Light Extraction Structures Research Articles

Modeling the co-assembly of binary nanoparticles

In this work, we present a binary assembly model that can predict the co-assembly structure and spatial frequency spectra of monodispersed nanoparticles with two different particle sizes. The approach relies on an iterative algorithm based on geometric constraints, which can simulate the assembly patterns of particles with two distinct diameters, size distributions, and at various mixture ratios on a planar surface. The two-dimensional spatial-frequency spectra of the modeled assembles can be analyzed using fast Fourier transform analysis to examine their frequency content. The simulated co-assembly structures and spectra are compared with assembled nanoparticles fabricated using transfer coating method are in qualitative agreement with the experimental results. The co-assembly model can also be used to predict the peak spatial frequency and the full-width at half-maximum bandwidth, which can lead to the design of the structure spectra by selection of different monodispersed particles. This work can find applications in fabrication of non-periodic nanostructures for functional surfaces, light extraction structures, and broadband nanophotonics.

Maximizing the Light Extraction Efficiency for AlGaN-Based DUV-LEDs with Two Optimally Designed Surface Structures under the Guidance of PSO

A novel method of utilizing an intelligent algorithm to guide the light extraction surface structure designing process for deep-ultraviolet light emitting diodes (DUV-LEDs) is proposed and investigated. Two kinds of surface structures based on the truncated pyramid array (TPA) and truncated cone array (TCA) are applied, which are expected to suppress the total internal reflection (TIR) effect and increase the light extraction efficiency (LEE). By addressing particle swarm optimization (PSO), the TPA and TCA microstructures constructed on the sapphire layer of the flip-chip DUV-LEDs are optimized. Compared to the conventional structure design method of parameter sweeping, this algorithm has much higher design efficiency and better optical properties. At the DUV wavelength of 280 nm, as a result, significant increases of 221% and 257% on the LEE are realized over the two forms of optimized surface structures. This approach provides another design path for DUV-LED light extraction structures.

Hybrid Tandem White OLED with Long Lifetime and 150 Lm W−1 in Luminous Efficacy Based on TADF Blue Emitter Stabilized with Phosphorescent Red Emitter

AbstractModern lighting requires high efficiency and long lifetime simultaneously. In this paper, highly efficient white organic light emitting diodes (WOLEDs) are demonstrated based on a modified hybrid tandem structure constructed with a phosphorescent yellow‐red unit using an exciplex host and a blue‐red unit based on a thermally activated delayed fluorescence (TADF) blue emitter, and the devices are properly built with internal and external light extraction structures. The resulting four‐color WOLED exhibits an external quantum efficiency and luminous efficacy of 126.2% and 150.7 Lm W−1, respectively, and the color rendering index is close to 80 at a brightness of 1000 cd m−2. This device also shows a very good color stability with increasing luminance up to 15 000 cd m−2 and there is only a small angular color variation for viewing angles changing from 0° to 70°. The T50 lifetime (defined as the time corresponding to 50% decrease in luminance from an initial value) of the WOLED at an initial luminance of 1000 cd m−2 is extrapolated as 12 600 h.

Highly Efficient Tandem White OLED Using a Hollow Structure

AbstractA simple fabrication method for a light extraction layer is required. In this report, an internal light extraction layer composed of a high refractive index material and an air void is fabricated in five steps. A direct printing process followed by annealing of the randomly arrayed poly(benzyl methacrylate) pillars after a planarization process using TiO2‐nanoparticle dispersed resist and sol is used. These substrates are used for light extraction in white tandem organic light emitting diodes (WOLEDs). By combining the hollow structure and hemi‐spherical lens, WOLEDs without and with the light extraction structures are found to show maximum external quantum efficiencies of 35.6% and 103%, respectively. The power efficiencies at 100 cd m−2 of the WOLEDs without and with the light extraction structures are found to be 26.5 and 93.2 lm W−1, respectively. A color rendering index of 86.4, correlated color temperature of 4860 K, and CIE of (0.353, 0.389) are achieved in the WOLEDs with light extraction structures.

Organic Light-Emitting Diodes: Pushing Toward the Limits and Beyond.

Organic light-emitting diodes (OLEDs) are established as a mainstream light source for display applications and can now be found in a plethora of consumer electronic devices used daily. This success can be attributed to the rich luminescent properties of organic materials, but efficiency enhancement made over the last few decades has also played a significant role in making OLEDs a practically viable technology. This report summarizes the efforts made so far to improve the external quantum efficiency (EQE) of OLEDs and discusses what should further be done to push toward the ultimate efficiency that can be offered by OLEDs. The study indicates that EQE close to 58% and 80% can be within reach without and with additional light extraction structures, respectively, with an optimal combination of cavity engineering, low-index transport layers, and horizontal dipole orientation. In addition, recent endeavors to identify possible applications of OLEDs beyond displays are presented with emphasis on their potential in wearable healthcare, such as OLED-based pulse oximetry as well as phototherapeutic applications based on body-attachable flexible OLED patches. OLEDs with fabric-like form factors and washable encapsulation strategies are also introduced as technologies essential to the success of OLED-based wearable electronics.

A Solution Processed Flexible Nanocomposite Substrate with Efficient Light Extraction via Periodic Wrinkles for White Organic Light‐Emitting Diodes

AbstractA new flexible nanocomposite substrate is reported to enhance the light outcoupling efficiency of white organic light‐emitting diodes (OLEDs). The substrate comprises silver nanowires as electrode and periodic wrinkles as external structures for light extraction. It can outcouple both the waveguide and substrate modes. With the optimized external light extraction structures, white OLEDs based on the nanocomposite substrates show external quantum efficiency (EQE) enhancement factor of 2.34 compared to control devices on indium tin oxide (ITO)/glass, with an EQE of (38.4 ± 2.7)%, a power efficiency of 85.8 ± 2.9 lm W−1, and a current efficiency of 100.3 ± 7.1 cd A−1 at luminance of 1000 cd m−2. Moreover, the external wrinkles improve the spectral uniformity over a wide range of viewing angles, which is an important feature for lighting applications. The nanocomposite substrate is fully solution‐processed and thus can potentially lower the manufacturing cost of high‐efficiency flexible OLEDs.

26.4: Invited Paper: Light Extraction Improvement by High Refractive Index Nanocrystal Materials for OLED Displays and Lightings

While there are many known ways to improve light extraction, however, the cost‐effective manufacturing process and the requirement to achieve high quality image severely restrict how light can be extracted. Pixelligent's solution processable high refractive index zirconia materials allow for the creation of high index, transparent, low‐haze nanocomposites that can build the necessary light extraction structures. The application of Pixelligent's PixClear® high‐refractive index (> 1.7) zirconia nanocomposite films onto glass or flexible substrates by inkjet printing is discussed in this paper. The optically clear nanocomposite formulation comprises zirconia nanocrystals (< 10 nm in diameter), a UV‐curable binder material, a high‐boiling point solvent, and with (or without) scatters. Clear films with > 90% transmission and films containing scattering particles have been printed as uniform layers and patterned structures by inkjet printing. A broad range of film thicknesses are achievable for inkjet printing, ranging as low as tens of nanometers and up to 100 microns. Main advantages of inkjet printing are the material conservation during deposition and complex patterning of the printed material.For this study, we demonstrate the ability of Pixelligent's zirconia nanocomposites to improve the light extraction and can be inkjet printed, emphasizing a new way in which high‐refractive index materials are being incorporated into devices for OLED lighting and displays.

Internal light extraction structures based on SiO2 nanoparticles-conducting polymers for organic light-emitting diodes

Internal light extraction structures based on SiO2 nanoparticles-conducting polymers for organic light-emitting diodes

150 mW deep-ultraviolet light-emitting diodes with large-area AlN nanophotonic light-extraction structure emitting at 265 nm

High-power 265 nm deep-ultraviolet (DUV) AlGaN-based light-emitting diodes (LEDs) with large-area AlN nanophotonic light-extraction structures that were fabricated by a nanoimprint lithography process are presented. Each DUV-LED has a large active area (mesa size of ∼0.35 mm2) and a uniform current spreading design that allows high injection current operation. We have shown that these DUV-LEDs with their large-area nanoimprinted AlN nanophotonic structures exhibit wider near-field emitting areas, stronger far-field extracted light intensities, and an approximately 20-fold increase in output power when compared with a conventional flat-surface DUV-LED. A large-area nanoimprinted single-chip DUV-LED operating in the UV-C wavelength regime has demonstrated a record continuous-wave output power in excess of 150 mW for an injection current of 850 mA at a peak emission wavelength of 265 nm.

Enhanced light extraction in tunnel junction-enabled top emitting UV LEDs

The efficiency of ultraviolet (UV) light-emitting diodes (LEDs) is critically limited by absorption losses in p-type and metal layers. In this work, surface-roughening-based light extraction structures were combined with tunneling-based top-layer contacts to achieve highly efficient top-side light extraction in UV LEDs. By using self-assembled Ni nanoclusters as an etch mask, the top surface-roughened LEDs were found to enhance the external quantum efficiency by over 40% for UV LEDs with a peak emission wavelength of 326 nm. The method described here can be used for fabricating highly efficient UV LEDs without the need for complex manufacturing techniques such as flip chip bonding.

Color-stable, ITO-free white organic light-emitting diodes with enhanced efficiency using solution-processed transparent electrodes and optical outcoupling layers

In this work, we demonstrate color-stable, ITO-free white organic light-emitting diodes (WOLEDs) with enhanced efficiencies by combining the high-conductivity conducting polymer PEDOT:PSS as transparent electrode and a nanoparticle-based scattering layer (NPSL) as the effective optical out-coupling layer. In addition to efficiency enhancement, the NPSL is also beneficial to the stabilization of electroluminescent spectra/colors over viewing angles. Both the PEDOT:PSS and the NPSL can be fabricated by simple, low-temperature solution processing. The integration of both solution-processable transparent electrodes and light extraction structures into OLEDs is particularly attractive for applications since they simultaneously provide manufacturing, cost and efficiency advantages.

We Want Our Photons Back: Simple Nanostructures for White Organic Light‐Emitting Diode Outcoupling

Conventional planar organic light‐emitting diodes (OLEDs) suffer from poor light extraction due to the total internal reflection at the waveguided interfaces. Therefore, the development of efficient light extraction structures is of great necessity and significance to realize practical applications in large area and cost‐effective light sources. In this paper, a high‐performance internal light outcoupling system for white OLEDs with spontaneously formed metal oxide nanostructures is developed. The fabrication process of the outcoupling system is simple and can be scaled to large area manufacturing. The enhancement of external quantum efficiency in white OLEDs comprising the outcoupling system reaches a factor of 1.7, and it is further increased to 2.9 when a hemispherical lens is employed. Together with the improvement of light extraction, excellent color stability over broad viewing angles is achieved.

Nano-particle based scattering layers for optical efficiency enhancement of organic light-emitting diodes and organic solar cells

The performance of both organic light-emitting diodes (OLEDs) and organic solar cells (OSC) depends on efficient coupling between optical far field modes and the emitting/absorbing region of the device. Current approaches towards OLEDs with efficient light-extraction often are limited to single-color emission or require expensive, non-standard substrates or top-down structuring, which reduces compatibility with large-area light sources. Here, we report on integrating solution-processed nano-particle based light-scattering films close to the active region of organic semiconductor devices. In OLEDs, these films efficiently extract light that would otherwise remain trapped in the device. Without additional external outcoupling structures, translucent white OLEDs containing these scattering films achieve luminous efficacies of 46 lm W−1 and external quantum efficiencies of 33% (both at 1000 cd m−2). These are by far the highest numbers ever reported for translucent white OLEDs and the best values in the open literature for any white device on a conventional substrate. By applying additional light-extraction structures, 62 lm W−1 and 46% EQE are reached. Besides universally enhancing light-extraction in various OLED configurations, including flexible, translucent, single-color, and white OLEDs, the nano-particle scattering film boosts the short-circuit current density in translucent organic solar cells by up to 70%.

A bipolar host containing carbazole/dibenzothiophene for efficient solution-processed blue and white phosphorescent OLEDs

The photophysical properties of a novel small molecule host containing carbazole/dibenzothiophene (CzDBS) were studied. Solution processing of CzDBS found well-balanced bipolar functionalities, high triplet bandgap, and good thin-film morphology. By using this compound as a host material, high-performance solution-processed blue and white phosphorescent organic light emitting devices were fabricated. The blue and white emitting devices showed maximum efficiencies, with 13.9% external quantum efficiency (EQE), 29.7 cd A−1 CE, 18.6 lm W−1 PE and 13.8% EQE, 29.1 cd A−1 CE, and 17.7 lm W−1 PE, respectively. In addition, through incorporation of the light-extraction structures, white-light emission with the Commission Internationale de I'Eclairage chromaticity coordinates (0.38, 0.40) and satisfactory efficiencies of 22.9% EQE, 48.4 cd A−1 CE, and 30.4 lm W−1 PE were demonstrated.

Review paper: Recent developments in light extraction technologies of organic light emitting diodes

Organic light emitting diodes (OLEDs) have rapidly progressed in recent years due to their potential applications in flat panel displays and solid-state lighting. In spite of the commercialization of OLEDs, they still have a low out-coupling efficiency of about 20% due to factors such as the total internal reflection, absorption, and surface plasmon coupling. This light out-coupling efficiency is a major limitation on the high efficiency levels of OLEDs. Hence, enhancing the light out-coupling efficiency of OLEDs offers the greatest potential for achieving a substantial increase in the external quantum efficiency and power efficiency of OLEDs. Accordingly, significant advancements in OLEDs have driven the development of light extraction technologies as well as highly transparent conducting electrode materials. Recent efforts to combine light extraction structures with the improved out-coupling efficiency of OLEDs have produced OLEDs with an efficiency level that matches the efficiency of a fluorescent tube (>100 lm/W). This paper reviews the technical issues and recent progress in light extraction technologies and discusses ways of enhancing the out-coupling efficiency of OLEDs.

Emission Characteristics of OLEDs on Planar and Non-Planar Substrates

AbstractWe report on light extraction experiments with polyfluorene based organic light emitting diodes (OLEDs). The intensity distribution of emission within the substrate and within air is determined experimentally. For the devices investigated in this study, we find that the external quantum efficiency can be increased by a factor of 1.82, in the case of complete suppression of waveguiding within the substrate. The experimental data is in good agreement with modeling results based on dipole emission within a multilayer system. As a consequence of these findings, we modified the device topology by introducing light extraction structures between glass substrate and indium tin oxide anode. This results in redirection of photon trajectories and thus in a higher probability to extract waveguided radiation. At a luminance of 100 cd/m2, the photometric efficiency for emission in forward direction is enhanced by a factor of 1.45 to 8.9 cd/A.