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Micro-LED (light-emitting diode) is a potentially disruptive display technology, while power consumption is a critical issue for all display devices. In this paper, we develop a physical model to evaluate the power consumption of micro-LED displays under different ambient lighting conditions. Both power efficiency and ambient reflectance are investigated in two types of full color display structures: red/green/blue (RGB) micro-LEDs, and blue-LED pumped quantum dots color-conversion. For each type of display with uniform RGB chip size, our simulation results indicate that there exists an optimal LED chip size, which leads to 30–40% power saving. We then extend our model to analyze different RGB chip sizes, and find that with optimized chip sizes an additional 12% average power saving can be achieved over that with uniform chip size.
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crystals
Article
Improving the Power Eciency of Micro-LED
Displays with Optimized LED Chip Sizes
En-Lin Hsiang 1, Ziqian He 1, Yuge Huang 1, Fangwang Gou 1, Yi-Fen Lan 2
and Shin-Tson Wu 1,*
1College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA;
enlinhsiang@knights.ucf.edu (E.-L.H.); zhe@Knights.ucf.edu (Z.H.); y.huang@knights.ucf.edu (Y.H.);
fangwang.gou@knights.ucf.edu (F.G.)
2AU Optronics Corp. Hsinchu Science Park, Hsinchu 300, Taiwan; Even.YF.Lan@auo.com
*Correspondence: swu@creol.ucf.edu
Received: 22 May 2020; Accepted: 4 June 2020; Published: 8 June 2020


Abstract:
Micro-LED (light-emitting diode) is a potentially disruptive display technology, while power
consumption is a critical issue for all display devices. In this paper, we develop a physical model to
evaluate the power consumption of micro-LED displays under dierent ambient lighting conditions.
Both power eciency and ambient reflectance are investigated in two types of full color display
structures: red/green/blue (RGB) micro-LEDs, and blue-LED pumped quantum dots color-conversion.
For each type of display with uniform RGB chip size, our simulation results indicate that there exists
an optimal LED chip size, which leads to 30–40% power saving. We then extend our model to analyze
dierent RGB chip sizes, and find that with optimized chip sizes an additional 12% average power
saving can be achieved over that with uniform chip size.
Keywords: micro-LED display; color-conversion; power consumption; ambient contrast ratio
1. Introduction
Micro-LED displays with high peak luminance, true dark state, high resolution, wide color gamut
and long lifetime are emerging as next-generation displays [
1
4
]. Two approaches are commonly
used to achieve full color: red/green/blue (RGB) subpixels and blue-LED pumped color-conversion.
In the first scheme, RGB micro-LED chips are fabricated from dierent semiconductor materials,
according to their lattice constants and energy band gaps. For examples, red LEDs are typically
fabricated by growing AlGaInP epilayers on GaAs substrates, while green and blue LEDs are produced
by depositing InGaN epilayers on sapphire substrates. In such a RGB display panel, millions of
micro-LED chips are transferred from the corresponding semiconductor wafers to the display substrate
through mass transfer processes [
5
7
]. In the color-conversion scheme, we can use UV or blue
LEDs to excite the down-conversion materials, such as quantum dots (QDs) or phosphors [
8
11
].
These color- conversion materials can be fabricated by inject printing or photolithography [
12
,
13
].
These micro- LED displays have found potential applications in ultra-large size and seamlessly
tiled video walls [
14
,
15
], 75-inch modular TVs, medium-size sunlight readable vehicle displays,
smart watches, and high-resolution-density microdisplays [
16
21
] for augmented reality and virtual
reality, just to name a few.
In addition to above-mentioned properties, low power consumption is always desirable for
micro-LED display to compete with its counterparts such as liquid crystal displays (LCDs) and organic
LED (OLED) displays [
22
24
]. Especially for a mobile display, its operating time is governed by the
battery capacity. Thus, power consumption is a critical issue for all mobile display devices. Although
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TVs and desktop monitors are usually connected to wall plugs, low power consumption helps to save
the ecosystem.
Generally, LED’s eciency decreases as the chip size shrinks due to sidewall defects [
25
30
].
To improve LED’s eciency, several approaches have been investigated, such as enhancing the light
extraction eciency and boosting the internal quantum eciency [
31
,
32
]. However, an obvious
tradeois the increased fabrication complicity. In addition, the external quantum eciency (EQE)
of LEDs depends on the driving current density. In order to keep driving at peak EQE, pulse width
modulation (PWM) is a common method for power saving [
14
,
33
,
34
]. In PWM, the driving current
density stays at peak EQE, while the luminance is modulated by changing the duty ratio in each frame.
In this paper, we evaluate the power consumption of both types of micro-LED displays, including
RGB chips and blue LEDs pumped color conversion, under dierent ambient lighting conditions. First,
we evaluate the power consumption of uniform LED chip size, i.e. RGB subpixels having the same
chip size, as the baseline for comparison. The optimum LED chip sizes corresponding to the lowest
power consumption for three applications: smartphones, laptop computers, and TVs, are analyzed.
For TV applications, the LED chip size studied ranges from 5
µ
m to 50
µ
m. The optimal LED chip size
is found to be 16
µ
m for the RGB type and 20
µ
m for the color conversion type. At the optimal chip
size, the power saving can reach 30–40%. Next, we extend our model to evaluate RGB subpixels with
dierent chip sizes. Through the same optimization procedures, our proposed micro-LED display with
dierent RGB chip sizes further reduces ~ 12% average power consumption than that with uniform
LED chip size.
2. Device Modeling
2.1. RGB Micro-LED Display
Figure 1depicts the device structure of our proposed micro-LED display, where W
R
, W
G
, and W
B
represent the chip size of RGB LEDs, respectively. The gap between micro LEDs is filled with black
matrix to reduce ambient light reflection. Because of the small aperture ratio of micro-LED displays,
the circular polarizer normally used in OLED displays to reduce the ambient light reflection is not
required here.
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Generally, LED’s efficiency decreases as the chip size shrinks due to sidewall defects [25–30]. To
improve LED’s efficiency, several approaches have been investigated, such as enhancing the light
extraction efficiency and boosting the internal quantum efficiency [31,32]. However, an obvious
tradeoff is the increased fabrication complicity. In addition, the external quantum efficiency (EQE) of
LEDs depends on the driving current density. In order to keep driving at peak EQE, pulse width
modulation (PWM) is a common method for power saving [14,33,34]. In PWM, the driving current
density stays at peak EQE, while the luminance is modulated by changing the duty ratio in each
frame.
In this paper, we evaluate the power consumption of both types of micro-LED displays,
including RGB chips and blue LEDs pumped color conversion, under different ambient lighting
conditions. First, we evaluate the power consumption of uniform LED chip size, i.e. RGB subpixels
having the same chip size, as the baseline for comparison. The optimum LED chip sizes
corresponding to the lowest power consumption for three applications: smartphones, laptop
computers, and TVs, are analyzed. For TV applications, the LED chip size studied ranges from 5 µm
to 50 µm. The optimal LED chip size is found to be 16 µm for the RGB type and 20 µm for the color
conversion type. At the optimal chip size, the power saving can reach 30–40%. Next, we extend our
model to evaluate RGB subpixels with different chip sizes. Through the same optimization
procedures, our proposed micro-LED display with different RGB chip sizes further reduces ~ 12%
average power consumption than that with uniform LED chip size.
2. Device Modeling
2.1. RGB Micro-LED Display
Figure 1 depicts the device structure of our proposed micro-LED display, where WR, WG, and
WB represent the chip size of RGB LEDs, respectively. The gap between micro LEDs is filled with
black matrix to reduce ambient light reflection. Because of the small aperture ratio of micro-LED
displays, the circular polarizer normally used in OLED displays to reduce the ambient light reflection
is not required here.
Figure 1. Device structure of our proposed RGB micro-LED display.
It is rare that a display device is used under a completely dark room. Thus, ambient contrast
ratio (ACR) is a more realistic way to compare the performance of different display devices. The ACR
of a display is defined as [35]:
,
on ambient
off ambient
L L R
ACR L L R
 
(1)
where Lon (Loff0) represents the on (off)-state luminance of the display, Lambient is the ambient
luminance, and R is the ambient light reflectance which depends on the surface reflectivity. From
Equation 1, in a dark room (Lambient = 0), an emissive display can easily achieve over 10 : 1 contrast
ratio. However, as the ambient luminance increases, the ACR declines sharply [36].
From Figure 1, the incident ambient light will be partially reflected by the LED chips, and
absorbed by the black matrix. Thus, the total ambient light reflection of the display panel depends on
the micro-LED chip size. To investigate the ambient light reflection of RGB micro-LED displays, we
build a ray-tracing simulation model based on Light Tools. The device structure of flip-chip RGB
micro-LED is similar to that reported in [37]. The device material characteristics of the flip-chip RGB
Figure 1. Device structure of our proposed RGB micro-LED display.
It is rare that a display device is used under a completely dark room. Thus, ambient contrast ratio
(ACR) is a more realistic way to compare the performance of dierent display devices. The ACR of a
display is defined as [35]:
ACR =Lon +Lambient ×R
Lo f f +Lambient ×R, (1)
where L
on
(L
o
0) represents the on (o)-state luminance of the display, L
ambient
is the ambient luminance,
and R is the ambient light reflectance which depends on the surface reflectivity. From Equation (1), in a
dark room (L
ambient
=0), an emissive display can easily achieve over 10
6
: 1 contrast ratio. However,
as the ambient luminance increases, the ACR declines sharply [36].
From Figure 1, the incident ambient light will be partially reflected by the LED chips, and absorbed
by the black matrix. Thus, the total ambient light reflection of the display panel depends on the
micro-LED chip size. To investigate the ambient light reflection of RGB micro-LED displays, we build
a ray-tracing simulation model based on Light Tools. The device structure of flip-chip RGB micro-LED
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is similar to that reported in [
37
]. The device material characteristics of the flip-chip RGB micro-LEDs
are summarized in Table 1, where nand krepresent the real part and imaginary part of the refractive
index of the corresponding material [38,39].
Table 1. Material parameters used in RGB micro-LED display simulations.
Material Parameters 626 nm 529 nm 465 nm
nknknk
Molding layer 1.48 0 1.49 0 1.50 0
Red LED chip 3.30 0 3.56 0.16 3.76 0.28
Blue/Green LED chip 2.35 4×1052.34 4×1052.42 4×105
Bounding metal 0.15 3.52 0.44 2.29 1.43 1.85
Glass substrate 1.5 0 1.5 0 1.5 0
Let us assume the ambient light is a standard illuminant D65 white light. The intensity spectrum
of ambient light and reflected light by RGB micro-LEDs are shown in Figure 2a. In the 400 nm to
560 nm blue-green region, the absorption of AlGaInP-based red micro-LED is much stronger than that
of InGaN-based green/blue micro-LEDs, as Table 1shows.
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micro-LEDs are summarized in Table 1, where n and k represent the real part and imaginary part of
the refractive index of the corresponding material [38,39].
Table 1. Material parameters used in RGB micro-LED display simulations.
Material Parameters
626 nm 529 nm 465 nm
n k n k n k
Molding layer 1.48 0 1.49 0 1.50 0
Red LED chip 3.30 0 3.56 0.16 3.76 0.28
Blue/Green LED chip 2.35 4 × 10−5 2.34 4 × 105 2.42 4 × 105
Bounding metal 0.15 3.52 0.44 2.29 1.43 1.85
Glass substrate 1.5 0 1.5 0 1.5 0
Let us assume the ambient light is a standard illuminant D65 white light. The intensity spectrum
of ambient light and reflected light by RGB micro-LEDs are shown in Figure 2a. In the 400 nm to 560
nm blue-green region, the absorption of AlGaInP-based red micro-LED is much stronger than that of
InGaN-based green/blue micro-LEDs, as Table 1 shows.
The luminance ambient reflectance can be calculated from
( ) ( ) ,
( ) ( )
reflect
L
ambient
I K d
RI K d
 
 
 
 
(2)
where I() is the radiant flux and K() represents the photopic human eye sensitivity function. From
Equation 2, we find the luminance ambient reflectance of the AlGaInP-based red micro-LED is 29.94%
and the InGaN-based blue/green micro-LEDs is 66.07%. As mentioned above, the ambient light is
partially reflected at the LED chip area but is absorbed in the black matrix region. Therefore, the
luminance ambient reflectance is proportional to the aperture ratio of the micro-LED display, as
plotted in Figure 2b. At a given aperture ratio, the luminance ambient reflectance of blue and green
LEDs is about 2.2x stronger than that of red LED.
Figure 2. (a) The intensity spectrum of ambient light, ambient light reflected by AlGaInP based LED,
and ambient light reflected by InGaN based LED. (b) The luminance ambient reflectance of RGB
micro-LEDs at different LED chip aperture ratio.
From Equation 1, if the off-state luminance of a display is zero (Loff = 0), then the on-state
luminance can be expressed as:
Figure 2.
(
a
) The intensity spectrum of ambient light, ambient light reflected by AlGaInP based LED,
and ambient light reflected by InGaN based LED. (
b
) The luminance ambient reflectance of RGB
micro-LEDs at dierent LED chip aperture ratio.
The luminance ambient reflectance can be calculated from
RL=RIre f lect(λ)·K(λ)·dλ
RIambient(λ)·K(λ)·dλ, (2)
where I(
λ
) is the radiant flux and K(
λ
) represents the photopic human eye sensitivity function.
From Equation (2), we find the luminance ambient reflectance of the AlGaInP-based red micro-LED
is 29.94% and the InGaN-based blue/green micro-LEDs is 66.07%. As mentioned above, the ambient
light is partially reflected at the LED chip area but is absorbed in the black matrix region. Therefore,
the luminance ambient reflectance is proportional to the aperture ratio of the micro-LED display,
as plotted in Figure 2b. At a given aperture ratio, the luminance ambient reflectance of blue and green
LEDs is about 2.2x stronger than that of red LED.
From Equation (1), if the o-state luminance of a display is zero (L
o
=0), then the on-state
luminance can be expressed as:
Ldisplay =Lambient ×
Rs+ (1Rs)×X
i=RGB
RL(i)×AP(i)
×(ACR 1), (3)
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where R
S
is the surface ambient reflectance from the cover glass, R
L(RGB)
is the luminance ambient
reflectance from RGB LEDs respectively when the aperture ratio is 1, and AP
(RGB)
is the aperture ratio
of RGB LEDs. For touch-panel smartphones and laptop computers, the cover glass usually does not
have anti-reflection (AR) coating. Thus, we assume their surface reflection is around 4%. However,
most of TVs use remote control so that we can apply AR coating to reduce the surface reflection.
Here, we assume their surface reflection is 1.2%. Besides, the ambient light illuminance could vary
wildly depending on the environment lighting conditions, e.g., direct sunlight, oce light, and living
room light. From Equation (3), to achieve the same ACR, the display luminance should increase as
the ambient reflectance increases, which in turn is proportional to the aperture ratio. That is to say,
a smaller aperture ratio in the RGB micro-LED display and a lower ambient reflectance would lead to
a lower display luminance for the same ACR. The display luminance as a function of aperture ratio
under three dierent ambient illuminances: 2000 lux for outdoor, 450 lux for oces, and 200 lux for
living rooms, is plotted in Figure 3.
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( ) ( )
(1 ) ( 1),
display ambient s s L i i
i RGB
L L R R R AP ACR
 
 
 
 
(3)
where RS is the surface ambient reflectance from the cover glass, RL(RGB) is the luminance ambient
reflectance from RGB LEDs respectively when the aperture ratio is 1, and AP(RGB) is the aperture ratio
of RGB LEDs. For touch-panel smartphones and laptop computers, the cover glass usually does not
have anti-reflection (AR) coating. Thus, we assume their surface reflection is around 4%. However,
most of TVs use remote control so that we can apply AR coating to reduce the surface reflection. Here,
we assume their surface reflection is 1.2%. Besides, the ambient light illuminance could vary wildly
depending on the environment lighting conditions, e.g. direct sunlight, office light, and living room
light. From Equation 3, to achieve the same ACR, the display luminance should increase as the
ambient reflectance increases, which in turn is proportional to the aperture ratio. That is to say, a
smaller aperture ratio in the RGB micro-LED display and a lower ambient reflectance would lead to
a lower display luminance for the same ACR. The display luminance as a function of aperture ratio
under three different ambient illuminances: 2000 lux for outdoor, 450 lux for offices, and 200 lux for
living rooms, is plotted in Figure 3.
Figure 3. The required luminance of display, as a function of aperture ratio, for achieving the listed
ACR under three ambient light illuminances: (a) 2000 lux, (b) 450 lux, and (c) 200 lux.
Meanwhile, the power consumption of the display is also affected by the efficiency of LED chips.
The power efficiency of LEDs is defined as the ratio of luminance intensity over the power
consumption (cd/W) as:
/.
RGB sys RGB RGB
RGB RGB
RGB
LED RGB RGB
EQE T h K
P q V
 
  (4)
In Equation 4, η stands for the luminance efficiency,
the luminance flux, P the power
consumption of LED, q the elementary charge, hv the photon energy, K the luminance efficacy, α
conversion efficiency from luminance intensity [unit: cd] to luminous flux Φ [unit : lm], and V the
driving voltage of LED. Since PWM driving scheme is employed in our simulations, the EQE in
Equation 4 represents the peak EQE of the LED while the voltage is fixed at the optimal driving
condition.
Generally, the peak EQE of LED in Equation 4 depends on the chip size. Therefore, to investigate
the power efficiency of different LED chip sizes, we have to take this peak EQE variation into
consideration. The peak EQE deceases as the micro-LED chip size decreases, resulting from the
nonradiative recombination at the etched sidewall. The ratio of sidewall surface to the total surface
increases as the LED chip size shrinks. As a result, the sidewall effect becomes more significant when
the LED chip size is smaller than ~ 100 µm. The chip size dependent efficiency of InGaN based micro-
LEDs has been widely discussed in [25–28]. Because AlGaInP exhibits a higher surface recombination
velocity than InGaN, the efficiency drop of red micro-LED is more serious than the blue and green
ones as the chip size decreases [29]. Detailed theoretical analyses and experimental results have been
reported in [30]. Using these published results, we plot the peak EQE as a function of LED chip size
for RGB LEDs in Figure 4.
Figure 3.
The required luminance of display, as a function of aperture ratio, for achieving the listed
ACR under three ambient light illuminances: (a) 2000 lux, (b) 450 lux, and (c) 200 lux.
Meanwhile, the power consumption of the display is also aected by the eciency of LED chips.
The power eciency of LEDs is defined as the ratio of luminance intensity over the power consumption
(cd/W) as:
ηRGB =ΦRGB/αRGB
PLED
=EQERGB ×Tsys ×hνRGB ×KRGB
q×VRGB ×αRGB
. (4)
In Equation (4),
η
stands for the luminance eciency,
Φ
the luminance flux, Pthe power
consumption of LED, qthe elementary charge, hv the photon energy, K the luminance ecacy,
α
conversion eciency from luminance intensity [unit: cd] to luminous flux
Φ
[unit:lm], and V
the driving voltage of LED. Since PWM driving scheme is employed in our simulations, the EQE
in Equation (4) represents the peak EQE of the LED while the voltage is fixed at the optimal
driving condition.
Generally, the peak EQE of LED in Equation (4) depends on the chip size. Therefore, to investigate
the power eciency of dierent LED chip sizes, we have to take this peak EQE variation into
consideration. The peak EQE deceases as the micro-LED chip size decreases, resulting from the
nonradiative recombination at the etched sidewall. The ratio of sidewall surface to the total surface
increases as the LED chip size shrinks. As a result, the sidewall eect becomes more significant
when the LED chip size is smaller than ~ 100
µ
m. The chip size dependent eciency of InGaN
based micro-LEDs has been widely discussed in [
25
28
]. Because AlGaInP exhibits a higher surface
recombination velocity than InGaN, the eciency drop of red micro-LED is more serious than the blue
and green ones as the chip size decreases [
29
]. Detailed theoretical analyses and experimental results
have been reported in [
30
]. Using these published results, we plot the peak EQE as a function of LED
chip size for RGB LEDs in Figure 4.
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Figure 4. The peak EQE of RGB micro-LED as a function of LED chip size.
2.2. Color-Conversion Micro-LED Display
Figure 5 illustrates the device structure of color conversion micro-LED displays. The QD color
conversion layer is deposited above the blue micro-LED array in green and red subpixels. These
quantum dots have isotropic scattering. Thus, to match the same scattering property, we also fill
scattering particles in the blue subpixels to make the angular spectrum of RGB lights consistent. The
color filter array is aligned with the QD layer to prevent the ambient light excitation and the blue
light leakage. The black matrix filled between LEDs helps suppress ambient light reflection and color
crosstalk [40]. The reflected ambient light consists of two major parts. (1) QD excitation (Iemit). When
the ambient light is incident on the QD layer, a portion is absorbed and down-converted to green or
red light by the QD materials. The down-converted light escapes from the QD layer and enters the
air. From Figure 5, the ambient light traverses through the QD layer twice. Therefore, the excitation
can occur in both traveling routes. (2) Reflection from LED chips (IR). As Figure 5 depicts, the incident
ambient light, which is not completely absorbed by the QD material, is reflected back by the blue
micro-LED chip.
Figure 5. Device structure of our color-converted micro-LED display.
As Equation 3 illustrates, in order to define the luminance of display at different ambient
conditions, we have to know the luminance ambient reflectance of display. Therefore, we build a
Light Tools raytracing model to quantitatively analyze the luminance ambient reflectance [41]. In the
model, the mean path method is used to simulate the photo-luminance of QD materials. The mean
path represents the average distance that a ray propagates before it hits the QD particle [42,43]. In the
0 20 40 60 80 100
Chip size (
m)
0
0.1
0.2
0.3
0.4
0.5
Red LED
Green LED
Blue LED
Figure 4. The peak EQE of RGB micro-LED as a function of LED chip size.
2.2. Color-Conversion Micro-LED Display
Figure 5illustrates the device structure of color conversion micro-LED displays. The QD
color conversion layer is deposited above the blue micro-LED array in green and red subpixels.
These quantum dots have isotropic scattering. Thus, to match the same scattering property, we also
fill scattering particles in the blue subpixels to make the angular spectrum of RGB lights consistent.
The color filter array is aligned with the QD layer to prevent the ambient light excitation and the
blue light leakage. The black matrix filled between LEDs helps suppress ambient light reflection and
color crosstalk [
40
]. The reflected ambient light consists of two major parts. (1) QD excitation (I
emit
).
When the ambient light is incident on the QD layer, a portion is absorbed and down-converted to green
or red light by the QD materials. The down-converted light escapes from the QD layer and enters the
air. From Figure 5, the ambient light traverses through the QD layer twice. Therefore, the excitation
can occur in both traveling routes. (2) Reflection from LED chips (I
R
). As Figure 5depicts, the incident
ambient light, which is not completely absorbed by the QD material, is reflected back by the blue
micro-LED chip.
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Figure 4. The peak EQE of RGB micro-LED as a function of LED chip size.
2.2. Color-Conversion Micro-LED Display
Figure 5 illustrates the device structure of color conversion micro-LED displays. The QD color
conversion layer is deposited above the blue micro-LED array in green and red subpixels. These
quantum dots have isotropic scattering. Thus, to match the same scattering property, we also fill
scattering particles in the blue subpixels to make the angular spectrum of RGB lights consistent. The
color filter array is aligned with the QD layer to prevent the ambient light excitation and the blue
light leakage. The black matrix filled between LEDs helps suppress ambient light reflection and color
crosstalk [40]. The reflected ambient light consists of two major parts. (1) QD excitation (Iemit). When
the ambient light is incident on the QD layer, a portion is absorbed and down-converted to green or
red light by the QD materials. The down-converted light escapes from the QD layer and enters the
air. From Figure 5, the ambient light traverses through the QD layer twice. Therefore, the excitation
can occur in both traveling routes. (2) Reflection from LED chips (IR). As Figure 5 depicts, the incident
ambient light, which is not completely absorbed by the QD material, is reflected back by the blue
micro-LED chip.
Figure 5. Device structure of our color-converted micro-LED display.
As Equation 3 illustrates, in order to define the luminance of display at different ambient
conditions, we have to know the luminance ambient reflectance of display. Therefore, we build a
Light Tools raytracing model to quantitatively analyze the luminance ambient reflectance [41]. In the
model, the mean path method is used to simulate the photo-luminance of QD materials. The mean
path represents the average distance that a ray propagates before it hits the QD particle [42,43]. In the
0 20 40 60 80 100
Chip size (
m)
0
0.1
0.2
0.3
0.4
0.5
Red LED
Green LED
Blue LED
Figure 5. Device structure of our color-converted micro-LED display.
As Equation (3) illustrates, in order to define the luminance of display at dierent ambient
conditions, we have to know the luminance ambient reflectance of display. Therefore, we build a
Light Tools raytracing model to quantitatively analyze the luminance ambient reflectance [
41
]. In the
model, the mean path method is used to simulate the photo-luminance of QD materials. The mean
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path represents the average distance that a ray propagates before it hits the QD particle [
42
,
43
]. In the
simulations, we define the mean path value, which can totally absorb the blue light emitted by the
micro-LED. Because the QDs are nanometer-sized particles, the ray does not change direction after
hitting the QD particle. In addition, the down-converted light has isotropic scattering. The material
properties of blue LED are listed in Table 1. We plot the spectra of RGB color filters in Figure 6a.
The emission and absorption spectra of QD materials are depicted in solid and dashed lines in Figure 6b.
The photoluminescence quantum yield (PLQY) is 88% for green QD and 90% for red QD [44].
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simulations, we define the mean path value, which can totally absorb the blue light emitted by the
micro-LED. Because the QDs are nanometer-sized particles, the ray does not change direction after
hitting the QD particle. In addition, the down-converted light has isotropic scattering. The material
properties of blue LED are listed in Table 1. We plot the spectra of RGB color filters in Figure 6a. The
emission and absorption spectra of QD materials are depicted in solid and dashed lines in Figure 6b.
The photoluminescence quantum yield (PLQY) is 88% for green QD and 90% for red QD [44].
Figure 6. (a) The transmittance spectra of color filters and (b) the emission and absorption spectra of
green and red quantum dots.
Figure 7 shows the calculated intensity of ambient light and reflected ambient light from RGB
subpixels. The reflected light is separated into two categories: QD excitation (Iemit) and reflected light
without QD absorption (IR). Without luminescent material, the Iemit is zero in blue subpixels. We only
show IR in Figure 7a. For green subpixels, because the cutoff wavelength of green color filter is ~ 450
nm, some of the blue and most of the green light can transmit through the green color filters. From
Figure 6b, the transmitted green light can still excite the green QD materials. Therefore, the reflected
light is dominated by Iemit as Figure 7b depicts. For red subpixels, the cutoff wavelength of red color
filter is ~ 570 nm, thus, the transmitted light can hardly excite the QD materials. Therefore, Iemit is
small and IR dominates the reflected light as Figure 7c shows.
Figure 7. Calculated intensity spectrum of ambient light, reflected light for (a) blue, (b) green and (c)
red subpixels.
From Equation 2, we find the luminance ambient reflectance of red, green and blue subpixels is
6.83%, 16.73%, and 2.88%, respectively. Because the ambient light is reflected in the emission area but
is absorbed in the black matrix region, the luminance ambient reflectance is proportional to the
aperture ratio of the micro-LED display. The relationship between aperture ratio and luminance
ambient reflectance of color conversion micro-LED display is shown in Figure 8.
Figure 6.
(
a
) The transmittance spectra of color filters and (
b
) the emission and absorption spectra of
green and red quantum dots.
Figure 7shows the calculated intensity of ambient light and reflected ambient light from RGB
subpixels. The reflected light is separated into two categories: QD excitation (I
emit
) and reflected
light without QD absorption (I
R
). Without luminescent material, the I
emit
is zero in blue subpixels.
We only show I
R
in Figure 7a. For green subpixels, because the cutowavelength of green color
filter is ~450 nm, some of the blue and most of the green light can transmit through the green color
filters. From Figure 6b, the transmitted green light can still excite the green QD materials. Therefore,
the reflected light is dominated by I
emit
as Figure 7b depicts. For red subpixels, the cutowavelength
of red color filter is ~570 nm, thus, the transmitted light can hardly excite the QD materials. Therefore,
Iemit is small and IRdominates the reflected light as Figure 7c shows.
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simulations, we define the mean path value, which can totally absorb the blue light emitted by the
micro-LED. Because the QDs are nanometer-sized particles, the ray does not change direction after
hitting the QD particle. In addition, the down-converted light has isotropic scattering. The material
properties of blue LED are listed in Table 1. We plot the spectra of RGB color filters in Figure 6a. The
emission and absorption spectra of QD materials are depicted in solid and dashed lines in Figure 6b.
The photoluminescence quantum yield (PLQY) is 88% for green QD and 90% for red QD [44].
Figure 6. (a) The transmittance spectra of color filters and (b) the emission and absorption spectra of
green and red quantum dots.
Figure 7 shows the calculated intensity of ambient light and reflected ambient light from RGB
subpixels. The reflected light is separated into two categories: QD excitation (Iemit) and reflected light
without QD absorption (IR). Without luminescent material, the Iemit is zero in blue subpixels. We only
show IR in Figure 7a. For green subpixels, because the cutoff wavelength of green color filter is ~ 450
nm, some of the blue and most of the green light can transmit through the green color filters. From
Figure 6b, the transmitted green light can still excite the green QD materials. Therefore, the reflected
light is dominated by Iemit as Figure 7b depicts. For red subpixels, the cutoff wavelength of red color
filter is ~ 570 nm, thus, the transmitted light can hardly excite the QD materials. Therefore, Iemit is
small and IR dominates the reflected light as Figure 7c shows.
Figure 7. Calculated intensity spectrum of ambient light, reflected light for (a) blue, (b) green and (c)
red subpixels.
From Equation 2, we find the luminance ambient reflectance of red, green and blue subpixels is
6.83%, 16.73%, and 2.88%, respectively. Because the ambient light is reflected in the emission area but
is absorbed in the black matrix region, the luminance ambient reflectance is proportional to the
aperture ratio of the micro-LED display. The relationship between aperture ratio and luminance
ambient reflectance of color conversion micro-LED display is shown in Figure 8.
Figure 7.
Calculated intensity spectrum of ambient light, reflected light for (
a
) blue, (
b
) green and (
c
)
red subpixels.
From Equation (2), we find the luminance ambient reflectance of red, green and blue subpixels
is 6.83%, 16.73%, and 2.88%, respectively. Because the ambient light is reflected in the emission area
but is absorbed in the black matrix region, the luminance ambient reflectance is proportional to the
aperture ratio of the micro-LED display. The relationship between aperture ratio and luminance
ambient reflectance of color conversion micro-LED display is shown in Figure 8.
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.
Figure 8. The luminance ambient reflectance of RGB subpixels in color-conversion micro-LED display
at different aperture ratio.
By substituting the luminance ambient reflectance from Figure 8 into Equation 3, we calculate
the display luminance as a function of aperture ratio under three different ambient illuminances: 2000
lux for outdoor, 450 lux for offices and 200 lux for living rooms. Results are plotted in Figure 9.
Figure 9. The required luminance of color-conversion micro-LED display as a function of aperture
ratio for achieving the listed ACR under three ambient light illuminances: (a) 2000 lux, (b) 450 lux and
(c) 200 lux.
The power efficiency of RGB subpixels in color-conversion micro-LED display is defined as the
ratio of luminance intensity over the power consumption (cd/W), as:
/.
B RG RG sys RGB RGB
RGB RGB
RGB
LED RGB RGB
EQE PLQY LEE T h K
P q V
 
    (5)
In Equation 5, PLQY is the photoluminescence quantum yield of QD materials, and LEE is the
light extraction efficiency of QD film. The definition of other parameters are the same as Equation 4.
Unlike a RGB micro-LED display, which uses different LED semiconductor materials, the color
conversion micro-LED display generates full color by using blue LEDs to pump QD materials.
Besides, the color conversion efficiency of QD material remains the same and is independent of the
emission area. As a result, in a color conversion micro-LED display, RGB subpixels have the same
peak EQE decreasing trend, which is determined by the blue LED property shown in Figure 4. In
contrast, in a RGB micro-LED display, as the LED chip size decreases, RGB subpixels have different
decreasing trends in peak EQE.
3. Simulation Results and Discussion
3.1. Optimization Process of Uniform LED Chip Size in RGB Subpixels
Figure 8.
The luminance ambient reflectance of RGB subpixels in color-conversion micro-LED display
at dierent aperture ratio.
By substituting the luminance ambient reflectance from Figure 8into Equation (3), we calculate the
display luminance as a function of aperture ratio under three dierent ambient illuminances: 2000 lux
for outdoor, 450 lux for oces and 200 lux for living rooms. Results are plotted in Figure 9.
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.
Figure 8. The luminance ambient reflectance of RGB subpixels in color-conversion micro-LED display
at different aperture ratio.
By substituting the luminance ambient reflectance from Figure 8 into Equation 3, we calculate
the display luminance as a function of aperture ratio under three different ambient illuminances: 2000
lux for outdoor, 450 lux for offices and 200 lux for living rooms. Results are plotted in Figure 9.
Figure 9. The required luminance of color-conversion micro-LED display as a function of aperture
ratio for achieving the listed ACR under three ambient light illuminances: (a) 2000 lux, (b) 450 lux and
(c) 200 lux.
The power efficiency of RGB subpixels in color-conversion micro-LED display is defined as the
ratio of luminance intensity over the power consumption (cd/W), as:
/.
B RG RG sys RGB RGB
RGB RGB
RGB
LED RGB RGB
EQE PLQY LEE T h K
P q V
 
    (5)
In Equation 5, PLQY is the photoluminescence quantum yield of QD materials, and LEE is the
light extraction efficiency of QD film. The definition of other parameters are the same as Equation 4.
Unlike a RGB micro-LED display, which uses different LED semiconductor materials, the color
conversion micro-LED display generates full color by using blue LEDs to pump QD materials.
Besides, the color conversion efficiency of QD material remains the same and is independent of the
emission area. As a result, in a color conversion micro-LED display, RGB subpixels have the same
peak EQE decreasing trend, which is determined by the blue LED property shown in Figure 4. In
contrast, in a RGB micro-LED display, as the LED chip size decreases, RGB subpixels have different
decreasing trends in peak EQE.
3. Simulation Results and Discussion
3.1. Optimization Process of Uniform LED Chip Size in RGB Subpixels
Figure 9.
The required luminance of color-conversion micro-LED display as a function of aperture ratio
for achieving the listed ACR under three ambient light illuminances: (
a
) 2000 lux, (
b
) 450 lux and (
c
)
200 lux.
The power eciency of RGB subpixels in color-conversion micro-LED display is defined as the
ratio of luminance intensity over the power consumption (cd/W), as:
ηRGB =ΦRGB/αRGB
PLED
=EQEB×PLQYRG ×LEERG ×Tsys ×hνRGB ×KRGB
q×VRGB ×αRGB
. (5)
In Equation (5), PLQY is the photoluminescence quantum yield of QD materials, and LEE is the
light extraction eciency of QD film. The definition of other parameters are the same as Equation (4).
Unlike a RGB micro-LED display, which uses dierent LED semiconductor materials, the color
conversion micro-LED display generates full color by using blue LEDs to pump QD materials. Besides,
the color conversion eciency of QD material remains the same and is independent of the emission
area. As a result, in a color conversion micro-LED display, RGB subpixels have the same peak EQE
decreasing trend, which is determined by the blue LED property shown in Figure 4. In contrast, in a
RGB micro-LED display, as the LED chip size decreases, RGB subpixels have dierent decreasing
trends in peak EQE.
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3. Simulation Results and Discussion
3.1. Optimization Process of Uniform LED Chip Size in RGB Subpixels
From Figure 4, the power eciency of micro-LED display decreases as the chip size decreases.
Therefore, a larger LED chip size is helpful to enhance the power eciency. However, as shown in
Equation (3), the micro-LED display with a larger LED chip size needs to deliver a higher luminance to
maintain the same ACR because of its higher reflectance. In order to find the optimal LED chip size
with minimum power consumption for white (D65) image content, in the following, we define two
functions f(x) and g(x) to describe the power eciency and luminance intensity of micro-LED display
at dierent LED chip sizes, respectively. Because the power consumption is defined as the luminance
intensity divided by power eciency, we can find the optimal LED chip size when the ratio of g(x) to
f(x) has a minimum. Detail of this function is discussed as follows.
The luminance intensity of a display is the product of display luminance (Equation (3)) and display
area. We define g(x), to describe the luminance intensity of display at dierent LED chip sizes, as:
g(x) = Ldisplay ×Adisplay =Lambient ×
Rs+ (1Rs)×X
i=RGB
RL(i)×
x(i)2
p2
×(ACR 1)×p2×N, (6)
where x
RGB
is the LED chip size of RGB subpixels, p is the pixel width, R
s
is the surface ambient
reflectance of the cover glass, R
L(RGB)
is the luminance ambient reflectance from RGB subpixels
respectively when aperture ratio is 1, L
ambient
is the ambient luminance, and Nis the number of pixels.
In order to define the function g(x) for RGB micro-LED display and color conversion micro-LED display,
we have to substitute the corresponding RLto Equation (6).
Because the power eciency of RGB subpixels is dierent, thus the eciency of display strongly
depends on the image contents. Moreover, dierent colors can be obtained by mixing the ratios of RGB
primaries. Therefore, the power eciency of a mixed color can be determined by
1
f(x)=1
ηpixel
=γR
ηR
+γG
ηG
+γB
ηB
, (7)
where ηis the power eciency and γrepresents the luminance intensity ratio of RGB primaries.
3.1.1. RGB Micro-LED Display
In our RGB micro-LED model, the D65 white light consists of around 25% red, 68% green, and
7% blue. Based on Equations (4) and (7), we depict the power eciency of each RGB micro-LED and
the power eciency of display at white image content as a function of LED chip size in Figure 10.
As Figure 10 shows, the power eciency of micro-LED display with dierent LED chip sizes displaying
white image content can be represented by the function f(x) (black curve). Here, the variable x
represents the LED chip size.
In the following, we investigate the chip size eect on the power consumption of RGB micro-LED
display. In this study, we assume the RGB subpixels have the same chip size. By varying the chip
size from 5
µ
m to 50
µ
m, we hope to find an optimal chip size for the lowest power consumption.
Three kinds of applications are evaluated: smartphone, laptop computer, and TV. Table 2lists the
specifications of each application. For smartphone at outdoor sunlight (2000 lux), laptop at oce
(450 lux), and TV at family room (200 lux) lighting conditions, the specified ACR=30:1, 120:1, and 800:1
should give excellent readability. Here, we take the specific ambient conditions listed in Table 2as
examples. In practice, the optimized results are still valid in all ambient conditions. This is because,
in Equation (6)
, the parameters outside the square brackets including ambient light intensity and ACR
can be treated as a constant and be normalized. As a result, the normalized illuminance function g(x) is
still the same for dierent ambient conditions.
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Figure 10. Chip size-dependent power efficiency of RGB micro-LED display.
In the following, we investigate the chip size effect on the power consumption of RGB micro-
LED display. In this study, we assume the RGB subpixels have the same chip size. By varying the
chip size from 5 µm to 50 µm, we hope to find an optimal chip size for the lowest power consumption.
Three kinds of applications are evaluated: smartphone, laptop computer, and TV. Table 2 lists the
specifications of each application. For smartphone at outdoor sunlight (2000 lux), laptop at office (450
lux), and TV at family room (200 lux) lighting conditions, the specified ACR=30:1, 120:1, and 800:1
should give excellent readability. Here, we take the specific ambient conditions listed in Table 2 as
examples. In practice, the optimized results are still valid in all ambient conditions. This is because,
in Equation 6, the parameters outside the square brackets including ambient light intensity and ACR
can be treated as a constant and be normalized. As a result, the normalized illuminance function g(x)
is still the same for different ambient conditions.
Table 2. The specifications of smartphone, laptop and TV evaluated.
Applications Smartphone laptop TV
Resolution 2688 × 1242 3840 × 2160 3840 × 2160
Pixel size 55.45
µm
90
µm
373
µm
Ambient illuminance 2000 lux 450 lux 200 lux
ACR 30 120 800
Display surface reflection 4% 4% 1.2%
Figure 11 depicts the function of f(x), g(x), and g(x)/f(x) as micro-LED chip size increases from 5
µm to 50 µm for TV applications. As the micro-LED chip size increases, the power consumption
[g(x)/f(x)] decreases first and then bounces back. The optimal LED chip size with minimum power
consumption takes place at 16 µm (Figure 11(c)). The power saving at this optimum LED chip size
over that at 50 µm chip size is 48% and over that at 5 µm is 32%. These results manifest the advantage
of using optimized LED chip size. With the same analysis process, we find the optimal LED chip size
for smartphone is 6 µm, and for laptop is 8 µm.
Normalized Power efficiency (cd/W)
Figure 10. Chip size-dependent power eciency of RGB micro-LED display.
Table 2. The specifications of smartphone, laptop and TV evaluated.
Applications Smartphone Laptop TV
Resolution 2688 ×1242 3840 ×2160 3840 ×2160
Pixel size 55.45 µm 90 µm 373 µm
Ambient illuminance 2000 lux 450 lux 200 lux
ACR 30 120 800
Display surface reflection
4% 4% 1.2%
Figure 11 depicts the function of f(x), g(x), and g(x)/f(x) as micro-LED chip size increases from
5
µ
m to 50
µ
m for TV applications. As the micro-LED chip size increases, the power consumption
[g(x)/f(x)] decreases first and then bounces back. The optimal LED chip size with minimum power
consumption takes place at 16
µ
m (Figure 11c). The power saving at this optimum LED chip size over
that at 50
µ
m chip size is 48% and over that at 5
µ
m is 32%. These results manifest the advantage of
using optimized LED chip size. With the same analysis process, we find the optimal LED chip size for
smartphone is 6 µm, and for laptop is 8 µm.
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Figure 11. Normalized (a) power efficiency fitting function f(x), (b) luminance intensity function g(x),
and (c) power consumption function g(x)/f(x) at different LED chip sizes.
3.1.2. Color-Conversion Micro-LED Display
In our color conversion micro-LED display, the D65 white light consists of around 30% red, 61%
green, and 9% blue. These ratios are slightly different from those for RGB micro-LEDs because the
emission spectra of red and green QD materials are different from those of red and green micro-
LEDs. From Equations 5 and 7, the power efficiency of each individual RGB subpixel and the power
efficiency of the display at white image content as a function of LED chip size is shown in Figure 12.
Because only blue LED is applied in color conversion micro-LED and the color conversion efficiency
of QD is independent of the emission area; the RGB subpixels have the same decreasing trend in
power efficiency at different LED chip sizes. From Equation 7, the same decreasing trend in RGB
subpixels leads to the same power-efficiency decreasing trend for all the colors. The power efficiency
of color conversion micro-LED display with different LED chip sizes displaying white image content
can be described by the function f(x) (black curve) shown in Figure 12. Here, the variable x represents
the LED chip size.
Figure 12. LED chip size-dependent power efficiency of color-conversion micro-LED display.
We also evaluate the applications listed in Table 2 in our color conversion micro-LED model. We
depicts the function of f(x), g(x), and g(x)/f(x) as micro-LED chip size increases from 5 µm to 50 µm
for TV applications in Figure 13. As the micro-LED chip size increases, the power consumption
[g(x)/f(x)] decreases first and then bounces back. The optimal LED chip size with minimum power
consumption takes place at 20 µm. The power saving at this optimum LED chip size over that of 50
Figure 11.
Normalized (
a
) power eciency fitting function f(x), (
b
) luminance intensity function g(x),
and (c) power consumption function g(x)/f(x) at dierent LED chip sizes.
3.1.2. Color-Conversion Micro-LED Display
In our color conversion micro-LED display, the D65 white light consists of around 30% red,
61% green, and 9% blue. These ratios are slightly dierent from those for RGB micro-LEDs because the
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emission spectra of red and green QD materials are dierent from those of red and green micro-LEDs.
From Equations (5) and (7), the power eciency of each individual RGB subpixel and the power
eciency of the display at white image content as a function of LED chip size is shown in Figure 12.
Because only blue LED is applied in color conversion micro-LED and the color conversion eciency of
QD is independent of the emission area; the RGB subpixels have the same decreasing trend in power
eciency at dierent LED chip sizes. From Equation (7), the same decreasing trend in RGB subpixels
leads to the same power-eciency decreasing trend for all the colors. The power eciency of color
conversion micro-LED display with dierent LED chip sizes displaying white image content can be
described by the function f(x) (black curve) shown in Figure 12. Here, the variable x represents the
LED chip size.
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Figure 11. Normalized (a) power efficiency fitting function f(x), (b) luminance intensity function g(x),
and (c) power consumption function g(x)/f(x) at different LED chip sizes.
3.1.2. Color-Conversion Micro-LED Display
In our color conversion micro-LED display, the D65 white light consists of around 30% red, 61%
green, and 9% blue. These ratios are slightly different from those for RGB micro-LEDs because the
emission spectra of red and green QD materials are different from those of red and green micro-
LEDs. From Equations 5 and 7, the power efficiency of each individual RGB subpixel and the power
efficiency of the display at white image content as a function of LED chip size is shown in Figure 12.
Because only blue LED is applied in color conversion micro-LED and the color conversion efficiency
of QD is independent of the emission area; the RGB subpixels have the same decreasing trend in
power efficiency at different LED chip sizes. From Equation 7, the same decreasing trend in RGB
subpixels leads to the same power-efficiency decreasing trend for all the colors. The power efficiency
of color conversion micro-LED display with different LED chip sizes displaying white image content
can be described by the function f(x) (black curve) shown in Figure 12. Here, the variable x represents
the LED chip size.
Figure 12. LED chip size-dependent power efficiency of color-conversion micro-LED display.
We also evaluate the applications listed in Table 2 in our color conversion micro-LED model. We
depicts the function of f(x), g(x), and g(x)/f(x) as micro-LED chip size increases from 5 µm to 50 µm
for TV applications in Figure 13. As the micro-LED chip size increases, the power consumption
[g(x)/f(x)] decreases first and then bounces back. The optimal LED chip size with minimum power
consumption takes place at 20 µm. The power saving at this optimum LED chip size over that of 50
Figure 12. LED chip size-dependent power eciency of color-conversion micro-LED display.
We also evaluate the applications listed in Table 2in our color conversion micro-LED model.
We depicts the function of f(x), g(x), and g(x)/f(x) as micro-LED chip size increases from 5
µ
m to
50
µ
m for TV applications in Figure 13. As the micro-LED chip size increases, the power consumption
[g(x)/f(x)] decreases first and then bounces back. The optimal LED chip size with minimum power
consumption takes place at 20
µ
m. The power saving at this optimum LED chip size over that of 50
µ
m
chip size is 16% and over that of 5
µ
m is 17%. With the same analysis process, we find the optimal LED
chip size for smartphone is 7 µm, and for laptop is 11 µm.
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µm chip size is 16% and over that of 5 µm is 17%. With the same analysis process, we find the optimal
LED chip size for smartphone is 7 µm, and for laptop is 11 µm.
Figure 13. Normalized (a) power efficiency fitting function f(x), (b) luminance intensity function g(x),
and (c) power consumption function g(x)/f(x) at different LED chip sizes.
3.2. Optimization Process of Different LED Chip Sizes in RGB Subpixels
In this section, we analyze the power consumption of RGB micro-LED displays with different
LED chip sizes. To do so, we have to consider following two important factors: (1) The chip-size
dependent power efficiency is different for the RGB micro-LEDs, and (2) the ambient light reflectance
is dependent on the chip size. Therefore, we need to optimize the LED chip sizes in RGB subpixels to
minimize the power consumption for achieving the same ACR. In RGB micro-LED display, when the
chip size decreases, the power efficiency declines for RGB micro-LEDs, but at different rates as shown
in Figure 10. The decreasing rate of red micro-LED is more significant than that of green and blue,
due to its faster surface recombination rate. For example, as the LED chip size decreases from 15 µm
to 5 µm, the power efficiency of red, green, and blue LEDs drops 46.23%, 41.52%, and 18.69%,
respectively. Therefore, using different chip sizes (with red being the largest) could improve the
overall power efficiency. On the other hand, in color conversion micro-LED displays, the chip-size
dependent power efficiency is the same in RGB subpixels and the slop of chip-size dependent power
efficiency, as shown in Figure 12, is smaller than the RGB micro-LED displays. Therefore, applying
different LED chip size in RGB subpixels is not help for reducing power consumption in color
conversion micro-LED displays. In the following paragraph, we focus on the optimization process
for the RGB micro-LED displays.
As mentioned above, increasing the LED chip size leads to enhanced display luminance for
maintaining the same ACR. However, from Figure 2b, the slope of red micro-LED is the smallest.
Therefore, among the RGB primaries, the enhancement of display luminance originated from
enlarging the chip size is the smallest for the red micro-LED.
By lifting the restrictions on micro-LED chip size, we conducted a systematic optimization for
achieving the lowest power consumption. The optimal LED chip size in RGB subpixels is (10,5,5) µm,
respectively, for the smartphone, (14,7,5) µm for the laptop, and (26,13,8) µm for the TV studied.
Next, we compare these power consumption results with that of uniform chip size. The power saving
at different chromaticity coordinates in DCI-P3 color space is shown in Figure 14. Overall, the power
saving covers about 94.46% of DCI-P3 color space. More specifically, the power saving over 10%
covers 68% area of the DCI-P3 color space.
Figure 13.
Normalized (
a
) power eciency fitting function f(x), (
b
) luminance intensity function g(x),
and (c) power consumption function g(x)/f(x) at dierent LED chip sizes.
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3.2. Optimization Process of Dierent LED Chip Sizes in RGB Subpixels
In this section, we analyze the power consumption of RGB micro-LED displays with dierent
LED chip sizes. To do so, we have to consider following two important factors: (1) The chip-size
dependent power eciency is dierent for the RGB micro-LEDs, and (2) the ambient light reflectance
is dependent on the chip size. Therefore, we need to optimize the LED chip sizes in RGB subpixels to
minimize the power consumption for achieving the same ACR. In RGB micro-LED display, when the
chip size decreases, the power eciency declines for RGB micro-LEDs, but at dierent rates as shown
in Figure 10. The decreasing rate of red micro-LED is more significant than that of green and blue,
due to its faster surface recombination rate. For example, as the LED chip size decreases from 15
µ
m to
5
µ
m, the power eciency of red, green, and blue LEDs drops 46.23%, 41.52%, and 18.69%, respectively.
Therefore, using dierent chip sizes (with red being the largest) could improve the overall power
eciency. On the other hand, in color conversion micro-LED displays, the chip-size dependent power
eciency is the same in RGB subpixels and the slop of chip-size dependent power eciency, as shown
in Figure 12, is smaller than the RGB micro-LED displays. Therefore, applying dierent LED chip size
in RGB subpixels is not help for reducing power consumption in color conversion micro-LED displays.
In the following paragraph, we focus on the optimization process for the RGB micro-LED displays.
As mentioned above, increasing the LED chip size leads to enhanced display luminance for
maintaining the same ACR. However, from Figure 2b, the slope of red micro-LED is the smallest.
Therefore, among the RGB primaries, the enhancement of display luminance originated from enlarging
the chip size is the smallest for the red micro-LED.
By lifting the restrictions on micro-LED chip size, we conducted a systematic optimization for
achieving the lowest power consumption. The optimal LED chip size in RGB subpixels is (10,5,5)
µ
m,
respectively, for the smartphone, (14,7,5)
µ
m for the laptop, and (26,13,8)
µ
m for the TV studied. Next,
we compare these power consumption results with that of uniform chip size. The power saving at
dierent chromaticity coordinates in DCI-P3 color space is shown in Figure 14. Overall, the power
saving covers about 94.46% of DCI-P3 color space. More specifically, the power saving over 10% covers
68% area of the DCI-P3 color space.
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Figure 14. The decreased power consumption (unit: %) of RGB micro-LED displays with different
chip sizes and uniform chip size.
When we compare the power saving between uniform LED chip size and different LED chip
sizes, we have to consider the image contents. From Figure 14, the power saving varies with color
contents and the maximum power saving (>20%) occurs at red color. Therefore, if the image content
is rich in red, the power saving will be more obvious. In the following, we use two test images shown
in Figure 15 to compare the power consumptions of micro-LED TVs.
Figure 15. Two tested images for micro-LED TVs: (a) Christmas, and (b) Maldives beach.
The calculated power saving of red-dominated Christmas image and blue/green-dominated
Maldives beach is around 20% and 7%, respectively. To explain the power saving difference between
these two images, we plot the color histograms of these images in Figure 16. As Figure 16a shows, in
comparison with the blue/green color histogram, the red color histogram in the Christmas image
mainly covers the higher gray level region. This indicates the image content is rich in red. From Fig.
8, the red color exhibits a larger power saving. Therefore, the Christmas image leads to a significant
power saving (~20%). On the other hand, from Figure 16b, the blue histogram in the Maldives beach
covers mostly the higher gray levels and many red pixels are at zero gray level (black). As a result,
the power saving is only ~ 7%.
Figure 14.
The decreased power consumption (unit: %) of RGB micro-LED displays with dierent chip
sizes and uniform chip size.
When we compare the power saving between uniform LED chip size and dierent LED chip sizes,
we have to consider the image contents. From Figure 14, the power saving varies with color contents
and the maximum power saving (>20%) occurs at red color. Therefore, if the image content is rich
in red, the power saving will be more obvious. In the following, we use two test images shown in
Figure 15 to compare the power consumptions of micro-LED TVs.
Crystals 2020,10, 494 12 of 15
Crystals 2020, 10, x FOR PEER REVIEW 12 of 15
Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals
Figure 14. The decreased power consumption (unit: %) of RGB micro-LED displays with different
chip sizes and uniform chip size.
When we compare the power saving between uniform LED chip size and different LED chip
sizes, we have to consider the image contents. From Figure 14, the power saving varies with color
contents and the maximum power saving (>20%) occurs at red color. Therefore, if the image content
is rich in red, the power saving will be more obvious. In the following, we use two test images shown
in Figure 15 to compare the power consumptions of micro-LED TVs.
Figure 15. Two tested images for micro-LED TVs: (a) Christmas, and (b) Maldives beach.
The calculated power saving of red-dominated Christmas image and blue/green-dominated
Maldives beach is around 20% and 7%, respectively. To explain the power saving difference between
these two images, we plot the color histograms of these images in Figure 16. As Figure 16a shows, in
comparison with the blue/green color histogram, the red color histogram in the Christmas image
mainly covers the higher gray level region. This indicates the image content is rich in red. From Fig.
8, the red color exhibits a larger power saving. Therefore, the Christmas image leads to a significant
power saving (~20%). On the other hand, from Figure 16b, the blue histogram in the Maldives beach
covers mostly the higher gray levels and many red pixels are at zero gray level (black). As a result,
the power saving is only ~ 7%.
Figure 15. Two tested images for micro-LED TVs: (a) Christmas, and (b) Maldives beach.
The calculated power saving of red-dominated Christmas image and blue/green-dominated
Maldives beach is around 20% and 7%, respectively. To explain the power saving dierence between
these two images, we plot the color histograms of these images in Figure 16. As Figure 16a shows,
in comparison with the blue/green color histogram, the red color histogram in the Christmas image
mainly covers the higher gray level region. This indicates the image content is rich in red. From Figure 8,
the red color exhibits a larger power saving. Therefore, the Christmas image leads to a significant
power saving (~20%). On the other hand, from Figure 16b, the blue histogram in the Maldives beach
covers mostly the higher gray levels and many red pixels are at zero gray level (black). As a result,
the power saving is only ~7%.
Crystals 2020, 10, x FOR PEER REVIEW 13 of 15
Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals
Figure 16. The color histogram of (a) Christmas and (b) Maldives beach images.
We also evaluated some frequently displayed images for smartphones, laptop computers, and
TVs. For smartphones, we compared the image contents of Facebook homepage, Google map, Google
search, YouTube homepage, and iPhone homepage with app icons. Due to copyright issue, we do
not show these images here. The average power saving is 12.9%. For laptop computers, we evaluated
the image contents of Amazon homepage, Gmail, Facebook homepage, YouTube homepage, and
computer game PUBG, and the average power saving is 13.2%. For TVs, we evaluated the image
contents of CNN news, NBA game, football game, TV show, and weather forecast, and the average
power reduction is 11.7%. Therefore, by employing various RGB micro-LED chip sizes, we can obtain
about 12% average power saving in all the three intended applications
4. Conclusion
We developed a model for evaluating the power consumption of RGB and color-conversion
based micro-LED displays. In the model, we investigate the power efficiency and luminance ambient
reflection of RGB subpixels in each type of micro-LED display. The optimal chip sizes corresponding
to the lowest power consumption are found in three application scenarios: smartphones, laptop
computers, and TVs. The major findings are twofold: (1) For TV applications, the optimized chip size
(16 m) leads to 48% and 32% power saving, as compared to uniform LED chip size at 50 µm and 5
µm, respectively. Similar analysis shows 16% and 17% power reduction in color-conversion based
micro-LED display with uniform chip size at 50 µm and 5 µm, respectively. (2) Our proposed micro-
LED display employing different RGB LED chip sizes further reduces ~ 12% average power
consumption over the optimized RGB micro-LED display with uniform LED chip size.
Author Contributions: Conceptualization, E.-L.H., Y.H., S.-T.W.; methodology, E.-L.H., F.G., Z.H.; software,
E.-L.H, F.G.; writing—original draft preparation, E.-L.H.; writing—review and editing, S.-T.W.; supervision, Y.-
F.L, S.-T.W. All authors have read and agreed to the published version of the manuscript.
Funding: This research is funded by a.u.Vista, Inc.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Lin, J.Y.; Jiang, H.X. Development of microLED. Appl. Phys. Lett. 2020, 116, 100502.
2. Jiang, H.X.; Lin, J.Y. Nitride micro-LEDs and beyond-a decade progress review. Opt. Express. 2013, 21,
A475–A484.
3. Huang, Y.; Tan, G.; Gou, F.; Li, M.C.; Lee, S. L.; Wu, S. T. Prospects and challenges of mini-LED and micro-
LED displays. J. Soc. Inf. Disp. 2019, 27, 387–401.
4. Huang, Y.; Hsiang, E.L.; Deng, M.Y.; Lin, C.L.; Wu, S.T. Mini-LED, Micro-LED and OLED displays: Present
status and future perspectives. Light: Sci. Appl. 2020, in press.
Figure 16. The color histogram of (a) Christmas and (b) Maldives beach images.
We also evaluated some frequently displayed images for smartphones, laptop computers, and TVs.
For smartphones, we compared the image contents of Facebook homepage, Google map, Google search,
YouTube homepage, and iPhone homepage with app icons. Due to copyright issue, we do not show
these images here. The average power saving is 12.9%. For laptop computers, we evaluated the image
contents of Amazon homepage, Gmail, Facebook homepage, YouTube homepage, and computer game
PUBG, and the average power saving is 13.2%. For TVs, we evaluated the image contents of CNN
news, NBA game, football game, TV show, and weather forecast, and the average power reduction is
11.7%. Therefore, by employing various RGB micro-LED chip sizes, we can obtain about 12% average
power saving in all the three intended applications
4. Conclusions
We developed a model for evaluating the power consumption of RGB and color-conversion
based micro-LED displays. In the model, we investigate the power eciency and luminance ambient
reflection of RGB subpixels in each type of micro-LED display. The optimal chip sizes corresponding
to the lowest power consumption are found in three application scenarios: smartphones, laptop
computers, and TVs. The major findings are twofold: (1) For TV applications, the optimized chip
Crystals 2020,10, 494 13 of 15
size (16
µ
m) leads to 48% and 32% power saving, as compared to uniform LED chip size at 50
µ
m
and 5
µ
m, respectively. Similar analysis shows 16% and 17% power reduction in color-conversion
based micro-LED display with uniform chip size at 50
µ
m and 5
µ
m, respectively. (2) Our proposed
micro-LED display employing dierent RGB LED chip sizes further reduces ~ 12% average power
consumption over the optimized RGB micro-LED display with uniform LED chip size.
Author Contributions:
Conceptualization, E.-L.H., Y.H., S.-T.W.; methodology, E.-L.H., F.G., Z.H.; software,
E.-L.H., F.G.; writing—original draft preparation, E.-L.H.; writing—review and editing, S.-T.W.; supervision,
Y.-F.L., S.-T.W. All authors have read and agreed to the published version of the manuscript.
Funding: This research is funded by a.u.Vista, Inc.
Conflicts of Interest: The authors declare no conflict of interest.
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article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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Book
This book focuses on basic fundamental and applied aspects of micro-LED, ranging from chip fabrication to transfer technology, panel integration, and various applications in fields ranging from optics to electronics to and biomedicine. The focus includes the most recent developments, including the uses in large large-area display, VR/AR display, and biomedical applications. The book is intended as a reference for advanced students and researchers with backgrounds in optoelectronics and display technology. Micro-LEDs are thin, light-emitting diodes, which have attracted considerable research interest in the last few years. They exhibit a set of exceptional properties and unique optical, electrical, and mechanical behaviors of fundamental interest, with the capability to support a range of important exciting applications that cannot be easily addressed with other technologies. The content is divided into two parts to make the book approachable to readers of various backgrounds and interests. The first provides a detailed description with fundamental materials and production approaches and assembly/manufacturing strategies designed to target readers who seek an understanding ofof essential materials and production approaches and assembly/manufacturing strategies designed to target readers who want to understand the foundational aspects. The second provides detailed, comprehensive coverage of the wide range of device applications that have been achieved. This second part targets readers who seek a detailed account of the various applications that are enabled by micro-LEDs.
Chapter
This chapter consists of 6 sections. Section 2 describes the manufacturing process of micro-LED displays and emphasizes the transfer step. Section 3 explains the fundamental mechanics of the transfer technology, including the essential elements of the transfer and their selection flowchart. Section 4 presents some promising technologies applicable to micro-LED transfer, such as electrostatic transfer, laser transfer, rubber stamp transfer, self-assembly transfer, and roll transfer. Section 5 provides three practical examples of the roll transfer for micro and mini-LEDs, demonstrating face-up and face-down transfer of micro-LEDs and face-down transfer of mini-LEDs. Finally, section 6 summarizes this chapter with some research suggestions for researchers and engineers studying transfer technology.
Article
Full-text available
Micro-light-emitting diodes (Micro-LEDs) based on gallium nitride (GaN) materials offer versatile platforms for various applications, including displays, data communication tools, photodetectors, and sensors. In particular, the introduction of Micro-LEDs in the optoelectronic industry enables the development of novel short-distance wireless communication applications for the Internet of Things as well as near-to-eye displays for virtual reality and augmented reality. Micro-LEDs used in conjunction with colloidal quantum dots (QDs) as color-conversion layers provide efficient full-color displays as well as white LEDs for high-speed visible light communications (VLCs). Here, the latest progress on full-color Micro-LED displays with a printed QD color conversion layer, GaN material-based Micro-LEDs for VLC systems, and the photostability of novel QD materials for Micro-LEDs is comprehensively reviewed. Outlooks on the efficiency of Micro-LEDs with sizes ≤10 µm, QD stability issues, and flexible Micro-LED displays are also provided.
Article
In this Letter, we have successfully realized the full-color micro-LED display on a single-chip utilizing multi-wavelength multi-quantum wells (MQWs). The epitaxial wafer used for micro-LED array chips is designed with two types of MQWs including In0.1Ga0.9N/GaN and In0.55Ga0.45N/GaN grown by metal-organic chemical vapor deposition (MOCVD). A single-chip broad-spectrum multi-wavelength emission from 620 to 450 nm can be realized by changing the injection current to realize the regulation of carrier injection in the MQWs with different emission wavelengths. And the full-color micro-LED display with uniform brightness can be achieved by adopting the pulse width modulation (PWM) to adjust the duty cycle of micro-LED pixels at different pulse voltages. We expect this study will provide a promising research direction for full-color micro-LED displays, thus effectively avoiding the problems caused during the massive transfer and color conversion.
Article
Thick‐shell quantum dots (QDs) with high mass absorption of 450 nm blue light (absorbance >1.5 for green QD and absorbance > 2.0 for red QD at concentration of 1 mg QD/1 mL solvent) have been adopted to formulate high resolution QD photoresist (QDPR), which can achieve color gamut higher than 90% BT.2020 coverage at film feature resolution of 5 µm and thickness less than 2 µm. The thermal stability and photostability of the QD color converter films were tested to evaluate for the feasibility of applications in color converters for full‐color µ‐LED microdisplays.
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Full-text available
Presently, liquid crystal displays (LCDs) and organic light-emitting diode (OLED) displays are two dominant flat panel display technologies. Recently, inorganic mini-LEDs (mLEDs) and micro-LEDs (μLEDs) have emerged by significantly enhancing the dynamic range of LCDs or as sunlight readable emissive displays. “mLED, OLED, or μLED: who wins?” is a heated debatable question. In this review, we conduct a comprehensive analysis on the material properties, device structures, and performance of mLED/μLED/OLED emissive displays and mLED backlit LCDs. We evaluate the power consumption and ambient contrast ratio of each display in depth and systematically compare the motion picture response time, dynamic range, and adaptability to flexible/transparent displays. The pros and cons of mLED, OLED, and μLED displays are analysed, and their future perspectives are discussed. Mini and micro light-emitting diodes (LEDs) could move to the centre-stage of display screen technologies once they mature. Shin-Tson Wu of the University of Central Florida and colleagues analysed the pros, cons, and future prospects of the latest display screen technologies, especially for use in smartphones, smart watches, virtual and augmented reality, and heads-up vehicle displays. These applications require bright, flexible, transparent, and power-efficient displays. The currently dominant liquid crystal displays (LCDs) require a backlight unit, dictating their shape and flexibility. LCDs with a backlight unit made from mini LEDs are becoming rapid contenders to the conventional technology. So are displays using organic light-emitting diodes, but these are limited in their brightness and lifespans. Emissive displays made from mini and micro-LEDs show huge potential once manufacturing costs can be brought down.
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A 31.5″ 8K LCD with 10,000 nit peak luminance, based on the Perceptual Quantization format in the ITU‐R Recommendation BT2100, has been developed with local dimming driving to perceive a high dynamic range in bright ambient environments. Good and uniform image quality has been confirmed due to excellent light‐resistance characteristics of IGZO TFT.
Article
This perspective provides an overview of early developments, current status, and remaining challenges of microLED (μLED) technology, which was first reported in Applied Physics Letters in 2000 [S. X. Jin, J. Li, J. Z. Li, J. Y. Lin and H. X. Jiang, "GaN Microdisk Light Emitting Diodes," Appl. Phys. Lett. 76, 631 (2000)]. Today, microLED is recognized as the ultimate display technology and is one of the fastest-growing technologies in the world as technology giants utilize it on a wide range of products from large flat panel displays and televisions, wearable displays, and virtual reality displays to light sources for the neural interface and optogenetics. It is anticipated that the collective R&D efforts worldwide will bring microLED products not only to the mass consumer electronic markets but also to serve the society on the broadest scale by encompassing sectors in medical/health, energy, transportation, communications, and entertainment.
Conference Paper
We report on Quantum Photonic Imager (QPI®) device, which is comprised of a spatial array of digitally addressable multicolor micro-scale pixels, wherein each pixel is a vertical stack of Red, Green, Blue light emitting diodes (LEDs) with RGB light emission sharing the same optical aperture. Starting with Blue, Green and Red epi-wafers first processed to create any desired size micro-LED arrays with 5-10 µm pixel pitch (pixelated), each of the 3 processed epi-wafers are sequentially bonded to a single receiving handle wafer followed by substrate removal and backside process to create a handle wafer with Blue, Green and Red micro-LED pixel arrays stacked vertically on top of each other. The handle wafer, which encapsulates stacked RGB pixel array, is also monolithically pre-processed to incorporate micro-scale pixel-level optical elements array that is designed to collimate and directionally modulate the multi-color light emitted from the individual pixels of the LED array. A proprietary CMOS image processor is then bonded to the handle wafer combining the vertically stacked arrays of micro-scale pixel-level optics and LEDs. The QPI® device alleviates inefficiencies associated with spatially or temporally multiplexed color pixel architectures, enabling high pixel density leading to small form-factor display system design. Low power display system operation is enabled by the QPI® device. Small form-factor multi-color “wearable” AR displays with sub-1W power consumption utilizing QPI® optically coupled to the edge of the AR combining lens have been demonstrated. Additionally, QPI® enabled compact light field displays, head-up displays and pico projectors have also been demonstrated.
Article
The development of glass‐free organic LCDs (OLCDs) promises a future of thin, lightweight, and shatterproof screens that are scalable to large areas and capable of sidestepping flat screens' limitations to embrace curved surfaces.
Article
Micro-light-emitting diodes (μLEDs) are semiconductor devices that have been shown to have higher luminous efficacy, higher contrast ratio, and higher energy efficiency than existing mainstream technologies based on liquid crystals or organic LEDs (OLEDs). Portable display applications such as wearable devices and head-up display are some of the interesting applications of μLED displays. However, this technology has not yet been mass-produced for commercial devices due to process yields, costs, and manufacturability issues. This article presents a novel technology for the heterogeneous integration of a μLED matrix display with bipolar complementary metal-oxide-semiconductor (CMOS) DMOS (BCD) circuits that could improve manufacturability by eliminating the need for a dedicated bond stack in the bump-bonding process. To validate the concept, custom high-performance, 2-D arrays of parallel-addressed GaN blue μLEDs matrices were fabricated. The individual μLED pixel diameters are 20 and 50 μm, respectively, and the overall dimension of the array is 650 μm². In addition, a μLED display driver-integrated circuit (IC) with a compact size of 3x4.4 mm² has been designed, implemented, and verified experimentally for the μLED matrices. Measured output optical power-forward bias current-forward bias voltage (P-I-V) curves of the individual μLED pixel are shown. The $4x4$ μLED matrix has also been successfully driven using active-matrix driving and display pictures to demonstrate the function of active-matrix driving are presented.
Article
Emissive displays based on light‐emitting diodes (LEDs), with high pixel density, luminance, efficiency, and large color gamut, are of great interest for applications such as watches, phones, and virtual displays. The high pixel density requirements of some emissive displays require a particular class of LEDs that are sub‐20‐micrometers in length, called micro‐LEDs. While state‐of‐the‐art emissive displays incorporate organic LEDs, an alternative is inorganic III‐nitride LEDs with potential reliability and efficiency benefits. Here we explore the performance, challenges, and prospective outcomes for III‐nitride micro‐LEDs to produce efficient emissive displays and provide insight to advance this technology. Calculations are performed to determine the operating points for the micro‐LEDs and the efficiency of the overall emissive display. It is shown that III‐nitride micro‐LEDs suffer from some of the same problems as their larger‐sized solid‐state lighting LED cousins; however, the operating conditions of micro‐LEDs can result in different challenges and research efforts. These challenges include improving efficiency at low current densities; improving the efficiency of longer wavelength (green and red) LEDs; and creating device designs that can overcome low coupling efficiency, high surface recombination, and display assembly difficulties. III‐Nitride micro light‐emitting diodes (micro‐LEDs) are investigated to create high pixel density, luminance, efficiency, and large color gamut emissive displays. Challenges for implementation include improving efficiency at low current densities; improving the efficiency of green and red LEDs; and creating device designs that can overcome low coupling efficiency, high surface recombination, and display assembly difficulties.
Article
A simulation scheme was developed to explore the light distribution of full-color micron-scale light-emitting diode (LED) arrays. The influences of substrate thickness, patterning, and cutting angle of the substrate on several important features, such as light field pattern, light extraction efficiency, and color variation, were evaluated numerically. An experiment was conducted; the results were consistent with simulation results for a 225 × 125 µm2 miniLED and those for an 80 × 80 µm2 microLED. Based on the simulation results, the light extraction efficiency of LED devices with a substrate increases by 67.75% over the extraction efficiency of those without a substrate. The light extraction efficiency of LED devices with a substrate increases by 113.55% when an additional patterned design is used on green and blue chips. The calculated large angle Δu′v′ can be as low as 0.015 for miniLED devices.
Article
OLEDs suffer from viewing angle dependent spectral shift due to microcavity effects. To address this issue, we introduce a novel top‐emitting OLED with a dielectric spacer that forms multiple cavity modes. The resulting device shows almost no color shift at different viewing angles. A multi‐mode top‐emitting OLED having an 8.5‐μm dielectric spacer with a metal reflector is proposed. This structure induces multi‐mode effect and eliminates the spectrum shift with viewing angles caused by the single microcavity.