Ultra Gas-Proof Polymer Hybrid Thin Layer - Nano Letters (ACS

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Ultra gas-proof polymer hybrid thin layer Lynn Lee, Kwan Hyuck Yoon, Jin Won Jung, Hong Rho Yoon, Hongbum Kim, Seung Hun Kim, Kyung Sun Park, and Myung Mo Sung Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01855 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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Ultra gas-proof polymer hybrid thin layer Lynn Lee1, Kwan Hyuck Yoon1, Jin Won Jung1, Hong Rho Yoon1, Hongbum Kim1, Seung Hun Kim2, Kyung Sun Park1, Myung Mo Sung1* 1

Department of Chemistry, Hanyang University, Seoul 04763, Republic of Korea

2

Display Research Center, Samsung Display, Yongin-si, Gyeonggi-do 17113, Republic of Korea

Keywords: Hermetic sealing, organic-inorganic hybrid thin layer, atomic layer infiltration, flexible OLED, encapsulation film

Hermetic sealing is an important technology for isolating and protecting air-sensitive materials and is key in the development of foldable and stretchable electronic devices. Here we report an ultra gas-proof polymer hybrid thin layer prepared by filling the free volume of the polymer with Al2O3 using gas-phase atomic layer infiltration. The high-density polymer– inorganic hybrid shows extremely low gas transmission rate, below the detection limit of the Ca corrosion test (water vapor transmission rate < 10−7 g m−2 day-1). Furthermore, due to the remarkable nanometer-scale thinness of the complete polymer–inorganic hybrid, it is highly flexible, making it useful for hermetic sealing of stretchable and foldable devices.

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Hermetic sealing, having a long history from ancient times, is a technique to provide a moisture-tight and air-tight barrier between environments. Hermetic seals are broadly used in foods, pharmaceuticals, chemicals, consumer goods, semiconductor electronics, thermostats, optical devices, MEMS, photovoltaics, batteries, and switches 1-3. Recently, hermetic sealing of electronic devices has attracted interest for use with the freely foldable and stretchable devices that represent the next generation of electronics and displays 4-7. Most organic materials used in flexible devices degrade in performance when exposed to ambient air and thus require complete hermetic isolation with flexible barriers. Among the hermetic sealing technologies, thin film encapsulation (TFE) is the most efficient sealing method for electronic devices, solar cells, displays, and especially flexible devices 8-11. TFE has emerged as a core method for flexible hermetic sealing because thin films can act as conformal and stable encapsulating layers over flexible devices. Ideally, a defect-free, single-crystalline monolayer of Al2O3 should be the perfect hermetic barrier, but its production has never been achieved 12. Most inorganic thin films have pinholes and defects arising from either the deposition process or substrate imperfections; such defects serve as paths for gas diffusion, and films having any defects exhibit poor barrier properties. Direct pinholes, channels that extend completely through the barrier film, are especially representing the main gas diffusion path through inorganic thin films. Direct pinholes arise from point defects that propagate stably throughout the film growth process

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studies have suggested using multilayer structures of alternating inorganic and organic layers as an efficient solution to produce hermetic thin films. In such structures (Vitex or organicinorganic multilayers), each inorganic layer serves as a thin barrier layer while the organic polymer layer provides an open space for gas diffusion. As a result, defects in the inorganic layers are not connected; thus, there are no direct pinholes, meaning that gas diffusion 2 ACS Paragon Plus Environment

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through the film occurs by means of much slower and more complex paths. The gas diffusion paths through the film can be dramatically complicated by increasing the number of organic– inorganic layer pairs. Moreover, the organic layers impart flexibility to the films, reducing the film stress compared to that of solely inorganic films

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. However, thin film encapsulation

with organic–inorganic multilayers still suffers from problems such as poor uniformity, particle problems, difficulty of fabrication, and outgassing of the physically adsorbed organic layers. Especially, the use of thick and rigid inorganic layers prevents such structures from satisfy typical industrial requirements of extremely high flexibility (bending radius < 1 mm) and ultra-low gas permeation rate (water vapor transmission rate (WVTR) < 10−7 g m−2 day−1) 17-19. Therefore, the development of ultrathin and flexible polymer hybrid films with substantially enhanced hermetic sealing ability is currently in great demand. This may require a breakthrough in organic–inorganic hybrid structures, beyond traditional stacked multilayers. We suppose that a polymer hybrid thin layer, prepared by filling the free volume of a polymer with inorganic material, may be a solution. In polymer films, the free volume represents the main path of gas diffusion. Inorganic material could be infiltrated into the free volume of polymer chains to form a compact organic–inorganic hybrid, which might efficiently block the gas diffusion through the polymer. The polymer film containing such a complete hybrid thin layer is expected to maintain the excellent flexibility of the original polymer. We developed an extremely highly gas-proof and foldable polymer hybrid by employing atomic layer infiltration (ALI) of inorganic materials into polymer films. Polymer–inorganic hybrid thin layers were produced by infiltrating inorganic materials into already formed polymer matrixes in the subsurface regions of the polymers. In particular, we


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found that infiltration of Al2O3 into polyimide (PI) film produced a highly flexible hybrid film with excellent gas diffusion barrier ability. Atomic layer deposition (ALD) is a gas-phase thin film deposition method based on sequential saturated chemical reactions of precursors on substrate surfaces, resulting in the sequential formation of monolayers 20, 21. ALI is a modified ALD process employing a vapor infiltration step in which a polymer substrate is exposed to high concentrations of gaseous precursors in a closed chamber. At the infiltration step, called an exposing step, the gaseous small molecules can penetrate into complicated 3-dimensional polymer chains and undergo chemical reactions in the subsurface region of the polymer film. Here, the polymer structure consists of polymer chains and free volume, empty space between the chains, according to the general free volume theory 22-24. The fractional free volume (FFV) is the ratio of the free volume within the polymer to its total volume. PI film is a heterochain polymer containing an imide group on the backbone, which results in low FFV under 20%, making it a suitable material for barrier applications

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. Furthermore, PI exhibits very high glass transition

temperature, excellent thermal and chemical stability, and outstanding physico-mechanical properties, which are crucial features for a flexible device substrate

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. In this study, a

transparent PI film of thickness 20 µm was used as the polymer substrate. Scheme 1 illustrates the ALI process whereby Al2O3 was formed in the PI substrate, producing a PI–Al2O3 hybrid thin layer in accordance with free volume theory. The ALI precursors are small gas-phase molecules that can permeate the polymer chains in the subsurface region. The gas permeation proceeds through the free volume. More specifically, each gas molecule spends most of its time in free volume holes and occasionally jumps into a neighboring hole through an occasionally formed channel

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. The procedure for each ALI

cycle to produce the PI–Al2O3 hybrid is detailed as follows. First, the aluminum precursor 4 ACS Paragon Plus Environment

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(trimethyl aluminum, TMA) is dosed at a desired high concentration into a reaction chamber containing a PI film. The outlet gate, which is connected to the vacuum pump, is closed to increase the precursor pressure. Then, the chamber is completely closed for a certain period of time to allow the TMA molecules to permeate the PI film. After this infiltration step, the excess gas molecules in the chamber and the polymer are completely removed by a purging step including 5 cycles of evacuation followed by Ar exposure. Second, an oxygen precursor, H2O, is dosed and introduced into the PI film in the same way. The degree of gas permeation depends on ALI conditions such as precursor vapor pressure, exposing time, and temperature. To test the combined effects of the ALI parameters upon the depth of Al2O3 infiltration into the PI films, the infiltration depths were measured as functions of the ALI parameters using cross-sectional transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) (Figure S1). The infiltration depth increased proportionally with the precursor pressure and exposing time, but decreased with increasing temperature. In this experiment the number of the ALI cycles was fixed at 50, a number deemed adequate to allow sufficient infiltration of Al2O3 into the PI films. TMA molecules inside the free volume of the PI film react with C=O groups on the polymer chain to form C–O–Al bonds, as found in previously reported studies using in situ FT-IR spectroscopy 21, 28. In the same way, water as an oxygen precursor can penetrate the PI film and react with chemically bonded methyl aluminum to form Al2O3. After several ALI cycles, the Al2O3 that formed inside the PI polymer nearly fills the free volume. The chemical bonds between PI and Al2O3 also decrease the segmental movements of polymer chains, decreasing the formation of gas diffusion paths. Eventually, Al2O3 completely fills the free volume and blocks gas permeation paths in the subsurface region accessible to the precursors.


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The blocking of gas diffusion paths in PI film by means of Al2O3 infiltration is mainly influenced by the penetration depth and packing density of the PI–Al2O3 hybrid within the polymer, which depend on the ALI parameters (temperature, precursor vapor pressure, exposing time, and number of ALI cycles). We quantitatively estimated the barrier performance of the PI–Al2O3 hybrid thin layers as functions of the ALI parameters of temperature, precursor vapor pressure, exposing time, and number of ALI cycles. WVTR was measured using a Ca dot corrosion testing to investigate the barrier performance of the hybrid thin layer (Figure S2). Based on the results, the optimized ALI conditions were determined to be 100 °C temperature, 3 Torr precursor pressure, 400 s precursor exposing time, and 50 ALI cycles.


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Scheme 1. Schematic illustration of the fabrication of a polymer hybrid thin layer using atomic layer infiltration (ALI). A flexible polyimide (PI) film represents the polymer substrate; the detailed polymer structure is illustrated below so as to demonstrate the ALI process. ALI precursors are small gas-phase molecules that can penetrate into the polymer chains in the subsurface region of the PI film. Gas permeation proceeds through the free volume to form a PI–Al2O3 hybrid thin layer; this can be interpreted as gradual accumulation of Al2O3 in the free volume of the PI polymer.


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PI films with infiltrated Al2O3 (hybrid/PI film) were prepared using the optimized ALI conditions and were compared with PI films having overlying layers of pure Al2O3, prepared by a conventional ALD process conducted at 100 °C. The morphology and composition of these two specimens were studied by TEM (Figure 1a and 1e) with EDS analyses (Figure 1b–d and 1f–h). Cross-sectional TEM analysis of a representative hybrid/PI film showed the presence of a 22-nm-thick hybrid layer (22 ± 2 nm) in the subsurface region of the PI film (Figure 1a), and similar analysis of a PI film with a deposited Al2O3 layer (Al2O3/PI film) showed the 22-nm-thick overlying Al2O3 layer (Figure 1e). The thicknesses of the two thin layers were determined from TEM measurements performed at 10 different spots on films of each type. Specific element mappings of each sample were performed by EDS conducted at the same positions as in TEM images. In EDS element maps (Figure 1b– d), the hybrid thin layer showed not only aluminum (Figure 1c) and oxygen (Figure 1d) peaks attributable to Al2O3, but also a carbon peak (Figure 1b) attributed to the PI polymer. The coexistence of C, Al, and O peaks in the EDS data demonstrated that Al2O3 penetrated into the subsurface region of the PI film during the ALI process to form an organic–inorganic hybrid thin layer, presumably by means of Al2O3 formation in the free volume of the PI polymer. Aluminum (Figure 1g) and oxygen (Figure 1h) peaks but no recognizable carbon peak (Figure 1f) were observed in the separate Al2O3 thin layer deposited over the PI film in the other sample type, indicating that it was probably composed of pure Al2O3. The quantitative composition and area density of the thin layers in both sample types were determined using Rutherford backscattering spectrometry (RBS). The RUMP software simulation package was used to fit the simulation to experimental data and to provide information regarding the stoichiometry and area concentration of the samples. The experimental setup is described in detail in the supproting information, experimental section. RBS signals were obtained from 10 different spots in samples of both types, and the thickness 8 ACS Paragon Plus Environment

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of the layer was set to be 22 nm, as determined by TEM. The random backscattered ion yields from both samples are plotted as a function of the backscattered ion energy in Figure 1i and 1j, respectively, where solid black lines represent the experimental data, and red curves represent simulated results from RUMP. The RUMP simulation results fit almost perfectly to the experimental data (Figure 1i and 1j). From the distinct nature of the peaks shown in the spectra, the chemical composition of the PI–Al2O3 hybrid thin layer was calculated to be Al2.00O3.79C4.35N0.495H2.24, indicating that Al2O3 had infiltrated the free volume and occupied around 39 vol% of the 22-nm-thick subsurface region of the PI film. In contrast, the chemical composition of the Al2O3 thin layer in the Al2O3/PI film was almost the same as that of a representative pure Al2O3 thin film, with little or no trace impurities such as carbon and hydrogen, as confirmed by elastic recoil detection analysis. Further, the area density of the PI–Al2O3 hybrid thin layer was 2.4 × 1017 atoms cm−2, much higher than that of the pure Al2O3 thin layer (1.9 × 1017 atoms cm−2) and comparable to a single-crystalline Al2O3 thin film of the same thickness (2.58 × 1017 atoms cm−2). This data indicates that Al2O3 infiltration into the free volumes of the PI matrix increases the density of the hybrid thin layer. Overall, the results suggest that Al2O3 infiltrates into PI, forming a dense PI–Al2O3 hybrid thin layer that efficiently blocks gas diffusion paths in the polymer. We note that the hybrid/PI film is transparent enough to be used for transparent devices, with transmittance above 80% throughout most of the visible and IR region, as shown in Figure S3.


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Figure 1. Electron microscopy analysis and Rutherford backscattering characterization of hybrid/polyimide (PI) films or Al2O3/PI films. (a) Cross-sectional TEM image and (b) – (d) EDS maps of a hybrid/PI film: (b) carbon, (c) aluminum, (d) oxygen. (e) Cross-sectional TEM image and (f) – (h) EDS maps of a Al2O3/PI film. (i) Rutherford backscattering spectra of a hybrid/PI film, including both experimental data (black line) and simulation data obtained based upon the elemental composition Al2.00O3.79C4.35N0.495H2.24 (red line). (j) Rutherford backscattering spectra of a Al2O3/PI film, including both experimental (black line) and simulated (red line) data.


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To investigate the barrier properties of hybrid/PI films, their WVTRs were measured using a Ca dot corrosion test. Ca corrosion testing relies on the color change from dark grey to transparent due to the oxidation of Ca metal dots. Each corrosion test sample was prepared as follows. An array of 144 square Ca dots was deposited on a glass substrate by thermal evaporation. The Ca dot array was then covered with a sealing film using epoxy resin as an adhesive layer. The sealing films tested were the hybrid/PI films and Al2O3/PI films with 22or 100-nm-thick pure Al2O3 layers; also, a bare PI film was similarly applied as a blank sample. These samples were exposed to an accelerated water permeation environment of constant temperature and humidity (85 °C and 85% RH). Figure 2a shows photographs of the samples at exposing times of 0, 24, 120, 240, 480, 720, and 1080 h. The yellow-red color of the Ca test samples is due to the epoxy resins used as a glue layer between the Ca-bearing glass substrates and PI films. WVTRs of the samples were calculated from Ca corrosion test results obtained from samples subjected to two different environmental conditions (85 °C/85% RH and 70 °C/90% RH), as described previously in detail 17. Notably, the PI–Al2O3 hybrid thin layer sample showed no color change in any of its 144 Ca dots even after 1080 h; thus, its water permeation was below the detection limit of the Ca dot corrosion test (WVTR < 10−7 g m−2 day−1) 8, 29, 30. This result demonstrates the excellent barrier properties of the PI– Al2O3 thin layer, which contains Al2O3 tightly packed within the free volume of the polymer chains. In comparison, the PI samples containing pure Al2O3 thin layers showed significantly worse barrier performance; nearly 100% of the Ca dots were completely oxidized and became transparent after 240 and 720 h exposure under the films containing 22- and 100-nmthick pure Al2O3 layers, respectively. The hybrid/PI films maintained flexibility as good as that of the bare PI film due to the complete polymer–Al2O3 hybridization in the polymer matrix and the much lower thickness 11 ACS Paragon Plus Environment

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of the hybrid (22 nm) compared to the bare film (20 µm). To demonstrate the excellent flexibility of the hybrid/PI films, we investigated their barrier properties after an extreme bending process close to folding, comprising 10,000 bending cycles with the bending radius of 1 or 5 mm and carried out using a previously described homemade bending machine

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The Ca dot corrosion test was then performed with the environmental conditions of 85 °C and 85% RH. For comparison, the barrier properties of a Al2O3/PI film with a 22-nm-thick pure Al2O3 layer were also investigated under the same bending conditions. All the Ca dots encapsulated by the hybrid/PI film survived the full 1080 h of exposure after bending to 1 mm radius, whereas the Ca dot array covered by the Al2O3/PI film was completely oxidized after only 24 h of exposure (Figure 2b). Additionally, optical microscopic (OM) images acquired after the rigorous bending processes showed many cracks in the Al2O3/PI film, whereas there were no visible surface cracks on the hybrid/PI film (Figure S4). This is convincing evidence that the PI film containing the 22-nm-thick hybrid layer had excellent mechanical flexibility and suffered no damage even under extreme bending.


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Figure 2. Measurements of water vapor transmission rate using a Ca corrosion test. Arrays of 144 Ca dots on glass substrates, encapsulated by hybrid/polyimide (PI) films or Al2O3/PI films using red epoxy resins. Photographs were taken at various times during exposure to an accelerated water permeation environment of 85% RH at 85 °C. Visible color changes indicate the corrosion of Ca (dark grey) into calcium oxide (transparent) by water vapor. (a) Ca dot corrosion tests for PI films not subjected to bending. (b) Ca dot corrosion tests for PI films after they were subjected to 10,000 bending cycles with 5 or 1 mm bending radius.


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To evaluate the hybrid thin layer in a practical setting, we fabricated an actual flexible OLED display incorporating the hybrid/PI film as a backplane substrate and tracked the display's operation over time. Specifically, lighting tests of the OLED display were conducted to provide realistic information about the encapsulation performance of the PI– Al2O3 hybrid thin layer. A test specimen was prepared by fabricating a 5.5-inch flexible OLED panel on a hybrid/PI film (Figure 3a). The device structure comprised hybrid/PI film as the substrate, upon which the TFT layer, OLED layer, and traditional multilayer encapsulation of the OLED display device were fabricated (Figure 3a inset). To characterize the encapsulation performance of the PI backplane substrate, the OLED display was exposed to 85 °C and 85% RH. The flexible OLED display produced clear white lighting throughout the 1080 h period of exposure to the accelerated water permeation conditions (Figure 3b). This result demonstrates that the PI film with the hybrid thin layer exhibited sufficient gas barrier performance for commercialization standards (10−7 g m−2 day −1) when applied to an actual flexible device.


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Figure 3. Photographs of a flexible OLED display device on a backplane of hybrid/polyimide (PI) film. (a) Photograph of a flexible OLED display device on the hybrid/PI film, showing its mechanical flexibility; (inset) cross-sectional diagram of the proposed flexible OLED display device structure, comprising a TFT layer, an OLED layer, and traditional stacked multilayer encapsulation on the hybrid/PI film. (b) Photographs of the lighting performance of the OLED display device at various times during exposure to an accelerated water permeation environment of 85 °C and 85% RH. A Al2O3/PI film was applied as the encapsulation layer of the backplane substrate. OLED photos were taken in the dark to clearly display the RGB lighting.


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In conclusion, we developed a high-density polymer hybrid thin layer with extremely low gas permeation (water vapor transmission rate < 10−7 g m−2 day−1) and extremely high flexibility (bending radius < 1 mm). The ultra gas-proof polymer hybrid thin layer was fabricated using gas-phase atomic layer infiltration (ALI), in which inorganic ALI precursors penetrated into the free volume of the polymer, forming a compact polymer–inorganic hybrid in a subsurface region of the polymer. The area density of this PI–Al2O3 hybrid thin layer (2.4 × 1017 atoms cm−2) was higher than that of a pure Al2O3 thin layer produced by atomic layer deposition (ALD) and approached the area density of single-crystalline Al2O3, thereby demonstrating the tight packing of Al2O3 within the polymer. This packing efficiently blocked gas diffusion paths within the polymer. Even after extreme bending, the thin and flexible polymer hybrid thin layer provided a completely airtight environment. Further, application of the hybrid film to a flexible OLED display imparted the device with high durability, demonstrating the film's applicability to real foldable devices. This hermetic sealing film can be applied to broad areas ranging from simple food seals to flexible solar cells and displays.


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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol)

Notes The authors declare no competing financial interest.

Funding Sources This work was supported by the Creative Materials Discovery Program (2015M3D1A1068061) and by a research grant (No. 2014R1A2A1A10050257), administered by the National Research Foundation of Korea and funded by the Ministry of Science, ICT, and Future Planning of South Korea. This work was also supported by Samsung Display Co., Ltd.

Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acs.nanolett.xxxxxxx. Detailed experimental procedures and supplementary figures (S1 – S8)

References (1) Guenha, R.; Salvador, B. d. V.; Rickman, J.; Goulao, L. F.; Muocha, I. M.; Carvalho, M. O. J. Stored Prod. Res. 2014, 59, 275-281. (2) Ko, W. H. Mater. Chem. Phys. 1995, 42, 169-175. 17 ACS Paragon Plus Environment

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Ultra Gas-Proof Polymer Hybrid Thin Layer - Nano Letters (ACS

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