The document summarizes the results of a research program to develop an UV-curing epoxy adhesive for sealing OLED displays with low water vapor transmission rates (WVTR). Key findings include: 1) An epoxy adhesive called DELO-KATIOBOND AD690 was formulated and found to have a WVTR of 0.8 g/m2d at 50°C/95% RH, meeting requirements for long OLED lifetimes. 2) Tests on the adhesive showed it maintained high shear strengths even after humidity and temperature cycling stresses. 3) Calcium mirror tests of the adhesive encapsulating an OLED found WVTRs of 0.03 g/

1. P-64 / R. Doerfler
P-64: New UV-Curing OLED Encapsulation Adhesive with Low Water
Permeation
Rainer Doerfler
DELO Industrial Adhesives, Ohmstr. 3, 86899 Landsberg am Lech, Germany
Siegfried Barth
Optrex Europe GmbH, Seligenstaedter Str. 40, 64832 Babenhausen, Germany
Christine Boeffel
Fraunhofer Institute for Reliability and Microintegration, Kantstr. 55, 14513 Teltow, Germany
Armin Wedel
Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, 14476 Golm, Germany
WVTR
Abstract Size of display (active display area)
[g/m²d]
The results of a research program for the development of an UV
curing epoxy adhesive for the sealing frame of OLED displays 21 mm x 30 mm 0.052
with a preferably low water vapor transmission rate (WVTR)
will be presented. The material was analyzed in substantial 28.8 mm x 73.8 mm 0.19
lifetime investigations both in calcium mirror tests and in PLED Edge encapsulation
devices under different climatic environments and its capability 5‘‘ diagonal 0.24
was verified.
10‘‘ diagonal 0.49
1. Introduction
independent of size,
Due to their manifold advantages, OLED displays are currently Full encapsulation transmission rate for 5.5*10-5
replacing LCDs to an increasing degree in various applications. Thus, the given surface
they were meanwhile able to gain a significant market share especially
in the field of mobile phones and MP3 players. In 2005, for example, Table 1: Calculated upper limit permeation values for OLED
OLED passive-matrix (PM) displays were already used in 42 % of the encapsulation adhesives (see text for the assumptions made).
produced MP3 players [1]. However, several OLED producers are
working on lifetime improvements in order to enable also a substantial 2. Adhesive formulation
penetration into the premium OEM market, such as for automotive
The water vapor permeability of adhesives can be influenced in
applications. One remaining critical issue is the high sensitivity of the
many ways. Thus, fillers can act as a barrier or water molecules
functional layers against moisture and oxygen. Therefore, OLEDs
can be bonded chemically or physically to the filler. The
need to be encapsulated with materials showing low water vapor
polymer itself can be influenced by inserting highly regular
permeability in the range of 10-5 g/m²d [2]. Those values are hard to
molecule structures, by the degree of cross-linking or by low
achieve with standard organic adhesives. The sealing frame
intermolecular interactions with water molecules. However,
accordingly represents a weak point in relation to the OLED lifetime
these degrees of freedom are significantly limited by further
and further optimizations of the adhesive materials are necessary.
demands concerning functionality and processibility of the
The following table 1 gives an overview of the required permeation adhesive. Regarding the functionality, a high adhesive strength
rates of adhesives for an edge and full encapsulation of OLED on the relevant substrates as well as a preferably high
devices, respectively. The calculation is based on an expected OLED insensitivity against aging and climatic influences must be
lifetime of 10.000 h, which have to be reached for the relevant guaranteed. Moreover, with respect to the processibility the
temperature and humidity of the environment. The assumption was material must be suitable for dispensing and screen printing with
made, that a 100 nm thick magnesium cathode may be reduced by a standard flat panel display equipment. A decisive influence is
maximum of 20 % of its layer thickness. The adhesive layer is exerted by the fact that the material can be pressed to very thin
supposed to be 5 µm high and 1 mm wide. layers, setting boundaries to the use of fillers. Due to the high
significance of the adhesive layer thickness a value of ≤ 10 µm
The calculated permeation values will not be reached by standard was formulated for the latter as a development objective.
adhesives in particular at higher temperatures and humidity. Typical
achievable values at the start of our project were 3 - 10 g/m2d at 60°C UV curing epoxies were chosen as base material. They allow an
and 80 % humidity. As a consequence further protective measures, extremely fast curing at low temperatures and can therefore
such as integrated absorbers, are essential for long-lived OLEDs. fulfill the demands for a high throughput production of OLED
Furthermore the WVTR of the adhesives has to be further examined displays. Compared to UV curing acrylates, epoxies exhibit
and optimized. Therefore the aim of the research program was the fundamental advantages regarding low water permeability.
development of encapsulation adhesives with significantly lower
water vapor permeability.
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2. P-64 / R. Doerfler
3. Results 35
In the framework of the adhesive improvement, permeation 30
Shear Strength [MPa]
measurements were carried out according to ASTM E 96. The
25
adhesive strengths were determined on compression shear
specimens of 20 x 20 x 5 mm³ based on sodium silicate glass, 20 glass/glass
according to DELO standard 5 as well as on ITO-coated display 15 ito/glass
glasses. The specimens were conditioned at 120°C for one hour. 10
The product chosen from these pre-examinations, DELO-
5
KATIOBOND AD690, was subject to detailed analyses both on
calcium mirror tests and PLED devices. 0
temperature 500 h 85/85 16 h
3.1 Material characteristic values cycling pressure
cooker
DELO-KATIOBOND AD690 shows a WVTR of
0.8 g/m2d according to ASTM E 96 at 50 °C/95 % r.h. referring
to a layer thickness of 1 mm. Further important material Figure 1: Adhesive strengths after different stress tests.
characteristics are summarized in table 2.
3.3 Calcium mirror tests
Viscosity Brookfield 7/5 mPas 260000
ASTM E 96 and the so called Mocon test allow for the relatively
Tensile DIN EN ISO 527 MPa 32 comfortable determination of the water vapor permeation on
strength adhesive foils. What cannot be measured with these methods,
Elongation at DIN EN ISO 527 % 0.7 are effects on the boundary layer between adhesive and
tear substrate, so that no conclusion can be drawn about the
Young DIN EN ISO 527 MPa 4800 reliability of an adhesive sealing. The calcium (Ca) mirror test is
modulus better suited for such analyses [3]. Here, a 100 nm thick Ca-
Shear strength DELO standard 5 MPa 28 layer is vacuum-deposited onto a glass substrate. The glass is
glass/glass then sealed by a cover glass via a circular sealing frame. The
Shear strength DELO standard 5 MPa 15 measurement of penetrating moisture and oxygen is determined
glass/ITO indirectly via the transmission change of the Ca-layer.
Glass TMA °C 102 According to the following chemical equations
transition Ca + 2 H2O → Ca(OH)2 + H2 and
temperature
CTE TMA Ppp/K 30 2 Ca + O2 → 2 CaO
[30-80 °C]
the opaque Ca-layer is changed to transparent calcium hydroxide or
Water DIN EN ISO 62 % <0.1 calcium oxide, respectively, meaning that also penetrating oxygen is
absorption recorded by this measuring method. For the measurement the Ca-layer
Filler size d98 µm ≤8 is divided into various sectors. The optical transmission of the layer in
Table 2: Characteristic values of DELO-KATIOBOND AD690. each single sector is determined via a laser and a photodiode. The used
measurement set-up is shown in figure 2. Via the averaged
3.2 Humidity resistance transmission, a water equivalent and therefore the permeation of the
Adhesive sealings may peel off under the influence of air adhesive layer can be determined.
humidity, if water diffuses in the boundary layer between Thus the architecture of the devices used for the Ca-mirror test is
adhesive and substrate. Therefore, not only high initial adhesive similar to the one of edge sealed OLED displays (however, without
strengths are crucial for the reliability of the bonding, but also the organic functional layers). Consequently useful conclusions can be
the durability of the adhesive sealing. Accordingly, test deduced for the behavior of the adhesive material.
specimens and display glasses were subject to the following
stress tests: Detailed measurements were carried out at different temperature
/ humidity environments (all devices without integrated
- temperature shock test, 10 cycles – 40/+85 °C; hold time absorbers!). The results are summarized in table 3. As expected
2 h, change time 5 s an increase of the WVTR can be observed at higher
- 500 h storage at 85 °C, 85 % r. h. temperatures and air humidity.
- 16 h Pressure Cooker storage at 100 °C, 1.2 bar, 100 % What is revealing as well is the process of the reduction of the
humidity. Ca-layer thickness. The decrease of the Ca-layer thickness and
the concomitant increase of the relative transmission of the Ca-
After the storage tests all specimens exhibit shear strengths of > layer over the storage time at 25 °C/30-50 % r. h., 60 °C/80 % r.
5 MPa (see figure 1). For an OLED device of 21 mm x 30 mm h. and 85 °C/85 % r. h. are shown in figure 3-5, respectively.
with a circular, 1 mm wide sealing frame, this means a load
capacity of more than At a storage temperature of 25 °C and ambient humidity (30 -
500 N! No delaminations and cracks could be detected on the 50 %), a WVTR of 0.03 g/m²d has been determined for DELO-
sealed devices after storage. KATIOBOND AD690 deposited as a 150 µm thick layer on a
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3. P-64 / R. Doerfler
At 60 °C and 80 % r. h. a significantly higher WVTR of 1.0
g/m2d can be observed (deduced from figure 4). In a direct
comparison to other products used for edge sealing of OLED
Laser devices, the value of DELO-KATIOBOND AD690 lies on the
best level, as well as for 25 °C/30 - 50 % (see table 4).
Photodiode
Encapsulated
Calcium mirror
Figure 2: Optical transmission set-up for the analysis of Ca-
mirror test devices. Up to 15 samples can be measured at the
time in one frame.
Temperature Humidity WVTR
°C % g/m2d Figure
25 30-50 0.03*) 4: Ca-mirror decrease and relative transmission increase over the
storage time using DELO-KATIOBOND AD690 at 60 °C and
60 80 1.0 80 % r. h. The glue line was 1.2 mm wide and 80 µm high.
70 20-30 0.5
85 85 14.4
*) measured on adhesive covered Ca-layer WVTR in g/m2d
Product Base material 25 °C/30- 60 °C/
Table 3: WVTR of DELO-KATIOBOND AD690 at different
50 % r. h. 80 % r. h
climatic conditions.
DELO-
UV-curing
PHOTOBOND 0.15 4.3
Acrylate
VE 55478
UV-curing
Competitor 1 Urethane 0.46 10
Acrylate
UV-curing
Competitor 2 0.15 10
Epoxy
UV-curing
Competitor 3 0.08 1.0
Epoxy
DELO-
UV-curing
KATIOBOND 0.03 1.0
Epoxy
AD690
Table 4: Comparison of the WVTR of different adhesives for
edge encapsulation of OLED displays.
Figure 3: Ca-mirror decrease and relative transmission increase The WVTR determined for DELO-KATIOBOND AD690 at 85
over the storage time using DELO-KATIOBOND VE 15223 at °C and 85 % r. h. lies at 14.4 g/m2d (deduced from figure 5).
25°C and 30 - 50 % r. h. In this sample the glue line was 1.5 mm The measuring temperature is close to the glass transition
wide and 0.3 mm high. temperature of 102 °C. In this measurement a significant
acceleration of the Ca-layer decrease at the end of the
Ca-film. The required values introduced above for a display measurement can be observed. A reason for this could be the
lifetime of 10.000 h can therefore be achieved. The sub-surface migration of the adhesive due to irregularities
investigations with edge sealed devices have not been completed induced during the sealing process.
yet. As an example, the Ca-mirror decrease by using DELO-
KATIOBOND VE 15223, a material identical to DELO- 3.4 Tests on PLED-devices
KATIOBOND AD690 besides the filler size distribution, is
presented in figure 3. The complete reduction occurred over For a confirmation of the quality of the adhesive sealing by
more than 300 days. During the first 100 days, nearly no using DELO-KATIOBOND AD690 4-pixel PLED test devices
decrease of the Ca-layer takes place. During this phase the were prepared and afterwards stored at 70 °C in an ambient
penetrating moisture can be absorbed by the adhesive. Only after humidity of 20 - 50 %. Moreover, storage at changing
the phase of saturation moisture reaches the Ca-layer in a temperatures of -20 and +70 °C at 0% humidity was carried out.
significant amount. The results are shown in figure 6 and 7, respectively.
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4. P-64 / R. Doerfler
Figure 5: Ca-mirror decrease and relative transmission increase
over the storage time using DELO-KATIOBOND AD690 at 85 Figure 6: PLED luminance versus storage time at 70 °C and 20 -
°C and 85 % r. h. The glue line was 2.5 mm wide and 30 µm 30 % r.h.
high.
3.4.1 Device Preparation
The PLED devices were prepared by deposition of several layers
onto ITO glass substrates by spin-coating and vacuum
deposition. The ITO substrates were cleaned and then directly
transferred into a dry nitrogen glove box for further processing.
The substrates were deposited with a 60 nm hole-injecting layer
of poly(3,4-ethylenedioxythiophene) (PEDOT; HC Starck,
Baytron®) and dried by heating at 110 °C for 30 minutes. A
green emitting polymer was dissolved in p-xylene and deposited
by spin-coating. The spin coating parameters were adjusted to
realize a thickness of the electroluminescent layer of around 80
nm, because the thickness has a great influence on the electrical
properties and efficiency of a PLED. The solvent was removed
Figure 7: PLED luminance versus storage time at temperature
by heating at 110 °C for 60 minutes. A thin Ca-layer (50 nm)
change stress -20/+70 °C in dry environment (hold time 1 h,
was vacuum-deposited at first onto the emitting polymer layer.
change time 5 s).
Finally, a silver metal layer (100 nm) was vacuum-deposited on
the top of the device before encapsulation. 5. References
[1] K. Allen, iSuppli Corp., OLEDs 2005, Conference
3.4.2 Results on Test Device Proceedings, San Diego, CA (USA), November 14-16, 2005.
Storage of PLEDs at 70 °C under ambient atmosphere as well as [2] M.S. Weaver, L.A. Michalski, K. Rajan, et al. “Organic
during temperature cycling leads to the decrease of the light-emitting devices with extended operating lifetimes on
luminance of approximately 50% after 700 to 800 h. The plastic substrates” Appl. Phys. Lett. 81(16), pp 2929-2931,
comparison with Ca-mirror test data imply, that the degradation 2002.
is not mainly affected by the diffusion of water and/or oxygen [3] G. Nisato, P.C.P. Bouten, P.J. Slikkerveer, et al.
into the devices. Only a few literature data are available on „Evaluating high performance diffusion barriers: the calcium
PLED lifetimes at elevated temperatures. For example Haskal et test” Proceeding Asia Display/IDW’01, pp 1435-1438, 2001.
al. [4] reported on PLED lifetimes of a polyfluorene-based [4] E.I. Haskal, H.J. Bolink, M. Büchel, P.C. Duineveld, B.
polymer, which exhibits lifetimes of approximately 500 - 700 h Jacobs, M.M. de Kok, E.A. Meulenkamp, E.H.J. Schreurs, S.I.E.
at 20 mA/cm² and 70°C. Lifetimes presented in this paper are of Vulto, E. van de Weijer, S.H.P.M. de Winter, “Passive-matrix
the same order as the ones measured by Haskal et al. Further polymer light-emitting displays” in Journal of the SID, 11/1,
investigations of the encapsulated PLEDs at high humidity as 2003, p.155.
well as lifetime investigations under inert atmosphere for the
separation of the OLED degradation and the influence of the 6. Acknowledgment
encapsulation are in progress. The authors would like to thank their colleagues, namely Ralf
Müller (IZM) and Bert Fischer (IAP), involved in the device
4. Conclusion preparations as well as samples measurements.
An UV-curing epoxy adhesive was developed which exhibits
excellent low WVTR in Ca-mirror tests. Lifetime tests on PLED Part of this work was financially supported by the Federal
devices under elevated temperatures and temperature change Ministry for Education and Research (bmb+f, 01 BK 916/919)
stress showed promising results. For detailed interpretation of the German Government.
further examinations are in progress. The adhesive should be of
value for the development of more reliable OLED devices for
the integration into premium applications.
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