Advanced Display Technology

In this chapter we discuss phosphorescent OLED (PHOLED) materials and technologies. Over the past several decades the OLED has transitioned from the lab to the marketplace and is now the leading technology for a wide range of display applications. While many technological developments have led to this success, none is more crucial than the development of highly efficient PHOLEDs. We discuss here the physics and chemistry of phosphorescent emitters, their application in PHOLEDs, the inherent advantages of these materials and their future.

TADF (Thermally Activated Delayed Fluorescence) is recognized as the third generation of OLED-emitting technology, which provides highly efficient emission without using any rare metals, such as iridium. Fundamental molecule design strategies of TADF were based on the introduction of electron donor and acceptor units in which the π-conjugation was significantly distorted by steric hindrance introduced through bulky substituents. In this design, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals can be located around donor and acceptor moieties, respectively, leading to a small ΔEst while maintaining a reasonably high radiative decay rate (k_r). This concept enabled efficient spin up-conversion from triplet to singlet. As a result, highly efficient emission from singlet as high as 100% internal quantum efficiency was achieved. One disadvantage of TADF is low color purity if it is applied to displays because of its wide emission spectrum. The emission color purity is indicated by the index full width at half maximum (FWHM). FWHM of TADF, in general, is from 80 to 100 nm, wider than that of fluorescence. Hyperfluorescence (HF) combines TADF and fluorescence and is considered as the 4th-Gen OLED emitting technology. TADF acts as exciton generator and transfers excitons to fluorescence by Förster resonance energy transfer (FRET). Fluorescence molecules receive excitons and emit light as high as 100% internal quantum efficiency which is four times higher efficiency than a conventional fluorescence emitting technology. Color purity of HF, as indicated by FWHM, is similar to fluorescence—from 30 to 40 nm.

In this chapter, we will show that RGB side-by-side IJP is a very promising technology for large-scale, top-emission, and high-resolution applications.

The basic guidelines of solution-processible OLED material design for higher efficiency and longer lifetime based on the conjugated polymer technology, and the latest progress and status are discussed. The materials are highly suitable for printing OLED panel fabrication, especially for mid- to large-sized displays. We also show the comparison of the performance between inkjet (IJ) and spin devices, and its theoretical fundamentals to secure high performance in IJ printing. Finally we discuss the expectation for IJP-OLED panel production from a viewpoint of its process, resolution and panel size, and performance as well as the comparison with WOLED.

The short operation lifetime of organic light-emitting devices (OLEDs) remains as a major hurdle. The poor device longevity results from accumulations of defects during normal operation. Strategies to improve device stability require understanding of the chemical processes underlying the generation and annihilation of defects. This chapter summarizes the current knowledge about the chemical degradation of organic materials in OLEDs. Focus is placed on the chemical mechanisms of the defect generation. Unimolecular degradation from excitons or polarons is discussed with key examples. Chemical degradation by bimolecular processes, including exciton-exciton annihilation and exciton-polaron annihilation, are summarized. Strategies toward minimizing bimolecular degradation are also introduced. Finally, a recently identified bimolecular degradation pathway involving exciton-mediated electron transfer is overviewed.

With the ongoing advances in nanomaterials and fabrication technologies, the form factor of electronic devices is also evolving. Displays have evolved from rigid flat panels to flexible, rollable, and foldable formats. Such changes in form factor can provide improvements in consumer utility and convenience, including portability and ease of use. Developing a stable flexible display has attracted considerable attention for these reasons. Comprehensive studies have investigated various approaches, including flexible liquid crystal displays (LCD), displays employing light-emitting diodes (LED), and organic light-emitting diodes (OLED). Because they provide superior flexibility, OLEDs are highly desirable in the flexible display industry. But to realize the full potential of flexible OLEDs, they not only require enhanced characteristics to withstand rolling and folding, but a highly effective thin-film encapsulation barrier is also essential. To date, most existing encapsulation studies have focused on the low water vapor transmission rate (WVTR) characteristic, which is related to gas barrier properties. This book chapter covers developments in encapsulation technologies; their structure designs; and materials for realizing flexible, rollable, and foldable OLEDs. Special focus is given to the existing hurdles to flexibility and how to overcome these limitations. Finally, further insights on the evolution of encapsulation technologies are discussed.

Since the invention of IGZO by Prof. Hosono in 2004, the development of IGZO thin-film transistors has been accelerated by material optimization including cation composition, processing conditions, and careful architecture design such as a self-aligned structure. In this chapter, the baseline of the current IGZO backplane technology for AMOELD TVs will be addressed including the architecture and process optimization of IGZO TFTs. In particular, the next-generation backplane technology beyond IGZO and SiO₂ will be briefly presented from the viewpoint of high mobility and low-voltage operation. Finally, the device instability of IGZO TFTs will be discussed, which is critical for their implementation in AMOLED products. The degradation mechanisms for the bias thermal stress and light illumination will be summarized including the carrier trapping/injection, defect creations such as oxygen vacancies, oxygen interstitials, and a hydrogen complex model.

The structures, operating principles and technical issues of OLED pixel circuits are explained in this chapter. First, the necessity and role of the OLED pixel circuit is introduced. Then, three operating principles: diode-connection, source-follower and current programming schemes of in-pixel compensation techniques are explained. Other compensation techniques utilizing a complicated system outside the display panel are also introduced. Moreover, three examples of OLED pixel circuits employed in mobile display products are analysed in detail. Finally, the effect of the V_T extraction time and dimming method on low-grey-level mura, is discussed.

Currently, commercially available large-sized OLED displays use white OLED devices. This chapter intends to cover the core technologies of tandem white OLED devices and their applications through white OLED devices have been developed in order to improve performance. The first half explains two-stack and three-stack tandem WOLED structures, which were developed to improve the efficiency of bottom-emitting WOLEDs, and the configuration of a light-emitting layer to increase color reproduction. In addition, it introduces features of products with this technology such as wallpaper OLED, cinematic sound OLED, rollable OLED, and 8K OLED displays. The second half deals with issues such as the viewing angles and power consumption of microcavity top-emitting WOLEDs and introduces the sputtered transparent electrode technology and OLED device characteristics that make up the non-microcavity top-emitting WOLED devices. Additionally, we introduce transparent and flexible transparent OLED displays.

Quantum dot emitters (QDs) have become common in wide color gamut LCD displays using quantum dot enhancement film. Several new display architectures using QDs are also under development. QDs have the potential to impact many different future display designs including LCD backlight units as is in the marketplace today; pixel-based color conversion of OLED, LCD, or micro-LED technologies; or as electroluminescent emitters in “OLED-like” displays. This chapter will describe the structure, benefits, and development status of each type of QD display, including the challenges each one faces relative to competitive technologies. A comparison of the emitter properties of QDs and alternative display emitters will also be discussed.

Colloidal quantum dots (QDs) are a few nanometer-sized semiconductor nanocrystals whose electronic states are subject to change depending on their dimension. QDs have been of great interest as light-emitting materials in future displays owing to their superb optical properties such as near-unity photoluminescence quantum yield and narrow emission spectra, as well as their solution processability. The present chapter focuses on the emerging display technologies based on quantum dots. Specifically, this chapter covers the brief introduction of quantum dots for light-emitting applications, photophysical properties of quantum dots relevant to light-emitting applications, and the state-of-the-art of, and the perspectives on QD-based display technologies.

Light-emitting diodes (LEDs), which emit efficient and bright light ranging from infrared to ultraviolet, have been adopted in various fields to improve quality of life. In particular, micro-LEDs, which are III–V inorganic-based micro-miniaturized light sources, which can form clear and vivid displays for smart portable devices and can be used with artificial intelligence, are drawing great attention. The micro-LEDs can be used in applications where it is difficult to use existing inorganic LEDs, such as in medicine, clothing, and miniature electronics. Research on developing micro-LEDs is underway around the world as the demand for micro-miniaturized light sources increases, especially for display applications. However, problems are also emerging that hinder the mass production of micro-LED displays. Herein, we introduce the current research trends and issues involved in overcoming various problems associated with using micro-LEDs as full-color light sources in display applications. We also briefly review newly presented research and development strategies and technologies for micro-LEDs by research institutes and companies, including their patents and products.

Augmented reality (AR) and virtual reality (VR) have received a lot of attention and expectation with the experiences never possible before. The recent development of the display technology has resulted in high-resolution images indistinguishable from reality. However, unlike the table-top displays, the head-mounted AR and VR displays have additional factors to be considered such as the form factor of the device, the field of view, and the visual fatigue. In this chapter, some of the key technical issues for optically configuring the display in VR and AR are discussed.