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Journal of Information Display
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White organic light-emitting diode (OLED)
microdisplay with a tandem structure
Hyunsu Cho, Chun-Won Byun, Chan-Mo Kang, Jin-Wook Shin, Byoung-Hwa
Kwon, Sukyung Choi, Nam Sung Cho, Jeong-Ik Lee, Hokwon Kim, Jeong Hwan
Lee, Minseok Kim & Hyunkoo Lee
To cite this article: Hyunsu Cho, Chun-Won Byun, Chan-Mo Kang, Jin-Wook Shin, Byoung-Hwa
Kwon, Sukyung Choi, Nam Sung Cho, Jeong-Ik Lee, Hokwon Kim, Jeong Hwan Lee, Minseok
Kim & Hyunkoo Lee (2019): White organic light-emitting diode (OLED) microdisplay with a tandem
structure, Journal of Information Display, DOI: 10.1080/15980316.2019.1671240
To link to this article: https://doi.org/10.1080/15980316.2019.1671240
© 2019 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Group on behalf of the Korean Information
Display Society
Published online: 05 Oct 2019.
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JOURNAL OF INFORMATION DISPLAY
https://doi.org/10.1080/15980316.2019.1671240
White organic light-emitting diode (OLED) microdisplay with a tandem structure
Hyunsu Cho a, Chun-Won Byuna, Chan-Mo Kanga, Jin-Wook Shina, Byoung-Hwa Kwona, Sukyung Choia,Nam
Sung Choa, Jeong-Ik Leea,HokwonKim
b, Jeong Hwan Leeb, Minseok Kimband Hyunkoo Leea
aFlexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea; bRAONTECH,
Seongnam-si, Republic of Korea
ABSTRACT
Microdisplay is a key technology for realizing augmented reality (AR) and mixed reality (MR) devices,
which have attracted much attention of late. Even though the operating voltage in the tandem struc-
ture is higher than that in the single structure, 2-stack tandem OLED exhibited 20,000 cd/m2at 9 V,
which is compatible with CMOS circuit driving. Due to its top-emitting geometry with a tandem
structure, the OLED device with a well-controlled thickness exhibited a white spectrum with (0.26,
0.26) color coordinates. The pixel density of the fabricated microdisplay panel with a white tandem
OLED was about 2350 pixels per inch, and the active area of the panel was 0.7 inch diagonally. The
resolution of the panel was 1280 ×1024, corresponding to SXGA, and the maximal luminance was
3,000 cd/m2.
ARTICLE HISTORY
Received 29 July 2019
Accepted 15 September 2019
KEYWORDS
Organic light-emitting diode
(OLED);tandem;whiteOLED;
microdisplay
1. Introduction
Augmented reality (AR) and mixed reality (MR) devices
are key applications for the future display technol-
ogy. Liquid crystal on silicon (LCoS) microdisplays
were initially applied, but organic light-emitting diode
(OLED) microdisplays have attracted much attention
[1–3]. Most AR/MR devices have been used in head-
mounted or glass-type wearable devices. Hence, thin
and lightweight form factor as well as superior image
quality are important properties, which are the advan-
tages of the OLED display compared with the LC-based
display.
There are two approaches to achieving the OLED
microdisplay: using a white OLED structure with a color
lter (C/F) and using direct color patterning of evapo-
rating emitters via shadow masks. To realize lively and
realistic images, a higher resolution has been essentially
demanded in AR/MR devices. From a resolution per-
spective, the former is more favorable than the latter
becausetheC/Fisfabricatedviaphotolithography[1].
The luminance of the microdisplay is another impor-
tant parameter, especially in AR/MR devices. To ensure
visibility outdoors, a minimum of over 2,000 cd/m2lumi-
nance is required [3]. Using a white OLED structure
withaC/F,however,hasadisadvantageforachieving
CONTACT Hyunkoo Lee lhk108@etri.re.kr Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon,
Republic of Korea
ISSN (print): 1598-0316; ISSN (online): 2158-1606
higher luminance because the transmittance of the C/F
is not 100% and the blocked light is completely lost. If
losses due to the C/F are inevitable, fabricating an e-
cient white OLED is important. The tandem-structure
white OLED has important merits: improved lifetime
and eciency [4]. Theoretically, the current density at a
xed luminance in the tandem OLED is less than that
in the single-element OLED, which reduces the device
degradation.
A microdisplay was fabricated on an opaque sin-
gle crystalline silicon (Si) substrate through comple-
mentary metal-oxide-semiconductor (CMOS) processes,
which is dierent from the conventional OLED pan-
els with glass substrates and thin-lm transistors (TFTs)
[5]. Therefore, to apply a white OLED with a tandem
structure, it must be veried that the OLED devices
are suciently driven by CMOS circuits. In addition,
although OLEDs use a top-emitting geometry with a
tandem structure, it must be veried that a broadband
white emission spectrum is obtained. In this study, a
white OLED with a 2-stack tandem structure was applied
in the OLED microdisplay. The tandem white OLED
exhibited 20,000 cd/m2at 9 V, which is compatible with
CMOS circuit driving. The OLED device with a well-
controlled thickness exhibited a white spectrum with
© 2019 The Author(s). Published by Informa UK Limited, trading as Taylor& Francis Group on behalf of the Korean Information Display Society
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
2H. CHO ET AL.
(0.26, 0.26) color coordinates. The fabricated microdiplay
panel with a white tandem OLED had a 0.7-inch diagonal
active area. The resolution of the panel was 1280 ×1024,
corresponding to SXGA, and the maximal luminance
was 3,000 cd/m2.
2. Experiment
The backplanes of the OLED microdisplay panels were
fabricated on 8-inch Si wafers by a commercial foundry
company, and contained a CMOS integrated circuit (IC)
Figure 1. Structure of the white tandem OLED microdisplay.
Figure 2. (a) Simplified OLED structure for optical simulation. (b) Simulated emission spectra of OLEDs. (c) Emission spectra and (b) CIE
coordinates of tandem OLEDs for different HTL thicknesses.
J. INF. DISP. 3
for OLED microdisplay driving. The 0.11 µmCMOS
process was used for the CMOS ICs, and 1.2–5.5 V
dual voltages were available [5]. The diced wafer sub-
strates were sequentially cleaned with acetone, methanol,
and deionized water, and were transferred to the vac-
uum thermal evaporator for the deposition of all the
organic materials and top cathode metals. A white tan-
dem OLED is shown in Figure 1. The OLED device
consists of uorescent blue and yellow-green phospho-
rescent emitters connected by a charge generation layer
[6]. The fabricated device was encapsulated using ultra-
violet (UV)-curable epoxy in an inert-environment glove
box. The chip-on-board (COB) bonding process was
applied for module packaging. The OLED microdisplay
panel was connected to the panel mounting printed cir-
cuit board (PCB) through wire bonding. A exible PCB
(FPCB) was used for connecting the OLED microdis-
play panel with the driving circuit board. Low-voltage
dierential signaling (LVDS) was used for the signal
interface [5].
The current density (J)-voltage (V) characteristics of
the device were measured using a source-measure unit
(Keithley-238, Keithley), and the luminance (L) and elec-
troluminescence (EL) spectra were examined using a
spectroradiometer (CS-2000, Konica Minolta).
3. Results and discussion
3.1. White emission spectrum of the tandem OLED
microdisplay
A top-emitting geometry should be applied in the
microdisplay because the OLED device is deposited
on an opaque Si wafer. In TEOLEDs, thin-metal-based
semi-transparent electrodes are typically used instead of
transparent conductive oxides (TCOs), whose deposition
Figure 3. J-V characteristics of YG single OLEDs for different HILs.
can damage the underlying organic layers [7,8]. A cer-
tain level of metal lm thickness is required to obtain
sucient sheet conductance. At such thickness, the thin
metal lm has non-negligible reectance, forming a
rather strong microcavity. Therefore, it is important to
account for the inuence of the microcavity environment
on the optical properties of TEOLEDs. In addition, the
total organic layer in the tandem structure is more than
twice as thick as that in the single structure, resulting in
a stronger microcavity environment [9].
When designing the device structure, optical simu-
lation was conducted to obtain the guidelines for the
approximate thickness. Assuming the simplied struc-
ture as shown in Figure 2(a), the change of the spec-
trum according to the thickness of the organic layer
between two emitters was conrmed via optical simu-
lation. The thicknesses of the electron transport layer
(ETL) and hole transport layer (HTL) for the simplied
model were assumed to be 30 and 60 nm, respectively, for
Figure 4. J-V characteristics of tandem OLEDs for (a) different Yb
thicknesses and (b) the ETL structure. The inset figure shows the
transmittance of the Yb/Ag bilayer.
4H. CHO ET AL.
the blue and yellow-green cavity length. As the thickness
of the organic layer between the two emitters increases,
the cavity length of the OLED device also increases.
The increased cavity length leads to the red shift of
the resonance wavelength. As a result, cool white emis-
sion spectra can be obtained when the blue emission is
enhanced, but the yellow-green emission is suppressed,
as shown in Figure 2(b).
Figure 2(c) shows the emission spectra of the tandem
OLEDs for dierent HTL thicknesses in a blue-emitting
unit. The trend of spectral change is similar to the simu-
lation result. The CIE coordinates of tandem OLEDs with
a 20-nm-thick HTL are (0.270, 0.290). When the HTL
thickness increases by 10 nm, the CIE coordinates of tan-
dem OLEDs change to (0.292, 0.398), (0.314, 0.499), and
(0.318, 0.541), as shown in Figure 2(b). As the thicknesses
ofotherorganiclayersarelessthan40nm,itisnotpos-
sible to reduce the total organic layer thickness. In other
words, a white emission spectrum is obtained at the min-
imum thickness to form a tandem structure. If the total
organic layer thickness further increases, it will be hard
to obtain broad band emission.
3.2. Lowering the operating voltage of tandem
OLEDs
The silver (Ag)/indium tin oxide (ITO) bilayer is widely
usedasabottomelectrodefortop-emittingOLEDs
owingtothehighreectanceofAgandthehighwork
function of ITO [10]. These materials, however, are
not currently available in the general CMOS foundry.
Instead, an aluminium (Al)/titanium nitride (TiN)
bilayercompatiblewiththeCMOSprocessisusedas
thebottomelectrodefortheOLEDmicrodisplay[11]. Al
contributes to low sheet resistance and high reectance,
and TiN contributes to the proper work function for
using an anode in OLEDs [12]. To lower the operating
voltage of tandem OLEDs, the selection of a hole injec-
tion layer (HIL) is important to eciently inject holes
from TiN to the HTL.
Figure 3shows the J-V characteristics of single OLED
devices. To focus on the hole injection property from
the Al/TiN anode, a single OLED structure based on
theYGunitinthetandemOLEDstructurewasused.
Compared with Dipyrazino [2,3-f:2,3-h] quinoxaline-
2,3,6,7,10,11-hexacarbonitrile (HAT-CN), widely used as
a hole injection material in OLEDs, the p-doping HIL
showedalargerJatthesameV.Whenthedopingratio
was optimized as 15%, the operating voltage was reduced
from 4.5–3.4 V at the J of 10 mA/cm2.
Ecient electron injection from the top cathode is
also signicantly important. For the top-emitting geom-
etry, an ytterbium (Yb)/Ag bilayer was used in this study
because Ag alone is not favorable for injecting electrons
into an ETL. Figure 4(a) shows the J-V characteristics of
thetandemOLEDfordierentYbthicknesses.Asthe
Figure 5. (a) J-V-L characteristics, (b) spectrum, and (c) current efficiency of white tandem OLEDs. (d) Operating image of the fabricated
white OLED microdisplay panel (0.7 inch, SXGA (1280 ×1024), 2350 ppi resolution).
J. INF. DISP. 5
Yb thickness increases, the electron injection property is
enhanced, but the transmittance of the bilayer cathode
decreases, as shown in the inset in Figure 4(a). Due to
the trade-o between the electron injection property and
the cathode transmittance, the change in the Yb thickness
is not suitable for decreasing the operating voltage while
maintaining the same luminance. Therefore, a method
capable of improving electron injection while minimiz-
ing the thickness of Yb is required. As applied in the p-i-n
OLED structure [13], an ETL/n-doped ETL was applied
while maintaining the same thickness as the ETL single
layer. Figure 4(b) shows the J-V characteristics of the tan-
dem OLED devices for a dierent ETL structure. When
the n-doping ETL replaced some ETL thicknesses, the
operating voltage was reduced from 9.6–8.7V at the J of
100 mA/cm2.
3.3. White OLED microdisplay panel
The 2-stack tandem OLED exhibited 20,000 cd/m2at 9 V,
whichiscompatiblewithCMOScircuitdriving,asshown
in Figure 5(a). The well-controlled OLED structure
exhibited a white spectrum with (0.26, 0.26) CIE coor-
dinates and 28 cd/A current eciency at 1,000 cd/m2,as
shown in Figure 5(b). Figure 5(c) shows the operating
image of the fabricated microdisplay panel with white
tandem OLED. The pixel density was about 2350 pixels
per inch, and the active area of the panel was 0.7 inch
diagonally. The resolution of the panel was 1280 ×1024,
correspondingtoSXGA,andthemaximalluminancewas
about 3,000 cd/m2.
4. Conclusion
A white organic light-emitting diode (OLED) microdis-
play with a tandem structure was successfully demon-
strated. In spite of its top-emitting geometry with
atandemstructure,awhiteemissionspectrumwas
obtained through well-controlled cavity design. In addi-
tion, by using p- and n-doped organic layers as the
HIL and electron injection layer (EIL), respectively,
the operating voltage of the tandem OLEDs was su-
ciently reduced for CMOS circuit driving. The fabricated
OLED microdisplay had a 0.7 inch size and an SXGA
resolution.
Acknowledgement
This work was supported in part by a National Research Coun-
cil of Science & Technology (NST) grant from the South Korean
government (MSIT) (Development of OLED Microdisplay
Technology for Integrated Helmets in Military Application)
and in part by a Institute for Information & Communications
Technology Promotion (IITP) grant also funded by the South
Korean government (MSIT) (Development of an Augmented-
reality-based Life Safety Content Service Platform for Low
Vision).
Funding
This work was supported in part by a National Research Coun-
cil of Science & Technology (NST) grant from the South Korean
government (MSIT) (Development of OLED Microdisplay
Technology for Integrated Helmets in Military Application)
and in part by a Institute for Information & Communications
Technology Promotion (IITP) grant also funded by the South
Korean government (MSIT) (Development of an Augmented-
reality-based Life Safety Content Service Platform for Low
Vision).
Notes on contributors
Hyun su Cho received his B.S. and Ph.D.
Electrical Engineering degrees from Korea
Advanced Institute of Science and Tech-
nology (KAIST), Daejeon, South Korea
in 2008 and 2014, respectively. He joined
Electronics and Telecommunications
Research Institute (ETRI) in Daejeon,
South Korea in 2014, where he is cur-
rently a senior researcher. His research interests include the
device physics and optical design of optoelectronics devices like
organic light-emitting diodes (OLEDs).
Chun-Won Byun received his B.S. and
M.S. Electrical and Computer Engineer-
ing degrees from Hanyang University,
Seoul, South Korea in 2002 and 2007,
respectively. From2007 to 2010, he worked
for ETRI, and from 2011 to 2013, he
worked for Samsung Display, South Korea.
Since 2013, he has been working at the
Reality Device Research Division of ETRI as a senior researcher.
His research interests include the microdisplay for AR/VR
(augmented reality/virtual reality), driving methods for new
displays, and implantable biomedical devices.
Chan-mo Kang received his B.S. and
Ph.D. Electrical and Computer Engineer-
ing degrees from Seoul National Univer-
sity (SNU) in 2008 and 2014, respec-
tively. He is currently a senior researcher
at ETRI. His research interests include
device engineering and physics in organic
electronics, thin-lm transistors (TFTs),
and organic/inorganic hybrid devices.
Jin-Wook Shin received his B.S. and M.S.
degrees from Myongji University and
Kwangwoon University, respectively. In
2018, he received his Ph.D. degree from
Tohoku U niversity, Jap a n . H e j o ined ETRI
in 2009. His current research interests
include exbile OLED displays with
graphene lms, microdisplays for AR/VR,
and implantable biomedical devices.
6H. CHO ET AL.
Byoung-Hwa Kwon received his B.S., M.S.,
and Ph.D. degrees from the Department
of Materials Science & Engineering of
Hanyang University, Pohang University
of Science and Technology (POSTECH),
and Korea Advanced Institute of Science
and Technology (KAIST), respectively, in
2006, 2008, and 2012. After graduating, he
worked in the Department of Materials Science & Engineering
of the University of Florida in Gainsville, USA as a postdoctoral
associate, and at LG Chem Ltd. as a senior researcher. He has
been with ETRI since 2014. His current research interests are
optoelectronic materials and devices like OLEDs, QD (quan-
tum dot)-LEDs, photodetectors, and thin-lm encapsulation
for exible devices.
Sukyung Choi received her B.S. Nano-
materials Engineering degree from Pusan
National University, South Korea in 2011,
and her Ph.D. Chemistry degree from
POSTECH in 2016. She is currently a
researcher at ETRI. Her current research
interests are optoelectronic devices with
QDs, and organics like QLEDs and OLEDs.
Nam Sung Cho received his B.S. degree
from Chung-Ang University, Seoul, South
Korea in 2000, and his M.S. and Ph.D.
Chemistry degrees from KAIST in 2002
and 2006, respectively. From 2006 to 2008,
he was a postdoctoral associate at Univ.
of California Santa Barbara. He worked
on materials development for OLEDs at
LG Display R&D Center, Paju from 2008 to 2011. He joined
ETRI in 2011. His current research interests include the OLED
structure, OLED materials, and white OLEDs.
Jeong-Ik Lee received his B.S., M.S., and
Ph.D. Chemistry degrees from KAIST
in 1992, 1994, and 1997, respectively.
After graduating, he joined IBM Almaden
Research Center in San Jose, CA, USA
as a postdoctoral associate, and worked
on OLED materials. He joined ETRI in
1999 and continued his research on OLED
materials and devices. He has been leading the Reality Devices
Research Division of ETRI since 2017 and has worked on the
convergence of display and sensor technologies.
Hokwon Kim received his B.S. degree from
Korea University in 2005. He is currently a
senior engineer at RAONTECH Research
and Development Group.
Jeong-Hwan Lee received his B.S. and
M.S. Electronic Engineering degrees from
KoreaUniversityinSeoul,SouthKoreain
1997 and 1999, respectively. He worked at
Hynix Semiconductor from 1999 to 2002,
and at Integrant Technology (merged with
Analog Devices) from 2002 to 2009. He
has been with RAONTECH since 2013 as
a principal circuit designer.
Min-Seok Kim received his B.S. and M.S.
Electrical Engineering degrees from Korea
University in 1990 and 1992, respec-
tively,andhisPh.D.ElectricalEngineering
degree from Dankook University in 2011.
He was the key engineer at Samsung SDI’s
Iljin Display, developing display driving
circuits, HTPS TFT LCDs, and LCoS dis-
play modules for microdisplay applications. Since 2012, he has
been the head of the Research and Development Group of
RAONTECH. He oversees the engineering departments and
focuses on developing microdisplay technologies for AR (aug-
mented reality) devices.
Hyunkoo Lee received his B.S., M.S., and
Ph.D. Electrical Engineering degrees from
SNU in 2004, 2006, and 2013, respec-
tively. He was a junior researcher with
theMonitorCircuitDesignTeamofLG
Display, South Korea from 2006 to 2009.
HehasbeenwithETRIasasenior
researcher since 2013. His research inter-
ests include the device physics of organic semiconductors and
organic/inorganic hybrid materials and their applications to
optoelectronics devices like OLEDs, QD-LEDs, and printed
electronics; optoelectronics devices with graphene and con-
ducting polymer electrodes; thin-lm encapsulation for exible
devices; and microdisplays for AR/VR.
ORCID
Hyunsu Cho http://orcid.org/0000-0003-0182-6376
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