Liquid Crystalline Semiconductors: Materials, properties and applications (Springer Series in Materials Science Book 169)

This is an exciting stage in the development of organic electronics. It is no longer an area of purely academic interest as increasingly real applications are being developed, some of which are beginning to come on-stream. 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. Within the family of organic electronic materials, liquid crystals are relative newcomers. The first electronically conducting liquid crystals were reported in 1988 but already a substantial literature has developed. The advantage of liquid crystalline semiconductors is that they have the easy processability 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. With these matters in mind this is an opportune moment to bring together a volume on the subject of ‘Liquid Crystalline Semiconductors’.^ The field is already too large to cover in a comprehensive manner so the aim has been to bring together contributions from leading researchers which cover the main areas of the chemistry (synthesis and structure/function relationships), physics (charge transport mechanisms and optical properties) and potential applications in photovoltaics, organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs). This book will provide a useful introduction to the field for those in both industry and academia and it is hoped that it will help to stimulate future developments. Liquid Crystals [LCs] are synthetic functional materials par excellence and are to be found in many types of LCDs; LCs self-assemble into ordered, but fluid, supramolecular structures and domains; they can be oriented in large homogeneous monodomains by electric and magnetic fields, Langmuir Blodgett techniques and also by self-orientation on suitable alignment layers; they are also anisotropic with preferred axes of light absorption, emission and charge transport with excellent semiconducting properties; they are soluble in organic solvents and can be deposited as uniform thin layers on device substrates, including plastic, by low-cost deposition processes, such as spin coating and doctor blade techniques; reactive mesogens [polymerisable LC monomers] can be photopatterned and fixed in position and orientation as insoluble polymer networks. LCs are increasingly being used as active components in electronic and photonic organic devices, such as Organic Light-Emitting Diodes [OLEDs], Organic Field Effect Transistors [OFETs], Thin Film Transistors [TFTs] and photovoltaic cells [PVs]. Such devices on plastic substrates represent a major component of the plastic electronics revolution. The self-assembling properties and supramolecular structures of liquid crystals can be made use of in order to improve the performance of such devices. The relationships between chemical structure, liquid crystalline behaviour and other physical properties, such as charge-transport, photoluminescence and electroluminescence are discussed and explained. For example, high carrier-mobility, polarised emission and enhanced output-coupling are identified as the key advantages of nematic and smectic liquid crystals for electroluminescence. The advantageous use of anisotropic polymer networks formed by the polymerisation of reactive mesogens [RMs] in devices with multilayer capability and photopatternability is described. The anisotropic transport and high carrier mobilities of columnar liquid crystals make them promising candidates for photovoltaics and transistors. The issues in the design and processing of liquid crystalline semiconductors for such devcies with improved performance are described. The photonic properties of chiral liquid crystals and their use as mirror-less lasers are also discussed. Annotation This is an exciting stage in the development of organic electronics. It is no longer an area of purely academic interest as increasingly real applications are being developed, some of which are beginning to come on-stream. 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. Within the family of organic electronic materials, liquid crystals are relative newcomers. The first electronically conducting liquid crystals were reported in 1988 but already a substantial literature has developed. The advantage of liquid crystalline semiconductors is that they have the easy processability 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. With these matters in mind this is an opportune moment to bring together a volume on the subject of Liquid Crystalline Semiconductors. The field is already too large to cover in a comprehensive manner sothe aim has been to bring together contributions from leading researchers which cover the main areas of the chemistry (synthesis and structure/function relationships), physics (charge transport mechanisms and optical properties) and potential applications in photovoltaics, organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs). This book will provide a useful introduction to the field for those in both industry and academia and it is hoped that it will help to stimulate future developments Front Matter....Pages i-ix Introduction to Liquid Crystalline Phases....Pages 1-37 Charge Carrier Transport in Liquid Crystalline Semiconductors....Pages 39-64 Columnar Liquid Crystalline Semiconductors....Pages 65-96 Synthesis of Columnar Liquid Crystals....Pages 97-144 Charge Transport in Reactive Mesogens and Liquid Crystal Polymer Networks....Pages 145-172 Optical Properties of Light-Emitting Liquid Crystals....Pages 173-196 Organic Light-Emitting Diodes (OLEDs) with Polarised Emission....Pages 197-218 Liquid Crystals for Organic Photovoltaics....Pages 219-245 Liquid Crystals for Organic Field-Effect Transistors....Pages 247-268 Back Matter....Pages 269-272

Preface 1 Introduction 2 Charge Transport in Liquid Crystalline Semiconductors 3 Columnar Liquid Crystalline Semiconductors 4 Synthesis of Columnar Liquid Crystals 5 Charge Transport in Reactive Mesogens and Liquid Crystal Polymer Networks 6 Optical Properties of Liquid Crystals 7 Organic Light-Emitting diodes (OLEDs) and OLEDs with Polarised Emission 8 Liquid Crystals for Organic Photovoltaics 9 Liquid Crystals for Organic Field-Effect Transistors.

. This is the first book ever written on this subject. It describes recent progress in the use of liquid crystals as active components in electronic and photonic organic devices such as organic light-emitting diodes, thin film transistors and photovoltaic cells.