Liquid Crystalline Semiconductors: Materials, properties and applications

Richard J. Bushby, Stephen M. Kelly and Mary O’Neill (eds.)Springer Series in Materials ScienceLiquid Crystalline Semiconductors2013Materials, properties and applications10.1007/978-90-481-2873-0© Springer Science+Business Media Dordrecht 2013

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Editors

Richard J. Bushby, Stephen M. Kelly and Mary O’Neill

Liquid Crystalline SemiconductorsMaterials, properties and applications

Editors

Richard J. Bushby

School of Chemistry, University of Leeds, Leeds, UK

Stephen M. Kelly

Department of Chemistry, University of Hull, Hull, UK

Mary O’Neill

Departments of Physics and Mathematics, University of Hull, Hull, UK

ISSN 0933-033X

ISBN 978-90-481-2872-3e-ISBN 978-90-481-2873-0

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Preface

This is an exciting stage in the development of organic electronics. No longer is it an area of purely ‘academic interest’ but increasingly real applications are being developed and some of these are beginning to come on-stream. There have been several drivers for the surge of interest shown by the electronics industry. Perhaps the greatest is the promise afforded by organic systems for low-cost processing, e.g., methods of fabrication such as roll-to-roll contact printing or ink-jet printing in which the components are ‘assembled’ on low-cost substrates at room temperature. The long-term vision is for the creation of integrated devices in which, for example, a photovoltaic cell, a sensor and a display, all made of organic materials, is printed onto a plastic film in a single operation. Another factor in the recent development of organic electronics is the huge advances made in organic synthesis over the last half century, which make it easy to fine-tune the properties of organics at a molecular level in a way that is simply not possible for their metallic and semi-metallic counterparts. Finally, organics are much easier to interface with biomaterials, which is perhaps the key problem in the development of new generation biosensors. Another consideration in the development of organic electronics has been ‘Green Issues’: not only the need to develop energy-efficient methods of manufacture but also the need to develop methods based on renewable (organic) rather than finite (inorganic and metallic) components.

Although it seems improbable that organics will ever compete with silicon-based devices in terms of high-end computing applications, where device speed is of the essence, there are many low-end areas of application, where we can look to see traditional inorganic components increasingly replaced by organics. Areas that have already been commercially developed or which are under intensive development include organic light emitting diodes (for flat panel displays and solid state lighting), organic photovoltaic cells, organic thin film transistors (for smart tags and flat panel displays) and sensors. Potentially this is a field that will affect every aspect of our lives and have an impact in every home and in every business.

Within the family of organic electronic materials, liquid crystals are relative newcomers. The first electronically conducting liquid crystals were only reported in 1988, but already a substantial literature has developed. The potential advantage of liquid crystalline semiconductors is that they have the easy processabilty of amorphous and polymeric semiconductors, but they usually have higher charge carrier mobilities. Their mobilities do not reach the levels seen in crystalline organics, but they circumvent all of the difficult issues of controlling crystal growth and morphology. Liquid crystals self-organise, they can be aligned by fields and surface forces and, because of their fluid nature, defects in liquid crystal structures readily self-heal.

Because this is a relatively young field, there are still issues which need to be understood. In particular, the theory of electronic conduction in liquid crystals is much less well developed than that of electronic conduction in other organic materials and, although the relationship between molecular structure and conductivity is mostly understood, some issues still remain to be resolved and understood.

With these matters in mind, this is an opportune moment to bring together a monograph on the subject of ‘Liquid Crystalline Semiconductors’. The field is already too large to cover in a comprehensive manner, so our aim has been to bring together contributions from leading workers, which cover the main areas of the chemistry (synthesis and structure/function relationships), physics and potential applications. A general introduction to liquid crystals and the nature and kinds of their mesomorphic behaviour and structure (mesophases) is given in Chap. 1 . A description of the nature and mechanisms of different kinds of charge transport in calamitic (nematic and smectic) liquid crystalline semiconductors is given in Chap. 2 followed by a similar treatment of columnar (discotic) liquid crystalline semiconductors in Chap. 3 . The different approaches and methods of determining charge transport in liquid crystals are also described in detail. Chap. 4 provides a comprehensive description of the synthesis of a wide range of columnar liquid crystalline semiconductors. A series of reaction schemes are used to illustrate different synthetic strategies and approaches to the synthesis of this special type of liquid crystal. Chapter 5 gives an extensive discussion of the nature and magnitude of charge transport in reactive mesogens (monomers) and how they are determined. A similar treatment of the corresponding liquid crystal polymer networks is also given in this chapter. An insight into the nature and complexity of the optical properties of liquid crystals is provided in Chap. 6 . Chapters 7 , 8 and 9 describe the modes of operation, configuration and performance of a range of electrooptic devices, i.e., organic light-emitting diodes (OLEDs), especially with polarised emission, organic photovoltaics and organic field-effect transistors, incorporating the types of liquid crystalline semiconductors described in the preceding chapters. Our hope is that this book will provide a useful introduction to the field both for those in industry and for those in academia and that it will help to stimulate future developments.

Richard J. Bushby

Stephen M. Kelly

Mary O’Neill

Contents

1 Introduction to Liquid Crystalline Phases 1

John E. Lydon

2 Charge Carrier Transport in Liquid Crystalline Semiconductors 39

Jun-Ichi Hanna

3 Columnar Liquid Crystalline Semiconductors 65

Richard J. Bushby and Daniel J. Tate

4 Synthesis of Columnar Liquid Crystals 97

Sandeep Kumar

5 Charge Transport in Reactive Mesogens and Liquid Crystal Polymer Networks 145

T. Kreouzis and K. S. Whitehead

6 Optical Properties of Light-Emitting Liquid Crystals 173

Mary O’Neill and Stephen M. Kelly

7 Organic Light-Emitting Diodes (OLEDs) with Polarised Emission 197

E. Scheler and P. Strohriegl

8 Liquid Crystals for Organic Photovoltaics 219

Mary O’Neill and Stephen M. Kelly

9 Liquid Crystals for Organic Field-Effect Transistors 247

Mary O’Neill and Stephen M. Kelly

Index269

Contributors

Richard J. Bushby

School of Chemistry, University of Leeds, Leeds, UK

Jun-Ichi Hanna

Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, Yokahama, Japan

Stephen M. Kelly

Department of Chemistry, University of Hull, Hull, UK

T. Kreouzis

Department of Physics, Queens Mary, University of London, London, UK

Sandeep Kumar

Soft Condensed Matter Group, Raman Research Institute, Bangalore, India

John E. Lydon

Faculty of Biological Sciences, University of Leeds, Leeds, UK

Mary O’Neill

Department of Physics and Mathematics, University of Hull, Hull, UK

E. Scheler

Lehrstuhl fuer Makromolekular Chemie I, Bayreuther Institut fuer Makromolekulforschung BIMF, University of Bayreuth, Bayreuth, Germany

P. Strohriegl

Lehrstuhl fuer Makromolekular Chemie I, Bayreuther Institut fuer Makromolekulforschung BIMF, University of Bayreuth, Bayreuth, Germany

Daniel J. Tate

Organic Materials Innovation Centre, School of Chemistry, University of Manchester, Manchester, UK

K. S. Whitehead

Department of Physics, Queens Mary, University of London, London, UK

Richard J. Bushby, Stephen M. Kelly and Mary O'Neill (eds.)Springer Series in Materials ScienceLiquid Crystalline Semiconductors2013Materials, properties and applications10.1007/978-90-481-2873-0_1© Springer Science+Business Media Dordrecht 2013

1. Introduction to Liquid Crystalline Phases

John E. Lydon¹  

(1)

Faculty of Biological Sciences, University of Leeds, Leeds, LS2 9JT, UK

John E. Lydon

Email: [email protected]

Abstract

This chapter deals principally with structural aspects of liquid crystalline phases – the necessary asymmetry of the molecular structure, the way in which molecules assemble into layers or columns, and the larger-scale structures visible in the optical microscope.

There is a discussion of the molecular asymmetry which makes liquid crystalline behaviour possible, and then the structures of nematic, smectic and columnar phases are described. The hierarchy of smectic structures is outlined, together with the corresponding scheme for columnar structures.

Un-aligned samples of liquid crystalline phases spontaneously adopt characteristic ‘textures’ which have relaxed down to their lowest accessible energy state. For nematic phases these tend to have linear ‘disclinations’ and for smectic phases, the layers curve into ‘focal conic’ structures. The geometry of these structures is apparent when they are viewed with polarized light in an optical microscope, and the principal ‘optical textures’ of liquid crystal are explained.

The final section concerns X-ray diffraction patterns. As the crystalline solid is heated and passes through a succession of phases, the loss of order causes a stepwise change in the appearance of the diffraction pattern. Sharp features in the diffraction pattern of the crystalline solid, broaden and then disappear, one by one, until only very diffuse features remain for the isotropic liquid.

1.1 Introductory Comments

Introductory comments about liquid crystals usually assert that they are states of matter intermediate between solids and liquids. In some senses this is, of course, correct and, in phase diagrams, liquid crystal phases do lie between solids and liquids as far as both temperature and composition are concerned, but the statement carries many wrong implications about them. They do indeed have some degree of molecular alignment between the long-range, three-dimensional order of a crystalline solid and the total disorder of the (isotropic) liquid and they are neither as rigid as a solid nor as fluid as a liquid, but not all properties of liquid crystals can be represented by a histogram of the form shown in Fig. 1.1a. There are significant properties, which are qualitatively different to those of either classical solids or liquids. The influential general overview of liquid crystalline systems by Peter Collings had the inspired sub title – Nature’s Delicate Phase of Matter [1] – and it is their unique sensitivity, coupled with their self-organising, self-aligning and self–healing properties, which is responsible for most of the commercial usefulness of liquid crystals. Consider, for example, the ease with which the molecules in some liquid crystalline phases can be oriented by electric or magnetic fields. A field of the order of 100 mV μm−1 will reorient the molecules in a typical liquid crystal display (LCD) cell. In contrast, fields of 4 or 5 orders of magnitude larger are usually unable to align the molecules in either crystalline solids or (isotropic) liquids. Similarly, the response of liquid crystalline phases to small changes of temperature or to the addition of small quantities of chiral dopants is often appreciable in liquid crystals and insignificant in solids and liquids. The interest in liquid crystals lies in the fact that their properties are not always intermediate between those of solids and those of liquids and some of their properties are unique.

Fig. 1.1

Not all properties of liquid crystalline phases are intermediate between those of solids and liquids. (a) Order and viscosity, (b) response to electric fields

A second general point is that there are frequent references to ‘the liquid crystalline phase’ as if there were only one type of mesophase, with possibly a few very minor variations in detail. This again is a misleading picture. There are dozens of thermodynamically stable liquid crystalline phases, each sufficiently different to the others, in structure and properties, to be classified as a distinct bona fide phase in its own right. This range of structures is perhaps, easiest to survey in terms of dimensions of ordering. A perfect crystalline solid has molecules lying in fixed orientations on fixed lattice sites. It could be said to have six dimensions of ordering, i.e., three in terms of the positioning along the x, y and z axes and three in terms of the orientation about the three orthogonal axes. At the other end of the scale, in the isotropic liquid there is no positional or orientational order, i.e., no dimensions of ordering at all. In the majority of cases, the melting of a crystalline solid is a synergistic process in that an individual molecule cannot move out of its lattice site or rotate out of its prescribed alignment without disrupting its neighbours and, once melting has started, the whole crystalline pattern collapses catastrophically, i.e., all six dimensions of ordering are lost simultaneously. The cause of this phenomenon lies in the fact that the potential energy wells in which the molecules sit, tend to be of more or less equal depth for translational and rotational motion. The energy barriers that prevent small-scale oscillations from becoming free translational movement and small angle oscillations from becoming completely free rotations, tend to be of comparable height.

However, this is not universally true. If, for example, the molecules are approximately spherical, and if there are no highly-directional intermolecular forces, it may be possible for the molecules to rotate freely without disturbing their neighbours enough to destroy the long-range positional ordering. Such systems are termed plastic crystals. If, on the other hand, the molecules are highly asymmetric rods or discs, it may be possible for the structure to become more disordered and fluid in some dimensions than others. Such systems are termed liquid crystals . Molecules which are rod-like or lath-like are said to be calamitic (from the Greek καλαμοσ for reed). They may form liquid crystal phases, which are highly fluid, but which are still able to maintain an extended pattern of alignment with the molecular long-axes all lying parallel, like a box of pencils being shaken. Conversely discotic molecules, that are flattened and disc-like, may form liquid crystal phases where the planes of the molecules tend to lie parallel, but where the molecules are free to slide over one another, e.g., like a box of coins being shaken (Fig. 1.2).

Fig. 1.2

The importance of asymmetry in the formation of liquid crystalline phases

In the nematic calamitic and discotic phases, there is sufficient thermal energy to prevent the attractive forces between molecules locking them into a rigid pattern and the molecules are free to move more or less as individuals. But in the majority of types of liquid crystal phases there is a pattern of aggregation of molecules, which imposes some ordered structure on the phase, whilst at the same time, allowing considerable molecular mobility and disorder. Calamitic molecules tend to aggregate into layers, giving smectic phases and discotic molecules tend to form stacks, giving columnar phases (Fig. 1.3).

Fig. 1.3

Molecular aggregation in thermotropic liquid crystalline phases – calamitic molecules aggregating into a layer (left) and discotic molecules aggregating into a column (right)

In both types of aggregation, it is possible for large amounts of thermal motion to be accommodated without destroying the structure of the phase. In particular, rotational motion about the long axes of calamitics, and about the short axes of discotics, can occur without destroying the overall pattern of molecular alignment.

Fig. 1.4

The molecular structures of some typical lyoytropic mesogens, (a) sodium dodecylsulphonate (a sodium soap), (b) a non-ionic synthetic detergent, (c) dimyrisoylphosphatidylcholine (a biologically–occurring phospholipid)

In principle, there are two distinct ways of converting a solid crystalline material into a liquid crystalline phase, i.e., either by heating or by adding a solvent. Mesophases created by heating are said to be thermotropic and those formed by adding solvent are said to be lyotropic (Fig. 1.4). The usual solvent involved is water. These mesophases are therefore restricted to the temperature range over which water is liquid. Some lyotropic phases exist over narrow temperature ranges and the mesophase-forming properties of lyotropics are usually summarised in terms of a two-dimensional temperature/composition phase diagram like that shown in Fig. 1.5.

Fig. 1.5

Lyotropic phase diagram.This sketch shows the typical form of one of the simpler types of phase diagrams for lyotropic systems. As the amphiphile concentration increases, from left to right across the diagram, there is a progressive rise in the level of aggregation of the molecules. At extreme dilution, individual amphiphile molecules are dispersed in the solvent (a). At higher concentrations, small sub-micellar aggregates of three or four molecules build up. These increase in size and concentration until spherical micelles (typically composed of 60 or 70 molecules) arise (b). This stage (indicated by the dashed line), is termed the critical micelle concentration (CMC) and marks a drastic change in the properties of the solution The micelle is a stable arrangement because the hyrodrophobic parts of the molecules are shielded from the solvent – and because it is energetically more favourable for a single molecule to join a micelle than to exist alone, the pattern of distribution of cluster sizes changes dramatically as virtually all of the molecules present become incorporated into micelles. As these spherical assemblies increase in number they begin to fuse together into cylinders which pack in a hexagonal array in the H1 liquid crystal phase (c). At higher concentrations, these cylinders fuse together to produce extended sheets giving the lamellar, Lα phase (d). In order to fill the awkward spaces in the centres of spherical and cylindrical micelles, the alkyl chains must be fluid and disordered. As a sample is cooled, it reaches a state, where the flexibility of the chains is reduced to such an extent that micelles can no longer form and hence the mesophases, which are composed of micellar units, are no longer stable. The phase therefore separates into water and crystalline solid. The temperature at which this occurs is known as the Krafft point and the Krafft point boundary shown marks the lower limit of mesophase formation

The major distinction between thermotropic and lyotropic systems lies in the patterns of aggregation of the molecules. In conventional lyotropic systems, the molecules have both hydrophobic and hydrophilic regions. This causes them to aggregate into micelles, allowing the hydrophobic parts to reduce their contact with the water subphase.

There are some specific amphiphile systems which form exceptionally stable pancake-like micelles which, as one might expect, give anisotropic solutions analogous to discotic nematic phases. Conversely, there are other lyotropic systems containing elongated stacks of aromatic molecules, i.e., chromonic systems, which are analogous to thermotropic calamitic nematic phases.

There are also anisotropic solutions formed by hydrophilic polymers such as cellulose derivatives, collagen, α-helical polypeptides, and nucleic acids (DNA and RNA), but, as far as I am aware, there are no lyotropic liquid crystalline phases analogous to the thermotropic nematics, in which small solute molecules are free to move individually.

Lyotropic phase diagrams can be much more complex than the example given. At higher amphiphile concentrations, inverse structures can occur where the micelles are turned inside out, giving phases with a similar geometry, but of a water-in-oil type instead of an oil-in-water type. Also, there are other types of mesophase structure, which can occur between the hexagonal and lamellar and at the low concentration side of the hexagonal phase. These fall into two groups, i.e., those formed from individual micelles and those formed by extended networks of jointed cylinders or sheets. Some in the latter category are particularly complex, with very high symmetry patterns of interlocking lattices. Because both the water and the lipid regions extend continuously throughout the phase, these are known as bicontinuous structures. One of these, with space group Im3m, is aptly nicknamed the ‘plumber’s nightmare’. Many of these structures have cubic symmetry and as a consequence are optically isotropic. They are therefore difficult to miss in optical microscopy because they show up as solid black regions when samples are viewed between crossed polars.

1.2 Calamitic Mesophases

1.2.1 Nematic Phases

The word nematic comes from the Greek νημα (nema), meaning thread. This is a reference to the linear topological defects, termed disclination lines, which can be seen with the naked eye in bulk samples of this phase. The nematic phase is one of the commonest thermotropic liquid crystalline states. It is formed by elongated rodlike molecules, such as the cyanobiophenyls shown in Fig. 1.7. In this phase, the molecules spontaneously align so that their long axes lie more of less parallel over considerable distances. When bulk samples are viewed between crossed polars in the optical microscope, the director (the mean direction of the molecular long axis) can be seen gradually curving over distances of the order of 0.1 mm.

This phase generally has low viscosity. The molecules are as free to flow as those in the isotropic liquid. The positions of their centres of mass are randomly distributed, but the long-range directional order is still maintained (Fig. 1.6).

Fig. 1.6

A stylised picture of a group of molecules in a nematic array

The vital step, which turned the liquid crystal display industry from a dream into a reality, was the synthesis, in 1972, of the first stable room temperature nematic material by Gray and Harrison. This was pentylcyanobiphenyl (5CB) shown in Fig. 1.7a. Subsequently, some thousands of nematogenic compounds have been synthesised. The vast majority of these have similar structures, e.g., they are often calamitic molecules with a rigid half, consisting of two or three aromatic rings, and a flexible half of more or less the same length, consisting of a C5 or C6 alkyl chain. Some variants of this structure are shown in Fig. 1.7. These include the incorporation of a rigid aliphatic ring, replacing one of the aromatic rings for compound (b), the substitution of fluorine for hydrogen atoms, to give a molecule with negative dielectric properties (c) and the presence of chiral centres (d). In general, nematic phases of such compounds are miscible. This makes it possible to fine tune the properties for each particular type of display requirement. A typical commercial material will be a mixture of three or four components.

Fig. 1.7

Structures of four nematogenic molecules. (a) 5 CB the archetypal nematic mesogen; (b) MBBA; (c) a fluorinated mesogen with a negative dialect susceptibility; (d) a chiral mesogen-forming a twisted nematic (cholesteric phase) phase

The majority of nematic molecules are actually more blade-like than rod-like, but the more-or-less free rotation of each molecule about its molecular long axis means that the molecules effectively see each other as circular cylinders. There has been a long search for a biaxial nematic phases, where the planes of the blades lie parallel, and, although it is thought that credible examples of such phases have been found, they are very few in number. In general, nematic phases can be easily aligned by relatively weak electric fields and they usually spontaneously align on prepared substrate surfaces. It is the combination of these two properties that make them uniquely useful in liquid crystal display devices. The classical calamitic nematic phase is optically uniaxial and the prolate shape of the indicatrix usually matches the prolate shape of an individual molecule.

It is hardly surprising that the first thermotropic liquid crystal phase to be investigated, over a century ago, were cholesteryl esters. The iridescent colours, which flash out when molten material is cooled, is a beautiful phenomenon and it cried out for further investigation. These naturally-occurring sterols are chiral, optically-active molecules. Their asymmetry causes them to form a twisted nematic phase, as sketched in Fig. 1.8. When this has a pitch within the optical range, it produces iridescent interference colours. When first observed, this property was regarded as being so notable that the generic term ‘cholesteric’ was used to describe all twisted mesophases of this type. The discovery that the addition of small quantities of chiral dopants to non chiral nematic phases produces similar phases, led to the feeling that they should be treated as a subdivision of nematics and they are now generally termed twisted nematics indicated by a symbol followed by asterisk (N*).

Fig. 1.8

The twisted nematic N* (cholesteric) structure, formed by chiral modifications of nematic compounds and by non-chiral nematagens doped with chiral solutes. Where the pitch of this helicoidal structure is comparable with the wavelength of light, the phase gives iridescent reflections. Larger pitch structures, where the repeat distance can be resolved in the optical microscope, appear as ‘fingerprint textures’ with bands separated by half a pitch. Helicoidal structures of this type are common in biological material, most obviously in the carapaces of iridescent beetles. Their electron microscope pictures show a characteristic pattern of nested arcs, known as Bouligand patterns [4]

1.2.2 Smectic Phases

In general, as the temperature of a nematic phase is lowered, a new dimension of ordering is introduced as a phase change occurs where the molecules spontaneously organize themselves into layered ‘smectic’ patterns. The molecular structures of commercial smectic materials are essentially those of nematics with higher transition temperatures. In the smectic A phase, abbreviated to SmA, the molecules in each layer are disordered and mobile and it can be regarded as a two-dimensional liquid. On average the molecules lie normal to the layer, but there is considerable variation of alignment, with a spread of up to 10° or 15°. There is rapid rotational motion of the molecules about their long axes on a timescale of about 10¹¹ times per second. This is to be compared with the much slower motion about their short axes, which occurs on a timescale of ~ 10−6s. Although there is