Organic Electroluminescence

Zakya H. Kafafi

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780367392833
• eBook ISBN: 9781351836777
• Topic: Technology & Engineering
• Subtopic: Optics & Light
• Index: Technology & Engineering

Organic light-emitting diode(OLED) technology has achieved significant penetration in the commercial market for small, low-voltage and inexpensive displays. Present and future novel technologies based on OLEDs involve rigid and flexible flat panel displays, solid-state lighting, and lasers. Display applications may range from hand-held devices to large flat panel screens that can be rolled up or hung flat on a wall or a ceiling.

Organic Electroluminescence gives an overview of the on-going research in the field of organic light-emitting materials and devices, covering the principles of electroluminescence in organic thin films, as well as recent trends, current applications, and future potential uses. The book begins by giving a background of organic electroluminescence in terms of history and basic principles. It offers details on the mechanism(s) of electroluminescence in thin organic films. It presentsin-depth discussions of the parameters that control the external electroluminescence quantum efficiency including the photoluminescence quantum yield, the light-output coupling factor, carrier/charge injection and transport, and electron and hole recombination processes in organic semiconductors.

The authors address the design and the characterization of amorphous charge transport materials with high glass transition temperatures, light-emitting small molecules and conjugated polymers. The book covers state-of-the-art concepts and technologies such as fluorescent and phosphorescent OLEDs, various approaches for patterning organics, and active matrix organic emissive displays including their back panel thin film transistors and pixel electronics. It concludes by summarizing future directions for OLEDs in organic light-emitting displays, large area distributed solid state light sources, and lasers using organic thin films, nanostructures, and photonic crystals.

Organic Electroluminescence is an excellent resource and reference for stu

1

Electroluminescence in Small Molecules

TETSUO TSUTSUI

CONTENTS

1.1 Introduction

1.2 Recombination and Emission in Bulk

1.3 Multilayer Structures

1.4 Design of Hole Transport, Electron Transport, and Emissive Molecules

1.5 EL Quantum Efficiency and Power Conversion Efficiency

1.6 Concluding Remarks

References

1.1 INTRODUCTION

Small-size color displays (e.g., mobile phones) based on organic light-emitting diodes (OLEDs) have recently penetrated the commercial market. The attraction with this new technology is based on the bright, full-color light emission from OLEDs, which originates from the radiative relaxation of the electronic excited states of π-conjugated molecular systems. The size of the OLED displays is expected to expand in the near future and will include large TV screens, other information displays, and even solid state lighting.

In the last century, much effort has been devoted to developing a variety of artificial lighting sources such as gas lamps, electric light bulbs, fluorescent lamps, neon lamps, cathode-ray tubes, inorganic light-emitting diodes, and semiconductor lasers. Every artificial light source is based on simple mechanisms. For example, incandescence is based on short-wavelength edge emission of blackbody radiation from high temperature substances. Light emission from excited states of atoms or inorganic solids is another mechanism. In contrast, a variety of colors of living species are based on reflection or transmission, which are due to optical transitions between the excited and ground electronic states of molecules. There are animals and plants, such as moonlight mushrooms, luminous bacteria, lantern fish, sea firefly, firefly squid, firefly, and railroad worm that have the ability to continuously emit light and thus are quite visible in the dark. Even the advanced technologies achieved in the 20th century have not been successful in imitating the lighting mechanisms that take place in such living species. Fortunately, however, just at the end of the 20th century, the technology on OLEDs, the artificial version of living light, has been successfully established.

The mechanism for charge injection electroluminescence (EL) in covalently bonded molecular materials is common and is found among many molecular systems, including mainchain π-conjugated polymers. In this chapter, we describe charge injection EL in molecular glass films composed of small molecules. In the case of thin solid films composed of small molecules with unsaturated bonds, there is no need to describe their electronic structure using an extended π-electron system. Their electronic properties can simply be given in terms of their molecular orbitals, in particular the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Thus, charge transport across the molecular solid can be described as a hopping process among localized π-electron systems, where the location for opposite charge recombination and light emission is assumed to be on specific molecules. This simple picture for charge-injection EL serves as a useful guide. It is helpful when using different molecular design concepts and provides a reasonable basis for device efficiency considerations.

1.2 RECOMBINATION AND EMISSION IN BULK

It is useful to look at the early work (1960s) on electroluminescence (EL) in organic single crystals to understand the high performance exhibited by OLEDs based on very thin films. Helfrich and Schneider reported bright EL from a single-crystal anthracene, where they used anthracene cation and anion containing electrolyte solutions as anode and cathode, respectively.^1,2 Figure 1.1 shows the data replotted from this study where the emission intensity scales linearly with the injected current density over more than three decades. This result is quite valuable and it provides clear-cut evidence for a charge-injection process, which is the basis for determining the EL quantum efficiency defined as the ratio of photons emitted per injected charges. Helfrich and Schneider^1,2 reported that the emission arose predominantly from a region near the positive electrode (anode), indicating that electrons are injected from the cathode and transported through the crystal to meet the holes entering from the opposite electrode. This result clearly shows that bulk-controlled charge transport and recombination govern the process of charge injection EL.

Figure 1.1 The relationship between the brightness and the current density in an OLED made of an anthracene single crystal. (Data from figure 2 in Reference 1 is re-plotted.) The thickness and size of electrodes are 5 mm and 0.2 cm², respectively.

Following this early work, numerous reports on electroluminescence from various organic single crystals have appeared.^{3, 4, 5, 6, 7, 8} It is worthwhile noticing that the reported EL quantum efficiencies decreased as the troublesome and unstable liquid contacts were replaced with their more stable but less efficient solid counterparts. The development of stable solid state contacts with good injection characteristics for both electrons and holes is an important area deserving further attention. Further improvements will not only lead to better EL efficiency but also to overall improved device stability. In molecular systems, which have very low carrier densities, the injected positive and negative charges (referred to as electrons and holes for simplicity) are expected to exist as space charges with no local charge neutrality. This case is clearly different from inorganic semiconductor LEDs, in which minority charge carrier injection at a p–n junction determines the charge recombination process.

OLEDs based on single crystals (µm) are not useful for practical applications. The high voltages, the small light-emitting areas, and the difficulty of single crystal processing are some of the shortfalls that would prevent their use as pixel elements in displays and solid state lighting. This led to the exploration of means to fabricate thin film (nm) structures, and charge injection EL was reported in Langmuir-Blodgett films, vacuum-sublimed polycrystalline films, and vacuum-sublimed amorphous glassy films.^{9, 10, 11, 12, 13, 14} Both the EL quantum efficiencies and stabilities of those thin-film devices remained low until the breakthrough work of Tang and VanSlyke.¹⁵ In 1987, they reported the fabrication of high-performance OLEDs using a double-layer vacuum-sublimed heterostructure based on organic dyes.¹⁵ Following this demonstration, Adachi and co-workers extended and generalized the concept to multilayer structures,^{16, 17, 18, 19, 20, 21} and developed high-efficiency devices. It should be stressed, however, that charge recombination and light emission still originate from molecules in the bulk.

1.3 MULTILAYER STRUCTURES

The structure of OLEDs containing two organic layers consists of a transparent indium-tin-oxide (ITO) anode, an organic hole transpo...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Electroluminescence in Small Molecules
• Chapter 2 Emission Mechanisms in Organic Light-Emitting Diodes
• Chapter 3 Physical Properties of Organic Light-Emitting Diodes in Space Charge–Limited Conduction Regime
• Chapter 4 Amorphous Molecular Materials for Carrier Injection and Transport
• Chapter 5 Chemistry of Electroluminescent Conjugated Polymers
• Chapter 6 Organic Electrophosphorescence
• Chapter 7 Patterning of OLED Device Materials
• Chapter 8 AMOLED Display Pixel Electronics
• Chapter 9 Past, Present, and Future Directions of Organic Electroluminescent Displays
• Chapter 10 Organic Electroluminescent Devices for Solid State Lighting
• Chapter 11 Photoexcited Organic Lasers
• Index