Reliable thin-film encapsulation of flexible OLEDs and enhancing their bending characteristics through mechanical analysis

Abstract

Thin film encapsulation of flexible organic light-emitting diodes (FOLEDs) with a moisture barrier, incorporating a silica nanoparticle-embedded sol–gel organic–inorganic hybrid nanocomposite (S–H nanocomposite) and Al₂O₃ were demonstrated, and their reliability and mechanical characteristics were assessed. The bending stress of the multi-layer structure for both the case of moisture barriers and encapsulated FOLEDs was investigated based on nonlinear finite-element analysis (FEA). To minimize the bending stress at the desired region, the neutral axis (NA) position could be strategically adjusted by the introduction of a buffer layer of UV-curable cycloaliphatic epoxy hybrid materials (hybrimer), synthesized via a sol–gel reaction. The optimized multi-layer structure, proposed as a result of FEA was validated by related experiments. Regarding the bending characteristics of the moisture barrier structure, the water vapor transmission rate (WVTR) of the hybrimer-coated moisture barrier was much lower than that of a non-coated sample, as a result of calcium corrosion tests after bending. The structure of encapsulated FOLEDs, which are coated by the hybrimer achieved an almost identical performance to that of non-bending samples in spite of 30 days exposure to 30 °C and 90% R.H. after a bending test with a radius of 1 cm. During this period, the occurrence of dark spots caused by moisture penetration was effectively suppressed. Collectively, these results suggest that the bending characteristics of hybrimer-coated multi-layer structures are remarkably improved with the theoretical prediction of the NA position.

---

Introduction

There is currently substantial interest in developing flexible organic electronic devices.^1,2 Over the past few years, significant advances have been made in flexible organic light-emitting diodes (FOLEDs), based on a flexible substrate with outstanding performance.^3–5 One of the most critical issues in realizing reliable flexible OLEDs is to find an alternative for breakable glass-lid encapsulation.⁶ Therefore, much research has been focused on the development of this alternative. The thin-film encapsulation (TFE) technique is recognized as a promising alternative to glass-lid encapsulation because it prevents gas permeation and provides mechanical flexibility. Various types of inorganic-based barrier system have been utilized for the encapsulation of OLED devices, such as Al₂O₃ (ref. 7–9) and SiO₂.^10,11 Since the organic/inorganic multi-barrier (dyad) system was proposed as a gas barrier for OLED applications by Burrows et al.,⁶ several research groups have reported promising results in relation to this approach.^12–14 So far, it is the most effective method of encapsulating OLEDs. However, most previous studies have concentrated on obtaining a low water vapor transmission rate (WVTR), rather than optimizing bending characteristics. Even though FOLEDs are implemented on flexible substrates with thin-film encapsulation, some components are still vulnerable to mechanical strain, such as ITO electrodes and inorganic materials of multi-barrier systems. The bending characteristics of these elements are limited by the breakability of the oxide despite being several tens of nanometers thick.^15,16 To overcome the limitation of the materials, the design of flexible OLED devices must consider structural aspects, such as the bending stress distribution of the entire structure. Recently, composite beam theory¹⁷ has been receiving renewed attention for analyzing nano-scale multi-layer structures. The mechanical bending stress of ITO electrodes has been analyzed according to this theory.¹⁸ There has also been a report on the potential to improve the bending characteristics of organic solar cells by utilizing a neutral axis (NA) position that has a bending stress of zero.¹⁹

With respect to these issues, we report the reliable thin film encapsulation of flexible OLEDs. We particularly focus on enhancing robustness against cyclic bending stress after thin film encapsulation of flexible OLEDs. Regarding multi-layer structure for both the case of moisture barriers and encapsulated flexible OLEDs, the mechanical stress was interpreted in accordance with the introduction of the buffer layer. UV-curable cycloaliphatic-epoxy hybrid materials (hybrimer) was utilized as a buffer layer, which allows multi-layer structures to release the cyclic bending stress. Strategies to optimize the entire multi-layer structures with theoretical prediction of NA behavior based on the finite-elements method (FEM) analysis was also discussed. The finite-element model was solved using ANSYS 16.0 (ANSYS Inc.) to analyze the bending stress distribution. For the moisture barrier composed of a silica nanoparticle-embedded sol–gel organic–inorganic hybrid nanocomposite (S–H nanocomposite) and Al₂O₃, the improvement of bending characteristics was theoretically and experimentally proven by investigating the change of WVTR values regarding the optimized hybrimer coating. Finally, flexible OLEDs were successfully encapsulated and structurally optimized. They also showed superior mechanical characteristics despite exposure to a harsh environment at 30 °C and 90% R.H. after the bending test.

Results and discussion

Finite-element method (FEM) analysis of multi-layer structure

Nonlinear two-dimensional finite-element analysis (FEA) based on ANSYS was performed to investigate the mechanical stress distribution of the multi-layer structure. Fig. 1a shows a local region of the multi-layer structure for the finite-element model and the schematics of bending simulations, where the two types of multi-layer structures, composed of a 3.5 dyad moisture barrier and encapsulated FOLEDs, were sandwiched between the PET substrate and the hybrimer. The 3.5 dyad moisture barrier was configured such that an additional Al₂O₃ layer was deposited on three dyads of Al₂O₃ and an S–H nanocomposite as shown in Fig. 1b. The bending strain was generated by the application of force of 25 mN at the center of the bottom surface, and only elastic deformation was considered. To avoid rigid body motion, the degrees of freedoms (DOFs) at the origin of the FEA model were all fixed. Constraint of x-directional DOFs at the other node was applied. During the numerical calculation, the internal stress of each layer was not considered. It was also assumed that all layers are firmly bounded and do not penetrate each other. The mechanical properties of the PET and the hybrimer were evaluated using the nano-indentation technique, based on Oliver and Pharr analysis,²⁰ as shown in Fig. 1c and d, respectively. The Young's modulus values were calculated to be 3.2 GPa and 4 GPa, respectively, with indentation depths ranging from 200 nm to 300 nm. Poisson's ratio was assumed to be 0.3 to calculate the Young's modulus values because Poisson's ratio cannot dominantly affect the indentation results.²¹ The Young's modulus of the S–H nanocomposite was almost identical to that of the hybrimer. The Young's modulus values of other materials were taken from the results of previous studies.^18,22–26 Thicknesses less than 5 nm were not considered for the FEA. The material properties and thickness of the multi-layer structure used in the FEA are listed in ESI (Table 1†). Fig. 1b shows FIB-SEM images of the 3.5 dyad moisture barrier on a Si wafer. The thicknesses of the Al₂O₃ and S–H nanocomposite layers were measured to be 30 nm and 120 nm, respectively. The overall thickness of the entire moisture barrier structure was 480 nm. Each layer was well formed in the moisture barrier configuration.

Fig. 1 (a) Schematic illustration of the FEM analysis with a local region of the finite-element model for each multi-layer structure. (b) FIB-SEM cross-section images of the moisture barrier on a Si wafer. Young's modulus versus displacement curve of (c) 125 μm PET substrate and (d) organic–inorganic hybrid materials (hybrimer).

Bending characteristics of flexible moisture barriers

Fig. 2 shows the FEM analysis results for the 3.5 dyad moisture barrier structure. Al₂O₃ and S–H nanocomposite layers are sequentially indicated by symbols from A1 to A4 and from S1 to S3, respectively. The bending stress near the center of the entire barrier structure was examined with hybrimer thicknesses ranging between 0 and 200 μm. The bending stress values at the center point of the S–H nanocomposite (S2) and the Al₂O₃ (A3) layer, when the samples experienced bending loading conditions, are presented in Fig. 2a and b, respectively. It was expected that the bending stress of each point, indicated by the red triangle and blue diamond would change significantly in relation to hybrimer thickness. The bending stress showed a tendency to decrease as the hybrimer thickness increased. It can be also seen that the reversal of the sign of bending stress occurred with the ref. 111 μm-thick hybrimer. The stress value fell almost to zero for that hybrimer thickness. These results reveal that the neutral axis (NA) is positioned near the corresponding point that has a bending stress of zero. The NA position in the moisture barrier structure was observed in a similar manner with calculation to find the point at which the bending stress is zero. The effect of hybrimer thickness on the position of the NA is shown in Fig. 2c. In the absence of a hybrimer layer, the NA was located near the center of the PET substrate. Notice that the NA position moved toward the hybrimer side when the hybrimer layer thickness increased. For hybrimer thicknesses less than 111 μm, the NA was located in the PET region. For hybrimer thicknesses greater than 111 μm, the NA position moved to the hybrimer region. It should be noted that the NA is located near the center of the moisture barrier structure for the case of 111 μm-thick hybrimer. In terms of protecting the moisture barrier from bending stress, the optimized hybrimer thickness is 111 μm because bending stress can be minimized when the NA is close to the moisture barrier.

Fig. 2 Bending stress under bending loading condition as a function of hybrimer thickness. (a) The stress at the center of the S–H nanocomposite layer (S2) indicated by red triangle, (b) the stress at the center of the Al₂O₃ layer (A3), indicated by the blue diamond. Inset: schematics of multi-layer stacks on PET substrate utilized in FEM analysis. (c) Neutral axis (NA) position in the moisture barrier structure with varying hybrimer thickness.

To validate the FEM analysis of optimization of the hybrimer thickness that allows the NA to be located near the center of the moisture barrier structure, the water vapor transmission rate (WVTR) after the bending test was characterized in relation to a 111 μm-thick hybrimer layer. It was quantified by an electrical corrosion test of Ca at 30 °C and 90% R.H. No bending moisture barrier of 3.5 dyads was considered as a reference. For two other sample groups, a bending test with a radius of 1 cm was conducted using a custom-made bending machine. After 1000 iterations of bending, the WVTR was estimated. The normalized conductance curves over 10 days for each sample are shown in Fig. 3a. The resistance of a sample which underwent the bending test without a hybrimer layer changed dramatically. For another sample that included a hybrimer layer, the function of the barrier was well maintained. Fig. 3b compares the WVTRs of three samples of each group. The average WVTRs are marked by colored boxes with maximum and minimum values. The lowest WVTRs were recorded to be 4.4 × 10⁻⁵ g per m² per day, 8.2 × 10⁻⁵ g per m² per day, and 2.1 × 10⁻² g per m² per day, respectively. To identify the lower limit of WVTR and the reliability of the sealing method for the Ca test in our experiment, cavity glass was also tested with identical experimental conditions. The lowest WVTR value was 3 × 10⁻⁶ g per m² per day as seen in Fig. 3a and b. The transmittance was measured with a baseline of air. The moisture-barrier coated film showed transmittance values of 88.7% at wavelength of 550 nm as seen in Fig. 3c. The average transmittance of the moisture-barrier coated film with a hybrimer layer was determined to be 88.2% in the visible region (λ = 400–700 nm). A photograph of the sample is shown in the inset of Fig. 3c. The red dashed line indicates the hybrimer-coated region of the barrier film.

Fig. 3 (a) Normalized conductance versus time curve as a result of electrical corrosion test of Ca. (b) Estimated WVTR from the Ca test. The reference was measured without bending test. For other samples, the bending test was performed with a bending radius of 1 cm. (c) Optical transmittance of full stacks of moisture barrier film. Air was used as a baseline. The inset shows a photograph of a logo image under barrier film. (d) Photographs of the Ca sensor and schematics of the Ca sensor. The Ca was oxidized over time.

Optimize bendability of flexible OLED devices

The bending stress in the flexible OLED structure was investigated by analogy with the previous sections. The bending stress of all marked points was reduced by varying the hybrimer layer thickness from 0 to 200 μm as shown in Fig. 4. Zero-crossing of the bending stress at the center of Alq₃ occurred in the case of the 109 μm-thick hybrimer layer, as shown in Fig. 4a. This result can be attributed to the fact that the NA was located around the center of the Alq₃ layer. The bending stress of the outermost Al₂O₃ layers from the Alq₃ were suppressed almost to zero for that thickness of hybrimer as seen in Fig. 4b. The NA position of the encapsulated flexible OLEDs in relation to hybrimer thickness was also examined as shown in Fig. 4c. The hybrimer layer significantly affected the NA position as expected. It can be observed that changes in the NA position had the same tendency to move toward the hybrimer region when the thickness of the hybrimer layer varied from 0 to 200 μm. It is remarkable that, with a hybrimer layer thickness of 109 μm, the NA position is near the center of the encapsulated FOLED structure. Considering the relations between the bending stress and the distance from the NA, these results indicate that the optimal hybrimer thickness is 109 μm to suppress the bending stress of FOLEDs.

Fig. 4 Bending stress as a function of hybrimer thickness. (a) The stress at the center of the Alq₃ layer, indicated by black diamond, (b) the stress in the outermost Al₂O₃ layer from the center of Alq₃, indicated by red circle and black square, respectively. Inset shows the schematics of the passivated OLED structure on the barrier-coated PET. (c) NA position in the flexible OLED structure with increasing hybrimer thickness.

Based on the FEM analysis, a hybrimer layer of 109 μm thickness was coated on the encapsulated FOLEDs. The device characteristics immediately after encapsulation of the FOLEDs on the barrier-coated PET film were considered as a reference. Other comparison samples were exposed to harsh-environmental conditions of 30 °C and 90% R.H. for a month. To verify the effectiveness of the hybrimer layer as a buffer, the comparison samples were divided into two experimental sets. One set was not subjected to the bending test. The other set was subjected to 1000 iterations of bending with a bending radius of 1 cm. After 30 day of storage in the climate chamber, the devices' performance was investigated, and the results are shown in Fig. 5a and b. All samples exhibited stable operation. Regardless of the bending test, it can be also seen that the comparison group showed almost identical device performance in spite of exposure to harsh environmental conditions. To observe dark spots caused by the moisture penetration, active area images were taken for each experimental condition. For the case of encapsulated FOLEDs with a 109 μm-thick hybrimer layer stored in the climate chamber at 30 °C and 90% R.H., the initial state of the active area of the devices that were not subjected to the bending test was fully maintained. For the devices that underwent 1000 iterations of bending with a radius of 1 cm, the active area was almost identical to that of the non-bending devices as seen in Fig. 6a. However, the FOLED encapsulated without the hybrimer layer resulted in device failure after the bending test as seen in Fig. 6b. For the non-encapsulated device, a large area of dark spots was observed after 6 hour storage at 30 °C and 90% R.H. as seen in Fig. 6c.

Fig. 5 The characteristics of FOLED devices in relation to the bending test. (a) Comparison of voltage versus current density and luminance curve. (b) Efficiency comparison of FOLEDs. The performance of reference samples was measured immediately after encapsulation, and other comparison samples were measured after 30 day storage in a climate chamber at 30 °C and 90% R.H. (c) Schematics and photographs of flexible OLEDs.

Fig. 6 Active area images of flexible OLEDs for each experimental condition (a) comparison of active area images in relation to the bending test. The images were taken at 10 days intervals during storage in the climate chamber. (b) The active area images of the FOLED encapsulated without the hybrimer coating. (c) Images of the non-encapsulated FOLED, which was exposed to 30 °C and 90% R.H.

As discussed in the previous sections, the bending characteristics for both the case of the moisture barrier and encapsulated FOLEDs were investigated using a combination of experimental and mechanical analysis. Based on neutral axis engineering, the multi-layer structure, which effectively enhances robustness against cyclic bending stress, were demonstrated. By introducing a hybrimer as a buffer layer, the NA was strategically positioned near the Al₂O₃ layer which is the most vulnerable to bending. By using Hook's law (σ = Eε), the bending stress can be obtained from the bending strain by the following equation:²⁷

where, σ is the bending stress, E is Young's modulus, ρ is the radius of curvature, and y is the distance from the NA. The bending stress at any location can be expressed as a function of ρ and y. However, the radius of curvature (ρ) is difficult to determine at a given location because the differential equations should be solved.²⁸ Based on the moment equilibrium equation regarding the NA, the bending stress equation can be expressed as²⁹
where, M is the bending moment, I is the moment of inertia in relation to the NA, and y is the distance from the NA. The bending moment and the moment of inertia are constant values, which can be obtained from the geometrical factor and bending loading conditions. Note that the bending stress can be described as a linear function of the distance from the NA. The FEM analysis results for the vertical distribution of the bending stress are well matched with the theoretical expectation as shown in the ESI (Fig. S1 and S2†). The bending stress was linearly distributed along the vertical dashed line for each thickness of the hybrimer layer. With respect to the multi-layer structure, variation of the hybrimer layer thickness affected the NA position as seen in Fig. 2c and 4c. These also led to the change in the y term in the above equation. Therefore, the bending stress values changed as a result of the change in the NA position as seen in Fig. 2a and b and 4a and b. As the NA position moved toward the center of the multi-layer structure, the bending stress values at the corresponding point were reduced almost to zero for both the case of the 3.5 dyad moisture barrier and the encapsulated FOLED structures. It was also observed that the bending stress had positive values when the corresponding point was located above the NA. For the opposite case, when the point was located below the NA, the bending stress had negative values. This can be recognized by comparing Fig. 2 and 4. The opposite sign of the bending stress can be explained by the fact that the direction of the bending stress was reversed. These facts also suggest that the bending stress is converted between tensile stress and compressive stress. For the case of the moisture barrier structure, as shown in Fig. 2, the tensile bending stress was reduced as the NA position shifted to the corresponding point in the hybrimer thickness range of 0 to 111 μm. After zero-crossing of the bending stress, the compressive bending stress increased because the NA position gradually shifted away from the corresponding point with variation of the hybrimer layer thickness from 111 to 200 μm. In this regard, the bending stress changed in relation to the relative position of the NA. This tendency of the results for the relation between the bending stress and the NA position was also observed in the case of the encapsulated FOLED structure as seen in Fig. 4.

There are two factors that influence flexible moisture barrier performance. It is generally known that, during the deposition process, defects in the inorganic layer occur due to the imperfection of the process.³⁰ These defects can serve as a diffusion path for gas permeation. An organic–inorganic multi-barrier can effectively compensate for the degradation of barrier performance caused by these defects.³¹ Alternating stacks of organic and inorganic layer prevent defects occurring in the adjacent layers. The 3.5 dyad moisture barrier was fabricated based on these approach. The flexible OLEDs were successfully encapsulated with the 3.5 dyad moisture barrier, which has low WVTR value. After 30 day storage in the climate chamber at 30 °C and 90% R.H., the encapsulated FOLEDs showed a performance almost identical to that achieved before exposure to such a harsh environment as seen in Fig. 5a and b. There were also no dark spots during the acceleration test for the non-bending sample. Another factor, that can affect the flexible moisture barrier performance is cracks, caused by mechanical strain. The inorganic layer that has a relatively high Young's modulus values is easily cracked.³² These cracks can function as a direct propagation channel for moisture penetration, which leads to low WVTR values. However, it can be minimized by modification of the multi-layer. We have introduced a hybrimer layer as a buffer, which can control the NA position, as a result of FEM analysis. Major benefits of adjusting the NA position are that the bending stress at the desired position can be reduced almost to zero and the stress for the region adjacent to the NA can be effectively suppressed because the bending stress is linearly proportional to the distance from the NA. Thus, the formation of cracks in the moisture barrier can be prevented. We first aimed to position the NA at the center of the moisture barrier structure and then position the NA at the center of the encapsulated FOLEDs by using the same approach as that shown in Fig. 2 and 4. The optimized hybrimer layer thicknesses that make the NA position shift to the desired position were determined to be 111 μm for the moisture barrier structure and 109 μm for the encapsulated FOLEDs, respectively. For the case of the hybrimer-coated sample, the bending characteristics showed far superior performance compared to that of the non-coated sample. The WVTR values of the two samples differed by more than two orders of magnitude after the bending test as shown in Fig. 3. The moisture barrier performance in the encapsulated FOLEDs, containing the hybrimer coating was relatively well retained after bending notwithstanding 30 day storage in the climate chamber. Almost identical I–V–L characteristics were recorded and only a few dark spots were observed during the acceleration test. These results suggest that the effect of reduction of the bending stress by adjustment of the NA position leads to the suppression of crack formation in the moisture barrier.

Experimental

Materials and sample preparation

Cycloaliphatic epoxy oligosiloxane resin was synthesized by sol–gel condensation reaction of [2-(3,4-epoxycyclohexyl)ethyl]trimethoxysilane (ECTS) (Gelest, USA) and diphenylsilanediol (DPSD) (Gelest, USA). Cycloaliphatic epoxy hybrid materials (hybrimer) were produced by photo-induced polymerization of the cycloaliphatic epoxy oligosiloxane with a cationic photo-initiator. The details of the optimization of the synthesis method and characteristics of the hybrimer were described in previous work.³³ The hybrimer was bar-coated to form a buffer layer on the passivated FOLEDs and UV-cured by I-line (λ = 365 nm, optical power density = 20 mW cm⁻²) for 100 s. To promote adhesion between the Al₂O₃ and hybrimer, the O₂ plasma treatment was applied to the Al₂O₃ (RF power = 250 W, pressure = 1.19 Torr) for 5 min. The gap between a plate of the bar coater and levelling blade was controlled at intervals of 1 μm. The thickness of the hybrimer was measured by a dial thickness gauge immediately after UV-curing. A silica nanoparticle-embedded sol–gel organic–inorganic hybrid nanocomposite (S–H nanocomposite) was prepared by mixing the hybrimer and nanopox® E600 (Nanoresins, Germany). Methyl-terminated silica nanoparticles distributed in 3,4-epoxycyclohexyl methyl 3,4-epoxycyclohexane carboxylate (EMEC) was homogeneously dispersed for high silica contents of 100%. The details about characteristics of the S–H nanocomposite were presented in earlier work.³⁴ A 3.5 dyad moisture barrier was fabricated on a 125 μm-thick PET substrate. An S–H nanocomposite and Al₂O₃ were alternately coated. To control the viscosity for coating the S–H nanocomposite layer at desired thickness, propylene glycol monoether acetate (PGMEA, Aldrich, USA) was added as a solvent. Spin-coated S–H nanocomposite was UV-cured by identical condition to that of the hybrimer. After the spin-coating process, the film was dried in a vacuum oven to remove residual solvent. Thermal ALD Al₂O₃ was deposited at the low temperature of 70 °C. During the deposition of Al₂O₃, trimethylaluminum (TMA) and H₂O were successively flowed with a carrier gas of N₂. The FOLEDs were thermally evaporated on the moisture barrier coated PET substrate. The FOLEDs had a configuration of ZnS(25 nm)/Ag(7 nm)/MoO₃(5 nm)/NPB(50 nm)/Alq₃ (50 nm)/Liq(1 nm)/Al(100 nm). The FOLEDs were directly passivated with an identical moisture barrier coating method.

Characterization

The WVTR value was quantified by electrical corrosion test of the Ca.³⁵ The schematics of the Ca test is represented in Fig. 3d. The Ca layer, which has identical area to water vapour penetration had a dimension of 1.5 cm² area and 250 nm height. A 100 nm-thick Al electrode and the Ca layer were sequentially deposited on a glass substrate by thermal evaporation method. The Ca layer was encapsulated with moisture-barrier coated film using a UV-curable sealant, which was applied by a dispenser. To prevent oxidation of the Ca, these process was carried out in an N₂-filled glove box, integrated with thermal evaporation system. During measuring the change of resistance of the Ca, the sample was stored in the climate chamber, which was kept at 30 °C and 90% R.H. The bending test was carried out with tensile stress mode at a bending speed of 0.5 Hz. A custom-made bending machine was used to apply mechanical strain to the samples, as shown in the inset of Fig. 3a. The device performance (I–V–L) was characterized by a source meter (Keithley 2400, USA) and a spectrophotometer (CS-2000, Konica Minolta, Japan). Silver paste was used to ensure contact. The active area of the FOLEDs was observed by digital microscope. The moisture barrier was cross-sectioned and observed using FEB-SEM (Helios Nanolab 450, FEI company, USA). The Young's modulus values were calculated from the results of indentation obtained using a nanoindenter (Nano indenter XP, MTS, USA). To minimize surface effects, the Young's modulus values were determined with indentation depths ranging from 200 nm to 300 nm.

Conclusions

We have investigated effective enhancement of the bending characteristics based on FEM analysis after reliable thin film encapsulation of flexible OLEDs. We demonstrated that introducing a hybrimer as a buffer layer into both the moisture barrier and the encapsulated FOLEDs could adjust the neutral axis (NA) position. It has been experimentally proved that a change in the NA position can result in minimal bending stress at the desired region of the entire structure. The optimized hybrimer layer thickness was determined by FEM analysis; hence, the bending stress near the inorganic Al₂O₃ layer could be effectively suppressed. After the bending test with a radius of 1 cm, the WVTR of the moisture barrier coated with the hybrimer layer was comparable to that of the non-bending sample. Even though the encapsulated FOLEDs coated with the hybrimer were stored in the climate chamber at 30 °C and 90% R.H. for a month after the bending test, the FOLEDs showed a performance almost identical to that of reference devices. It was also observed that dark spots caused by moisture penetration were effectively suppressed during exposure to harsh environmental conditions. These results suggest that the hybrimer layer can prevent the formation of cracks caused by mechanical stress during bending. In a flexible display, a structure design that considers the NA position is essential for enhancement of bending characteristics.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (CAFDC 5-1(0), NRF-2007-0056090). This work was also supported by the IT R&D program of MOTIE/KEIT[10042412, more than 60” transparent flexible display with UD resolution, transparency 40% for transparent flexible display in large area].

References

1. Y. H. Kim, C. Sachse, M. L. Machala, C. May, L. Müller-Meskamp and K. Leo, Adv. Funct. Mater., 2011, 21, 1076–1081.
2. Y. Fujisaki, H. Koga, Y. Nakajima, M. Nakata, H. Tsuji, T. Yamamoto, T. Kurita, M. Nogi and N. Shimidzu, Adv. Funct. Mater., 2014, 24, 1657–1663.
3. T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B. H. Hong, J.-H. Ahn and T.-W. Lee, Nat. Photonics, 2012, 6, 105–110.
4. K.-H. Choi, H.-J. Nam, J.-A. Jeong, S.-W. Cho, H.-K. Kim, J.-W. Kang, D.-G. Kim and W.-J. Cho, Appl. Phys. Lett., 2008, 92, 223302.
5. H. Zhu, Z. Xiao, D. Liu, Y. Li, N. J. Weadock, Z. Fang, J. Huang and L. Hu, Energy Environ. Sci., 2013, 6, 2105–2111.
6. P. E. Burrows, V. Bulovic, S. R. Forrest, L. S. Sapochak, D. M. McCarty and M. E. Thompson, Appl. Phys. Lett., 1994, 65, 2922–2924.
7. J. Meyer, P. Görrn, F. Bertram, S. Hamwi, T. Winkler, H.-H. Johannes, T. Weimann, P. Hinze, T. Riedl and W. Kowalsky, Adv. Mater., 2009, 21, 1845–1849.
8. P. F. Carcia, R. S. McLean, M. H. Reilly, M. D. Groner and S. M. George, Appl. Phys. Lett., 2006, 89, 031915.
9. M. D. Groner, S. M. George, R. S. McLean and P. F. Carcia, Appl. Phys. Lett., 2006, 88, 051907.
10. A. A. Dameron, S. D. Davidson, B. B. Burton, P. F. Carcia, R. S. McLean and S. M. George, J. Phys. Chem. C, 2008, 112, 4573–4580.
11. J.-H. Choi, Y.-M. Kim, Y.-W. Park, T.-H. Park, J.-W. Jeong, H.-J. Choi, E.-H. Song, J.-W. Lee, C.-H. Kim and B.-K. Ju, Nanotechnology, 2010, 21, 475203.
12. N. Kim, W. J. Potscavage Jr, B. Domercq, B. Kippelen and S. Graham, Appl. Phys. Lett., 2009, 94, 163308.
13. M. S. Weaver, L. A. Michalski, K. Rajan, M. A. Rothman, J. A. Silvernail, J. J. Brown, P. E. Burrows, G. L. Graff, M. E. Gross, P. M. Martin, M. Hall, E. Mast, C. Bonham, W. Bennett and M. Zumhoff, Appl. Phys. Lett., 2002, 81, 2929–2931.
14. Y. G. Lee, Y.-H. Choi, I. S. Kee, H. S. Shim, Y. Jin, S. Lee, K. H. Koh and S. Lee, Org. Electron., 2009, 10, 1352–1355.
15. Z. Chen, B. Cotterell, W. Wang, E. Guenther and S.-J. Chua, Thin Solid Films, 2001, 394, 201–205.
16. D. C. Miller, R. R. Foster, Y. Zhang, S.-H. Jen, J. A. Bertrand, Z. Lu, D. Seghete, J. L. O'Patchen, R. Yang, Y.-C. Lee, S. M. George and M. L. Dunn, J. Appl. Phys., 2009, 105, 093527.
17. J. Gere and B. Goodno, in Mechanics of Materials, Cengage Learning, 2012, pp. 508–512.
18. C.-J. Chiang, C. Winscom, S. Bull and A. Monkman, Org. Electron., 2009, 10, 1268–1274.
19. S. Lee, J.-Y. Kwon, D. Yoon, H. Cho, J. You, Y. T. Kang, D. Choi and W. Hwang, Nanoscale Res. Lett., 2012, 7, 1–7.
20. W. c. Oliver and G. m. Pharr, J. Mater. Res., 1992, 7, 1564–1583.
21. S. D. J. Mesarovic and N. A. Fleck, Proc. R. Soc. London, Ser. A, 1999, 455, 2707–2728.
22. C. F. Herrmann, F. W. DelRio, S. M. George and V. M. Bright, Properties of atomic-layer-deposited Al2O3/ZnO dielectric films grown at low temperature for RF MEMS, 2005, vol. 5715, pp. 159–166.
23. Y. Cao, C. Kim, S. R. Forrest and W. Soboyejo, J. Appl. Phys., 2005, 98, 033713.
24. C. Kim, Y. Cao, W. O. Soboyejo and S. R. Forrest, J. Appl. Phys., 2005, 97, 113512.
25. Y. Cao, S. Allameh, D. Nankivil, S. Sethiaraj, T. Otiti and W. Soboyejo, Mater. Sci. Eng., A, 2006, 427, 232–240.
26. T. Mandal, P. K. Maiti and C. Dasgupta, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, 86, 024101.
27. J. Gere and B. Goodno, in Mechanics of Materials, Cengage Learning, 2012, vol. 412, pp. 407–409.
28. J. Gere and B. Goodno, in Mechanics of Materials, Cengage Learning, 2012, pp. 730–735.
29. J. Gere and B. Goodno, in Mechanics of Materials, Cengage Learning, 2012, pp. 412–415.
30. B. Singh, J. Bouchet, G. Rochat, Y. Leterrier, J.-A. E. Månson and P. Fayet, Surf. Coat. Technol., 2007, 201, 7107–7114.
31. G. L. Graff, R. E. Williford and P. E. Burrows, J. Appl. Phys., 2004, 96, 1840–1849.
32. S.-H. Jen, J. A. Bertrand and S. M. George, J. Appl. Phys., 2011, 109, 084305.
33. K. Jung, J.-Y. Bae, S. J. Park, S. Yoo and B.-S. Bae, J. Mater. Chem., 2011, 21, 1977–1983.
34. J. Jin, J. J. Lee, B.-S. Bae, S. J. Park, S. Yoo and K. Jung, Org. Electron., 2012, 13, 53–57.
35. R. Paetzold, A. Winnacker, D. Henseler, V. Cesari and K. Heuser, Rev. Sci. Instrum., 2003, 74, 5147–5150.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06571f

---