Design of Highly Water Resistant, Impermeable, and Flexible Thin

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 3251−3261

Design of Highly Water Resistant, Impermeable, and Flexible ThinFilm Encapsulation Based on Inorganic/Organic Hybrid Layers Jeong Hyun Kwon,† Eun Gyo Jeong,‡ Yongmin Jeon,‡ Do-Geun Kim,† Seunghun Lee,† and Kyung Cheol Choi*,‡ †

Advanced Nano-Surface Department, Korea Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea School of Electrical Engineering, KAIST, Daejeon 34141, Republic of Korea

‡

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ABSTRACT: The lack of a transparent, flexible, and reliable encapsulation layer for organic-based devices makes it difficult to commercialize wearable, transparent, flexible displays. The reliability of organic-based devices sensitive to water vapor and oxygen must be guaranteed through an additional encapsulation layer for the luminance efficiency and lifetime. Especially, one of the major difficulties in current and future OLED applications has been the absence of thin-film encapsulation with superior barrier performance, mechanical flexibility, and water-resistant properties. In this work, we fabricated highly waterresistant, impermeable, and flexible inorganic/organic multilayers with optimized Al2O3 and functional organic layers. The key properties of the fabricated multilayers were compared according to the thickness and functionality of the inorganic and organic layers. Improvement of the barrier performance is mainly attributed to the optimized thickness of the Al2O3 films, and is additionally due to the increased lag time and effective surface planarization effects caused by the use of micrometer-thick organic layers. As a result, the 3-dyad multilayer structure composed of 60 nm-thick Al2O3 layers deposited at 70 °C and 2-μm-thick silane-based inorganic/organic hybrid polymer (silamer) layers with layered silica exhibited the lowest WVTR value of 1.11 × 10−6 g/m2/ day in storage conditions of 30 °C/90% relative humidity. In addition, the multibarrier exhibited good mechanical stability through the use of alternating stacks of brittle inorganic and soft organic layers, without showing a large increase in the WVTR after bending tests. In addition, silamer layers improved the environmental stability of the Al2O3 ALD film. The silamer layer coated on the Al2O3 film effectively worked as a protective layer against harsh environments. The effective contact at the interface of Al2O3/silamer makes the barrier structure more impermeable and corrosion-resistant. In this study, we not only demonstrated an optimized multilayer based on functional organic layers but also provided a methodology for designing a wearable encapsulation applicable to wearable organic electronics. KEYWORDS: water vapor transmission rate (WVTR), thin film encapsulation, aluminum oxide (Al2O3), water-resistant, wearable encapsulation for transparent flexible displays due to issues of rigidity and lack of transparency. To develop transparent flexible encapsulations, various gas diffusion barriers (GDBs), and thin film encapsulations (TFEs) have been proposed.5,8−13 However, inorganic GDBs can be corroded under exposure to harsh environments and are difficult to render flexible. In addition, although inorganic GDBs formed on plastic substrates are thicker at a critical thickness, their barrier performance is not continually improved because of cracks caused by the buildup of internal stress.3,14 However, a TFE structure alternately stacking organic and inorganic layers, which was barix encapsulation technology proposed by Vitex systems Inc., showed increasingly better barrier performance

1. INTRODUCTION As organic electronics are being actively researched, organic light-emitting diodes (OLEDs) are considered to have strong potential for the realization of current and future displays.1 In particular, OLEDs are advantageous for realizing transparent flexible displays as next-generation displays. However, because OLEDs are based on organic materials sensitive to reactive gases, such as water vapor and oxygen, encapsulation technologies with high-level barrier functionality are required to block OLED degradation against the ambient atmosphere.2−5 The reliability and stability of environmentally sensitive OLEDs have been ensured by using a separate encapsulation layer to prevent the diffusion of water vapor and oxygen.3,6,7 The encapsulation layers are typically formed from thick and rigid materials such as glass and metal lids or from thin-film multilayers consisting of deposited thin films. However, glass-lid or metal-lid encapsulation is not suitable © 2018 American Chemical Society

Received: July 17, 2018 Accepted: September 6, 2018 Published: September 6, 2018 3251

DOI: 10.1021/acsami.8b11930 ACS Appl. Mater. Interfaces 2019, 11, 3251−3261

Research Article

ACS Applied Materials & Interfaces

bending tests with respect to fracture and fatigue in mechanical stress. Finally, the silamer layer improved the environmental stability of the Al2O3 ALD film. As the silamer layer demonstrated synergistic combination at the interface with the Al2O3 ALD film, the multilayer based on the Al2O3/silamer structure can be used as a wearable encapsulation that withstands harsh environments and water. Therefore, we optimized Al2O3-based TFEs using functional inorganic/ organic hybrid polymer layers and evaluated their characteristics and reliability.

with an increasing number of inorganic/organic dyads due to the stress relief effect of organic layers.5,9,15 Therefore, TFE structures have been a strong candidate as a transparent flexible encapsulation. However, to fabricate highly impermeable and flexible TFEs with a WVTR value of 1 × 10−6 g/m2/day, the multilayer structure needs to be optimized in terms of barrier and mechanical properties through optimal thickness of the constituent inorganic and organic layers. Weijer et al. reported a 1.5-dyad multilayer consisting of 150 nm-thick silicon nitride and a 20-μm-thick organic layer for OLED reliability.16 The multilayer based on the thick organic layer exhibited an effective defect-decoupling effect and a significant delay in the formation of dark spots. Therefore, the multilayer structure based on thick organic layers can work as an effective TFE for long-term reliability of OLEDs. However, in fact, inorganic layers in the multilayer structure act as a substantial GDB against water vapor and oxygen in comparison to organic layers.3 Using optimized inorganic layers in the multilayer allows the TFE to be fabricated simply and effectively using decreasing the number of inorganic/organic pairs. The atomic layer deposition (ALD) method is considered a potential candidate for densely packed GDB formation. Although the ALD film quality is influenced by deposition temperature, precursor, plasma, and so forth, the ALD film exhibited good barrier performance at a low temperature.17−19 Among various ALD films, Al2O3 ALD films have long been used as a GDB that satisfies barrier requirements for device passivation.6,7,10,11,20 Al2O3 ALD films can be formed such that they exhibit good barrier performance at a low temperature of less than 80 °C because of the amorphous growth and ALD growth mechanism. However, the Al2O3 ALD film is known to be corroded because of the hydrolysis reaction with water under exposure to high temperature and high relative humidity (RH).3,8,9,21,22 Therefore, there have been many reports on the use of Al2O3-based nanolaminates to improve the environmental stability of Al2O3 ALD films.3,8,22−24 However, no significant improvements in the environmental stability have been shown. E.G. Jeong et al. have been reporting for the first time on improving the environmental stability of inorganic layers using an organic layer in the thin film multilayer system.25,26 The poor environmental stability of the Al2O3 ALD film can be more significantly overcome by the introduction of organic layers based on layered silica in the inorganic/organic multilayer system, rather than the material and structural design such as an ALD nanolaminate structure3,8,24 We herein tried to fabricate Al2O3-based multilayer structures alternated with silane-based organic/inorganic hybrid polymer (silamer) layers to investigate thickness effects of the inorganic/organic layers in the multilayer system. First, we investigated the optimized temperature condition for ALDprocessed Al2O3 films without thermal damage to OLEDs. The Al2O3 film was then analyzed and optimized in terms of moisture-resistant and mechanical properties using various analysis techniques. Second, silamer organic layers are introduced to the multilayer system for effective surface planarization and tortuous diffusion path extension for water vapor and oxygen. As a result, these optimized multilayers with micrometer-thick organic layers resulted in a very low WVTR value of 1.11 × 10−6 g/m2/day, which is comparable to that of glass-lid encapsulation. We also analyzed the mechanical flexibility of the Al2O3 single layer and multilayers through

2. EXPERIMENTAL SECTION 2.1. Fabrication of Glass-Encapsulated Green Fluorescent OLED and Thin-Film Encapsulated Blue Fluorescent OLED. To investigate the thermal stability of the OLEDs, a green fluorescent OLED (FOLED) device with a structure of indium tin oxide (ITO)/ 4,4′- bis(N-phenyl-1-naphthylamino)biphenyl (NPB)/tris(8- hydroxyquinoline) aluminum (Alq3 )/lithium fluoride (LiF)/Al was fabricated on a glass substrate, and then encapsulated with glass-lids. In addition, to demonstrate the feasibility of the fabricated TFE, we fabricated FOLEDs with a structure of ITO(150 nm)/ HAT-CN (5 nm)/NPB (45 nm)/2-methyl-9,10-di(2-naphthyl)anthracene (MADN):p-bis(p-N,N-diphenyl-aminostyryl) (DSA-ph) (25 nm, 3%)/Alq3 (10 nm)/lithium quinolate (Liq, 1 nm)/Al (100 nm) using thermal evaporation. 2.2. Al2O3 ALD Film Deposition. All barrier films were formed on the 250-μm-thick PET substrate for WVTR evaluation and surface analysis. The Al2O3 thin film was grown using H2O as a reactant, where its metal source was trimethylaluminum (TMA) using thermal ALD (Lucida D100 from NCD). Al2O3 films through sequential selflimiting surface reaction of TMA and H2O sources were deposited at a chamber temperature of 70 °C. In general, one deposition cycle for the Al2O3 film comprised sequences of ts1, tp1, ts2, and tp2, where ts1 and ts2 are the exposure times of the metal source and H2O reactant, respectively, and tp2 and tP2 are the purging times for N2 flow. The exposure times of ts1, tp1, ts2, and tp2 for the Al2O3 thin films were 0.25, 10, 0.25, and 10 s at the base pressure of 3.1 × 10−1 Torr, resulting in the growth rate of 0.85 Å/cycle. 2.3. Formation of Silamer and S−H Nanocomposite Layers. The silamer layer was sysnthesized by a sol−gel method and was spincoated with 2-μm-thickness using an acceleration period of 30 s and a maintaining period of 3 s for 5000 rpm. The silamer layer was thermally cured at 70 °C under 5 × 10−2 Torr for 10 min in an ALD chamber and this resulted in a three-dimensional SiO2-embedded organic layer. The Silica-nanoparticle-embedded sol−gel organic/ inorganic hybrid nanocomposite (S−H nanocomposite) layer was also synthesized via a sol−gel reaction and was coated through a spincoating process and then was cured by UV light with the wavelength of 365 nm for 110 s.5,27 2.4. Method for Measuring Barrier Properties, Optical Transmittance, Film Density, and Mechanical Flexibility. For measurement of the WVTR values, electrical Ca testing with a low WVTR measurement limit of 1 × 10−6 g/m2/day was conducted. The thin films to be measured were attached to a Ca film deposited on the glass substrate using a UV-curable sealant.18 The WVTR measurements for multilayer optimization and bending tests were mainly measured in the climate chamber at 30 °C/90% RH and WVTR evalulation in harsher environments of 60 °C/90% RH was conducted for Al2O3 corossion tests. The optical transmittance of the barrier film was measured using a UV−vis spectrophotometer (UV-2550, SHIMADZU) in the visible wavelength range of 400 to 800 nm. The film density of the Al2O3 ALD films was measured by an X-ray reflectivity (XRR) analysis using high-resolution X-ray diffractometer (SmartLab, RIGAKU) based on Cu Kα radiation. The XRR spectrum was fitted by “Rigaku Global Fit” software obtaining the reflection intensity curves depending on incidence angle for determining film thickness, density, and surface/ interface roughness.28 3252


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Figure 1. Current−voltage curves of glass-encapsulated OLEDs before and after storage for 10 h at an ALD chamber of (a) 90 and (b) 70 °C, respectively. Inset: Schematic of ITO/NPB/Alq3/LiF/Al structure encapsulated with glass-lid.

Figure 2. Electrical Ca test configuration attached with PET/barrier film (a) exposed and (b) unexposed to a storage environment. (c) WVTR comparison of directly exposed Al2O3 and unexposed Al2O3 thin films directly in a storage condition of 30 °C/90% RH.

temperature of 90 °C is below the Tg of NPB, temperature of 90 °C causes performance degradation of the OLEDs. Therefore, we determined that the stable and reliable deposition temperature for the Al2O3 film is 70 °C. We investigated the storage conditions for Ca testing to minimize the environmental effect of the Al2O3 ALD film. The Al2O3 ALD film is known to be corroded and degraded under exposure to high temperature and high RH. Therefore, harsh storage conditions for Ca testing make reliable WVTR measurement difficult. WVTRs of directly exposed Al2O3 and unexposed AlO3 to a storage environment were evaluated using a Ca test conducted at 30 °C/90% RH. Figure 2a, b show the electrical Ca test configuration attached with a PET/barrier film using UV curable sealant. As shown in Figure 2a, the barrier film exposed to a harsh environment for a long time shows further degradation of barrier performance due to its own film degradation. Therefore, to distinguish the defectbased permeation from the corrosion-induced permeation, minimizing the degradation of barrier films against the storage environment for WVTR evaluation is important. We conducted the Ca test on Al2O3 ALD films directly exposed and unexposed to a 30 °C/90% RH. The results of the measurement show that the WVTR value of the exposed Al2O3 thin film was better in comparison to that of the unexposed Al2O3 thin film. In other words, the storage conditions of 30 °C/90% RH did not cause Al2O3 corrosion during Ca tests, having little effect on the obtained WVTRs. Therefore, in this study, all Ca tests were conducted at 30 °C/90% RH for multilayer optimization and bending tests. The Al2O3 film deposited at 70 °C was optimized through WVTR calculation in terms of barrier performance. The barrier film formed on plastic substrates showed the lowest or

We conducted bending tests to evaluate the mechanical stability of Al2O3 single layers and the proposed multilayers. Bending tests were performed by bending the barrier films with 100 iterations by the bending machine. The WVTR value of the bent film was compared with the WVTR value of the unbent film to indirectly investigate the degradation level induced by bending stress. To understand the mechanical limitations of Al2O3 ALD films, the critical strains at which the Al2O3 ALD films fracture were determined through WVTR comparison and SEM surface analysis. 2.5. Residual Stress Measurement in the Film Using the Wafer Curvature Method. The change in the curvature of the 6-in. Si wafer, measured by a stress gauge equipment (FSM500TC, FSM), was obtained before and after thin films on a wafer and then the residual stress in the film was calculated by the Stoney formula.17

3. RESULTS AND DISCUSSION 3.1. Optimization of Temperature and Thickness for Al2O3 ALD Films and Surface Planarization Effect Induced by Organic Layer. The deposition temperature for the Al2O3 film should be optimized to establish a process temperature that can be applied without resulting in performance degradation of OLEDs. Therefore, we investigated the thermal stability of fluorescent OLEDs before and after storage for 10 h in an ALD chamber at 90 and 70 °C, respectively. In addition, the OLEDs were encapsulated using a UV-curable sealant and glass-lids in a glovebox to prevent the permeation of water vapor and oxygen. Figure 1 shows the electrical characteristics of the OLEDs after thermal treatment. The OLEDs that were stored in the chamber at 90 °C exhibited significant performance degradation due to the thermal degradation of NPB with a glass transition temperature (Tg) of 96 °C, whereas the OLEDs stored at 70 °C showed no change in electrical performance.6 Although the chamber 3253


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Figure 3. (a) WVTR values of Al2O3 ALD films as a function of film thickness. Inset: change in gas transmission rate of barrier film deposited on plastic substrates with increasing film thickness. (b) XRR data measured for the 40 nm-thick Al2O3 ALD film deposited on a Si wafer at 70 °C. (c) Changes in residual stress and membrane force as a function of film thickness. (Inset: Schematic of the concave wafer due to tensile stress induced by the formation of Al2O3 ALD film).

Figure 4. AFM images and surface roughness values of (a) bare PET, (b) PET/silamer (2 μm), (c) PET/Al2O3 (30 nm), (d) PET/silamer (2 μm)/Al2O3 (30 nm), (e) PET/Al2O3 (60 nm), and (f) PET/silamer (2 μm)/Al2O3 (60 nm).

saturated WVTR value at a certain thickness as the film thickness increases. Thus, using the ALD film of the optimal

thickness to obtain the lowest WVTR value helps enable a simple and low-cost TFE process for practical applicability of 3254


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Figure 5. (a) Normalized conductances of Ca sensors with Al2O3, Al2O3/S−H nanocomposite, and Al2O3/silamer in storage conditions of 60 °C/ 90% RH. (b) Ca test results of 3-dyad Al2O3/silamer multilayer under 30 °C/90% RH before and after immersion in water for 30 min. (c) Schematic of the PET/Al2O3/silamer structure and chemical bonding at the interface of Al2O3/silamer. The inset displays a cross-sectional TEM image of the silamer layer (d) Schematic of the PET/Al2O3/silamer structure and chemical bonding at the interface of Al2O3/silamer. The inset displays a cross-sectional TEM image revealing the S−H nanocomposite layer inserted with SiO2 nanoparticles.

TFEs. We compared the barrier performance of the thin films using WVTR values obtained by Ca testing. In general, the WVTR values of films are related to film density, thickness, the number of defects or voids, and so on.29,30 We also measured the film density of the Al2O3 films by XRR for comparison with the previously reported Al2O3 ALD film. Figure 3a shows the changes in the WVTR values of the Al2O3 films with various film thicknesses. As the thickness of the Al2O3 film increased toward the critical thickness (dc), the WVTR value of the Al2O3 film decreased more, eventually reaching the saturated WVTR value of 8.065 × 10−5 g/m2/day at a thickness of 60 nm. However, when the Al2O3 film became thicker than the fracture thickness (df), the WVTR value of the Al2O3 film increased due to the increased tensile stress in the thin film. The buildup of tensile stress in an Al2O3 film with increasing thickness results in fracture induced by tensile stress and membrane force (Figure 3a). Film densities of the Al2O3 ALD film deposited at 70 and 150 °C were 2.753 and 2.989 g/cm3 obtained from XRR measurements, respectively, which are in agreement with the measurement results of previous studies (Figure 3b). In particular, Al2O3 ALD film densities are related to compounds such as hydroxyl groups and carbon to become

incorporated after chemical reactions between TMA and H2O. However, hydroxyl groups and carbon are further reduced due to their volatility at higher deposition temperature, resulting in denser films.11,29 Although the film density of the Al2O3 ALD film deposited at 70 °C was slightly lower than that of the Al2O3 ALD film deposited at 150 °C, a low-temperature process below 70 °C is required for application as TFEs for various organic electronics. Low film quality and barrier properties can be overcome through optimization of film thickness and introduction of a multilayer system. The membrane force, which was calculated as the stressthickness-product (σ·h), is related to the probability of crack formation in the film structures.17 Therefore, the membrane force of the Al2O3 ALD film with increasing thickness continuously increases because of increased tensile stress and film thickness. In other words, the increase of membrane force makes crack formation in the film easier. Therefore, the thickness of the Al2O3 film was investigated below the critical membrane force value, which is the critical membrane force for the onset of cracking. In WVTR measurements, increased WVTRs of Al2O3 films deposited on PET substrates were observed for thicknesses between 60 and 90 nm, correspond3255


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in terms of environmental stability and high tensile stress. Therefore, introduction of a functional organic layer can be a good alternative to improve the environmental stability of the multilayer and control the film stress. Therefore, we developed the silamer layer to improve the environmental stability of TFEs. To compare the reliability improvement effect between organic layers, a silamer layer inserted with layered SiO2 and a S−H nanocomposite layer inserted with SiO2 nanoparticles were coated on the PET/Al2O3 (60 nm). Although the S−H nanocomposite layer worked as an effective organic layer for the multilayer system, the S−H nanocomposite layer did not protect the Al2O3 ALD film in the multilayer structure. Figure 5a shows Ca test results of Al2O3, Al2O3/silamer, and Al2O3/ S−H nanocomposite in measurement conditions of 60 °C/ 90% RH, respectively. As shown in Figure 2c, measurement conditions of 30 °C/90% RH did not influence the Al2O3 corrosion. However, a measurement temperature beyond 60 °C in a humid environment is enough to cause corrosion of the Al2O3 thin fim. In Ca tests at 60 °C/90% RH, Ca sensors attached with a bare Al2O3 film and a S−H nanocompositecapped Al2O3 film were oxidized completely within 2 days, whereas the Ca sensor with the silamer-capped Al2O3 film took 6 days to oxidize. In other words, the S−H nanocomposite layer did not help improve the environmental stability of the Al2O3 thin film. On the other hand, the silamer layer with the randomly formed three-dimensional silica layers significantly blocked corrosion of the Al2O3 thin film. Because S−H nanocomposite and silamer layers showed little barrier performance, as shown in Figure S1, the difference in the environmental stability in Al2O3 capped with different organic layers can be attributed to the structural difference in the organic layers. In additin to investigation on the corrosionresistant effet of the silamer layer, to confirm the waterresistant effect of the silamer layer, we conducted the water immersion test of the 3-dyad Al2O3/silamer multilayer. The multilayer were immersed in water for 30 min and then dried in a dry oven. As a result of measurements, the multilayer protected by the silamer layer maintained the WVTR on the order of 1 × 10−6 g/m2/day withouth degradation even after exposure to water for 30 min (Figure 5b). Through storage test in harsh environments and water immersion test, the silamer layer effectively worked as water-resistant and corrsionresistant layer. As shown in Figure 5c, chemical reaction at the interface of the Al2O3/silamer makes the Al2O3 film more reliable and impermeable. However, the S−H nanocomposite with silica nanopaticles did not interact with the Al2O3 ALD film at the interface (Figure 5d). Dameron et al. reported that the SiO2 ALD films exhibited synergistic effects with the Al2O3 ALD film.33 The WVTR resulting from the Al2O3/SiO2 ALD bilayer was effectively reduced. In the case of our Al2O3/ silamer structure, the silamer layer worked as a protective layer against harsh environments as well as planarization and a defect-decoupling layer through the effective and synergistic combination between Al2O3/silamer. In addition, filling of pinhole defects in the Al2O3 ALD film in the course of the silamer being thermally cured made the TFE structure impermeable and corrosion-resistant. Notably, although various ALD nanolaminate structures have been proposed to improve their environmental stability by alternately depositing various barrier films at a few nanometer thickness, there have been no reports on environmentally stability improvement of barrier films using functional organic layer.3,8,23,34,35 Therefore, we can fabricate a highly water-resistant and flexible inorganic/

ing to deposition cycles of 720 and 1080, respectively. Therefore, the critical membrane force was identified at the values between (σh)t=60 nm = 168.95 GPaÅ and (σh)t=90 nm = 439.05 GPaÅ (Figure 3c). Stress management and thickness optimization are very important for optimization of the barrier and mechanical characteristics of inorganic layers. An inorganic/organic multilayer system is effective for the fabrication of a highly reliable and moisture-resistant TFE with an extremely low WVTR of 1 × 10−6 g/m2/day by reducing the internal stress using an organic layer film. We used an Al2O3 film of 60 nm corresponding to 720 ALD cycles as an inorganic layer for the inorganic/organic multilayer. Figure 4 shows the root-mean-square (RMS) roughness (Rrms), peak-to-valley roughness (Rp‑v) and atomic layer force (AFM) images of the surface of Al2O3 films with 30 and 60 nm coated on a bare PET and PET/silamer (2 μm). In general, commercial PET substrates have a rough surface with high RMS and peak-to-valley values (Figure 4a). Because these rough surfaces on PET substrates make devices unreliable and unstable, for reliable device operation, surface planarization is required. As shown in Figure 4b, c, both the Al2O3 and silamer contributed to surface planarization; however, the 2-μm-thick silamer layer showed a significant surface planarization effect, reducing the RMS roughness from 5.9 to 0.3 nm and reducing the peak-to-valley value from 41.234 to 1.046 nm. Therefore, surface planarization by a thick silamer organic layer results in denser and pinhole-free Al2O3 films in the multilayer (Figure 4d, f). 3.2. Fabrication of Corrosion-Resistant and Impermeable Al2O3-Based Multilayer Using Silamer Layers. In this paper, we introduced the silamer layer as an organic layer to fabricate reliable and flexible multibarrier suitable for all OLED device structures. Recently, our group reported electrical characteristics and operational lifetimes of the bottom-emitting OLEDs encapsulated with S−H nanocomposite-based TFEs.3,5,6,31,32 However, the previously proposed UV-curable S−H nanocomposite layer is difficult to use as an encapsulation layer applicable to top-emitting OLEDs due to the direct UV exposure of organic emitting layers through the transparent top electrode. Therefore, introduction of thermally curable organic layers at low temperatures is required for forming the inorganic/organic multilayer to OLEDs emitting on both sides. The silamer layer is thermally cured at 120 °C during 10 min for complete layer formation in air. However, use of an ALD chamber with a vacuum level below 5 × 10−2, which lowers the boiling point in vacuum, enables the silamer layer to be cured at 70 °C within 10 min. Furthermore, the silamer layer improved the corrosion-resistant and water-resistant properties of TFEs due to the synergistic effect with the Al2O3 layer. As explained in section 3.1, the Al2O3 ALD film was easily degraded in a harsh environment. As the storage temperature increases, the corrosion of the Al2O3 ALD film was much more accelerated and water vapor more easily penetrated the thin film. To delay or protect against the corrosion of the Al2O3 thin film, various methods have been proposed. In particular, nanolaminatebased GDBs were mainly fabricated by alternately depositing Al2O3 and other sublayers to improve the environmental stability of GDBs. At the interfaces of the nanolaminates, aluminate phases are formed and make the nanolaminate corrosion-resistant. However, although ALD nanolaminates with aluminate phases showed improved environmental stability, the ALD nanolaminate showed limited improvement 3256


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layers provided much longer diffusion pathways for permeating water vapor and oxygen and a more effective surface planarization effects in comparison to the multilayer fabricated with 150 nm-thick silamer layers. Water vapor and oxygen are difficult to pass due to random collisions with internally formed three-dimensional SiO2 layers in the silamer layer. To fabricate a better multilayer structure through further optimization, we fabricated the Al2O3 (60 nm)/silamer (2 μm) multilayer. Because barrier properties of the Al2O3 film were optimized at the thickness of approximately 60 nm corresponding to 720 ALD cycles, the Al2O3 (60 nm)/silamer (2 μm) multilayer can show significant improvement in barrier properties. As a result, the averaged WVTR values of the Al2O3 (60 nm)/silamer (2 μm) multilayer stacks of 1, 2, and 3 dyads were 6.36 × 10−5, 9.28 × 10−6, and 3.11 × 10−6 g/m2/day, respectively (Table 1). These synergistic effects with Al2O3 films were further enhanced with an increasing number of dyads, resulting in a WVTR value of 1 × 10−6 g/(m2 day) for the 3-dyad Al2O3/silamer multilayer. Furthermore, our optimized multilayers exhibited an average transmittance of approximately 90% in the visible range using UV−vis spectrophotometer (UV-2550, Shimadzu), as shown in Figure 6b. Therefore, highly impermeable Al2O3/silamer multilayer with a WVTR of 1 × 10−6 g/(m2 day) can be used as the encapsulation layer for transparent flexible OLEDs. 3.4. Mechanical Stability of Al2O3 Single Layer and Multilayers. To apply TFE to transparent and flexible OLEDs, the mechanical stability of the encapsulation is the key property. Therefore, the mechanical properties and limitations of encapsulation layers must be investigated using various analysis techniques. To investigate the mechanical stability of brittle Al2O3 single-layer and 3-dyad multilayer based on Al2O3/silamer, we evaluated WVTR values and surface SEM images of Al2O3 single layer and multilayer after bending tests. Elastic modulus (E) and hardness (H) of the brittle Al2O3 layer were 134.36 and 8.58 GPa, whereas E and H of the soft silamer layer were 1.8 and 0.1 GPa using nanoindentation method. Figure 7a shows WVTR changes of 60 nm-thick Al2O3 thin film according to the tensile strain. Although visible cracks were not observed in the Al2O3 surface at a strain of 0.75% seen as a threshold for the onset of internal crack formation, the WVTR value of the Al2O3 film largely starts to increase. Although the thin films after bending do not show visible cracks, considering the size of oxygen and water vapor molecules with diameters of 0.32 and 0.33 nm, respectively, water vapor and oxygen can easily penetrate through invisible nanosized cracks.37 Then, as the strain increases up to a strain of 1.25%, visible cracks and an abrupt increase in the WVTR value of the film were observed. In the case of the 3-dyad Al2O3/silamer multilayer, the significant WVTR continually increased at the bending strain of 1% (Figure 7b). The multilayer structure showed the relatively improved mechanical reliability in comparison to the Al2O3 single layer. This mechanical improvement seems to be attributed to the effective bonding at the interfaces of Al2O3/ silamer. As shown in Figure 5c, when the silamer layer is thermally cured, the Al2O3 and silamer are strongly bonded together at the interface by filling the surface pinhole defects of Al2O3 surface. Therefore, the silamer layers in the multilayer helped improve mechanical performance as well as barrier performance. 3.5. Electrical Characteristics and Operational Lifetime of a Thin-Film Encapsulated Fluorescent OLED. To

organic multilayer applicable to wearable encapsulations using a silamer layer and an ALD Al2O3 film. 3.3. Moisture-Resistant Properties and Transmittance of Multilayers in Relation to Number of Dyads and Organic Layer Thickness. Generally, in single inorganic layers, the diffusion of water vapor and oxygen inside films is mainly affected by film quality factors, such as the film density and the number of defects and pinholes.36 An ALD-deposited inorganic layer has internal defects caused by imperfections of the deposition temperature and the presence of impurities on the substrate. As a result, these defects make use of such layers as GDBs difficult. Therefore, in an organic/inorganic multilayer system, defects and pinholes can be reduced by introducing a smooth and thick organic layer. In addition, lag times are remarkably extended with increasing numbers of organic/inorganic pairs, resulting in a significantly reduced WVTR. After we optimized the thickness of the Al2O3 film, WVTR measurements for multilayers with various numbers of dyads were carried out. An inorganic/organic layer pair is referred to as a dyad. In our multilayer, one dyad was composed of an Al2O3/silamer stack. To fabricate a multilayer barrier structure with a WVTR value of 1 × 10−6 g/(m2 day), we deposited 60 nm thick Al2O3 films and thick silamer layers with the thickness of approximately 4 μm by ALD and spin-coating, respectively. In multilayers, optimized Al2O3 films work as excellent gas diffusion barriers against water vapor and oxygen. In addition, thick organic layers in the multilayer provide effective surface planarization and defect-decoupling, and result in increased lag time. To investigate the thickness effects of layers in multilayers, we obtained normalized conductance curves of various barrier structures from Ca tests, and the calculated the WVTR values by plotting conductance curves from the constant steady state. First, we fabricated Al2O3 (30 nm)/silamer (150 nm and 2 μm) multilayers to investigate the thickness effect of the organic layers. The WVTR values of the Al2O3 (30 nm)/silamer (150 nm) multilayer and Al2O3 (60 nm)/silamer (2 μm) multilayer stacks of 1, 2, and 3 dyads are shown in Table 1. The multilayer based on thick silamer layers Table 1. WVTR Values of Multilayers Based on Al2O3 and Silamer Layers of Various Thicknesses As a Function of the Number of Dyads under 30°C/90% RH averaged WVTR value at 30 °C/90% RH [g/ m2/day] multilayer Al2O3 (30 nm)/silamer (150 nm) Al2O3 (30 nm)/silamer (2 μm) Al2O3 (60 nm)/silamer (2 μm)

1 dyad

2 dyads

3 dyads

3.14 × 10−4

7.43 × 10−5

1.14 × 10−5

2.98 × 10−5

7.18 × 10−5

7.18 × 10−6

6.36 × 10−5

9.28 × 10−6

3.11 × 10−6

started to show increasingly greater improvement in barrier performance with an increasing number of dyads in comparison to the multilayer based on the Al2O3 (30 nm)/ silamer (150 nm) (Figure 6a). Therefore, coating of thick organic layers leads to a meaningful decrease in the WVTR value due to surface planarization and lag-time effects. As shown in Figure 4b, 2-μm-thick silamer layers make the surface very smooth and effectively separate the particles and defects of the surface, producing an Al2O3 film of better quality. In other words, the multilayer fabricated with 2-μm-thick silamer 3257


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Figure 6. (a) Comparison of WVTR values of multilayer stacks fabricated with different thicknesses of Al2O3 and silamer as a function of the number of dyads. (b) Average transmittance values of multilayers coated on a PET substrate with an increasing number of dyads.

Figure 7. Changes in WVTR values of (a) PET (250 μm)/ Al2O3 (60 nm) and (b) PET (250 μm)/ 3-dyad Al2O3/silamer multilayer as a function of applied tensile strain (measured after bending 100 times at tensile strain). SEM surface images of Al2O3 single layer bent with bending radii of (c) 1.67 and (d) 1 cm.

and water vapor, whereas the FOLED encapsulated with the Al2O3/silamer structure showed greatly improved lifetime by blocking water vapor and oxygen, resulting in half-lifetimes of 67 and 897 h for unencapsulated and encapsulated FOLEDs, respectively (Figure 8d). Therefore, the fabricated barrier structure using functional organic layers and Al2O3 thin film effectively protected the OLED without damaging the OLEDs. Therefore, our inorganic/organic multilayer demonstrated a process reliability suitable for OLEDs as well as effective key properties that can be used as a wearable encapsulation.

investigate the feasibility of the barrier structure based on thermally curable silamer layers to OLEDs, we applied the Al2O3 (60 nm)/silamer (2 μm) structure to blue-emitting FOLEDs and measured their electrical characteristics and operational lifetimes. The structure of the encapsulated FOLED is shown in Figure 8a and the OLED performance was compared together to investigate the process reliability before and after the encapsulation process. As shown in Figures 8b, c, a deposition condition of 70 °C for the formation of Al2O3 and silamer layers did not influence OLED degradation and performance. In other words, the ALD process and thermal curing at 70 °C showed good process reliability in OLEDs. Finally, operational lifetimes of FOLEDs according to the presence of the barrier structure were measured at the initial luminance of 3000 cd/m2 at room temperature. As a result of measurements, the unencapsulated FOLED showed rapid degradation due to exposure to oxygen

4. CONCLUSION In summary, we fabricated a highly impermeable, waterresistant, and flexible Al2O3/silamer multilayer through thickness optimization of Al2O3 and silamer layers. The optimized 3-dyad multilayer fabricated at low temperature of 3258


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Figure 8. (a) Schematic of the encapsulated blue-emitting fluorescent OLED. Luminance vs voltage and current efficiency vs current density for bare OLED and encapsulated OLED are shown in b and c, respectively. (d) Operational lifetime test results of bare OLED and encapsulated OLED at room temperature.

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70 °C achieved an averaged WVTR value of 3.11 × 10−6 g/ m2/day, which is comparable to that of glass-lid encapsulation. In addition, the Al2O3/silamer multilayer exhibited good mechanical stability in comparison to the Al2O3 single layer, showing a slight increase in the WVTR within a magnitude of one order after bending tests with 0.75% bending strain. The significant improvement is attributed to the synergistic effect of optimized Al2O3 and silamer layers at the interface. The silamer layers showed synergistic and effective effects at the interfaces with the Al2O3 ALD film as well as well-known functions such as surface planarization, increased lag time of permeation, as well as stress relief effects in the multilayer structure. In the case of the Al2O3/silamer structure, the generated chemical bonding at the interface and pinhole-filling effect significantly delayed the Al2O3 corrosion against an extremely harsh environment of 60 °C/90% and effectively prevent the penetration of water in a water immersion test. Therefore, methodologies for designing a low-temperature TFE applicable to all organic electronics were provided through functional layer introduction and layer thickness optimization, and will be particularly used to improve the longterm reliability of real wearable and flexible displays based on fabric and fiber substrates, requiring highly mechanical and environmental reliability.

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AUTHOR INFORMATION

Corresponding Author

*Email: kyungcc@ kaist.ac.kr. Phone: +82-42-350-3482. Fax: +82-42-350-8082. ORCID

Seunghun Lee: 0000-0002-1356-2331 Kyung Cheol Choi: 0000-0001-6483-9516 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Engineering Research Center of Excellence (ERC) Program (Grant NRF2017R1A5A1014708) and the Nano·Material Technology Development Program (Grant NRF-2016M3A7B4910635) through the National Research Foundation (NRF) funded by Korean Ministry of Science & ICT (MSIT). In addition, this work was supported by the Technology Innovation Program (20000489, Interactive fiber based wearable display platforms for clothing displays) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11930. Ca test results of S−H nanocomposite and silamer layers under 60 °C/90% RH (PDF) 3259


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Design of Highly Water Resistant, Impermeable, and Flexible Thin

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