Functional Design of DielectricâMetalâDielectric-Based Thin-Film

Research Article www.acsami.org

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 34133, Republic of Korea

‡

S Supporting Information *

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, which are considered to be essential requirements for organic light-emitting diode (OLED) encapsulation. The DMDTFE, 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 0.41% 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 the thin-film encapsulated and glass-lid encapsulated PHOLEDs are 832 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 difference in lifetime of the OLED devices in relation to the presence and thickness of a Ag film was analyzed by applying various TFE structures to fluorescent organic light-emitting diodes (FOLEDs) that could generate high amounts of heat. To demonstrate the difference in heat dissipation effect among the TFE structures, the saturated temperatures of the encapsulated FOLEDs were measured from the back side surface of the glass substrate, and were found to be 67.78, 65.12, 60.44, and 39.67 °C after all encapsulated FOLEDs were operated at an initial luminance of 10 000 cd/ m2 for sufficient heat generation. Furthermore, the operational lifetime tests of the encapsulated FOLED devices showed results that were consistent with the measurements of real-time temperature profiles taken with an infrared camera. A multifunctional 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. KEYWORDS: Thin-film encapsulation, water vapor transmission rate (WVTR), dielectric/metal/dielectric (DMD), residual stress, lifetime, heat dissipation, flexible displays

1. INTRODUCTION

and heat transfer properties, in spite of the many strengths of OLEDs. 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 To effectively protect flexible OLED devices using TFE layers, organic layers and a nanolaminate system were added to the TFE, which significantly enhanced the gas barrier and mechanical properties of the existing TFE layers based on

Currently, organic electronics based on organic materials are considered as strong candidates for flexible displays due to the flexibility and smoothness of the organic materials. However, these organic electronics require flexible electrodes and thinfilm 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 to be 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 enabling gas diffusion barrier properties, mechanical superiority, © 2017 American Chemical Society

Received: May 2, 2017 Accepted: July 18, 2017 Published: July 18, 2017 27062

DOI: 10.1021/acsami.7b06076 ACS Appl. Mater. Interfaces 2017, 9, 27062−27072

Research Article

ACS Applied Materials & Interfaces

compared to other flexible electrodes.22 Therefore, DMD electrodes have been found to be suitable to replace conventional indium tin oxide (ITO) electrodes, and show 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 attempted to fabricate a customized TFE suitable for a flexible OLED by inserting a DMD structure into the TFE. Metals, in general, have a high thermal conductivity, malleability, and ductility due to their freely conducting electrons. Therefore, metals have many benefits for OLED encapsulation, such as flexibility and high thermal conductivity, which enable several issues related to TFE to be resolved for the 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 a Ag film for use in the TFE and investigate the bending characteristics and heat dissipation effect of the DMD-TFE layer. Regarding the use of the TFE, aluminum oxide (Al2O3) was used to create a dielectric film in the DMD structure due to its 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 optical transmittance through light interference and resonance effects between the Al2O3 and Ag films to overcome the low transmittance due to the use of an opaque metal. Second, we investigated the key performance of the DMD-TFE as a gas diffusion barrier 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 comprising 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 changes in the water vapor transmission rate (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

oxide.4,5 The introduction of soft organic materials and the robust nanolaminate system provided superior flexibility to the 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 the basic performance of the TFEs, OLED devices are susceptible to damage by heat stemming from the deposition process and device operation.3,8 Specifically, flexible OLED devices based on plastic substrates with very low thermal conductivity may face serious performance degradation if they do not contain 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 in OLED efficiency and lifetime.9,10 Thick polymer substrates interfering with heat flow out of OLED devices can cause thermal degradation due to heat trapped inside the OLEDs, accelerating the degradation of device efficiency and lifetime.10 To ensure stable OLED operation and lifetime by ensuring thermal conduction out of the device, an additional heat sink system is necessarily required for effective heat transfer, and this becomes increasingly important for high resolution and large-area OLEDs.11 There have been several attempts to solve the thermal issue of OLEDs. Park et al.12 showed 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 reported OLED lifetime improvement by efficient heat dissipation through a metal substrate. However, the use of a rigid metal substrate or metal sheet has several limitations when 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 rigid thick metal sheet on an OLED device can damage the device by physical pressure. A very thin, transparent, and flexible heat sink system is required for practical application of transparent, flexible OLEDs. Unfortunately, there have been no reports on designing a thin heat sink system or a new TFE technique to address major issues related to flexible OLEDs, especially their heat dissipation property. To resolve the reliability issues of flexible OLEDs, in this study, our research aims to combine a TFE and a heat sink system together without a separate heat sink system. Our existing research only considered the basic performance of TFEs by alternating an inorganic layer prepared by an atomic layer deposition (ALD) technique and a solutionprocessed 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 synthesized on ultrathin plastic substrates and fabric substrates.14,15 Among the many flexible electrodes, including metal nanowires, 16 metal grids, 17 graphene,18 carbon nanotubes,19 multilayer electrodes,15 and so forth, in particular, the dielectric/metal/dielectric (DMD) multilayer electrode is a strong candidate for the fabrication of highly robust, transparent, and flexible electrodes due to its combination of good mechanical flexibility and the high conductivity of the 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 can be easily patterned on the microscale using a shadow mask and photolithography, 27063


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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).

Figure 2. (a) Comparison of the normalized conductance curves, vs time, of the sequential 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 3 cm. The normalized conductance curve was obtained from a Ca electrical test. Inset: a schematic of the bending test.

tion 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, on the basis of the optimized thickness of each layer, an optical simulation was conducted using MATLAB software (MathWorks) based on the multilayer matrix formula. There were few 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 that of 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 demonstrated nearly identical optical transmittance to that 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 had 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 was improved in the visible wavelength range [4, 19]. The transmittance improvement of the thickness-optimized DMD structure based on an antireflection condition was achieved by suppressing the surface plasmon coupling of the interfaces between the metal and the dielectric. Dielectric films with a high refractive index help to suppress strong reflection between the metal and the

long-term reliability for practical applications compared to conventional TFE technology based on a simple oxide/organic multilayer structure.

2. RESULTS AND DISCUSSION 2.1. Design of the Thin-Film Encapsulation Structure. The multibarrier structure was designed considering barrier performance, transmittance, flexibility, and heat transfer effects. 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, an Al2O3 single film or Al2O3-based nanolaminate film grown by the ALD technique was mainly used owing to the formation of a highly dense film free of defects.28,29 ALD fabricates high-quality films by minimizing pinholes, which allow the passage of water vapor and oxygen. 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 microscopy (SEM) surface image of the Ag film deposited onto an Al2O3 layer showed formation of the first full coverage film at a thickness of around 15 nm. To investigate the formation of the Ag film on the Al2O3 layer in detail, atomic force microscopy (AFM) analysis and surface sheet resistance measurements were also conducted. (See Supporting Informa27064


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ACS Applied Materials & Interfaces Table 1. Measured WVTR Values of Each Barrier Structure before and after Bending at 3 cm barrier structure Al2O3

Al2O3/Ag

Al2O3/Ag/Al2O3

Al2O3/Ag/Al2O3/S-H

30

30/15

30/15/30

30/15/30/30/120

Al2O3/Ag/Al2O3/Al2O3

Al2O3/Ag/Al2O3/S-H/Al2O3

30/15/30/30

30/15/30/30/120/30

3.72 × 10−5 3.95 × 10−4

8.70 × 10−6 4.46 × 10−5

thickness [nm]

before bending after bending

8.53 × 10−4

7.64 × 10−4

8.64 × 10−5

WVTR [g/m2/day] 7.32 × 10−4

dielectric.30 Therefore, the optical transmittance of the DMDTFE can be improved due to the suppressed reflection of the 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), which are commonly used as dielectric layer materials in DMD structures.26,30 In summary, the proposed DMD-TFE structure showed an average transmittance of 64.6% in the visible range of 400−700 nm, demonstrating its applicability not as an electrode, but as a transparent TFE layer through the use of an Al2O3 layer as an electrical insulator. 2.2. Moisture-Resistant Barrier, Mechanical Properties, and the Reliability of the Thin-Film Encapsulation Method. To provide the key properties required of a flexible TFE for the long-term reliability of flexible OLEDs, Al2O3, an S-H nanocomposite, and Ag were integrated. An encapsulation for OLEDs should essentially have barrier properties that are equivalent to 10−6 g/m2/day for an OLED lifetime of more than 10 000 h 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 2a shows the normalized conductance curve used 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 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 the barrier structure was improved by the sequential formation of each layer, from the initial Al2O3 layer to the final Al2O3/Ag/Al2O3/S-H nanocomposite/Al2O3 structure. The WVTR values of the barrier structures with the sequential formation of each layer are represented in Table 1, and were calculating from the plotted conductance curves of Figure 2. As expected, the barrier properties were significantly improved with the formation of each layer. Because the 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 storage conditions of 30 °C and 90% RH, according to an electrical Ca test. Stress management of the TFE structure is required for longterm 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 the high tensile stress of Al2O3 deposited by ALD at a low temperature.26,32 In contrast, organic layers in multibarriers based on inorganic/organic

layers show 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 provides the DMD-TFE with 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 stress on the 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, a 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

Figure 3. Change in residual stress and membrane force in multilayer structures with formation of each subsequent layer.

of Al2O3 (A), Al2O3/Ag (A/Ag), and Al2O3/Ag/Al2O3 (A/Ag/ A) were 248.5, 338.8, and 268.2 MPa, respectively. These results show that the multilayer structures composed of inorganic layers were experiencing 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. High residual stress in a film serves as the driving force for mechanical failure.35 Therefore, to reduce the residual stress in the multibarrier 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 spincoating, the residual stress of the multilayer structure was sharply reduced to 41.36 MPa. We also calculated the change in 27065


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Figure 4. (a) Ca test of the Al2O3-based multilayer before and after exposure to 60 °C/90% RH for 7 days. (b) The normalized conductance curve of bare Ag film and the Al2O3/Ag/Al2O3 passivated Ag film under harsh conditions of 60 °C/90% RH. The inset displays a schematic for the measurement of conductivity. AFM images of the surface of a single Al2O3 film sample (c) before and (d) after storage in 60 °C/90% RH.

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), and it is important to lower this value below the critical membrane force by the introduction of organic layers with compressive stress.7 Figure 3 shows the change in membrane force and residual stress according to the barrier structure. The presence of the organic layer in the multibarrier structure improved the stress reliability of the barrier structures, resulting in a sharp reduction of the residual stress and the membrane force between 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 of 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 1 order. Because the membrane force of the A/Ag/A/A structure is 3 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 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, and this is because of degradation due to hydrolysis, which causes a phase change from an amorphous structure to a crystalline structure. We measured the real-time resistance change to investigate the degradation of the Al2O3-based multibarrier structure using a Ca test. The degradation level of a 1.5 dyads Al2O3-based multibarrier structure was evaluated through a comparison of WVTR values of the DMD-TFE before and after storage in a humidity chamber of 60 °C/90% RH for 7 days. As shown in Figure 4a, the barrier performance of the 1.5 dyad multibarrier structure was largely degraded by corrosion reactions, showing an increase of the WVTR on a magnitude of 2 orders. To reveal the reason for the degradation of the multibarrier structure, we analyzed the surface of an Al2O3 film upon exposure to a hot and humid environment using an AFM technique (Figure 4c,d). The amorphous Al2O3 film has a very smooth roughness of less than 1 nm, whereas the corroded Al2O3 film showed a 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, the 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 the Al2O3 27066


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Figure 5. (a) Structure of the thin-film encapsulated PHOLED device, (b, c) electrical properties of the PHOLED 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 encapsulated PHOLED on a glass substrate. (f) Cell image of a not-encapsulated PHOLED and a DMD-TFE encapsulated PHOLED over time.

film, the stability of the Ag thin film against 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 4b, the bare Ag film degraded rapidly within 10 h 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 degradation when exposed to a harsh temperature/ humidity level, the Ag thin film was protected by the Al2O3/SH nanocomposite/Al2O3 multilayer structure. According to the results 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 degradation of the Ag film. 2.3. Electrical Characteristics and Lifetime of a ThinFilm Encapsulated Phosphorescence OLED. To demon-

strate the feasibility of the DMD-TFE technology, a PHOLED was fabricated with 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) (8 wt %)/2,2′,2″-(1,3,5-benzinetriyl)-tris(1phenyl-1-H-benzimidazole) (TPBi)/lithium quinolate (Liq)/ aluminum (Al) as a bottom-emitting, microcavity structure.36,37 To prevent the degradation of the PHOLED 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 DMDTFE layer was formed on the device after taping the contact edge of the ITO electrode using Kapton tape with good hightemperature stability. Generally, the OLED devices showed thermal degradation induced by the process temperature and joule heating generated during the operation of the device.8,12 The phosphorescence OLEDs contain CBP, NPB, Ir(ppy)3, and TPBi, which have glass-transition temperature (Tg) values of 62, 96, 359, and 142 °C, respectively.8,39 Therefore, the Al2O3 film was formed at a chamber temperature of 70 °C, which is close to the Tg of CBP and below the Tg of NPB, and which did not cause performance degradation of the PHOLED devices, resulting in good process reliability. 27067


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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 10 000 cd/m2.

We measured the J−V−L characteristics, operational lifetimes, and shelf lifetimes of the encapsulated PHOLED to evaluate the barrier performance and reliability of the DMDTFE layer. The PHOLED device did not show performance degradation after TFE application with a 70 °C 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 °C for the ALD process and UVcuring did not damage the OLED devices. Furthermore, the lifetimes of the PHOLEDs with DMD-TFE and glass-lid encapsulation were 832 and 754 h, respectively. The PHOLEDs had turn-on voltages of around 3.0 V, and the maximum luminance was 74 608 cd/m2. The maximum current efficiency value of the PHOLED was 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 the 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. Although the WVTR value of the DMD-based thin-film encapsulation is nearly 10 times higher than that of the glass-lid encapsulation, the operational lifetime of the PHOLED with the DMD-TFE was longer compared to that of the glass-lid encapsulated PHOLED at the initial luminance of 1000 cd/m2 at room temperature. The half-lifetimes of the PHOLED with the DMD-TFE and glass-lid encapsulation were 832 and 754 h, respectively. Specifically, the half-lifetime of the PHOLED with the DMD-TFE was 10% longer than that of the PHOLED with the glass-lid encapsulation. This interesting result stems 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 of the OLED devices using DMD-TFE, a shelflifetime test was conducted by storage for 80 days at room temperature. As shown in Figure 5f, cell images of OLED devices with TFE clearly showed a difference in device reliability after external exposure to the ambient environment. Although the Al/NPB double layer shows proper gas barrier functionality against oxygen and water vapor, the notencapsulated PHOLED device showed many dark spots after 24 h. 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. 2.4. Lifetime of Thin-Film Encapsulated FOLEDs Due to Heat Dissipation Performance of Thin-Film Encapsulation. To evaluate the heat dissipation ability of the DMDTFE, a FOLED device with the structure of ITO/NPB/tris-(8hydroxyquinoline) aluminum (Alq3)/lithium fluoride (LiF)/Al was fabricated on a glass substrate as a heat generator (Figure 6a). A FOLED with poor current efficiency generates excessive heat by electrical stress and thermionic emission, which results in thermal degradation and short lifetimes of 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. Thinfilm encapsulation with various structures was applied to FOLEDs to compare the heat dissipation ability among the TFE layers. Four types of thin-film encapsulations were undertaken to analyze the thermal conductivity effect according to the presence and thickness of the Ag thin film (Figure 6b).25 The first TFE structure was fabricated using only Al2O3 and an 27068


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differ significantly from those of bulk Ag, especially in terms of 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.41,42 Therefore, the heat dissipation effect of the DMD-TFE is proportional to the Ag film thickness, with a trade-off between the optical transmittance and the heat dissipation effect. To demonstrate the association between OLED lifetime and the real-time temperature measurement results, the operational lifetimes of the FOLEDs were measured at room temperature and at an initial luminance of 10 000 cd/m2 by applying four types of thin-film encapsulations to the FOLEDs. The tested FOLEDs showed rapid degradation within 10 h 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 a Ag film was inserted into the TFE. Therefore, the TFE should be designed considering the heat dissipation effect as well as barrier quality, transmittance, and flexibility for the realization of flexible displays.

S-H nanocomposite with very low thermal conductivity (TFE without Ag). The second TFE structure incorporated 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/(m K) at a temperature of 300 K, which is the highest thermal conductivity of any metal.35 This is one of the reasons that we utilized Ag film as the metal in the DMDTFE 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 Ag film thickness. Finally, a TFE with a flexible graphite heat spreader of 600 W/(m K) (eGRAF Spreadershield) was prepared to compare the heat dissipation effect of the heat sheet used in the product. Figure 6c,d shows the realtime temperature measurements and the lifetimes of the encapsulated FOLEDs, respectively. Generally, heat can only be transferred though 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 radiation barrier by reflecting the light of the infrared wavelength region due to the increasing absorption coefficient of the metal film with an increase in wavelength. Therefore, for the DMD structure, it was estimated that there was almost no heat dissipation effect by thermal radiation. However, the main heat transfer occurs mainly by thermal conduction through the DMD-TFE by contact between the very thin films. We calculated the optical reflectance of the DMD structure in the wavelength range of 300−1000 nm by simulation (see Supporting Information, Figure S3 for the calculated transmittance of the Al2O3 (30 nm)/Ag (x nm, x = 15, 100 nm)/ Al2O3 (30 nm) structure). The optical reflectance of the DMD structure increased with an increase in Ag film thickness. The reflectance value of Al2O3(30 nm)/Ag(100 nm)/Al2O3(30 nm) was nearly 100% at a wavelength of 1000 nm. Likewise, the reflectance value of Al2O3(30 nm)/Ag(15 nm)/Al2O3(30 nm) was estimated to be closer to 100% at wavelengths of more than 1 μm due to the increasing Ag absorption coefficient with an increase in wavelength. In other words, both the Al2O3(30 nm)/Ag(100 nm)/Al2O3(30 nm) structure and the Al2O3(30 nm)/Ag(15 nm)/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 thermal radiation effects but thermal conduction between the thin films due to the thermal radiation barrier properties of the DMD structure. The real-time temperatures of all encapsulated FOLED devices were saturated in less than 1 min after an 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 6c). As mentioned earlier, although the TFE structures with a Ag film reflect the infrared light, the thermal conduction mechanism due to contact between the thin films mainly transfers and spreads heat through the TFE layer. The differences in heat dissipation effect among the 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 had a tendency to be improved. In general, the physical properties of a Ag thin film

3. CONCLUSIONS 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 an optical transmittance of more than 60% in the visible range despite an opaque Ag layer in the TFE structure. The water vapor transmission rate of the 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 cycle 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 the 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 for 80 days at room temperature. Finally, the proposed DMDTFE was analyzed with respect to its heat transfer property and showed a thermally conductive property by incorporation of a Ag film with high thermal conductivity. The various TFE structures showed differences in heat dissipation ability according to the presence and thickness of the Ag film, which was consistent with the OLED lifetime measurement results. The DMD-TFE with a heat dissipation effect showed a thermal stress relaxation effect on the OLED lifetime. Therefore, our work shows the remarkable potential of this new flexible encapsulation technology to achieve flexible and wearable OLEDs for highly reliable displays designed considering flexibility, stress relaxation, barrier quality, and heat dissipation. 27069


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wafer curvature before and after the deposition process. A thin-film measurement system (FSM 500TC) with laser scanning technology measured 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 4.6. Temperature-Profile and IR Image Measurement. The real-time temperatures and images of the operating OLEDs were measured every second and captured by an IR thermal image camera device (SC-5000, FLIR) at the Korea Basic Science Institute.

However, to completely resolve the technical issues raised above, further research on the fabrication of flexible and thermally conductive TFEs beyond the proposed DMD-TFE is necessary.

4. EXPERIMENTAL SECTION 4.1. Preparation of Thin-Film Encapsulation Layer. DMDTFE was deposited on a 125 μm thick poly(ethylene terephthalate) (PET) substrate after the PET substrate was cleaned with acetone, isopropyl alcohol, and deionized water in an ultrasonic sonicator. The S-H nanocomposite layer, Al2O3, and Ag were coated by a spin-coater, ALD, and thermal evaporation, respectively. Generally, the ALD method requires a long process time due to a long purging time and atomic monolayer growth per cycle for a highly dense film. An Al2O3 ALD film was deposited using the chemical reaction between a TMA precursor and a H2O reactant at a low temperature of 70 °C, which did not cause OLED degradation (Lucida D 100, NCD). The growth rate of Al2O3 at a chamber temperature of 70 °C was 0.83 Å/cycle; the purge time and precursor exposure time were 10 and 0.2 s. A 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 a tungsten boat at a constant deposition rate of 2 Å/s to deposit a 15 nm thick thin film. Finally, spin-coating conditions for coating the 120 nm thick S-H nanocomposite layer were 4000 rpm for 3 s and acceleration for 30 s. The coated S-H nanocomposite layer was cured by UV light with a wavelength of 365 nm for 100 s. Thus, the combination of three materials with different effects made it possible to fabricate the multifunctional hybrid encapsulation. 4.2. 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 microcavity effect for increasing the external quantum efficiency and the phosphorescent emission layer (EML) for increasing the internal quantum efficiency.36−38,43 The PHOLED was composed of a 5 nm thick MoO3 layer as a hole injection layer, a 70 nm thick NPB layer 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 as an electron transport layer, a 1 nm thick Liq as an electron injection layer (EIL), and a highly reflective Al cathode.44,45 4.3. Thin-Film Encapsulated FOLED Fabrication. Bottomemitting 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 was composed of a 50 nm thick NPB layer as a HTL, a 50 nm thick Alq3 layer as an EML, a 1 nm thick LiF layer as an EIL, and a 100 nm thick Al layer as a cathode, which were all deposited by a thermal evaporation method under a vacuum of 10−6 Torr.8 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. 4.4. OLED Performance and WVTR Measurement. The electrical−optical characteristics of the thin-film encapsulated OLEDs were measured using a source meter (Keithley 2400) and a spectrophotometer (CS-2000, 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 °C and 90% RH for WVTR measurement (see Supporting Information, Figure S4 for a schematic diagram of the Ca test cell for the electrical Ca test). 4.5. Stress Measurement in Thin Film Using Wafer Curvature Measurement. The residual stress in the thin film or multilayer structure was determined by measuring the change in the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06076. 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 (30 nm)/Ag (x nm, x = 15, 100 nm)/Al2O3 (30 nm) structure (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-42-350-3482. Fax: +82-42-350-8082. ORCID

Kyung Cheol Choi: 0000-0001-6483-9516 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 through National Research Foundation (NRF), Korean Ministry of Science, ICT & Future Planning (MSIP) [Grant No. NRF-2017R1A5A1014708] and 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].

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REFERENCES

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