Application of Nanostructured Parylene‐C Films for Improved Optical Characteristics of Organic Light‐Emitting Diodes

Anti‐reflection (AR) characteristics on the display surface and light outcoupling efficiency are significant factors for controlling external and internal light in organic light‐emitting diodes (OLEDs), respectively. This paper presents the fabrication of moth‐eye nanostructured parylene‐C films that simultaneously achieve optimized AR characteristics and enhanced OLED performance. AR performance is optimized by varying the nanostructure dimensions, resulting in a nanostructured film with a reflectance of <1%. Furthermore, applying this nanostructured film to OLEDs improved the light outcoupling efficiency by 40% compared with the reference device. Therefore, employing nanostructured parylene‐C films to enhance the optical characteristics of OLEDs is proposed.


Introduction
Owing to their lightweight and flexibility compared to inorganic electronic devices, organic electronic devices have attracted considerable attention as essential components in the development of next-generation flexible devices. [1,2]][10] DOI: 10.1002/admi.202400089   The performance of optoelectronic devices, such as organic light-emitting diodes (OLEDs) and organic solar cells (OSCs), is significantly influenced by the refractive index of the thin film.Optoelectronic devices that incorporate a parylene-C film (with a refractive index of n = 1.64) exhibit high optical reflection on the surface owing to the substantial refractive index difference with air (n = 1). [11]hese reflections can cause several issues, including reduced visibility on display screens and decreased light absorption efficiency in OSCs. [12][14] These AR characteristics help reduce eye fatigue by mitigating glare caused by external light sources.Moreover, this technology helps users utilize the display more effectively by enhancing visual quality through an improved contrast ratio with the surrounding environment. [15]hese nanostructures, with dimensions comparable to the wavelength of visible light, suppress Fresnel reflections at the surface by introducing gradual changes in the refractive index. [16,17]o exhibit AR characteristics, the dimensions of the nanostructure must be optimized, which are typically described in terms of changes in shape or height.[20] However, these studies exclusively examined the characteristics related to the modification of nanostructures and did not investigate the implementation of these nanostructured films in practical devices.Therefore, assessing the device performance of nanostructured films when applied to optoelectronic devices, such as OLEDs and OPVs, is imperative.
The optical film used in OLED displays is essential for its AR characteristics in regulating external light as well as for implementing an outcoupling film to control the extraction of internal light.[23][24] In general, films that enhance the outcoupling efficiency have predominantly been polymer films with microstructures, such as microlens arrays (MLAs), or metal compound films with nanostructures. [25,26]However, MLA-based outcoupling technology has the disadvantage of requiring a separate AR film.Furthermore, the film formation process for metal compound nanostructures is more complex than that for polymer films, and fabricating them uniformly is challenging.In contrast, the parylene-C membrane fabrication process is highly advantageous for the fabrication of nanostructured films because of its favorable characteristics of excellent penetrability and uniform deposition capability. [27]Therefore, nanostructured parylene-C films can potentially simultaneously enhance the outcoupling efficiency and AR characteristics of OLEDs.
This study successfully achieved uniform optical parylene-C patterning on nanostructures for the first time, thereby overcoming the limitations of microstructured parylene patterning.Our fabrication technology for parylene-C films enables the design of nanostructures, facilitating the optimization of the optical performance of the device.The performance of the nanostructured films was examined by designing films with various nanopillar diameters.As the diameter of the nanopillars decreased, the nanostructured film exhibited a reflectance of <1%, owing to the optimized AR characteristics.A reflectance simulation was successfully conducted using the effective refractive index.In addition, these nanostructures exhibit hydrophobic characteristics owing to their rough surfaces.The application of nanostructured films to OLEDs resulted in a 40% increase in luminance while maintaining the internal stability of the device.Consequently, we simultaneously optimized the AR performance and improved the outcoupling efficiency of OLEDs using a nanostructured parylene-C film.

Fabrication of Nanostructured Films
The nanostructured film was fabricated using an anodic aluminum oxide (AAO) template, a porous substrate commonly employed for fabricating nanostructures, such as moth-eye structures. [28]The fabrication process of the nanostructured films began with the deposition of parylene-C onto a porous sur-face of AAO using the chemical vapor deposition (CVD) method, as shown in Figure 1a.Parylene-C deposited AAO was then rapidly etched using 3 m NaOH at 60 °C, resulting in the fabrication of the nanostructured film shown in Figure 1b.
The AR characteristics of the nanostructures are optimized when the size of the pillars falls within the visible wavelength range.Based on previous studies, the pore dimensions of AAO were determined to fabricate the nanostructures.The pore depth and interpore distance were set to 500 and 450 nm, respectively.20] The diameter of nanopillars is associated with the aspect ratio, and higher aspect ratios are known to lead to optimized AR characteristics. [29]The pore diameters we used varied from 400 to 100 nm, with the most significant difference in aspect ratio being fourfold, as indicated in Table 1.

SEM Images of Nanostructures
The formation of nanostructures was confirmed by surface and cross-sectional FE-SEM images.Figure 2 shows arrays of nanopillars for each AAO pore diameter.The diameters of the fabricated nanopillars are denoted as D = 400, 300, 170, and 100 nm.
As the pore diameter of the AAO template increased, the nanostructures exhibited nanopillar shapes with flat and uniform tops.Conversely, as the pore diameter of the AAO template decreased to 100 nm, the nanostructures transformed into a cone  shape with varied volume fractions and heights.As shown in the cross-sectional SEM image in Figure 2e, the uniform arrangement of the cone-shaped nanopillars in 3D space can be expected to have AR characteristics owing to a gradual change in the refractive index.

Reflectance Measurement
We used a spectrophotometer to measure reflectance for a comparative analysis between the flat and nanostructured parylene-C films.Figure 3a shows the influence of the nanopillar diameter on the AR characteristics.For the flat parylene-C film, an interference pattern of reflectance emerges because of resonance occurring at the interface between the upper and lower boundaries of the film. [30]In addition, parylene-C has a higher refractive index (n = 1.64) than glass (n = 1.5), which leads to increased reflectance.All the nanostructured parylene-C films exhibited a decrease in reflectance compared to the flat parylene-C films.Furthermore, the reflectance decreased as the pore diameter (D) decreased.Interestingly, in nanostructured films with different diameters, a consistent trend was observed, wherein the reflectance approached 0% as the wavelength decreased.This behavior is typical of nanostructures, where periodic nanopillars promote more frequent interference from short-wavelength reflected light at the top and bottom surfaces. [20]o investigate the effect of the underlying film thickness on the nanostructures, the reflectance was analyzed based  on the total film thickness.We could precisely control the thickness of the parylene-C films to fabricate nanostructured films with total thicknesses of 1.7 and 3 μm.The minimum required thickness for device application processes such as lamination and delamination is 1 μm excluding the height of nanostructures.Figure 3b confirms that the total film thickness affected the reflectance as the pore diameter increased.
The influence on the AR performance can be explained by the effective medium theory, as described by Equation (1). [17]eff where n 1 and n 2 are the refractive indices of parylene-C (n 1 = 1.64) and air (n 2 = 1) at 550 nm, and f is the volume fraction of the nanopillars.The variation in the volume fraction of the nanopillars leads to a gradual change in the effective refractive index, effectively reducing the surface reflection. [17,18]20] For the fabricated nanostructures, as the diameter decreases, the top of the nanopillar exhibits a parabolic shape, as shown in the SEM images in Figure 2, thereby enhancing the AR characteristics.However, as the diameter increases, abrupt changes in the volume fraction of the nanopillars may occur owing to deviations from the columnar shape.This can hinder the gradual change in the refractive index and lead to a higher reflectance compared with smaller diameters.Consequently, we demonstrated that the reflectance of the nanostructured film was reduced by <1% compared with that of the flat parylene-C film.Moreover, these results underscore the possibility of fabricating films with specific reflectance values based on changes in diameter.

Optical Reflectance Simulation
AFM is a powerful method for verifying the relationship between the optical characteristics and the morphology of nanostructures.Surface analysis of a film with a diameter (D) of 100 nm and a height of 500 nm was conducted using AFM, with measurements taken in an analysis area of 10 μm × 10 μm. Figure 4a confirms the uniform arrangement of the nanopillars in 3D space.
According to the effective medium theory, a nanostructure can be considered an optical medium that modifies the effective refractive index of the film surface.The volume fraction of the nanopillars must be determined to conduct reflectance simulations.One method used for this purpose is the AFM bearing ratio. [31]Figure 4b shows a histogram of the height distribution of the fabricated nanopillars and their corresponding bearing ratio.The simulation was performed by dividing the nanopillars into 100 layers, with each layer at intervals of 5 nm, and the volume fraction of the nanopillars in each layer was substituted with the bearing ratio of the AFM.Based on the effective refractive index calculated using Equation (1), the reflectance was successfully simulated, and the results were in good agreement with the measured reflectance spectrum, as shown in Figure 4d.Thus, we optically validated the AR characteristics.

Improving Hydrophobic Surfaces
A notable feature, in addition to the AR characteristics, is the low surface energy resulting from the surface roughness of the nanostructured surface. [32,33]This imparts hydrophobic characteristics, which can be confirmed through contact angle measurements.To measure the angle, a 20 μL water droplet was placed onto both the flat parylene-C film and the nanostructured parylene-C film surface, as shown in Figure 5.
The water contact angles of the flat parylene-C and nanostructured films were measured to be 86°and 112°, respectively, demonstrating the hydrophobic nature of the nanostructured surface.Moreover, an increase in the contact angle led to a decrease in the surface energy.We calculated the surface energy of each film using Equation (2). [34] where  sv is the surface energy of the film,  lv is the surface energy of DI water (= 72.7 × 10 −3 J m −2 ), and  = 123.4m 4 J −2 is the empirical constant for various solid-liquid combinations.The surface energies calculated from Equation (2) for the flat parylene-C film and the nanostructured parylene-C film are 31.5 and 15.8 mJ m −2 , respectively.An increase in the contact angle leads to a decrease in surface energy, which effectively facilitates the removal of surface impurities. [35]Consequently, the hydrophobic characteristics of nanostructured films contribute to maintaining device stability.

Application of OLEDs
[23] One method to overcome this limitation is to introduce a nanostructured film as the outer layer. [24]The improvement in light outcoupling efficiency of nanostructures can be intuitively investigated by simulation using the finitedifference time-domain (FDTD) method. [36][39][40] Therefore, the nanostructured film developed in this study can be used for external light outcoupling in OLEDs based on the same principle.
Figure 6a shows the current density-voltage-luminance (JVL) characteristics of the OLED devices.The OLED with the nanostructured film exhibited increased luminance while maintaining the same current density as the reference OLED.This indicates that the film lamination process did not affect the electrical characteristics of the device, and improved the efficiency of external light outcoupling.The current efficiencies measured in the normal direction were 22.6 and 28.7 cd A −1 for the reference OLED and OLED with the nanostructured film, respectively, showing a 27% improvement.Furthermore, the emission characteristics in the entire direction resulting from light scattering can be measured using an integrating sphere.Figure 6c shows the EL spectra measured using an integrating sphere at a voltage of 5 V.When comparing the shape of the EL spectrum in the normal direction and the entire direction, they were almost identical.Through this, the device is expected to exhibit Lambertian emission characteristics.The measured EL spectra of the OLED showed a 40% improvement in the total directional emission characteristics owing to the application of nanostructured films.Therefore, our nanostructured parylene-C film successfully improved the efficiency of OLEDs while simultaneously reducing reflection from the external environment to increase the visibility of the display.

Conclusion
In this study, we fabricated nanostructured films using parylene-C to enhance the optical performance of OLED.The AR characteristics of these nanostructured films were achieved by optimizing the diameter of the nanopillars, resulting in a reflectance of <1%.The application of the nanostructured film to OLEDs improved the light outcoupling efficiency in the total direction by 40% compared with the reference device.Consequently, the nanostructured parylene-C film fabricated in this study has been demonstrated to improve the optical characteristics of OLEDs.Furthermore, we propose the application of these films as optical films in various organic optoelectronic devices.

Experimental Section
The surface morphology was characterized using field-emission scanning electron microscopy (FE-SEM) (SU8230, Hitachi) and atomic force microscopy (AFM) (SPM-9600, Shimadzu).Reflectance was measured using a UV-vis-NIR spectrophotometer (Lambda 950, Perkin Elmer) in the wavelength range of 400-700 nm and an incident angle of 8°.Reflectance simulation was conducted using Essential Macleod software (DigiClassic, Korea) to demonstrate the consistency of the measured reflectance.A contact-angle measurement instrument (Phoenix 300plus, SEO) was used to evaluate the hydrophobicity of the sample surfaces.

Figure 1 .
Figure 1.The fabrication process of a) parylene-C deposition on AAO substrate using CVD and b) nanostructured film with AAO etching.

Figure 3 .
Figure 3. AR characteristics of the fabricated nanostructured film.a) Measured reflectance as a function of diameter.b) Measured reflectance at 550 nm wavelength as a function of total thickness.

Figure 4 .
Figure 4. a) AFM image of D = 100 nm film.b) Histogram-based AFM bearing ratio.c) Refractive index variation graph calculated from the bearing ratio using volume fraction of nanopillars in Equation (1) (n 1 = 1.64, wavelength at 550 nm).d) Comparison graph of measured reflectance and simulated reflectance.

Figure 5 .
Figure 5. Water contact angle measurement for a) flat parylene-C film and b) nanostructured parylene-C film.

Figure 6 .
Figure 6.a) Energy diagram of OLEDs.Electrical characteristics: b) current density-voltage-luminance (J-V-L) characteristics and illustration of OLED outcoupling enhancement by nanostructured film and c) current efficiency-current density.d) EL spectrum and images with and without a nanostructured film integrated into OLEDs.

Table 1 .
AAO template size and aspect ratio (AAO template depth is fixed at 500 nm).