Top-emitting Devices Research Articles

Graphene-Enhanced UV-C LEDs.

Light-emitting diodes in the UV-C spectral range (UV-C LEDs) can potentially replace bulky and toxic mercury lamps in a wide range of applications including sterilization and water purification. Several obstacles still limit the efficiencies of UV-C LEDs. Devices in flip-chip geometry suffer from a huge difference in the work functions between the p-AlGaN and high-reflective Al mirrors, whereas the absence of UV-C transparent current spreading layers limits the development of UV-C LEDs in standard geometry. Here it is demonstrated that transfer-free graphene implemented directly onto the p-AlGaN top layer by a plasma enhanced chemical vapor deposition approach enables highly efficient 275nm UV-C LEDs in both, flip-chip and standard geometry. In flip-chip geometry, the graphene acts as a contact interlayer between the Al-mirror and the p-AlGaN enabling an external quantum efficiency (EQE) of 9.5% and a wall-plug efficiency (WPE) of 5.5% at 8V. Graphene combined with a ≈1nm NiOx support layer allows a turn-on voltage <5V. In standard geometry graphene acts as a current spreading layer on a length scale up to 1mm. These top-emitting devices exhibit a EQE of 2.1% at 8.7V and a WPE of 1.1%.

9‐3: The Latest Technology Breakthroughs for 31” 4K Flexible Printed OLED TV Display Technology

The flexible printed OLED TV display technology will be able to create new application market, such as rollable/foldable TV, in the future. It's TV Mobiles. Since the TCL (TCL CSOT, Guangdong Juhua Printed Display Technology Co. Ltd. &amp; TCL Corporate Research) has successfully demonstrated the 31” FHD flexible printed OLED TV, there are a lot of technology breakthroughs for improving the panel performance and cost reduction. The co‐planar top gate Oxide TFT device architecture, printing OLED top emission device architecture with new OLED materials, MURA free ink jet printing technology with unique bank design, advanced encapsulation technology with new inorganic material for panel performance improvement are developed. Especially innovative mechanical release technology without using laser lift off technology is developed for cost reduction and production yield up in flexible printed OLED TV MP line. Based on these technology breakthroughs, 31” 4K flexible printed OLED TV with high resolution(142ppi) are fabricated.The performance of 31” flexible printed OLED TV is, 4K high resolution (3840x2160, 142ppi), luminance: FW 150nits, excellent C/R; &gt; 1M:1, color gamut: DCI‐P3: &gt;99%, wide viewing angle (Δu'v' &lt; 0.02): ≥ +/‐ 45°, A/R: 47.75%, and curvature radius: 20mmIn this paper, the panel performance of 31” 4K high resolution flexible printed OLED TV and the latest technology breakthroughs are discussed in details.

Twisted vibration of Aza-azulene promotes ultra-fast triplet-singlet up-conversion for extremely stable red electroluminescence

The fast triplet-singlet up-conversion based on thermally activated delayed fluorescence (TADF) molecules is a unique effort to improve the utilization of triplet excitons. We construct two TADF molecules using nonbenzenoid aromatic aza-azulene as donor and triazine as acceptor. The distortion structural resonance of aza-azulene enhances the reverse intersystem crossing (RISC) process, resulting in efficient inter/intramolecular charge transfer (CT) emission. The rate constant of RISC (kRISC) is 9.0 × 106 and 8.4 × 106 s−1 in solid states. Efficient Förster Energy Transfer (FRET) and/or Dexter Energy Transfer (DET) between these molecules and dopants make them suitable host materials to improve the performances of red phosphorescent organic light-emitting diodes (OLEDs). Especially, the top-emitting device achieves an external quantum efficiency (EQE) of 51.3% and a current efficiency of 68.2 cd/A at 1000 cd m−2 with a long device lifetime of LT95 = 52,350 and LT50 = 536,200 h.

Efficient Flexible Fabric-Based Top-Emitting Polymer Light-Emitting Devices for Wearable Electronics.

Low-cost fabric-based top-emitting polymer light-emitting devices (Fa-TPLEDs) have aroused increasing attention due to their remarkable potential applications in wearable displays. However, it is still challenging to realize efficient all-solution-processed devices from bottom electrodes to top electrodes with large-scale fabrication. Here, a smooth reflective Ag cathode integrated on fabric by one-step silver mirror reaction and a composite transparent anode of polydimethylsiloxane/silver nanowires/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) via a water-assisted peeling method are presented, both of which possess excellent optoelectrical properties and robust mechanical flexibility. The Fa-TPLEDs are constructed by spin-coating functional layers on the bottom reflective cathodes and laminating the top transparent anodes. The Fa-TPLEDs show a current efficiency of 16.3cdA-1 , an external quantum efficiency of 4.9% and angle-independent electroluminescence spectra. In addition, the Fa-TPLEDs possess excellent mechanical stability, maintaining a current efficiency of 14.3cdA-1 after 200 bending cycles at a radius of 4mm. The results demonstrate that the integration of solution-processed reflective cathodes and transparent anodes sheds light on a new avenue to construct low-cost and efficient fabric-based devices, showing great potential applications in emerging smart flexible/wearable electronics.

Optical performance evaluation of blue tandem top-emitting OLED device

Optical performance evaluation of blue tandem top-emitting OLED device

Top-Emitting Quantum Dot Light-Emitting Diodes: Theory, Optimization, and Application.

The superior optical properties of colloidal quantum dots (QDs) have garnered significant broad interest from academia and industry owing to their successful application in self-emitting QD-based light-emitting diodes (QLEDs). In particular, active research is being conducted on QLEDs with top-emission device architectures (TQLEDs) owing to their advantages such as easy integration with conventional backplanes, high color purity, and excellent light extraction. However, due to the complicated optical phenomena and their highly sensitive optoelectrical properties to experimental variations, TQLEDs cannot be optimized easily for practical use. This review summarizes previous studies that have investigated top-emitting device structures and discusses ways to advance the performance of TQLEDs. First, theories relevant to the optoelectrical properties of TQLEDs are introduced. Second, advancements in device optimization are presented, where the underlying theories for each are considered. Finally, multilateral strategies for TQLEDs to enable their wider application to advanced industries are discussed. This work believes that this review can provide valuable insights for realizing commercial TQLEDs applicable to a broad range of applications.

Influence of Light-Matter Interaction on Efficiency of Quantum-Dot Light-Emitting Diodes.

Light-matter interaction can affect the radiative decay rate of excitons and thus the emission quantum yield (QY) of quantum dots (QDs). In this work, light-matter interaction and outcoupling efficiency of QD light-emitting diodes with different structures are investigated experimentally and theoretically. We show that the external quantum efficiency (EQE) of top-emitting devices is higher than that of the bottom-emitting devices, which is mainly due to the stronger light-matter interaction and higher light outcoupling efficiency of the top-emitting structures. In addition, we show that the QY enhancement induced by light-matter interaction is more significant for low-QY QDs. Our results suggest that top-emitting structures are more powerful for improving the EQE of devices built with low-QY QDs.

Top-Emitting Active-Matrix Quantum Dot Light-Emitting Diode Array with Optical Microcavity for Micro QLED Display.

An electroluminescent quantum-dot light-emitting diode (QLED) device and a micro QLED device array with a top-emitting structure were demonstrated in this study. The QLED device was fabricated in the normal structure of [ITO/Ag/ITO anode]/PEDOT:PSS/PVK/QDs/[ZnO nanoparticles]/Ag/MoO3, in which the semi-transparent MoO3-capped Ag cathode and the reflective ITO/metal/ITO (IMI) anode were designed to form an optical microcavity. Compared with conventional bottom-emitting QLED, the microcavity-based top-emitting QLED possessed enhanced optical properties, e.g., ~500% luminance, ~300% current efficiency, and a narrower bandwidth. A 1.49 inch micro QLED panel with 86,400 top-emitting QLED devices in two different sizes (17 × 78 μm2 and 74 × 40.5 μm2) on a low-temperature polysilicon (LTPS) backplane was also fabricated, demonstrating the top-emitting QLED with microcavity as a promising structure in future micro display applications.

Highly conductive and transparent thin metal layer for reducing microcavity effect in top-emitting white organic light-emitting diode

The optical and electrical properties of the top electrode are crucial for the structural design of top-emitting organic light-emitting diodes (OLEDs). A uniform metal layer, if obtainable, can be electrically excellent and optically well-tuned. In this study, Ag deposited with Mg at a concentration of 9:1 to form a metal thin film exhibits higher conductivity and higher transmittance than those of a neat Ag thin film within a thickness of 10 nm or less. In addition, the thin Mg:LiF (1:1) layer renders the Mg:Ag metal film more uniform and enhances its electron injection property. If a Mg:LiF/Mg:Ag thin film is applied to top-emitting white OLEDs, the driving voltage is lowered and the efficiency is improved. In addition, color shifts for viewing angles are reduced, and the emission characteristics are close to Lambertian. • 10-nm-thick Mg:Ag (1:9) metal forms a more uniform film compared with neat Ag thin metal of the same thickness. • Thin Mg:LiF (1:1) layer improves the optical and electrical properties of the Mg:Ag thin film. • Top-emitting white OLED device with thin Mg:Ag shows higher efficiency and superior angular emission characteristics.

Preview the Products on Display at Display Week 2022

Preview the Products on Display at Display Week 2022

All inkjet‐printed 6.95″ 217 ppi active matrix QD‐LED display with RGB Cd‐free QDs in the top‐emission device structure

AbstractElectroluminescence quantum dot devices (QD‐LEDs) show the promise in the development of future displays. For commercialization, the top emission device structure is more advantageous than the bottom emission device structure to realize the high aperture ratio full color displays. Moreover, it is essential to understand not only the device structure and characteristics of QD‐LEDs but also the challenging issues in inkjet‐printed devices. Here, we demonstrated the world first all inkjet‐printed Cd‐free active matrix QD‐LEDs based on getting over the hurdles of all inkjet printing process. Especially, we have fabricated QD‐LED devices by stacking 4‐soluble layers without intermixing and studied the factors affecting QD efficiency and QD printing and the influence of ETL solvent on QD. Inkjet‐printed QD‐LED devices have achieved 8 cd/A, 70 cd/A, and 8 cd/A current efficiency of red, green, and blue, respectively, on the basis of 250 nits of full white RGB panels. We believe that our work clarifies an encouraging step toward the practical application of Cd‐free based QD‐LED displays.

Top-Emitting Microcavity Light-Emitting Diodes Based on All-Thermally Evaporated Lead-Free Copper Halide Self-Trapped-Exciton Emitters.

Lead-free metal halide light-emitting diodes (LEDs) based on cesium copper halide (CsCu2I3) self-trapped-exciton (STE) emissions show great potential in lighting and color display applications, especially because of their nontoxicity and earth abundance. However, so far, the efficiency and color purity of CsCu2I3-based LEDs remain low. Here we demonstrate the emission of a CsCu2I3 emitter can be enhanced and narrowed in a top-emitting microcavity device. Consequently, the CsCu2I3-based LED device with the assistance of a top-emitting microcavity has significantly narrowed and enhanced the emission spectrum with a full width at half-maximum of 59 nm and a maximum forward brightness of 14767 cd m-2. To the best of our knowledge, this work achieves the narrowest CsCu2I3 LED spectra and demonstrates the potential of employing the microcavity effect to increase the efficiency and color purity of STE-based light-emitting devices.

Reflective 3D pixel configuration for enhancing efficiency of OLED displays

Although active-matrix organic light-emitting diode displays (AMOLEDs) have been widely applied, effective optical out-coupling techniques possessing both integration compatibility and image quality remain highly desired for further improving their efficiency and power consumption performances. In our previous simulation studies, a 3-dimensional (3D) reflective pixel structure filled with a patterned high-index material was proposed for boosting light extraction efficiency of common top-emitting AMOLED pixel devices. In this work, we conducted experimental studies on the proposed 3D OLED pixel configuration to validate the simulation and design. 3D OLED pixel devices with varied pixel dimensions were implemented and their structures, electrical properties, efficiencies, and EL characteristics were characterized. Significant efficiency gains were obtained with the μm-scale reflective 3D pixel devices and experiment results well corroborated multiscale optical simulation results, confirming effectiveness of the 3D OLED pixel for enhancing light extraction of OLED displays. • Implemented reflective 3D OLED pixel configurations for enhancing light extraction of OLED displays. • Significant gain in light extraction was obtained with reducing pixel sizes. • Experiment results validate optical simulation and effectiveness of 3D pixel for light extraction.

Curved Mirror Arrays for Light Extraction in Top-Emitting Organic Light-Emitting Diodes.

The light outcoupling efficiency of a top-emitting organic light-emitting diode (OLED) is only about 20%, and the majority of the light is trapped in the waveguide modes and surface plasmon polariton (SPP) modes. Extracting the trapped modes can reduce the device power consumption and improve the operating lifetime. In this study, we demonstrate a top-emitting OLED structure with a dielectric spacer to suppress the SPP mode and with a patterned back mirror to extract the waveguide modes. We examine and compare several curved mirror arrays and conclude that a micromirror array (μMA) can efficiently extract the waveguide modes while minimizing the absorption loss. The optimized μMA device with a semi-transparent top electrode shows a 36% external quantum efficiency, 2 times higher than the referenced device. This optical design can be easily incorporated into a top-emitting device and has a great potential for displays and lighting applications.

Ultrapure Blue Phosphorescent Organic Light-Emitting Diodes Employing a Twisted Pt(II) Complex.

Described herein is a stable complex, Pt(mpzpyOczpy-mesi), embodying efficient, narrow blue emission. The highly twisted structure of the complex improves the stability and efficiency of photo- and electroluminescence by reducing the intermolecular interactions. The complex in solution shows high photoluminescence efficiency (>95%) and radiative decay rate (Kr = 2.9 × 105 s-1) with a narrow emission spectrum. The bottom-emitting phosphorescent device, BE1, exhibits durable deep blue emission with CIE coordinates of (0.145, 0.166) and 5.2 h of LT50 at an initial luminance of 685 cd/m2. Top-emitting devices, TE1 and TE2, achieve ultrapure blue color with CIEx,y values of (0.141, 0.068) and (0.140, 0.071), respectively. TE4 shows high brightness of 3405 cd m-2 at 50 mA m-2, EQE of 10.2% at 1000 cd/m2, and almost negligible color deviation around (0.135, 0.096) at viewing angles of 0°-60°.

12.1: Invited Paper: Efficiency enhancement in dual emission OLEDs

Transparent displays are attracting more and more attentions nowadays, benefited from its widely application in TVs, automobiles and shopwindows. Dual emission Organic Light Emitting Devices (OLEDs) are efficient solution to increase the transmittance of OLED displays, while the device efficiency is always low. Here we introduce a thin metal film to enhance the device efficiency by building a weak micro‐cavity. To further increase the efficiency, lifetime and maximum luminance, tandem device is established with the efficiency comparable to single top emission device

26‐4: High Performance Multiple‐Sensitizing Process Red Hyperfluorescence Device with Assistant Emission Layer (AEL)

In this work, through analysis of excition distribution mechanism, the multiple‐sensitizing process red hyperfluorescence top–emission device achieved unprecedent high current efficiency of 53.6 cd/A with CIE (0.666, 0.334), LT95 of 900 hrs and low voltage of 4.15 V at current density of 15mA/cm2 among the reported Hyperfluorescence OLEDs.

Study of beam shaped of large-aperture top-emitting laser device

The output beam of the large-aperture top emitting laser is a kind of annular beam, the beam quality of which is poor and the spot energy distribution is not uniform. This characteristic of large-aperture top-emitting laser restricts its application in many fields. In order to improve the annular light profile to flat-top beam, a beam shaped DOE designed by modified G–S algorithm which based geometric mapping is applied on the surface of the output aperture. The simulation results show that the designed diffractive optical elements have good shaping effect and high light energy utilization. The output characteristics of the devices with and without DOE are tested in the lab. It found that the beam of the device with DOE appears more uniform than the device without DOE and as the injected current increased the light intensity of the center maintain a good uniformity.

Luminance compensation in top-emitting organic light-emitting diodes with high color purity as quantum dots using horizontal dipole orientation

Abstract Organic light-emitting diodes (OLEDs) and quantum dots (QDs) are mainstream applications in large-sized displays. Owing to their narrow photoluminescence (PL), QDs offer a wide color gamut. Although OLEDs have a wider PL spectrum bandwidth than that of QDs, a high color purity can be obtained through an optical design of the top-emitting device. However, the luminance improvement from the strong cavity effect in a top-emitting geometry is not significant because of the wide PL spectrum. This insufficient luminance improvement can be compensated by applying organic light-emitting materials with horizontally oriented dipoles. Luminance compensation is color dependent. In a blue light-emitting structure, the luminance improvement by a narrow PL spectrum is more advantageous than by a horizontal dipole orientation. However, in green and red light-emitting structures, even if the full width at half maximum of the PL is about 20-nm-wide, luminance can be compensated using a horizontal dipole orientation while maintaining high color purity.

Surface ligand engineering-assisted CsPbI3 quantum dots enable bright and efficient red light-emitting diodes with a top-emitting structure

All-inorganic CsPbX3 (X = Cl, Br, I) quantum dots (QDs) are promising materials for light-emitting diodes (LEDs), but the long insulating ligands such as oleic acid (OA) and oleylamine (OLA) on the QD surface limit the device performances. They reduce the electrical conductivity of QDs, while their loss deteriorates the photoluminescence and stability. To overcome these issues, octylphosphonic acid (OPA) with a shorter carbon chain was added in the synthesis of perovskite QDs to partially replace OA ligands. Owing to the strong interaction between OPA and Pb atoms, the under­coordinated Pb atoms on the surface of CsPbI3 QDs can be effectively passivated by OPA, leading to improved fluorescence quantum efficiency (close to unity) and solution stability. Furthermore, the electrical conductivity of QD films was also enhanced from 5.3 × 10−4 to 1.1 × 10−3 S/m by using shorter chain OPA ligands. Finally, highly efficient red perovskite QD LEDs with peak external quantum efficiency (EQE) of 12.6% and maximum luminance of 10171 cd m−2 were achieved in a top-emitting device structure, a nearly 3-fold enhancement in luminance compared with pristine CsPbI3 QDs based LEDs.