, is given by
After calculation, the BLA of the green QDCCL is increased from 88.3% to 99.6%, and the LCE is increased from 28.3% to 31.6%. The color purity is also improved, with the chromaticity coordinates changing from (0.172, 0.451) to (0.187, 0.745). A similar performance improvement is clearly found for the red QDCCL, in which the BLA and LCE of the red QDCCL increased from 94.3% to 99.3% and from 34.7% to 39.5%, respectively. The chromaticity coordinate changes from (0.543, 0.236) to (0.677, 0.300). It means that the DBR can greatly reduce the number of blue photons passing through the QDCCL and improve the color purity of the QDCCL. When ${\theta _R}:{\theta _G}:{\theta _B} = {1}:{1}:{1}$ is set for the QD-CW device sample, the average LCE is 57.0%. Compared to a conventional CF-based phosphor-converted CW, in which the loss in CF layers is over 74.0% [54], the overall LCE is improved by 31.0% when the optimized duty cycle, ${\theta _R}:{\theta _G}:{\theta _B} = {3.286}:{4.286}:{1}$, is set.
The working mechanism of the DBR is illustrated in Fig. 4(d). The incident blue light with the intensity ${I_B}$ can be divided into two parts after passing through the green QDCCL: the converted green light with the intensity ${I_G}$, and the remaining blue light with the intensity ${I_b}$. The remaining blue light cannot pass through DBR and would excite the QDCCL again. ${I_g}$ is the intensity of the converted light after reuse of the blue light. That is why the converted light intensity shown in Fig. 3(c) can be improved after introducing this DBR.
C. White Balance
The QD-CW device sample is electrically driven by a small motor and rotated at a uniform speed to achieve the white balance output. Figure 5(a) shows the explosive view of the working QD-CW device and the corresponding testing platform (SRC-200M, Everfine Corporation). The blue light passes through the red, green, and blue segment of the QD-CW sample in turn, and then the emitted light is collected by the spectral color luminance meter.
Fig. 5. White balance realization of the QD-CW device sample. (a) Explosive view of the QD-CW device sample and the corresponding testing platform. (b) Working principle of the QD-CW device sample. (c) Measured spectrum of the white light emitted. (d) Output spatial light intensity distribution. (e) Color gamut area of the QD-CW device sample.
Figure 5(b) shows the working principle of the QD-CW device sample for color conversion. In a complete rotation cycle T, the QD-CW device sample will output blue, green, and red light in turn according to the pre-determined duty cycle. When the transparent part of the QD-CW device sample is rotated above the blue mini-LED backlight, the QD-CW device sample will output blue light in T1. Similarly, T2 and T3, respectively, represent the red and green light output periods when the red or green QDCCL on the QD-CW is aligned with the blue mini-LED backlight. More details about the output red, green, and blue light from the QD-CW device sample can be found in Supplement 1.
The final emission spectrum of the QD-CW device sample is shown in Fig. 5(c). The chromaticity coordinate is (0.317, 0.338), which matches well with the chromaticity coordinate (0.313,0.329) of the D65 white balance. The inset of Fig. 5(c) shows the fast-spinning QD-CW device sample achieving white balance. When the QD-CW device sample is rotating at a low speed, the color separation can be clearly observed. As the speed increases, the white light is observed without color separation. The three primary colors are mixed uniformly in the space. The complete video clip recorded from the QD-CW device sample achieving white balance can be found in Visualization 1.
For the QD-CW device, the color breakup phenomenon cannot be ignored when the CW’s rotation speed is slower than the refresh frequency of the projector’s display panel. Increasing the color wheel’s rotation speed to synchronize with the display panel should be a straightforward approach to mitigate the color breakup. In this case, other issues that are worth noting should be the mechanical stability and noise of the QD-CW device. After a good matching between the color wheel’s rotation speed and the refresh frequency of the projector’s display panel, another main cause of the color breakup should be the visual stability of the human eye [55]. In this case, the occurrence of the color breakup phenomenon is mainly attributed to the action of eye position signals during the saccade movement. To mitigate this kind of color breakup, one approach is to use a multiple primary color wheel or swirl color wheel to enhance color overlay and balance, where the multiple primary color QDCCL has been demonstrated before [56]. The second method is the image motion compensation, which first calculates the motion vector through the pixel displacement between adjacent frames and then compensates for the pixel position in the current frame to offset the displacement generated by the eye position signal.
As shown in Fig. 5(d), the output spatial light intensity distribution of the QD-CW device sample is measured by the photometer (GO-I300, Everfine Corporation). It can be seen that the spatial light intensity distribution matches the standard Lambertian distribution, which is beneficial for displays with a large viewing angle. At the same time, the color gamut area of the QD-CW system is defined as the comparison between the three primary color triangular area of the QD-CW based display and the triangular area of the NTSC standard [57]. Therefore, the color gamut area is calculated to reach 116.6% NTSC, which is approximately 40% higher than a conventional CW system.
D. Projection Display
Figure 6(a) shows the schematic of a projection display prototype that is combined by a blue laser pumped projector and the prepared QD-CW device sample. The blue imaging pattern emitted by the laser projector will pass through the chromatic segments on the QD-CW device in turn. The corresponding color-converted images on the screen are shown in Figs. 6(b)–6(d). The blue image in Fig. 6(b) is clear, because the blue light can directly pass through the QD-CW device sample. However, the red and green images become somewhat fuzzy because of the QD scattering. The QD scattering also brings some blue light leakage, resulting in the color deviation from pure red and green in Figs. 6(c) and 6(d). This prototype shows a straightforward projection application based on the QD-CW device sample. In addition, this QD-CW can also generate white-balanced light by a blue backlight, which makes it compatible with passive luminous projection architectures, such as DMD, LCoS, and LCD.
Fig. 6. Projector prototype based on the prepared QD-CW device sample. (a) Schematic of the projector. (b)–(d) Projection images generated by the QD-CW device sample.
For this prototype, both the QD scattering and the relative location with projection optics will affect the projector contrast to some extent, which is discussed in detail and experimentally compared to a conventional phosphor-based CW. In Supplement 1, it is found that the QD-CW provides a higher image contrast than a conventional phosphor-based CW device. The main reason is that the phosphor powders have larger particle sizes than QDs, which leads to stronger scattering characteristics and more color conversion light leaking into the background area. On the other part, the relative position between the blue laser projector and the QD-CW affects the size of the excited region, and the relative position of the QD-CW and the projection screen determines whether the scattered light will affect the overall area of the projection screen. Inserting a focusing optical element could help to shield scattered light outside the excitation area and further improve the projector contrast.
Another concern is the thermal quenching and photodegradation of the QD-CW device. In this work, the rated power of the laser projector used is 25 W, and the luminous flux of the blue light that excites the QD-CW is measured to be only 0.35 lm. Under such low blue light excitation, there is not much of a decrease in the brightness from the potential thermal quenching of QD materials. Moreover, the QDCC materials encapsulated in the photoresist also have certain advantages in thermal stability compared to pure QDs. The reliability testing details of the QD-CW device can be found in Supplement 1. Note that when using a laser projector as a blue excitation source, the stability of the QD-CW device largely depends on the intensity of the laser source. When a high-power blue laser is used, the lattice destruction of the QDs would be promoted by water and oxygen, causing fluorescence quenching of the QD-CW device. Atomic layer deposition or a polymer matrix are possible approaches to lengthen the QD’s lifetime [8,58,59].
5. CONCLUSION
This paper presents and systematically studies a newly proposed QD-CW device by theoretical derivation, simulation analysis, and experimental verification. The duty cycle of the QD-CW device is theoretically calculated as ${\theta _R}:{\theta _G}:{\theta _B} = {3.286}:{4.286}:{1}$ to achieve the white balance, and the CIE chromaticity coordinate point is proven to be (0.313, 0.328) from ray-tracing simulation. For experimental verification, a QD-CW device sample was prepared using multiple rounds of a photolithography process. By integrating a DBR, the QD-CW device can provide the white light output with a chromaticity coordinate point of (0.317,0.338), which matches well with the standard D65 light source and the simulation result. The color gamut area of the QD-CW device sample reaches 116.6% NTSC, and the average LCE is 57.0%. The proposed QD-CW device has 1.2× higher LCE and ${\sim}{40}\%$ higher color gamut area than a conventional CW device. A direct-view laser pumped projector is assembled by this QD-CW device sample, where the full-color projection images are realized. It is expected that this novel QD-CW device with its improved efficiency and color performance will be used for applications in projection displays and related display fields.
Funding
National Natural Science Foundation of China (62175032); Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China (2020ZZ111).
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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