Light Extraction Layer Research Articles

Spontaneously formed cellulose-based random micro-textured film for light extraction in organic light-emitting diodes

This study utilized cellulose-based external light extraction films to enhance the outcoupling efficiency of organic light-emitting diodes (OLEDs). These films were created through a straightforward fabrication employing the biopolymers hydroxyethyl cellulose (HEC) and tannic acid (TA). We observed that the hydrogen bonding between HEC and TA could spontaneously generate a random microtextured surface on the HEC: TA films. Furthermore, the surface morphology of the HECTA film can be adjusted by varying the TA content. When applying the HECTA films as an external light-extraction layer for OLEDs, we noticed a remarkable improvement in the external quantum efficiency and current efficiency of the OLEDs up to 54% compared to those without the HECTA film. Additionally, the HECTA film displayed an outstanding ability to absorb ultraviolet light. Therefore, it is anticipated that the HECTA film can significantly contribute to prolonging the stability of OLEDs.

Enhancing light extraction efficiency and color stability for OLED by truncated micro-cone array films

A newly truncated micro-cone array film as light extraction layer for organic light emitting diodes (OLEDs) has been developed, which was fabricated by process combined with photolithography, polydimethylsiloxane (PDMS) molding, self-assembled anti-adhesion, and ultraviolet (UV) forming techniques. In this study, by varying the sidewall angle and fill factor of this film, their influences on the optical properties and light extraction efficiency for OLED are fully investigated. As a result, device performance of OLED employed truncated micro-cone array film, with a high fill factor and sidewall angle of 76.49⁰, was found to be significantly enhanced up to 52% compared with the reference OLED. Notably, the proposed truncated micro-cone array film also stabilized emissive color stability while the viewing angle changes from 0° to 70°, achieving an excellent angular shift with CIE-indices of (0.0025, 0.0066). Aforementioned findings contribute to the development in advanced OLEDs for display and lighting applications.

A morphological study of random nanostructured external light extraction layers for enhancing optical characteristics of OLEDs

This study presents an in-depth investigation of nano-pillar-based external light extraction techniques, focusing on the analysis of light extraction characteristics based on the pillar and plane structures.

Optical and Electrical Characterization of Visible Parylene Films.

Poly-dichloro-para-xylylene (parylene-C) film is formed through a chemical vapor deposition process, where monomeric gases are polymerized on the target surface at room temperature and are used as transparent insulating coating films. The thin parylene-C films exhibit uniform conformal layers even when deposited on substrates or surfaces with fine cracks, structures, and bumps. However, the film is highly transparent in the visible range (transmittance > 90%); thus, it is difficult to visually identify, inspect the coating process and check for any defects when used as an insulation film. Some reports have demonstrated the deposition of visible (hazy) parylene films through the control of the vaporization or pyrolysis of the parylene-C powder and sublimed dimers, respectively. Even though these films have been applied as device substrates and light extraction layers in organic light-emitting diodes (OLEDs), their optical and electrical characteristics have not been extensively explored, especially for their applications as insulation coatings. In this study, the characteristics of visible parylene films produced by tuning the ratio of dimer to monomer gases via the adjustments of the pyrolysis temperature are analyzed with electrical and optical methods. Parylene-C films deposited within the pyrolysis temperature of 400–700 °C exhibited a haze range of 10–90%. A relative reflectance of 18.8% at 550 nm of the visible light region was achieved in the visible parylene film deposited with a pyrolysis temperature of 400 °C. Resistivity in the order of 1010 Ω cm was achieved for the visible parylene films measured with the transmission line measurement (TLM) method. The films can be applied in advanced insulation coatings for various optical systems and electronic devices.

Island-like random scattering layer for light extraction in organic light-emitting diodes with enhanced luminous efficiencies and angular spectral stability

Optical outcoupling efficiency of organic light-emitting diode (OLED) can be improved using nanoscale or sub-micrometer quasi-periodic structures. An embossed nano/micro-structure inserted between the substrate and anode is known to be effective in extracting the waveguide mode. However, the techniques to fabricate these nanoscale or sub-micrometer structures are usually complicated, and the light extraction is wavelength-oriented. Herein, we develop a simple process to enhance the performance of OLED. We texture the glass surface with shrinking Ag droplets inthe form of island structures and then deposit SiNx planarization film to form an island-like random scattering layer (IRSL), which changes the light path of emitting light in a way to reduce light attenuation and extract emission outward. Furthermore, IRSL has a low haze and a high optical transparency, both of which are beneficial to light output. The OLEDs constructed with IRSL as an internal light extraction structure exhibit a significant improvement of current and power efficiencies by 52% and 62%, respectively, compared to the planar OLED device. In addition, compared to the conventional pillar-structured RSL, the IRSL not only facilitates the extraction of light for a broad range of visible light region, but also favors better angular spectral stability and Lambertian emission.

Enhanced Light Extraction from Organic Light-Emitting Diodes with Micro-Nano Hybrid Structure.

In this study, an external light extraction layer with a micro-nano hybrid structure was applied to improve the external light extraction efficiency of organic light-emitting diodes (OLEDs). A reactive ion-etching (RIE) process, using O2 and CHF3 plasma, was performed on the surface of the micro-scale pattern to form micro-nano hybrid structures. According to the results of this study, the nanostructures formed by the treatment of O2 and CHF3 were different, and the efficiency according to the structures was analyzed experimentally and theoretically. As a result, the OLED, to which the micro-nano hybrid structure, manufactured through a simple process, is applied, improved the external light extraction efficiency by up to 38%, and an extended viewing angle profile was obtained. Additionally, an effective method for enhancing the out-coupling efficiency of OLEDs was presented by optimizing the micro-nano hybrid structure according to process conditions.

Efficient and Stable Quantum‐Dot Light‐Emitting Diodes Enabled by Tin Oxide Multifunctional Electron Transport Layer

AbstractZinc oxide (ZnO) based nanoparticles (NPs) are the most commonly used electron transport layer (ETL) materials in quantum dot light‐emitting diodes (QLEDs). However, numerous defects, severe quenching, and poor stability of ZnO NPs limit the commercialization of QLEDs. Herein, tin oxide (SnO2) NPs are prepared as a substitute to ZnO. The obtained SnO2 NPs exhibit high conductivity, good transparency, and excellent stability, thus enabling them to be applied as multifunctional ETLs for various structured QLEDs, e.g., 1) as a phase tuning layer to tune the microcavity effect in inverted QLEDs, 2) as a thick protection layer to improve the stability of noninverted QLEDs, 3) as a sputtering buffer layer to protect the functional layers from the ion bombardment damage in transparent QLEDs, and 4) as an internal light extraction layer to enhance the light coupling efficiency of top‐emitting QLEDs. With the multifunctional SnO2 ETL, the resultant QLEDs are highly efficient (external quantum efficiency of 27.6%) and stable (half lifetime of 323 h at 4,459 cd m−2), which are comparable and even better than ZnMgO based devices. The results suggest that SnO2 is a promising ETL material for realizing efficient and stable QLEDs.

Enhanced light extraction from organic light emitting diodes using a flexible polymer-nanoparticle scattering layer

Efficient optical outcoupling from organic light emitting diodes (OLEDs) is an important challenge in solid state lighting technology. In this work, the effect of an external light extraction layer, prepared using polydimethylsiloxane (PDMS) and zinc oxide (ZnO) nanoparticles, was studied on the performance of red, green, blue and white OLEDs. The self-standing flexible film can effectively address the loss modes at the substrate-air interface. With the application of the layer, a maximum enhancement of about 80% in light outcoupling was observed in white OLEDs. Monochrome OLEDs also showed appreciable enhancement in emission with green having the highest improvement (102%), followed by red (56%) and blue (35%). Finite-difference time-domain (FDTD) simulations were performed for elucidating the mechanism behind the improvement in light extraction due to nanoparticle scattering layer. Total-Field Scattered-Field (TFSF) source was used to simulate the scattering cross-section for different wavelengths. Reduced Fresnel reflections at the air-glass interface and advantageous Mie scattering by ZnO nanoparticles seem to increase the optical outcoupling. This film could be a potential candidate for light extraction in OLEDs because of its facile fabrication, economical precursors, and adaptability to flexible substrates and absence of any possible optical artefacts to the emission.

Light extraction efficiency of nanostructures on OLED prepared using nanoimprinting and thermal expansion

In this study, three diverse structures of exterior light extraction layers (LELs) were fabricated to enhance the electroluminescent efficiency of organic light emitting diodes (OLEDs). The light extraction characteristics of LELs, based on the transmittance, reflectance, and haze properties determined using Snell’s law, Fresnel’s equations, and the envelop method, are discussed. The deduced regular transmittances of the Al- and ZnR-LELs were equivalent to their measured counterparts. Although the sand-LEL exhibited a low regular transmittance, its high optical scattering effect increased the efficiency of light extraction. The ZnR-LEL, possessing the high transmittance and low reflectance, exhibited higher light extraction efficiency than those of the sand- and Al-LELs. The external quantum efficiencies of OLEDs fabricated with a sand-, Al-, and ZnR-LELs were enhanced by 20.9, 11.2%, and 61.1%, respectively, compared to those of OLEDs without an LEL.

Single-step-fabricated disordered metasurfaces for enhanced light extraction from LEDs

While total internal reflection (TIR) lays the foundation for many important applications, foremost fibre optics that revolutionised information technologies, it is undesirable in some other applications such as light-emitting diodes (LEDs), which are a backbone for energy-efficient light sources. In the case of LEDs, TIR prevents photons from escaping the constituent high-index materials. Advances in material science have led to good efficiencies in generating photons from electron–hole pairs, making light extraction the bottleneck of the overall efficiency of LEDs. In recent years, the extraction efficiency has been improved, using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum. However, the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and time-consuming fabrication, which causes a gap between lab demonstration and industrial-level applications. Inspired by lightning bugs, we propose and realise a disordered metasurface for light extraction throughout the visible spectrum, achieved with single-step fabrication. By applying such a cost-effective light extraction layer, we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs, demonstrating a substantial potential for global energy-saving and sustainability.

Lambertian Extraction of Light from Organic Light-Emitting Devices Using Randomly Dispersed Sub-Wavelength Pillar Arrays.

A simple light extraction method is reported to minimize the variation in emission spectrum with viewing angle by embedding a light extraction layer, consists of randomly dispersed nano-pillars on a substrate. Nano-imprint lithography technique was employed to fabricate the nano-pillars using ultra-violet curable acrylate. An organic light emitting diode device employing the imprint technology showed 46% enhancement in light extraction efficiency in the forward direction with Lambertian emission pattern and no color change with viewing angle. Moreover, the emitted light does not show any specific periodic patterns.

Enhancing the light out-coupling efficiency of organic light-emitting devices with random corrugated structures

The mismatch of refractive index at the interfaces of substrate, electrode/organic material will cause nearly 80% light losses in the organic light-emitting device (OLED). To solve this problem, a light extraction layer with random corrugated structures is fabricated with a quite simple method to enhance the light out-coupling efficiency of OLED. Compared with the planar OLED, about 21% enhancement of current efficiency is achieved in the corrugated OLED. This improvement comes from the scattering effect of polymer phase separation film, which can suppress surface plasmon polariton losses and reduce waveguide mode optical losses. In addition, the structures can decrease the angular dependence of the emission spectra in the planar OLED. With these advantages, the random corrugated structures provide the potential to be applicable to OLED for high-efficiency lighting and full-color display.

Light extraction enhancement in organic light-emitting diodes through polyimide/porous silica hybrid films

In this work, light extraction efficiency of organic light-emitting diodes (OLEDs) with a spin-coated polyimide/porous silica hybrids were enhanced. The polyimide/porous silica thin films ( C1 and C2 ) were accomplished by spin coating a hybrid film composed of a polyimide-silica composite blended with various amount of porous silica nanoparticles into the opposite site of ITO glass. The optical, thermal, and morphology properties were controlled by adding various amount of porous silica. The incorporation of light extraction layer improved the maximum external quantum efficiency (EQE) by as much as 24.8% based on integrating sphere measurement condition when compared to that of a reference device without a light extraction layer. Furthermore, the device utilizing the light extraction layer showed identical EL spectra as the reference device did. Additionally, the optical emission distribution of the device was close to the Lambertian profile. The polyimide/porous silica hybrids demonstrated in this work are useful and efficient for OLED device for the enhancement of EL performance, indicating the potential in a wide range of applications, such as displays, lightings and so on. • The polyimide/porous silica light extraction films were accomplished by spin coating a hybrid film composed of a polyimide-silica composite blended with various amount of porous silica nanoparticles. • The incorporation of light extraction layer improved the maximum external quantum efficiency (EQE) by as much as 24.8%. • The device utilizing the light extraction layer showed identical EL spectra as the reference device did.

Phase-Separated Nanophotonic Structures by Inkjet Printing.

The spontaneous phase separation of two or more polymers is a thermodynamic process that can take place in both biological and synthetic materials and which results in the structuring of the matter from the micro- to the nanoscale. For photonic applications, it allows forming quasi-periodic or disordered assemblies of light scatterers at high throughput and low cost. The wet process methods currently used to fabricate phase-separated nanostructures (PSNs) limit the design possibilities, which in turn hinders the deployment of PSNs in commercialized products. To tackle this shortcoming, we introduce a versatile and industrially scalable deposition method based on the inkjet printing of a polymer blend, leading to PSNs with a feature size that is tuned from a few micrometers down to sub-100 nm. Consequently, PSNs can be rapidly processed into the desired macroscopic design. We demonstrate that these printed PSNs can improve light management in manifold photonic applications, exemplified here by exploiting them as a light extraction layer and a metasurface for light-emitting devices and point-of-care biosensors, respectively.

Planarized and Compact Light Scattering Layers Based on Disordered Titania Nanopillars for Light Extraction in Organic Light Emitting Diodes

AbstractIn this work, the extraction of waveguided and substrate modes in organic light emitting diodes (OLEDs) is improved by using compact light scattering layers composed of a disordered 2D array of TiO2 nanopillars. The TiO2 nanopillars are fabricated by combining a self‐assembly and a solvent‐assisted lift‐off process, and are further planarized by a 250 nm thin epoxy‐based photoresist layer to facilitate their anode deposition and integration within the OLED stack. This fabrication route allows engineering internal light outcoupling elements with a limited amount of parasitic absorption and with easily tunable light scattering properties that are effective over a broad spectral and angular range. Taking the example of a monochromatic bottom emitting OLED ( = 520 nm), the authors demonstrate an efficiency enhancement of +22%rel upon the incorporation of the planarized light extraction layer as well as ameliorated angular emission characteristics. This approach can be integrated in a high‐throughput fabrication routine and straightforwardly extended to other OLED layouts.

Efficiency enhancement in blue phosphorescent organic light emitting diode with silver nanoparticles prepared by plasma-assisted hot-filament evaporation as an external light-extraction layer

The uniform growth of silver (Ag) nanoparticles in large areas is important for improving the flat panel devices, mainly organic light emitting diodes (OLED). In this work, two dimensional (2D) arrays of Ag nanoparticles were grown on glass surface substrates via plasma-assisted hot-filament evaporation technique at different plasma environments: no plasma, plasma cleaning (PC), plasma deposition (PD) and PC&PD, which were utilized as an external light-extraction layer for blue phosphorescent OLED device. The surface morphology of the Ag nanoparticles revealed the formation of spherical-like shapes of uniform size which ranged from 7 to 14 nm at different plasma conditions. X-ray diffraction showed that the nanoparticles were mainly single crystals and their crystallinity was significantly enhanced by employing plasma process in PC&PD. The localized surface plasmon resonance peaks of Ag nanoparticles were located at around 380–430 nm which suggests a dependency on diameter and interparticle spacing. The variation of refractive index of Ag nanoparticles 2D array in the visible wavelength from 1.9 to 2.4 plays a key role in high performance OLED. The light-extraction of the OLED device with Ag nanoparticles at PC&PD condition appeared to increase by 82.5% the luminance efficiency compared to that of a reference device without the Ag nanoparticle layer. The effects of the plasma environment on the morphological, structural and optical properties of Ag nanoparticles layer are discussed in the following sections by taking into consideration the effects of glass modification by these Ag nanoparticles on OLED performance.

Random organic nano-textured microstructures formed by photoexcitation for light extraction of blue OLEDs

Organic light-emitting diodes require efficient and inexpensive light extraction methods for applications toward lighting and displays. Here, a facile and effective method for fabricating random organic microstructures for efficient light extraction from blue OLEDs is presented. Simple drop casting of a TCTA and B4PyMPM mixed solution followed by UV curing results in films with irregular-shaped microstructures (DACMs), ideal for light extraction without diffraction patterns. An external quantum efficiency (EQE) of 44.3% is realized by attaching DACMs formed on a polymer film to a blue phosphorescent OLED. The efficiency is improved by 35% compared to a planar device without the light extraction layer, greater than the 22% improvement obtained by using hemispherical-shaped microlens arrays. The method is useful for OLED lighting and potentially in displays because of the simple fabrication method that is applicable to a large area on rigid or flexible substrates, the low material cost, the insolubility of the microstructure in alkyl halide solvents such as chloroform, and the controllability of the structure through the solution process. Furthermore, we fully discuss on the EQE, enhancement ratio and the efficiency of light extraction structure (ELOS). We show that enhancement ratio and ELOS should be obtained using the optimized device structure as the reference device to be useful indicators for the effectiveness of a light extraction structure.

8‐4: Invited Paper: Flexible glass Substrate for OLED Lightning Application ans Efficient Internal Light Extraction for OLED Lightning Devices

We discuss application of Corning® Willow® Glass for OLED Lighting integrated substrate which includes an internal light extraction layer for high efficacy. Two technologies – (1) glass carrier, and (2) edge healing, needed for successful product development will be discussed.

Preparation of various morphological films at nanoscale by phase separation method

A film consisting of nanopore and nanopillar structures was produced from a binary immiscible polymer solution of poly(amic acid) (PAA) and polyimide (PI), which can be used as a potential light extraction layer or flexible substrate in organic light-emitting diode (OLED) devices. A phase separation method was applied to create uniform and random pattern structures in nanoscale via a simple spin-coating technique. Firstly, the binary immiscible polymer solutions of PAA and PI whose precursor is carbazole-based dianiline were prepared with various volume ratios, and then the polymer mixtures were spin-coated onto substrates to form transparent films with various morphologies and dimensions, as observed by a field emission scanning electron microscope technique. In addition, after annealing PAA/PI films at 300 °C, the homogenous and flexible characteristic of nanopore and nanopillar structures could be achieved.

Enhanced optical efficiency and color purity for organic light-emitting diodes by finely optimizing parameters of nanoscale low-refractive index grid

To extract the confined waveguided light in organic light-emitting diodes (OLEDs), inserting a low refractive index (RI) periodic structure between the anode and organic layer has been widely investigated as a promising technology. However, the periodic-structure-based light extraction applied inside devices has been shown to severely distort spectrum and affect EL characteristics. In this study, a simple light extraction technology using periodic low-RI nanodot array (NDA) as internal light extraction layer has been demonstrated. The NDA was fabricated simply via laser interference lithography (LIL). The structural parameters of periodic pattern, distance, and height were easily controlled by the LIL process. From computational analysis using finite-difference time-domain (FDTD) method, the NDA with 300 nm pitch and 0.3 coverage ratio per unit cell with 60 nm height showed the highest enhancement with spectral-distortion-minimized characteristics. Through both computational and experimental systematic analysis on the structural parameters of low-RI NDA-embedded OLEDs, highly efficient OLEDs have been fabricated. Finally, as representative indicators, hexagonal and rectangular positioned NDA-embedded OLEDs showed highly improved external quantum efficiencies of 2.44 (+29.55%) and 2.77 (+57.38%), respectively. Furthermore, the disadvantage originating from the nanoscale surface roughness on the transparent conductive oxide was minimized.