A flexible moisture barrier comprised of a SiO2-embedded organicâ ...

Organic Electronics 14 (2013) 1435–1440
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Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
A flexible moisture barrier comprised of a SiO 2 -embedded
organic–inorganic hybrid nanocomposite and Al 2 O 3 for
thin-film encapsulation of OLEDs
Yun Cheol Han a , Eungtaek Kim a , Woohyun Kim a , Hyeon-Gyun Im b , Byeong-Soo Bae b ,
Kyung Cheol Choi a,⇑
a Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea
b Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701,
Republic of Korea
article
info
abstract
Article history:
Received 18 January 2013
Received in revised form 8 February 2013
Accepted 3 March 2013
Available online 29 March 2013
Keywords:
OLED
Ca test
Nanocomposite
Moisture barrier
Encapsulation
We demonstrated a high performance flexible multi-barrier containing a silica nanoparticle-embedded
organic–inorganic hybrid (S–H) nanocomposite and Al 2 O 3 . The multi-barrier
was prepared by low-temperature Al 2 O 3 atomic layer deposition and with a spin-coated
S–H nanocomposite. The moisture barrier properties were investigated with a water vapor
transmission rate (WVTR), estimated by a Ca test at 30 °C, 90% R.H.. Moisture diffusion was
effectively suppressed by the sub-700 nm thick multi-barrier incorporating well-dispersed
silica nanoparticles in the organic layer. A low WVTR of 1.14 10 5 g/m 2 day and average
transmittance of 85.8% in the visible region were obtained for the multi-barrier. After
bending under tensile stress mode, the moisture barrier property of the multi-barriers
was retained. The multi-barrier was successfully applied to thin-film encapsulation of
OLEDs. The thin-film encapsulated OLEDs showed practicable current–voltage–luminance
(I–V–L) characteristics and stable real operation over 700 h under ambient conditions.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
Since the first report of electroluminescent organic
thin-films [1], the development of organic light-emitting
diodes (OLEDs) has progressed rapidly. Flexible OLEDs
are currently considered to be the most promising candidate
for next-generation displays. One of the major obstacles
to the realization of flexible OLEDs is reliable thin-film
encapsulation of OLEDs. Due to sensitivity to moisture and
oxygen, organic materials are easily oxidized and crystalized
by exposure to external environment. These effects
can directly lead to the formation of dark spots, which
are known as non-emissive regions [2–4]. Burrows et al.
reported a simple encapsulation method using glass and
epoxy adhesive in a N 2 -filled glove box [5]. Based on this
⇑ Corresponding author. Tel.: +82 42 350 3482; fax: +82 42 350 8082.
E-mail address: kyungcc@kaist.ac.kr (K.C. Choi).
encapsulation technique, glass-lid encapsulation is generally
used with a desiccant such as BaO and CaO. However,
the glass-lid is not suitable for flexible encapsulation due
to its mechanically breakable nature. Furthermore, the
water vapor transmission rate (WVTR) of the polymer substrate
is substantially below the requirement for OLED
applications (10 6 g/m 2 day) [6]. A thin-film barrier coating
is essential for realizing reliable flexible OLEDs. Inorganic
single barriers [7,8] and inorganic nanolaminate
barriers [9,10] have shown potential to replace glass-lid
encapsulation. Although these inorganic barriers have
low WVTR values, the formation of defects such as
pinholes cannot be avoided due to the limitations of the
vacuum deposition process. An organic–inorganic multibarrier
approach was reported as an alternative to prevent
the formation of pinholes [11]. Multi-barrier encapsulation
of OLEDs has demonstrated long lifetime comparable to
that achieved with glass-lid encapsulation [12,13]. The
1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.03.008

1436 Y.C. Han et al. / Organic Electronics 14 (2013) 1435–1440
permeation mechanisms of the multi-barriers were
discussed by Graff et al., on the basis of transient and
steady-state vapor permeation analyses [14]. The multibarriers
delay permeation in the transient period rather
than reducing the steady-state permeation rate. In the
transient period, lag time is extremely increased due to
the effectively long diffusion paths. Permeation is consequently
delayed by up to few years. A multi-barrier design
having a complicated diffusion path is crucial for the moisture
barrier.
Here, we report an organic–inorganic flexible multibarrier
based on a solution-processed silica nanoparticleembedded
organic–inorganic hybrid (S–H) nanocomposite
as an organic layer coupled with a low-temperature atomic
layer deposited Al 2 O 3 layer. To reinforce the effects provided
by the multi-barrier approach, we additionally introduced
silica nanoparticles as filler into the multi-barrier
structure to create complicated diffusion paths. These barriers
resulted in a low WVTR value for a sub-700 nm thinfilm.
We also discussed the barrier performance in terms of
optical transmittance and flexibility. Finally, we applied
the multi-barrier to thin-film encapsulation of OLEDs.
The passivated OLEDs showed identical current–voltage–
luminance (I–V–L) characteristics and stable operation.
2. Experimental
2.1. Preparation of organic–inorganic hybrid nanocomposite
In a previous study, we prepared a silica nanoparticleembedded
sol–gel organic–inorganic hybrid nanocomposite
(S–H nanocomposite) to improve the moisture barrier
performance of a cyclo-aliphatic epoxy based hybrid material
[15]. Nanopox Ò E600 (Nanoresins, Germany), based on
methyl-terminated silica nanoparticles dispersed in a
reactive diluent of 3,4-epoxycyclohexyl methyl 3,4-
epoxycyclohexane carboxylate (EMEC) was mixed with
UV-curable cycloaliphatic-epoxy hybrid materials (hybrimer),
synthesized via a sol–gel reaction between [2-(3,4-
epoxycyclohexyl)ethyl]trimethoxysilane (ECTS) and diphenylsilanediol
(DPSD). Arylsulfoninum hexafluorophosphate
salt was used as an initiator for photo-cationic polymerization.
Propylene glycol monoether acetate was added as a
solvent to control the thickness of the spin-coated layer.
The details of the synthesis method and characteristics of
the hybrimer are provided in earlier studies [16]. We have
also reported the average particle diameters and dispersion
morphology of the S–H nanocomposite in relation to
silica content. For 100% silica content, the nanoparticles,
which have an average diameter of 19 nm, were homogeneously
dispersed in the oligosiloxane resin. The optimized
S–H nanocomposite showed high transmittance and a low
WVTR value due to the relatively low degree of light scattering
and extended diffusion paths of moisture compared
to those observed with low silica contents, respectively.
2.2. Ca test
The WVTR value was estimated by a Ca test, which is
based on the electrical corrosion of the Ca layer [17]. The
Ca sensor was prepared by thermal evaporation of a Ca
layer on a glass substrate. The Ca layer had an area of
1.5 cm 2 and a height of 250 nm. The window area for permeation
was identical to that of the Ca layer. An Al layer of
100-nm thickness was deposited as an electrode. 100-lm
thick PET was employed as a substrate for the barrier coating
throughout the experiment. The Ca sensor was encapsulated
by the barrier-coated film using a UV-curable
sealant. To ensure encapsulation of the Ca layer, a thin line
of sealant was applied from a syringe along the sealing line
by a dispenser. The width of the sealing line was 2.5 mm. A
glass-lid with a hole at the center was used to support the
barrier-coating film. To apply pressure on the sealing line,
the Ca sensor was clamped to the large glass substrate
while UV-curing was performed. The entire process was
conducted in a N 2 -filled glove box, which was integrated
with a thermal evaporator. During degradation of the Ca
sensor in the climate chamber at 30 °C and 90% R.H., the
resistance of the sensor was monitored in situ using the
four-point probe system.
2.3. Barrier coating and device fabrication
For the multi-barrier structure, the S–H nanocomposite
and Al 2 O 3 were used as an organic and inorganic layer,
respectively. Al 2 O 3 was deposited using a thermal ALD system.
Trimethylaluminum (TMA) and H 2 O were used as a
precursor and reactant, respectively. One cycle for Al 2 O 3
consists of a TMA pulse, TMA purge, H 2 O pulse, and H 2 O
purge successively. Nitrogen gas was used as both the carrier
and purge gas. Pulse and purge sequences of 0.2–10 s
were used and the temperature of the main chamber was
maintained at 70 °C during the deposition process. The
S–H nanocomposite with a predetermined amount of diluent
was spin-coated at 4500 RPM for 5 s and subsequently
UV-cured by I-line UV light (k = 365 nm, optical power
density = 20 mW/cm 2 ) for 100 s. The thickness of the
spin-coated S–H nanocomposite was around 190 nm, measured
by a surface profiler (Dektak-8, Veeco, USA). Following
the spin-coating process, the barrier-coated film was
dried in a vacuum chamber with a high purity N 2 gas flow
(99.9999%, 100 sccm) for 30 min to remove the remaining
solvent. While drying the sample, the vacuum level of the
chamber was maintained at 1.2 Torr. These organic and
inorganic layers were coated on a 100-lm thick PET substrate,
alternately.
The OLEDs were fabricated on a glass substrate in a
bottom-emission type. The OLED devices had a structure
of ITO(150 nm)/2-TNATA(60 nm)/NPB(30 nm)/Alq 3 doped
with C545t(35 nm)/Alq 3 (30 nm)/Liq(0.8 nm)/Al(100 nm),
wherein 4,4 0 ,4 00 -tris(2-naphthylphenyl-phenylamino)-
triphenylamine (2-TNATA) and N,N 0 -bis(1-naphthyl)-N,N 0 -
diphenyl-1,1 0 -biphenyl – 4,4 0 -diamine (NPB) respectively
functioned as a hole-injection layer (HIL) and a hole-transport
layer (HTL). The emitting layer was co-deposited with
tris(8-hydroxyquinolinato)aluminum (Alq 3 ) and 10-(2-
benzothiazolyl)-2,3,6,7- (C545t) at 2% as a green emission
dopant. Alq 3 was subsequently used as an electron
transport layer (ETL). 8-Hydroxyquinolinolato-lithium
(Liq) and Al were used as an electron-injection layer (EIL)
and as the cathode, respectively. The devices were

Y.C. Han et al. / Organic Electronics 14 (2013) 1435–1440 1437
encapsulated using a glass lid with a desiccant in a nitrogen
globe box in the reference device. For thin-film encapsulation
(TFE) of OLEDs, the multi-barrier was directly
coated on the devices with an identical barrier coating
method. The I–V–L characteristics were measured with a
source meter (Keithley 2400, USA) and a spectrophotometer
(CS-2000, Konica Minolta, Japan). The lifetime was recorded
by Si-photo diodes (Polaronix M6000S, McScience,
Korea).
3. Results and discussion
3.1. Permeation barrier properties of the multi-barrier
Permeation barrier properties of the S–H nanocomposite
and Al 2 O 3 were characterized by the Ca test. The results of
the Ca test are plotted as normalized conductance vs. time
curves and shown in Fig. 1. WVTR values of 0.58 g/m 2 day
and 7.94 10 4 g/m 2 day were observed for the S–H
nanocomposite and Al 2 O 3 , respectively. These values were
estimated from the relation between the WVTR and slope
of the curve [17].
The normalized conductance curve for the glass-lid
encapsulation of the Ca sensor is illustrated in Fig. 2a. To
evaluate the long term stability of the sealing method described
in Section 2.2, a Ca test was carried out over
72 days. After WVTR measurement, the sample was stored
Fig. 2. Results of WVTR measurements: (a) Glass-lid encapsulation of
the Ca sensor, and (b) multi-barrier encapsulation of the Ca sensor with
an increase of the barrier stacks. Insets show a photograph of the
encapsulated Ca sensor with a glass and a cross-sectional SEM image of
the multi-barrier on a Si substrate, respectively.
Fig. 1. Ca test results of (a) 190-nm thick S–H nanocomposite and (b)
330-cycles Al 2 O 3 on the 100-lm thick PET substrate. Insets show a
photograph and schematic diagram of a Ca test sample, respectively.
under ambient conditions for 5 months. The photograph in
the inset of Fig. 2a shows that the initial conditions of the
Ca layer were fully maintained. Among the three samples
encapsulated by a glass-lid, the lowest WVTR value,
2.96 10 6 g/m 2 day, was recorded with a 0.985% change
in conductance after 72 days. The WVTR of the multibarrier
was also measured with application of an identical
sealing method while varying the multi-barrier stacks, as
shown in Fig. 2b. One dyad was composed of Al 2 O 3 (330
cycles)/S–H nanocomposite resin (190 nm). The multibarrier
stacks were composed of 1.5 dyads to 3.5 dyads
by depositing an Al 2 O 3 layer as a top layer. The lowest
WVTR values were determined to be 1.14 10 4 g/m 2
day, 5.43 10 5 g/m 2 day, and 1.14 10 5 g/m 2 day for
each multi-barrier stack, respectively. The inset in Fig. 2b
depicts a scanning electron microscope (SEM) image of
the 3.5 dyads multi-barrier on a Si wafer. The image was
taken using a FEI Company instrument (Magellan400,
USA). Each layer was well defined in the multi-barrier configuration.
Sub-700 nm thickness was observed to be in the
entire barrier structure.
To ensure reproducibility and reliability of the multibarrier
performance, the WVTR measurement was evaluated
for three samples of each multi-barrier stack. The
resistance of each sample was monitored for more than
10 days. The average WVTR values of the measurements
are shown in Fig. 3. The WVTR values decreased as the

1438 Y.C. Han et al. / Organic Electronics 14 (2013) 1435–1440
Fig. 3. The average WVTR values as a function of the multi-barrier stacks
and the glass. Each sample was measured three times. The average WVTR
value is indicated as a black box with maximum and minimum values.
multi-barrier stack was increased. The average WVTR
values were calculated to be 1.6 10 4 g/m 2 day, 7.07
10 5 g/m 2 day, and 1.26 10 5 g/m 2 day as the multibarrier
stacks were increased from 1.5 dyads to 3.5 dyads.
For the glass-lid encapsulated Ca sensor, the average WVTR
value was 3.54 10 6 g/m 2 day. The lower limit of the Ca
test almost reached a WVTR of 10 6 g/m 2 day at 30 °C,
90% R.H. in our experimental conditions. The variation of
the WVTR values for the same multi-barrier structure
showed reasonable values.
3.2. Bending characteristics of the multi-barrier
Fig. 4. Ca test results after bending test while varying the multi-barrier
stacks; (a) Normalized conductance versus time curve after 100 iterations
of bending, and (b) comparison of the WVTR values after bending test.
The inset shows a photograph of the bending test set-up.
To identity the multi-barrier’s resistivity to bending
stress, the bending test was conducted with a custommade
bending machine, as shown in the inset of Fig. 4a.
Tensile stress was applied to the multi-barrier with a bending
radius of 3 cm. Fig. 4a presents the results of the WVTR
measurement after 100 iterations of bending at a bending
speed of 0.5 Hz. The WVTR was estimated to be
2.23 10 4 g/m 2 day, 6.97 10 5 g/m 2 day, and
1.76 10 5 g/m 2 day for 1.5 dyads, 2.5 dyads, and 3.5
dyads, respectively. As shown in Fig. 4b, the WVTR values
after the bending test were comparable to those before
bending test.
3.3. Optical properties of the multi-barrier
The optical transmittance for the normal incidence light
was measured using a spectrophotometer (UV-2550,
Shimadzu, Japan). The air was estimated as a baseline. All
the barrier-coated samples on PET showed optical transmittance
of above 80% in the wavelength region from
400 nm to 900 nm, as illustrated in Fig. 5. The average
transmittance in the visible region (k = 400–700 nm) for
the 3.5 dyads multi-barrier was measured to be 85.8%.
The inset in Fig. 5 shows a photograph of the logo images.
The images in the square region indicated by the red
dashed line are under the 3.5 dyads multi-barrier film.
These images are almost identical to the original image
owing to the high transmittance of the multi-barrier.
Fig. 5. Optical transmittance of the multi-barrier with different stacks. In
the photograph of the logo images, the red-dashed region shows the logo
images under the multi-barrier film and the others are original images.
3.4. Thin-film encapsulation of OLEDs
A2mm 2 mm active area of the OLED was defined by
crossing the cathode and anode electrode. A photo resist
was used as an insulator, which allows the charge to flow
only through the active area, as shown in the inset of
Fig. 6a. Fig. 6 shows the I–V–L characteristics of the thinfilm
encapsulated OLED and a glass-lid encapsulated OLED
with a desiccant. Both devices showed diode characteristics
in the current density versus voltage curve, as illustrated
in Fig. 6a. The devices had a turn-on voltage of

Y.C. Han et al. / Organic Electronics 14 (2013) 1435–1440 1439
Fig. 7. Comparison of the lifetime with an initial luminance of 600 cd/m 2
under ambient conditions. The inset shows 2 mm 2 mm microscope
images of the active area before and after driving, respectively.
Fig. 6. (a) A comparison of I–V characteristics between the thin-film
encapsulated device and glass-lid encapsulated device. (b) Plot of the
luminance versus current density. Insets show a schematic diagram and
photograph of passivated OLEDs, respectively.
around 2.5 V. The dc-current sweep measurement showed
that the luminance values linearly increased. The maximum
luminance level was measured to be around
58,000 cd/m 2 at a current density of 382 mA/cm 2 . These results
indicate that the two devices show almost identical
operation.
To verify the performance of the multi-barrier for thinfilm
encapsulation of OLEDs, a lifetime test was carried out.
The devices were operated by a constant current source of
0.15 mA under ambient conditions. The relative light
intensity was calculated using a calibrated Si-photo diode
with an initial luminance value of L 0 = 600 cd/m 2 . The error
of the initial luminance value was less than 20 cd/m 2 .Itis
known that the luminance degradation in OLEDs shows
stretched exponential decay behavior under constant current
driving [18]. The light intensity exponentially decreased
over time for the two devices. After 720 h of
driving for the two devices, the luminance of the OLEDs reduced
to 55.2% of the initial value for the glass-lid encapsulated
device and 50.5% of the initial value for the thin-film
encapsulated device, respectively. The microscope images
of the active area for the thin-film encapsulated device,
presented in the inset of Fig. 7, suggest that the OLED
was successfully passivated without dark spots and did
not degrade by external factors during real operation.
We discussed the characteristics of a high performance
multi-barrier. In the case of single inorganic barriers, permeation
of moisture is dominantly affected by the defects
density [19]. A vacuum-deposited inorganic layer has
intrinsic and extrinsic defects caused by imperfections of
the deposition process and the presence of impurities on
the substrate, respectively. These defects function as permeation
paths. They can be blocked by introducing an organic
layer. In the organic–inorganic multi-barrier
approach, lag times are remarkably extended by adding organic–inorganic
dyads due to suppression of diffusion,
which is the slowest and the most critical step in the permeation
process [14]. Because nanofillers in the polymer
matrix force moisture to detour around the fillers in this
layer, the barrier property can be enhanced by the tortuous
diffusion path [20]. In this regard, the enhancement of the
moisture barrier property is primarily attributed to extended
diffusion paths. The experiments verified that the
diffusion of moisture is effectively inhibited by organic–
inorganic dyads in the multi-barrier. The diffusion path
in the organic layer of the multi-barrier is complicated by
the silica nanoparticles, which serve as the filler in the
cyclo-aliphatic epoxy oligosiloxane resin (hybrimer).
Consequently, a low WVTR value of 1.14 10 5 g/m 2 day
was achieved for the 3.5 dyad multi-barrier with sub-
700 nm overall thickness. The organic layer also functioned
as a buffer layer for the bending stress. The moisture
barrier property was retained after a bending test with a
bending radius of 3 cm.
4. Conclusion
We have investigated the characteristics of a high performance
flexible multi-barrier consisting of the S–H nanocomposite
and Al 2 O 3 as an organic and inorganic layer,
respectively. The moisture barrier property was characterized
through a Ca test, optimized with the sealing method
of the Ca sensor. The Ca test showed a low limit of
2.96 10 6 g/m 2 day and favorable reproducibility over a
long time. The sub-700 nm thick multi-barrier showed a
low WVTR and high transmittance. After a bending test
with a bending radius of 3 cm, the WVTR value did not
significantly change. This multi-barrier was successfully
applied to thin-film encapsulation of OLEDs. The encapsulated
OLEDs comparable performance to that of glass-lid
encapsulated OLEDs.

1440 Y.C. Han et al. / Organic Electronics 14 (2013) 1435–1440
Acknowledgements
This research was supported by Basic Science Research
Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science
and Technology (CAFDC-20120000820). This work was also
supported by the Global Leading Technology Program of
the Office of Strategic R&D Planning (OSP) funded by the
Ministry of Knowledge Economy, Republic of Korea
(10042412).
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