Functional Design of DielectricâMetalâDielectric-Based Thin-Film

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Functional Design of Dielectric-Metal-Dielectric based Thin Film Encapsulation with Heat Transfer and Flexibility for Flexible Displays Jeong Hyun Kwon, Seungyeop Choi, Yongmin Jeon, Hyuncheol Kim, Ki Soo Chang, and Kyung Cheol Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06076 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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ACS Applied Materials & Interfaces

Functional Design of Dielectric-Metal-Dielectric based Thin Film Encapsulation with Heat Transfer and Flexibility for Flexible Displays Jeong Hyun Kwon†, Seungyeop Choi†, Yongmin Jeon†, Hyuncheol Kim†, Ki Soo Chang‡, and Kyung Cheol Choi*,†

†

School of Electrical Engineering, KAIST, Daejeon 34141, Republic of Korea

‡

Division of Instrument Development, Korea Basic Science Institute, Daejeon, Republic of Korea

Keywords. Thin film encapsulation, Water vapor transmission rate (WVTR), Dielectric/Metal/Dielectric (DMD), Residual stress, life time, Heat dissipation, Flexible displays

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Abstract

In this study, a new and efficient dielectric-metal-dielectric-based thin-film encapsulation (DMD-TFE) with an inserted Ag thin film is proposed to guarantee the reliability of flexible displays by improving the barrier properties, mechanical flexibility, and heat dissipation, considered as essential requirements for OLED encapsulation. The DMD-TFE, which is composed of Al2O3, Ag, and a silica nanoparticle-embedded sol-gel hybrid nanocomposite, shows a water vapor transmission rate of 8.70 × 10−6 g/m2/day and good mechanical reliability at a bending radius of 30 mm corresponding to 0.16 % strain for 1000 bending cycles. The electrical performance of a thin-film encapsulated phosphorescent organic light-emitting diode (PHOLED) was identical to that of a glass-lid encapsulated PHOLED. The operational lifetimes of thin-film encapsulated and glass-lid encapsulated PHOLEDs are 832 h and 754 h, respectively. After 80 days, the thin-film encapsulated PHOLED did not show performance degradation or dark spots on the cell image in a shelf-lifetime test. Finally, the differences in the lifetimes of OLED devices according to the presence and thickness of an Ag film was analyzed by applying various TFE structures to fluorescent organic light-emitting diodes (FOLEDs) sufficient to generate high amounts of heat. To demonstrate the differences in the heat dissipation effect among TFE structures, the saturated temperatures of encapsulated FOLEDs measured from the backside surface of the glass substrate were 67.78, 65.12, 60.44 and 39.67 °C, after all encapsulated FOLEDs were operated at an initial luminance of 10000 cd/m2 for sufficient heat generation. Furthermore, operational lifetime tests of encapsulated FOLED devices showed results consistent with the measurements of real-time temperature profiles taken with an infrared camera. A multi-functional hybrid thin film encapsulation based on a dielectric-metal-dielectric structure was thus effectively designed considering the transmittance, gas-permeation barrier properties, flexibility, and heat dissipation effect by exploiting the advantages of each separate layer. 1. INTRODUCTION Currently, organic electronics based on organic materials are considered as strong candidates for flexible displays due to the flexibility and smoothness of organic materials. However, such organic electronics require flexible electrodes and thin-film encapsulation (TFE) to be reliable for flexible operation. Recently, many studies have been conducted in an effort to fabricate flexible displays using transparent, flexible TFEs and electrodes, which are considered hurdles to the realization of flexible displays.1 The development of a highly reliable and flexible TFE is most important for fabricating transparent, flexible OLEDs and improving the device lifetime.2,

3

More specifically, one of the major hurdles preventing the

realization of transparent, flexible OLEDs has been the absence of a customized TFE method with gas diffusion barrier properties, mechanical superiority, and heat transfer properties, in spite of the many strengths of OLEDs many strengths. First, very sensitive OLED devices must be protected from reactants such as water vapor, oxygen and reactive gases, using a moisture-resistant encapsulation layer to ensure long device lifetimes. Furthermore, although organic materials and metal electrodes in OLEDs have somewhat mechanical characteristics, brittle encapsulation layers based on oxide films can easily break due to external bending stress.4 In order to effectively protect flexible OLED devices applying TFE layers, the application of organic layers and


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nanolaminate system to the TFE significantly enhanced gas barrier and mechanical properties of the existing TFE layers based on oxide.4,5 the introduction of soft organic materials and robust nanolaminate system provided superior flexibility to encapsulation layers.6,

7

Thus, many reports pertaining to encapsulation

technology for OLEDs have focused primarily on improving barrier performance and flexibility until now. However, in addition to basic performances of TFEs, OLED devices are susceptible to damage by heat stemming from the deposition process and device operation.3, 8 Specifically, flexible OLEDs based on plastic substrates with very low thermal conductivity may cause serious performance degradation of OLED devices without an additional heat sink system for heat conduction out of the devices. A few studies have reported that thermal stress induced by heat generation inside OLEDs can cause thermal degradation, resulting in a reduction of the OLED efficiency and lifetime.9, 10 Thick polymer substrates interfering with heat flow out of OLED devices cause thermal degradation due to heat trapped inside OLEDs, accelerating the degradation of device efficiency and lifetime.10 To ensure stable OLED operation and lifetime, thermal conduction out of the device, an additional heat sink system is necessarily required for effective heat transfer, which becomes increasingly important for the high resolution and large-area of OLEDs.11 There have been several attempts to solve the thermal issue of OLEDs. Park et al.12 have shown that the efficiency and lifetime of OLED devices were improved by attaching a thick metal sheet of several tens of micrometers to encapsulated OLED devices. Chung et al.13 have reported about OLED lifetime improvement by efficient heat dissipation through the metal substrate. However, the use of a rigid metal substrate or metal sheet has several limitations for fabricating flexible OLEDs. First, the surface of the metal substrate needs to be planarized by a solution process to fabricate a stable OLED device due to the very rough surface of metal substrates. Also, the physical attachment of a thick metal sheet on an OLED device can be rigid and damage to the device by physical pressure. A very thin, transparent, and flexible heat sink system is required for practical application to transparent, flexible OLED. Unfortunately, there have been no reports on designing a thinned heat sink system or a new TFE technique to address major issues related with flexible OLEDs, especially their heat dissipation property. To resolve the reliability issues of flexible OLED, our research is to combine a TFE and a heat sink system together without a separate heat sink system in this study. Our existing research only considered basic performances of TFEs by alternating an inorganic layer prepared by an atomic layer deposition (ALD) technique and a solution-processed organic layer.3,5 In other words, earlier studies have limitations such as flexibility, low thermal conductivity, and the requirement of many layers of TFE structures. For highly flexible OLEDs based on flexible electrodes, flexible electrodes have been presented on and ultrathin plastic substrates and fabric substrates.14, 15 Among many flexible electrode, including metal nanowire16, metal

grids17,

graphene18,

carbon

nanotube19,

multilayer

electrode15

etc.,

in

particular,

the

dielectric/metal/dielectric (DMD) multilayer electrode is strong candidate for the fabrication of highly robust, transparent, and flexible electrodes through its combination of good mechanical flexibility and the high conductivity of metal with oxide layers with a high refractive index, ensuring light interference and a resonance effect in the sandwiched DMD structure.20, 21 Furthermore, DMD electrode structures are easily patterned on the micro-scale order using a shadow mask and photolithography compared to other flexible electrodes.22 Therefore, DMD electrodes were found to be suitable to replace conventional ITO electrodes showing many benefits in terms of electrical conductivity, optical transmittance, mechanical flexibility, and patterning for practical applications.22,

23

Thus, most DMD multilayer structures are applied to flexible electrode fabrication. We



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attempted to fabricate a customized TFE suitable for a flexible OLED by inserting a DMD structure into the TFE. Metals in general have high thermal conductivity, malleability, and ductility by free conduction electrons. Therefore, metals have many benefits for OLED encapsulation such as flexibility and high thermal conductivity, which enable several issues related to the TFE to be resolved for realization of flexible displays. In this manner, advantageous DMD structures can be applied to various applications such as transparent and flexible electrodes, TFEs, and heat mirrors depending on the properties of the dielectric and metal used.24 Here, we propose a DMD-based TFE (DMD-TFE) as a new attempt to solve technical problems preventing the realization of flexible OLEDs by combining three materials with different effects together.4, 25 We introduce an Ag film for use in the TFE and investigate the bending characteristics and heat dissipation effect of the DMDTFE layer. Regarding the use of the TFE, an aluminum oxide (Al2O3) was used to create a dielectric film of DMD structure due to electrically insulating properties.26 Furthermore, an Al2O3 film has superior barrier properties suitable for TFE fabrication.3 A functional hybrid TFE was fabricated by means of mutual supplementation utilizing the advantages of each layer: l2O3 with superior barrier properties, a silica nanoparticle-embedded organic/inorganic hybrid (S-H) nanocomposite with stress relaxation and planarization effects, and Ag with good ductility and high thermal conductivity.4, 27 First, the proposed DMD-TFE was designed to improve the optical transmittance through light interference and resonance effect between the Al2O3 and Ag film to overcome the low transmittance due to the use of opaque metal. Second, we investigated key performances of the DMD-TFE as gas diffusion barriers and the degradation of the TFE film itself. In addition, an organic layer was introduced to reduce the residual stress and membrane force of the TFE for mechanical stability. The residual stress of each layer composing the DMD-TFE was measured to investigate the correlation between flexibility and total residual stress using laser curvature measurements. The mechanical characteristics of the DMD-TFE were indirectly confirmed by the change in the WVTR values before and after a bending test. Third, we determined the electrical characteristics and lifetime of a green phosphorescent OLED (PHOLED) encapsulated with a DMD-TFE and compared the results with those of a glass-lid encapsulated PHOLED to assess the practical applicability of the proposed method. Finally, we compared the heat transfer effects of a DMD-TFE applied to fundamental fluorescent organic light-emitting diodes (FOLEDs) with sufficient heat generation according to the presence and thickness of the Ag film. The Ag film in the DMD-TFE structure caused a difference in the heat dissipation effect, which was analyzed by measuring the real-time temperature profiles of encapsulated FOLEDs using an infrared (IR) camera, and an operational lifetime test was conducted at room temperature to investigate the impact of thermal degradation on the OLED efficiency and lifetime. As a result, OLEDs encapsulated with the DMD-TFE showed improved long-term reliability for practical applications compared to conventional TFE technology based on a simple oxide/organic multilayer structure. 2. RESULTS AND DISCUSSION Design of the thin film encapsulation structure. The multi-barrier structure was designed considering the barrier performance, transmittance, flexibility and heat transfer effect. To optimize the DMD-TFE with an inserted Ag thin film, the thickness of each layer was optimized based on various material characteristic evaluations. For superior barrier properties of the TFE, Al2O3 single film or Al2O3-based nanolaminate film grown by the ALD technique is mainly used owing to the formation of a highly dense film free of defects.28, 29


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ALD can fabricate high-quality films by minimizing pinholes which allow the passage of water vapor and oxygen. An Al2O3 as a dielectric film on the DMD structure was deposited to a thickness of 30 nm in light of its mechanical characteristics, barrier properties, and optical simulation results. The Ag film was optimized based on various analysis techniques. A scanning electron microscope surface (SEM) image of Ag film deposited onto an Al2O3 layer showed the formation of the first full coverage film at a thickness of around 15 nm. To investigate the formation of Ag film on the Al2O3 layer in detail, an atomic force microscopy (AFM) analysis and surface sheet resistance measurements were also conducted. (See Supporting Information Figure S1; the SEM image, AFM data, and sheet resistance value clearly show that a 15-nm-thick Ag film forms a continuous layer fully covering the Al2O3 surface). Furthermore, based on the optimized thickness of each layer, an optical simulation was conducted using MATLAB software (MathWorks) based on the multilayer matrix formula. There are little differences between the simulated transmittance data and the measured transmittance data for the Al2O3/Ag/Al2O3 structure. The Al2O3/Ag/Al2O3 (ASA) structure showed improved transmittance by 33% compared with the Al2O3/Ag structure (Figure 1a). As shown in Figure 1b, the final DMD-TFE structure was Al2O3/Ag/Al2O3/S-H nanocomposite/Al2O3, which was nearly identical to the optical transmittance of ASA (Figure 1). Although the S-H nanocomposite and Al2O3 layers were additionally introduced on the DMD structure to enhance the flexibility and barrier properties, additional layers have little impact on the optical transmittance. In general, a thin Ag single layer has low transmittance due to its high reflectance, and the optical transmittance of the Ag thin film sandwiched by dielectric layers with high refractive indexes is improved in the visible wavelength range [4, 19]. The transmittance improvement of the thickness-optimized DMD structure based on an anti-reflection condition is achieved by suppressing the surface plasmon coupling of the interfaces between the metal and the dielectric. Dielectric films with a high refractive indexes help to suppress strong reflection between the metal and the dielectric.30 Therefore, the optical transmittance of the DMD-TFE can be improved due to the suppressed reflection of an Ag thin film embedded between Al2O3 films. However, Al2O3 film (n ~ 1.6, in the visible wavelength region) has a low refractive index compared to those of ZnS (n ~ 2.5, in the visible wavelength region), WoO3 (n ~ 2.05, in the visible wavelength region), and TiO2 (n ~ 2.03, in the visible wavelength region), commonly used as dielectric layer materials in DMD structures.26, 30 In summary, the proposed DMD-TFE structure showed average transmittance of 64.6% in the visible range of 400-700 nm, demonstrating its applicability as not an electrode but a transparent TFE layer through the use of an Al2O3 layer as an electrical insulator.



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Figure 1. (a) Comparison of the experimental transmittance levels of Al2O3/Ag, Al2O3/Ag/Al2O3, and the DMD-TFE with the simulation results. (b) Schematic of DMD-TFE (Al2O3/Ag/Al2O3/S-H nanocomposite/Al2O3). Moisture-resistant barrier, mechanical properties and the reliability of the thin film encapsulation method. To provide the key properties required as a flexible TFE for long-term reliability of flexible OLEDs, Al2O3, an S-H nanocomposite, and Ag were intergrated. An encapsulation for OLEDs should essentially have barrier properties equivalent to 10-6 g/m2/day for OLED lifetime of more than 10000 hours and very low residual stress for long-term reliability of the OLEDs.7, 31 As mentioned above, a 30 nm thick Al2O3 film was used as an inorganic layer to improve the barrier properties of the DMD-TFE. Figure 2(a) shows the normalized conductance curve for calculating the WVTR values. The WVTR values were easily measured through an electrical calcium (Ca) test, which makes use of the initial resistance and the resistance change by the oxidation of a Ca film.28, 29 The WVTR value was calculated using the slope of the conductance as a function of time. An electrical Ca test is an effective method for relative comparison between samples. The barrier performance of barrier structure was improved by the sequential formation of each layer, from initial Al2O3 to the final Al2O3/Ag/Al2O3/S-H nanocomposite/Al2O3.

Figure 2. (a) Comparison of the normalized conductance curves versus the time according to the formation of each layer. (b) The normalized conductance curves of A/Ag/A/S-H/A and A/Ag/A/A before and after bending at


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3 cm. The normalized conductance curve is obtained from a Ca electrical test. Inset: a schematic of the bending test. Table 1. Measured WVTR values of each barrier structure before and after bending at 3 cm according to the formation of each layer

The WVTR values of the barrier structures with the sequential formation of each layer are represented in Table 1 by calculating the plotted conductance curves of Figure 2. As expected, the barrier properties were significantly improved with the formation of each layer. Because Ag and S-H nanocomposite films have almost no barrier properties, three layers of Al2O3 film were deposited for the DMD-TFE fabrication to obtain a WVTR value of 10−6 g/m2/day. As a result, the WVTR value of the DMD-TFE structure was 8.7 × 10−6 g/m2/day in a storage condition of 30℃ and 90% RH according to an electrical Ca test. Stress management of the TFE structure is required for long-term reliability. The DMD-TFE structure contains a smooth organic layer to improve its flexibility by reducing the total residual stress of the structure due to high tensile stress of ALD Al2O3 deposited at a low temperature.26, 32 In contrast, organic layers in multi-barriers based on inorganic/organic layers have the effects of defect-decoupling, stress relaxation and extending the water vapor diffusion path.31, 33 Therefore, the introduction of the S-H nanocomposite layer lends the DMD-TFE improved barrier performance and flexibility though surface planarization and stress relaxation. The residual stress of the barrier structures can be calculated by measuring the curvature change induced in a wafer by the stress in a thin film.32 If the absolute magnitude of the measured stress exceeds a certain level, voids (tensile stress) or buckling (compressive stress) occur immediately after the formation of the thin film or during the use of the thin film.34 When extrinsic tensile stress is applied to a thin film, the thin film can show cracks and pores that permit the passage of water vapor and oxygen. To relax high residual tensile stress in the multilayer structure, the thermally evaporated Ag film was deposited, which is known to have high thermal conductivity and compressive stress with a negative value.7 Therefore, we used a compressively strained and thermally conductive Ag film to reduce the tensile stress in the Al2O3 layer. As shown in Figure 3, the residual stress values of Al2O3 (A), Al2O3/Ag (A/Ag), and Al2O3/Ag/Al2O3 (A/Ag/A) were 248.5 MPa, 338.8 MPa, and 268.2 MPa, respectively. These results show that the multilayer structures composed of inorganic layers were undergoing high tensile stress. Contrary to our expectations, the insertion of the Ag film increased the residual stress in the structure. We concluded that the thermal stress caused by the difference in the thermal expansion coefficients (Al2O3 : 4.2 ppm/K and Ag : 18.9 ppm/K) led to increased stress in the multilayer structure. The high residual stress in a film serves as the driving force for mechanical failure.35 Therefore, to reduce the residual stress in the multi-barrier structure and improve the mechanical reliability, an S-H nanocomposite layer was coated on the A/Ag/A structure. After the S-H nanocomposite was coated by means of spin-coating, the residual stress of the multilayer structure was sharply reduced to 41.36 MPa. We also calculated the change in



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the membrane force of the multilayer structures, which is related to the probability of crack formation in the film structures. The membrane force is calculated as the stress-thickness-product (σ · h), which is important to lower the value below the critical membrane force by the introduction of organic layers with compressive stress.7

Figure. 3 The change of the residual stress and membrane force in multilayer structures according to the formation of each layer. Figure 3 shows the change of the membrane force and the residual stress according to the barrier structure. The presence of the organic layer in the multi-barrier structure improved the stress reliability of the barrier structures, resulting in a sharp reduction of the residual stress and the membrane force between the Al2O3/Ag/Al2O3/Al2O3 (A/Ag/A/A) and Al2O3/Ag/Al2O3/S-H nanocomposite/Al2O3 (A/Ag/A/S-H/A). Furthermore, we conducted a bending test to measure the WVTR before and after bending the A/Ag/A/A and A/Ag/A/S-H/A to evaluate the practical flexibility under external stress such as bending. The WVTR value of the A/Ag/A/S-H/A structure changed little before and after the bending test with a bending radius of 30 mm corresponding to 0.16 % strain for 1000 bending cycles. However, the A/Ag/A/A structure without an organic layer showed an increase of the WVTR on a magnitude of one order. Because the membrane force of the A/Ag/A/A structure is three times greater than that of A/Ag/A/S-H/A, the relative barrier degradation was much larger with identical bending stress. As a result of several measurements, it was determined that the insertion of the organic layer into the DMD-TFE structure contributed to improved flexibility through a reduction of residual stress in the barrier structure. However, although the A/Ag/A/S-H/A structure had lower residual stress compared to the A/Ag/A/A structure, the A/Ag/A/S-H/A structure started to show considerable degradation of the barrier performance at a radius of curvature of less than 20 mm. To investigate the degradation of the DMD-TFE structure, the environmental reliability of the TFE film needed to be considered upon exposure to harsh conditions. There have been reports of the corrosion of Al2O3 films in harsh environments.26, 29 Upon exposure to harsh environments, an Al2O3 film does not show good reliability as a result of degradation due to hydrolysis, causing a phase change from an amorphous structure to a crystalline


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structure. We measured the real-time resistance change to investigate the degradation of the Al2O3-based multibarrier structure using a Ca test. By comparing the WVTR values of the DMD-TFE before and after storage in a humidity chamber of 60 ℃/90% RH during seven days, the degradation level of a 1.5 dyads Al2O3-based multibarrier structure was evaluated. As shown in Figure 4(a), the barrier performance of the 1.5 dyads multi-barrier structure was largely degraded by corrosion reaction, showing an increase of the WVTR on a magnitude of two orders. To reveal the reason for the degradation of the multi-barrier structure, we analyzed the surface of an Al2O3 film upon exposure to a hot and humid environment using an AFM technique (Figures 4(c) and (d)). An amorphous Al2O3 film has very smooth roughness of less than 1 nm, whereas a corroded Al2O3 film showed rough surface roughness of 7.787 nm and a peak-to-valley value of 55.525 nm due to crystallization by a chemical reaction with water vapor, producing cracks and defects in the film. Therefore, an Al2O3 layer should be minimized in terms of its thickness and the number of layers for long-term reliability of the TFE film. In addition to the degradation of Al2O3 film, the stability of an Ag thin film by oxidation is an important issue under harsh conditions due to the importance of the contribution of the Ag film to the heat transfer and bending characteristics of the TFE structure. As shown in Figure 4(b), the bare Ag film degraded rapidly within 10 hours due to the oxidation reaction between water vapor and Ag molecules in a hot and humid environment.23 However, the Ag film in the DMD-TFE was well protected by the Al2O3/S-H nanocomposite/Al2O3 multilayer structure formed on the Ag film. The Al2O3/S-H nanocomposite/Al2O3 multilayer structure effectively prevented the permeation of water vapor and oxygen, which cause Ag oxidation, resulting in little change of the electrical conductivity. Although the Al2O3 film itself showed the degradation when exposed to a harsh temperature/humidity level, an Ag thin film was protected by Al2O3/S-H nanocomposite/Al2O3 multilayer structure. According to the result of the measurements, Al2O3 films in the TFE structure must be replaced by a water-resistant oxide with a new structure and new materials for reliable TFE fabrication. Therefore, the proposed DMD-TFE showed good mechanical reliability for application to flexible displays though a stress analysis as well as good environmental reliability by minimizing the degradation of the Ag film.



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Figure 4. (a) Ca test of the Al2O3-based multilayer before and after exposure to 60 ℃/90% RH for seven days. (b) The normalized conductance curve of bare Ag film and Al2O3/Ag/Al2O3 passivated Ag film under a harsh condition of 60 ℃/90% RH. The inset displays a schematic of for a measurement of conductivity. AFM images of the surface of a single Al2O3 film sample (c) before and (d) after storage in a 60 ℃/90% RH.

The electrical characteristic and lifetime of thin-film encapsulated phosphorescence OLED. To demonstrate the feasibility of the DMD-TFE technology, a PHOLED was fabricated with a structure of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/4, 4’-bis(N-phenyl-1-naphthylamino)biphenyl (NPB)/4, 4’bis(carbazol-9-yl)biphenyl (CBP) : Tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) (8wt%)/2, 2’, 2’’-(1,3,5benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)/lithium quinolate (Liq)/aluminum (Al) as a bottomemitting, micro-cavity structure [30-32]. To prevent the degradation of PHOLEDs upon exposure to reactant gases such as water vapor, oxygen, and trimethylaluminum (TMA), during the formation of the thin film encapsulation, a 100-nm-thick NPB layer was additionally deposited as a protective layer (Figure 5a). Finally, the DMD-TFE layer was formed on the device after taping the contact edge of the ITO electrode using Kapton tape with good high-temperature stability. Generally, OLED devices showed thermal degradation induced by the process temperature and joule heating generated during the operation of the device [12, 33]. The phosphorescence OLEDs contain CBP, NPB, Ir(ppy)3, and TPBi, which have glass-transition temperature (Tg) values of 62, 96, 359, and 142 ℃, respectively.8, 39 Therefore, the Al2O3 film was formed at a chamber temperature of 70 ℃, close to the Tg of CBP and below the Tg of NPB, which did not cause performance degradation of the PHOLED devices, resulting in good process reliability.


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Figure 5. (a) The structure of the thin-film encapsulated PHOLED device, (b - c) Electrical properties of PHOLEDs before and after the encapsulation process: (b) current density-voltage-luminance curve, (c) current efficiency-current density. (d) Operational lifetime test of a glass-lid encapsulated PHOLED and a DMD-TFE encapsulated PHOLED at an initial luminance level of 1000 cd/m2. (e) Emitting image of a PHOLED encapsulated DMD-TFE Inset: photograph of a DMD-TFE encapsulate PHOLED on a glass substrate. (f) Cell image of not-encapsulated PHOLED and DMD-TFE encapsulated PHOLED over time. We measured the J-V-L characteristics, operational lifetimes, and shelf lifetimes of encapsulated PHOLEDs to evaluate the barrier performance and reliability of the DMD-TFE layer. The PHOLED device did not show performance degradation after TFE application with a 70℃ process, presenting identical J-V-L characteristics. In other words, the reactant species ((i.e., H2O and TMA) for Al2O3 formation and the process conditions of 70℃ ALD process and UV-curing did not damage the OLED devices. Furthermore, the lifetimes of PHOLEDs for the DMD-TFE and glass-lid encapsulation were 832 h and 754 h, respectively. The PHOLEDs had turn-on voltages of around 3.0 V, and the maximum luminance was 74608 cd/m2. The maximum current efficiency values of the PHOLEDs were 46.3 cd/A at 4.5 V. To investigate the dominant factor between the operation stability of the device structure and the barrier performance of the DMD-TFE in terms of the lifetime of PHOLEDs, constant current-driving lifetime measurements in which both the device efficiency and the barrier properties of the TFE were considered were conducted together with shelf lifetime measurements to evaluate the barrier properties of the TFEs in PHOLED devices. Although the WVTR value of the DMD-based thin film encapsulation is nearly ten times higher than that of the glass-lid encapsulation, the operational lifetime of the PHOLED with the DMDTFE was longer compared to that of the glass-lid encapsulated PHOLED at the initial luminance of 1000 cd/m2



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at room temperature. The half-lifetimes of the PHOLED with the DMD-TFE and glass-lid encapsulation were 832 h and 754 h, respectively. Specifically, the half-lifetime of the PHOLED with the DMD-TFE is 10 % longer than that of the PHOLED with the glass-lid encapsulation. This interesting result stemmed from the side permeation of oxygen and moisture through the UV-curable sealant (XNR5570-B1, Nagase Chemtex) used to attach the glass lid onto the glass substrate during the process of glass-lid encapsulation (see Supporting Information Figure S2 for schematics of the glass-lid encapsulated OLED and thin-film encapsulated OLED). For the DMD-TFE, thin films were deposited on OLED devices directly and side permeation by water vapor and oxygen was less likely to occur compared to the glass-lid encapsulation. From these results, the DMD-TFE shows better long-term stability than the conventional glass-lid encapsulation when applied to OLED devices. Further, to evaluate the barrier properties for OLED devices of the DMD-TFE, a shelf-lifetime test was conducted by storage for 80 days at room temperature. As shown in Figure 5 (f), cell images of OLED devices according to the presence of the TFE clearly showed the difference in device reliability by external exposure to the ambient environment. Although the Al/NPB double layer has a proper gas-barrier function against oxygen and water vapor, the non-encapsulated PHOLED device showed many dark spots after 24 hours. However, the J-V-L characteristics and the current efficiency of the encapsulated PHOLEDs did not show degradation after 80 days, with a clean cell image noted with no dark spots. The DMD-TFE demonstrated strong potential as an encapsulation layer for OLED devices based on the measurement and analysis results of both the operational lifetime and the shelf-lifetime. The lifetime of thin-film encapsulated FOLEDs by heat dissipation performance of thin film encapsulation. To evaluate the heat dissipation ability of DMD-TFE, the FOLED device with the structure of ITO/NPB/ tris-(8-hydroxyquinoline) aluminum (Alq3)/ lithium fluoride (LiF)/Al was fabricated on a glass substrate as a heat generator (Figure 6 (a)). The FOLED with poor current efficiency generates excessive heat by electrical stress and thermionic emission, which results in thermal degradation and short lifetimes of the OLEDs.8, 12 The real-time temperature of all FOLED devices was measured from the back side of the glass substrate using an IR camera. Thin film encapsulations with various structures were applied to FOLEDs to compare the heat dissipation ability among the TFE layers. Four types of thin film encapsulations were fabricated to analyze the thermal conductivity effect according to the presence and thickness of the Ag thin film (Figure 6 (b)).25 The first TFE structure was fabricated only using Al2O3 and a S-H nanocomposite with very low thermal conductivity (TFE without Ag). The second TFE structure incorporates the proposed DMD-TFE structure with a 15-nm-thick Ag film (TFE with Ag 15 nm). The thermal conductivity of silver is known to be 374 W/mK at a temperature of 300 K, which is the highest thermal conductivity of any metal [35]. This is one of the reasons we utilized Ag film as the metal in the DMD-TFE layer. A TFE with 100 nm of Ag was also fabricated to investigate the difference in the heat dissipation effect according to an increase in the Ag thickness. Finally, a TFE with a flexible graphite heat spreader of 600 W/mK (eGRAF® Spreadershieldtm) was prepared to compare the heat dissipation effect of the heat sheet used in the product. Figures 6(c) and 6(d) show the realtime temperature measurements and the lifetimes of the encapsulated FOLEDs, respectively. Generally, heat can only be transferred though the mean conduction, convection, and radiation from high to low temperatures.40 Specifically, conduction and radiation are two important heat transfer mechanisms in solid materials. We designed a heat-transferable DMD-TFE using the thermal conduction effect through physical contact between very thin layers. As the DMD structure is well known as a heat mirror, the DMD structure functions as a thermal


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radiation barrier by reflecting the light of the infrared wavelength region due to the increasing absorption coefficient of metal film with an increase in wavelength. Therefore, for the DMD structure, it is estimated that there is almost no heat dissipation effect by thermal radiation. However, main heat transfer occurs mainly by thermal conduction through the DMD-TFE when in contacting between very thin films. We calculated the optical reflectance of the DMD structure in the wavelength range of 300 ~ 1000 nm by a simulation (See Supporting Information Figure S3 for the calculated transmittance of the Al2O3 (30nm) /Ag (x nm, x = 15, 100 nm)/Al2O3 (30 nm) structure). The optical reflectance of the DMD structure increased with an increase in the Ag thickness. The reflectance value of Al2O3(30nm)/ Ag(100nm)/ Al2O3(30 nm) is nearly 100% at a wavelengths of 1000 nm. Likewise, the reflectance value of Al2O3(30nm)/Ag(15nm)/Al2O3(30 nm) is estimated to be closer to 100 % at wavelengths of more than 1 um due to the increasing Ag absorption coefficient with an increase in the wavelength.

In

other

words,

both

the

Al2O3(30nm)/Ag(100nm)/Al2O3(30nm)

structure

and

the

Al2O3(30nm)/Ag(15nm)/Al2O3(30 nm) structure function as perfect thermal radiation barriers at infrared wavelengths radiated by a heat source. Therefore, the DMD-TFE reduced the generated heat using not the thermal radiation effect but the thermal conduction between thin films due to thermal radiation barrier properties of the DMD structure. The real-time temperatures of all encapsulated-FOLED devices were saturated in less than one minute after electrical input power of approximately 80 mW was applied to the FOLED devices. The maximum temperatures of the aforementioned encapsulated FOLED devices were 66.78, 64.12, 59.44 and 39.67 °C, respectively ( Figure 6(c)). As mentioned earlier, although the TFE structures with Ag film reflect the infrared light, the thermal conduction mechanism by contact between the thin films mainly transfers and spreads heat through the TFE layer. The differences in the heat dissipation effect among TFE structures according to the presence and the thickness of the Ag film led to the temperature differences. As the Ag film thickness in a DMD-TFE structure increases, the heat dissipation effect of the DMD-TFE have a tendency to be improved. In general, the physical properties of Ag thin film differ significantly from those of bulk Ag, especially thermal and electrical conductivity. The thermal conductivity of a metal thin film is estimated to be reduced due to free electron scattering at the surfaces and grain boundaries in a thin metal film.41, 42 Therefore, the heat dissipation effect of the DMD-TFE is propositional to the Ag film thickness, with a trade-off between the optical transmittance and the heat dissipation effect.



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Figure 6. (a) Schematic diagram of a thin-film encapsulated FOLED device according to infrared camera measurements. (b) Schematics of various TFE structures for a comparison of the heat dissipation effects. (c) Real-time temperature profiles of FOLEDs with various TFE structures operated at an input power of 80 mW over time. (d) The operational lifetime test results of FOLEDs with various TFE structures operated at an initial luminance of 10000 cd/m2.

To demonstrate the association between the OLED lifetime and the real-time temperature measurement results, the operational lifetimes of FOLEDs were measured at room temperature and at an initial luminance of 10000 cd / m2 by applying four types of thin film encapsulations to the FOLEDs. The tested FOLEDs showed rapid degradation within ten hours due to the harsh operation conditions. In particular, the lifetime of the FOLED without the TFE was dramatically shortened by exposure to air. However, FOLEDs with TFE layers showed more than twofold enhanced lifetimes through their minimized exposure to oxygen and water vapor. The lifetimes of the FOLEDs with the DMD-TFE increased as a thicker Ag film was incorporated. Although the DMD-TFE encapsulated FOLED did not show a significant improvement in its OLED lifetime, the heat dissipation effect on the OLED lifetime and efficiency was meaningful when inserting Ag film into the TFE. Therefore, the TFE should be designed considering the heat dissipation effect as well as the barrier quality, transmittance, and flexibility for the realization of flexible displays. 3. CONCLUSION In summary, we demonstrated a highly impermeable, flexible, and thermally conductive hybrid encapsulation based on a DMD structure for a flexible OLED. A reliable hybrid encapsulation combined with oxide, metal, and organic layers was designed to solve major issues such as lifetime, flexibility, and thermal degradation in realizing a flexible OLED. The DMD-TFE structure showed the optical transmittance of more than 60 % in a range of visible range despite an opaque Ag layer in the TFE structure. The water vapor transmission rate of the


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240-nm-thick DMD-TFE was 8.86 × 10−6 g/m2/day which nearly meets the WVTR requirement for OLEDs, and the DMD-TFE showed good reliability in a 1000 cyclic bending test with a bending radius of 3 cm by reducing the residual stress of the DMD-TFE through the introduction of the S-H nanocomposite layer. The DMD-TFE was applied to a PHOLED and a FOLED for applicability to flexible OLEDs, and it enhanced the stability and reliability of OLED devices. The operational lifetime of the PHOLED with a DMD-TFE was longer than that of the glass-encapsulated PHOLED due to side permeation through the sealant of glass encapsulation. Moreover, the PHOLED with the DMD-TFE did not show performance degradation or dark spots even after being stored during 80 days at room temperature. Finally, the proposed DMD-TFE was analyzed with respect to its heat transfer property and showed a thermally conductive property by incorporation of an Ag film with high thermal conductivity. The various TFE structures showed differences of heat dissipation ability according to the presence and thickness of the Ag film, consistent with the OLED lifetime measurement results. The DMD-TFE with heat dissipation effect showed a thermal stress relaxation effect on the OLED lifetime. Therefore, our work shows remarkable potential for new flexible encapsulation technology to achieve flexible and wearable OLEDs toward highly reliable displays designed considering flexibility, stress relaxation, barrier quality, and heat dissipation. However, to completely resolve technical issues raised above, further research on the fabrication of flexible and thermally conductive TFE beyond the proposed DMD-TFE is necessary. 4. EXPERIMENTAL SECTION Preparation of thin film encapsulation layer. DMD-TFE was deposited on a 125-um-thick PET substrate after PET substrate was cleaned with acetone, isopropyl alcohol and de-ionized water in an ultrasonic sonicator. S-H nanocomposite layer, Al2O3 and Ag were coated by spin-coater, ALD, and thermal evaporation, respectively. Generally, the ALD method requires long process time due to long purging time and atomic monolayer growth per cycle for highly dense film. An Al2O3 ALD film is deposited using the chemical reaction between TMA precursor and an H2O reactant at a low temperature of 70℃ which does not occur OLED degradations (Lucida D 100, NCD). The growth rate of Al2O3 at a chamber temperature of 70 ℃ were 0.83 Å/cycle; the purge time and precursor exposure time were 10 s and 0.2 s. An Ag thin film was coated by a thermal evaporation method at vacuum pressures below 1 × 10−5 Torr. In our experiment, silver sources were evaporated from tungsten boat at constant deposition rate of 2 Å/s to deposit a 15-nm-thick thin film. Finally, spin-coating condition for coating 120 nm-thick S-H nanocomposite layer were 4000 rpm for 3s and acceleration for 30s. The coated S-H nanocomposite layer should be cured by UV light with a wavelength of 365nm during 100 s. Thus, the combination of three materials with different effects makes it possible to fabricate multi-functional hybrid encapsulation. PHOLED Fabrication. Bottom-emitting PHOLEDs on a 150-nm-thick ITO-coated glass substrate (2.5 cm × 2.5 cm) (a 150-nm-thick ITO film on a glass substrate in the case of the FOLED) were fabricated using a thermal evaporation technique. Highly efficient PHOLEDs were designed considering the micro-cavity effect for increasing the external quantum efficiency and the phosphorescent emission layer for increasing the internal quantum efficiency.38, 43 The PHOLED is composed of a 5-nm-thick MoO3 for a hole injection layer (HIL), a 70 nm-thick NPB as a hole transport layer (HTL), a 20 nm-thick host-guest energy transfer system, CBP : Ir(ppy)3 (8%) as an emission layer (EML), a 40 nm-thick TPBi for an electron transport layer(ETL), a 1 nm-thick Liq for



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an electron injection layer(EIL), and a highly reflective Al cathode.44-45 Thin-film encapsulated FOLEDs Fabrication. Bottom-emitting FOLEDs for sufficient heat-generation were fabricated on a 150-nm-thick ITO-patterned glass substrate (2.5 cm × 2.5 cm). The FOLED structure is composed of 50-nm-thick NPB for a HTL, 50-nm-thick Alq3 for an EML, 1-nm-thick LiF for an EIL, and 100nm-thick Al for a cathode, which were all deposited by a thermal evaporation method under a vacuum of 10-6 torr [33]. A 100-nm-thick NPB layer was additionally deposited to prevent device degradation in the process of encapsulation. After fabricating the FOLED devices on a glass substrate, TFE layers with various structures were directly formed on the OLED devices. OLED Performance and WVTR Measurement. The electrical-optical characteristics of thin-filmencapsulated OLEDs were measured using a source meter (Keithley 2400, USA) and a spectrophotometer (CS2000, Konica Minolta, Japan). The operational lifetime test was performed by using Si-photo diodes (Polaronix M6000S, McScience, Korea) at room temperature. The WVTR was measured through an electrical Ca test, which makes use of the initial resistance and the resistance change by the oxidation of a Ca film. The patterned Al film and Ca film were deposited sequentially on a glass substrate. The passivated plastic film was attached on a Ca sensor using UV-curable sealant. The Ca tests were conducted at 30℃ and 90% RH for WVTR measurement (See the Supporting Information Figure S4 for a schematic diagram of the Ca test cell for the electrical Ca test). Stress Measurement in thin film using Wafer Curvature Measurement. The residual stress in the thin film or multilayer structure has been determined by measuring the change in the wafer curvature before and after the deposition process. Thin film measurement system (FSM 500TC, USA) with laser scanning technology measures the changes in the radius of curvature of the wafer substrate induced by the deposition process on the wafer. The resulting change in curvature is related to the film stress, which is calculated using the Stoney relation, or Stoney formula.46 Temperature-profile and IR Image Measurement. The real-time temperatures and images of operating OLEDs every second were measured and captured by an IR thermal image camera device (SC-5000, FLIR) in Korea Basic Science Institute. 5. ASSOCIATED CONTENT 6. AUTHOR INFORMATION Corresponding Author K.C. Choi.* Author is with the School of Electrical Engineering, the Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 34141, Republic of Korea (corresponding author phone: +82-42-350-3482; fax: +82-42-350-8082; e-mail: kyungcc@ kaist.ac.kr). Supporting Information Surface SEM and AFM image of Ag film according to thickness change, schematic of the glass-lid encapsulated OLED and the thin-film encapsulated OLED, schematic diagram of Ca test cell for the electrical Ca test, and the calculated transmittance of the Al2O3 (30nm) /Ag (x nm, x = 15, 100 nm)/Al2O3 (30 nm) structure. This material is available free of charge via the Internet at http://pubs.acs.org. Notes


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The authors declare no competing financial interest. 7. ACKNOWLEDGEMENT This work was supported by the IT R&D program of MOTIE/KEIT[10042412, More than 60〞transparent flexible display with UD resolution, transparency 40% for Transparent Flexible Display in Large Area]. References (1) Lee, S. M.; Kwon, J. H.; Kwon, S.; Choi, K. C. A Review of Flexible OLEDs Toward Highly Durable Unusual Displays. IEEE Trans. Electron Devices 2017, 64 (5), 1922–1931. (2) Jeong, E. G.; Kwon, S.; Han, J. H.; Im, H.-G.; Bae, B.-S.; Choi, K. C. Mechanically Enhanced Hybrid Nano-Stratified Barrier with Defect Suppression Mechanism for Highly Reliable Flexible OLEDs. Nanoscale 2017, 6370–6379. (3) Han, Y. C.; Kim, E.; Kim, W.; Im, H. G.; Bae, B. S.; Choi, K. C. A Flexible Moisture Barrier Comprised of a SiO2-Embedded Organic-Inorganic Hybrid Nanocomposite and Al2O3 for Thin-Film Encapsulation of OLEDs. Org. Electron. 2013, 14 (6), 1435–1440. (4) Kwon, J. H; Kim, E.; Im, H.-G.; Bae, B.-S.; Soo Chang, K.; Ko Park, S.-H.; Cheol Choi, K. MetalContaining Thin-Film Encapsulation with Flexibility and Heat Transfer. J. Inf. Disp. 2015, 16, 123–128. (5) Kim, E.; Han, Y.; Kim, W.; Choi, K. C.; Im, H. G.; Bae, B. S. Thin Film Encapsulation for Organic Light Emitting Diodes Using a Multi-Barrier Composed of MgO Prepared by Atomic Layer Deposition and Hybrid Materials. Org. Electron. 2013, 14 (7), 1737–1743. (6) Yoon, K. H.; Kim, H. S.; Han, K. S.; Kim, S. H.; Lee, Y. E. K.; Shrestha, N. K.; Song, S. Y.; Sung, M. M. Extremely High Barrier Performance of Organic-Inorganic Nanolaminated Thin Films for Organic LightEmitting Diodes. ACS Appl. Mater. Interfaces 2017, 9 (6), 5399–5408. (7) Behrendt, A.; Meyer, J.; Van De Weijer, P.; Gahlmann, T.; Heiderhoff, R.; Riedl, T. Stress Management in Thin-Film Gas-Permeation Barriers. ACS Appl. Mater. Interfaces 2016, 8 (6), 4056–4061. (8) Kwak, K.; Cho, K.; Kim, S. Analysis of Thermal Degradation of Organic Light-Emitting Diodes with Infrared Imaging and Impedance Spectroscopy. Opt. Express 2013, 21 (24), 29558–29566. (9) Vamvounis, G.; Aziz, H.; Hu, N.-X.; Popovic, Z. D. Temperature Dependence of Operational Stability of Organic Light Emitting Diodes Based on Mixed Emitter Layers. Synth. Met. 2004, 143, 69–73. (10) Moore, A. L.; Shi, L. Emerging Challenges and Materials for Thermal Management of Electronics. Mater. Today 2014, 17 (4), 163–174. (11) Park, J. W.; Shin, D. C.; Park, S. H. Large-Area OLED Lightings and Their Applications. Semicond. Sci. Technol. 2011, 26 (3), 34002. (12) Park, J.; Ham, H.; Park, C. Heat Transfer Property of Thin-Film Encapsulation for OLEDs. Org. Electron. 2011, 12 (2), 227–233. (13) Chung, S.; Lee, J.-H.; Jeong, J.; Kim, J.-J.; Hong, Y. Substrate Thermal Conductivity Effect on Heat Dissipation and Lifetime Improvement of Organic Light-Emitting Diodes. Appl. Phys. Lett. 2009, 94 (25), 253302. (14) Kim, D. Y.; Han, Y. C.; Kim, H. C.; Jeong, E. G.; Choi, K. C. Highly Transparent and Flexible Organic Light-Emitting Diodes with Structure Optimized for Anode/Cathode Multilayer Electrodes. Adv. Funct.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

Mater. 2015, 25, 7145–7153. (15) Kim, W.; Kwon, S.; Han, Y. C.; Kim, E.; Choi, K. C.; Kang, S. H.; Park, B. C. (3)Reliable Actual FabricBased Organic Light-Emitting Diodes: Toward a Wearable Display. Adv. Electron. Mater. 2016, 2, 1–7. (16) Zeng, X. Y.; Zhang, Q. K.; Yu, R. M.; Lu, C. Z. A New Transparent Conductor: Silver Nanowire Film Buried at the Surface of a Transparent Polymer. Adv. Mater. 2010, 22 (40), 4484–4488. (17) Kang, M.-G.; Guo, L. J. Nanoimprinted Semitransparent Metal Electrodes and Their Application in Organic Light-Emitting Diodes. Adv. Mater. 2007, 19 (10), 1391–1396. (18) Bae, S.; Kim, H. K.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Im, D.; Lei, T.; Song, Y. Il; Kim, Y. J.; Kim, K. S.; Özyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. 30 Inch Roll-Based Production of High-Quality Graphene Films for Flexible Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574-578. (19) Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H. J.; Dennler, G.; Sariciftci, N. S.; Hu, L.; Gruner, G. Organic Solar Cells with Carbon Nanotube Network Electrodes. Appl. Phys. Lett. 2006, 88 (23), 23–25. (20) Cattin,

L.;

Bernède,

J.

C.;

Morsli,

M.

Toward

Indium-Free

Optoelectronic

Devices:

Dielectric/metal/dielectric Alternative Transparent Conductive Electrode in Organic Photovoltaic Cells. Phys. Status Solidi Appl. Mater. Sci. 2013, 210 (6), 1047–1061. (21) Guillén, C.; Herrero, J. TCO/metal/TCO Structures for Energy and Flexible Electronics. Thin Solid Films 2011, 520 (1), 1–17. (22) Han, J. H.; Kim, D.; Jeong, E. G.; Lee, T.-W.; Lee, M. K.; Park, J. W.; Lee, H.; Choi, K. C. Highly Conductive Transparent and Flexible Electrodes Including Double-Stacked Thin Metal Films for Transparent Flexible Electronics. ACS Appl. Mater. Interfaces 2017, acsami.7b04725. (23) Behrendt, A.; Friedenberger, C.; Gahlmann, T.; Trost, S.; Becker, T.; Zilberberg, K.; Polywka, A.; Görrn, P.; Riedl, T. Highly Robust Transparent and Conductive Gas Diffusion Barriers Based on Tin Oxide. Adv. Mater. 2015, 27, 5961–5967. (24) Zhao, J.; Brinkmann, K. O.; Hu, T.; Pourdavoud, N.; Becker, T.; Gahlmann, T.; Heiderhoff, R.; Polywka, A.; G??rrn, P.; Chen, Y.; Cheng, B.; Riedl, T. Self-Encapsulating Thermostable and Air-Resilient Semitransparent Perovskite Solar Cells. Adv. Energy Mater. 2017, No. March. (25) Kwon, J. H.; Im, H.; Bae, B.; Chang, K. S.; Park, S. K.; Choi, K. C. P-96: Heat Transferable Thin Film Encapsulation Inserted Ag Thin Film to Improve Reliability of Flexible Displays. In: SID Symposium Digest of Technical Papers, 2016, 47, 1491–1494. (26) Kim, L. H.; Kim, K.; Park, S.; Jeong, Y. J.; Kim, H.; Chung, D. S.; Kim, S. H.; Park, C. E. Al2O3/TiO2 Nanolaminate Thin Film Encapsulation for Organic Thin Film Transistors via Plasma-Enhanced Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2014, 6, 6731–6738. (27) Jin, J.; Lee, J. J.; Bae, B. S.; Park, S. J.; Yoo, S.; Jung, K. Silica Nanoparticle-Embedded Sol-Gel Organic/inorganic Hybrid Nanocomposite for Transparent OLED Encapsulation. Org. Electron. 2012, 13 (1), 53–57. (28) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Groner, M. D.; George, S. M. Ca Test of Al 2O 3 Gas Diffusion Barriers Grown by Atomic Layer Deposition on Polymers. Appl. Phys. Lett. 2006, 89 (3), 15–17. (29) Meyer, J.; Görrn, P.; Bertram, F.; Hamwi, S.; Winkler, T.; Johannes, H. H.; Weimann, T.; Hinze, P.; Riedl, T.; Kowalsky, W. Al2O3/ZrO2 Nanolaminates as Ultrahigh Gas-Diffusion Barriersa Strategy for Reliable


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Encapsulation of Organic Electronics. Adv. Mater. 2009, 21 (18), 1845–1849. (30) Yang, D. Y.; Lee, S.-M.; Jang, W. J.; Choi, K. C. Flexible Organic Light-Emitting Diodes with ZnS/Ag/ZnO/Ag/WO3 Multilayer Electrode as a Transparent Anode. Org. Electron. 2014, 15 (10), 2468– 2475. (31) Lewis, J. Material Challenge for Flexible Organic Devices. Mater. Today 2006, 9 (4), 38–45. (32) Ylivaara, O. M. E.; Liu, X.; Kilpi, L.; Lyytinen, J.; Schneider, D.; Laitinen, M.; Julin, J.; Ali, S.; Sintonen, R. L. Aluminum Oxide from Trimethylaluminum and Water by Atomic Layer Deposition: The Temperature Dependence of Residual Stress, Elastic Modulus, Hardness and. Thin Solid Films 2014, 552, 124–135 (33) Graff, G. L.; Williford, R. E.; Burrows, P. E. Mechanisms of Vapor Permeation through Multilayer Barrier Films: Lag Time versus Equilibrium Permeation. J. Appl. Phys. 2004, 96 (4), 1840. (34) Lahiri, S. K. Mechanical Stress Induced Void and Hillock Formations in Thin Films. Proc. IEEE Int. Work. Mem. Technol. Des. Test 1994, 22–25. (35) Jen, S.-H. H.; Bertrand, J. A.; George, S. M. Critical Tensile and Compressive Strains for Cracking of Al2O3 Films Grown by Atomic Layer Deposition. J. Appl. Phys. 2011, 109 (8), 84305. (36) Kwong, R. C.; Nugent, M. R.; Michalski, L.; Ngo, T.; Rajan, K.; Tung, Y. J.; Weaver, M. S.; Zhou, T. X.; Hack, M.; Thompson, M. E.; Forrest, S. R.; Brown, J. J. High Operational Stability of Electrophosphorescent Devices. Appl. Phys. Lett. 2002, 81 (1), 162–164. (37) Peng, H. J.; Zhu, X. L.; Sun, J. X.; Yu, X. M.; Wong, M.; Kwok, H. S. Efficiency Improvement of Phosphorescent Organic Light-Emitting Diodes Using Semitransparent Ag as Anode. Appl. Phys. Lett. 2006, 88 (3), 1–3. (38) Song, W.; Lee, J. Y. Light Emission Mechanism of Mixed Host Organic Light-Emitting Diodes. Appl. Phys. Lett. 2015, 106, 123306. (39) Mu, H.; Rao, L.; Li, W.; Wei, B.; Wang, K.; Xie, H. Electroluminescence Dependence of the Simplified Green Light Organic Light Emitting Diodes on in Situ Thermal Treatment. Appl. Surf. Sci. 2015, 357, 2241–2247. (40) Jamal-Abad, M. T.; Saedodin, S.; Aminy, M. Heat Transfer in Concentrated Solar Air-Heaters Filled with a Porous Medium with Radiation Effects: A Perturbation Solution. Renew. Energy 2016, 91, 147–154. (41) Lin, H.; Xu, S.; Li, C.; Dong, H.; Wang, X. Thermal and Electrical Conduction in 6.4 Nm Thin Gold Films. Nanoscale 2013, 5 (11), 4652. (42) Lin, H.; Xu, S.; Wang, X.; Mei, N. Thermal and Electrical Conduction in Ultrathin Metallic Films: 7 Nm down to Sub-Nanometer Thickness. Small 2013, 9 (15), 2585–2594. (43) Lin, C. L.; Lin, H. W.; Wu, C. C. Examining Microcavity Organic Light-Emitting Devices Having Two Metal Mirrors. Appl. Phys. Lett. 2005, 87 (2), 1–4.. (44) Kröger, M.; Hamwi, S.; Meyer, J.; Riedl, T.; Kowalsky, W.; Kahn, A. Role of the Deep-Lying Electronic States of MoO3 in the Enhancement of Hole-Injection in Organic Thin Films. Appl. Phys. Lett. 2009, 95 (12), 2009–2011. (41) Liu, Z.; Salata, O. V.; Male, N. Improved Electron Injection in Organic LED with Lithium Quinolate/aluminium Cathode. Synth. Met. 2002, 128 (2), 211–214. (45) Zheng, X.; Wu, Y.; Sun, R.; Zhu, W.; Jiang, X.; Zhang, Z.; Xu, S. Efficiency Improvement of Organic Light-Emitting Diodes Using 8-Hydroxy-Quinolinato Lithium as an Electron Injection Layer. Thin Solid



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Films 2005, 478 (1–2), 252–255. (46) Ardigo, M. R. M.; Ahmed, M.; Besnard, A. Stoney Formula: Investigation of Curvature Measurements by Optical Profilometer. Adv. Mater. Res. 2014, 996, 361–366.


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Functional Design of DielectricâMetalâDielectric-Based Thin-Film

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