Tài liệu PHYSICAL AND CHEMICAL ASPECTS OF ORGANIC ELECTRONIC doc

func-Organic Electronics: Using Molecules as Semiconductors Today, applications related to organic electronics can be divided into tially three major fields, namely organic light emittin

Edited by

Christof Wöll

Semiconducting Polymers

Chemistry, Physics andEngineering

1999 ISBN: 978-3-527-29507-4

Müllen, K., Wegner, G (eds.)

Electronic Materials: The Oligomer

Approach

1998 ISBN: 978-3-527-29438-1

produced Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to

be free of errors Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

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Composition Druckhaus Thomas Müntzer, Bad Langensalza

Printing Strauss GmbH, Mörlenbach

Bookbinding Litges & Dopf GmbH, Darmstadt

Printed in the Federal Republic of Germany Printed on acid-free paper

ISBN: 978-3-527-40810-8

Prof Dr Christof Wöll

Lehrstuhl für Physikalische Chemie I

der Ruhr-Universität Bochum

Universitätsstrasse 150

44780 Bochum

Germany

List of Contributors XXXIII

Part I Industrial Applications

1 Organic Transistors as a Basis for Printed Electronics 3

Walter Fix, Andreas Ullmann, Robert Blache, and K Schmidt

How Does it Distinguish Itself from a Conventional One? 5

1.4 Basic Logical Integrated Circuits: Ring Oscillators 6

1.5 Complex Organic Circuits: the 64-Bit RFID Tag 9

1.8 Application and Future Prospects 13

References 14

2 Printable Electronics: Flexibility for the Future 17

Mark A.M Leenen, Heiko Thiem, Jürgen Steiger,

and Ralf Anselmann

Thin Films 37

H Brinkmann, C Kelting, S Makarov, O Tsaryova,

G Schnurpfeil, D Wöhrle, and D Schlettwein

3.2.1.4 2,29,20,2-,3,39,30,3-Octafluorophthalo-cyaninato Zinc(II)

(F8PcZn) 40

3.2.1.5

1,19,10,1-,2,29,20,2-,3,39,30,3-,4,49,40,4-Hexadecafluoro-phthalocyaninato Zinc(II) 40

3.2.2 Calculation of Energy Levels 40

3.2.3 Thin Film Preparation and Measurements 41

3.3.1 Synthesis and Molecular Characterisation 42

3.3.2 Thin Evaporated Films of Zinc(II) Phthalocyanines

with a Different Degree of Fluorination 44

3.3.3 Growth of F16PcZn Thin Films 51

3.3.5 Measurements of the Field Effect 55

References 58

4 Novel Organic Semiconductors and Processing Techniques

for Organic Field-Effect Transistors 61

H N Tsao, H J Räder, W Pisula, A Rouhanipour,

and K Müllen

4.2 Molecular Alignment from Solution Through

the Zone-Casting Technique 62

4.3 Solution Processed Donor – Acceptor Copolymer

Film Organic Field-Effect Transistor (OFET) Based

on Substituted Oligothiophenes 75

K Haubner, E Jaehne, H.-J P Adler, D Koehler, C Loppacher,

L M Eng, J Grenzer, A Herasimovich, and S Scheiner

6.5 Mobilities from Radiation Induced Conductivity

Electrodes onto Self-Assembled Monolayers – A StepTowards the Fabrication of SAM-Based OrganicField-Effect Transistors 115

Heidi Thomas, Jan Müller, and A Terfort

7.2.3 Stabilising Layer of the Nanoparticles 120

7.4.8 Electroless Deposition of Gold 134

7.4.9 Chemical Vapour Deposition of Gold 134

References 135

of Pentacene Organic Field Effect Transistors with

RF Sputtered Aluminium Oxide Gate Insulators

8.3.1 Structural and Morphological Properties of the Pc Films 142

8.3.1.1 X-Ray Diffraction 142

8.3.1.2 Scanning Force Microscopy 145

8.3.2 Analysis of the Electrical Characteristics 148

8.3.2.1 Overview of the ID–VD Characteristics 148

8.3.2.2 Temperature Dependence of the Mo-bilities 151

8.3.2.3 Detailed Analysis of the Field Effect Mobilities as a Function

8.3.3.1 Correlation of the Electrical Transport Properties and the Film

Alexander Gerlach, Stefan Sellner, Stefan Kowarik,

and Frank Schreiber

9.3.2 Thin Film Growth and Dynamic Scaling 165

9.3.3 Growth of Organic Molecular Materials 166

9.4.1 Pentacene on Silicon Oxide 167

9.5.2.1 Degradation of Devices 177

9.5.2.2 Encapsulation of Devices 177

9.5.2.3 Aluminium Oxide Capping Layers 178

9.5.2.4 Thermal Stability of Capped Organic Films 180

References 184

10 X-Ray Structural and Crystallinity Studies of Low

and High Molecular Weight Poly(3-hexylthiophene) 189

S Joshi, S Grigorian, and U Pietsch

10.3 X-Ray Grazing-Incidence Diffraction Studies 191

10.4 Structure Determination for LMW Fraction 195

References 204

of Thin Organic Films on Metals for Applications

Deposition (OMBD) on Metal Substrates:

Case Studies for Rubrene, Perylene and Pentacene 21211.4.1 Rubrene Deposition on Au(111) 213

11.4.2 Adsorption-Induced Restructuring of Metal Substrates:

Perylene on Cu(110) 214

11.4.3 Organic Molecular Beam Deposition of Pentacene

on Clean Metal Surfaces 216

11.5 Organic Molecular Beam Deposition of Perylene 22011.6 Growth of Other Molecules of Interest for Organic Electronics

Bonding, Structure and Function of Molecular AdsorbateLayers on Solid Surfaces 235

S Soubatch, R Temirov, and F S Tautz

12.2.1 Bonding: What can be Learned for OFETs? 243

13 Metal/Organic Interface Formation StudiedIn Situ

by Resonant Raman Spectroscopy 263

G Salvan, B.A Paez, D.R.T Zahn, L Gisslen, and R Scholz

13.2.1 Sample Preparation and Characterisation 263

13.2.2 Theoretical Methods 264

13.3.1 Chemistry of Metal/Organic Interfaces 264

13.3.2 Morphological Properties and Indiffusion of Metals

at the Interfaces with Organic Semiconductors 270

13.3.3 Assignment of Raman Intensities with DFT Calculations 276

References 279

on Well-Ordered Sapphire Substrates 281

S Sachs, M Paul, F Holch, J Pernpeintner, P Vrdoljak,

M Casu, A Schöll, and E Umbach

14.1.1 The Present Micro-OFET Concept 282

14.3.1 Realisation of the Micro-OFET Concept 284

14.3.1.1 Sapphire Substrate 284

14.3.1.2 Growth of DIP on Sapphire 286

14.3.1.3 Contacts – the Au/DIP Interface 289

Part IV Device Performance and Characterisation

and Photo Response 301

Bert Nickel

15.2.1 Film Formation on Inert Surfaces 301

15.2.2 Film Formation on Metallic and Conductive Surfaces 305

15.3.1 Mobility and Charge Carrier Density 307

15.3.2 Influence of Trap States and Fixed Interface Charges 309

16.3.1 Organic Field-Effect Transistors 320

16.3.1.1 Short Channel OFET Based on P3HT 320

16.3.1.2 OFET Based on a Modified PPV and with Silanised

Gate Oxide 322

16.3.2.1 Quasi-Static CV Curves for a Capacitor with

16.3.2.2 Dynamic CV Curves 325

16.4.1 Simulations for the MIS Capacitor 327

16.4.2 Simulations for Thin-Layer OFETs and the Corresponding

Capacitor 329

16.5 Equilibrium of Polarons With Doubly Charged States

of the Polymer Chain 331

16.5.1 Polarons and Bipolarons or Polaron Pairs 332

16.5.1.1 Polarons and Bipolarons 332

16.5.1.2 Polarons and Polaron Pairs 333

16.5.2 Polarons, Bipolarons and Polaron Pairs 335

16.5.3 Polarons and General Dipolarons 337

16.6 Bipolaron Mechanism for Hysteresis 339

16.6.1 Formation and Dissociation of Bipolarons 339

16.6.1.1 Kinetics of Formation and Dissociation 339

16.6.1.2 The Bipolaron Mechanism 340

16.6.2 Formation of Complexes With Counter Ions 341

16.6.2.1 The Kirova – Brazovskii Scenario of Complex Formation 341

16.6.2.2 Slow Ion Capture by an Overcharged Complex 342

17.2 Materials, Device Preparation and Experimental Methods 348

17.3 Unipolar Field-Effect Transistors 352

17.4 Ambipolar Field-Effect Transistors 353

17.5 Charge Carrier Mobility and Threshold Voltage 354

References 370

18 Gate Dielectrics and Surface Passivation Layers for Organic

Field Effect Transistors 373

T Diekmann and U Hilleringmann

18.3.1 Inorganic Gate Dielectric Layers 377

18.3.1.1 Thermally Grown Silicon Dioxide 378

19 Influence of Metal Diffusion on the Electronic Properties

of Pentacene and Diindenoperylene Thin Films 401

M Scharnberg, R Adelung , and F Faupel

Charge Carrier Mobilities and Injection Barriers in Bottomand Top Contact Configurations 427

R Scholz, D Lehmann, A.-D Müller, F Müller, and D R T Zahn

20.2 Device Geometries and Sample Preparation 429

20.3.1 Potentiometry and Electrical Probes 431

20.3.2 Mobility Estimates 431

20.3.3 Two-Dimensional Device Simulation 433

20.3.4 Charge Transient Spectroscopy 436

20.4 Investigations of Top-Contacted Pentacene OFETs 438

20.4.1 Electrical Characterisation In Situ 438

20.4.2 Potentiometry Measurements Ex Situ 439

20.4.3 Charge Transient Spectroscopy 441

References 443

21 Microscopic and Spectroscopic Characterisation of Interfaces

and Dielectric Layers for OFET Devices 445

K Müller, Y Burkov, D Mandal, K Henkel, I Paloumpa,

A Goryachko, and D Schmeißer

21.2.2.2 Electrical Characterisation (CV, IV) 449

22.4.2 Influence of the Insulator Thickness 478

22.5.1 Semiconductor Related Performance 479

22.5.2 Tuning the Contact Resistance 481

22.6 Influence of the Semiconductor Thickness 483

23 Aluminium Oxide Film as Gate Dielectric for Organic FETs:

Anodisation and Characterisation 499X.-D Dang, W Plieth, S Richter, M Plötner, and W.-J Fischer

23.2.2 Characterisation 500

23.3.1 Influence of Formation Current Density 501

23.3.2 Influence of the Formation Voltage 504

23.3.3 Influence of Anodisation Time 506

23.3.4 Influence of Surface Roughness 508

23.3.5 Barrier Aluminium Oxide Films as Gate Dielectrics for Organic

Transistors 509

References 511

24 Electronic States at the Dielectric/Semiconductor

Interface in Organic Field-Effect Transistors 513Niels Benson, Christian Melzer, Roland Schmechel,and Heinz von Seggern

25 Aspects of the Charge Carrier Transport

in Highly-Ordered Crystals of Polyaromatic Molecules 539

J Pflaum, J Niemax, S Meyer, and A.K Tripathi

25.2.4.1 Gate Insulator Thickness 547

25.3.1 Tetracene Crystals: Surface Versus Bulk Transport 548

25.3.2 Diindenoperylene Crystals: Structural Impact on Transport 554

References 562

Functionalisation and Device Characterisation 567

Kannan Balasubramanian, Eduardo J H Lee, Ralf Thomas Weitz,Marko Burghard, and Klaus Kern

26.2.1 Physical and Electronic Structure 568

26.2.2 Field-Effect Transistors Based on Single SWCNTs 569

26.2.3 CNT-FETs Based on Electrochemical Field-Effect 572

26.2.4 Role of Capacitances 573

26.3.1 Motivation and Strategies 575

26.3.2 Chemically Modified Devices 576

26.3.3 Electrochemical Functionalisation 577

26.3.4 Selective Electrochemical Functionalisation 579

26.3.6 Sensors Based on Functionalised SWCNT-FETs 585

26.4.1.1 Saturation 586

26.4.1.2 Transconductance 586

26.4.1.3 Sub-Threshold Swing 586

26.4.1.4 Mobility 587

26.4.2 Electrochemically Gated Devices 587

26.4.3 Scanning Photocurrent Microscopy 587

References 590

Artur Hefczyc, Lars Beckmann, Eike Becker, Hans-HermannJohannes, and Wolfgang Kowalsky

27.2.6 Planar Device Structure 604

27.2.7 Localisation of Switching Region 605

28.2 Concepts and Progress of Spintronics 614

28.3 Organic Semiconductors in Spintronics Applications 61628.4 OFET Concept for Spin-Polarised Transport 617

Introduction

After the ground breaking discovery of electrical charge carrier transport inpolymers in the late 1980s by Alan J Heeger, Alan G MacDiarmid and HidekiShirakawa [1–3], who were awarded the Nobel Prize in chemistry in 2000, thequestion arose as to whether organic materials would also find applications asorganic semiconductors This field really started to attract major attention afterthe demonstration of the first organic light emitting device (OLED) in 1987 byTang and Van Slyke [4] Since then, the field of organic electronics has stimu-lated a tremendous research interest into organic semiconductors

Today, owing to the constant improvements of the particular properties ofmolecular materials – including the synthesis of new compounds – organicsemiconductors have made their way from a rather exotic and academic topicstudied by a few specialists in molecular physics to a mature research field[5–8] There are already numerous applications of organic semiconductors and

a number of products have reached the market Probably the first commercialproducts employing charge transport in organic materials and thus utilising thesemiconducting properties of organic materials were laser printers In this ap-plication the photoactive organic material is deposited on the imaging drum.After the charging of this photoactive organic layer by means of a corona dis-charge, selected areas are illuminated using the intense light of a laser beam.The drum is then brought into contact with toner particles that adhere to thecharged parts of the drum but not to the illuminated regions In the last step thetoner particles are transferred to a sheet of paper and fixed there using a tem-perature treatment, thus yielding high quality prints

In the future, there are several potential properties of organic electronics thatmay become very important The most often quoted prospect for organic elec-tronics is related to their cost Many companies invest a substantial amount ofmoney and effort into developing functioning organic electronic devices, mo-tivated by the prospect of “cheap” electronics The vision of being able to sim-ply print electronic circuits on a substrate, using existing print technology, isvery attractive Organic materials can be processed at low temperatures (below

200°C), thus allowing the combination of organic electronics into the flexible

plastic substrate This particular property carries a huge potential with regard

to the fabrication of e-paper (electronic paper) and the manufacture of tional clothing The prospect of integrating electronic circuitry (e.g organic so-lar cells with the corresponding electronics) into dresses and coats thus allow-ing mobile electronic equipment to be charged is certainly very attractive [9]

func-Organic Electronics: Using Molecules as Semiconductors

Today, applications related to organic electronics can be divided into tially three major fields, namely organic light emitting devices (such asOLEDs), organic field effect transistors (OFETs) and organic solar cells Thelast is still in its infancy but is receiving a substantial and strongly increasingamount of attention Despite their lower efficiency compared with silicon-based devices, organic solar cells offer the advantage of low cost large areaproduction, which might lead to a cost-effective use Organic light emittingdevices have so far had the largest impact in the field of organic electronics.After intense research on OLEDs throughout the last decade, these deviceshave already entered the market and are presently being used mainly for dis-play applications, e.g in car radios (Pioneer) or in mobile phone displays Re-cently, also large-scale displays based on OLEDs have reached the market, e.g

essen-in TV screens, computer laptop displays or mobile DVD-player monitors (see[10] for the first commercial OLED TV)

In contrast to such electro-luminescent devices, in many instances the sation of organic field effect transistors (OFETs) for applications requires thatthese OFETs can be operated at a minimum switching speed, which in turn re-quires fairly large charge carrier mobilities of the organic materials used in thedevice Unfortunately, these charge carrier mobilities are found to depend verysensitively on a number of parameters, including the degree of ordering, con-taminations due to charge carrier traps, and other structural imperfections such

reali-as domain boundaries A real breakthrough with regard to understanding andoptimising charge carrier mobilities in organic materials has thus not beenachieved as yet and electronic devices using organic semiconductors as an ac-tive material are still in the laboratory rather than on the market Charge carriermobilities are, however, not the key issue for all applications For the elec-tronic circuitry required to control a mobile OLED display, parameters such aslow operation voltage and stability are, for example, more important

Organic Field Effect Transistors – Prototype Devices in Organic

Electronics

This book demonstrates fundamental physical and chemical aspects in organicelectronics by mainly concentrating on one fundamental device, the organicfield effect transistor This important device is prototypical for organic elec-

tronics and was originally designed in analogy to field effect transistors usingconventional semiconducting materials, Si, Ge, GaAs A typical design of such

a device is shown in Figure 1 Whereas the general concept of operation is milar to inorganic thin film FET devices, there are some distinct differences:one is fairly strong charge localisation, limiting the carrier transport to the firstfew layers above the gate electrode [11, 12], and the particular nature of theorganic/inorganic interfaces

si-The analysis of OFET device characteristics allows many of the key lems in organic electronics to be stressed, ranging from fundamentals (nature

prob-of the charge transport) to applied issues (long-term stability prob-of organic cules) All articles collected in this book are in one way or the other related toorganic field effect transistors

mole-OFETs are the basis of all logic devices that are required to control, forexample, the intensity of a display pixel (the so called all organic display)

or the realisation of a radio frequency identification (RFID) tag Since for thelatter applications only limited frequency bands are available (essentially13.56 MHz or 900 MHz) this places rather strong constraints on the requiredswitching properties of the OFETs

localizedstatesband

DOS(E)hopping

Figure 1 Schematic presentation

of the two different mechanisms governing charge transport in organic semiconductors The so-called hop- ping transport assumes a thermally activated hopping between charge- traps and is always present, the resulting mobilities are very small.

For certain materials a mechanism yielding much higher mobilities has been observed, which is commonly referred to as “band-like”, thus im- plying a similarity to band-transport observed in conventional semicon- ductors.

To reach this frequency is in no way trivial, and in particular requires a nimum mobility of charge carriers within the organic field effect transistors.This important issue will be discussed below It has to be noted, however, that

mi-in view of the technological applications, a reliable and constant device formance may be even more important than a peak performance of the indi-vidual circuits

per-Before we start to address particular properties of organic field effect tors, or present the limitations and the prospects of improving the performance

transis-of these devices, we would like to point out that the interest in OFETs goes farbeyond solving technological problems With regard to determining chargecarrier mobilities in organic materials and, more importantly, understandingthe physics behind, in particular, the band-like transport observed in organicsemiconductors, OFETs play a key role in experimentally determining theseparameters While there are other ways to determine conductivities, chargecarrier mobilities, and temperature dependences in organic materials, for most

of the molecules investigated so far these parameters have been determined ing organic field effect transistors

us-The Three Key Aspects of Organic Electronic Devices

With regard to applications as organic semiconductors, there are severalproperties of molecular materials that have been improved in the past andwhich still need to be optimised further The most important characteristic of

an organic semiconductor is the charge carrier mobility The processability oforganic compounds comes second; the question of whether high-performancethin organic films can be prepared in a straightforward fashion is very impor-tant not only for technological applications but also for fundamental studies.Third, the formation of electrical contacts where either electrons or holesare injected into the organic materials is also critical when it comes to fabricat-ing a functioning device – for this reason the interaction between organicmolecules and metals is a topic of key interest with regard to the development

of functioning devices In the following these three main aspects – which arealso the key topic of the articles collected in this book – will by briefly dis-cussed

(i) Charge Carrier Mobilities in Organic Semiconductors

Charge carrier mobilities are, as already noted above, a key parameter ing the performance of a semiconductor This quantity describes the mobility

describ-of charges, electrons or holes in the presence describ-of an electric field The moststraightforward way to measure these charge carrier mobilities is to literallymeasure the speed of the charge carriers in a semiconductor in the presence of

an electric field A classical method, which can also be used for organic conductors, is to excite charge carriers at a defined point in space, e.g by a la-

semi-ser pulse, and then measure the time needed to travel a certain distance, using atime-of-flight technique This method is well established for conventionalsemiconductors and has also successfully been used for organic semiconduc-tors, see the chapter by Pflaum et al (Chapter 25) for a more detailed descrip-tion of this technique A major disadvantage of the time-of-flight method todetermine mobilities in organic materials is the fact that rather large millime-tre-sized single crystals are required in order to apply this technique Unfortu-nately, in the case of organic semiconductors, the fairly small amounts of anorganic material available and the difficulties encountered in obtaining singlecrystals of millimetre-sizes frequently prohibit the application of the method Itshould also be noted that in the presence of defects (domain boundaries, impu-rities, vacancies), the applicability of the technique is severely hampered oreven made impossible because the signals then become so broadened that adistinct time-of-flight can no longer be determined.

Because of these fundamental difficulties in applying the time-of-flightmethod for arbitrary molecular materials, today most mobilities known for mo-lecular materials have in fact been extracted from the electrical characteristics

of organic field effect transistors There are several methods to extract the tron and hole mobilities from the electrical device characteristics of an OFET.For a more thorough description of the different procedures, the reader is re-ferred to the paper by Scheinert and Paasch [13]

elec-Quite often the mobilities determined for a given molecule, e.g rubrene orpentacene, differ for different devices in different laboratories, reflecting theproblems related to the extraction of the mobility data from the electrical char-acteristics Typically, these characteristics are not only defined by the mobili-ties but also by the contact resistance and of course the presence of domainboundaries defects and impurities in the organic semiconductor The determi-nation of the true intrinsic mobility of an organic semiconductor is still a chal-lenge, which has only been overcome in a very few cases

Over the last two decades the charge carrier mobilities of organic materialshave shown a rather rapid increase; today organic field effect transistors can bebuilt with performances [14, 15] that can compete with FETs based on poly-crystalline silicon [16]

Today, the search for new organic materials, both molecules and polymers,largely proceeds on a trial and error basis; see the review by Anthony [17].There are very few theoretical guidelines that allow to predict charge carriermobilities in a molecule or a polymer Very often one finds the suggestion thatcharge mobilities are large if there is a substantial overlap between the systems

of flat aromatic molecules It should be noted, however, that rubrene, one ofthe molecules with the highest charge carrier mobility known today, is in factnot a flat aromatic molecule, but is distinctly non-planar; in the bulk there isbasically no overlap between the core tetracene units of adjacent rubrenemolecules In addition to the lack of theoretical guidelines, there is also the se-vere problem of the measurement of the intrinsic mobilities, for a new mole-cule that has been synthesised it can be quite difficult to build OFETs that

yield the same mobilities Since the performance of an OFET device is alsoseverely affected by phenomena occurring at the electrodes (charge injection),the presence of defects and morphological parameters, it is of utmost impor-tance to determine these mobilities reliably Whereas in most polymeric mate-rials and many molecular systems there is clear evidence that charge transport

at room temperature is brought about by a hopping-type transport, there is alsoclear evidence for at least some molecular materials that a band-like transport

is present The main difference from the hopping transport is that the ties either increase or at least stay constant when lowering the temperature,whereas the hopping of electrons or holes from one molecule to the next isstrongly activated by temperature, thus leading to an increase of mobilitieswith temperature While there are many models for a hopping transport ofcharge carriers in organic materials [18], only recently have ab initio calcula-tions describing a band-like transport in organic materials become available[19, 20] The main result of these calculations is that the relative orientation ofthe molecules and also the morphology of organic semiconductor films arevery important since these electronic properties are highly anisotropic and de-pend strongly on the relative orientation of the molecules

mobili-(ii) Processability of Organic Compounds for Applications

in Organic Electronics

In general, for the realisation of OFETs two classes of materials are available:polymers and small molecules (so-called oligomers) Polymers exhibit the ad-vantage that the deposition of the molecular material on a substrate is muchmore straightforward Polymers at that stage are typically liquid and have – ofcourse – a very low vapour pressure Therefore the coating of a substrate cansimply be performed by spin-coating a suitable substrate possibly followed by

a drying process In addition, polymers are suitable for printing techniques andtherefore existing technology can be used to print structures with semiconduct-ing polymers on substrates Owing to their low vapour pressure it is impossible

or at least very difficult to prepare polymers using organic molecular beamdeposition, OMBD As a result it is very difficult to obtain thin polymer layersthat exhibit a high degree of crystalline order Generally, probably as a result

of the pure ordering, polymeric organic semiconductors exhibit fairly smallcarrier mobilities, typically less than 0.1 cm2/Vs

Oligomers on the other hand can be synthesised in a more pure form Moreimportantly, because of their high vapour pressure, these smaller molecules arecompatible with OMBD and in many cases allow the preparation of highly or-dered crystalline films with a high degree of crystallinity Such well-defined,highly ordered thin layers of organic molecules make it possible to carry outdetailed studies on structure/property interrelations, which can be used to iden-tify the microscopic physical properties of such devices

Note that recently soluble precursors of some oligomers have also been thesised, which, in addition, allow the fabrication of highly ordered polycrys-

syn-talline films, thus combining the advantage of flexible processing (i.e printing)and their superior electronic properties [17].

Organic Molecular Beam Deposition (OMBD)

The fabrication of organic thin layers from small organic molecules usingOMBD is in principle fairly straightforward, but considerable problems arefrequently observed A particularly interesting case is rubrene, a molecule thatbelongs to the class of organic materials for which mobilities have been re-ported [21, 22] However, when using rubrene to fabricate ultrathin organiclayers substantial problems with regard to nucleation and obtaining homoge-neous films are observed [23, 24] These problems could be traced back to thefact that the rubrene molecules adopt a different conformation in the bulk and

in the gas phase Whereas in the bulk the tetracene backbone of the molecule isessentially planar, for the free molecule the steric repulsion between the fourphenyl units attached to the tetracene backbone cause a substantial tilt whendepositing on the surface In the initial stage of the deposition, the moleculesmaintain the conformation of the free molecule so that nucleation is signi-ficantly delayed and thin, well ordered, or even epitaxial films cannot beachieved

(iii) Metal/Organic and Oxide/Organic Interfaces

Another important aspect in connection with the design and fabrication ofOFETs is the role of interfaces, either between the OSC (organic semiconduc-tor) and the dielectric or OSC and electrode The structural properties of thedielectric/OSC interface have important consequences for the charge transportalong the active layer just above the gate and at the metal/OSC interface Theinjection of charge carriers, electrons or holes takes place, which significantlycontributes to the overall performance of the OFET device In particular, therelative position of the electronic levels of the metal and the molecules at theinterface are fairly important for the injection properties Originally it had beenthought that in simple cases this electron level alignment could be understood

by just aligning the respective ionisation potentials and then simply using theelectronic structure determined for the systems separately Unexpectedly, evenfor supposedly simple systems, such as a saturated hydrocarbon, for the proto-type of an unreactive molecule deposited on a noble metal like gold there aresignificant deviations from this simple model, which goes back to Mott Thesimplest way to demonstrate these problems is to consider the work function.Using this simple Mott-model mentioned above, the adsorption of a saturatedhydrocarbon on gold should not lead to a change in the work function It hasbeen discovered earlier, however, by Seki and coworkers [25] and Kahn andcoworkers [26] that there can be rather substantial changes in the work func-tion even in this simple case, which, of course, have important consequencesfor the electronic level alignment at the metal/organic interface By using pre-cise ab initio electronic structure calculations, recently it was possible to un-

Figure 2 Scheme of a bottom-contact OFET illustration of the current research topics including the structure and morphology of organic semiconductor films as well as charge carrier transport and injection mechanism, which are the subject of this book.

ravel the origin of these unexpected phenomena: there are considerable change phenomena (or Pauli repulsions [27]), which have been dubbed the

ex-“cushion” effect [28] Therefore, the prediction of the relative positions ofelectronic levels at metal/molecule interfaces is not straightforward and re-quires more detailed investigations [29, 30]

OFETs and Organic Electronics

Organic electronics and the preparation of organic thin films for applications inorganic electronics have recently not only been the topic of a number of reviewarticles [31–33], but in addition a number of journals have recently devotedspecial issues to this subject (J Mat Res [34], Phys Stat Sol [35, 36], Chem.Mater [37], Chem Rev [38]) Also, several books [7, 8, 39, 40] have beenpublished in the last few years on this topic These are all strong indicators thatthat this field constitutes a “hot” topic in current research

To provide a more fundamental understanding of the basic mechanism andproperties of such OFETs, a national research initiative (supported by the DFGwithin the framework of the focus program 1121 OFET) was established in

2001 In this interdisciplinary research network various aspects of OFETs, cluding the synthesis of new materials, preparation and characterisation of or-ganic thin films, characterisation of device properties and the development ofnew device concepts, have been addressed The results obtained by the variousgroups collaborating in this effort provide the basis for the present book

in-Organization of the Book

As already mentioned above, in the present book the results obtained duringsix years of research within the framework of a national focus program entitled

“Organic field effect transistors: Structural and dynamic characteristics” arepresented We have augmented the book with two contributions from compa-

nies who are either about to place products on the market or have definitiveplans to do so.

Section I: Industrial Applications

The first section of the book is devoted to industrial applications In two cles written by two of the major companies active in this field, PolyIC andEvonik, the applications that presently attract the most interest from acommercial point of view are described At the same time, the key problemsrelated to the manufacturing of “cheap electronics” through a printing processare addressed These two chapters provide an excellent introduction to themore applied aspects of the field and also define the framework for thefollowing chapters in the book, which all address problems that in one way orthe other are related to producing organic field effect transistors and toimproving their performance and stability

arti-Section II: Molecular Compounds

In the second section of the book entitled “Molecular compounds” there arefour papers describing molecular aspects of the materials needed to fabricateorganic field effect transistors From these contributions it becomes clear thatthere are quite a few different routes to producing the basic material for an or-ganic semiconductor Today the search for molecules that allow the fabrication

of organic field effect transistors with superior properties is an important topic

In particular, in the contribution from the Müllen-group (Chapter 4, by Tsao etal.) it becomes clear that it is not only the desire to reach high-charge carriermobilities, but also to improve the processing properties of the compounds aswell as their stability against environmental influences (i.e degradation or oxi-dation), but which class of molecules will be incorporated in future organicsemiconductors is not yet clear There are a number of different molecularclasses that are presently being investigated One of the most of importantclasses of materials, oligothiophenes are discussed in the chapters from theScheinert and Jaehne group (Chapter 5, by Haubner et al.) and in the contribu-tion from the Scherf and Neher groups (Chapter 6, by Zen et al.) Phthalocya-nines have also been of interest for such applications, mostly because of the ra-ther large flexibility of the molecular properties, which can be changed byslightly modifying the composition, and are discussed in the contribution fromthe Wöhrle and Schlettwein groups (Chapter 3, by Brinkmann et al.)

Section III: Structural and Morphological Aspects

The third part of the book concentrates on structural and morphological aspects

of thin organic layers deposited on solid substrates, either metals or oxides.Here the emphasis is not – as in the previous part – on the particular com-pounds but on the ways to fabricate well defined organic thin layers from such

molecules suitable for applications in organic electronics The contributions inthis section of the book are all based on molecules that are fairly well under-stood, in some cases even for decades The experimental work described in thechapters contained in this section make it clear that it is in no way a trivialprocess to produce well defined organic thin layers suitable for applications inorganic electronics from a given molecule For virtually all molecule/substratecombination examples, a careful optimisation of processing parameters is re-quired and in many cases the detailed characterisation of organic films madefrom materials that are considered to be well known carries a number of sur-prises Some of these surprises result from the phenomena that occur whenmolecules generally considered as “soft” matter are deposited on a solid sub-strate that typically is considered to be “hard”.

A particularly important problem is the fabrication of electrical contacts tween metal and organic materials While the deposition of organic molecules

be-on a metallic substrate is fairly straightforward, the opposite depositibe-on ofmetals on an organic material is significantly non-trivial A very interestingapproach to this problem that uses a rather sophisticated procedure is presented

in the contribution from the Terfort group (Chapter 7, by Thomas et al.).For the characterisation of the organic/metal interphase in the onset of or-ganic growth on solid substrates the technique of X-ray scattering is fairly sig-nificant The most important aspects of such studies are described in the paper

by the Schreiber group (Chapter 9, by Gerlach et al.) In the article by Joshi et

al (Chapter 10) particular aspects of the structure and morphology of phenes are discussed A more general overview of organic molecular beamdeposition (OMBD) is presented in the chapter by Witte and Wöll (Chapter11) where results of other techniques are also discussed

thio-Of course, a proper understanding of the interphase between a metal and anorganic adlayer requires a careful experimental investigation of the first layer

of molecules in direct contact with the metal, because in this case the differentenvironment of the molecule may cause distortion and corresponding elec-tronic structure changes This is the topic of the article from the Tautz group(Chapter 12, by Soubatch et al.) where a careful characterisation of molecules

in the first monolayer, in particular with scanning tunnelling microscopy, ispresented

Surface enhanced Raman spectroscopy is a particularly useful spectroscopicmethod used to characterise the interface between metal and organics In thechapter from the Scholz and Zahn groups (Chapter 13, by Salvan et al.) a fairlycomprehensive review of work carried out in this field is provided

The active organic layer in OFET devices is frequently deposited on thinsilicon oxide layers on top of a silicon substrate These SiO2-layers are rarelystructurally well characterised For this reason the investigation of organic ad-layers grown on well defined model substrates for oxides is mandatory Hencethe results described in the contribution from the Umbach group (Chapter 14,

by Sachs et al.) for the growth of organic molecules on ordered, well definedsapphire substrates are very interesting

In the article by the Pflaum and Sokolowski groups (Chapter 8, by Voigt

et al.) a related dielectric, Al2O3deposited on a transparent conductor, indiumtin oxide, ITO, is used to fabricate pentacene-based OFETs Such devices areparticularly interesting in connection with the manufacture of drivers forOLED displays

Section IV: Device Performance and Characterisation

The fourth section of the book is entitled “Device performance and sation” and describes the next level of complication, namely producing organicthin layers with electrodes attached to them that can be used to determine thedevice characteristics of at least prototype devices Presently, there are a num-ber of different designs for organic field effect transistors and there are also anumber of parameters with regard to which of them need to be optimised Thissection of the book is the largest and reflects, in part, the fact that there arequite a few different, and sometimes conflicting, requirements for devices Itwill also become clear in this section that there is a pronounced difference be-tween organic electronic devices based on monomeric materials (i.e singlemolecules with a molecular mass below about 400 amu) and polymeric materi-als Basically, monomeric compounds can be deposited on a given substrateusing molecular beam deposition techniques, allowing for the preparation offairly well defined systems This approach, on the other hand, has the draw-back of being rather complicated and typically requiring ultrahigh vacuumequipment Using polymeric materials, on the other hand, has the advantage of

characteri-a fcharacteri-airly strcharacteri-aightforwcharacteri-ard scharacteri-ample prepcharacteri-archaracteri-ation; typiccharacteri-al scharacteri-amples ccharacteri-an be prepcharacteri-ared byspin coating the respective polymer on a pre-patterned substrate This ap-proach, however, carries the disadvantage that the polymeric thin films aretypically not very well defined structures and that the interface in particular atthe electrodes can be less well defined With the exception of gold, which doesnot form an oxide stable under ambient conditions, all other metals are covered

by such an oxide layer, which makes predictions about the precise structuralarrangement at the molecule substrate interface impossible or at least very dif-ficult

In the contribution by Pflaum et al (Chapter 25) the charge carrier mobilities

in organic compounds are addressed, as pointed out in the Introduction, bilities are one of the key parameters describing the suitability of a given mo-lecular compound for organic electronics The focus in this article is on oli-gomers Also in the contribution from the Nickel group (Chapter 15) oligomersare used to fabricate devices, in this case pentacene is used Also in this article,the use of a novel technique, recording the photo response, is used to charac-terise device performance In the chapter by Scheinert and Paasch et al (Chap-ter 16) fairly thorough general considerations about the general aspects inpolymer based field effect transistors are presented The contribution from theBrütting group (Chapter 17, by Opitz et al.) focuses on the question of whethercharge carrier transport can be obtained for both polarities, a stringing re-

mo-quirement which has to be fulfilled, for example for building logic deviceswithin organic electronics In the paper from the Hilleringmann group (Chap-ter 18, Diekmann et al.) an interesting approach is pursued where organic ma-terials are also used for the dielectric As pointed out earlier, the metallisation

of molecular compounds are a very interesting topic with regard to devicemanufacturing Here, the contribution by Scharnberg, Faupel and Adelung(Chapter 19) describes a rather interesting approach where radioactive label-ling is used to study the interdiffusion of metal into the films For the charac-terisation of real devices, potentiometry based on scanning probe instruments

is a very promising method In the chapter by the Scholz and Zahn group(Chapter 20, by Scholz et al.) results from applying this approach on carriermobilities and injection barriers within real devices are described Also in thecontribution from the Schmeißer group (Chapter 21, by Müller et al.) real de-vices are at the focus of interest Here a combination of microscopic and spec-troscopic methods is used to characterise the corresponding interphases In thecontribution from the Wagner group (Chapter 22, by Hoppe et al.) the focus isagain on thiophenes, one of the most promising materials within organic elec-tronics, where in particular the implications of minimising the size of the oper-ating devices are discussed In a working organic field effect transistor one ofthe most important parts, aside from the organic semiconductors, is the gatedielectrics In the contribution from Dresden (Chapter 23, by Plötner et al.) thefabrication and performance of aluminium oxide gate films are described Also

in the chapter from the Schmechel and von Seggern group (Chapter 24, byBenson et al.) the dielectric/organic semiconductor interphase is the topic.Section V: Novel Devices

The book ends with a section called “Novel devices” where either standard organic materials are used to produce organic field effect transistors(e.g carbon nanotubes), or organic materials are used to fabricate devicesother than organic field effect transistors One of the examples described in thearticle from the Kowalsky group (Chapter 27, by Hefczyc et al.) are non-volatile memory devices fabricated from organic materials The contributionfrom the Molenkamp and Geurts group (Chapter 28, by Michelfeit et al.) dem-onstrates a particularly interesting application at present, namely building or-ganic field effect transistors employing spin-polarised transport

non-Another particularly interesting approach for novel devices is described inthe contribution from the Kern group (Chapter 26, by Burghard et al.) whereinstead of single molecules or polymers, carbon nanotubes are used as the ac-tive semiconducting material The particular properties in contacting and ad-dressing these nanotubes to obtain a functioning device are described in detail

in this contribution

We would like to thank the German Science Foundation, DFG, for funding ofthe research projects described here within the framework of the National Fo-cus Programme (Schwerpunktprogramm SPP 1121: “Organic field effect tran-sistors”) In particular we would like to acknowledge the support of Dr KlausWefelmeier, the programme officer at the DFG in charge of this programme,for the support and advice provided throughout the years We would also like

to thank the referees who have evaluated the project three times and have vided valuable hints and suggestions for the work of the different groups in thefield Mrs Knödlseder-Mutschler has provided very competent technical sup-port through the years in managing the OFET-project We thank her also forhelping to compile this book

pro-Bochum, March 2009Gregor Witte and Christof Wöll

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Chemistry and Textile Chemistry

Technical University of Dresden

Campus Ring 1

28759 BremenGermanyEike BeckerTechnical University ofBraunschweig

Schleinitzstraße 22

38106 BraunschweigGermany

Lars BeckmannTechnical University ofBraunschweig

Schleinitzstraße 22

38106 BraunschweigGermany

Niels BensonInstitute of Materials ScienceDarmstadt University of TechnologyPetersenstraße 23

64287 DarmstadtGermanyRobert BlachePolylC GmbH & Co KGTucherstraße 2

90763 FuerthGermany

01062 DresdenGermanyandDepartment of ChemistryUniversity of CaliforniaSanta Barbara, CA 93106USA

T DiekmannDepartment EIM-E,Sensor TechnologyUniversity of PaderbornWarburger Straße 100

33098 PaderbornGermany

L M EngInstitute of Applied PhotophysicsTechnical University of Dresden

01062 DresdenGermany

F FaupelChair for Multicomponent MaterialsInstitute for Materials SciencesUniversity of Kiel

24118 KielGermanyW.-J FischerTechnical University of DresdenInstitute of Semiconductors andMicrosystems

01062 DresdenGermanyWalter FixPolylC GmbH & Co KGTucherstraße 2

90763 FuerthGermany

Walter Schottky Institute

Technical University of Munich

Institute of Ion Beam Physics

and Materials Research

01062 DresdenGermanyArtur HefczycTechnical University ofBraunschweig

Schleinitzstraße 22

38106 BraunschweigGermany

K HenkelBrandenburg University ofTechnology at CottbusDepartment of Applied Physicsand Sensors

P.O Box 101344

03013 CottbusGermany

A HerasimovichSolid State Electronics and Center

of Micro- and Nano-TechnologiesTechnical University of IlmenauP.O Box 100565

98684 IlmenauGermany

U HilleringmannDepartment EIM-E,Sensor TechnologyUniversity of PaderbornWarburgerstraße 100

33098 PaderbornGermany

F HolchUniversity of WürzburgExperimental Physics II

Am Hubland

97074 WürzburgGermany

A Hoppe

School of Engineering and Science

Jacobs University Bremen

Campus Ring 1

28759 Bremen

Germany

I Hörselmann

Institute of Solid State Electronics

Technical University of Ilmenau

Chemistry and Textile Chemistry

Technical University of Dresden

70569 StuttgartGermanyandInstitute of Physics of NanostructuresÉcole Polytechnique Fédérale deLausanne

1015 LausanneSwitzerland

D KoehlerInstitute of Applied PhotophysicsTechnical University of Dresden

01062 DresdenGermanyWolfgang KowalskyTechnical University ofBraunschweig

Schleinitzstr 22

38106 BraunschweigGermany

Stefan KowarikInstitute of Applied PhysicsUniversity of TübingenAuf der Morgenstelle 10

72076 TübingenGermanyEduardo J H LeeMax-Planck-Institute forSolid State ResearchHeisenbergstraße 1

70569 StuttgartGermanyMark A M LeenenEvonik IndustriesCreavis Technology and InnovationPaul-Baumann-Straße 1

45764 MarlGermany

Leibniz Institute for Solid State and

Materials Research IFW Dresden

P.O Box 270016

01171 Dresden

Germany

C Loppacher

Institute of Applied Photophysics

Technical University of Dresden

Institute of Materials Science

Darmstadt University of Technology

Petersenstraße 23

64287 Darmstadt

Germany

S Meyer3rd Department of PhysicsUniversity of Stuttgart

70550 StuttgartGermany

M MichelfeitUniversity of WürzburgInstitute of Physics

Am Hubland

97074 WürzburgGermany

L W MolenkampUniversity of WürzburgInstitute of Physics

Am Hubland

97074 WürzburgGermany

T MuckSchool of Engineering and ScienceJacobs University Bremen

Campus Ring 1

28759 BremenGermany

K MüllenMax-Planck Institute forPolymer ResearchAckermannweg 10

55128 MainzGermanyA.-D MüllerAnfatec Instruments AGMelanchthonstraße 28

08606 Oelsnitz (V)Germany

F MüllerAnfatec Instruments AGMelanchthonstraße 28

08606 Oelsnitz (V)Germany