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Improved Color Purity of Monolithic Full Color Micro-LEDs Using Distributed Bragg Reflector and Blue Light Absorption Material

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Shao-Yu Chu
Shao-Yu Chu
Shao-Yu Chu
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Hung-Yu Wang
Hung-Yu Wang
Hung-Yu Wang
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Ching-Ting Lee
Ching-Ting Lee
Ching-Ting Lee
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Hsin-Ying Lee at University of California, Merced
Hsin-Ying Lee
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Abstract and Figures

In this study, CdSe/ZnS core-shell quantum dots (QDs) with various dimensions were used as the color conversion materials. QDs with dimensions of 3 nm and 5 nm were excited by gallium nitride (GaN)-based blue micro-light-emitting diodes (micro-LEDs) with a size of 30 μm × 30 μm to respectively form the green and red lights. The hybrid Bragg reflector (HBR) with high reflectivity at the regions of the blue, green, and red lights was fabricated on the bottom side of the micro-LEDs to reflect the downward light. This could enhance the intensity of the green and red lights for the green and red QDs/micro-LEDs to 11% and 10%. The distributed Bragg reflector (DBR) was fabricated on the QDs color conversion layers to reflect the non-absorbed blue light that was not absorbed by the QDs, which could increase the probability of the QDs excited by the reflected blue light. The blue light absorption material was deposited on the DBR to absorb the blue light that escaped from the DBR, which could enhance the color purity of the resulting green and red QDs/micro-LEDs to 90.9% and 90.3%, respectively.
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coatings
Article
Improved Color Purity of Monolithic Full Color
Micro-LEDs Using Distributed Bragg Reflector and
Blue Light Absorption Material
Shao-Yu Chu 1, Hung-Yu Wang 1, Ching-Ting Lee 1,2, Hsin-Ying Lee 1,* , Kai-Ling Laing 3,
Wei-Hung Kuo 3, Yen-Hsiang Fang 3and Chien-Chung Lin 3,4
1Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan;
kevinvicky168@gmail.com (S.-Y.C.); aodhan20015@gmail.com (H.-Y.W.); ctlee@ee.ncku.edu.tw (C.-T.L.)
2Department of Electrical Engineering, Yuan Ze University, Taoyuan 320, Taiwan
3Electronic and Optoelectronic System Research Laboratories, Industrial Technology Research Institute,
Hsinchu 310, Taiwan; KL@itri.org.tw (K.-L.L.); GuoWeiHong@itri.org.tw (W.-H.K.);
YHFang@itri.org.tw (Y.-H.F.); chienchunglin@faculty.nctu.edu.tw (C.-C.L.)
4Institute of Photonic System, National Chiao Tung University, Tainan 711, Taiwan
*Correspondence: hylee@ee.ncku.edu.tw; Tel.: +886-6-2082368
Received: 26 February 2020; Accepted: 28 April 2020; Published: 29 April 2020


Abstract:
In this study, CdSe/ZnS core-shell quantum dots (QDs) with various dimensions were used
as the color conversion materials. QDs with dimensions of 3 nm and 5 nm were excited by gallium
nitride (GaN)-based blue micro-light-emitting diodes (micro-LEDs) with a size of 30
µ
m
×
30
µ
m to
respectively form the green and red lights. The hybrid Bragg reflector (HBR) with high reflectivity at
the regions of the blue, green, and red lights was fabricated on the bottom side of the micro-LEDs to
reflect the downward light. This could enhance the intensity of the green and red lights for the green
and red QDs/micro-LEDs to 11% and 10%. The distributed Bragg reflector (DBR) was fabricated on
the QDs color conversion layers to reflect the non-absorbed blue light that was not absorbed by the
QDs, which could increase the probability of the QDs excited by the reflected blue light. The blue
light absorption material was deposited on the DBR to absorb the blue light that escaped from the
DBR, which could enhance the color purity of the resulting green and red QDs/micro-LEDs to 90.9%
and 90.3%, respectively.
Keywords:
blue light absorption material; color conversion layer; distributed Bragg reflector; hybrid
Bragg reflector; micro-light-emitting diodes; quantum dots
1. Introduction
In recent years, display screens have been widely used in daily life, and liquid crystal display
(LCD) and organic light-emitting diode (OLED) displays are the mainstream of display technology [
1
].
However, breakthrough progress has been made in semiconductor manufacturing technology, leading
to the abrupt rise of micro-light-emitting diodes (micro-LEDs). In particular, the micro-LEDs have
advantages of long lifetime, high luminous efficiency, and smaller volume [
2
–
4
]. On the other hand,
micro-LEDs can effectively reduce energy consumption and improve pixel characteristics, which can
be expected to become the mainstream of next-generation display technology. In general, each pixel of
the full color micro-LED display is constructed by red, green, and blue light sources; then different
light source colors can be obtained by controlling the ratio of the three primary colors. However,
this method includes disadvantages such as different lifetime and complex driver circuits.
To overcome the above-mentioned problem, the blue light emitted from the gallium nitride
(GaN)-based micro-LEDs was used to excite the quantum dots (QDs) with various dimensions to form
Coatings 2020,10, 436; doi:10.3390/coatings10050436 www.mdpi.com/journal/coatings
Coatings 2020,10, 436 2 of 9
the monolithic red and green QDs/micro-LEDs [
5
–
7
]. In this work, the QDs were filled in the regions
around the black photoresist with very low transmittance in the visible light region. The function of
the black photoresist is to prevent the light emitted from the sidewall of the micro-LEDs to reduce the
crosstalk between the micro-LEDs. However, since the excitation efficiency of the red and green QDs
was still low, a large amount of non-absorbed blue light would respectively blend with the green light
and red light and be simultaneously emitted from the green and red QDs/micro-LEDs. Consequently,
the color purity of red and green QDs/micro-LEDs would be not good. The distributed Bragg reflector
(DBR) with high reflectivity in the blue light region was deposited on the QD color conversion layer to
solve the problem of the non-absorbed blue light [
8
,
9
]. However, since the DBR reflectivity depends
on the incident angle of the light, the DBR reflectivity will be reduced if the light is not incident
perpendicularly into the DBR structure [
10
,
11
]. Consequently, in this study, the blue light absorption
layer was utilized and deposited on the DBR to absorb the escaped blue light from the DBR, which
could improve the color purity of the monolithic full color micro-LEDs. To further enhance the output
light intensity, a hybrid Bragg reflector (HBR) was deposited on the bottom side of the micro-LEDs to
reflect the downward propagation of red, green, and blue lights.
2. Materials and Methods
2.1. Materials
In this work, the epitaxial wafers of the GaN-based blue micro-LEDs were supported by Epistar
Co., Hsinchu, Taiwan. CdSe/ZnS core-shell quantum dots with an average dimension of 5 nm and
3 nm were purchased from Taiwan Nanocrystal Inc., Tainan, Taiwan. The granules of titanium dioxide
(TiO
2
) (99.9%) and silicon dioxide (SiO
2
) (99.999%) were purchased from Admat Inc., Pennsylvania,
PA, USA. The black photoresist (model: ABK408X) was supported by Daxin Materials Co., Taichung,
Taiwan. The blue light absorption material (model: Eusorb UV-1995) was supported by Eutec Chemical
Co., Taipei, Taiwan.
2.2. Experimental Procedure
Figure 1a illustrates the schematic configuration of the monolithic full color micro-LEDs. A metal
organic chemical vapor deposition system was used to epitaxy the GaN-based blue micro-LEDs
structure on c-plane sapphire substrates. The structure of the micro-LEDs was constructed by a
2.8
µ
m-thick GaN buffer layer, a 4
µ
m-thick n-GaN layer, an undoped InGaN/GaN (3/7 nm, 10 pairs)
multiple quantum wells (MQW) active layer, a 33 nm-thick p-AlGaN layer, and a 150 nm-thick p-GaN
layer. A 500 nm-thick Ni metal was deposited on the p-GaN layer as the metal mask to protect the mesa
(30
µ
m
×
30
µ
m) of the micro-LEDs through an electron-beam evaporator. After the mesa dry etching,
the remaining Ni metal was removed by using aqua regia. The Ti/Al/Pt/Au (25/100/50/150 nm) metals
were deposited on the patterned n-GaN layer by an electron-beam evaporator and then annealed in
a pure N
2
environment at 850
◦
C for 2 min using a rapid temperature annealing (RTA) system to
obtain n-electrode ohmic contact [
12
]. Afterward, the thin Ni/Au (3/3 nm) metals were deposited on
the p-GaN surface treated by (NH
4
)
2
S
x
solution (S =6%) for 30 min [
13
] as the current spreading
layer, and the Ni/Au (20/100 nm) metals were deposited as the p-electrode. To obtain the p-electrode
ohmic contact, the samples were annealed in an air environment at 500
◦
C for 10 min using the RTA
system. In this work, an extended p-electrode pattern was designed to facilitate electrical measurement
of the micro-LEDs (as shown in Figure 1b). The 2.5
µ
m-thick black photoresist was spun on the
samples and patterned between the monolithic full color micro-LEDs through a photolithography
method. The black photoresist was solidified through the RTA system in a N
2
environment at 150
◦
C
for 10 min. Since black photoresist had a low average transmittance of 0.56% in the visible light region,
it could prevent crosstalk among the full color micro-LEDs. Subsequently, the HBR composed of
(TiO
2
/SiO
2
)
m-pairs
(48/77 nm) and a Ag (200 nm) metal reflector was deposited on the bottom side of
the samples through the electron beam evaporator. The red (green) CdSe/ZnS core-shell QDs with
Coatings 2020,10, 436 3 of 9
an average dimension of 5 nm (3 nm) and the poly(methyl methacrylate) (PMMA) were added into
toluene, and then the red (green) QD slurry was filled into the openings in the black photoresist layer
to form the red (green) QD color conversion layer. Subsequently, the electron beam evaporator was
used to alternately deposit (TiO
2
/SiO
2
)
n-pairs
(48/77 nm)/TiO
2
(48 nm) on the red (green) QDs at 100
◦
C
as the DBR structure. Finally, a 100 nm-thick blue light absorption layer was formed on the DBR
by spin-coating.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 9
photoresist layer to form the red (green) QD color conversion layer. Subsequently, the electron beam
evaporator was used to alternately deposit (TiO
2
/SiO
2
)
n-pairs
(48/77 nm)/TiO
2
(48 nm) on the red (green)
QDs at 100 °C as the DBR structure. Finally, a 100 nm-thick blue light absorption layer was formed
on the DBR by spin-coating.
(a) (b)
Figure 1. (a) The schematic configuration of the monolithic full color micro-light-emitting diodes
(micro-LEDs). (b) Top view photograph of the blue micro-LEDs.
3. Results and Discussion
Figure 2a and the inset figure show the electroluminescence (EL) spectrum and the current–
voltage (I–V) characteristics of the blue micro-LEDs, respectively. In this work, an EL measurement
system was installed in a black box and equipped with an Agilent 4156C semiconductor parameter
analyzer, an optical fiber, and a spectrometer (Ocean Optics USB2000, Florida, FL, USA) system. In
the EL measurement process, the resulting samples were placed on the stage in the black box and the
optical fiber was fixed above the micro-LEDs with a distance of 1 mm to catch the light emitted from
the micro-LEDs. The maximum EL intensity of all the micro-LEDs at the applied current of 1.2 mA
was measured by adjusting the sample stage. The emission wavelength and the threshold voltage of
the blue micro-LEDs was 451 nm and 2.5 V, respectively. The emission wavelengths of 625 and 550
nm for the red and green QDs excited by a tunable laser with a wavelength of 451 nm were obtained,
respectively (Figure 2b). The full width at half maximum (FWHM) of the blue, green, and red lights
emitted from the blue micro-LEDs, red and green QD micro-LEDs was 27.0, 32.2, and 34.5 nm,
respectively.
400 500 600 700 800 400 500 600 700 800
Wavelength (nm)
(b)
Blue micro-LEDs
Emission Intensity (a.u.)
Wavelength (nm)
(a)
012345
0
5
10
15
20
Current (mA)
Voltage (V)
red QDs
green QDs
Emission Intensity (a.u.)
Figure 2. (a) The electroluminescence (EL) spectra of the GaN-based blue micro-LEDs operated at a
bias of 3 V. The inset figure shows the current–voltage characteristics. (b) The emission spectrum of
the green and red QDs.
Figure 1.
(
a
) The schematic configuration of the monolithic full color micro-light-emitting diodes
(micro-LEDs). (b) Top view photograph of the blue micro-LEDs.
3. Results and Discussion
Figure 2a and the inset figure show the electroluminescence (EL) spectrum and the current–voltage
(I–V) characteristics of the blue micro-LEDs, respectively. In this work, an EL measurement system
was installed in a black box and equipped with an Agilent 4156C semiconductor parameter analyzer,
an optical fiber, and a spectrometer (Ocean Optics USB2000, Florida, FL, USA) system. In the EL
measurement process, the resulting samples were placed on the stage in the black box and the optical
fiber was fixed above the micro-LEDs with a distance of 1 mm to catch the light emitted from the
micro-LEDs. The maximum EL intensity of all the micro-LEDs at the applied current of 1.2 mA was
measured by adjusting the sample stage. The emission wavelength and the threshold voltage of the blue
micro-LEDs was 451 nm and 2.5 V, respectively. The emission wavelengths of 625 and 550 nm for the
red and green QDs excited by a tunable laser with a wavelength of 451 nm were obtained, respectively
(Figure 2b). The full width at half maximum (FWHM) of the blue, green, and red lights emitted from
the blue micro-LEDs, red and green QD micro-LEDs was 27.0, 32.2, and 34.5 nm, respectively.
Generally, the QD density in the color conversion layer would affect the color conversion efficiency
of the QDs. Consequently, the various weight ratios (10:20, 10:10, 20:10, and 40:10 mg, hereafter referred
to as 1:2, 1:1, 2:1, and 4:1) of the green and red QDs:PMMA were added into toluene (1 mL) to form
the QD slurry with various QD densities. Figure 3a,b present the emission spectra of various green
QDs:PMMA ratios and red QDs:PMMA ratios excited at the tunable laser with a wavelength of 451 nm.
As shown in Figure 3, the emission intensity of the green QDs:PMMA =1:1 and red QDs:PMMA
=1:1 was larger than that of the other green and red QDs:PMMA ratios. This phenomenon can be
attributed to the excessive QDs in the QDs slurry with QDs:PMMA ratios of 2:1 and 4:1, which would
reabsorb the converted green and red lights, reducing the output intensity of the converted green and
red lights [
14
]. In contrast, the fewer QDs in the QD slurry with the QDs:PMMA ratio of 1:2 would
lead to the absorbed amount of the blue light being too low. Consequently, the emission intensity of
the QD slurry with QDs:PMMA ratio of 1:2 was smaller in comparison with the QD slurry with the
QDs:PMMA ratio of 1:1.
Coatings 2020,10, 436 4 of 9
Coatings 2020, 10, x FOR PEER REVIEW 3 of 9
photoresist layer to form the red (green) QD color conversion layer. Subsequently, the electron beam
evaporator was used to alternately deposit (TiO
2
/SiO
2
)
n-pairs
(48/77 nm)/TiO
2
(48 nm) on the red (green)
QDs at 100 °C as the DBR structure. Finally, a 100 nm-thick blue light absorption layer was formed
on the DBR by spin-coating.
(a) (b)
Figure 1. (a) The schematic configuration of the monolithic full color micro-light-emitting diodes
(micro-LEDs). (b) Top view photograph of the blue micro-LEDs.
3. Results and Discussion
Figure 2a and the inset figure show the electroluminescence (EL) spectrum and the current–
voltage (I–V) characteristics of the blue micro-LEDs, respectively. In this work, an EL measurement
system was installed in a black box and equipped with an Agilent 4156C semiconductor parameter
analyzer, an optical fiber, and a spectrometer (Ocean Optics USB2000, Florida, FL, USA) system. In
the EL measurement process, the resulting samples were placed on the stage in the black box and the
optical fiber was fixed above the micro-LEDs with a distance of 1 mm to catch the light emitted from
the micro-LEDs. The maximum EL intensity of all the micro-LEDs at the applied current of 1.2 mA
was measured by adjusting the sample stage. The emission wavelength and the threshold voltage of
the blue micro-LEDs was 451 nm and 2.5 V, respectively. The emission wavelengths of 625 and 550
nm for the red and green QDs excited by a tunable laser with a wavelength of 451 nm were obtained,
respectively (Figure 2b). The full width at half maximum (FWHM) of the blue, green, and red lights
emitted from the blue micro-LEDs, red and green QD micro-LEDs was 27.0, 32.2, and 34.5 nm,
respectively.
400 500 600 700 800 400 500 600 700 800
Wavelength (nm)
(b)
Blue micro-LEDs
Emission Intensity (a.u.)
Wavelength (nm)
(a)
012345
0
5
10
15
20
Current (mA)
Voltage (V)
red QDs
green QDs
Emission Intensity (a.u.)
Figure 2. (a) The electroluminescence (EL) spectra of the GaN-based blue micro-LEDs operated at a
bias of 3 V. The inset figure shows the current–voltage characteristics. (b) The emission spectrum of
the green and red QDs.
Figure 2.
(
a
) The electroluminescence (EL) spectra of the GaN-based blue micro-LEDs operated at a
bias of 3 V. The inset figure shows the current–voltage characteristics. (
b
) The emission spectrum of the
green and red QDs.
Coatings 2020, 10, x FOR PEER REVIEW 4 of 9
Generally, the QD density in the color conversion layer would affect the color conversion
efficiency of the QDs. Consequently, the various weight ratios (10:20, 10:10, 20:10, and 40:10 mg,
hereafter referred to as 1:2, 1:1, 2:1, and 4:1) of the green and red QDs:PMMA were added into toluene
(1 mL) to form the QD slurry with various QD densities. Figure 3a,b present the emission spectra of
various green QDs:PMMA ratios and red QDs:PMMA ratios excited at the tunable laser with a
wavelength of 451 nm. As shown in Figure 3, the emission intensity of the green QDs:PMMA = 1:1
and red QDs:PMMA = 1:1 was larger than that of the other green and red QDs:PMMA ratios. This
phenomenon can be attributed to the excessive QDs in the QDs slurry with QDs:PMMA ratios of 2:1
and 4:1, which would reabsorb the converted green and red lights, reducing the output intensity of
the converted green and red lights [14]. In contrast, the fewer QDs in the QD slurry with the
QDs:PMMA ratio of 1:2 would lead to the absorbed amount of the blue light being too low.
Consequently, the emission intensity of the QD slurry with QDs:PMMA ratio of 1:2 was smaller in
comparison with the QD slurry with the QDs:PMMA ratio of 1:1.
400 450 500 550 600 650 700
Emission Intensity (a.u.)
Wavelength(nm)
(a)
1:2
1:1
2:1
4:1
Green QDs:PMMA
400 450 500 550 600 650 700
Red QDs:PMMA
Emission Intensity (a.u.)
Wavelength (nm)
(b)
1:2
1:1
2:1
4:1
Figure 3. The emission spectra of various (a) green core-shell quantum dots: poly (methyl
methacrylate) (QDs:PMMA) ratios and (b) red QDs:PMMA ratios excited by a tunable laser with a
wavelength of 451 nm.
To improve the output light intensity of the monolithic full color micro-LEDs, the HBR with high
reflectivity for the red, green, and blue lights was deposited on the bottom side of the samples. The
HBR was constructed by (TiO2/SiO2)m-pairs (48/77 nm) and Ag (200 nm) metal. Figure 4 shows the
reflectivity spectra of the Ag metal and (TiO2/SiO2)m-pairs (m = 1, 2, and 3)/Ag measured using an UV–
Visible spectrophotometer. The reflectivity of all of the (TiO2/SiO2)m-pairs/Ag HBR structures was higher
than that of the single Ag metal. Furthermore, the highest reflectivity of (TiO2/SiO2)2-pairs/Ag was 97.5%,
98.9%, and 98.8% at the wavelengths of 451, 550, and 625 nm, respectively. Consequently, the
downward propagation of the red, green, and blue lights could be effectively reflected by the HBR,
which could improve the output light intensity.
Figure 3.
The emission spectra of various (
a
) green core-shell quantum dots: poly (methyl methacrylate)
(QDs:PMMA) ratios and (
b
) red QDs:PMMA ratios excited by a tunable laser with a wavelength of
451 nm.
To improve the output light intensity of the monolithic full color micro-LEDs, the HBR with
high reflectivity for the red, green, and blue lights was deposited on the bottom side of the samples.
The HBR was constructed by (TiO
2
/SiO
2
)
m-pairs
(48/77 nm) and Ag (200 nm) metal. Figure 4shows
the reflectivity spectra of the Ag metal and (TiO
2
/SiO
2
)
m-pairs
(m=1, 2, and 3)/Ag measured using an
UV–Visible spectrophotometer. The reflectivity of all of the (TiO
2
/SiO
2
)
m-pairs
/Ag HBR structures was
higher than that of the single Ag metal. Furthermore, the highest reflectivity of (TiO
2
/SiO
2
)
2-pairs
/Ag
was 97.5%, 98.9%, and 98.8% at the wavelengths of 451, 550, and 625 nm, respectively. Consequently,
the downward propagation of the red, green, and blue lights could be effectively reflected by the HBR,
which could improve the output light intensity.
Coatings 2020,10, 436 5 of 9
Figure 4. The reflectivity spectra of the Ag metal and (TiO2/SiO2)m-pairs (m=1, 2, and 3)/Ag.
In this work, the green and red QD color conversion layers were used to convert the blue light
into the green and red lights, respectively. To avoid the red and green lights that emitted from
the QDs/micro-LEDs blended with the non-absorbed blue light, the DBR with high reflectivity at
the wavelength of 451 nm should be deposited on the red and green QDs color conversion layers.
Simultaneously, the DBR with high transmittance at wavelengths of 550 nm and 625 nm was requested.
The DBR was composed of (TiO
2
/SiO
2
)
n-pairs
(48/77 nm)/TiO
2
(48 nm). Figure 5presents the reflectivity
spectra of the (TiO
2
/SiO
2
)
n-pairs
(n=2, 3, 4, and 5)/TiO
2
DBR structure measured using an UV–Visible
spectrophotometer. The number of n-pairs would obviously affect the reflectivity of the DBR at the
blue light range. The DBR reflectivity at the wavelength of 451 nm increased with an increase in the
number of n-pairs. Although the highest DBR reflectivity at the wavelength of 451 nm was 98.6%
as n=6, the DBR reflectivity at wavelengths of 550 and 625 nm was 31.9% and 15.8%, respectively,
which adversely affected the emitting of the green light. The reflectivity of the (TiO
2
/SiO
2
)
5-pairs
/TiO
2
DBR structure at the wavelengths of 451, 550, and 625 nm was 96.4%, 13.7%, and 7.6%, respectively.
Consequently, the 5-pairs (TiO
2
/SiO
2
) were more suitable for the DBR structure than the 6-pairs
(TiO2/SiO2).
Coatings 2020, 10, x FOR PEER REVIEW 5 of 9
m = 1
m = 2
m = 3
300 350 400 450 500 550 600 650 700 750 800
0
10
20
30
40
50
60
70
80
90
100
(TiO
2
/SiO
2
)
m-pairs
/Ag
Ag metal
Reflectivity (%)
Wavelengh (nm)
Figure 4. The reflectivity spectra of the Ag metal and (TiO2/SiO2)m-pairs (m = 1, 2, and 3)/Ag.
In this work, the green and red QD color conversion layers were used to convert the blue light
into the green and red lights, respectively. To avoid the red and green lights that emitted from the
QDs/micro-LEDs blended with the non-absorbed blue light, the DBR with high reflectivity at the
wavelength of 451 nm should be deposited on the red and green QDs color conversion layers.
Simultaneously, the DBR with high transmittance at wavelengths of 550 nm and 625 nm was
requested. The DBR was composed of (TiO2/SiO2)n-pairs (48/77 nm)/TiO2 (48 nm). Figure 5 presents the
reflectivity spectra of the (TiO2/SiO2)n-pairs (n = 2, 3, 4, and 5)/TiO2 DBR structure measured using an
UV–Visible spectrophotometer. The number of n-pairs would obviously affect the reflectivity of the
DBR at the blue light range. The DBR reflectivity at the wavelength of 451 nm increased with an
increase in the number of n-pairs. Although the highest DBR reflectivity at the wavelength of 451 nm
was 98.6% as n = 6, the DBR reflectivity at wavelengths of 550 and 625 nm was 31.9% and 15.8%,
respectively, which adversely affected the emitting of the green light. The reflectivity of the
(TiO2/SiO2)5-pairs/TiO2 DBR structure at the wavelengths of 451, 550, and 625 nm was 96.4%, 13.7%, and
7.6%, respectively. Consequently, the 5-pairs (TiO2/SiO2) were more suitable for the DBR structure
than the 6-pairs (TiO2/SiO2).
(
TiO
2
/SiO
2
)
n-pairs
/TiO
2
300 350 400 450 500 550 600 650 700 750 800
0
10
20
30
40
50
60
70
80
90
100
n = 2
n = 3
n = 4
n = 5
n = 6
Reflectivity (%)
Wavelengh (nm)
Figure 5. The reflectivity spectra of the (TiO2/SiO2)n-pairs (n = 2, 3, 4, and 5)/TiO2 distributed Bragg
reflector (DBR) structure.
Figure 5.
The reflectivity spectra of the (TiO
2
/SiO
2
)
n-pairs
(n=2, 3, 4, and 5)/TiO
2
distributed Bragg
reflector (DBR) structure.
The EL spectra of the resulting blue micro-LEDs and the resulting QDs/micro-LEDs were measured
to verify the function of the HBR, DBR and blue light absorption layer. Figure 6shows the EL spectra of
Coatings 2020,10, 436 6 of 9
the resulting green and red QDs/micro-LEDs and the blue micro-LEDs with HBR. As shown in Figure 6,
the output intensity of the green and red QDs/micro-LEDs with HBR was respectively increased 11%
and 10% in comparison with the QDs/micro-LEDs without HBR. This proved that the HBR could
reflect the downward emitted red, green, and blue lights. Furthermore, it was worth noting that only
part of the blue light was absorbed by the green and red QDs, which leads to non-absorbed blue light
blending with the green light and red light simultaneously being emitted from the green and red
QDs/micro-LEDs with HBR. Therefore, the non-absorbed blue light emitted from the QDs/micro-LEDs
with HBR could be effectively reflected back into the QD color conversion layer by using the DBR
with high reflectivity at the wavelength of 451 nm and excite more red and green lights. Compared
with the QDs/micro-LEDs with HBR and without DBR, the output intensity of the green and red
QDs/micro-LEDs with HBR and DBR was increased 20% and 23%, respectively. However, since the
reflectivity of the DBR was not 100% and the reflectivity of the DBR was decreased as the light was
not perpendicularly incident into the DBR structure, a small portion of blue light still escaped, which
would affect the color purity of the QDs/micro-LEDs. In this work, the blue light absorption layer
was deposited on the DBR to reduce the escaped blue light. The absorptivity spectrum of the blue
light absorption layer is shown in Figure 7. The absorptivity of the blue light absorption layer at
wavelengths of 451, 550, and 625 nm was 77.2%, 1.3%, and 1.8%, respectively. Since the blue light
escaped from the DBR could be absorbed by the blue light absorption layer, the intensity of the blue
light for the QDs/micro-LEDs was reduced, as shown in Figure 6, which could improve the red and
green color purity.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 9
The EL spectra of the resulting blue micro-LEDs and the resulting QDs/micro-LEDs were
measured to verify the function of the HBR, DBR and blue light absorption layer. Figure 6 shows the
EL spectra of the resulting green and red QDs/micro-LEDs and the blue micro-LEDs with HBR. As
shown in Figure 6, the output intensity of the green and red QDs/micro-LEDs with HBR was
respectively increased 11% and 10% in comparison with the QDs/micro-LEDs without HBR. This
proved that the HBR could reflect the downward emitted red, green, and blue lights. Furthermore, it
was worth noting that only part of the blue light was absorbed by the green and red QDs, which leads
to non-absorbed blue light blending with the green light and red light simultaneously being emitted
from the green and red QDs/micro-LEDs with HBR. Therefore, the non-absorbed blue light emitted
from the QDs/micro-LEDs with HBR could be effectively reflected back into the QD color conversion
layer by using the DBR with high reflectivity at the wavelength of 451 nm and excite more red and
green lights. Compared with the QDs/micro-LEDs with HBR and without DBR, the output intensity
of the green and red QDs/micro-LEDs with HBR and DBR was increased 20% and 23%, respectively.
However, since the reflectivity of the DBR was not 100% and the reflectivity of the DBR was decreased
as the light was not perpendicularly incident into the DBR structure, a small portion of blue light still
escaped, which would affect the color purity of the QDs/micro-LEDs. In this work, the blue light
absorption layer was deposited on the DBR to reduce the escaped blue light. The absorptivity
spectrum of the blue light absorption layer is shown in Figure 7. The absorptivity of the blue light
absorption layer at wavelengths of 451, 550, and 625 nm was 77.2%, 1.3%, and 1.8%, respectively.
Since the blue light escaped from the DBR could be absorbed by the blue light absorption layer, the
intensity of the blue light for the QDs/micro-LEDs was reduced, as shown in Figure 6, which could
improve the red and green color purity.
400 450 500 550 600 650 700
Relative Intensity (a.u.)
Wavelength
(nm)
(a)
blue light absorption layer
Blue micro-LEDs with HBR
Green QDs/micro-LEDs
without HBR and DBR
with HBR and without DBR
with HBR and DBR
with HBR, DBR, and
400 450 500 550 600 650 700
Relative Intensity (a.u.)
Wavelength
(nm)
(
b
)
blue light absorption layer
Blue micro-LEDs with HBR
Red QDs/micro-LEDs
without HBR and DBR
with HBR and without DBR
with HBR and DBR
with HBR, DBR, and
Figure 6. The EL spectra of the resulting (a) green QDs/micro-LEDs and (b) red QDs/micro-LEDs.
Blue light absorption layer
300 350 400 450 500 550 600 650 700 750 800
0
10
20
30
40
50
60
70
80
90
100
Absorptivity (%)
Wavelength (nm)
Figure 7. The absorptivity spectrum of the blue light absorption layer.
Figure 6. The EL spectra of the resulting (a) green QDs/micro-LEDs and (b) red QDs/micro-LEDs.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 9
The EL spectra of the resulting blue micro-LEDs and the resulting QDs/micro-LEDs were
measured to verify the function of the HBR, DBR and blue light absorption layer. Figure 6 shows the
EL spectra of the resulting green and red QDs/micro-LEDs and the blue micro-LEDs with HBR. As
shown in Figure 6, the output intensity of the green and red QDs/micro-LEDs with HBR was
respectively increased 11% and 10% in comparison with the QDs/micro-LEDs without HBR. This
proved that the HBR could reflect the downward emitted red, green, and blue lights. Furthermore, it
was worth noting that only part of the blue light was absorbed by the green and red QDs, which leads
to non-absorbed blue light blending with the green light and red light simultaneously being emitted
from the green and red QDs/micro-LEDs with HBR. Therefore, the non-absorbed blue light emitted
from the QDs/micro-LEDs with HBR could be effectively reflected back into the QD color conversion
layer by using the DBR with high reflectivity at the wavelength of 451 nm and excite more red and
green lights. Compared with the QDs/micro-LEDs with HBR and without DBR, the output intensity
of the green and red QDs/micro-LEDs with HBR and DBR was increased 20% and 23%, respectively.
However, since the reflectivity of the DBR was not 100% and the reflectivity of the DBR was decreased
as the light was not perpendicularly incident into the DBR structure, a small portion of blue light still
escaped, which would affect the color purity of the QDs/micro-LEDs. In this work, the blue light
absorption layer was deposited on the DBR to reduce the escaped blue light. The absorptivity
spectrum of the blue light absorption layer is shown in Figure 7. The absorptivity of the blue light
absorption layer at wavelengths of 451, 550, and 625 nm was 77.2%, 1.3%, and 1.8%, respectively.
Since the blue light escaped from the DBR could be absorbed by the blue light absorption layer, the
intensity of the blue light for the QDs/micro-LEDs was reduced, as shown in Figure 6, which could
improve the red and green color purity.
400 450 500 550 600 650 700
Relative Intensity (a.u.)
Wavelength
(nm)
(a)
blue light absorption layer
Blue micro-LEDs with HBR
Green QDs/micro-LEDs
without HBR and DBR
with HBR and without DBR
with HBR and DBR
with HBR, DBR, and
400 450 500 550 600 650 700
Relative Intensity (a.u.)
Wavelength
(nm)
(
b
)
blue light absorption layer
Blue micro-LEDs with HBR
Red QDs/micro-LEDs
without HBR and DBR
with HBR and without DBR
with HBR and DBR
with HBR, DBR, and
Figure 6. The EL spectra of the resulting (a) green QDs/micro-LEDs and (b) red QDs/micro-LEDs.
Blue light absorption layer
300 350 400 450 500 550 600 650 700 750 800
0
10
20
30
40
50
60
70
80
90
100
Absorptivity (%)
Wavelength (nm)
Figure 7. The absorptivity spectrum of the blue light absorption layer.
Figure 7. The absorptivity spectrum of the blue light absorption layer.
Coatings 2020,10, 436 7 of 9
Figure 8shows the international commission on illumination (CIE) chromaticity coordinates of the
resulting micro-LEDs. Compared with the green and red QDs/micro-LEDs with HBR and without DBR,
the CIE chromaticity coordinate of the green and red QDs/micro-LEDs with HBR and DBR shifted from
(0.211, 0.221) to (0.271, 0.531) and from (0.311, 0.109) to (0.564, 0.226), respectively. The CIE chromaticity
coordinate of the green and red QDs/micro-LEDs with HBR, DBR, and blue light absorption layer
was (0.292, 0.632) and (0.646, 0.279), respectively. The color purity of the LEDs could be estimated by
Equation (1) as follows [15–17]:
color purity =q(X−Xi)2−(Y−Yi)2
q(Xd−Xi)2−(Yd−Yi)2
×100% (1)
where (X, Y) is the CIE chromaticity coordinate of the resulting QDs/micro-LEDs, (X
i
, Y
i
) is the CIE
chromaticity coordinate of the blue micro-LEDs, (X
d
, Y
d
) is the CIE chromaticity coordinate of green
light and red light at color purity of 100%. The CIE chromaticity coordinate of the green light with
wavelength of 550 nm and the red light with wavelength of 625 nm was (0.301, 0.692) and (0.701, 0.299),
respectively. Compared with the green and red QDs/micro-LEDs with HBR and without DBR, the color
purity of the green and red QDs/micro-LEDs with HBR and DBR was improved from 28.2% to 75.4%
and from 27.7% to 74.2%, respectively. Consequently, the DBR could effectively reflect the non-absorbed
blue light to improve the color purity. The color purity of the green and red QDs/micro-LEDs with
HBR, DBR, and blue light absorption layer was further improved to 90.9% and 90.3%, respectively.
Coatings 2020, 10, x FOR PEER REVIEW 7 of 9
Figure 8 shows the international commission on illumination (CIE) chromaticity coordinates of
the resulting micro-LEDs. Compared with the green and red QDs/micro-LEDs with HBR and without
DBR, the CIE chromaticity coordinate of the green and red QDs/micro-LEDs with HBR and DBR
shifted from (0.211, 0.221) to (0.271, 0.531) and from (0.311, 0.109) to (0.564, 0.226), respectively. The
CIE chromaticity coordinate of the green and red QDs/micro-LEDs with HBR, DBR, and blue light
absorption layer was (0.292, 0.632) and (0.646, 0.279), respectively. The color purity of the LEDs could
be estimated by Equation (1) as follows [15–17]:
  =
( − )−(−
)
(−
)−(
−
)× % (1)
where (X, Y) is the CIE chromaticity coordinate of the resulting QDs/micro-LEDs, (Xi, Yi) is the CIE
chromaticity coordinate of the blue micro-LEDs, (Xd, Yd) is the CIE chromaticity coordinate of green
light and red light at color purity of 100%. The CIE chromaticity coordinate of the green light with
wavelength of 550 nm and the red light with wavelength of 625 nm was (0.301, 0.692) and (0.701,
0.299), respectively. Compared with the green and red QDs/micro-LEDs with HBR and without DBR,
the color purity of the green and red QDs/micro-LEDs with HBR and DBR was improved from 28.2%
to 75.4% and from 27.7% to 74.2%, respectively. Consequently, the DBR could effectively reflect the
non-absorbed blue light to improve the color purity. The color purity of the green and red QDs/micro-
LEDs with HBR, DBR, and blue light absorption layer was further improved to 90.9% and 90.3%,
respectively.
0.0 0.2 0.4 0.6 0.8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
blue light absorption layer
Blue micro-LEDs
with HBR and without DBR
with HBR and DBR
with HBE, DBR, and
Red QDs/micro-LEDs
Green QDs/micro-LEDs
CIE Y
CIE X
Figure 8. The international commission on illumination (CIE) chromaticity coordinates of the
resulting micro-LEDs.
4. Conclusions
In this study, monolithic full color micro-LEDs were successfully fabricated by using blue micro-
LEDs to excite various dimensions of CdSe/ZnS core-shell QDs. To increase the output intensity and
improve the color purity, HBR and DBR were used as the bottom reflector for the full colors and the
top reflector for the blue light, respectively. The downward emitted full colors could be reflected by
the bottom reflector HBR, which could increase the probability of blue light exciting the red and green
QDs. Consequently, the output intensity of the green and red QDs/micro-LEDs with HBR was
respectively increased 11% and 10% in comparison with the QDs/micro-LEDs without HBR.
Furthermore, the output intensity of the green and red QDs/micro-LEDs with HBR and DBR was
further increased 20% and 23%, respectively, as the blue light reflected by the DBR could excite QDs
Figure 8.
The international commission on illumination (CIE) chromaticity coordinates of the
resulting micro-LEDs.
4. Conclusions
In this study, monolithic full color micro-LEDs were successfully fabricated by using blue
micro-LEDs to excite various dimensions of CdSe/ZnS core-shell QDs. To increase the output intensity
and improve the color purity, HBR and DBR were used as the bottom reflector for the full colors
and the top reflector for the blue light, respectively. The downward emitted full colors could be
reflected by the bottom reflector HBR, which could increase the probability of blue light exciting the
red and green QDs. Consequently, the output intensity of the green and red QDs/micro-LEDs with
HBR was respectively increased 11% and 10% in comparison with the QDs/micro-LEDs without HBR.
Furthermore, the output intensity of the green and red QDs/micro-LEDs with HBR and DBR was
Coatings 2020,10, 436 8 of 9
further increased 20% and 23%, respectively, as the blue light reflected by the DBR could excite QDs to
form more red and green lights. Finally, the blue light absorption layer with absorptivity of 77.2% at
the wavelength of 451 nm was developed to absorb the blue light that escaped from the DBR. The color
purity of the green and red QDs/micro-LEDs with HBR, DBR, and blue light absorption layer was
further improved to 90.9% and 90.3%, respectively.
Author Contributions:
Conceptualization, H.-Y.L., C.-T.L., K.-L.L., W.-H.K., Y.-H.F., and C.-C.L.; Data curation,
S.-Y.C. and H.-Y.W.; Funding acquisition, H.-Y.L., C.-T.L., K.-L.L., W.-H.K., Y.-H.F., and C.-C.L.; Investigation,
S.-Y.C., H.-Y.W., H.-Y.L., and C.-T.L.; Writing—original draft, S.-Y.C. and H.-Y.W.; Writing—review and editing,
H.Y.L. All authors have read and agreed to the published version of the manuscript.
Funding:
This work was supported by the Ministry of Science and Technology (MOST), Taiwan under Nos.
MOST-108-2221-E-006-196-MY3 and MOST 108-2221-E-006-215-MY3, and the Industrial Technology Research
Institute, Taiwan.
Conflicts of Interest: The authors declare no conflict of interest.
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©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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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The implementation of high efficiency and high‐resolution displays has been the focus of considerable research interest. Recently, micro‐LEDs, which are inorganic light‐emitting diodes of size less than 100 μm2, have emerged as a promising display technology owing to their superior features and advantages over other displays like liquid crystal displays and organic light‐emitting diodes. Although many companies have introduced micro‐LED displays since 2012, obstacles to mass production still exist. Three major challenges, i.e., low quantum efficiency, time consuming transfer, and complex color conversion, have been overcome with technological breakthroughs to realize cost‐effective micro‐LED displays. In the review, methods for improving the degraded quantum efficiency of GaN based micro‐LEDs induced by the size effect are examined, including wet chemical treatment, passivation layer adoption, LED structure design, and growing LEDs in self‐passivated structures. Novel transfer technologies, including pick‐up transfer and self‐assembly methods, for developing large‐area micro‐LED displays with high yield and reliability are discussed in depth. Quantum dots as color conversion materials for high color purity, and deposition methods such as electrohydrodynamic jet printing or contact printing on micro‐LEDs have also been addressed. This review presents current status and critical challenges of micro‐LED technology and promising technical breakthroughs for commercialization of high performance displays. This article is protected by copyright. All rights reserved
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Tunable distributed Bragg reflectors with wide-angle and broadband high-reflectivity using nanoporous/dense titanium dioxide film stacks for visible wavelength applications
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Full-text available
Highly-tolerant distributed Bragg reflectors (DBRs) based on the same materials consisting of nanoporous/dense titanium dioxide (TiO2) film pair structures with wide-angle and broadband highly-reflective properties at visible wavelengths are reported. For a high refractive index contrast, the two dense and nanoporous TiO2 film stacks are alternatingly deposited on silicon (Si) substrates by a oblique angle deposition (OAD) method at two vapor flux angles (θα) of 0 and 80° for high and low refractive indices, respectively. For the TiO2 DBRs at a center wavelength (λc) of 540 nm, the maximum level in reflectance (R) band is increased with increasing the number of pairs, exhibiting high R values of > 90% for 5 pairs, and the normalized stop bandwidth (∆λ/λc) of ~17.8% is obtained. At λc = 540 nm, the patterned TiO2 DBR with 5 pairs shows an uniform relative reflectivity over a whole surface of 3 inch-sized Si wafer and a large-scalable fabrication capability with any features. The angle-dependent reflectance characteristics of TiO2 DBR at λc = 540 nm are also studied at incident angles (θinc) of 20-70° for p-, s-, and non-polarized lights in the wavelength region of 350-750 nm, yielding high R values of > 70.4% at θinc values of 20-70° for non-polarized light. By adjusting the λc/4 thicknesses of nanoporous and dense films, for λc = 450, 540, and 680 nm, tunable broadband TiO2 DBRs with high R values of > 90% at wavelengths of 400-800 nm are realized.
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Photoluminescence enhancement of Ca3Sr3(VO4)4:Eu3+,Al3+ red-emitting phosphors by charge compensation
Article
  • Oct 2019
  • OPT LASER TECHNOL
In this study, Ca3Sr3(VO4)4:0.05Eu³⁺, Ca3Sr3(VO4)4:0.05Eu³⁺,xAl³⁺, and Ca3Sr3(VO4)4:0.05Eu³⁺,0.06Al³⁺, yM⁺ (M = Li, Na, and K) red-emitting phosphors were synthesized by a citric acid-assisted sol combustion method. For the photoluminescence properties and crystal structure characterization of the prepared powder samples, the X-ray diffraction (XRD), photoluminescence (PL) spectroscopy and scanning electron microscopy (SEM) were used. The powder XRD analysis illustrated that the low content of doped Al³⁺ ions and charge compensators did not have a significant effect on the crystal structure of the prepared samples, and the samples still retained the rhombohedral crystal structure. The SEM images indicated that the charge compensator-doped samples were well crystallized, with a homogeneous particle size distribution. The PL studies revealed that the luminescence intensity was significantly improved by doping Al³⁺ into Ca3Sr3(VO4)4:Eu³⁺. The emission intensity of Ca3Sr3(VO4)4:0.05Eu³⁺,0.06Al³⁺ was nearly doubled upon co-doping with the charge compensator M⁺ (M = Li, Na, and K); the optimal concentration of the charge compensator was 12%. In particular, the maximum emission intensity and particle size were observed when Li⁺ was co-doped. In addition, the emission intensity of Ca3Sr3(VO4)4:0.05Eu³⁺,0.06Al³⁺,0.12Li⁺ is about 2.08 times higher than that of Ca3Sr3(VO4)4:0.05Eu³⁺,0.06Al³⁺. Its color purity is as high as 95% and the external quantum efficiency is 57.77%. The Ca3Sr3(VO4)4:Eu³⁺,Al³⁺,Li⁺ system provides potential red phosphors for near-ultraviolet excitation applied in white light-emitting diodes (LEDs).
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Temperature dependence of upconversion luminescence and sensing sensitivity of Ho3+/Yb3+ modified PSN-PMN-PT crystals
Article
  • Jun 2019
  • J ALLOY COMPD
In view of the vital role of temperature in actural applications of materials, a detailed investigation was conducted for Ho³⁺/Yb³⁺-modified Pb(Sc1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PSN-PMN-PT) crystals focusing on their temperature dependence of upconversion luminescence and sensing abilities. The results indicated that the luminescence intensity of grown crystals weakened with elevating temperature, and the fastest decrease rate occurred in green emission. The absolute and relative sensitivities calculated by S755/S767 (SA2 and SR2) were higher than that based on I665/I554 (SA1 and SR1) at low temperature, and the maximum of 0.0112 K⁻¹ and 0.0373 K⁻¹ were obtained at 93 K, comparable to that of some rare earth (RE) doped host materials reported. The study on chromaticity coordinate revealed the high colour purity of crystals, nearly up to 99%. Furthermore, the decay dynamics of crystals were investigated. The longest average decay time (157.44 μs ) of 554 nm emission was obtained at 293 K. These results manifest the promising applications of Ho³⁺/Yb³⁺-modified PSN-PMN-PT crystals as luminescent and optical thermometric materials at low and room temperature. It is also of significance for further investigation of photoelectric coupling performance of ferroelectric crystals.
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A combined experimental theoretical approach for energy gap determination, photophysical, photostable, optoelectronic, NLO, and organic light emitting diode (OLED) application: Synthesized coumarin derivative
Article
  • May 2019
  • J MOL STRUCT
In this technological world, there is a vast demand for organic materials due to their application in the field optoelectronic point of view. In this regard, we made an attempt to develop a new photostable, light emitting and highly efficient synthesized material to exhibit display property and NLO active materials with good energy gap energy. Therefore, we synthesised 3,3'-(naphthalen-2-ylmethylene) bis (4-hydroxy-2H-chromen-2-one) (3,3-NHC) molecule via simple catalytic method. The photophysical property of studied molecule in various organic solvents having different polarity exhibited considerably acceptable results. The energy band gap was determined by both experimentally and theoretically which is found to be an average of 3.0 eV which is well matched to the semiconducting materials band gap. The non-linear optical absorption properties of the synthesized 3,3-NHC molecule are dissolved in suitable solvents had been investigated using Z-scan technique. We have determined the sign and magnitude of nonlinear absorption coefficient by the method of open aperture (OA) Z‒scan setup using a continuous wave Nd:YAG laser at 532 nm. The OA Z‒scan profiles clearly demonstrate the presence of RSA kind nonlinearity in the investigated compound and are due to (2 PA) the two-photon absorption. Results of the non-linear optical studies confirm that the studied compound is more potential candidates for optical limiting applications in CW regime. Further, the optoelectronic properties were investigated the CIE, CRI, color purity and CCT results of the 3,3-NHC in all studied solvents revels that this compound exhibits blue color (CIE-coordinates x = 0.14, y = 0.08) and is suitable for OLEDs which can be preferably used for daylight applications.
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Visual-Attention-Based Pixel Dimming Technique for OLED Displays of Mobile Devices
Article
  • Oct 2018
Organic light-emitting diode (OLED) display has been widely used for mobile devices due to its features of wide color range and high contrast. However, display system dominates the significant power consumption on mobile devices which limits its usage time. Hence, this paper proposes a visual attention based pixel dimming (VAB-PD) technique to achieve power saving for real-time video displaying on OLED mobile devices. Firstly, the proposed method extracts motion vector (MV) array from bitstream. Next, considering the perception of human vision to moving objects, a saliency map can be calculated from MV array. Finally, a pixel dimming function is designed to reduce the brightness of pixels lying in the unnoticeable parts of video frames, thereby elevating the energy-efficiency without deteriorating the visual quality. On the other hand, an ACR11 subjective visual quality test is conducted to determine the optimal coefficient for dimming function. The experimental results demonstrate that VAB-PD can accomplish 13% of power reduction on average while maintaining acceptable visual quality. IEEE
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Monolithic Red/Green/Blue Micro-LEDs with HBR and DBR Structures
Article
  • Dec 2017
In this work, monolithic red, green, and blue (RGB) micro-light-emitting diodes (LEDs) were fabricated using gallium nitride (GaN)-based blue micro-LEDs and quantum dots (QDs). Red and green QDs were sprayed onto individual region surrounded by patterned black matrix photoresist on the blue micro-LEDs to form color conversion layers. Owing to its light-blocking capability, the patterned black matrix photoresist improved the contrast ratio of the micro-LEDs from 11 to 22. To enhance the color conversion efficiency and the light output intensity, a hybrid Bragg reflector (HBR) was deposited on the bottom side of the monolithic RGB micro-LEDs, thus reflecting the RGB light emitted to the substrate. To further improve the color purity of the red and green light, a distributed Bragg reflector (DBR) with high reflection for the blue light was deposited on the top side of the QDs/micro-LEDs. The red and green light output intensities of the micro-LEDs with HBR and DBR were enhanced by about 27%.
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Efficiency enhancement and angle-dependent color change in see-through organic photovoltaics using distributed Bragg reflectors
Article
  • Mar 2016
  • APPL PHYS LETT
A distributed Bragg reflector (DBR) is conducted as a bottom reflector in see-through organic photovoltaics (OPVs) with an active layer of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The DBR consists of alternative layers of the high- and low-refractive index materials of Ta2O5 (n = 2.16) and SiO2 (n = 1.46). The DBR selectively reflects the light within a specific wavelength region (490 nm–630 nm) where the absorbance of P3HT:PCBM is maximum. The see-through OPVs fabricated on DBR exhibit efficiency enhancement by 31% compared to the device without DBR. Additionally, the angle-dependent transmittance of DBR is analysed using optical simulation and verified by experimental results. As the incident angle of light increases, peak of reflectance shifts to shorter wavelength and the bandwidth gets narrower. This unique angle-dependent optical properties of DBR allows the facile color change of see-through OPVs.
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Color Conversion of GaN-Based Micro Light-Emitting Diodes Using Quantum Dots
Article
  • Nov 2015
Using CdSe/ZnS quantum dots (QDs) excited by the blue light emission of GaN-based micro light-emitting diodes (LEDs), quasi-monochromatic red light sources with emission wavelength of 625 nm were obtained. In order to enhance the blue light emission of GaN-based LEDs for further excitation of QDs, distributed/hybrid Bragg reflectors and zinc oxide (ZnO) nanorod array were, respectively, introduced to the top/bottom and top sides of GaN-based micro LEDs. The presence of ZnO nanorod array helps to channel the blue light to pump the CdSe/ZnS QDs for enhancing the red light emission. The electroluminescence of micro QDs-LEDs with reflectors and ZnO nanorod array was increased by 57.4% compared with the micro QDs-LEDs alone. The international commission on illumination chromaticity coordinate of the micro QDs-LEDs with reflectors and ZnO nanorod array was very close to the red light emission at (0.59, 0.26).
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White-light-emitting diodes using GaN-excited CdSe/CdS/ZnS quantum dots
Article
  • Aug 2014
  • PARTICUOLOGY
Fluorescence-based white-light-emitting diodes (WLEDs) were fabricated using blue GaN chips and green- and red-emitting CdSe/CdS/ZnS quantum dots (QDs). The coordinate and color temperature of the WLEDs could be varied because of the size-tunable emission of CdSe QDs from 510 to 620 nm. Warm and cold white emissions were confirmed with the color temperature ranging from 4000 to 9000 K. Color coordinates were analyzed at different bias. The fast enhancement of blue emission resulted in the shift of color coordinates to the cold side. The stability of white emission during operation was analyzed; stable spectra were achieved within 90 mm.
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