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Abstract
We have investigated an effective and a single-step chemical vapor deposition (CVD) method to achieve conformal visible poly-dichloro-para-xylylene (parylene C) film for light extraction enhancement in bottom-emitting organic light-emitting diodes (OLEDs) at room temperature. We report that sublimed parylene dimers pyrolyzed between 400 °C and 500 °C resulted in visible parylene films with tunable transmittance and haze, exhibiting light scattering properties due to the formation of uniformly distributed dimer crystals. We achieved a novel conformal visible parylene film with total transmittance and high haze of 79.5% and 93.6%, respectively. It is observed that the outcoupling efficiency of the OLEDs employing the visible parylene film is enhanced up to 45.8%. Additionally, the OLED with the visible parylene light extraction film shows limited angle-dependency of emission spectrum over viewing angles. The single-step room temperature fabrication process of this conformal outcoupling film paves the way to achieving commercial high-performance OLEDs.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic light emitting diodes (OLEDs) have become prominent in widely commercialized small, medium and large sized displays and in solid state lighting because of their unique advantages such as the ability to bend, curve, roll and produce wide color gamut [1–4]. Conventional bottom-emitting OLEDs consist of organic layers sandwiched between transparent conducting oxide-coated glass substrate and a reflective metal cathode. The light emitted travels out toward the substrate, through multi-layers with mismatched refractive indices [5]. Consequently, the light suffers various optical effects such as waveguide loss, surface plasmon absorption loss and total internal reflection [6–9]. These losses significantly limit the total outcoupling efficiency of the generated light from the device to almost 25% [10]. Therefore, from the perspective of power efficiency and large-scale commercial application, improving the outcoupling efficiency has been an important issue in OLEDs. As such, several light extraction technologies for improving the efficiency of OLEDs, have been explored [11–19]. The basic concept and mechanism of light extraction technique is the addition of an optical structure to reduce the refractive index mismatch between the layers of OLEDs, leading to reduction in light loss due to total internal reflection. In light with achieving advanced, stable and cheap OLED applications that have design freedom (bend, curve, roll, thin, etc), lightweight, wide viewing angle, and high efficiency, it is necessary to adopt a light extraction technology that ensures good conformal coverage, excellent homogeneity, high chemical stability, can be applied at room temperature at low cost. Meanwhile, many methods of integrating either an internal or external light extraction layer or combining both have been developed to enhance the light extraction efficiency and therefore improve the external quantum efficiency. Light extraction techniques including micro-lens array [11–12], silica microsphere [13], random surface [14–15], micro-cavity structure [16], photonic crystal [17–18], and scattering layer [19] have been reported so far. However, some of these solutions are expensive, clumsy in structure, involve multi-steps and additional surface treatment materials and methods which are normally applied at high substrate temperatures.
In this study, we applied visible poly-dichloro-para-xylylene (parylene C) film as a light extraction layer for enhancing the outcoupling efficiency of bottom-emitting OLEDs in a single-step chemical vapor deposition (CVD) process at room temperature. Parylene films are well known CVD polymers and have been used in the conformal coating industry to protect electrical devices and mechanical parts from outer stresses, such as chemical reactions or physical stress [20]. Parylene derivatives such as parylene D (poly-dichloro-para-xylylene), parylene C (poly-chloro-para-xylylene), parylene F (poly-tetrafluoro-para-xylylene), and parylene HT have been widely utilized in micro-electromechanical sensors, flexible electronics, and in the medical industry [21,22]. Parylene C is an increasingly popular choice due to its deposition method, low deposition temperature, transparency, and compatibility with standard nano- and micro-fabrication processes [23]. In the area of flexible electronics, wide-ranging applications such as implementation in encapsulation layers (OLED, OPV) [20,24], flexible substrates (sensor, OLED) [21,25], interlayers for imprinting processes, insulating layers (OTFT) [26], and planarization layers (OLED) [22] have been investigated. The chemical vapor deposition of parylene was introduced by Gorham and can generally be categorized in four steps: evaporation, pyrolysis, deposition, and trapping respectively [27,28].
Lee et al. [25], have investigated the structural and thermal characteristics of room temperature deposited parylene C films at various pyrolysis and sublimation conditions. A transparent parylene film exhibiting high transmittance and low haze was successfully deposited. It was revealed that temperature control of the evaporation, pyrolysis, and substrate can affect the optical properties of the parylene C films. In this study, we demonstrate that tuning the pyrolysis temperature between 400 °C and 500 °C yields corresponding visible parylene films exhibiting total transmittance and haze within the ranges of 77% ∼ 98% and 9 ∼ 95, respectively. It is investigated that pyrolysis of sublimed dimers (di-para-xylylene) at such relatively low temperatures results in parylene C films with high light scattering properties due to the formation of uniformly distributed dimer crystals. A visible parylene film with 79.5% and 93.6 of total transmittance and haze respectively, achieved 45.8% light intensity enhancement, and greatly suppressed the angle-dependent spectral distortion in a bottom-emitting OLED. The main advantage of our technique is the single-step CVD process of commercially available parylene C which does not include any combination with other materials and techniques such as application of an additional scattering layer. This technology is expected to be extended to highly efficient flexible OLEDs and other optoelectronic devices.
2. Experimental
Figure 1 shows the schematic mechanism for chemical vapor deposition process of visible parylene C film as light extraction layer on a bottom-emitting phosphorescent OLED at room temperature. The vaporization chamber sublimed dimers within a programmed temperature range of 190–270 °C, and the resulting vapor passed through the pre-heated pyrolysis chamber, where the dimers were cleaved into the monomeric gaseous form. The monomeric gas flowed into the main deposition chamber and spontaneously polymerizes on condensation to form high molecular weight, linear poly-para-xylylene on the substrate at room temperature. To protect the pump against parylene contamination by residual monomers, the unpolymerized monomer gases were drained to a vacuum pump, followed by condensation by liquid nitrogen in the trapping chamber. Depositions were made at high vacuum pressures by means of a diffusion pump and rotary pump (RP). The deposition flow and molecular structures of parylene C dimer (di-para-xylylene), monomer (para-xylylene) and polymer C (poly-chloro-para-xylylene) are also shown in Fig. 1. Three visible parylene films herein referred to as VP400, VP450 and VP500 were made by pyrolyzing the sublimed dimers at temperatures of 400 °C, 450 °C and 500 °C, respectively. Tuning the pyrolysis temperature controls the formation of dimer crystals in the parylene film; a technique that forms the basic mechanism of the deposition of the visible parylene films.
The visible parylene films were deposited on bare glass substrates and bottom-emitting phosphorescent OLEDs in order to analyze their optical and light extraction properties, respectively. Bare glass and indium tin oxide (ITO, 150 nm) layer patterned glass substrates were cleaned sequentially with acetone and isopropyl alcohol in an ultrasonic bath, boiled in isopropyl alcohol, and dried in an oven at 150 °C for 15 minutes, respectively. Phosphorescent OLEDs were fabricated by sequentially depositing the following organic layers on the ITO layer patterned glass substrates: molybdenum oxide (MoO3, 4 nm) as a hole injection layer, N,N'-dicarbazolyl-4–4'-biphenyl (CBP, 40 nm) as a hole transport layer, iridium (III) tris(2-phenylpyridine) (Ir(ppy)3)-doped CBP (CBP:Ir(ppy)3, 10%, 15 nm) as phosphorescent emitting layer and 2,2,2-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi, 55 nm) as electron transport layer. The MoO3 layer and the organic layers were evaporated through a shadow mask at 0.5Å/s and 1Å/s deposition rates, respectively. Lithium fluoride (LiF, 1.2 nm) as electron injection layer and aluminum (Al, 100 nm) as cathode, were evaporated onto the organic layer using a metal shadow mask, at 0.5 Å/s and 4 Å/s deposition rates, respectively. All depositions were made in an evaporation chamber under 10−7 torr. The schematic structure of the resultant phosphorescent bottom-emitting OLED equipped with visible parylene film is shown in Fig. 1.
The speed and thickness for the depositions were monitored using a quartz crystal monitor (STM-2XM, Instruments Sycon). Optical transmittance of the visible parylene films was analyzed by UV–vis–NIR spectrophotometer (Lambda 950 UV–vis–NIR spectrophotometer, PerkinElmer). The surface analysis was performed using optical microscope (OM) (Olympus BX-43). The current density–voltage–luminance characteristics (J-V-L) of the flexible OLEDs were measured by means of a voltage source and a current meter using a Source Meter (KEITHLEY 2400) and analyzed in the vertical direction using Photo Research (LMS PR 650) software. We used an integrating sphere (IS200-4) and a spectrometer (Thorlab, CCS200/M) to analyze the light extraction efficiency in all directions.
3. Results and discussion
First, optical measurement and surface analysis were performed to verify the optical and light extraction characteristics of the visible parylene films. The total (Tt), parallel (Tp) and diffuse (Td) transmittance measured for the visible parylene films are shown in Fig. 2(a). Total transmittance here means the amount of measured transmitted light gathered by the integrating sphere after passing through the sample. It represents how much absorption and reflection occurred at the sample including the glass substrate. It is calculated as the sum of parallel transmittance and diffuse transmittance [12,19]. A reference bare glass substrate was used for the baseline measurement. It is shown that VP400, VP450 and VP500 films exhibit total transmittance of 79.5%, 90.0% and 96.5% on glass substrate, respectively. The spectrum is almost flat in the visible range, which means the optical property does not change when it is used as lighting sources. The diffuse transmittance of the VP400 and VP450 films are similar and high as compared to the VP500 film. The diffuse transmittance of VP400, VP450 and VP500 films are 74.3%, 74.8% and 11.7% at 530 nm of the visible spectrum, respectively, which implies that VP400 and VP450 films could act as useful light extraction layers in OLEDs and other optoelectronic devices. The inset of Fig. 2(a) shows the photographic images of glass samples with VP400, VP450 and VP500 films placed on a background image, respectively. VP400 exhibits highest blurriness due to its high haze. Apart from high transparency, all visible parylene films exhibit high uniformity. Therefore, this technique proves to be especially useful for application in optoelectronic devices requiring scattering layers with high transmittance.
Fig. 2. (a) Total (solid lines), parallel (dash lines) and diffuse (dash dot lines) transmittance of VP400 (Olive), VP450 (Blue) and VP500 (Red) films (inset: photograph of glass samples with films placed on a background image). (b) Measured haze of VP400 (Olive dash line), VP450 (Blue dot line) and VP500 (Red dash dot line) films. (c) OM images of VP400, VP450 and VP500 films.
The haze of the visible parylene films are shown in Fig. 2(b). Haze is defined as the ratio of the measured diffuse transmittance to the total transmittance and can be calculated based on the equation:
As seen, VP400, VP450 and VP500 films show uniform haze of 93.6, 83.1 and 12.2 at 530 nm of the visible spectrum, respectively. It is demonstrated that haze increases with decreasing pyrolyzing temperature, which is in good step with reducing cost for fabricating such optical coating layers. The OM images of the visible parylene films are shown in Fig. 2(c). VP400 and VP450 films show randomly distributed dimer crystals. The average size of each crystal is about 50 µm. It is reported that di-para-xylylene is quantitatively cleaved by vacuum vapor-phase pyrolysis at 600 °C to two molecules of para-xylylene [28]. Therefore, the crystallinity exhibited by the visible parylene films could be attributed to the presence of (uncleaved) dimer crystals present in the film [25]. The Fourier transform infrared (FT-IR) spectroscopy to investigate the molecular structural characteristics of parylene films possessing dimer crystals was reported [25]. The report indicates that dimer crystals contribute to additional weak absorption peaks at 652, 675, and 709 cm-1 of the FT-IR spectrum. In this study, it can be confirmed from the OM images that a large cluster of dimer crystals filled the surface of VP400 than VP450 and VP500, respectively. Consequently, VP400 exhibited the highest haze, followed by VP450 and VP500, respectively. Generally, haze represents the fraction of light, which is diffused away from the normal incident direction, thus optical layers with high haze may disperse the incident light effectively. Conventional light extraction films reported required various complicated manufacturing process or expensive high refractive index nano-particle materials and surfactants to achieve films with high haze and reasonable transmittance. However, we have demonstrated that we can easily achieve optical films with high haze and considerable transmittance in a simple and tunable manner by simply changing the pyrolysis temperature of the dimer gases of parylene prepolymers.Generally, it is investigated that parylene C deposition proceeds best at ambient temperature and produces film of optimum performance [25,29,30]. From differential scanning calorimetry (DSC), it was confirmed that the parylene films containing dimer crystals exhibited lower melting temperatures and melting enthalpies than the parylene films with less or no dimer crystals [25].
The light extraction effect of our visible parylene films on bottom-emitting OLEDs was analyzed. Figure 3 shows the electroluminescence spectra (EL) of OLED devices with or without visible parylene films which were measured by an integrating sphere measurement setup at an applied current density of 2.77 mA/cm2. All EL measurements show same spectrum with phosphorescence emission peaks of green Irppy3 at 520 nm and 548 nm, respectively since the visible parylene films have nearly flat transmittance at the visible range as shown in Fig. 2(a). As shown in Fig. 3, total light emission of bottom-emitting OLEDs with VP400, VP450 and VP500 films is increased by 45.8%, 30.2%, and 8.3% respectively. Maximum outcoupling efficiency is achieved with VP400 film due to its high scattering centers provided by the dimer crystals as already discussed in Fig. 2(b). The increase in light extraction efficiency is attributed to the fact that the incidence angle of the light at the interface between the air and the visible parylene becomes smaller than the critical angle due to the dimer crystals formed on the substrate, which suppresses the light loss due to total internal reflection at the ITO glass-air interface.
Fig. 3. Electroluminescence spectra of reference OLED (Black solid line) and OLEDs with VP400 (Olive dash line), VP450 (Blue dot line) and VP500 (Red dash dot line) films.
The External Quantum Efficiency (EQE), Power Efficiency (PE) for OLED with and without VP400 film were also investigated. From the results’ reliability point of view, it is necessary to analyze the light extraction efficiency in all directions through an integrating sphere. As such, the enhanced Luminance, EQE, and PE are calibrated using integrating sphere measurement. Figures 4(a), (b) and (c) show the calibrated Luminance verse Voltage, EQE, and PE of OLED with and without VP400 film, respectively. The enhanced Luminance, EQE, and PE are calibrated with the enhancement factor which is achieved with the VP400 film in Fig. 3. The OLED equipped with the visible parylene film shows enhanced luminance and has higher efficiencies in the whole luminance range. EQE of OLED with or without the visible parylene at 63.1 mA/cm2 is 18.9% and 13.0%, respectively. The power efficiency is correspondingly enhanced as well. Maximum PE of 41.8 lm/W and 28.7 lm/W is achieved for OLED with or without the visible parylene, respectively. The PE and EQE reported here indicate that the visible parylene film can achieve similar or even better outcoupling efficiency enhancement than some conventional light extraction techniques [12,14,18].
Fig. 4. Calibrated (a) Luminance versus voltage (L-V); (b) EQE; (c) PE of reference OLED (Black square) and OLED with VP400 film (Olive circle).
The measured angle-dependent electroluminescence spectra of reference OLED and OLED with the visible parylene film are shown in Figs. 5(a) and (b), respectively. The reference OLED shows a considerably distorted angular spectrum. However, it is observed that the original electroluminescence spectra of the OLED equipped with the visible parylene film is preserved without significant distortion at different viewing angles. It is further investigated that the full width at half maximum (FWHM) of the reference OLED and the OLED with the visible parylene film at 0°, 20°, 40° and 60° are 67.2 nm, 68.3 nm, 71.8 nm, 74.8 nm and 70.7 nm, 71.3 nm, 71.9 nm, 72.1 nm, respectively. Thus, the standard deviation of the FWHM of the reference OLED and the OLED with the visible parylene film are calculated as 3.5 and 0.6, respectively. These results clearly indicate that the angle-dependent spectra of OLEDs equipped with visible parylene films show an outstanding spectral stability (almost six times) over viewing angles, owing to the randomly distributed prepolymer parylene dimer crystals which scatters the emitted light. Figure 5(c) shows the 1931 Commission internationale de l′éclairage (CIE) color coordinates, which were extracted from the EL spectra. The standard deviations of the x and y coordinates are 0.0061 and 0.0026, and 0.0012 and 0.0012 for the reference OLED and the OLED with the VP400 film, respectively. In the case of the OLED without any light extraction film, an obvious change in the CIE coordinates can be observed. As the viewing angle changes, the optical traveling length changes, which results in significant color changes. In the case of the bottom-emitting OLED with the VP400 film, the CIE coordinates are stabilized over viewing angle.
Fig. 5. Measured electroluminescence spectra of (a) reference OLED, (b) OLED with VP400 film over viewing angles of 0° (Black solid line), 20° (Red dash line), 40° (Blue dot line), and 60° (Olive dash dot line). (c) The CIE coordinates of OLED with (Olive circle) and without (Black square) the VP400 film over viewing angles.
Figure 6 shows the normalized angular intensity distribution of OLED with visible parylene film. The angular emission intensity of the OLED equipped with visible parylene film is significantly closer to the Lambertian characteristic as compared to the reference device. The angular emission intensity of OLED with the visible parylene film has profound Lambertian characteristics because the dimer crystals have a random distribution in the film. This means the use of our outcoupling film can make a Lambertian-like plane light source which is usually desired for lighting sources. The inset shows a photograph of the bottom-emitting OLED with (upper device) and without (lower device) VP400 film. We have demonstrated that visible parylene films with high haze can be easily applied in a single-step at room temperature for improving the outcoupling efficiency and spectral stability of OLEDs at a low cost.
Fig. 6. Angular distribution of luminance for reference OLED (Black square) and OLED with VP400 film (Olive circle). (Blue triangle is Lambertian distribution). Inset photograph is the OLED without (lower device) and with (upper device) VP400 film.
4. Conclusion
In this study, we have successfully applied visible parylene film as a light extraction layer for enhancing the outcoupling efficiency of a bottom-emitting OLED in a single-step CVD process at room temperature. It is investigated that pyrolyzing sublimed parylene dimers at 400 °C yielded visible parylene film with high light scattering properties due to the formation of uniformly distributed dimer crystals. We achieved a novel visible parylene film with total transmittance and high haze of 79.5% and 93.6, respectively. A bottom-emitting OLED equipped with the visible parylene film achieved 45.8% of light intensity enhancement, and greatly suppressed the angle-dependent spectral distortion. The single-step room-temperature fabrication process of the visible parylene film as a high-performance light extraction layer, using commercially available parylene C, without any combination with other materials and techniques, makes it adaptable to highly efficient commercial OLEDs and other optoelectronic devices.
Funding
Korea Ministry of SMEs and Startups (S2737207); Hanbat National University (2018).
Disclosures
The authors declare no conflicts of interest.
References
1. C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes,” Appl. Phys. Lett. 51(12), 913–915 (1987). [CrossRef]
2. T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B. H. Hong, J.-H. Ahn, and T.-W. Lee, “Extremely efficient flexible organic light-emitting diodes with modified graphene anode,” Nat. Photonics 6(2), 105–110 (2012). [CrossRef]
3. H. Sasabe and J. Kido, “Development of high performance OLEDs for general lighting,” J. Mater. Chem. C 1(9), 1699–1707 (2013). [CrossRef]
4. Y. Luo, C. Wang, L. Wang, Y. Ding, L. Li, B. Wei, and J. Zhang, “Flexible organic light emitting diodes with enhanced light out-coupling efficiency fabricated on a double sided nanotextured substrate,” ACS Appl. Mater. Interfaces 6(13), 10213–10219 (2014). [CrossRef]
5. S. Reineke, M. Thomschke, B. Lüssem, and K. Leo, “White organic light-emitting diodes: status and perspective,” Rev. Mod. Phys. 85(3), 1245–1293 (2013). [CrossRef]
6. K. Saxena, V. K. Jain, and D. S. Mehta, “A review on the light extraction techniques in organic electroluminescent devices,” Opt. Mater. 32(1), 221–233 (2009). [CrossRef]
7. H.-W. Chang, K.-C. Tien, M.-H. Hsu, Y.-H. Huang, M.-S. Lin, C.-H. Tasi, Y.-T. Tasi, and C.-C. Wu, “Organic light-emitting devices integrated with internal scattering layers for enhancing optical out-coupling,” J. Soc. Inf. Disp. 19(2), 196–204 (2011). [CrossRef]
8. H.-W. Chang, J. Lee, S. Hofmann, Y. H. Kim, L. Müller-Meskamp, B. Lüssem, C.-C. Wu, K. Leo, and M. C. Gather, “Nano-particle based scattering layers for optical efficiency enhancement of organic light-emitting diodes and organic solar cells,” J. Appl. Phys. 113(20), 204502 (2013). [CrossRef]
9. M. C. Gather and S. Reineke, “Recent Advances in Light Outcoupling from White Organic Light-emitting Diodes,” J. Photonics Energy 5(1), 057607 (2015). [CrossRef]
10. R. Windisch, P. Heremans, A. Knobloch, P. Kiesel, G. H. Döhler, B. Dutta, and G. Borghs, “Light-emitting diodes with 31% external quantum efficiency by outcoupling of lateral waveguide modes,” Appl. Phys. Lett. 74(16), 2256–2258 (1999). [CrossRef]
11. K.-H. Han, Y.-S. Park, D.-H. Cho, Y. Han, J. Lee, B. Yu, N. S. Cho, J.-I. Lee, and J.-J. Kim, “Optical Analysis of Power Distribution in Top-Emitting Organic Light Emitting Diodes Integrated with Nanolens Array Using Finite Difference Time Domain,” ACS Appl. Mater. Interfaces 10(22), 18942–18947 (2018). [CrossRef]
12. A. Kim, G. Huseynova, J. Lee, and J.-H. Lee, “Enhancement of out-coupling efficiency of flexible organic light-emitting diodes fabricated on an MLA-patterned parylene substrate,” Org. Electron. 71, 246–250 (2019). [CrossRef]
13. F. Li, X. Li, J. Zhang, and B. Yang, “Enhanced light extraction from organic light-emitting devices by using microcontact printed silica colloidal crystals,” Org. Electron. 8(5), 635–639 (2007). [CrossRef]
14. K. Lee, J. Lee, C. W. Joo, J. Y. Kim, D.-H. Cho, J.-I. Lee, H. Y. Chu, J. Moon, and B.-K. Ju, “Organic Light Emitting Diode with Uniform Luminance Distribution and Enhanced Efficiency via Random Embossing Structure,” ECS Solid State Lett. 3(11), R56–R59 (2014). [CrossRef]
15. K. Lee, J. Lee, E. Kim, J.-I. Lee, D.-H. Cho, J. T. Lim, C. W. Joo, J. Y. Kim, S. Yoo, B.-K. Ju, and J. Moon, “Simultaneously enhanced device efficiency, stabilized chromaticity of organic light emitting diodes with lambertian emission characteristic by random convex lenses,” Nanotechnology 27(7), 075202 (2016). [CrossRef]
16. J. Lim, S. S. Oh, D. Y. Kim, S. H. Cho, I. T. Kim, S. H. Han, H. Takezoe, E. H. Choi, G. S. Cho, Y. H. Seo, S. O. Kang, and B. Park, “Enhanced out-coupling factor of microcavity organic light-emitting devices with irregular microlens array,” Opt. Express 14(14), 6564–6571 (2006). [CrossRef]
17. Y.-J. Lee, S.-H. Kim, J. Huh, G.-H. Kim, Y.-H. Lee, S.-H. Cho, Y.-C. Kim, and Y. R. Do, “A high-extraction-efficiency nanopatterned organic light-emitting diode,” Appl. Phys. Lett. 82(21), 3779–3781 (2003). [CrossRef]
18. A. Gasonoo, H.-S. Ahn, J. Lee, M.-H. Kim, J.-H. Lee, and Y. Choi, “Polymer Dispersed Liquid Crystal for Enhanced Light Out-Coupling Efficiency of Organic Light Emitting Diodes,” j.inst.Korean.electr.electron.eng. 24(1), 140–146 (2020).
19. C.-H. Shin, E. Y. Shin, M.-H. Kim, J.-H. Lee, and Y. Choi, “Nanoparticle scattering layer for improving light extraction efficiency of organic light emitting diodes,” Opt. Express 23(3), A133–A139 (2015). [CrossRef]
20. A. Hogg, T. Aellen, S. Uhl, B. Graf, H. Keppner, Y. Tardy, and J. Burger, “Ultra-thin layer packaging for implantable electronic devices,” J. Micromech. Microeng. 23(7), 075001 (2013). [CrossRef]
21. E. G. Kim, J. K. John, H. Tu, Q. Zheng, J. Loeb, J. Zhang, and Y. Xu, “A hybrid silicon–parylene neural probe with locally flexible regions,” Sens. Actuators, B 195, 416–422 (2014). [CrossRef]
22. Y. S. Yoon, H. Y. Park, Y. C. Lim, K. G. Choi, K. C. Lee, G. B. Park, C. J. Lee, D. G. Moon, J. I. Han, Y. B. Kim, and S. C. Nam, “Effects of parylene buffer layer on flexible substrate in organic light emitting diode,” Thin Solid Films 513(1-2), 258–263 (2006). [CrossRef]
23. E. Meng, P.-Y. Li, and Y.-C. Tai, “Plasma removal of Parylene C,” J. Micromech. Microeng. 18(4), 045004 (2008). [CrossRef]
24. J. Ahmad, K. Bazaka, L. J. Anderson, R. D. White, and M. V. Jacob, “Materials and methods for encapsulation of OPV: A review,” Renewable Sustainable Energy Rev. 27, 104–117 (2013). [CrossRef]
25. J.-H. Lee and A. Kim, “Structural and thermal characteristics of the fast-deposited parylene substrate for ultra-thin organic light emitting diodes,” Org. Electron. 47, 147–151 (2017). [CrossRef]
26. J. A. DeFranco, B. S. Schmidt, M. Lipson, and G. G. Malliaras, “Photolithographic patterning of organic electronic materials,” Org. Electron. 7(1), 22–28 (2006). [CrossRef]
27. J. S. Song, S. Lee, S. H. Jung, G. C. Cha, and M. S. Mun, “Improved biocompatibility of parylene-C films prepared by chemical vapor deposition and the subsequent plasma treatment,” J. Appl. Polym. Sci. 112(6), 3677–3685 (2009). [CrossRef]
28. W. F. Gorham, “A New, General Synthetic Method for the Preparation of Linear Poly-p-xylylenes,” J. Polym. Sci., Part A-1: Polym. Chem. 4(12), 3027–3039 (1966). [CrossRef]
29. A. K. Sharma, “Parylene C at subambient temperatures,” J. Polym. Sci., Part A-1: Polym. Chem. 26(11), 2953–2971 (1988). [CrossRef]
30. A. Kim, G. Huseynova, J. Lee, and J.-H. Lee, “Enhancement of out-coupling efficiency of flexible organic light-emitting diodes fabricated on an MLA-patterned parylene substrate,” Org. Electron. 71, 246–250 (2019). [CrossRef]
