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OLEDs vs. μLEDs and mLEDs
Researchers from UCF presented a paper that compares the performance of mLEDs, OLEDs and μLEDs display technologies, which support a fast Motion Picture Response Time (MPRT), a high ppi, a high contrast ratio, a high bit depth, an excellent dark state, a wide color gamut, a wide viewing angle, a wide operating temperature range and a flexible form factor.
In realizing HDR, high peak brightness can be obtained on all mLED/μLED/OLED displays, except that mLED-LCDs require careful thermal management, and OLED displays experience a trade-off between lifetime and luminance. For transparent displays, all emissive mLED/μLED/OLED types work well. By removing the CP, the CC type and CP-free RGB-chip type mLED/μLED emissive displays are 3 ~ 4× more efficient. In addition, OLED displays, and mLED-LCDs have advantages in terms of cost and technology maturity. UCF evaluated the power consumption and Ambient Contrast Ratio (ACR) of each display in depth and systematically compare the dynamic range, MPRT, and adaptability to flexible and transparent displays. The pros and cons of mLED, μLED, and OLED displays were analyzed. In this paper, UCF appears to use mLEDs two ways, 1) as a definition of the emissive technology as practiced by Sony, Samsung and TCL for very large TVs with 50um LEDs and 2) the mini LED category of backlights.
According to research published in Nature by Huang et al., µLED technology has some serious advantages over AMOLED. However, numerous drawbacks are preventing mass fabrication and the adoption of the technology. The paper first looks at the different configurations.
Figure 1: Layers & Configurations: LCD, OLED, µLED
Researchers from UCF presented a paper that compares the performance of mLEDs, OLEDs and μLEDs display technologies, which support a fast Motion Picture Response Time (MPRT), a high ppi, a high contrast ratio, a high bit depth, an excellent dark state, a wide color gamut, a wide viewing angle, a wide operating temperature range and a flexible form factor.
In realizing HDR, high peak brightness can be obtained on all mLED/μLED/OLED displays, except that mLED-LCDs require careful thermal management, and OLED displays experience a trade-off between lifetime and luminance. For transparent displays, all emissive mLED/μLED/OLED types work well. By removing the CP, the CC type and CP-free RGB-chip type mLED/μLED emissive displays are 3 ~ 4× more efficient. In addition, OLED displays, and mLED-LCDs have advantages in terms of cost and technology maturity. UCF evaluated the power consumption and Ambient Contrast Ratio (ACR) of each display in depth and systematically compare the dynamic range, MPRT, and adaptability to flexible and transparent displays. The pros and cons of mLED, μLED, and OLED displays were analyzed. In this paper, UCF appears to use mLEDs two ways, 1) as a definition of the emissive technology as practiced by Sony, Samsung and TCL for very large TVs with 50um LEDs and 2) the mini LED category of backlights.
According to research published in Nature by Huang et al., µLED technology has some serious advantages over AMOLED. However, numerous drawbacks are preventing mass fabrication and the adoption of the technology. The paper first looks at the different configurations.
Figure 1: Layers & Configurations: LCD, OLED, µLED
Beyond the physical configuration, it is necessary to explore some of the trade-offs before getting excited about the future of your next commercial display. What the configuration for μLEDs doesn’t account for is the poor efficiency of green and red, which has led to the use of blue μLEDs, with some type of color converter, most likely red and green QDs. The paper does go on to discuss the differences in efficiency for the three primary colors.
Huang et al., believe that under the proper driver characteristics, µLED can overtake AMOLED technologies quickly in maximum luminance, ACR, HDR, and response time. This conclusion should not be a surprise to anyone knowledgeable in display performance. Although the term HDR was not qualitatively defined and is subject to interpretation.
Figure 2: Relative efficiency with respect to chip size demonstrates the power advantages of color conversion (CC) µLED films.
Huang et al., believe that under the proper driver characteristics, µLED can overtake AMOLED technologies quickly in maximum luminance, ACR, HDR, and response time. This conclusion should not be a surprise to anyone knowledgeable in display performance. Although the term HDR was not qualitatively defined and is subject to interpretation.
Figure 2: Relative efficiency with respect to chip size demonstrates the power advantages of color conversion (CC) µLED films.
The paper calls out that AMOLED technologies suffer from two major weaknesses that affect their durability, including screen “burn-in” and a shorter lifespan for organic materials which produce blue wavelengths.
µLEDs also have drawbacks from an electrical standpoint. Although µLEDs have a significantly higher peak potential luminance (solid lines figure [a]), the external quantum efficiency (EQE) varies considerably over the operating luminance range. Secondly, unless a blue light source with a color conversion configuration is used green and red µLED are less efficient than OLED.
Figure a) external quantum efficiency (EQE) versus luminance; OLED (dashed lines) and µLED (solid lines). Figure b) normalized EQE / Vf (dashed) vs current for µLED.
The paper calls out that AMOLED technologies suffer from two major weaknesses that affect their durability, including screen “burn-in” and a shorter lifespan for organic materials which produce blue wavelengths.
µLEDs also have drawbacks from an electrical standpoint. Although µLEDs have a significantly higher peak potential luminance (solid lines figure [a]), the external quantum efficiency (EQE) varies considerably over the operating luminance range. Secondly, unless a blue light source with a color conversion configuration is used green and red µLED are less efficient than OLED.
Figure a) external quantum efficiency (EQE) versus luminance; OLED (dashed lines) and µLED (solid lines). Figure b) normalized EQE / Vf (dashed) vs current for µLED.
Two primary modulations used to drive LEDs, pulse amplitude modulation (PAM) and pulse width modulation (PWM). The research found that PWM can be a superior drive technology for µLEDs, improving their relative efficiency over PAM, with an increase of 30%, 91%, and 28% in efficiency (circles vs. triangles in figure [b]). UCL used the following metrics to compare displays:
Both mLED, μLED and OLED chips can be used as emissive displays, while mLEDs can also serve as a BLU for LCDs. The next figure illustrates three commonly used device configurations: red, green and blue (RGB)-chip emissive displays, color conversion (CC) emissive displays and mLED-backlit LCDs. In emissive displays mLED/μLED chips and patterned OLEDs serve as subpixels. In a non-emissive LCD, an mLED backlight is segmented into a zone structure; each zone contains several mLED chips to control the panel luminance, and each zone can be turned on and off selectively. The LC panel consists of M and N pixels, and each RGB subpixel, addressed independently by a thin-film transistor (TFT), regulates the luminance transmittance from the backlight. The full-color images are generated differently in these three types. RGB LED chips are adopted. Each LED will emit light in both the upward and downward directions. To utilize downward light, a reflective electrode is commonly deposited at the bottom of each LED chip. However, such a reflector also reflects the incident ambient light, which could degrade the ACR32. One solution is to adopt tiny chips to reduce the aperture ratio and cover the non-emitting area with a black matrix to absorb the incident ambient light26. This strategy works well for inorganic LEDs. However, for OLED displays, a large chip size helps to achieve a long lifetime and high luminance. Under such conditions, to suppress the ambient light reflection from bottom electrodes, a circular polarizer (CP) is commonly laminated on top of the OLED panel to block the reflected ambient light from the bottom electrodes.
Figure 3: Display system configurations. an RGB-chip mLED/μLED/OLED emissive displays. b CC mLED/μLED/OLED emissive displays. c mini-LED backlit LCDs
- HDR and a high ambient contrast ratio
- High resolution or a high resolution density for virtual reality to minimize the screen-door effect
- Wide color gamut
- Wide viewing angle and an unnoticeable angular color shift
- Fast motion picture response time (MPRT) to suppress image blur
- Low power consumption, which is particularly important for battery-powered mobile displays,
- Thin profile, freeform, and lightweight system, and (8) low cost.
Both mLED, μLED and OLED chips can be used as emissive displays, while mLEDs can also serve as a BLU for LCDs. The next figure illustrates three commonly used device configurations: red, green and blue (RGB)-chip emissive displays, color conversion (CC) emissive displays and mLED-backlit LCDs. In emissive displays mLED/μLED chips and patterned OLEDs serve as subpixels. In a non-emissive LCD, an mLED backlight is segmented into a zone structure; each zone contains several mLED chips to control the panel luminance, and each zone can be turned on and off selectively. The LC panel consists of M and N pixels, and each RGB subpixel, addressed independently by a thin-film transistor (TFT), regulates the luminance transmittance from the backlight. The full-color images are generated differently in these three types. RGB LED chips are adopted. Each LED will emit light in both the upward and downward directions. To utilize downward light, a reflective electrode is commonly deposited at the bottom of each LED chip. However, such a reflector also reflects the incident ambient light, which could degrade the ACR32. One solution is to adopt tiny chips to reduce the aperture ratio and cover the non-emitting area with a black matrix to absorb the incident ambient light26. This strategy works well for inorganic LEDs. However, for OLED displays, a large chip size helps to achieve a long lifetime and high luminance. Under such conditions, to suppress the ambient light reflection from bottom electrodes, a circular polarizer (CP) is commonly laminated on top of the OLED panel to block the reflected ambient light from the bottom electrodes.
Figure 3: Display system configurations. an RGB-chip mLED/μLED/OLED emissive displays. b CC mLED/μLED/OLED emissive displays. c mini-LED backlit LCDs
In Fig. 1b, each blue LED chip pumps a subpixel in the patterned CC layer (quantum dots or phosphors)44. An absorptive color filter (CF) array is registered above to absorb unconverted blue light and suppress ambient excitations. This filter also enhances the ACR so that no CP is required. In some designs, a distributed Bragg reflector (DBR) is inserted to selectively recycle the unconverted blue light or to enhance the red/green output efficiency. In Fig. 1c, blue mLED chips pump a yellow CC layer48 to generate white backlight. Additionally, a DBR could be optionally applied. In such a BLU, the mLED zones do not need to register with the subpixels so that a larger LED chip can be used. Because the CC layer scatters light, up to two brightness enhancement films (BEFs) can be employed to collimate light onto the on-axis direction. A dual brightness enhancement film (DBEF)49 can be inserted to transmit the preferred polarization, which is parallel to the transmission axis of the first polarizer and to recycle the orthogonal polarization. The transmitted light is modulated by the LCD with an absorptive CF array. In some designs, RGBW CFs instead of RGB CFs are employed to enhance the optical efficiency.
The power consumption of mLED/μLED/OLED displays is primarily determined by the driving circuitry designs, LED quantum efficiency and optical system efficiency.
Pulse amplitude modulation (PAM) driving schemes
The power consumption of mLED/μLED/OLED displays is primarily determined by the driving circuitry designs, LED quantum efficiency and optical system efficiency.
Pulse amplitude modulation (PAM) driving schemes
- PAM is also called analogue driving, is commonly used in emissive OLED displays. PAM is also an intuitive choice for μLED drivers. Figure 2a shows a basic 2 transistors and 1 capacitor (2T1C) subpixel circuitry in AM addressing. In an emissive display panel with M by N pixels, the circuitry in Fig. 2a is arrayed by 3M columns (each pixel contains RGB subpixels) and N rows. TS denotes the switching TFTs to sequentially turn on the LEDs, and TD stands for the driving TFTs regulating the current flowing to the LEDs. For each row, TS is only open for 1/N of the whole frame time (Tf), during which the data voltage (Vdata) is loaded to the gate of TD, and then TS is switched off. A storage capacitance (Cs) holds the voltage so that TD is kept open for the remainder of the frame time. Therefore, in AM addressing, the LED emits light for a Tf. Figure 2b illustrates the arrayed PM driving circuitry. Here, no storage capacitance is employed. Thus, each LED only emits light for a short period (Tf/N). To achieve the same effective luminance, the instant luminance in the PM should be N times higher than that of the AM.
Figure 5: Operating Spots Of OLED Displays And mLED Displays
VDS: the TFT drain-to-source voltage. VF, OLED: the OLED forward voltage. VF, μLED: the μLED forward voltage
The blue curve shows the OLED I-VF characteristics with the flipped voltage. The intersection of the black dashed lines and the blue curve denotes the I and VDS_min at full brightness. Then, the minimal required voltage across the TD and LED is: 𝑉DD_min=𝑉DS_min+𝑉𝐹VDD_min=VDS_min+VF
(3) where VDD is determined by the highest grey level and remains unchanged at lower grey levels.
Figure 6: Illustration of VDD voltage drop
The blue curve shows the OLED I-VF characteristics with the flipped voltage. The intersection of the black dashed lines and the blue curve denotes the I and VDS_min at full brightness. Then, the minimal required voltage across the TD and LED is: 𝑉DD_min=𝑉DS_min+𝑉𝐹VDD_min=VDS_min+VF
(3) where VDD is determined by the highest grey level and remains unchanged at lower grey levels.
Figure 6: Illustration of VDD voltage drop
Figure 7: OLEDs and μLED characteristics
a. EQEchip as a function of chip luminance. The RGB dashed lines are for RGB OLEDs. The RGB solid lines are for RGB mLEDs. b Current-dependent EQEchip (solid lines) and normalized EQEchip/VF (dashed lines) of RGB mLEDs, as denoted by RGB colors, respectively
The strong variation in EQEchip makes operation spot optimization critical for mLED/μLED displays.
Color conversion emissive displays
As Fig. 1b depicts, the red/green colors are converted from blue LED chips, which bypasses the need for high EQEchip red mLEDs/μLEDs. However, OLED displays rely on blue chips, which have lower efficiency and shorter lifetimes. In Fig. 1b, the patterned CC film is normally a quantum dot color filter (QDCF)44. The overall EQE becomes a product of the blue chip EQE (EQEchip,B) and QDCF’s CC efficiency (EQEQDCF). Above that, the absorptive CF could be presented by its transmittance (TCF).
Contrast ratio
The CR of an emissive display is inherently high. In a nonemissive LCD, its CR is limited by the depolarization effect mainly from the employed LC material, surface alignment and CF. Normally, the CR of an LCD is approximately 5000:1, 2000:1 and 1000:1 for the multidomain vertical alignment (MVA) mode36, fringe-field switching (FFS) mode37 and twisted-nematic (TN) mode2, respectively. To further enhance the CR, local dimming technology can be applied to reduce light leakage in the dark state. A local dimming display system consists of dual modulation units, i.e., a segmented low-resolution mLED backlight and a high-resolution LCD panel. As discussed previously, this pre-modulation can be realized by a 2D arrayed mLED BLU. With a proper number of local dimming zones, the troublesome halo effect and clipping effect can be suppressed to an unnoticeable level28,79. Another method is to cascade two LCD panels80,81,82: a black-and-white low-resolution panel (e.g., 2K1K) to provide a local dimming effect and a high-resolution (8K4K) full-color panel. Unlike an mLED backlight that can provide thousands of zones, such a dual-panel LCD can offer millions of zones at a fairly low cost, but the traded-off is the increased thickness.
Ambient contrast ratio
In practical applications, the reflected ambient light (either from the external surface or from internal electrodes) is also perceived in addition to the displayed contents. The ACR is defined as:
ACR=𝐿on+𝐼am𝜋⋅𝑅𝐿𝐿off+𝐼am𝜋⋅𝑅𝐿≈1+𝜋⋅𝐿on𝐼am⋅𝑅𝐿ACR=Lon+Iamπ⋅RLLoff+Iamπ⋅RL≈1+π⋅LonIam⋅RL
This result is because the LED reflectance in the RGB-chip type is strong, while the CF array in the CC-based μLED emissive displays partially suppresses ambient excitations. For the applications, the most power-efficient chip size is located at <20 μm. mLED-LCDs (green curves) were added for comparison, although the actual chip size of the mLED (~200 μm) in the BLU is beyond the horizontal scale plotted in Fig. 6.
Figure 8: Chip Size-Dependent LED Power Consumption With Different Display Technologies
The strong variation in EQEchip makes operation spot optimization critical for mLED/μLED displays.
Color conversion emissive displays
As Fig. 1b depicts, the red/green colors are converted from blue LED chips, which bypasses the need for high EQEchip red mLEDs/μLEDs. However, OLED displays rely on blue chips, which have lower efficiency and shorter lifetimes. In Fig. 1b, the patterned CC film is normally a quantum dot color filter (QDCF)44. The overall EQE becomes a product of the blue chip EQE (EQEchip,B) and QDCF’s CC efficiency (EQEQDCF). Above that, the absorptive CF could be presented by its transmittance (TCF).
Contrast ratio
The CR of an emissive display is inherently high. In a nonemissive LCD, its CR is limited by the depolarization effect mainly from the employed LC material, surface alignment and CF. Normally, the CR of an LCD is approximately 5000:1, 2000:1 and 1000:1 for the multidomain vertical alignment (MVA) mode36, fringe-field switching (FFS) mode37 and twisted-nematic (TN) mode2, respectively. To further enhance the CR, local dimming technology can be applied to reduce light leakage in the dark state. A local dimming display system consists of dual modulation units, i.e., a segmented low-resolution mLED backlight and a high-resolution LCD panel. As discussed previously, this pre-modulation can be realized by a 2D arrayed mLED BLU. With a proper number of local dimming zones, the troublesome halo effect and clipping effect can be suppressed to an unnoticeable level28,79. Another method is to cascade two LCD panels80,81,82: a black-and-white low-resolution panel (e.g., 2K1K) to provide a local dimming effect and a high-resolution (8K4K) full-color panel. Unlike an mLED backlight that can provide thousands of zones, such a dual-panel LCD can offer millions of zones at a fairly low cost, but the traded-off is the increased thickness.
Ambient contrast ratio
In practical applications, the reflected ambient light (either from the external surface or from internal electrodes) is also perceived in addition to the displayed contents. The ACR is defined as:
ACR=𝐿on+𝐼am𝜋⋅𝑅𝐿𝐿off+𝐼am𝜋⋅𝑅𝐿≈1+𝜋⋅𝐿on𝐼am⋅𝑅𝐿ACR=Lon+Iamπ⋅RLLoff+Iamπ⋅RL≈1+π⋅LonIam⋅RL
This result is because the LED reflectance in the RGB-chip type is strong, while the CF array in the CC-based μLED emissive displays partially suppresses ambient excitations. For the applications, the most power-efficient chip size is located at <20 μm. mLED-LCDs (green curves) were added for comparison, although the actual chip size of the mLED (~200 μm) in the BLU is beyond the horizontal scale plotted in Fig. 6.
Figure 8: Chip Size-Dependent LED Power Consumption With Different Display Technologies
a 50-μm pitch smartphone under 1500-lux overcast daylight for ACR=40:1. b 90-μm pitch notebook under 500-lux office light for ACR=100:1. c 375-μm pitch (65-inch 4K) TV under 150-lux living room ambient for ACR = 1000:1. The most power efficient choice was the RGB-chip μLED display. Both the small-chip CP-free design (blue curves) and large-chip CP-laminated structure (red curves) are outstanding. The intersection point of with/without-CP designs can be calculated by the following method.
Response time and MPRT
The response time of mLED/μLED/OLED chips is several orders faster than that of LCs. However, we cannot conclude that mLED/μLED/OLED emissive displays provide a much smoother visual experience than LCDs. A
Response time and MPRT
The response time of mLED/μLED/OLED chips is several orders faster than that of LCs. However, we cannot conclude that mLED/μLED/OLED emissive displays provide a much smoother visual experience than LCDs. A
widely used metric for the visual response time is MPRT41,42. MPRT is jointly determined by pixel response time (τ) and frame rate (f = 1/Tf), and it can be calculated by a simplified equation proposed by Peng et al.42:
MPRT=𝜏2+(0.8𝑇𝑓)2‾‾‾‾‾‾‾‾‾‾‾‾‾√MPRT=τ2+(0.8Tf)2
(29)
However, these displays are still much slower than the impulse driving CRT whose MPRT is approximately 1 ms
Figure 9: Pixel Response Time-Dependent MPRT At Different Frame Rates
MPRT=𝜏2+(0.8𝑇𝑓)2‾‾‾‾‾‾‾‾‾‾‾‾‾√MPRT=τ2+(0.8Tf)2
(29)
However, these displays are still much slower than the impulse driving CRT whose MPRT is approximately 1 ms
Figure 9: Pixel Response Time-Dependent MPRT At Different Frame Rates
An alternative method to shorten the MPRT is to globally dim the panel when the LC response is in transition and only illuminate the panel when the LC is ready. The ratio between the light emission time and the frame time is called the DR. In this way, the MPRT is shortened to
MPRT=0.8×𝑇𝑓×DRMPRT=0.8×Tf×DR
(30)
Still taking the 60-fps display as an example, its MPRT can be dramatically shortened to 1.33 ms by applying a 10% DR, regardless of the LCDs or emissive displays. Recently, sub millisecond MPRT has been achieved on LCDs by material development operation mode innovations and DR reductions. However, the trade-off of using a 10% DR is decreased luminance. To achieve the same pixel luminance, the peak brightness of mLED backlight or the OLED (or μLED) pixels should be boosted by 10×. The lifetime degradation and efficiency droop effect should be taken into consideration.
High dynamic range
Currently, a variety of HDR formats coexist87, such as the basic HDR10, the superb Dolby Vision, the broadcast-friendly Hybrid Log Gamma (HLG), and the rising Advanced HDR by Technicolor. An HDR display may support one or more HDR formats, but the hardware specs are more crucial to the final performance than the format adopted. In this section, we will discuss the necessities of the HDR display hardware88,89, namely, the high peak luminance, excellent dark state, high bit depth and wide color gamut.
Luminance. The human eye has a very wide dynamic range, covering an absolute specular highlight (10 000 cd/m2) to an extreme dark state (0.005 cd/m2). In contrast, the standard dynamic range display only offers a 100 cd/m2 peak luminance. As a manufacturer-friendly target, Ultra HD Premium defined the HDR luminance range as 0.05~ 1000 cd/m2 for LCDs and 0.0005~ 540 cd/m2 for OLED displays. This standard can be satisfied by all mLED/μLED/OLED display technologies. Dolby Vision is mastered at a 4000-cd/m2 peak luminance88. In 2020, Sharp’s 8 K LCD TV achieved over 10,000 cd/m2 by employing indium-gallium-zinc-oxide (IGZO) TFTs with an extremely low dark current and by boosting the backlight luminance92. The low optical efficiency-caused thermal issue can be partially addressed by local dimming technology. On the other hand, OLEDs suffer from efficiency roll-off93 and fast ageing43 at a high luminance, so they are more suitable for frequent-update devices. As a result, the mLED/μLED emissive displays demonstrate the best quality HDR preference for high luminance with high efficiency.
Bit depth
With the expansion of the luminance range, 8 bits per color is no longer sufficient to provide a smooth color change. While 10 bits are applied in current HDR display systems, 12 bits per color is highly desired to avoid banding artefacts according to the Barten model and the Perceptual Quantizer (PQ) curve90,94. Technically, at least 10 bits are required on the hardware if 2 bits are handled by dithering95. In conventional LCDs, the bit depth is limited by a large voltage swing and a slow grey-to-grey response time. Fortunately, the dual modulation units in local dimming LCDs share the burden equivalently so that the 12-bit PQ curve has been achieved82,96. In emissive displays, achieving 10-bit or 12-bit requires ultra-accurate current control in the PAM and ultra-short pulse generation in the PWM, leading to a high electronics cost. In 2018, JDC demonstrated a 10-bit µLED on a silicon backplane with PWM65. High bit depth is especially challenging when a low DR is applied to the PWM because it further reduces the shortest pulse width. Similar to the dual modulation in local dimming LCDs, hybrid driving71 could tackle the difficulties by combining PAM and PWM.
Color performance
Vivid color is another critical requirement of HDR displays. There are various standards to evaluate the color performance of a display panel, such as sRGB, NTSC, DCI-P3, and Rec. 202033,34,35. The color gamut coverage of the display is mainly defined by the central wavelength and full width at half maximum (FWHM) of the RGB emission spectrum. For example, Rec. 2020 is defined by red (630 nm), green (532 nm) and blue (467 nm) lasers33,34. In this section, we will report the color gamut (x, y area coverage in CIE 1931) and color shift of the mLED/μLED/OLED displays.
In 2017, SEL showed new materials to enable an OLED display with >101% (u’, v’) coverage, which corresponds to 91.8% (x, y) coverage in Rec. 2020. Such a large color gamut is achieved by material and device advancements: (1) Deep blue fluorescent and deep red phosphorescent OLED materials have been developed14,66,72, although further research is required to extend the device lifetime for commercial applications, and (2) the two metallic electrodes of the top emission OLED form a microcavity to significantly narrow the emission FWHM. The trade-offs are a compromised efficiency and a large angular color shift. Therefore, proper OLED structure parameter optimizations97 and better cavity designs for mitigating color shift are still needed.
Inorganic mLED/μLED inherently has a relatively narrow FWHM (18 ~ 30 nm). so, the color gamut mainly depends on the emission wavelength. Recently, 91.4% Rec. 2020 has been reported on the RGB-chip type100. A practical issue of PAM mLED/μLED displays is the central wavelength drift and the FWHM change with current100. As the current density increases, the central wavelength is blue shifted for the blue/green (InGaN) LEDs and redshifted for the red (AlGaInP) LEDs. As a result, the mixed white color (D65) may not appear as white. This current-dependent color shift can be minimized with the PWM. Inorganic mLEDs/μLEDs also have an angular-dependent color shift, which results from the LED material difference and angular spectrum mismatch of the red and green/blue LEDs70. This problem can be solved by adding a black matrix to absorb the side emission to compromise the light extraction efficiency.
For the CC-type mLED/μLED emissive displays, the color gamut is jointly determined by the blue LED chip and the green and red quantum dots. The narrow FWHM and high central wavelength tunability of QDs can theoretically enable >97% Rec. 202035, and 93.1% has been experimentally demonstrated101. In this CC emissive display, additional attention should be paid to blue light leakage. The QDCF should be thick enough to effectively convert the blue light to red and green44,102, and an additional absorptive CF44,45 or DBR46 is needed to clean up the unconverted blue light and to minimize ambient excitations. As discussed above, the current-sensitive spectrum of inorganic mLEDs/μLEDs causes a color shift on the blue subpixels under PAM so that PWM is still a preferred approach. In comparison, green and red quantum dots exhibit stable spectral emission profiles even though the wavelength and intensity of blue pumping light fluctuate. In addition, the color shift may come from the angular emission profile mismatch between the blue LED and green/red quantum dots. To address this issue, scattering particles are added to the blue subpixels in the CC film to generate the same Lambertian angular profile as the green/red subpixels.
The color gamut of mLED-LCD is dependent on the adopted CC material. From the Yttrium Aluminum Garnet (YAG) phosphor and K2SiF6 (KSF) phosphor to the QDs, the color gamut is improved from ~50% and 70 ~ 80% to 80 ~ 90% Rec. 2020103. Different from the patterned CC film in emissive displays, the white backlight and absorptive CF in LCDs may introduce color crosstalk and impair color purity. Narrower band absorptive CFs could reduce crosstalk at the cost of a lower transmittance. In 2017, Chen et al. designed a bandpass filter in conjunction with green perovskite and red QDs to generate >95% Rec. 2020104. At large viewing angles, the gamma shift of the LCDs has been addressed by multidomain designs36,37,39 and compensation films6,40 to achieve an unnoticeable color shift (<0.02).
A wide color gamut (>90% Rec. 2020) can be obtained on all of them. It is a matter of choice to balance the color gamut with the lifetime, color shift, system efficiency, luminous efficacy and cost.
Figure 10: Chromaticity Diagram Of mLED/uLED/OLED Displays With Rec. 2020
MPRT=0.8×𝑇𝑓×DRMPRT=0.8×Tf×DR
(30)
Still taking the 60-fps display as an example, its MPRT can be dramatically shortened to 1.33 ms by applying a 10% DR, regardless of the LCDs or emissive displays. Recently, sub millisecond MPRT has been achieved on LCDs by material development operation mode innovations and DR reductions. However, the trade-off of using a 10% DR is decreased luminance. To achieve the same pixel luminance, the peak brightness of mLED backlight or the OLED (or μLED) pixels should be boosted by 10×. The lifetime degradation and efficiency droop effect should be taken into consideration.
High dynamic range
Currently, a variety of HDR formats coexist87, such as the basic HDR10, the superb Dolby Vision, the broadcast-friendly Hybrid Log Gamma (HLG), and the rising Advanced HDR by Technicolor. An HDR display may support one or more HDR formats, but the hardware specs are more crucial to the final performance than the format adopted. In this section, we will discuss the necessities of the HDR display hardware88,89, namely, the high peak luminance, excellent dark state, high bit depth and wide color gamut.
Luminance. The human eye has a very wide dynamic range, covering an absolute specular highlight (10 000 cd/m2) to an extreme dark state (0.005 cd/m2). In contrast, the standard dynamic range display only offers a 100 cd/m2 peak luminance. As a manufacturer-friendly target, Ultra HD Premium defined the HDR luminance range as 0.05~ 1000 cd/m2 for LCDs and 0.0005~ 540 cd/m2 for OLED displays. This standard can be satisfied by all mLED/μLED/OLED display technologies. Dolby Vision is mastered at a 4000-cd/m2 peak luminance88. In 2020, Sharp’s 8 K LCD TV achieved over 10,000 cd/m2 by employing indium-gallium-zinc-oxide (IGZO) TFTs with an extremely low dark current and by boosting the backlight luminance92. The low optical efficiency-caused thermal issue can be partially addressed by local dimming technology. On the other hand, OLEDs suffer from efficiency roll-off93 and fast ageing43 at a high luminance, so they are more suitable for frequent-update devices. As a result, the mLED/μLED emissive displays demonstrate the best quality HDR preference for high luminance with high efficiency.
Bit depth
With the expansion of the luminance range, 8 bits per color is no longer sufficient to provide a smooth color change. While 10 bits are applied in current HDR display systems, 12 bits per color is highly desired to avoid banding artefacts according to the Barten model and the Perceptual Quantizer (PQ) curve90,94. Technically, at least 10 bits are required on the hardware if 2 bits are handled by dithering95. In conventional LCDs, the bit depth is limited by a large voltage swing and a slow grey-to-grey response time. Fortunately, the dual modulation units in local dimming LCDs share the burden equivalently so that the 12-bit PQ curve has been achieved82,96. In emissive displays, achieving 10-bit or 12-bit requires ultra-accurate current control in the PAM and ultra-short pulse generation in the PWM, leading to a high electronics cost. In 2018, JDC demonstrated a 10-bit µLED on a silicon backplane with PWM65. High bit depth is especially challenging when a low DR is applied to the PWM because it further reduces the shortest pulse width. Similar to the dual modulation in local dimming LCDs, hybrid driving71 could tackle the difficulties by combining PAM and PWM.
Color performance
Vivid color is another critical requirement of HDR displays. There are various standards to evaluate the color performance of a display panel, such as sRGB, NTSC, DCI-P3, and Rec. 202033,34,35. The color gamut coverage of the display is mainly defined by the central wavelength and full width at half maximum (FWHM) of the RGB emission spectrum. For example, Rec. 2020 is defined by red (630 nm), green (532 nm) and blue (467 nm) lasers33,34. In this section, we will report the color gamut (x, y area coverage in CIE 1931) and color shift of the mLED/μLED/OLED displays.
In 2017, SEL showed new materials to enable an OLED display with >101% (u’, v’) coverage, which corresponds to 91.8% (x, y) coverage in Rec. 2020. Such a large color gamut is achieved by material and device advancements: (1) Deep blue fluorescent and deep red phosphorescent OLED materials have been developed14,66,72, although further research is required to extend the device lifetime for commercial applications, and (2) the two metallic electrodes of the top emission OLED form a microcavity to significantly narrow the emission FWHM. The trade-offs are a compromised efficiency and a large angular color shift. Therefore, proper OLED structure parameter optimizations97 and better cavity designs for mitigating color shift are still needed.
Inorganic mLED/μLED inherently has a relatively narrow FWHM (18 ~ 30 nm). so, the color gamut mainly depends on the emission wavelength. Recently, 91.4% Rec. 2020 has been reported on the RGB-chip type100. A practical issue of PAM mLED/μLED displays is the central wavelength drift and the FWHM change with current100. As the current density increases, the central wavelength is blue shifted for the blue/green (InGaN) LEDs and redshifted for the red (AlGaInP) LEDs. As a result, the mixed white color (D65) may not appear as white. This current-dependent color shift can be minimized with the PWM. Inorganic mLEDs/μLEDs also have an angular-dependent color shift, which results from the LED material difference and angular spectrum mismatch of the red and green/blue LEDs70. This problem can be solved by adding a black matrix to absorb the side emission to compromise the light extraction efficiency.
For the CC-type mLED/μLED emissive displays, the color gamut is jointly determined by the blue LED chip and the green and red quantum dots. The narrow FWHM and high central wavelength tunability of QDs can theoretically enable >97% Rec. 202035, and 93.1% has been experimentally demonstrated101. In this CC emissive display, additional attention should be paid to blue light leakage. The QDCF should be thick enough to effectively convert the blue light to red and green44,102, and an additional absorptive CF44,45 or DBR46 is needed to clean up the unconverted blue light and to minimize ambient excitations. As discussed above, the current-sensitive spectrum of inorganic mLEDs/μLEDs causes a color shift on the blue subpixels under PAM so that PWM is still a preferred approach. In comparison, green and red quantum dots exhibit stable spectral emission profiles even though the wavelength and intensity of blue pumping light fluctuate. In addition, the color shift may come from the angular emission profile mismatch between the blue LED and green/red quantum dots. To address this issue, scattering particles are added to the blue subpixels in the CC film to generate the same Lambertian angular profile as the green/red subpixels.
The color gamut of mLED-LCD is dependent on the adopted CC material. From the Yttrium Aluminum Garnet (YAG) phosphor and K2SiF6 (KSF) phosphor to the QDs, the color gamut is improved from ~50% and 70 ~ 80% to 80 ~ 90% Rec. 2020103. Different from the patterned CC film in emissive displays, the white backlight and absorptive CF in LCDs may introduce color crosstalk and impair color purity. Narrower band absorptive CFs could reduce crosstalk at the cost of a lower transmittance. In 2017, Chen et al. designed a bandpass filter in conjunction with green perovskite and red QDs to generate >95% Rec. 2020104. At large viewing angles, the gamma shift of the LCDs has been addressed by multidomain designs36,37,39 and compensation films6,40 to achieve an unnoticeable color shift (<0.02).
A wide color gamut (>90% Rec. 2020) can be obtained on all of them. It is a matter of choice to balance the color gamut with the lifetime, color shift, system efficiency, luminous efficacy and cost.
Figure 10: Chromaticity Diagram Of mLED/uLED/OLED Displays With Rec. 2020
Applications in novel scenarios
Wearable displays
Wearable electronics, such as VR/AR headsets and smart wristbands, are believed to be next-generation information platforms. Common requirements for wearable displays are low power, light weight and high resolution density. Specifically, VR/AR near-eye displays demand a fast MPRT to reduce motion image blur, while smart wristbands prefer flexibility. We have already analyzed the power consumption and MPRT issues. Here, we discuss the remaining issues.
VR panels are operated in an immersed dark space so that the peak luminance of 150 ~ 200 cd/m2 should be adequate. This value corresponds to ~1000 cd/m2 instant luminance under a 15 ~ 20% DR. In Fig. 9, we plot the ηW of four different displays according to the peak EQE with different chip sizes. Ambient filters such as the CF on the CC μLED and the CP on the RGB-chip OLED/μLED are still laminated to clean up the ghost images. The efficiency ranks in the order of CC μLEDs, RGB-chip μLEDs, and mLED-LCDs to RGB-chip OLEDs when the LED chip size is over 7 μm. However, to eliminate the screen-door effect, an 100° field-of-view demands a 6K6K resolution, indicating 3000 ppi on a 2-inch panel and chip size < 5 μm. On such a small dimension, the CC μLED display is the most efficient, followed by the OLED display. On the other hand, foveation is an effective way to circumvent the high resolution/ppi hardware and software challenges105. This method releases 5× the burdens, embracing larger chips and LCDs59,106. Overall, a thin profile, high ppi, and high ηW make the performance of CC μLED emissive displays stand out, while the OLED display and mLED-LCD are mature and economic choices.
Figure 11: MPRT by Technology
Wearable displays
Wearable electronics, such as VR/AR headsets and smart wristbands, are believed to be next-generation information platforms. Common requirements for wearable displays are low power, light weight and high resolution density. Specifically, VR/AR near-eye displays demand a fast MPRT to reduce motion image blur, while smart wristbands prefer flexibility. We have already analyzed the power consumption and MPRT issues. Here, we discuss the remaining issues.
VR panels are operated in an immersed dark space so that the peak luminance of 150 ~ 200 cd/m2 should be adequate. This value corresponds to ~1000 cd/m2 instant luminance under a 15 ~ 20% DR. In Fig. 9, we plot the ηW of four different displays according to the peak EQE with different chip sizes. Ambient filters such as the CF on the CC μLED and the CP on the RGB-chip OLED/μLED are still laminated to clean up the ghost images. The efficiency ranks in the order of CC μLEDs, RGB-chip μLEDs, and mLED-LCDs to RGB-chip OLEDs when the LED chip size is over 7 μm. However, to eliminate the screen-door effect, an 100° field-of-view demands a 6K6K resolution, indicating 3000 ppi on a 2-inch panel and chip size < 5 μm. On such a small dimension, the CC μLED display is the most efficient, followed by the OLED display. On the other hand, foveation is an effective way to circumvent the high resolution/ppi hardware and software challenges105. This method releases 5× the burdens, embracing larger chips and LCDs59,106. Overall, a thin profile, high ppi, and high ηW make the performance of CC μLED emissive displays stand out, while the OLED display and mLED-LCD are mature and economic choices.
Figure 11: MPRT by Technology
Chip size versus the on-axis power efficacy (ηW) for the four specified display technologies
For AR devices, high luminance is critically important for the following reasons: (1) the displayed image overlays with environmental scenes so that the ACR matters. (2) In the space domain, a smaller panel means a higher luminance on the display if the same luminous flux is delivered to the human eye. The AR devices need much smaller panels than VR displays due to their increased optical system complexity. (3) In the time domain, a fast MPRT demands a high instant luminance. Numerically, we can use [AP · DR · Φ/ΦLED] to scale from the display luminance to the instant chip luminance, as discussed in the power consumption section. Because the lifetime of OLEDs is inversely related to their luminance43, inorganic LEDs have become the favored choice. Currently, projection displays dominate the AR market. Liquid-Crystal-on-Silicon (LCoS) feature high luminance (>40,000 cd/m2)107 and high ppi (>4000)108, but the system is bulkier because it is a reflective display24. Pursuing a slimmer profile, laser scanning is an option, except that the optical efficiency remains relatively low. In recent years, some high luminance and high resolution density emissive micro displays have been developed. In 2019, the BOE demonstrated a µOLED display with 5644 ppi and 3500-cd/m2 luminance109. On the other hand, μLED micro displays have fulfilled all the requirements of a high luminance (>10,000,000 cd/m2)23, a high ppi (>5000)110,111, a fast MPRT, low power and a long lifetime. Moreover, the small chip size opens a new door for transparent displays19,29, which would tremendously simplify the optical configuration.
Smart wristbands have viewing conditions similar to smartphones. The unique technical challenge is flexibility. To fulfil this requirement, first, the light source should better be 2D arrayed, opening the door for emissive displays and mLED-LCDs. Second, the light source requires good off-axis performance. As discussed in the HDR section, the color shift can be suppressed by various approaches. The main off-angle challenge comes from the quarter-wave plate in the CP. Therefore, CP-free small-aperture RGB-type and flexible QDCF112-laminated CC-type μLED emissive displays have the least physical limitations on flexibility and sunlight readability. On the other hand, the gamma shift on nonemissive LCDs has been well compensated6,38,39,40, and the integrated linear polarizer enhances the ACR. Researchers have developed organic TFTs for plastic substrates and flexible LCDs113. The so-called OLCDs have lower manufacturing costs and easier scalability for large panel sizes than do flexible OLED displays. Overall, OLEDs are the most mature flexible display technology, except their ACR is limited. New OLED materials with high EQE and long lifetimes are under active development14. The commercialization of flexible mLED-LCDs depends more on market strategies instead of technical challenges. Flexible μLED emissive displays are in the prototyping stage19,29. The CP-free small-aperture μLED is theoretically the best candidate.
Vehicle displays
Typical vehicle displays for automobiles and spacecraft include central cluster panels and head-up display (HUD) units. For these applications, reliability and sunlight readability are critically important for driver safety. A wide working temperature is an additional demand on vehicle displays. Inorganic LEDs have the widest temperature range. OLED displays function well in freezing cold environments and age fast if heated114,115. LCDs respond slowly in cold weather, and the upper limit depends on the clearing temperature (Tc). With extensive development efforts, LCs with Tc > 100 °C and 10-ms response times at −20 °C have been demonstrated83. Another drawback of LCDs is thermal management due to their low optical efficiency. Overall, mLED and μLED emissive displays show great advantages over OLED displays in luminance, lifetime and robustness in extreme environments.
In central clusters, a conventional LCD is the mainstream. With the alliance of the mLED BLU, a higher contrast ratio, lower power consumption, less heat generation and freeform factors are promising features to be realized. Micro-LED emissive displays may further enhance the HDR performance and power efficiency. Preferences with respect to the power efficiency can refer to the similar-pitch notebook in Fig. 6b.
The currently dominating HUDs in the market are LCD projection displays for the windshield or a postcard-size combiner116. There are several solutions to improve HUD quality: (1) Employing HDR panels to eliminate the postcard effect and gain higher peak brightness, where all mLED/μLED/OLED displays apply. (2) Enhancing the combiner reflectance of displays and smartly adjusting the ambient light transmission. An effective method is polarization modulation117. In this way, the display needs a polarizer at the output layer so that the optical efficiency of the CC μLED emissive display will be trimmed by half. Conceptually, transparent displays19,29,30 outperform projection displays with respect to the system complexity, optical efficiency, eye box, field-of-view, etc. Technically, high transparency can be realized by utilizing either high conductivity transparent electrodes in PM displays29 or patterned transparent electrodes in AM displays30. Generally, a large aperture lays the foundation of high luminance in OLED transparent displays30, while they can be minified by employing μLEDs. To date, an ~ 70% transparency has been achieved on OLED30 and μLED29 displays.
For AR devices, high luminance is critically important for the following reasons: (1) the displayed image overlays with environmental scenes so that the ACR matters. (2) In the space domain, a smaller panel means a higher luminance on the display if the same luminous flux is delivered to the human eye. The AR devices need much smaller panels than VR displays due to their increased optical system complexity. (3) In the time domain, a fast MPRT demands a high instant luminance. Numerically, we can use [AP · DR · Φ/ΦLED] to scale from the display luminance to the instant chip luminance, as discussed in the power consumption section. Because the lifetime of OLEDs is inversely related to their luminance43, inorganic LEDs have become the favored choice. Currently, projection displays dominate the AR market. Liquid-Crystal-on-Silicon (LCoS) feature high luminance (>40,000 cd/m2)107 and high ppi (>4000)108, but the system is bulkier because it is a reflective display24. Pursuing a slimmer profile, laser scanning is an option, except that the optical efficiency remains relatively low. In recent years, some high luminance and high resolution density emissive micro displays have been developed. In 2019, the BOE demonstrated a µOLED display with 5644 ppi and 3500-cd/m2 luminance109. On the other hand, μLED micro displays have fulfilled all the requirements of a high luminance (>10,000,000 cd/m2)23, a high ppi (>5000)110,111, a fast MPRT, low power and a long lifetime. Moreover, the small chip size opens a new door for transparent displays19,29, which would tremendously simplify the optical configuration.
Smart wristbands have viewing conditions similar to smartphones. The unique technical challenge is flexibility. To fulfil this requirement, first, the light source should better be 2D arrayed, opening the door for emissive displays and mLED-LCDs. Second, the light source requires good off-axis performance. As discussed in the HDR section, the color shift can be suppressed by various approaches. The main off-angle challenge comes from the quarter-wave plate in the CP. Therefore, CP-free small-aperture RGB-type and flexible QDCF112-laminated CC-type μLED emissive displays have the least physical limitations on flexibility and sunlight readability. On the other hand, the gamma shift on nonemissive LCDs has been well compensated6,38,39,40, and the integrated linear polarizer enhances the ACR. Researchers have developed organic TFTs for plastic substrates and flexible LCDs113. The so-called OLCDs have lower manufacturing costs and easier scalability for large panel sizes than do flexible OLED displays. Overall, OLEDs are the most mature flexible display technology, except their ACR is limited. New OLED materials with high EQE and long lifetimes are under active development14. The commercialization of flexible mLED-LCDs depends more on market strategies instead of technical challenges. Flexible μLED emissive displays are in the prototyping stage19,29. The CP-free small-aperture μLED is theoretically the best candidate.
Vehicle displays
Typical vehicle displays for automobiles and spacecraft include central cluster panels and head-up display (HUD) units. For these applications, reliability and sunlight readability are critically important for driver safety. A wide working temperature is an additional demand on vehicle displays. Inorganic LEDs have the widest temperature range. OLED displays function well in freezing cold environments and age fast if heated114,115. LCDs respond slowly in cold weather, and the upper limit depends on the clearing temperature (Tc). With extensive development efforts, LCs with Tc > 100 °C and 10-ms response times at −20 °C have been demonstrated83. Another drawback of LCDs is thermal management due to their low optical efficiency. Overall, mLED and μLED emissive displays show great advantages over OLED displays in luminance, lifetime and robustness in extreme environments.
In central clusters, a conventional LCD is the mainstream. With the alliance of the mLED BLU, a higher contrast ratio, lower power consumption, less heat generation and freeform factors are promising features to be realized. Micro-LED emissive displays may further enhance the HDR performance and power efficiency. Preferences with respect to the power efficiency can refer to the similar-pitch notebook in Fig. 6b.
The currently dominating HUDs in the market are LCD projection displays for the windshield or a postcard-size combiner116. There are several solutions to improve HUD quality: (1) Employing HDR panels to eliminate the postcard effect and gain higher peak brightness, where all mLED/μLED/OLED displays apply. (2) Enhancing the combiner reflectance of displays and smartly adjusting the ambient light transmission. An effective method is polarization modulation117. In this way, the display needs a polarizer at the output layer so that the optical efficiency of the CC μLED emissive display will be trimmed by half. Conceptually, transparent displays19,29,30 outperform projection displays with respect to the system complexity, optical efficiency, eye box, field-of-view, etc. Technically, high transparency can be realized by utilizing either high conductivity transparent electrodes in PM displays29 or patterned transparent electrodes in AM displays30. Generally, a large aperture lays the foundation of high luminance in OLED transparent displays30, while they can be minified by employing μLEDs. To date, an ~ 70% transparency has been achieved on OLED30 and μLED29 displays.