Organic Europium Complexes and Their Applications in Optoelectronic Devices [1 ed.] 9781617610691, 9781617289057

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Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

CHEMISTRY RESEARCH AND APPLICATIONS

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ORGANIC EUROPIUM COMPLEXES AND THEIR APPLICATIONS IN OPTOELECTRONIC DEVICES

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Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

CHEMISTRY RESEARCH AND APPLICATIONS

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ORGANIC EUROPIUM COMPLEXES AND THEIR APPLICATIONS IN OPTOELECTRONIC DEVICES

HAN YOU AND

DONGGE MA

Novinka

Nova Science Publishers, Inc. New York

Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

Copyright © 2010 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher.

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For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‟ use of, or reliance upon, this material. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA

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Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

CONTENTS

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Preface

vii

Chapter 1

Introduction

Chapter 2

Europium Complexes in OLEDs

17

Chapter 3

Europium Complexes in Organic Memories

35

Chapter 4

Summaries and Conclusion

45

1

References

47

Index

63

Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

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PREFACE Organic europium complexes are of considerable interest due to their sharp narrow emission spectrum and potentially high emission quantum efficiency, originated from 5D0-7F2 electronic transitions of the central ions. Therefore, organic europium complexes have well applications in organic light-emitting diodes. Furthermore, Organic europium complexes also show well electrical switching characteristics in diodes, potential applications as memory devices in information storage. Here, we will review the achieved progresses in organic europium complexes and their applications in organic light-emitting diodes and memory devices. It can be seen that organic europium complexes are promising materials as active medium in organic optoelectronic devices.

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Chapter 1

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INTRODUCTION The amazing success of microelectronics over the past 40 years is based to a large extent on advanced in the fabrication of silicon based integrated circuits. However, a number of physical and economic factors, which threaten the continued scaling down of silicon semiconductor devices, have motivated research into electronic systems based on other technologies [1, 2]. Organic materials are promising candidates for future molecular scale device application. Recent extensive studies have shown that organic materials exhibit a variety of interesting optical, electrical, photoelectric, and magnetic properties in the solid state. In contrast to inorganic materials that consist of covalent or ionic bonds of atoms over the entire expanse of solids, organic materials are based on independent molecules and characterized by weak intermolecular interactions. Therefore, the design of organic materials can be readily performed on the molecular level. It has been demonstrated that organic materials have a number of applications in electronic and optoelectronic devices such as sensors [3], solar cells [4], field-effect transistors [5], optical data storage [6], organic electroluminescent devices [7], memory devices [8], nonlinear optical materials [9], and many others. Compared with inorganic counterparts, emerging organic optoelectronic materials have exhibited advantages such as improved speed, reduced power consumption, increased brightness (for display), and improved processability, leading to conformal and flexible devices and the potential for low cost mass production. In the past several decades, research and development on organic electronic and optoelectronic materials and devices has grown rapidly. Plastic optoelectronic materials and devices are rapidly becoming a reality. One device that is on the cusp of widespread use is the organic light-emitting

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diodes (OLEDs). On the other side, the development of future information technology could come from data storage incorporating these advanced materials. So, we will describe the general OLEDs and organic memory first below.

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1.1. ORGANIC LIGHT-EMITTING DIODES Light-emitting diodes (LEDs) are optoelectronics devices, which generate light when they are electrically biased in the forward direction. The early commercial LEDs devices, in 1960s, were based on inorganic semiconductors such as gallium arsenide phosphide (GaAsP) as an emitter and the efficiency were very low [10]. After 50 years of development, the efficiencies of inorganic LEDs have been improved and they are used in a wide range of applications such as telecommunications, indicator lights, and more recently in solid state lighting. But for high resolution panel displays, the applications of LEDs have been limited as the cost of fabrication and the difficulty of high resolution. Light-emitting diodes made with organic materials are called OLEDs. Electroluminescence from organic crystals was first observed from anthracene in 1963 [11]. Since the efficiencies and lifetime of resulting devices are significantly lower than those obtained for inorganic systems, research activities were focused on the organic materials. Development of OLEDs was spurred on in the 1980s through the work of Tang and Van Slyke [12], who demonstrated efficient electroluminescence in two layer sublimed molecular film devices. Electroluminescence from conjugated polymer (also called as polymer light-emitting diode, PLED) was first reported in 1990 [13], using poly(p-phenylene vinylene), PPV, as the single semiconductor layer between metallic electrodes. In contrast to the first electroluminescent devices, the new OLED devices were based on a multilayer structure. During operation, electrons and holes are injected from a cathode and an anode, respectively, and the recombination of electrons and holes in the organic layer leads to efficient light generation. The operation principle of OLEDs is similar to that of inorganic LEDs. The OLED industry has been progressing forward rapidly and the commercialization of displays of 7 inches and below. Today OLED displays are already used mainly in small (2"-4") displays for mobile devices: cell phones, MP3 players, and others. OLED displays carry a price premium over LCDs, but offer brighter pictures and better power efficiency as it is self emission devices and do not need the backlight - making it ideal for battery

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Introduction

3

powered gadgets. Making larger OLEDs is also possible, Sony have lunched the word first 11 inch OLED TV XEL-1 in 2008. Many companies, including Sony, Samsung, and LG have the plan to manufacture the OLED TV in the near future. On the other side, OLED makers are seeking new opportunities in the lighting sector as OLEDs are very power efficient and they can be made very thin. Several companies are working towards white OLED light products, Such as Philips, which are now shipping OLED samples panels (which can be ordered online), with 'commercial' panels available in 2010; OSRAM is also shipping their first OLED panel, the ORBEOS, in 2009. Thin film OLED usually consists of multiple organic and metallic layers on an ITO covered glass plate. These layers may be deposited by various methods such as thermal evaporation, Langmuir-Blodget deposition, spin coating or ink-jet print from solutions. All layers should be chemically stable, especially under devices operation conditions. In order to maximize the OLED device efficiency, the balance of electron and hole transport is important. Light generation requires recombination of both types of carriers and dominance of electron or hole current will lead to nonradiative recombination. To achieve balance of carriers transport, it requires both balanced injection and transport of both types of carriers. But in most cases carrier transport in organic materials is highly imbalance, which means that the mobility of electrons and holes are very different. In many organic materials, they either preferentially transport electrons or holes. The imbalance transport will result in electron-hole recombination at either the cathode or anode interface, leading to quenching and significantly reduction in overall device efficiency. On the other hand, charge injection also plays an important role in determining the charge balance. Efficient hole injection from anode requires good matching of the anode‟s work function to the highest occupied molecular orbit (HOMO) level of the organic layer and efficient electron injection from cathode requires good matching of cathode‟s work function to the lowest unoccupied molecular orbit (LUMO) level of the organic layer. Fig. 1 shows the structure of a double-layer OLED. In this device, holes are injected from the ITO electrode through the hole transport layer (HTL) to electron transport/emissive layer (ETL) while electron are injected from the cathode into the electron transport layer. Electrons and holes meet at the HTL/ETL interface and recombine to give light. Although some PLEDs use single layer structure, the two-layer OLED device served as a basic building block for more sophisticated devices with optimum device performance. Depending on the emitting materials used in the device, a device consisting of two organic layers might not give optimum device performance

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and more layers are necessary. To facilitate carrier injection, carrier injection layers with proper energy alignment with the injection electrodes are necessary. Specifically, an electron injection layer (EIL) with its LUMO energy matching the work function of the cathode is need, while a hole injection layer (HIL) with its HOMO energy matching the work function of the transparent anode is needed. To transport the injected carriers from the injection layer to the emitting layer, electron and hole transport layers are need. To balance the carriers, a hole/exciton blocking layer (HBL) with a high HOMO level as a barrier between the emitting layer and the ETL is needed, while an electron blocking layer (EBL) with a high LUMO level as a barrier between the emitting layer and the HTL is needed. Today, an OLED device consisting of five layers or more is very common. In some case, a single transport layer serves the purpose of carrier transport, injection layer, and blocking layer, hence the total number of layers is reduced. A multiple-layer OLED device consist of an anode, a HIL, a HTL, an emitting layer, a HBL, an ETL, and a cathode is shown in Fig. 2.

Figure 1. Structure of a double-layer OLED device.

The hole-injection material (sometimes referred to as ITO or anode buffer layer) reduces the energy barrier in-between ITO/HTL, enhancing charge injection at the interfaces and ultimately improving power efficiency of the device. Thus, hole-injection can be promoted by inserting a thin layer of copper phthalocyanine (CuPc) [14], metal oxide [15], between the ITO/HTL

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Introduction

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interface. One of the widely used polymers for promoting hole injection is poly(3,4-ethylenedioxythiophene)–poly(styrene) known as PEDOT/PSS, which has been found to be useful in a hybrid OLED architecture combining both the advantages of PLED and multi-layered small molecule OLED [16].

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Figure 2. Structure of a multiple layer OLED device.

The hole transport layer mainly transports holes along the HOMO level. Therefore, the HTL materials should have a high hole mobility. In addition, HTL materials should also have a low ionization potential for the efficient injection of holes from the ITO anode. Ideally, after bipolar charge recombination, the HTL should help block the migration of excitons from the emitting layer and help confine excitons within the emitting layer. Finally, the HTL should have a high Tg for temperature stability and be transparent to the radiation emitted from the emitting layer to reduce the optical losses within the device. Numerous materials have been developed for holes transport layers in OLEDs. Most of these hole transport materials, which were originally developed for charge transport layer in xerography, are arylamine derivatives. Among them, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'diamine (TPD) and N,N'-di(naphthalene-2-yl)-N,N'-diphenylbipheynyl-4,4'diamine (NPB) have been studied extensively. The chemical structures some typical HTL and HIL materials are shown in Fig. 3. For electron transport layer, it should have high electron affinity and a wide optical band gap in addition of high electron mobility, and also should

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have a high Tg for temperature stability. Electron mobility in organic materials is generally orders of magnitude lower than hole mobility. In some case, the ETL also serves as HBL to block the hole or exciton. The most important electron-transport material is tris(8-hydroxyquinolinato)aluminum (Alq3) with its molecular structure shown in Fig. 4, which also shown some common ETL and HBL materials used in OLED devices.

Figure 3. The molecular structures of some typical hole injection layer (HIL) and hole transport layer (HTL) materials used in OLED devices.

One of the key developments in the advancement of OLED display technology can be attributed to the discovery of the guest–host doped emitter system [7]. This is because a single host material with optimized transport and luminescent properties may be used together with a variety of highly fluorescent or phosphorescent guest dopants leading to EL of desirable hues with very high efficiencies. Another reason is that most organic dye molecules exhibit very high fluorescence quantum yields in solution, however, when they

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Introduction

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exit in a condense phase, the fluorescence quantum yield can be drastically reduced due to formation of aggregates and exciplexes. Doping into a host matrix as an emitter layer in an OLED device can resolve this problem. Another advantage of the doped emitter system in OLED is the enhancement of its operational stability by transferring the electrogenerated exciton to the highly emissive and stable dopant site thus minimizing its possibility for nonradiative decay [17]. This doping principle has recently been successfully extended to the exploitation of highly phosphorescent materials leading to nearly 100% internal EL efficiency [18]. Doping can be achieved by coevaporating the dopant with the host for small organic molecule or by spincoating a mixture solution for the polymer molecule. Emission from the dopant molecule can be obtained by two possible pathways. One is called Förester energy transfer. Electrons and holes recombine at host molecule and form exciton, which then transfers the energy to the dopant molecule and form exciton in dopant molecule, resulting emission from the dopant molecule in the end. This process allows tuning of the emission color without changing the host materials. Another pathway is that electrons and holes are directly injected into the dopant molecules and the recombination occurs at the dopant molecular sites. Molecular doping enables the same host materials used with different color dopant molecules, which greatly simplifies the molecular design, especially in the realization of White OLED devices.

Figure 4. The molecular structures of some typical electron transport layer (ETL)/hole blocking layer (HBL) materials used in OLED devices.

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Light-emitting host materials should be able to transport electrons and holes, good film forming properties, high luminescence efficiency, and wide optical band gap compared to emitter dopants. Many electron transport materials are effective light-emitting host materials. For example, electron transport materials such as Alq3 and some imidzole derivative have been used as the host materials in OLED devices. Also, some hole transport materials can also be used for light-emitting host materials, such as NPB. In the case of phosphorescent devices, carbazole derivative, such as 4,4‟-bis(carbazol-9yl)biphenyl (CBP), is the most used host materials with iridium complexes. Some common host materials used in the OLED devices are shown in the Fig. 5.

Figure 5. The molecular structures of some typical host materials used in OLED devices.

Light-emitting dopant materials play an important role in the performance of the OLED device. Light-emitting dopant materials affect device efficiency and operational stability. The device emission color can be tuned to emit light covering the entire visible spectrum to realize the full color display devices. The dopant materials can be divided into the fluorescent and phosphorescent dopant materials. The green fluorescent dopant was amongst the first to be successfully demonstrated in a commercial product and it is also by far the most efficient. One of the best green dopants is 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl2,3,6,7-tetrahydro-1H,5H,11H-[l]benzo-pyrano[6,7,8-ij]quinolizin-11-one, known as C545T, which belongs to the highly fluorescent class of coumarin laser dyes. Among the RGB dopants used in OLED, red emission, due to its low efficiency, remains to be the weakest link in realizing the full potential of a full-colored OLED display. One of the best that comes close is 4(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran, better known as DCJTB. The blue doped emitter in OLED often necessitates the judicious selection or design of an appropriate blue host

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Introduction

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material which has a wide enough bandgap energy and a set of matching LUMO/HOMO level to effect the sensitization. It appears that there are a number of blue dopant/host systems that can achieve a high efficiency with CIE x = 0.15; y= 0.15. But, the status of their device operational lifetimes leaves much to be improved. The best dopant/host molecules in the literature are probably the BCzVBi/DPVBi system [19], but we should realize that many company in commercial circles have achieve some more better results, such as Idemitsu Kosan Co., which based on the basic structure of distyrylarylene (DSA) host doped with a hole transporting amine-substituted DSA dopant [20], and Kodak [21], which is based on 9,10-di(2-naphthyl)anthracene (βDNA) and tetrakis(t-butyl)perylene (TBPe). The chemical structures of some common fluorescent dye molecule used in OLEDs devices are shown in Fig. 6.

Figure 6. The molecular structures of some typical fluorescent dye materials used in OLED devices.

One of the key developments in the advance of modern OLED sciences and technology is the discovery of electrophosphorescence, which lifts the upper limit of the internal quantum efficiency of the usual fluorescent dopantbased devices from 25% to nearly 100%. Phosphorescence is inherently a slower and less efficient process, but triplet states constitute the majority of

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electrogenerated excited states (~75%), so the successful utilization of the triplet manifold to produce light should undoubtedly increase the overall luminance. The design and synthesis of triplet emitting materials containing heavy-metal complexes, where strong spin-orbit coupling leads to singlet– triplet state mixing which removes the spin-forbidden nature of the radiative relaxation of the triplet state, are therefore particularly important in achieving high-efficiency electrophosphorescence in OLEDs. One of the first examples of triplet emitting device is based on the red 2,3,7,8,12,13,17,18-octaethyl12H,23H-porhine platinum(II) (PtOEP) [22]. And then, osmium (Os) complexes [23] and iridium (Ir) complexes have been reported. To date, the Ir complexes is the most common phosphorescent material used in the phosphorescent OLEDs. One of the best electrophosphorescent green materials is fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) [24]; the bluest phosphorescent iridium complex is FIrpic [25]; the best red phosphorescent iridium complexes is Btp2Ir(acac) [26]. Their molecular structures were shown in the Fig. 7. Another promising material used in the electroluminescence devices is the conjugated polymer, which are fabricated by solution processes. For large area flat panel displays and solid state lighting, solution processed OLEDs are desirable because the fabrication processes are compatible with large area, low cost roll-to-roll processing. In particular, conjugated polymers as the emitters now show better charge transport properties and lower operating voltages compared to small molecule OLEDs. Another interesting property of PLEDs is the possibility of fabricating flexible devices by casting a polymer film on a plastic substrate, enabling the obtainment of displays in a variety of unusual shapes [27]. The most used conjugated polymer in PLEDs are poly(pphenylene vinylene) (PPV) [13] and polyfluorenes (PFO) [28, 29], and their derivatives. Fig. 8 show the chemical structures of some typical PPV and PFO polymers. It has been demonstrated [30, 31] that white electroluminescence with simultaneous blue, green, and red emission from a single polymer can be achieved by attaching a small amount of a green-emissive component to the pendant chain and incorporating a small amount of a red-emissive component into the main chain of a blue polyfluorene host, which will great simplify the fabrication processor. The nonconjugated polymers are also used as host material when phosphorescent materials are used in the solution processed OLEDs [32-34].

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Figure 7. The molecular structures of some typical phosphorescent materials used in OLED devices.

Figure 8. The molecular structures of some typical polymer materials used as the emitting layer in OLED devices.

It should also be mentioned that there appears to be growing interest and research effort in the academia to develop dendrimers which contain siteisolated chromophores [35] that can control charge-transport, exciton formation, fluorescence and intermolecular interaction[36]. These giant dendritic molecules synthesized by building convergently from a highly functionalized core through several generations have been demonstrated to form single-layered devices by solution processing. Although much work still needs to be done before this idea can be materialized and proven device worthy, the performance of such dendritic OLED reported to date is very good [37-39].

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1.2. ORGANIC MEMORY Memory devices can store, retain, and recall information. They became more and more important in this information technology age. Depending on the storage capability, memory devices are divided into two categories: volatile and nonvolatile. Volatile memory loses its information while nonvolatile memory retains its information, when the external power is turned off. Then SRAM (static random access memory) and DRAM (dynamic random access memory) are two typical volatile memories which wide used in the computer. Examples of nonvolatile memory include read-only memory, flash memory, optical discs, and most types of magnetic computer storage device such as hard disk and floppy disk drives. However, due to its device structure, the fabrication process of these devices is complicated and it is difficult to scale down the size. The demand for more efficient and faster memory structures is greater today than ever before. With the development of memory technology, more and more research interest has also been put into the development of novel memory device using organic materials. A wide variety of organic materials, including organic dyes [40], charge transfer complexes [41], conjugated oligomers [42], redox metal complexes [43], and other molecules [8, 44, 45], have been explored for memory application. Rather than encoding “0” and “1” as the amount of charge stored in a cell in silicon devices, the organic memory materials stored data in an entirely different form, for instance, based on the high- and low-conductivity response to an applied voltage. The devices use organic materials instead of inorganic semiconductors. The fabrication process of such device is simple and the devices are highly flexible. Moreover, these organic memory devices have a high response speed and can be easily made into high density memory array. All these advantage make organic memory as a strong candidate to become the next generation memory device. Organic memory devices have different device structure, principles, and operation mechanisms from inorganic memory devices. Fig. 9 shows the devices structure of an organic thin film memory. As we can see, the structure is very simple: an organic or polymer film sandwiched between two metal electrodes. Obviously, this two terminal device structure is different from the MOS of an inorganic semiconductor memory which is three terminal device. The simple device structure makes the device fabrication more easy and possibility of high device density.

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Figure 9. (a) The basic configuration of a organic memory cell. (b) Schematic diagram of a 4×4 cross point memory array. (c) Schematic diagram of a 2×4×4 stacked memory device.

The organic memory device utilizes electrical bistability of the organic or polymer film, which means that the device is stable in two electrical states. Fig. 10 illustrates the current-voltage (I-V) curve of such a device [8]. The original device first exhibits a low conductivity state in which very little current flows through it. The device then transits to a state of high conductivity in which high current flows through it when the external voltage is higher than the threshold voltage. The high conductivity state of the devices also can return to the low conductivity state (low current level) by applying a voltage of negative polarity. When the high and low conductivity states are defined as “1” and “0”, respectively, the device with electrical bistability can be used as a nonvolatile memory device. Also, the device has a switching time shorter than 0.5 ms and can switch several thousand times without significant degradation if the device is tested in vacuum. The performance of an organic memory device is strongly dependent on the properties of the materials. The active layer of an organic memory device has two components. One is the conjugated and nonconjugated polymer or these polymers mixed with some nanomaterials, and the other is the organic small molecules.

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Figure 10. Current –voltage (I-V) characteristics of the Au/MDCPAC/Al device (ref. [8]).

The mechanism of the electrical bistability in organic memory depends on the active materials in the device, which is still under investigated. For a device with a conjugated polymer containing electron donor and electron acceptor moieties, the bistability behavior is attributed to carrier trappingdetrapping processes [46] or field-induced charge transfer process [47]. For a non-conjugated polymer with a flexible spacer for bridging the electroactive pendant chromophore with the backbone, the switching process is related to the field induced conformational ordering (OFF to ON state switching) and retention of the ordered state (ON state) [48]. The redox effect of the polymer plays an important role in the switching process when some nanomaterials, such as fullerene [49], carbon nanotube [50], and gold nanoparticles [44], are doped into polymer matrices to form composite materials in organic memory. For an organic memory device using the small molecule as the active material [51], the conductance switching is related to the electrode materials and charge traps at the interface, which is more complicated and still under investigated. Several memory effects, including rewritable flash memory, write-once read-many-times (WORM) memory and dynamic random access memory (DRAM) have been realized by different organic materials. These memory devices exhibit a high ON/OFF current ratio, promise low misreading rates, and can endure more than several million read cycles. Both the ON-state and

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OFF state are stable under constant voltage stress. These organic memories also have a relatively fast response time in the microsecond range, making them suitable for many data storage applications.

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1.3. RERE EARTH COMPLEXES IN ORGANIC LEDS AND MEMORYS Since many developments have made in OLEDs, numerous investigations using organic dyes, organic metal complexes, and organic polymers as emitting materials have been carried out. Despite such great achievements, obtaining pure and sharp emission from these organic materials is difficult, especially for the red materials. The emission spectra are generally broad. In addition, it is difficult to fine tune the emission color without affecting the device and materials properties. Lanthanide metal ions, on the other hand, exhibit extremely sharp emission bands due to their 4f electrons in which orbital are effectively shielded from the influence of the external forces by the overlying 5s2 and 5p6 orbitals. Even the various states arising from the fn configurations are split by external fields only to the extant of ~100 cm-1. Therefore, emission bands (f-f transitions) are extremely sharp when electronic transitions occur from one J state to another J state in the same configuration. In addition to the spectral profile of the complexes, the excitation mechanism of the central metal ion also differs widely from the other organic materials. For lanthanide complexes with π-conjugated ligands such as β–diketonato, the lanthanide ions are excited via intramolecular energy transfer from the triplet excited states of the ligands. Fig. 11 shows the excitation and energy transfer mechanisms for the lanthanide complexes. The excitation energy of the ligand‟s triplet state, which may be directly generated by carrier recombination, can also be utilized to excite the emitting center. Thus, the internal quantum efficiency for devices using the lanthanide complex as emitters can reach to 100% theoretically as both single and triplet excitons are involved in the emission process. On the other side, it is found that some lanthanide complexes can be used to hold charges in the device [52], and also exhibit memory effects [43]. Actually, the lanthanide complex, especially the europium complex, has a very high electron affinity property and is a very good electron acceptor. So, the lanthanide complexes are also promising materials in the organic memory devices.

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Therefore, in the past decade, much work on OLEDs and organic memory devices using lanthanide ion chelates, especially the europium complexes, as the active materials has been reported. In following, we focus on the achieved progress in organic europium complexes and their application in organic lightemitting diodes and memory device.

Figure 11. Energy transfer in lanthanide complexes: Abs= absorption; PS= phosphorescence; PL= photoluminescence; a, b, c, d, e, and f = lanthanide ion energy level.

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Chapter 2

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EUROPIUM COMPLEXES IN OLEDS For commercial application of OLEDs, three primary colors of blue, green and red are basically required. Among red light emitting materials, europium (III) complexes are particular and attractive, because they can emit highly monochromatic red light around 612 nm with half peak bandwidths of 5-10 nm. Eu3+ ions are able to accept the energy with all of their 5D levels, depending on the triplet state energy of the donor. Although Eu3+ ions luminescence may arise from the 5D1 level, the main emission originates from electronic transitions from the lowest excited state, 5D0, to the ground state manifold, 7FJ (J=6-0) level, with the most intense emission line corresponding to the 5D0→7F2 transition, observed at 612 nm, in the red spectral region [53]. And also, the europium complexes can offer 100% emission quantum efficiency theoretically since this kind transition is not restricted by the spin inhibition rule. Red electroluminescence from a Eu complex was first observed using a tris(theotytrifluoroacetonate)Eu(III) (Eu(TTA)3) complex [54, 55], with the architecture ITO/PMPS : 20 wt% Eu(TTA)3 /PBD/Mg/Al. Red emission was observed with a turn on voltage of 12 V and a maximum intensity of 0.3 cd/m2 at 18 V. It was already known that in a series of europium (III) β-diketonate ternary complexes, relative PL emission intensities would increase by ~200 fold on introduction of a 1,10-phenanthroline as the second ligand, which contains lower-lying triplet states than the β-diketonate ligand. Thus, a brighter red EL has been observed using tris(dibenzoylmetanato)(phenanthroline) Europium (III), Eu(DBM)3phen [56], which structure is shown in Fig. 12, as the emitting material with ITO/TPD/ Eu(DBM)3Phen:PBD/Alq3/Mg:Ag structure. A luminance of 460 cd/m2 with an emission peak at 614 nm was

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observed with a driving voltage of 16 V. And then, a similar OLED based on Eu(TTA)3phen (structure in Fig. 12), was developed with optimized thickness [57] or microcavity structure [58]. It was quickly recognized that the use of pure compounds can lead to concentration quenching effects. Thus it is more efficient to incorporate the Eu(III) complexes into a host material, which can additionally function as charge-transport layers to facilitate the formation and confinement of excitons. Then the Eu(TTA)3phen doped into PVK host [52] or electron transport material [59] as the emitting layer in OLEDs were developed. Later, the Eu(DBM)3Bath (Bath represented bathophenanthroline) (Fig. 12) [60], was also developed and the device was more efficient compared with the device with Eu(TTA)3phen. It was also found [60, 61] that the Eu(DBM)3Bath has excellent electron injection and transport properties besides its luminescent performance. For best match between the lowest first ligand triplet state and the Eu (III) emitting level (5D0), the TTA and DBM were the basic first ligand, while the phen and Bath were the most common used second ligand in europium (III) complex for more sophisticated molecular and device structure design with optimum device performance. In the early stage, research was focus on looking for the better host materials to balance the hole and electron transport. The hole transporting polymer poly(Nvinylcarbazole) (PVK) was first used as the host material in Eu (III) complexbased OLEDs [52, 61, 62], and then oxadiazole derivative, 2-(4-bipheyl)-5-(4t-butylphenylyl)-1,3,4-oxadiazole (PBD) was generally mixed in PVK [63, 64] to improve the electron transport. Bathocuproine (BCP) was then used to block the hole and exciton [64, 65], which got a 417 cd/m2 at 25 V with a ITO/PVK: 3 wt% Eu(TTA)3phen: 32 wt% PBD/BCP/Ca:Al device. The phosphorescent materials were also used as host materials in Eu (III) complexes based OLEDs [66].

Figure 12. The molecular structures of some basic europium (III) complexes (Ref. [56, 57, 60]).

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In 2000, Forrest and co-workers achieved external quantum efficiency η=1.4% using Eu(TTA)3phen doped into 4,4'-N,N'-dicarbazole-biphenyl (CBP) as emitter layer, which device structure was shown in Fig. 13 [67]. They also elucidated the electroluminescence mechanisms of the europium (III) complex. The charge carriers were trapped by Eu(TTA)3phen, leading to subsequent exciton formation on the TTA ligand. Furthermore, a decrease of ηR with increasing current was found to be due to T-T annihilation on CBP molecules following back transfer from TTA due to the near resonance of TTA and CBP triplet states. Zhang and co-workers performed more detail research on the Eu3+ EL mechanism inside the ITO/TPD/Eu(TTA)3phen : CBP/BCP/Alq3/LiF/Al structure [68-70]. It was found [68] that only electrons could be trapped in Eu(TTA)3phen while the hole would be situated on CBP molecules, resulting an unbalanced carrier distribution. The distribution of holes and electrons on both Eu(TTA)3phen and CBP molecules would change gradually with increasing voltage. Therefore, the dominant EL mechanism for Eu3+ changed gradually from carrier trapping at relatively low voltage to Förster energy transfer at relatively high voltage. The increasing hole injection and decreasing electron injection would result in EL efficiency increase which was attributed to the improvement of the balance of holes and electrons on Eu(TTA)3phen molecules [69]. An external quantum efficiency of 5.15% was achieved by this way, which was the highest value for Eu(TTA)3phen. Also, the BCP emission, a result of the increasing hole penetration from EML into the BCP layer, would emerge with increasing hole injection and decrease gradually with decreasing electron injection [70]. By using a tris (di(bromo) benzoylmethane)mono(phenathroline)europium(III)(Eu(DBrBM)3phen), or tris(biphenoylmethane)mono(phenathroline)europium(III)(Eu(BDBBM)3phen) doped into a mixture of TPD and polymer PC (bisphenol-A-polycarbonate), Kalinowaski and co-workers realized a maximum EL quantum efficiency up to 5%, but they ascribed the efficiency roll off effect to the electric field-assisted dissociation of electron-hole pair precursors of europium (III) ion localized emissive states, which was at variance with the T-T annihilation mechanism [71, 72]. For the Eu(DBM)3Bath, which has excellent electron injection and transport properties [61], people usually mixed it with a hole transport materials as emitter layer to balance the hole and electron injection and transport. Liang and co-workers showed that the quenching effect of the metal cathode and the unstable nature of the Eu (III) complex under EL operation markedly influenced the EL efficiency, which could be significantly improved by keeping the emitting area far from the metal cathode and mixed the Eu(DBM)3Bath with TPD as the emitter layer [73]. They also shown that [74]

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biexcitonic quenching among the excited Eu3+ ions was an important channel in their decay process and this quenching process was a primary cause for the observation of rapid decrease in quantum efficiency with increasing current density. They also founded that holes injected into this europium complex film would be quenchers of luminescence and sources leading to poor stability. After mixed with hole transport materials TPD and optimize the ratio of the two materials, an external quantum efficiency of 4.6% was obtained [75]. More recently, by introducing the better electron injection layer 1,2,4-triazole derivative TAZ, Kido and co-worker has attained peak external quantum efficiency of 7.8% with Eu(DBM)3Bath :TPD (1 : 2) as the emitter layer [76].

Figure 13. The device structure and energy level diagram of the Eu(TTA)phen based OLED (Ref.67).

Based on the EL mechanism and the physical processes being understood inside the europium (III) complex based OLEDs, more and more research on europium (III) complexes is focused on the design of ligands to optimize their charge transport properties, luminescent efficiency, stability and volatility by incorporated some charge carrier transporting group, good conjugated system, or more excellent planar structure into the ligand molecule. These works are divided into two areas. One is design and optimization of first ligand in europium (III) complex to match the 5D0 level of Eu3+ ion, as it has been shown that the magnitude of the energy gap between the ligand triplet state and the lower energy europium (III) excited 5D0 state influenceed the luminescence quantum yield greatly [77]. The other is introduction some

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carrier transporting groups into the ligand molecule to improve the carrier transporting properties. In the early stage, different kind of β-diketones were used and compared to get the best match between the ligand T1 states and the excited state of central Eu (III) ions [78-80]. Zhang et al. [81-83] investigated the effect of the replacement of hydrogen by fluorine in the TTA ligand on the performance of europium (III) complex based OLEDs. It was found that [81] not only the brightness and efficiency was improved, but also strong emission from 5D1 → 7FJ (J=0-4) transitions was observed in addition to the 5D0 → 7FJ (J=0-4) lines. More recently, they substituted the thenoyl group by the long conjugate naphthyl group in the TTA ligand, the resulting europium complex Eu(HFNH)3phen (Fig. 14) shown a current efficiency of 2.15 cd/A with the structure ITO/TPD/ Eu(HFNH)3phen :CBP/BCP/Alq3/LiF/Al [84]. Inoue et al. [85] used 1,3-di(2-thienyl)propane-1,3-dione (DTP) as the first ligand to get the Eu(DTP)3Bath and applied it to OLED devices as emitting materials. More modifications on the DBM ligand were performed. Bazan et al. introduced a carbazole fragment to DBM ligand [86] to improve the hole transport, the hexyloxy groups were introduced to prevent crystallization and to increase solubility in common organic solvent (Fig. 14), resulting in the complex tris[(N-ethylcarazolyl)(3',5'-hexyloxybenzoyl)methane](phen)europium, where the phenanthroline ligand were used as second ligand to enhance the electron transportation in the EL process. Jen et al. [87] reported a quantum efficiency of 0.8% OLED based on polymer matrices doped with a new europium complex with dendron-substitued diketone ligands. They also investigated the effect of diketone ligand conjugation length on the performance of europium complex based OLED [88]. Butyl and methoxy group were also incorporated into the DBM ligand [89], the resulting europium (III) complex with TPPO second ligand as emitter in the OLEDs was reported. Some new ligands, such as N-(5-phenyl-1,3,4-oxadiazol-2-yl)-benzamide [90], was also developed for europium (III) complex. The pyrazolone based ligands were investigated by Huang and co-workers (Fig. 14) [91]. After modified the ligand structure, the triplet energy levels of the pyrazolone ligand was tuned to match the 5D0 energy level of Eu3+ properly, resulting in the high energy transfer efficiency and the better EL properties of the Eu (III) complex based OLEDs. Zheng and co-workers developed some other new DBM derivatives [92-94] by incorporating carbazole group in ligand. The resulting Eu complexes showed wide absorption profile, effective light harvesting chromophores, good hole transporting ability, effective suppress of the crystallization and excellent solubility in common organic solvents, which should result good performance

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for these Eu(III) complex based OLEDs although there did not present the device results.

Figure 14. The molecular structures of europium (III) complexes with different first ligand (Ref. [84, 85, 91]).

On the other hand, researches on the second ligand modification were also reported. In the early stage, triphenylphosphine oxide (TPPO), phen, and Bath are introduced and compared in order to get a high performance Eu(III) complex based OLED [60, 78, 95]. The best results were obtained from a series of europium (III) complexes (Fig. 15) containing some derivatives of phen, which were reported by Sun and co-workers [96, 97]. All the complexes were doped into CBP host. An external quantum efficiency 2.1%, current efficiency 4.4 cd/A, power efficiency 2.1 lm/W, and brightness 1670 cd/m2 was obtained from the device with a configuration of ITO/TPD/Eu (TTA)3(DPPz):CBP/BCP/Alq3/Mg:Ag [96]. Similar modifications on the phenanthroline ligand were also performed by Li and co-workers [98, 99]. With Eu(DBM)3pyzphen [99], (pyzphen=pyrazino[2,3-f]1,10-phenanthroline) (Fig. 15), a maximum current efficiency of 5.1 cd/A was obtain with a configuration ITO/TPD/ Eu(DBM)3pyzphen: Bath(1:3)/ Bath/LiF/Al. It also found that carrier trapping was a main process in this device in addition of energy transfer from the Bath to Eu(DBM)3pyzphen molecules [98]. Theory

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study found that these europium (III) complexes had better electron transporting properties and the electron trapping was the dominant EL mechanism in these materials due to their lower LUMO energies than CBP [100]. Huang and co-workers did many promising researches on the modification of phenanthroline structure. Beginning with 2-phenyl-imidazo[4,5-f]1,10-phenanthroline (L1) (Fig. 16) [101], they incorporated ethyl and dimethylanino group (L2) (Fig. 16) [102], then carbazole group (CPIP) (Fig. 16) [103-105] and triphenylanino group (TPIP) (Fig. 16) [106] into the L1 molecule, the resulting europium (III) complexes showed promising performance when used in OLEDs. A power efficiency of 2.7 lm/W and luminance of 2000 cd/m2 was obtained from a device with the configuration of ITO/TPD/Eu(DBM)3(CPIP)/BCP/Alq3/Mg:Ag [103]. With triphenylamine containing phenanthroline ligand, the europium complex, tis( dibenzoylmethanato) (2-(4'-triphenylamino) imidazo [4,5-f]1,10-phenanthroline) europium, Eu (DBM)3(TPIP), based OLED gave a external quantum yield of 0.85% with the configuration of ITO/TPD/Eu (DBM)3(TPIP)/BCP/Alq3/Mg:Ag (Fig. 16) [106]. They also developed 2-(2-pyridy)benzimidazole (HPBM) and 1-ethyl-2-(2-pyridyl)benzimidazole (EPBM) as the second ligand for the Eu (III) chelate (Fig. 17) [107]. These ligands may be easily substituted with other alkyl chains or groups on the benzimidazole in order to further improve. The maximum luminance of 180 cd/m2 for Eu(DBM)3EPBM was obtained at 18 V with structure ITO/TPD/Eu-(DBM)3EPBM/Alq3/Al. Similar molecular structure with N,N-chelated (benzothiazole) and N,O-chelated (benzoxazole) second ligand were also investigated by them for europium (III) complexes based OLED devices [108, 109]. Then, Wang and co-workers introduced an oxadiazole moiety into the HPBM ligand through a flexible spacer to improve the electron transporting properties and the solubility of europium (III) complex (Fig. 17) [110]. A quantum efficiency of 1.7% was obtained with a simple double layer structure device. A carbozole fragment was also introduced into the HPBM ligand and the EL performances of the devices using Eu(DBM)3(Car-PYBM) as an emitter was improved [111]. Ma et al. [112] investigated the effect of different substituted groups, such as methyl, chlorine, and nitryl, in phenanthroline ligand (Fig. 18) on the EL performance of devices based on the corresponding Eu (III) complexes. It was found that the more methyl-substituted groups on phenanthroline ligand led to higher device efficiency. Using 3,4,7,8-tetramethyl-1,10-phenanthroline (Tmphen) as the second ligand (Fig. 18) [113], the Eu(TTA)3Tmphen based OLED devices showed an external quantum efficiency of 2.5% and current efficiency of 4.7 cd/A with configuration ITO/TPD/CBP: Eu(TTA)3-

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Tmphen/BCP/Alq3/LiF/Al. Li et al. [114, 115] also incorporated the oxadiazole unit to the second ligand which was also designed to be a bulky molecular with spiro-structure derived from phenanthrline. When the resulting Eu (III) complex, Eu(DBM)3(OXD-Spiro-DF) (Fig. 19), was used as emitter, the brightness and EL efficiency were significantly improved compared with the device based on Eu(DBM)3(OMe-Spiro-DF) (Fig. 19), which had no oxadiazole unit. Some heterocyclic ligands containing pyridine, imidazole and triazine rings have been used to make ternary europium complex and the EL performance was also reported [116]. A rigid Lewis base with a large π system, 2,4,6-tri(2-pyridyl)-1,3,5-triazine (TPTZ) were also used as the second ligand for europium (III), forming a coordination polyhedron best describable as a monocapped square antiprism [117]. Not only PL quantum yield but also the EL performances were improved for the europium (III) complex using TPTZ as the second ligand [118]. Huang and co-workers also performed some promising research in the new second ligand design. First, they used the 2,2'bipyridine mono N-oxide as the second ligand, the resulting Eu(TTA)3(Obpy) based OLED gave a external quantum efficiency of 1.6% with a structure ITO/TPD/Eu(TTA)3(Obpy): CBP/BCP/Alq3/LiF/Al [119]. Then they synthesized a series of derivative of TPPO (triphenylphosphine oxide) and applied into the europium (III) complexes based OLED device. The bis(2(diphenylphosphino)phenyl)ether oxide (DPEPO) was used as a unit neutral ligand to prepare the complex Eu(TTA)3(DPEPO) first(Fig. 20) [120]. The DPEPO has the mezzo first triplet excited energy level (T1) lied between the first singlet excited energy level (S1) and triplet state (T1) of TTA, which resulted in the improvement of energy transfer in the europium (III) complex. The multilayer EL devices based on Eu(TTA)3(DPEPO) gave a maximum external quantum yield of 2.89% and maximum current current efficiency of 4.58 cd/A. Then 1,8-bis(diphenyl-phosphino)naphthalene oxide (NaPO) as neutral ligand was synthesized to prepare the Eu(TTA)3(NaPO) (Fig. 20) with aim to improve the thermal stability and the charge injecting and transporting ability of europium (III) complex [121]. More recently, two carbazole based phophine oxide ligands, 9-(4-(diphenylphosphinoyl)phenyl)-9H-carbazole (CPPO) and 9-(diphenyl-phosphoryl)-9H-carbazole (CPO), which have differrent bipolar structures, donor-π spacer-acceptor (D-π-A) or donor-acceptor (DA) systems respectively, were investigated in the europium (III) complex, Eu(TTA)3(CPO) and Eu(TTA)3(CPPO) (Fig. 20) [122]. It was found that the phosphine oxide ligands with D-π-A architecture were more appropriate than those with D-A architecture to achieve more EL efficient for europium (III) complex. A maximum current efficiency and external quantum efficiency of

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5.7 cd/A and 3.6% were obtained with Eu(TTA)3(CPPO) based OLED device. There are also some reports [123, 124] that incorporated the hole transporting group and electron transporting group into the first and second ligand, respectively, to prepare the europium (III) complex which has balance carrier transporting property to improve the performance of europium (III) complex OLEDs.

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Figure 15. The molecular structures of some typical europium (III) complexes (Ref. [96-99]).

Figure 16. The molecular structures of europium (III) complexes with modified phen ligand (Ref. [101-106]).

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Figure 17. The molecular structures of europium (III) complexes with novel second ligand (Ref. [107,110, 111]).

Figure 18. The molecular structures of europium (III) complexes with different phen ligand (Ref. [112, 113]).

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Figure 19. The molecular structures of europium (III) complexes with spiro-structure ligand (Ref. [116]).

Figure 20. The molecular structures of europium (III) complexes with TPPO derivative as the second ligand (Ref. [120-122]).

Actually, from above we can see that the devices structure for the europium (III) complex is complicated, which need the multiple layer structure and a complicated fabrication process. And also the doping process usually leads to phase separation when the device undergoes operation and the emission from EBL or HBL often appears as a result of the complicated fabrication processes of multilayer device. Therefore, polymers materials incorporated with the europium (III) complexes have attracted more and more

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attention in the past decade by inheriting the advantages of both the luminescence characteristics of europium (III) ions and the material properties of plastics. Besides the advantage of the desired mechanical flexibility, polymer-based europium (III) luminescent materials can be solution and spincoating processable, which is more attractive for the devices fabrication. Heeger et al. [125] first codissolved a blue emitting polymer, poly(2-(6'-cyano6'-methyl-heptyloxy)-1,4-phenylene) (CN-PPP) (Fig. 21), with a family of soluble Eu (III) complexes using β-diketonate ligands in a solvent, and casted film as the emitter layer in OLED device. Pure red emitting OLEDs with a quantum efficiency of 1.1% were made by doping Eu(DBM)3phen in CN-PPP. The hole conducting polymer PVK (Fig. 21) was also used as the host for Eu(TTA)3phen to get the red light emission [62]. Liu et al. [126] synthesized a serious of soluble europium (III) complexes, LEu(TTA)4(L=Li+, Na+, K+) (Fig. 22), and doped into polymer PVK to investigate the PL and EL properties. Salata et al. [64] doped the Eu(TTA)3phen and PBD into PVK to balance the carrier transportation and injection and also used the BCP layer as hole blocking layer to enhance the EL efficiency from Eu3+ ions in the OLEDs. Cao et al. introduced the fluorine unit in the first ligand and doped the resulting europium (III) complex Eu(FTA)3phen (Fig. 22) into the PVK:PBD host materials [127]. A maximum external quantum efficiency of 4.28% and maximum luminance efficiency of 4.6 cd/A at 9.0 V were obtained from a triple-layer device with a configuration of ITO/PEDOT:PSS/ PVK:PBD:Eu (FTA)3phen/TPBi/Ba/AL. They also used the polymer PFO and PBD as a cohost matrix for europium (III) complex containing bromine groups in the secondligand,tri(dibenzoylmethane)(3,9-dibromo-1,10-phenanthroline) europium (III) (Eu(DBM)3(DBrphen)), to prepare OLEDs [128]. A maximum external quantum efficiency of 1.4% was obtained. McGehee et al. [129] modified the first ligand DBM that incorporated four tert-butyl groups into each β-diketonate ligand. The emission center Eu3+ ions in the resulting complex, tris(di-(3,3',5,5'-tetra-tert-butyl)benzoylmethane) (monophenanthroline)europium(III) (Eu(t-t)3phen) (Fig. 22), was shielded from its environment relative to the Eu(DBM)3phen, resulting in CN-PPP doped Eu(tt)3phen OLED device with increased operational lifetime and decreased operational voltage. Another OLED with ITO/PEDOT/ PVK:Eu (DBM)3(Phen)/Ca/Al structure was fabricated by spin-coating the emission layer [130], which performance was comparable to that of devices fabricated using more sophisticated small molecule evaporation techniques. A long alkyl chains was incorporated into TTA ligand to increase site isolation of Eu3+ ion and reduce the strong queching effect, the resulting Eu (III) complex was blended with

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polyfluorene (PFO) (Fig. 21) as a single spin-coating emitting layer, the external quantum efficiency reached 1% [131]. It was also found that the radiation rates and PL emission quantum yields of Eu (III) complex can be greatly improved using asymmetric deuterated ligand [132]. Furthermore, to overcome the phase separation problem caused by the blending, the luminescent Eu (III)-containing copolymers were synthesized through the copolymerization of Eu (III)-complex monomers containing β-diketonate with other polymer monomers. First, the poly(MMA-co-AAEu(DBM)2) (Fig. 23) was synthesized through the reaction of europium isopropoxide with βdiketonate and acrylic acid with methyl methacrylate (MMA) [133]. It was found that no significant emission concentration quenching phenomenon was observed in the PL even the Eu (III) content reached 6.38 mol %. And then the PVK was incorporated into the copolymer (Fig. 23) and the characteristic bright-red EL from the Eu3+ ion at 614 nm was observed in a single layer OLED device [134]. More recently, the water soluble copolymer containing Eu (III) complex, poly(oxyethylene phosphate)tris(β-diketonate)europium(III), was also synthesized and the PL property was investigated [135, 136].

Figure 21. The molecular structures of some typical polymer host for europium (III) complexes based OLEDs (Ref. [64, 125, 131]).

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Figure 22. The molecular structures of some typical europium (III) complexes used in polymer Eu(III) OLEDs (Ref. [127-129]).

Figure 23. The molecular structures of some polymer containing europium (III) complex (Ref. [133-136]).

There are also many efforts were put on the device structure designs and improvements for the Eu (III) complex OLED devices. In the early stage, the hole injection layer CuPc [137], LiF/Mg cathode [138], hole blocking layer BCP (66) and Bath [139], mutilple-stacked emission unit [137], and

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microcavity device structure [140, 141] were introduced into the Eu (III) complex device to improve the performance. The thickness and growth rate evaporation of the Eu (III) complex were also investigated [142]. Lee et al. [143] reported efficient EL from OLED using red dye DCJTB doped into the Eu(DBM)3Bath as the emission layer in which both triplet and singlet energy could be converted into light emission. The EL mechanism inside the device was also investigated in detail. It was found that the fluorescent dye acts as efficient energy acceptor in the Eu (III) complex hosts and quenched the exciplex emission [144] which was formed by the charge transfer from HOMO of the donor (TPD molecule) to LUMO of the acceptor (Eu(III) complex) [145]. But the emission purity was great decreased and the efficiency was also lower compared with the DCJTB doped Alq3 devices. Ma and co-workers have done many works on the dye sensitilized europium (III) EL emission in the OLED device. By careful control the fluorescent dye doping concentration, they realized high efficient OLEDs (9.0 cd/A) based on europium (III) complex which emission comes only from the Eu (III) ions [146, 147]. The fluorescent dye DCJTB just acted as a carrier trapping which adjusted the local electric field within the emissive layer. Then they used the stacked device structure to further improve the efficiency of Eu (III) complex based OLED device [148]. With a two stacked emitter unit, they got a current efficiency of 14.5 cd/A (corresponding external quantum efficiency of 7.83%), which was the highest efficiency reported using a Eu (III) complex as a lumophore in an OLED device so far. More recently, they also used the blue electrophosphorescent complex FIrpic as sensitizer in europium (III) complex based OLED devices [149]. Due to the effective energy transfer assistance effects of the codoped FIrpic, the EL performance of Eu (III) complex based device was significantly enhanced. Huang et al. even synthesized some novel iridium-europium bimetallic complexes, {[(dfppy)2Ir(µ-phen5f)]3EuCl}Cl2 and (dfppy)2Ir(µ-phen5f)Eu(TFAcA)3 [dfppy represents 2-(4′,6′-difluorophenyl)-pyridinato-N,C2′, phen5f stands for 4,4,5,5,5-pentafluoro-1-(1′,10′phenanthrolin-2′-yl)-pentane-1,3-dionate and TFAcA represents trifluoroacetylacetonate] [150], in which the novel ligand Hphen5f with four coordination sites was designed as a bridge to link the Ir (III) center and the Eu (III) center. But no EL results were reported. Some new europium complex and new methods are also developed to improve the efficiency and the stability of the Eu complex based OLEDs. Christou et al. [151] reported a thin film EL devices prepared using a divalent europium species, bis[tris(dimethylpyrazolyl)borate]europium(II) (Fig. 24). This complex exhibited bright orange EL with λmax= 596 nm other than 614

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nm for the Eu3+ ion in the trivalent europium complex based OLEDs. Then the hybrid film which prepared by simultaneous vacuum deposition of the host materials CBP and the EuI2 nanoparticles was used as the emission layer to fabricate a three layer OLED device [152]. The maximum external quantum efficiency of 0.18% was achieved. Some Eu, Gd coordination complexes, (Eu0.1Gd0.9)(TTA)3TPPO2 [63], and Sm, Eu coordination complexes, (SmxEuy)(TTA)3TPPO2 (x=0.7,0.9; y=0.3,0.1) [153], were also used as emitting layer in the OLED devices. Also, a dinuclear europium complex, (TTA)3Eu(PYO)2Eu(TTA)3 (PYO=pyridine N-oxide) (Fig. 24) based OLED was reported [154]. The EL of some tetrakis β-diketonate complexes, M[Eu(DBM)4],M=Li+, Na+, and K+ [155], have also been investigated in the europium (III) based OLEDs. To solve the problem about the disassociation of neutral ligand, Adachi and co-workers reported the formation of ternary europium complex by vacuum co-deposition the Eu(DPM)3 and the neutral ligand in different source at the same time [156]. The result indicated that the neutral ligand coordinated with Eu(DPM)3 when they were deposited on substrate, and the red light emission from Eu (III) were observed from the device.

Figure 24. The molecular structures of a divalent europium (II) complex and a dinuclear europium (III) complex (Ref. [151, 154]).

Another interesting work in the europium complex based OLEDs is using Eu (III) complex as the red unit to realize white light OLEDs. Kido et al. [157] first report a white EL device using the europium (III) complex as the red emitting materials. Tian et al. [158] report that a white light emission with the CIE coordinates of (0.333, 0.348) was achieved with a single dendritic

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europium (III) complex dopand, Eu(TCPD)3(Phen) (tris[1-[3,4,5-tris[4-(9Hcarbazol-9-yl) butoxy] -phenyl] -3-phenylpropane-1,3-dione] (1,10-phenanthroline) europium (III)) (Fig. 25), which contained grafted carbazole units in the first ligand. With the EL configuration ITO/NPB/CBP : Eu(TCPD)3phen/BCP/Mg:Ag, a maximum external quantum efficiency of 1.1% was obtained. Ma et al. [159] also reported efficient white OLEDs using europium (III) complex as the red unit in a multilayer structure of ITO/NPB/CBP: (PPQ)2Ir(acac): Eu(TTA)3Tmphen/NPB/MADN: DSA-Ph /BCP/AlQ/LiF/AL. A maximum efficiency of 9 cd/A with a stable white light was generated. Some other potential white europium (III) complexes were also reported [160] in the OELD devices.

Figure 25. The molecular structures of typical europium (II) complex which used in white OLED devices (Ref. [158]).

The EL of europium complexes are always the focus of scientific and commercial interest. However, the stability and luminescence performance of their devices have not been satisfied until now, especially compared with the OLEDs based on some phosphorescent materials. If all the properties of europium complexes can be achieved simultaneously, including high PL and EL efficiency, excellent balanced carrier transfer properties, good stability, and proper device configuration, the europium complexes based OLEDs with good performance will be definitely anticipated.

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Chapter 3

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EUROPIUM COMPLEXES IN ORGANIC MEMORIES Besides the good applications of europium complexes in OLEDs, it is found that the europium complexes also exhibit very good memory effects in diode structure. It is expected that organic molecules could perform the functions of semiconductor electronics [161], such as assembling into prototypical memory and logic architectures [162, 163]. However, organic molecules have to face the fundamental and critical question of whether they can meet the minimum functionality standards that are expected of electronic devices. The europium-contained molecule memory device was first reported in 2003 [164]. It can be seen that porphyrin molecule with europium ions (Fig. 26)-based information-storage media was demonstrated to meet the processing and operating challenges required for use in electronic devices. In particular, this molecule was stable under extremes of temperature (400°C) and large numbers of read-write cycles (1012), which are attributed to the good redox behavior and robust molecular structure [164]. Actually, an important aspect that the memory devices based on organic semiconductor can receive a great deal of attention is due to their simplicity in structure, good scalability, low cost potential and large capacity for data storage. However, an organic memory device stores information in a manner entirely different from that of silicon devices. Rather than encoding “0” and “1” as the amount of charge stored in a cell, organic memory devices stores data, for instance, based on a high and low conductivity response to an applied voltage, called electrical bistability. This can be realized in polymer materials based on their resistance changes in response to an applied voltage [165]. The representative polymer material that exhibits memory effect in europium

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complex-based polymers is poly [N-vinylcarbazole-co-Eu (vinylbenzoate) (2thenoyltrifluoroacetone)2-phenanthroline] (PKEu) copolymer [166]. The molecular structure of PKEu and the configuration of the memory device are shown in Fig. 27. In the PKEu, the carbazole unit is widely used as an electron-donor and hole-transporting group, and the Eu complex acts as electron acceptor. It can be seen that the device ITO/PKEu/Al exhibits well memory effect. Fig. 28 shows the typical current density-voltage (J-V) characteristics of the Al/PKEu/ITO sandwich device. J increases progressively with the applied bias (stage I). A sharp decrease in current occurs at about 4 V (stage II), indicating the transition of the device from the high conductivity state (ON-state) to a low conductivity state (OFF-state). This transition from the ON-state to the OFF-state is equivalent to the „„writing‟‟ process in a digital memory cell. After this transition, the device remains in this state even after the power is turned off. This phenomenon can be seen in the third scan (stage III) in Fig. 28. The J-V characteristics define the electrical bistability of the PKEu and also reveal the non-volatile nature of the memory effect. The OFF-state can be recovered by the simple application of a reverse voltage pulse (at about -2 V, stage IV). This is equivalent to the „„erasing‟‟ process of a digital memory cell. Stage V of Fig. 28 shows the J-V characteristics of the device after application of a -2 V bias. The behavior is nearly identical to that of stage I. This feature allows the application of PKEu in a rewritable flash memory. It can be seen that the device showed a high ON/OFF current ratio, 4 about 10 , fast response time shorter than 20 μs from the ON state to the OFF state, and acceptable retention ability under ambient conditions. Also the device can operate well more than a million read cycles under ambient conditions without any encapsulation. This fully shows the advantages of Eu complex-based polymer as memory medium. Later, a similar non-conjugated methacrylate copolymer (PCzOxEu) containing carbazole moieties (electron donors), 1,3,4- oxadiazole moieties (electron acceptors), and europium complexes in the pendant groups was also synthesized [167]. When ITO/PCzOxEu/Al was fabricated, the written, read, and erased cycles were well realized in this device by electric bias. It was seen that the polymer 5 memory exhibited an ON/OFF current ratio up to 10 , switching response time 6 of ~1.5 μs, more than 10 read cycles, retention time of more than 8 h, and write/erase voltages of about 4.4 V/-2.8 V under ambient conditions. It was found experimentally that the oxadiazole moieties in the PCzOxEu played important role in improving the response time and retention time of the memory device. Fig. 29 gives the molecular structure of PCzOxEu. The

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ITO/PCzOxEu/Al device also showed similar J-V characteristics to ITO/PKEu/Al device [167]. The electrical switching characteristics show that PKEu and PCzOxEu are excellent active medium as flash memory.

Figure 26. Structure of the Si-tethered porphyrins with europium ions. The triple decker has four states below 1.6 V (8). The availability of multiple redox states affords the possibility of multibit information storage (Ref. [164]).

Figure 27. a) Molecular structure of the copolymer PKEu with the composition of x/y=0.987:0.013; b) schematic diagram of the memory device ITO/PKEu (50 nm)/Al (Ref. [166]).

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Figure 28. Current density-voltage (J-V) characteristics of the Al/PKEu/ITO device based on a spin-cast film of PKEu (w50 nm) for two sweep directions. Arrows indicate the sweep direction of the applied voltage (Ref. [166]).

Figure 29. Molecular structure of PCzOxEu copolymer (Ref. [167]).

Some conjugated copolymers with europium complex were also developed and their memory effects were studied. However, it was found that these conjugated polymers containing electron D-A structure and europium complex exhibit the WORM memory effect [168-171]. Fig. 30 shows the molecular structures of three conjugated copolymers with europium complex. Although they have similar molecular structure with donor-acceptor and europium complex, the J-V characteristics seem not to be the same. As the J-V characteristics of ITO/PF6Eu/Al device shown in Fig. 31, initially, the as-

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fabricated device was at its low conductivity state (OFF-state). The current density in the low voltage range on the first sweep was quite low (of the orders 2 of 10-10-10-6 A/cm ). When a switching threshold voltage of about 3 V was applied, an abrupt increase in J from 10-8-10-2 A/cm2 was observed, indicating the device transition from a low conductivity state to a high conductivity state (ON-state, the first sweep) [168]. This electrical transition from the OFF-state to the ON-state served as the „„writing‟‟ process for the memory device. The device exhibited good stability in this high conductivity state during the subsequent forward and reverse voltage scans (Fig. 31). It remained in the ONstate even after turning off the power (the second sweep) and did not return to the low conductivity state upon applying a negative bias (the third sweep). Thus, this device exhibited the WORM-type memory effect [168]. For the same class of conjugated copolymers containing europium complex with different substituted alkyl group [169], composition [170] or coordination ligands [171], the devices exhibited similar WORM memory effect. Additionally, a flexible WORM memory device based on P6FBEu has also been tried to fabricate with a conductive polypyrrole film as the bottom electrode and gold as the top electrode [171]. The flexible polymer memory is expected to meet the demand for data storage in memory devices of unique spatial construct or architecture, such as smart label and RFID.

Figure 30. Molecular structures of two conjugated copolymers with europium complex (Ref. [168-170]).

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Figure 31. Typical J-V characteristics of the Al/PF6Eu/ITO device in the ON- and OFF-state and the J-V curves of the Al/PF6/ITO device based on conjugated poly(9,9dihexylfluorene) (PF6) without the europium complex moiety (Ref. [168]).

As reported, these conjugated copolymers with europium complex exhibited rather good memory performances. For example, the memory device 7 of ITO/PF6Eu/Al showed a high ON/OFF current ratio up to 10 promises a 8 low misreading rate, both the ON and OFF states were stable up to 10 read cycles at a read voltage of 1 V, and both states were projected to be stable up to 10 years at a constant voltage stress of 1 V. It can be seen that these polymer WORM memory devices meet the demand for high capacity and inexpensive data storage in increasingly sophisticated handhold applications and disposable electronics. Why did the conjugated copolymers with europium complex show different memory characteristics from the non-conjugated copolymer with europium complex? Kang et al. gave explained well [169]. As explained, although they all might be explained well by trapping-detrapping mechanism, the electronic processes between conjugated copolymers and non-conjugated copolymers were completely different. For the case of conjugated copolymers, the electronic processes and experimental J-V curve fitting are shown in Fig. 32 (a), (b) and (c), respectively. Under a low bias voltage (0 V to 1.3 V), the JV curve is linear and can be fitted with the Ohmic model. At this time the

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Europium Complexes in Organic Memories

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device is at its OFF-state. When the voltage exceeds the Schottky barrier (1.3 V), electrons are injected into the LUMO of the Eu complexes and holes are injected into the HOMO of the fluorene moieties. The charged LUMO (radical anion) of the Eu complex and the charged HOMO (radical cation) of the fluorene moiety form a channel for charge carriers through nonradiative intersystem transition. The polymer becomes p-doped under the induction of the electric field and switches to the high conductive state (ON state). Because the europium complex in the conjugated copolymer may be considered as to be deep traps, thus, it is difficult to detrap (or detrapping will require a voltage higher than the break-down voltage of the device), and the device behaves as a WORM memory. For the case of non-conjugated polymers containing carbazole group as the electron donor and europium complex as the electron acceptor, the high conductivity state (ON) of PKEu is similar to that of PVK since the carbazole groups are the dominant moieties in the copolymer and the Eu complexes only act as the „„dopant‟‟ [166]. When a forward voltage is applied, the carbazole groups near the interface are oxidized and holes are generated, resulting in ON-state. However, when the voltage reaches the threshold voltage (4 V), the reduced Eu complex can further form a CT complex with the surrounding oxidized carbazole species. Charge carriers are trapped in the insulating CT complex. However, the CT complex can decompose under reverse bias. Thus it is a rewritable and nonvolatile memory or flash memory [166]. As seen, further incorporation of an electron acceptor, 1,3,4-oxadiazole moiety (Ox), in the non-conjugated copolymer further improved the performance of the memory device to resulted in a shorter response time, longer retention time and higher ON/OFF ratio [167]. This is because the additional electron acceptor can act as a mediator to facilitate carrier transport from the respective HOMOs and LUMOs of the carbazole (Cz) and Eu components. The mediation effect reduces internal energy barriers and accelerates hole transport from the HOMO of Cz and electron transport from the LUMO of Eu when a threshold voltage is applied [167]. Similar memory behavior has also been observed in a series of rare earth metal complexes, including Eu(DBM)3(tmphen), Sm(DBM)3(tmphen) and Gd(DBM)3(bath), doped into poly- (N-vinylcarbazole) (PVK) as active medium in the memory devices (DBM=dibenzoylmethane, tmphen=3,4,7,8tetramethyl- 1,10-phenanthroline, bath =4,7-diphenyl-1,10-phenanthroline) [172]. As shown in Fig. 32, although only simply physically doping organolanthanide complexes into proper conducting polymers, the repeatable electrical bistability can be well realized in J-V characteristics. It was found

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experimentally that these devices also exhibited excellent memory performances: the ON/OFF current ratio is over two orders of magnitude and the write-read-erase cycles are more than 106 without degradation. The mechanism investigation found that the interaction between PVK and organolanthanide complexes plays an important role on the electrically switching characteristics in these organolanthanide complexes doped devices, and an electrical-field-induced charge transfer between PVK and RE complexes is attributed to the electronic transit. Like the same electronic processes in memory device based on non-conjugated copolymers with europium complex, the high-conductance state of the devices is due to the higher hole mobility of PVK. The current increases with the increase of bias voltage, and when the bias reaches the threshold voltage, the reduced Eu complex can form a CT complex with surrounding oxidized carbazole groups. The CT complex is basically insulating, leading to the sharp decrease in the current of the devices. Now the devices are in its low-conductance state (OFF state). As known, due to the instability of the formed CT complex, a reversal voltage can result in the return of the carbazole group and Eu complex to their original state. Obviously, the electrically reversible processes are greatly crucial to repeat the transit between the ON state and OFF state.

Figure 32. Current-voltage characteristics of the devices ITO/PEDOT/PVK: Sm(DBM)3(Tmphen)/LiF/Ca/Ag, ITO/PEDOT/PVK: Eu(DBM)3(Tmphen)/LiF/Ca/Ag, and ITO/PEDOT/PVK: Gd(DBM)3(Bath)/LiF/Ca/Ag (Ref. [172]).

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All these results indicate fully that organolanthanide complexes are promising semiconductor materials for high-density, low-cost memory application besides the potential application as organic light-emitting materials in display devices.

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Chapter 4

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SUMMARIES AND CONCLUSION Organic europium complexes are of considerable interest due to their sharp narrow emission spectrum and potentially high emission quantum efficiency. Therefore, considerable progress has been achieved with organic light-emitting diodes by utilizing organic europium complexes as the emitting materials. For the practical application of the europium complexes based OLEDs, further development should be also focused on obtaining lower cost, easier fabrication, and better stability of europium complexes and their devices. Furthermore, Organic europium complexes also show well electrical switching characteristics in diodes. Several memory effects, including rewritable flash memory, write-once read-many-times (WORM) memory and dynamic random access memory (DRAM) have been realized by the europium complex. These memory devices exhibit a high ON/OFF current ratio, promise low misreading rates, and can endure more than one million read cycles. Both the ON-state and OFF-state are stable under constant voltage stress. These europium complexes based memories also have a relatively fast response time in the microsecond range, making them suitable for many data storage applications. So it can be seen that organic europium complexes are promising materials as active medium in organic optoelectronic devices.

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INDEX

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A access, 12, 14, 45, 50 acetonitrile, 50 acrylate, 51, 58, 59 acrylic acid, 29 aggregates, 7 aluminum, 6 applications, vii, 1, 2, 15, 35, 40, 45, 47, 53 aqueous solutions, 51 atoms, 1 availability, 37

B band gap, 5, 8 bandgap, 9 bandwidth, 58 behavior, 14, 35, 36, 41, 50 bias, 36, 39, 40, 41, 42 bisphenol, 19 blends, 58 bromine, 28 buffer, 4, 48

C candidates, 1, 50 carbon, 14, 51 carrier, 3, 14, 15, 19, 20, 22, 28, 31, 33, 41 cell, 12, 13, 35

cell phones, 2 chelates, 16, 53 chemical structures, 5, 9, 10 chlorine, 23 color, 7, 8, 15, 50 components, 13, 41 composition, 37, 39 compounds, 18 concentration, 18, 29, 31 conductance, 14, 42 conductivity, 12, 13, 35, 39, 41 configuration, 13, 15, 22, 28, 33, 36 confinement, 18, 49 conjugation, 21, 54 control, 11, 31 coordination, 24, 31, 32, 39, 47, 52 copolymers, 29, 38, 39, 40, 42, 58, 59 copper, 4 crystal structure, 56 crystallization, 21 crystals, 2 current ratio, 14, 36, 40, 42, 45 cycles, 14, 35, 36, 40, 42, 45

D decay, 7, 20 degradation, 13, 42 density, 12, 20, 36, 38, 39, 43, 55 density functional theory, 55 deposition, 3, 32, 60

Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

64

Index

derivatives, 5, 10, 21, 22 diodes, vii, 2, 45 dissociation, 19 distribution, 19 DNA, 9, 61 dominance, 3 dopants, 6, 8 doping, 7, 27, 31, 41, 55, 60 dyes, 8, 12, 15

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E earth, 41, 53, 57, 58, 59, 60 electric field, 19, 31, 41, 53 electrodes, 2, 4, 12 electroluminescence, 2, 10, 17, 19, 48, 49, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60 electron, 3, 5, 7, 8, 14, 15, 18, 19, 21, 23, 36, 38, 41, 49, 50, 51, 52, 55, 57 electronic materials, 47 electronic systems, 1 electrons, 2, 3, 7, 8, 15, 19, 41 emission, vii, 2, 7, 8, 10, 15, 17, 19, 21, 27, 30, 32, 45, 48, 49, 51, 54, 58, 59, 60 emitters, 10, 15, 52, 53 encapsulation, 36 encoding, 12, 35 energy, 4, 7, 9, 15, 16, 17, 19, 20, 22, 31, 41, 48, 51, 52, 55, 57, 58, 60 environment, 28 evaporation, 28, 31 exciton, 4, 6, 7, 11, 18, 19, 49

H harvesting, 21 host, 6, 8, 10, 18, 22, 28, 29, 32, 52, 55 hybrid, 5, 32, 48 hydrogen, 21

I induction, 41 information technology, 2, 12, 47 instability, 42 integrated circuits, 1 interaction, 11, 42, 49 interface, 3, 5, 14, 41 intermolecular interactions, 1 ionization, 5 ions, vii, 15, 17, 20, 21, 28, 31, 35, 37, 53 iridium, 8, 10, 31, 50, 60 isolation, 28, 49

L lanthanide, 15, 16, 53, 54, 57, 58, 61, 62 lifetime, 2, 28 ligand, 15, 17, 19, 20, 22, 25, 26, 27, 28, 31, 32, 33, 51, 53, 54, 55, 56, 57 light-emitting diodes, vii, 2, 16, 45, 48, 49, 53, 54, 56, 58, 60, 61 luminescence, 8, 17, 20, 21, 28, 33, 57, 58 luminescence efficiency, 8

M

F fabrication, 1, 2, 10, 12, 27, 45 fluorescence, 6, 11 fluorine, 21, 28 fullerene, 14

G gallium, 2 generation, 2, 3, 47, 49 gold, 14, 39 groups, 21, 23, 28, 36, 41, 42, 51, 57 growth rate, 31

magnetic properties, 1 majority, 9 matrix, 7, 28 memory, vii, 1, 12, 13, 14, 15, 16, 35, 37, 38, 40, 41, 43, 45, 47, 50, 51, 61, 62 memory performance, 40, 42 methyl methacrylate, 29, 58 microcavity, 18, 31, 59 microelectronics, 1 MMA, 29 mobile device, 2 mobility, 3, 5, 42

Organic Europium Complexes and Their Applications in Optoelectronic Devices, Nova Science Publishers, Incorporated, 2010.

Index model, 40 molecular structure, 6, 7, 8, 9, 10, 11, 18, 22, 23, 25, 26, 27, 29, 30, 32, 33, 35, 36, 38 molecules, 1, 6, 9, 11, 12, 13, 19, 23, 35 monomers, 29, 58 morphology, 54

N nanomaterials, 13, 14 nanoparticles, 32 nanotube, 14, 51 naphthalene, 5, 24, 51 next generation, 12

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O oligomers, 12 opportunities, 3 optimization, 20, 59 optoelectronic properties, 54 optoelectronics, 2 orbit, 3, 10 order, 3, 22 organic polymers, 15 organic solvents, 21 osmium, 10, 48 oxidation, 50

P pathways, 7 performance, 3, 8, 11, 13, 18, 21, 22, 28, 31, 33, 41, 53, 54, 56, 57, 59 perylene, 9 phosphorescence, 16 photoluminescence, 16, 49, 53, 55, 57 plastics, 28 platinum, 10 polarity, 13 polycarbonate, 19 polyimide, 50 polymer, 2, 7, 10, 11, 12, 13, 14, 18, 19, 21, 28, 29, 30, 35, 39, 40, 41, 49, 50, 51, 58, 61, 62 polymer materials, 11, 35

65

polymer matrix, 51 polymer molecule, 7 polymers, 5, 10, 13, 27, 36, 38, 41, 58 porphyrins, 37 power, 1, 2, 4, 12, 22, 36, 39 production, 1 propane, 21 properties, 6, 8, 10, 13, 15, 18, 19, 20, 23, 28, 33, 51, 52, 54, 55, 56, 57, 58, 59, 61 pulse, 36 purity, 31, 50

Q quantum yields, 6, 29

R radiation, 5, 29, 58 range, 2, 15, 39, 45 recall information, 12 recombination, 2, 3, 5, 7, 15 rresistance, 35, 51 response time, 15, 36, 41, 45 retention, 14, 36, 41

S Samsung, 3 scaling, 1 semiconductor, 1, 2, 12, 35, 43, 50, 60 semiconductors, 2, 12 sensitization, 9 sensors, 1 separation, 27 silicon, 1, 12, 35, 50 solar cells, 1, 47 solid state, 1, 2, 10 solubility, 21, 23 species, 31, 41 spectrum, vii, 8, 45 speed, 1, 12 spin, 3, 7, 10, 17, 28, 38, 52 stability, 5, 6, 7, 8, 20, 31, 33, 39, 45, 48 storage, vii, 1, 12, 15, 35, 37, 39, 40, 45 storage media, 35 stress, 15, 40, 45

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66

Index

styrene, 5 surfactant, 51 switching, vii, 13, 14, 36, 39, 42, 45, 47, 50, 61 synthesis, 10

transitions, vii, 15, 17, 21, 53 transport, 3, 5, 6, 7, 8, 10, 11, 18, 19, 20, 41, 49, 52 transportation, 21, 28 triphenylphosphine, 22, 55

T

V vacuum, 13, 32, 60 volatility, 20

W workers, 19, 21, 22, 31, 32

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telecommunications, 2 temperature, 5, 6, 35 thermal evaporation, 3 thermal stability, 24 thin films, 58, 61 threshold, 13, 39, 41, 42 trade-off, 50 transition, 17, 36, 39, 41

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