Journal of Encapsulation and Adsorption Sciences, 2011, 1, 23-28
doi:10.4236/jeas.2011.11003 Published Online June 2011 (
Copyright © 2011 SciRes. JEAS
New Organic Thin-Film Encapsulation for Organic Light
Emitting Diodes
Rakhi Grover1,2, Ritu Srivastava1, Omwati Rana1, D. S. Mehta2, M. N. Kamalasanan1
1Center for Organic Electronics, Physics of Energy Harvesting Division, National Physical Laboratory (Council of
Scientific and Industrial Research), New Delhi, India
2Instrument Design Development Center, Indian Institute of Technology Delhi, New Delhi, India
Received March 11, 2011; revised April 12, 2011; accepted April 20, 2011
Organic Light-Emitting diodes (OLEDs) are extremely sensitive to water vapour and oxygen, which causes
rapid degradation. Epoxy and cover glass with large amount of desiccant are commonly applied to encapsu-
late bottom emitting OLEDs which is not a viable option for flexible as well as top emitting OLEDs. This
paper reports a completely organic encapsulating layer consisting of four periods of alternate stacks of two
organic materials with different morphologies deposited by simple vacuum thermal evaporation technique.
Standard green OLED structures with and without encapsulation were fabricated and investigated using
structural, optical and electrical studies. Moreover, the encapsulation presented being organic is safe for un-
derlying organic layers in OLEDs and is ultrathin, transparent and without any cover glass and desiccant,
ensuring its application in flexible and top emitting OLEDs.
Keywords: OLED, Thin Film Encapsulation, Diffusion Barrier, Atomic Force Microscopy
1. Introduction
Organic Light-Emitting diodes (OLEDs) are considered
as one of the most potential display technology today due
to their low power consumption, low cost, and superior
viewing ability [1] especially for their possibility to build
flexible displays [2-6] as they are ultra-thin and light
weight. Top emitting OLED devices are also becoming
increasingly important because of the increase in aper-
ture ratio (the ratio of actual emitting area to the total
area of the pixel) obtained as compared to bottom emitter
approach. However one obstacle to these developments
is the susceptibility of these devices to water vapor and
oxygen, which causes rapid degradation.
Device reliability issues, in part, arise due to the envi-
ronmental instability of both the active materials and low
work function electrode in the devices [7-11]. Low work
function metals are highly reactive with oxygen and wa-
ter vapor and, thus, oxidize very quickly. This results in
the formation of insulating oxide barriers, making the
injection and collection of charge carriers less efficient.
Exposure to water vapor and oxygen in the environment
may also result in the formation of black spots in OLEDs,
reducing their light output and lifetimes [12]. Another
detrimental degradation mechanism which can arise from
environmental-induced oxidation is the delamination
within the device. When water vapor permeates through
defects into the interface formed by the cathode and an
active layer, it may cause chemical reactions which in-
duce out gassing or volumetric expansion leading to de-
lamination [13,14].
A number of approaches to encapsulation have been
developed including the use of metal lids, glass and the
sealing of devices between two glass substrates or plastic
substrates treated with barrier films [15,16]. Additionally,
getter materials such as calcium and barium are used to
remove any residual water in the encapsulated volume or
water vapour which diffuses through the epoxy sealant
[17]. However, these rigid materials are not amenable for
use in flexible OLEDs which require flexible covering as
well as for top emitting OLEDs in which the emitted
light has to pass through the cover glass and the opaque
Therefore Thin Film Encapsulation (TFE) technology
is the key technique to meet the requirements of
small-size flexible and top emitting electronic display
devices. For active layers and substrates with low glass-
transition temperatures and thermal stability, the proc-
essing temperatures at which the barrier layers can be
deposited should be low. However, processing at low
temperature may lead to more defects in the films, limit-
ing the overall barrier performance. Therefore, inorganic
films are limited in their performance mainly due to the
presence of defects in the films which provide pathways
for water vapour and oxygen to permeate through the
barrier layers.
By applying multilayer films with alternating stacks,
defects which span the entire thickness of the individual
inorganic layers are interrupted and do not channel con-
tinuously through the film structure. This structure cre-
ates a tortuous path resulting in very long effective diffu-
sion pathways, increasing the barrier performance (“de-
fect interruption layer”) [18]. These results suggest that
developing ultra-high barrier performance encapsulation
layer as well as more stable materials for organic device
is important for extended lifetime of organic devices.
Also, the deposition techniques of these inorganic mate-
rials can damage the underlying sensitive organic layers
used in OLEDs. Thus, the development and integration
of high-barrier encapsulation films with organic elec-
tronics remains a challenging endeavour.
The focus of the present work is to prepare completely
organic thin film encapsulation for OLEDs which has been
demonstrated by combining alternate stacks of two differ-
ent organic materials having different morphologies and
different thin film forming properties. One of the organic
material chosen is TPD (TPD-N, N’-diphenyl-N, N’-bis-3
-methylphenyl [1, 1’-bipheny]-4, 4’-diamine which is well
known as hole transporting material in OLEDs but also
known to get easily crystallized because of its low glass
transition temperature (Tg = 64˚C). Another organic mate-
rial chosen is newly synthesized material XP (2.2.6. 5, 5-(4,
ne))bis(2-(4-hexylphenyl)-1, 3, 4-oxadiazole) [19] having
highly amorphous and stable thin film forming ability with
a quite high glass transition temperature of 108˚C. Alter-
nate stacks of thin layers of these materials have been fab-
ricated and studied using Atomic force microscopy and
Calcium corrosion test involving UV-Visible absorption
spectroscopy. Finally the OLED devices with the opti-
mized encapsulation showed enhanced lifetime as com-
pared to the devices with no encapsulation layers.
2. Experimental
Plain glass plates were used as the starting substrates and
cleaned sequentially using de ionized water, acetone, tri-
chloroethylene and isopropyl alcohol for 20 minutes each
using an ultrasonic bath and then dried in vacuum oven.
Thin films of Ca (Calcium), TPD and XP were deposited
with thicknesses of 250 nm, 20 nm and 10 nm respectively
under a high vacuum (105 - 106 Torr).The correspond-
ing device structures being (also shown in Figure 2):
(a) Glass/Ca
(b) Glass/Ca/TPD
(c) Glass/Ca/TPD/XP
The thicknesses of these films were measured in situ
by a quartz crystal thickness monitor. Morphological
properties were examined using Atomic force micros-
copy (AFM) (NT-MDT). UV-Vis spectra were taken
using a high resolution UV-Vis spectrophotometer (Shi-
madzu 2401 PC) in the range of 200 - 800 nm.
Then OLEDs were fabricated incorporating the opti-
mized encapsulation layer. Indium-tin oxide (ITO) (thi-
ckness of 120 nm) coated glass plates with a sheet resis-
tance of 20 / (Vin Karola, USA) were used as starting
substrates and were patterned and cleaned using deion-
ised water, acetone, trichloroethylene and isopropyl al-
cohol sequentially for 20 min each using an ultrasonic
bath and dried in vacuum.The device structure of OLEDs
grown by vacuum thermal deposition was:
ITO/m-MTDATA (20 nm)/α-NPD (10 nm)/CBP +5%
Ir (ppy)3 (35 nm)/TPBi (30 nm) /LiF (1 nm)/Al (200 nm).
4, 4’, 4”-tris(3-methyl-phenylphenylamino) triphenyl-
amine (m-MTDATA) and N, N’-di-1-naphthalenyl-N,
N’- diphenyl-1.1’ biphenyl-4, 4’-diamine (α-NPD) were
used as hole injection and hole transporting layers re-
spectively. 5% (Ir (ppy)3) doped (CBP) was used as the
emissive layer. 2, 2’, 2’’-(l, 3, 5-benzenetriyl)-tris (L-ph-
enyl-l-H-benzimidazole (TPBi) and LiF were used as
electron transporting and electron injection layers re-
spectively. The size of each pixel was 5 mm 5 mm.
Then four periods of TPD (20 nm)/XP (10 nm) layers
were deposited. The current density-voltage-luminesc-
ence (J-V-L) characteristics have been measured with a
luminance meter (LMT-1009) interfaced with a Keithley
2400 programmable current-voltage digital source meter.
All the measurements were carried out at room tempera-
ture under ambient conditions.
3. Results and Discussion
3.1. Structural Studies
Figure 1 shows the morphology of the thin films of XP
as recorded from AFM with time .The films were
found to be highly amorphous and more significantly
stable also. As explained in Section 1, densely packed,
continuous, and highly conformal coatings are desir-
able for excellent barrier properties, so that all small
particles are well encapsulated. Table 1 shows the av-
erage roughness values of the thin film of XP with time
and corresponding AFM images are presented in Fig-
ure 1.
Copyright © 2011 SciRes. JEAS
Table 1 Average roughness values of thin film of XP with
S. No. Time Average Roughness value (nm)
1 After deposition 1.011
2 After 480 hours (20 days)1.937
3 After 960 hours (40 days)2.235
Figure 1. AFM images of 10nm thick films of XP. (a) after
deposition (b) after 20 days (c) after 40 days of storage.
Highly amorphous thin films of XP exhibited an av-
erage roughness of 2.235 nm even after 960 hours of
storage at room temperature (45% RH).
TPD is well known in literature to crystallize easily
due to its low glass transition temperature (64˚C) [20]
and hence the alternate arrangement of such a crystal-
line layer and highly amorphous layer of XP can prove
to be an efficient encapsulating layer which increases
the effective diffusion length for ambient water vapors
or oxygen molecules as explained in Section 1.
3.2. Optical Studies
Calcium corrosion test is a widely known method for
measuring ultra-low permeation rate of barrier films.
Calcium is a conducting and opaque metal which be-
comes transparent after oxidation and is very sensitive
for detecting the presence of oxygen and water vapour.
Hence, the measurement of Ca transparency with time
provides an indirect method to determine the oxidation
and corrosion rates of Ca. Thus well encapsulated films
of Ca show relatively lower values of transmittance as
compared to the Ca film without encapsulation. Figure 2
shows the device structures fabricated for the calcium
corrosion test. Figure 3(a) shows the Transmission spec-
tra against wavelength for the as deposited structures
2(a), 2(b) and 2(c). The films were found to have quite
good optical transparency. Figure 3(b) shows the
Transmission spectra with time for the structures 2(a),
2(b) and 2(c). The more rapid increase in optical trans-
parency of the structure with bare Ca in Figure 3(b)
shows that it rapidly oxidizes in air as compared to rela-
tively slow oxidation of structures 2(b) and 2(c) (slopes
of increment in oxidation with time being 0.13, 0.09 and
0.04 for the structures 2(a), 2(b) and 2(c) respectively).
Moreover, the device structure 2(c) exhibited greatest
resistance against moisture. This is an indication of im-
proved barrier properties of the alternate stack of the
(a) (b)
Figure 2. Device structure fabricated for Calcium corrosion
Copyright © 2011 SciRes. JEAS
Figure 3. (a) Transmission spectra for the as deposited films
and (b) Transmission values for the films degraded with
Figure 4. Schematic device structure of the fabricated
3.3. Electrical Studies / OLED Fabrication
Figure 4 shows the schematic device structure of the
OLED fabricated by vacuum thermal evaporation tech-
nique. Phosphorescent molecule Ir (ppy)3 was used as the
green emitter. On the top of the cathode layer, the opti-
mized encapsulating structure was deposited which con-
sisted of four periods of TPD (20 nm)/XP (10 nm) layers.
Figure 5 shows the Luminance-Voltage characteristics
of the devices with and without thin film encapsulation
layers. Both the devices with and without encapsulation
showed relatively similar initial L-V characteristics.
To further confirm the encapsulating properties of al-
ternate stacks of proposed TFE layers, the luminous de-
cay of the devices under a constant current operation
were investigated. The luminance decay curve of the
devices with and without TFE is shown in Figure 6. We
Figure 5. Luminance-Voltage characteristics of the OLEDs
fabricated with and without TFE.
Figure 6. Luminous decay of the devices with and without
TFE under a constant current density of 0.4 mA/cm2.
Copyright © 2011 SciRes. JEAS
have measured luminance of the devices at a continuous
constant current of 0.4 mA/cm2, which gave an initial
luminance of 140 Cd/m2 .As shown in Figure 6, the en-
capsulated device, could stay longer (12 hours) as com-
pared to the device without encapsulation layer (1.1
hours). The enhancement in lifetime of encapsulated
device being around ten times higher than that of the
device without encapsulation. Further studies on the
work are in progress. Although the obtained device life-
times are quite low in terms of efficient organic displays
but can be enhanced due to fact that encapsulation layers
were deposited without any further capping of the device
as reported in literature with additional aluminium foil
capping [21]. It is worth noting that the alternate stacking
type encapsulation is ultrathin, optically transparent and
flexible, suiting the encapsulation of flexible OLEDs as
well as top emitting OLEDs.
The barrier properties of the proposed TFE have been
attributed to the defect interruption due to different types
of film forming abilities of TPD and XP layers. TPD is
well known to have a glass transition temperature as low
as 64˚C whereas the newly synthesized material XP has a
high glass transition temperature of 108˚C. The alternate
stacks of these different types of films one highly amor-
phous and other crystalline basically increase the diffu-
sion lengths of ambient oxygen or water vapours thereby
protecting the device. Additionally the TFE being or-
ganic in nature can be deposited at much lower tempera-
tures as compared to earlier reported inorganic encapsu-
lation layers [22,23] and is therefore more compatible
with the organic transport and emissive layers used in the
organic based devices. Thus use of ultra-thin vacuum
thermally-deposited alternate organic thin films is effec-
tive as a barrier layer for significantly increasing the life-
time of an OLED device.
4. Conclusions
The encapsulation of an OLED device with the alternate
stacks of two organic thin films TPD and XP with dif-
ferent morphologies and different thin film forming
properties using the vacuum thermal evaporation method
has been carried out for the first time. Vacuum thermally
deposited organic films showed excellent barrier proper-
ties. The TFE was found to significantly slow down the
oxygen and moisture diffusion into the device leading to
a longer operational lifetime.
5. Acknowledgement
The authors wish to acknowledge the Director, NPL,
Delhi and Dr. S. S. Bawa for suggestions and discussions.
The authors would like to acknowledge Department of
Science and Technology DST and Council of Scientific
and Industrial Research CSIR, New Delhi, India for fi-
nancial support.
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