New uv curing oled encapsulation adhesive with low water


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New uv curing oled encapsulation adhesive with low water

  1. 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 WVTRAbstract Size of display (active display area) [g/m²d]The results of a research program for the development of an UVcuring epoxy adhesive for the sealing frame of OLED displays 21 mm x 30 mm 0.052with a preferably low water vapor transmission rate (WVTR)will be presented. The material was analyzed in substantial 28.8 mm x 73.8 mm 0.19lifetime investigations both in calcium mirror tests and in PLED Edge encapsulationdevices under different climatic environments and its capability 5‘‘ diagonal 0.24was verified. 10‘‘ diagonal 0.491. Introduction independent of size,Due to their manifold advantages, OLED displays are currently Full encapsulation transmission rate for 5.5*10-5replacing LCDs to an increasing degree in various applications. Thus, the given surfacethey were meanwhile able to gain a significant market share especiallyin the field of mobile phones and MP3 players. In 2005, for example, Table 1: Calculated upper limit permeation values for OLEDOLED 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 areworking on lifetime improvements in order to enable also a substantial 2. Adhesive formulationpenetration into the premium OEM market, such as for automotive The water vapor permeability of adhesives can be influenced inapplications. One remaining critical issue is the high sensitivity of the many ways. Thus, fillers can act as a barrier or water moleculesfunctional layers against moisture and oxygen. Therefore, OLEDs can be bonded chemically or physically to the filler. Theneed to be encapsulated with materials showing low water vapor polymer itself can be influenced by inserting highly regularpermeability in the range of 10-5 g/m²d [2]. Those values are hard to molecule structures, by the degree of cross-linking or by lowachieve 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 furtherand 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 strengthrates of adhesives for an edge and full encapsulation of OLED on the relevant substrates as well as a preferably highdevices, respectively. The calculation is based on an expected OLED insensitivity against aging and climatic influences must belifetime of 10.000 h, which have to be reached for the relevant guaranteed. Moreover, with respect to the processibility thetemperature and humidity of the environment. The assumption was material must be suitable for dispensing and screen printing withmade, that a 100 nm thick magnesium cathode may be reduced by a standard flat panel display equipment. A decisive influence ismaximum of 20 % of its layer thickness. The adhesive layer is exerted by the fact that the material can be pressed to very thinsupposed 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 µmThe 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. Typicalachievable 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 anand 80 % humidity. As a consequence further protective measures, extremely fast curing at low temperatures and can thereforesuch as integrated absorbers, are essential for long-lived OLEDs. fulfill the demands for a high throughput production of OLEDFurthermore the WVTR of the adhesives has to be further examined displays. Compared to UV curing acrylates, epoxies exhibitand optimized. Therefore the aim of the research program was the fundamental advantages regarding low water permeability.development of encapsulation adhesives with significantly lowerwater vapor permeability.440 • SID 06 DIGEST ISSN0006-0966X/06/3701-0440-$1.00+.00 © 2006 SID
  2. 2. P-64 / R. Doerfler3. Results 35In the framework of the adhesive improvement, permeation 30 Shear Strength [MPa]measurements were carried out according to ASTM E 96. The 25adhesive strengths were determined on compression shearspecimens of 20 x 20 x 5 mm³ based on sodium silicate glass, 20 glass/glassaccording to DELO standard 5 as well as on ITO-coated display 15 ito/glassglasses. The specimens were conditioned at 120°C for one hour. 10The product chosen from these pre-examinations, DELO- 5KATIOBOND AD690, was subject to detailed analyses both oncalcium mirror tests and PLED devices. 0 temperature 500 h 85/85 16 h3.1 Material characteristic values cycling pressure cookerDELO-KATIOBOND AD690 shows a WVTR of0.8 g/m2d according to ASTM E 96 at 50 °C/95 % r.h. referringto 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 inTable 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 averaged3.2 Humidity resistance transmission, a water equivalent and therefore the permeation of theAdhesive 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 isadhesive and substrate. Therefore, not only high initial adhesive similar to the one of edge sealed OLED displays (however, withoutstrengths are crucial for the reliability of the bonding, but also the organic functional layers). Consequently useful conclusions can bethe durability of the adhesive sealing. Accordingly, test deduced for the behavior of the adhesive material.specimens and display glasses were subject to the followingstress 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 loadcapacity 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 SID 06 DIGEST • 441
  3. 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 mirrorFigure 2: Optical transmission set-up for the analysis of Ca-mirror test devices. Up to 15 samples can be measured at thetime 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. hclimatic 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 85over 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 transitionwide and 0.3 mm high. temperature of 102 °C. In this measurement a significant acceleration of the Ca-layer decrease at the end of theCa-film. The required values introduced above for a display measurement can be observed. A reason for this could be thelifetime of 10.000 h can therefore be achieved. The sub-surface migration of the adhesive due to irregularitiesinvestigations 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-devicesKATIOBOND AD690 besides the filler size distribution, ispresented in figure 3. The complete reduction occurred over For a confirmation of the quality of the adhesive sealing bymore than 300 days. During the first 100 days, nearly no using DELO-KATIOBOND AD690 4-pixel PLED test devicesdecrease of the Ca-layer takes place. During this phase the were prepared and afterwards stored at 70 °C in an ambientpenetrating moisture can be absorbed by the adhesive. Only after humidity of 20 - 50 %. Moreover, storage at changingthe 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.442 • SID 06 DIGEST
  4. 4. P-64 / R. DoerflerFigure 5: Ca-mirror decrease and relative transmission increaseover 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 PreparationThe PLED devices were prepared by deposition of several layersonto ITO glass substrates by spin-coating and vacuumdeposition. The ITO substrates were cleaned and then directlytransferred into a dry nitrogen glove box for further processing.The substrates were deposited with a 60 nm hole-injecting layerof poly(3,4-ethylenedioxythiophene) (PEDOT; HC Starck,Baytron®) and dried by heating at 110 °C for 30 minutes. Agreen emitting polymer was dissolved in p-xylene and depositedby spin-coating. The spin coating parameters were adjusted torealize a thickness of the electroluminescent layer of around 80nm, because the thickness has a great influence on the electricalproperties and efficiency of a PLED. The solvent was removed Figure 7: PLED luminance versus storage time at temperatureby 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 onthe top of the device before encapsulation. 5. References [1] K. Allen, iSuppli Corp., OLEDs 2005, Conference3.4.2 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. “Organicduring temperature cycling leads to the decrease of the light-emitting devices with extended operating lifetimes onluminance 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 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 calciumPLED lifetimes at elevated temperatures. For example Haskal et test” Proceeding Asia Display/IDW’01, pp 1435-1438, [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, 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-matrixthe 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 theseparation of the OLED degradation and the influence of the 6. Acknowledgmentencapsulation are in progress. The authors would like to thank their colleagues, namely Ralf Müller (IZM) and Bert Fischer (IAP), involved in the device4. Conclusion preparations as well as samples measurements.An UV-curing epoxy adhesive was developed which exhibitsexcellent low WVTR in Ca-mirror tests. Lifetime tests on PLED Part of this work was financially supported by the Federaldevices 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 ofvalue for the development of more reliable OLED devices forthe integration into premium applications. SID 06 DIGEST • 443