Functional Design of Highly Robust and Flexible Thin-Film

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Functional Design of Highly Robust and Flexible Thin-Film Encapsulation Composed of Quasi-perfect Sublayers for Transparent, Flexible Displays Jeong Hyun Kwon, Yongmin Jeon, Seungyeop Choi, Jeong Woo Park, Hyuncheol Kim, and Kyung Cheol Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14040 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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ACS Applied Materials & Interfaces

Functional Design of Highly Robust and Flexible Thin-Film Encapsulation Composed of Quasi-perfect Sublayers for Transparent, Flexible Displays

Jeong Hyun Kwon†, Yongmin Jeon†, Seungyeop Choi†, Jeong Woo Park†, Hyuncheol Kim†, and Kyung Cheol Choi*,†

†

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

Keywords. Bio-inspiration, Water vapor transmission rate (WVTR), Nanolaminate structure, Thin film encapsulation, Organic light-emitting diodes (OLEDs)

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Abstract In this study, a structurally and materially designed thin film encapsulation is proposed to guarantee the reliability of transparent, flexible displays by significantly improving the barrier properties, mechanical stability, and environmental reliability, which are essential for organic light-emitting diode (OLED) encapsulation. We fabricated a bio-inspired, nacre-like ZnO/Al2O3/MgO laminate structure (ZAM) using atomic layer deposition for the microcrack toughening effect. The ZAM film was formed with intentional voids and defects through the formation of a quasi-perfect sublayer, rather than the simple fabrication of nanolaminate structures. The 240 nm thick ZAM-based multi-barrier (ZAM-TFE) with a compressively strained organic layer demonstrated an optical transmittance of 91.35% in the visible range, an extremely low water vapor transmission rate of 2.06 × 10−6 g/m2/day, mechanical stability enduring a strain close to 1% and residual stress close to zero, showing significant improvement of key TFE properties in comparison to an Al2O3-based multi-barrier. In addition, the ZAM-TFE demonstrated superior environment resistance without degradation of barrier properties in a severe environment of 85°C/90% relative humidity (RH). Thus, our structurally and materially designed ZAM film has been well optimized in terms of its applicability as a gas diffusion barrier as well as its mechanical and environmental reliability. Finally, we confirmed the feasibility of the ZAM-TFE through application in OLEDs. The low temperature ZAM-TFE technology showed great potential to provide highly robust and flexible TFE of TFOLEDs. 1. INTRODUCTION Encapsulation and passivation processes are necessary to protect organic electronics from their environment.1–3 In particular, transparent, flexible organic light-emitting diodes (TFOLEDs), which are considered next-generation displays, are composed of organic materials, metals, and metal compounds that are sensitive to moisture and oxygen permeating through plastic substrates and top electrodes.4,5 To block these reactive gases from permeating TFOLEDs, transparent and flexible thin film encapsulation (TFE) equivalent to water vapor transmission rates (WVTR) on the order of 10-6 g/m2/day must be applied to plastic substrates and OLEDs.5,6 TFE by alternately stacked inorganic/organic layers, called barix encapsulation technology, is a new alternative for TFOLEDs. For practical application to TFOLEDs, TFE must have key properties, such as a very low gas diffusion barrier, superior mechanical characteristics, and environmental reliability that does not degrade in a harsh environment.2,7 Barrier and mechanical properties are crucial in the design of encapsulation technology, where efficiency and the lifetime of TFOLEDs are of concern. In general, although inorganic layers have superior barrier properties compared to organic layers, they break easily when they are subjected to external and internal stress. In contrast, organic layers are very flexible and smooth and have a very low elastic modulus and hardness, but they provide many paths through which moisture and oxygen can permeate. The key properties of TFE can be effectively enhanced through supplementation between layers. Nevertheless, it is difficult to achieve all of the key properties of TFE to meet the requirements of TFOLEDs due to the limitations of the physical properties of the inorganic layers themselves.8 Recently, TFE layers based on inorganic layers, such as Al2O3 or MgO, have been fabricated by spin coating organic layers together.9–11 These multi-barriers are fabricated with more than three dyads of inorganic/organic stacks to achieve encapsulation properties suitable for OLEDs. Although multi-dyad barrier structures are able


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to achieve barrier properties of water vapor transmission rates on the order of 10-5 g/m2/day, they require a long, complex fabrication process, and their thickness is increased by multiple coatings of the organic layer.11 In particular, such TFE layers lose their barrier properties in bending tests due to the brittleness of the inorganic layers. Brittle inorganic layers are the origin of mechanical failure, which results in degradation of the barrier properties of TFE. Therefore, a significant improvement in the barrier and mechanical properties of the inorganic layer is necessary to fabricate highly flexible and reliable TFE for practical application to TFOLEDs. In addition to studies on flexible TFE fabrication, several studies have reportedly used a buffer layer or ultrathin substrates to fabricate highly flexible OLEDs. Ultra-thin flexible substrates have been mainly utilized to reduce the strain applied in thin films. As is generally known, the thinner a flexible substrate, the smaller the strain on a bent film on the substrate. However, the handling of an ultra-thin flexible substrate is very difficult at a thickness of less than 20 µm, and it makes device operation unstable. Han et al. reported a solution to control the neutral axis position by inserting a 109-µm-thick buffer layer into OLEDs.12 The buffer layer allowed the neutral axis to be placed in an OLED device. However, the additional buffer layer was very thick, and it was not easy to control the thickness. Thus, we tried to achieve a highly flexible OLED by improving the physical properties of the encapsulation itself without using additional processes such as the two methods described above. We significantly improved the key properties of the TFE by mimicking the structures of materials observed in nature.13 In nature, many structural materials, such as tooth enamel, bones, shells, fish scales, and so forth are mostly composed of fragile minerals, but contrary to our expectation, these materials are tough and durable.14–16 For example, nacre, which is the inner shell layer of mollusks, had high fracture toughness due to its unique structure, although nacre is composed of 95 wt% aragonite and 5 wt% organic materials.15,16 In other words, the exceptional mechanical properties of nacre are due to the formation of its layer-by-layer structure, which causes crack deflection and crack arrest by a microcrack toughening effect.13 Thus, we were inspired by the natural structures of various materials and set out to develop a bio-inspired gas diffusion barrier film. These structural designs can provide significant improvement in mechanical properties beyond the brittleness of previous oxide films. This paper presents TFE technology that exploits the structural features observed in natural materials to achieve the microcrack toughening effect. To mimic the layer-by-layer structure of nacre, the ALD process, which rapidly forms uniform, perfect films through precise thickness control at the angstrom-level, was used.1,2 In this study, a laminate structure was formed by the sequential deposition of ZnO, Al2O3, and MgO sublayers alternatively. The ALD deposition of Al2O3, ZnO, and MgO thin films is explained in detail in the experimental section. The sequential structure of ZnO, Al2O3, and MgO sublayers (ZnO/Al2O3/MgO) composes one period. The multi-periodic laminate structure composed of quasi-perfect ZnO, Al2O3, and MgO sublayers (ZAM) with intentional voids and defects achieved improved barrier performance with increasing periods due to the highly dense, defect-decoupled, and amorphous film formation (Figure 1a). It is very difficult to control the size or diameter of fine voids using ALD. In fact, all thin films deposited by physical vapor deposition (PVD) and chemical vapor deposition (CVD) have fine defects that allow the passage of moisture and oxygen. However, CVD techniques inhibit defect generation in thin films much more easily than PVD techniques. It has

been

reported that the defect densities in an ALD deposited Al2O3 layer decreased linearly with thickness until a critical thickness.17 Thus, we observed the cycle condition for forming the first ultra-thin film maximizing the defect densities in sublayers before the formation of a perfect film with full coverage through TEM analysis.



Finally, we fabricated ZAM films showing superior barrier performance. When quasi-perfect sublayers are formed, the number of interfaces is the greatest. The defect-decoupling effect by the layer-by-layer structure means that the positions of defects or pinholes in films are changed by the formation of layer stacks. This defect control makes the penetration of water vapor and oxygen more difficult. Generally, as a thin film becomes thicker, the initial defects inside a single layer grow long. However, a nanolaminate (NL) film with a multi-interface system enables the positions of defects be separated. In addition to the layer-by-layer structural design, the ZAM thin film was intended to improve the reliability and properties of the encapsulation through the material design. The Al2O3 sublayers were used as the main gas diffusion barrier. Specifically, the ZnO and MgO sublayers formed aluminate phases with the Al2O3 sublayers at the interfaces, enabling densification and corrosion delay of the ZAM film. In addition, unlike conventional NL structures consisting of two different layers, our NL structure consisting of three different layers contributes to the reduction of the Al2O3 ratio in the barrier film. The optimized ZAM film with a thickness of around 50 nm achieved a very low WVTR value of (3.60 ± 1.74) × 10−5 g/m2/day with the structural and material design. In addition, we fabricated a stress-minimized ZAM/S-H nanocomposite/ZAM (ZAM-TFE) multi-barrier structure with the insertion of an organic layer. The organic S-H nanocomposite was a silica nanoparticle-embedded organic/inorganic hybrid (S-H) nanocomposite, exhibiting high thermal stability and low coefficient of thermal expansion of 9.6 ppm/°C.18 Our results confirmed that the ZAM-TFE possessed significantly improved barrier and mechanical properties compared with Al2O3/S-H nanocomposite/Al2O3 (Al2O3-TFE). In addition, the ZAMTFE showed remarkable environmental reliability compared to neat Al2O3. The ZAM-TFE-encapsulated OLEDs exhibited excellent reliability through a shelf lifetime test without degradation of their electrical characteristics. Furthermore, the bending characteristics of flexible OLEDs on a 125-nm-thick polyethylene terephthalate (PET) substrate were ensured. The flexible OLEDs showed identical characteristics at a tensile strain of 0.78 %. Overall, with the optimization of the ZAM film, we demonstrated the integration of ZAM-TFE to flexible OLEDs. 2. Results and Discussion


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Figure 1. a) Schematic of the ZAM NL structure and defect-decoupling system by the layer-by-layer structure. b) Schematic of the nacre cross-section and bent ZAM film with energy dissipation system for crack deflection and crack arresting. 2.1. Design and moisture-resistant barrier properties of the ZAM laminate film and ZAM-TFE multilayer. Barrier properties, referred to as the WVTR, were measured using a calcium corrosion test (Ca test).19,20 A Ca test is an effective measurement method for comparing the WVTR values of barrier structures. All WVTR evaluations were conducted in an environment of 30 °C and 90 % RH. We fabricated the bio-inspired, nacre-like ZAM film using ALD, which allows thickness control at the angstrom level. To confirm the optimal cycle ratio of ZnO/Al2O3/MgO for forming a quasi-perfect film with intentional voids, cross-section images of the ZAM structure were obtained by high-resolution transmission electron microscopy (TEM) at resolutions down to 2 Å.

Figure 2. TEM cross-sectional image and FFT of ZAM thin films according to the cyclic ratio of ZnO:Al2O3:MgO. a) 7:3:2. b) 12:6:5. c) 20:12:10. d) 29:24:20. Figures 2a, b, c, and d show TEM images and fast Fourier transform (FFT) diffraction patterns with cyclic ratios of the ZnO/Al2O3/MgO of 7/3/2, 12/6/5, 20/12/10, and 29/24/20, respectively. Although the deposition rate per cycle for ZnO was higher than those for Al2O3 and MgO, more cycles were required to form the desired thickness of the ZnO thin film due to the chemical surface etching of the ZnO film by the trimethylaluminum



(TMA) precursor for the Al2O3 deposition.21 We observed the etching effect of ZnO thin films by exposure to TMA using x-ray photoemission spectroscopy (XPS) analysis (See Supporting Information, Figure S1). When the cyclic ratio of the ZnO/Al2O3/MgO was 12/12/10, Zn elements were scarcely detected in the ZAM film. Therefore, the number of cycles for the ZnO sublayer was increased considering the etching effect. Figures 2a and b show ZAM films composed of sublayers less than 1 nm, revealing an amorphous blend membrane structure, as seen in the FFT images, while Figures 2c and d, which show ZAM films composed of sublayers of more than 1 nm, confirm the layer-by-layer structure. In general, Al2O3 films grown by ALD have an amorphous state regardless of thickness.2 Therefore, the diffraction pattern in the FFT image was clear because of the crystal growth of ZnO and MgO sublayers. The FFT images shown in Figures 2c and d become clearer and clearer with the growth of the sublayers. As seen in Figure 2c, the ZAM film with a cyclic ratio of 20/12/10 showed that each sublayer forms a quasi–perfect film, resulting in a film structure close to the layer-by-layer structure.

Figure 3. a) WVTR values according to the cyclic ratio of ZnO:Al2O3:MgO. b) Changes in WVTR values according to thickness increase. c) Conductance curve versus time according to inorganic layers using Ca test. d) TEM cross-sectional image of the ZAM-TFE. e) EDS analysis of a ZAM film. Our explanations were supported by the Ca test results showing WVTR changes in 50-nm-thick ZAM films according to the cyclic ratio (Figure 3a). We measured the WVTR values on approximately 50-nm-thick ZAM films with variation of the sublayer thickness (cyclic ratio) to investigate the correlation between the barrier structure and the barrier properties (See Supporting Information, Table S1). ZAM films that were grown at ALD cyclic ratios of 7/3/2 and 12/6/5 where WVTRs on the order of 10-4 g/m2/day were achieved showed degradation


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of their barrier properties due to the formation of an unstable blend membrane. However, ZAM films with ALD cyclic ratios of 20/12/10 and 29/24/20 achieved WVTR values on the order of 10-5 g/m2/day. In particular, the ZAM structure with a cyclic ratio of 20/12/10 showed the best barrier properties of 1.92 × 10-5 g/m2/day. The superior barrier properties of the ZAM film structure can be attributed to the following three factors. First, the ZAM film produced an amorphous state by inhibiting the crystallization of each sublayer by the formation of the layer-by-layer structure (See Supporting Information, Figure S2). Second, the defect decoupling effect with multi-interface systems separated the defects and pinholes of the previous sublayer. Third, the formation of aluminate phases at the ZnO/Al2O3 and Al2O3/MgO interfaces made the ZAM film highly dense. Therefore, we optimized the cyclic ratio of the ZAM film based on the TEM analysis and WVTR values to design a highly robust and reliable TFE. In this study, ZAM films were fabricated with a cyclic ratio of 20/12/10. In addition, energy dispersive spectrometry (EDS) analysis demonstrated that zinc, aluminum (Al), and magnesium (Mg) atoms were uniformly distributed in the ZAM film (Figure 3e). After the cyclic ratio of the ZAM film was optimized, we measured its WVTR values with increasing numbers of Z/A/M periods. In general, as the thickness of an inorganic film becomes thicker on a plastic substrate, the residual stress applied inside the film increases.22 The increased stress causes cracks to form in the film, facilitating water vapor and oxygen penetration into the film. Therefore, finding the optimal thickness of an inorganic film is very important with respect to barrier properties. Our measurement results confirmed that the ZAM film achieved the lowest WVTR value of 1.92 × 10-5 g/m2/day with 14 periodic repetitions (Figure 3b). However, the ZAM film started to show degradation of barrier properties at thicknesses greater than 17 Z/A/M period repetitions. As previously mentioned, our results are consistent with the previous explanation. Notably, the WVTR value of the optimized ZAM film corresponds to the previously reported lowest WVTR values of inorganic films based on single-layer or NL structures (e.g., Al2O3, Al2O3/ZrO2, and Al2O3/SiO2) when the thickness of the ZAM film was around 50 nm.1,2,7,8,23,24 Therefore, all evaluations of ZAM-based barrier structures were conducted using ZAM films with a 14-period repetition of the Z/A/M unit structure with the cyclic ratio of 20/12/10. We conducted Ca tests to verify the superiority of the ZAM film by comparing 60-nm-thick inorganic layers, such as an Al2O3/MgO (1 nm/1 nm) laminated structure (AM), an Al2O3/ZnO (1 nm/1 nm) laminated structure (AZ), a ZnO/MgO (1 nm/1 nm) laminated structure (ZM), and Al2O3, ZnO, and MgO thin films. WVTR values were calculated using the gradient of the conductance change in the graph of the normalized conductance curve over time (See Supporting Information, Table S2). The Ca test results clearly showed differences in the conductance gradient between the inorganic layers (Figure 3c). Although the Al2O3 ratio in the ZAM film was lower than that in the AM film, the 50-nm-thick ZAM demonstrated better barrier properties than 60-nm-thick AM, AZ, and ZM thin films, as shown in Figure 3c. In particular, the ZAM film showed better barrier performance than AM and AZ thin films with Al2O3 sublayers despite the relatively lower ratio of the Al2O3 sublayer, which seems to be due to the more effective decoupling system by three different sublayers and the densification of the ZAM film by chemical aluminate phases at the interfaces. The lower Al2O3 ratio in the film resulted in greater improvement in the environmental reliability. The WVTR of the 60-nm-thick ZM film was a relatively high value of 1.87 × 10-3 g/m2/day, despite the introduction of a laminate structure, compared to the WVTR value of the 50-nm-thick ZAM film. This result fully reveals the need for Al2O3 thin film in the fabrication of laminate films. Al2O3 sublayers enabled the fabrication of an effective gas diffusion barrier film due to their highly dense and pinhole-



free film quality, which also formed aluminate phases in the interfaces with MgO sublayers in the ZAM film.

Figure 4. XPS spectra of Al2O3, MgO, ZnO, and ZAM of a) Al 2p, b) Mg 1s, and c) Zn 2p core levels.

The chemical composition of the interfaces between the Al2O3/MgO sublayers was observed by XPS analysis. The Al 2p and Mg 1s core-level spectra of the Al2O3, MgO, and ZAM thin films were studied. To remove carbon-based impurities from the film surfaces, the surface of each thin film was etched by Ar ions until the carbon element content reached zero percent. The XPS results revealed the formation of aluminate phases at interfaces between Al2O3/MgO and Al2O3/ZnO through the shift of the core-level peak. The aluminate phase or bonding at interfaces was formed for the following reason; the binding energy of the Al 2p and Mg 1s core levels significantly changed from 74.05 (Al2O3) to 73.25 (ZAM) eV and from 1302 (MgO) to 1302.95 (ZAM) eV, respectively (Figures 4a and b). It seems most likely that the core-level shifts resulted from the formation of an aluminate phase, whereby Al tends to gain an electron and Mg tends to donate an electron by the difference of the electronegativities between Al (1.61) and Mg (1.31).25,26 At the Al2O3/ZnO interfaces, chemical bonding also occurred due to the etching effect of the ZnO surface by TMA precursors. Through the etching effect, Zn-Al-O bonding was formed, where the shifts of Zn 2p core levels measured by XPS analysis showed the presence of Zn-Al-O bonding (Figure 4c).27 Remarkably, these aluminate phases or bonding at interfaces with Al2O3 sublayers contributed to the formation of the highly dense and reliable ZAM film. Thus, MgO and ZnO sublayers work effectively in the fabrication of highly dense gas diffusion barrier film by forming chemical bonding with the Al2O3 sublayers at the interfaces. In the next step, we fabricated a stress-minimized, reliable and flexible ZAM-TFE with an S-H nanocomposite inserted between two ZAM thin films to improve the reliability and durability of TFOLEDs. A cross-sectional


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TEM image of the ZAM-TFE is shown in Figure 3d. The organic S-H nanocomposite layer exhibited a WVTR of around 0.58 g/m2/day and the averaged transmittance of about 90% in the visible wavelength.18 In addition, the advantageous S-H nanocomposite layer had intrinsic compressive stress, where the compressive strain can relax the tensile stress in an ALD-grown oxide film. The S-H nanocomposite contained silica nanoparticles, which function as obstacles of water vapor and oxygen diffusion. For comparison with the ZAM-TFE, we also evaluated an Al2O3-TFE with an organic layer inserted between two Al2O3 films. While the ZAM-TFE achieved the averaged transmittance of 91.35 ± 1.5 % in the visible wavelength and an extremely low WVTR of (3.41 ± 1.35) × 10-6 g/m2/day, which is equivalent to the barrier properties of glass-lid encapsulation, the Al2O3-TFE achieved a comparatively high WVTR value of (7.26 ± 0.33) × 10-5 g/m2/day due to the relatively poor barrier properties of the Al2O3 film compared to the ZAM film. Remarkably, the 240-nm-thick ZAM-TFE showed a larger difference than a magnitude of one order in WVTR, compared to an Al2O3-TFE with the same thickness. Notably, the 50-nm-thick ZAM single-layer film demonstrated better barrier properties than the Al2O3-TFE multilayer-film. These WVTR differences were due to the clear difference between the barrier properties of ZAM and Al2O3.

Figure 5. a) WVTR values of 50-nm-thick Al2O3 and ZAM thin films at 30 °C and 90% RH after storage at elevated humidity/temperature levels. b) The change of conductance curves of Al2O3-TFE and ZAM-TFE structures depending on measured environment change. 2.2. Environmental stability of the ZAM-based barrier structure. It is very important to improve the environmental reliability of the TFE layer itself for the long-term stability and reliability of OLEDs. In this paper, the environmental reliability of the TFE layer means how well the original barrier properties of the barrier film are maintained at high temperatures and high humidity. The environment-induced degradation of barrier films results in the degradation of the barrier properties of those films. In other words, it is important to block or delay the degradation of the barrier films responsible for the main gas permeation barrier properties. Recently, there have been several reports on the environmental reliability of gas diffusion barrier films (e.g., Al2O3).1,2,7,20,23, It is well known that Al2O3 film is easily degraded at high temperatures and high humidity due to crystallization by the formation of -OH groups through the chemical reaction between Al2O3 and water vapor.2,23,28 It is very important to block or delay the corrosion of the Al2O3 film itself. Kim et al. presented an Al2O3/TiO2 laminate film for the passivation of thin-film transistors.1 After single Al2O3 and an Al2O3/TiO2 laminate structure were immersed in water at 90 °C, the surface of the single Al2O3 and the Al2O3/TiO2 laminate



was analyzed by atomic force microscopy and scanning electron microscopy. However, changes in WVTR values caused by immersion in water were not presented. Li et al. also reported that MgO film could absorb water vapor penetrating an Al2O3/MgO laminated film to protect against Al2O3 corrosion.23 However, they did not report on the degradation of barrier properties of thin films after storage in a harsh environment. Therefore, we first tried to evaluate the practical environment stability by measuring the WVTR values of ZAM and Al2O3 thin films before and after storage for 24 hours under various harsh conditions. In other words, we confirmed the correlation between film degradation and barrier property degradation by exposure to environmental conditions. In this study, all WVTR values were measured at a climate condition of 30°C/90% RH to minimize film degradation for the Ca test due to the rapid degradation of the neat Al2O3 thin films at severe temperature/humidity levels.1,8,23,28 After being stored for 24 hours in a harsh environmental conditions of 30 °C/90% RH, 60 °C/90% RH, or 85 °C/90% RH, each stored film was evaluated by a Ca test for WVTR calculation. The degradation levels of the barrier properties of the Al2O3 and ZAM thin films according to environment conditions are shown in Figure 5a. As expected, as the temperature of the storage environment increased at a fixed 90% RH, the degradation of the thin films sharply increased. Although the ZAM film showed a WVTR increase with exposure to high temperature and humidity, the ZAM film ensured much better environmental reliability through structural and material design compared to the neat Al2O3 thin film. Notably, the WVTR of the ZAM film was two orders of magnitude lower than that of the neat Al2O3 film after storage for 24 hours at 60 °C/90% RH. Therefore, the reduction of the Al2O3 component ratio to one-third the level with the laminate structure, the formation of water-resistant aluminate phases, and the insertion of a water-absorbing MgO sublayer contributed to the improvement of the environmental reliability of the ZAM film. As previously stated, more optimized multilayers were fabricated using the S-H nanocomposite, so we compared the environmental reliability of the ZAM-TFE and the Al2O3-TFE. Figure 5b shows the conductance curves of the multilayers according to measurement environments with variation of the climate conditions from 30°C/90% RH to 85°C/90% RH obtained by Ca tests. After the multilayers were stored for 24 hours at a 60 °C/90% RH, the Ca test of the multilayers was initially conducted under environmental conditions of 30 °C/90% RH. During the measurement, the environmental conditions were changed to 85 °C/90% RH for the accelerated degradation test. The WVTR values of the ZAM-TFE were 7.63 × 10-5 g/m2/day at 30 °C/90% RH and 1.18 × 10-3 g/m2/day at 85 °C/90% RH. On the other hand, the Al2O3-TFE showed rapid degradation due to the corrosion of the Al2O3 film as expected. Remarkably, the elevated temperature/humidity level resulted in a significant degradation of the Al2O3-TFE within a few hours, whereas the ZAM-TFE showed better environmental reliability against very severe temperature/humidity with no significant degradation. Thus, the results demonstrated that we successfully fabricated an environmentally reliable TFE based on ZAM film through structural and material design. 2.3. Mechanical properties of ZAM and ZAM-TFE structures. To confirm the mechanical reliability of the TFE layers, we evaluated the changes in WVTR before and after bending tests. The mechanical reliability of the barrier films was confirmed by comparing WVTR changes before and after bending of the barrier films according to strain. We evaluated the mechanical properties of multilayers (Al2O3-TFE and ZAM-TFE) containing an S-H nanocomposite layer. Bending tests were conducted at strains of 0.31, 0.63, 0.89, 1.04, and 1.25% whose bending radii were 20, 10, 7, 6, and 5 mm, respectively. Figure 6 and Table 1 show changes in the


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WVTR value of the multi-barrier vs. bending strain. A sharp WVTR increase in the Al2O3-TFE was observed under a 0.63% tensile strain due to crack formation in the Al2O3 thin films, which was considered the critical strain with mechanical failure. In striking contrast to Al2O3-TFE whose WVTR sharply increased by more than three orders of magnitude under a 0.63% tensile strain, the barrier properties of the ZAM-TFE maintained the WVTR value on the order of 10-6 g/m2/day even after a tensile strain of 0.63% was applied. However, a slight degradation of the barrier properties of the ZAM-TFE began to appear under the 1.04% tensile strain, which is considered as the onset of crack formation. Under a 1.25% tensile strain, the ZAM-TFE underwent mechanical failure, showing a sharp increase in WVTR values. We also applied a compressive strain in the TFE layers because a real flexible display should be able to be bent in both directions, inducing tensile or compressive strain. In the case of the Al2O3-TFE, no noticeable WVTR increase was observed until 0.89% compressive strain was applied, whereas an abrupt increase in the WVTR value was observed at 0.63% tensile strain due to the Al2O3 cracking. Similarly, WVTR changes in the compressively bent ZAM-TFE were less than those of the ZAM-TFE under tensile bending. This indicate that the TFE layer was more critical to tensile strain than compressive strain in bending, which agrees with the results of previous reports.29, 30 The difference in the level of WVTR variation according to the type of stress can explained by stress concentration.31 Stress concentration means that the applied stress may be fairly low in the thin film, whereas the effective local stress in defects of the thin film is very high. The stress concentration effect in the thin film occurs for the applied tensile stress mode, while no such stress concentration occurs for the compressive stress mode. Thus, both the ZAM-TFE and Al2O3-TFE are more robust to compression than tension. Note that the mechanical reliability of the ZAM-TFE was much better than that of the Al2O3-TFE because the mechanical limits of the inorganic layers were improved through the structural design. It also should be noted that the mechanical properties of ZAM-TFE are better than those of barrier film structures that have previously demonstrated superior mechanical properties.8,12,27,33–35 The multi-interfacial ZAM film with intentional voids and pinholes, where pinholes and voids generate highly tortuous crack propagation path, can dissipate the applied stress more efficiently than an Al2O3 single layer. As shown in Figure 1b, the continuous multilayered inorganic structure with relatively weak interfaces exhibits improved mechanical properties for crack deflection and crack arresting. Thus, our structurally and materially designed ZAM film was optimized in terms of barrier and mechanical properties.



Figure 6. Degradation of WVTR values of Al2O3-TFE and ZAM-TFE according to applied a) tensile and b) compressive strains. Table 1. Measured WVTR of Al2O3-TFE and ZAM-TFE according to applied strain.

In addition, we measured the mechanical properties of inorganic layers using nano-indentation and the wafer curvature method.14,15,24 The elastic modulus (E) and hardness (H) of each film were calculated using nanoindentation measurement (See Supporting Information, Figure S4a). The results revealed that the ZAM film had improved mechanical properties due to the laminate structure, recording average E and H values of 34.9 and 6.65 GPa, respectively. In general, the E value of the ZAM film is expected to be a value of 137.94 GPa, corresponding to the average E value of ZnO (107.42 GPa), Al2O3 (134.36 GPa), and MgO (172.05 GPa) films. However, the hierarchical ZAM film showed much lower E and H values compared to E and H values of ZnO, Al2O3, and MgO films with identical thickness. The significant improvement in mechanical properties is attributed to the inverse Hall-Petch softening effect. Although thick ZnO and MgO thin films with a certain thickness or more show crystalline growth, the thickness control of sublayers makes the ZAM film more flexible because of the structural transition from nanocrystalline to amorphous. Residual stress is the important criterion used to evaluate the mechanical properties of thin films for long-term reliability. Figure S4b shows the residual stress result of each 50-nm-thick layer measured by the wafer curvature method. The Al2O3, ZnO, and MgO thin films showed tensile stresses of 309.35, 260.56, and 80.78 MPa, respectively (Supporting Information). In general, tensile stress is critical to OLED lifetime and efficiency.29 Thus, the tensile stress of the TFE needs to be minimized to be as close as possible to zero stress to improve mechanical reliability. Notably, the fabricated ZAM thin film showed tensile stress under 117.2 MPa, which is one third lower than that of Al2O3 and MgO with the introduction of ZnO film and the laminate structure. In addition, we fabricated the ZAM-TFE with a compressively strained organic layer to improve overall mechanical reliability. The multi-barrier system allowed us to sharply lower the residual stress in the TFE structure by compensating for tensile stress in the ZAM film. Thus, the ZAM-TFE demonstrated an overall residual stress close to zero stress, with a reduced tensile stress of 30.37 MPa. 2.4. Electrical Characteristics and Shelf-Lifetime of a Thin- Film Encapsulated Phosphorescent OLED. To demonstrate the potential of the ZAM film for practical application in OLEDs as a TFE, we fabricated bottomemitting red phosphorescent OLEDs (PhOLEDs) composed of 4, 4’-bis(N-phenyl-1-naphthylamino)biphenyl (NPB, 50 nm)/silver (30 nm)/molybdenum trioxide (MoO3, 5 nm)/NPB (68 nm)/bis(10-hydroxybenzo[h] quinolinato)beryllium complex: tris(1-phenylisoquinoline)iridium (Bebq2: 8% Ir(ppy)3, 70 nm)/lithium


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quinolate (Liq, 1nm)/aluminum (100 nm)/NPB (100 nm).35 An NPB layer was additionally deposited to protect the device from degradation by water vapor and oxygen exposure in the TFE application process. The simple, damage-free, and low-temperature ZAM-TFE technology can be stably applied to OLEDs without any physical or chemical damage to the OLEDs (Figure 7a). The bottom TFE was formed to protect against the diffusion of water vapor and oxygen through the PET substrate before the device was fabricated. In addition, the bottom TFE was effective for the surface planarization of the rough PET substrate, which enabled stable device operation. After red PhOLEDs were fabricated on the bottom TFE, the electrode parts were covered with polyimide tape for the electrical contact. Finally, the top TFE was formed to prevent water vapor and oxygen penetrating directly into the OLEDs. To evaluate the key properties of the ZAM-TFE, we measured changes in electrical characteristics and shelf lifetime before and after the encapsulation process and bending tests. The electrical characteristics of the ZAM-TFE encapsulated PhOLEDs were almost identical to those of unencapsulated PhOLEDs (Figure 7b). This means that material sources and deposition conditions for the ZAM-TFE formation did not cause degradation of the OLEDs. The encapsulated PhOLEDs had a maximum luminance and an efficiency of around 14000 cd/m2 and 21 cd/A, respectively. Moreover, we compared the electrical characteristics over time for the shelf lifetime of OLEDs, which can confirm the practical effectiveness of the barrier properties. The measured results confirmed that the ZAM-TFE effectively protected the OLED device against the environment without a change in electrical characteristics or the occurrence of dark spots in the cell image even after 80 days (Figure 7b). In addition, cell images of PhOLEDs over time clearly demonstrated the importance of encapsulation (Figure 7c). The cell image of an unencapsulated PhOLED showed many dark spots after storage for 12 hours at room temperature. However, although the ZAM-TFE encapsulated PhOLED was exposed to air for 2000 hours, the PhOLED showed a clean cell image with no degradation.



Figure 7. a) The fabrication process of highly flexible ZAM-TFE encapsulated OLED. b) Emitting cell images of PhOLEDs with and without thin film encapsulation. c) Emitting cell images of PhOLEDs with and without TFE. d) Current density-voltage-luminance curves of PhOLEDs before and after the bending test. (Inset: The cell image of the Al2O3-TFE encapsulated OLED after bending at a tensile strain of 0.63%). e) Demonstration of ZAM-TFE encapsulated PhOLEDs using a 23-µm-thick PET substrate after bending. Finally, we evaluated the mechanical reliability of the encapsulated OLED device. The mechanical reliability of encapsulated OLEDs is of paramount importance for the realization of transparent, flexible OLEDs. As previously discussed, the ZAM-TFE showed good barrier properties even after a tensile strain of 1.25% was applied, while the Al2O3-TFE lost its barrier properties under a tensile strain of 0.63%. To confirm the mechanical reliability of encapsulated OLEDs according to the bending radius, the electrical characteristics of the encapsulated-OLEDs were measured seven days after the bending test because sufficient time is required for the diffusion of water vapor and oxygen through the TFE. Note that the electrical characteristics of the ZAMTFE-encapsulated PhOLEDs was maintained after a tensile strain of 0.78% was applied, whereas the Al2O3TFE-encapsulated PhOLEDs had a cracked cell image, which revealed unstable electrical characteristics under a tensile strain of 0.63% (Figure 7d). The ZAM-TFE improved the mechanical reliability of the encapsulated PhOLED due to its superior mechanical properties. Therefore, the application of the highly flexible TFE layer is very important for realization of TFOLEDs. 3. CONCLUSION In summary, we proved the superiority of ZAM-TFE through a comparative analysis with other inorganic films. The bio-inspired concept was exploited for the fabrication of highly flexible encapsulation not through the control of the neutral axis or the use of the ultra-thin substrate but through the structural and material design of the barrier film. The ZnO/Al2O3/MgO structure was fabricated by ALD with a cyclic ratio of 20/12/10, in which the cyclic ratio begins to form not a blend membrane but the first quasi-perfect layer-by-layer structure similar to the nacre hierarchical structure. The 50-nm-thick, multi-interfacial ZAM film with distributed voids and cracks enabled the barrier structure to have a very low WVTR of 1.92 × 10−5 g/m2/day and high mechanical flexibility through defect-decoupling and crack deflection. To significantly improve the key properties of TFE, we fabricated a hybrid ZAM-TFE composed of a ZAM/S-H nanocomposite/ZAM structure through the insertion of an organic layer. The ZAM-TFE showed an extremely low WVTR of 2.04 × 10−6 g/m2/day and a mechanical reliability that endured tensile strain close to 1%, which are better levels than those of previously reported gas diffusion barrier films in terms of barrier and mechanical properties. In striking contrast to other reports, we confirmed that the environmental reliability of the ZAM film was further improved through structural and material designs over that of the neat Al2O3 film by comparing WVTR values before and after storage under harsh environmental conditions. Finally, the ZAM-TFE was applied to OLEDs to verify the feasibility of practical application. The OLED device integrated with the ZAM-TFE demonstrated stable and identical operating characteristics after the encapsulation process and bending tests. Therefore, we realized thin film encapsulation that showed great improvement over previously reported encapsulation layers. This research will help pave the way to the realization of a real flexible OLED by providing a real flexible encapsulation technology.


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4. EXPERIMENTAL SECTION Materials and Thin Film Deposition. ZnO, Al2O3, and MgO thin films were deposited using a 6-inch ALD (Lucida D100 from NCD). All ZnO, Al2O3, and MgO thin films were grown using H2O as a reactant, where their metal sources were dimethylzinc (DEZ), trimethylaluminum (TMA), and bis-ethyl-cyclopentadienylmagnesium (Mg(EtCp)2). Prior thin films deposition, TMA, DEZ, and H2O sources were cooled to 10 °C, while the Mg(EtCp)2 source was heated to 90 °C due to its nonvolatile property. In general, the exposure times for the ALD film growth were sequences of ts1, tp1, ts2, and tp2, where ts1 and ts2 are the times of the metal source and reactant and tp2 and tP2 are the purging times for N2 flow. The exposure times of the Al2O3 and ZnO thin films were 0.25 s, 10 s, 0.25 s, and 10s, and those of the MgO thin film were 0.75 s, 15 s, 0.25 s, and 10 s at the base pressure of 3.1 × 10−1 torr. The growth rates of the ZnO, Al2O3, and MgO thin films were 1.03 Å, 0.83 Å, and 1.00 Å, respectively, per cycle in an ALD chamber at 70 °C. The S-H nanocomposite layer was applied using a spin coating method. The S-H nanocomposite was cured by UV exposure with propylene glycol monomethyl ether acetate (PGMEA) as the solvent. A 140-nm-thick S-H nanocomposite layer was applied using the conditions of 30 s for acceleration of 4000 rpm and a maintaining time of 3 s at 4000 rpm. OLED Device Fabrication and Characterization. The OLED was fabricated by the thermal evaporation method at a vacuum level of 1 × 10−6 torr. The OLED structure was composed of NPB for a microcavity effect (50 nm), Ag for an anode (30 nm), MoO3 for a hole-injection layer (5 nm), NPB for a hole-transport layer (68 nm), Bebq as a host, Ir(piq)3 for an emitting layer (70 nm, 6%), Liq for an electron injection layer (1 nm), and Al for a cathode (100 nm) on a PET substrate planarized by the bottom TFE. The active area of the PhOLEDs was 9 mm2 for evaluation of electrical characteristics and 69 mm2 for the fabrication of the large-area OLEDs, respectively. The electric-optical characteristics of the OLEDs were measured using a source meter (Keithley 2400) and a spectro-radiometer (CS-2000, Konica Minolta) together. Method for Measuring Barrier and Mechanical Properties. A Ca test was conducted for the calculation of WVTR values. All Ca tests were performed at a temperature/humidity level of 30 °C/90% RH. The mechanical properties of the thin films were measured in terms of elastic modulus, hardness, and residual stress. The elastic modulus and hardness were obtained using a nano-indentation system (iNano, Nanomechanics). The residual stress was measured using stress gauge equipment (FSM500TC, FSM) which measured the change levels of the curvature of the wafer substrate. Cross-sectional TEM and Elemental Analysis. TEM and EDS measurements were conducted by cs-corrected scanning transmission electron microscopy (JEM-ARM200F, JEOL) and EDX (Quantax 400, Bruker), respectively. XPS (Sigma probe, Thermo VG Scientific) measurement was conducted using a micro-focused Xray monochromator in an ultra-high vacuum of 10-9 torr. 5. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Depth profiles of ZAM film with the cyclic ratio of 12:12:10 and 20:12:10; WVTR of 50 nm thick ZAM films



with the cyclic ratio; WVTR comparison of ZAM, AM, AZ, ZM, Al2O3, MgO, and ZnO thin films; X-ray diffraction patterns of 50 nm thick ZnO, Al2O3, MgO, and ZAM thin films; Optical transmittance of ZAM-TFE; Elastic modulus and hardness of inorganic layers; Residual stress according to film structures; 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). ORCID Kyung Cheol Choi: 0000-0001-6483-9516 Notes The authors declare no competing financial interest.

7. ACKNOWLEDGEMENT This work was supported by the Engineering Research Center of Excellence (ERC) Program (Grant No. NRF2017R1A5A1014708) and Nano·Material Technology Development Program (Grant No. NRF2016M3A7B4910635) through the National Research Foundation (NRF) funded by the Ministry of Science, ICT and Future Planning.

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Table of Contents (TOC)



Figure 1. a) Schematic of the ZAM NL structure and defect-decoupling system by the layer-by-layer structure. b) Schematic of the nacre cross-section and bent ZAM film with energy dissipation system for crack deflection and crack arresting. 244x129mm (300 x 300 DPI)


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Figure 2. TEM cross-sectional image and FFT of ZAM thin films according to the cyclic ratio of ZnO:Al2O3:MgO. a) 7:3:2. b) 12:6:5. c) 20:12:10. d) 29:24:20. 205x157mm (300 x 300 DPI)



Figure 3. a) WVTR values according to the cyclic ratio of ZnO:Al2O3:MgO. b) Changes in WVTR values according to thickness increase. c) Conductance curve versus time according to inorganic layers using Ca test. d) TEM cross-sectional image of the ZAM-TFE. e) EDS analysis of a ZAM film. 207x169mm (300 x 300 DPI)


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Figure 4. XPS spectra of Al2O3, MgO, ZnO, and ZAM of a) Al 2p, b) Mg 1s, and c) Zn 2p core levels. 302x161mm (300 x 300 DPI)



Figure 5. a) WVTR values of 50-nm-thick Al2O3 and ZAM thin films at 30 °C and 90% RH after storage at elevated humidity/temperature levels. b) The change of conductance curves of Al2O3-TFE and ZAM-TFE structures depending on measured environment change. 210x84mm (300 x 300 DPI)


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Figure 6. Degradation of WVTR values of Al2O3-TFE and ZAM-TFE according to applied a) tensile and b) compressive strains. 208x82mm (300 x 300 DPI)



Figure 7. a) The fabrication process of highly flexible ZAM-TFE encapsulated OLED. b) Emitting cell images of PhOLEDs with and without thin film encapsulation. c) Emitting cell images of PhOLEDs with and without TFE. d) Current density-voltage-luminance curves of PhOLEDs before and after the bending test. (Inset: The cell image of the Al2O3-TFE encapsulated OLED after bending at a tensile strain of 0.63%). e) Demonstration of ZAM-TFE encapsulated PhOLEDs using a 23-µm-thick PET substrate after bending. 290x161mm (300 x 300 DPI)


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211x138mm (300 x 300 DPI)


Functional Design of Highly Robust and Flexible Thin-Film

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