Physics of Organic Semiconductors E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
The field of organic electronics has seen a steady growth over the last 15 years. At the same time, our scientific understanding of how to achieve optimum device performance has grown, and this book gives an overview of our present-day knowledge of the physics behind organic semiconductor devices. Based on the very successful first edition, the editors have invited top scientists from the US, Japan, and Europe to include the developments from recent years, covering such fundamental issues as:
- growth and characterization of thin films of organic semiconductors,
- charge transport and photophysical properties of the materials as well as their electronic structure at interfaces, and
- analysis and modeling of devices like organic light-emitting diodes or organic lasers.
The result is an overview of the field for both readers with basic knowledge and for an application-oriented audience. It thus bridges the gap between textbook knowledge largely based on crystalline molecular solids and those books focusing more on device applications.
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Seitenzahl: 1186
Veröffentlichungsjahr: 2012
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Contents
Cover
Related Titles
Title Page
Copyright
Foreword
Preface
List of Contributors
Part One: Film Growth, Electronic Structure, and Interfaces
Chapter 1: Organic Molecular Beam Deposition
1.1 Introduction
1.2 Organic Molecular Beam Deposition
1.3 Films on Oxidized Silicon
1.4 Films on Aluminum Oxide
1.5 Films on Metals
1.6 Films on Other Substrates
1.7 More Complex Heterostructures and Technical Interfaces
1.8 Summary and Conclusions
Acknowledgments
References
Chapter 2: Electronic Structure of Interfaces with Conjugated Organic Materials
2.1 Introduction
2.2 Energy Levels of Organic Semiconductors
2.3 Interfaces between Organic Semiconductors and Electrodes
2.4 Energy Levels at Organic Semiconductor Heterojunctions
2.5 Conclusions
Acknowledgments
References
Chapter 3: Electronic Structure of Molecular Solids: Bridge to the Electrical Conduction
3.1 Introduction
3.2 General View of Electronic States of Organic Solids
3.3 Electronic Structure in Relation to Charge Transport
3.4 Electron–Phonon Coupling, Hopping Mobility, and Polaron Binding Energy
3.5 Summary
Acknowledgments
References
Chapter 4: Interfacial Doping for Efficient Charge Injection in Organic Semiconductors
4.1 Introduction
4.2 Insertion of an Interfacial Layer in the Organic/Electrode Junction
4.3 Doped Organic/Electrode Junctions
4.4 Doped Organic/Undoped Organic Junction
4.5 Applications
4.6 Conclusions
References
Chapter 5: Displacement Current Measurement for Exploring Charge Carrier Dynamics in Organic Semiconductor Devices
5.1 Introduction
5.2 Displacement Current Measurement
5.3 Charge Accumulation at Organic Heterointerfaces
5.4 Conclusions
Acknowledgment
References
Part Two: Charge Transport
Chapter 6: Effects of Gaussian Disorder on Charge-Carrier Transport and Recombination in Organic Semiconductors
6.1 Introduction
6.2 Mobility Models for Hopping in a Disordered Gaussian DOS
6.3 Modeling of the Recombination Rate
6.4 OLED Device Modeling
6.5 Experimental Studies
6.6 Conclusions and Outlook
Acknowledgments
References
Chapter 7: Charge Transport Physics of High-Mobility Molecular Semiconductors
7.1 Introduction
7.2 Review of Recent High-Mobility Small-Molecule and Polymer Organic Semiconductors
7.3 General Discussion of Transport Physics/Transport Models of Organic Semiconductors
7.4 Transport Physics of High-Mobility Molecular Semiconductors
7.5 Conclusions
References
Chapter 8: Ambipolar Charge-Carrier Transport in Molecular Field-Effect Transistors
8.1 Introduction
8.2 Ambipolar Charge-Carrier Transport in Blends of Molecular Hole- and Electron-Conducting Materials
8.3 Ambipolar Charge-Carrier Transport in Molecular Semiconductors by Applying a Passivated Insulator Surface
8.4 Electrode Variation for Ambipolar and Bipolar Transport
8.5 Applications of Bipolar Transport for Ambipolar and Complementary Inverters
8.6 Realization of Light-Emitting Transistors with Combined Al and TTF-TCNQ Electrodes
8.7 Conclusion
Acknowledgments
References
Chapter 9: Organic Magnetoresistance and Spin Diffusion in Organic Semiconductor Thin-Film Devices
9.1 Introduction
9.2 Organic Magnetoresistance
9.3 Theory of Spin–Orbit Coupling in Singly Charged Polymer Chains
9.4 Theory of Spin Diffusion in Disordered Organic Semiconductors
9.5 Distinguishing between Tunneling and Injection Regimes of Ferromagnet/Organic Semiconductor/Ferromagnet Junctions
9.6 Conclusion
Acknowledgments
References
Part Three: Photophysics
Chapter 10: Excitons at Polymer Interfaces
10.1 Introduction
10.2 Fabrication and Structural Characterization of Polymer Heterojunctions
10.3 Electronic Structure at Polymer/Polymer Interfaces
10.4 Excitons at Homointerfaces
10.5 Type-I Heterojunctions
10.6 Type-II Heterojunctions
10.7 CT State Recombination
10.8 Charge Separation and Photovoltaic Devices based on Polymer:Polymer Blends
10.9 Future Directions
References
Chapter 11: Electronic Processes at Organic Semiconductor Heterojunctions: The Mechanism of Exciton Dissociation in Semicrystalline Solid-State Microstructures
11.1 Introduction
11.2 Experimental Methods
11.3 Results and Analysis
11.4 Conclusions
Acknowledgments
References
Chapter 12: Recent Progress in the Understanding of Exciton Dynamics within Phosphorescent OLEDs
12.1 Introduction
12.2 Exciton Formation
12.3 Distributing Excitons in the Organic Layer(s)
12.4 High Brightness Effects in Phosphorescent Devices
Acknowledgments
References
Chapter 13: Organometallic Emitters for OLEDs: Triplet Harvesting, Singlet Harvesting, Case Structures, and Trends
13.1 Introduction
13.2 Electroluminescence
13.3 Triplet Emitters: Basic Understanding and Trends
13.4 Case Studies: Blue Light Emitting Pt(II) and Ir(III) Compounds
13.5 Case Studies: Singlet Harvesting and Blue Light Emitting Cu(I) Complexes
13.6 Conclusion
Acknowledgments
References
Part Four: Device Physics
Chapter 14: Doping of Organic Semiconductors
14.1 Introduction
14.2 Doping Fundamentals
14.3 Organic p–n Junctions
14.4 OLEDs with Doped Transport Layers
14.5 Organic Solar Cells with Doped Transport Layers
14.6 Conclusion
14.7 Summary and Outlook
Acknowledgments
References
Chapter 15: Device Efficiency of Organic Light-Emitting Diodes
15.1 Introduction
15.2 OLED Operation
15.3 Electroluminescence Quantum Efficiency
15.4 Fundamentals of Light Outcoupling in OLEDs
15.5 Approaches to Improved Light Outcoupling
15.6 Conclusion
Acknowledgments
References
Chapter 16: Light Outcoupling in Organic Light-Emitting Devices
16.1 Introduction
16.2 Theories and Properties of OLED Optics
16.3 A Few Techniques and Device Structures to Enhance Light Outcoupling of OLEDs
16.4 Summary
References
Chapter 17: Photogeneration and Recombination in Polymer Solar Cells
17.1 Introduction
17.2 Photogeneration of Charge Carriers
17.3 Charge Carrier Transport in Disordered Organic Semiconductors
17.4 Recombination of Photogenerated Charge Carriers
17.5 Open-Circuit Voltage
17.6 Summary
References
Chapter 18: Light-Emitting Organic Crystal Field-Effect Transistors for Future Organic Injection Lasers
18.1 Introduction
18.2 Highly Photoluminescent Oligo(p-phenylenevinylene) Derivatives
18.3 Ambipolar Light-Emitting Field-Effect Transistors Based on Organic Single Crystals
18.4 Summary and the Outlook for Future Organic Injection Lasers
References
Index
Related Titles
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Foreword
Organic semiconductors now provide an important technology base, supported by a rapidly growing body of science. The field is not new; interest in the semiconducting properties of pi-conjugated molecules was already well established in the 1960s and the foundations of their semiconductor science were built then using molecular materials such as anthracene as model systems. As with inorganic semiconductors, the prospect of engineering toward applications has provided several major boosts to the field. The initial drive in the late 1970s was to use organic photoconductors in place of selenium as the photoactive drum in electrophotography applications. Molecular semiconductor “guests' in polymer “hosts” were successfully engineered and are now the ubiquitous technology for this application. They also provided the working systems for the understanding of electronic transport in disordered semiconductors.
The explosion of interest, dating from the late 1980s, was triggered by the observation of relatively efficient electroluminescence in thin-film diode structures, in both molecular semiconductors and solution-processed polymeric semiconductors. Though electroluminescence had been observed in single-crystal semiconductors in the 1960s, it was the prospect of practical materials processing to deliver useful devices such as pixelated displays that drew industrial and commercial attention, and this has supported a vibrant global research community. Besides light-emitting diodes, now in products such as smart phone displays, other devices have been brought to realistic levels of performance: field-effect transistors today match the performance of thin-film silicon, and solar cells offer realistic energy conversion efficiency, at 10%.
The level of device performance has been achieved on the back of a wide range of scientific and engineering breakthroughs. Perceived obstacles to performance (such as limits to solid-state luminescence efficiency or to field-effect carrier mobility) have been pushed aside and a remarkable landscape of new science and new phenomena is now revealed. This is captured by the excellent series of chapters in this book that cover both the richness of the physics-based science and the global reach of the field, with authors from leading research groups across North America, Europe. and Asia.
Richard Friend
Cavendish Laboratory
University of Cambridge, UK
Preface
With the invention of the transistor around the middle of the last century, inorganic semiconductors like Si or GaAs began to take over the role as dominant materials in electronics from the prevailing metals. At the same time, the replacement of vacuum tube-based electronics by solid-state devices initiated a development that by the end of the twentieth century led to the omnipresence of semiconductor microelectronics in our everyday life. Since the beginning of the twenty-first century, we are facing a new electronics revolution that has become possible due to the development and understanding of a new class of materials, commonly known as organic semiconductors. The enormous progress in this field has been driven by the expectation to realize new applications, such as large area, flexible light sources and displays, low-cost printed integrated circuits, or plastic solar cells from these materials.
Strictly speaking, organic semiconductors are not new. The first studies of the dark and photoconductivity of anthracene crystals (a prototype organic semiconductor) date back to the early twentieth century. Later on, triggered by the discovery of electroluminescence in the 1960s, molecular crystals were intensely investigated by many researchers. These investigations could establish the basic processes involved in optical excitation and charge carrier transport. Nevertheless, in spite of the principal demonstration of an organic electroluminescent diode incorporating even an encapsulation similar to the ones used in nowadays commercial display applications, there were several drawbacks preventing practical use of these early devices. Since the 1970s, the successful synthesis and controlled doping of conjugated polymers established the second important class of organic semiconductors. Together with organic photoconductors (molecularly doped polymers), these conducting polymers have initiated the first applications of organic materials as conductive coatings or photoreceptors in electrophotography. The interest in the semiconducting properties of molecular materials revived in the 1980s due to the demonstration of an efficient photovoltaic cell incorporating an organic heterojunction of “p- and n-type” semiconductors as well as the first successful fabrication of thin-film transistors from conjugated polymers and oligomers. The main impetus, however, came from the demonstration of high-performance electroluminescent diodes from vacuum-evaporated molecular films and from conjugated polymers. Owing to the large efforts of both academic and industrial research laboratories during the past two decades, organic semiconductor devices have progressed rapidly and meanwhile led to first commercial products incorporating displays and light sources made of organic light-emitting diodes (OLEDs), logic circuits utilizing organic field-effect transistors (OFETs) or solar energy harvesting modules on the basis of organic photovoltaic cells (OPVs).
This book focuses on the fundamental physics behind this rapidly developing field of organic electronics. It ties in with the previous edition of “Physics of Organic Semiconductors” published in 2005. Due to the big success of the first edition and the rapidly developing and still growing field, a new edition with completely restructured contents and new contributing authors was put together in order to include novel exciting developments over the past 6 years. In spite of the appearance of first commercial products, there is still a large interest in fundamental issues in the field of organic semiconductors. This book, therefore, tries to bridge the gap between textbook knowledge largely based on crystalline molecular solids on the one side (see, for example, Pope & Swenberg, Electronic Processes in Organic Crystals and Polymers, or Schwoerer & Wolf, Organic Molecular Solids) and other books focusing more on device applications.
The editors want to thank all contributing authors for writing high-quality up-to-date chapters of their work, including the state of the art in the respective field. Without their efforts this book would not have been possible. Furthermore, we want to thank Russell J. Holmes (University of Minnesota) who acted as consultant editor in the early stages of this book project. We are also indebted to our academic teachers, Prof. Em. Markus Schwoerer (Bayreuth University) and Prof. Em. Tetsuo Tsutsui (Kyushu University), who brought us in touch with this fascinating subject more than 20 years ago.
Wolfgang Brütting andChihaya Adachi
List of Contributors
Chihaya Adachi
Kyushu University
Center for Future Chemistry
744 Motooka
Nishi
Fukuoka 819-0395
Japan
Marc A. Baldo
Massachusetts Institute of Technology
Department of Electrical Engineering and Computer Science
Cambridge
MA 02139
USA
Andreas Baumann
Bavarian Center for Applied Energy Research (ZAE Bayern)
Am Hubland
D - 97074 Würzburg
Germany
David Beljonne
Université de Mons-Hainaut
Service de Chimie des Matériaux Nouveaux
Place du Parc 20
7000 Mons
Belgium
Peter A. Bobbert
Eindhoven University of Technology
Department of Applied Physics
P.O. Box 513
Eindhoven
5600 MB
The Netherlands
Wolfgang Brütting
University of Augsburg
Institute of Physics
Universitätsstr. 1
86159 Augsburg
Germany
Jui-Fen Chang
Department of Optics and Photonics National Central University
Chung-Li, 320, R.O.C.
Taiwan
Reinder Coehoorn
Philips Research Laboratories
High Tech Campus 4
Eindhoven
5656 AE
The Netherlands
and
Eindhoven University of Technology
Department of Applied Physics
P.O. Box 513
Eindhoven
5600 MB
The Netherlands
Rafał Czerwieniec
Universität Regensburg
Institut für Physikalische und Theoretische Chemie
Universitätsstr. 31
93053 Regensburg
Germany
Carsten Deibel
Experimental Physics VI
Julius-Maximilian-University of Würzburg
Am Hubland
D - 97074 Würzburg
Germany
Vladimir Dyakonov
Experimental Physics VI
Julius-Maximilian-University of Würzburg
Am Hubland
D - 97074 Würzburg
Germany
and
Bavarian Center for Applied Energy Research e.V. (ZAE Bayern)
Am Hubland
D - 97074 Würzburg
Germany
Jörg Frischeisen
University of Augsburg
Institute of Physics
Universitätsstr. 1
86159 Augsburg
Germany
Neil Greenham
University of Cambridge
Cavendish Laboratory
J.J. Thomson Avenue
Cambridge CB3 0HE
UK
Hisao Ishii
Chiba University
Center for Frontier Science
1-33 Yayoi-cho
Inage Chiba 263-8522
Japan
Paul-Ludovic Karsenti
Université de Montréal
Département de physique & Regroupement québécois sur les matériaux de pointe
C.P. 6128
Succursale Center-ville
Montréal
QC H3C 3J7
Canada
Jang-Joo Kim
Department of Materials Science and Engineering
Seoul National University
San 56-1
Shillim-dong
Gwanak-gu
Seoul 151-744
Korea
Norbert Koch
Humboldt-Universität zu Berlin
Institut für Physik
Newtonstr. 15
12489 Berlin
Germany
Gianluca Latini
Istituto Italiano di Tecnologia
Centre for Biomolecular Nanotechnologies @UNILE
Via Barsanti
73010, Arnesano (LE)
Italy
Jae-Hyun Lee
School of Global Convergence Studies
Hanbat National University
San 16-1
Duckmyoung-dong
Daejeon 305-719
Korea
Karl Leo
Technische Universität Dresden
Institut für Angewandte Photophysik
George-Bähr-Str. 1
01069 Dresden
Germany
Björn Lüssem
Technische Universität Dresden
Institut für Angewandte Photophysik
George-Bähr-Str. 1
01069 Dresden
Germany
Yukimasa Miyazaki
Chiba University
Graduate School of Advanced Integration Science
1-33 Yayoi-cho
Inage Chiba 263-8522
Japan
Hajime Nakanotani
Kyushu University
Center for Future Chemistry
744 Motooka
Nishi
Fukuoka 819-0395
Japan
Yasuo Nakayama
Chiba University
Center for Frontier Science
1-33 Yayoi-cho
Inage Chiba 263-8522
Japan
Yutaka Noguchi
Chiba University
Center for Frontier Science
1-33 Yayoi-cho
Inage Chiba 263-8522
Japan
Andreas Opitz
University of Augsburg
Institute of Physics
Universitätsstr. 1
86159 Augsburg
Germany
and
Humboldt-Universität zu Berlin
Institut für Physik
Newtonstr. 15
12489 Berlin
Germany
Francis Paquin
Université de Montréal
Département de physique & Regroupement québécois sur les matériaux de pointe
C.P. 6128
Succursale Center-ville
Montréal
QC H3C 3J7
Canada
Andreas F. Rausch
Universität Regensburg
Institut für Physikalische und Theoretische Chemie
Universitätsstr. 31
93053 Regensburg
Germany
Sebastian Reineke
Massachusetts Institute of Technology
Department of Electrical Engineering and Computer Science
Cambridge
MA 02139
USA
Moritz Riede
Technische Universität Dresden
Institut für Angewandte Photophysik
George-Bähr-Str. 1
01069 Dresden
Germany
Tomo Sakanoue
Organic Electronics Research Center Yamagata University, 4-3-16 Jonan
Yonezawa, Yamagata
992-8510
Japan
Maciej Sakowicz
Université de Montréal
Département de physique & Regroupement québécois sur les matériaux de pointe
C.P. 6128
Succursale Center-ville
Montréal
QC H3C 3J7
Canada
Naoki Sato
Chiba University
Graduate School of Advanced Integration Science
1-33 Yayoi-cho
Inage Chiba 263-8522
Japan
Frank Schreiber
Universität Tübingen
Institut für Angewandte Physik
Auf der Morgenstelle 10
72076 Tübingen
Germany
Carlos Silva
Université de Montréal
Département de physique & Regroupement québécois sur les matériaux de pointe
C.P. 6128
Succursale Center-ville
Montréal
QC H3C 3J7
Canada
Henning Sirringhaus
Cavendish Laboratory
University of Cambridge
Cambridge CB3 OHE
UK
Natalie Stingelin
Imperial College London
Department of Materials and Centre for Plastic Electronics
South Kensington Campus
London SW7 2AZ
UK
Yuya Tanaka
Chiba University
Graduate School of Advanced Integration Science
1-33 Yayoi-cho
Inage Chiba 263-8522
Japan
Chih-Hung Tsai
National Taiwan University
Graduate Institute of Photonics and Optoelectronics
No. 1, Sec. 4, Roosevelt Rd.
Taipei 10617, Taiwan
Republic of China
Nobuo Ueno
Chiba University
Graduate School of Advanced Integration Science
Inage-ku
Chiba 263-8522
Japan
Linjun Wang
Université de Mons-Hainaut
Service de Chimie des Matériaux Nouveaux
Place du Parc 20
7000 Mons
Belgium
Markus Wohlgenannt
University of Iowa
Department of Physics and Astronomy
Optical Science and Technology Center
205 North Madison Street
126 IATL
Iowa City
IA 52242
USA
Chung-Chih Wu
National Taiwan University
Department of Electrical Engineering
No. 1, Sec. 4, Roosevelt Rd.
Taipei 10617, Taiwan
Republic of China
Hartmut Yersin
Universität Regensburg
Institut für Physikalische und Theoretische Chemie
Universitätsstr. 31
93053 Regensburg
Germany
Part One
Film Growth, Electronic Structure, and Interfaces
1
Organic Molecular Beam Deposition
Frank Schreiber
1.1 Introduction
Organic semiconductors exhibit a range of interesting properties, and their application potential is rather broad, as seen in many other chapters in this book. For the crystalline “small-molecule” systems, grown by organic molecular beam deposition (OMBD), the subject of this chapter, it is generally agreed that the structural definition is important for the functional properties. The following list should serve to illustrate the various aspects:
Since the structure has a strong impact on the functional properties, understanding the structure formation, that is, the growth process, and finding ways to optimize the structural definition is a prerequisite for technological progress. Moreover, understanding the physics of the growth process raises several fundamental challenges.
We will mostly focus on “thicker” films, their growth modes, and the evolution of the morphology for thickness ranges that are typically employed in organic semiconductor applications. We will discuss only to a limited extent the work on the first monolayer, although as the “seed layer” for the following layers this is obviously important. Thus, some of the classical surface science issues, such as binding distances and associated interface dipoles, although very important [1, 2], epitaxial relation, and so on, are not the focus of this chapter. For these issues and also for information on the history of the field, we refer to Refs [3–15]. Also, we will not discuss issues related to chirality, although they are undoubtedly intriguing [11, 16–18].
In terms of growth technology, the equipment is essentially the same as for inorganic molecular beam epitaxy. Evaporation cells on a vacuum chamber are used to provide a flux of molecules at the substrate surface (typically some range around 1 Å/s to 1 Å/min), and ideally the growth can be monitored in situ. Virtually, all surface and interface techniques have been used for OMBD-grown films, and we refer to standard textbooks for details of the experimental methodology.
This chapter is organized as follows. We first present some of the general issues in thin film growth and then what is specific and potentially different for organics (Section 1.2). In Section 1.3, we give an overview of the most popular systems. Section 1.4 contains a number of case studies, trying to highlight the issues that we feel are particularly relevant and typical for OMBD. The case studies are based on a few selected compounds and are not intended as an exhaustive list. They are organized according to the (inorganic) substrates, covering, insulators, metals, and semiconductors. In Section 1.7, we briefly indicate the issues for organics-based heterostructures, inorganic–organic, and organic–organic. Some conclusions are given in Section 1.8.
In a review with limited space such as the present one, it cannot be our goal to give a complete and exhaustive overview. Instead, the examples are centered mostly around our own work, which we try to discuss in the context of the general field. This selection is obviously unbalanced, and we apologize for omissions of other important work.
We note that this chapter is an updated version of the 2005 edition of this book and related to Ref. [13]. Important developments since then are, inter alia, the precision determination of binding distances of organic semiconductors on metal contacts along with the associated electronic properties (e.g., Refs [1, 2]), the further development of real-time monitoring of growth (e.g., Ref. [19]) and an increased understanding of organic–organic heterostructures, as reviewed at the end of this chapter.
1.2 Organic Molecular Beam Deposition
1.2.1 General Concepts of Thin Film Growth
Crystal and thin film growth are enormously rich subjects with many different facets and theoretical approaches. For a thorough treatment of the underlying concepts, we refer to Refs [20–23]. Here, we shall only briefly touch upon selected aspects that we feel are important in the present context and help to appreciate the issues related to thin film growth (see also Figure 1.1).
Figure 1.1 Schematic of processes relevant in thin film growth, such as adsorption (as a result of a certain impingement rate), (re-)desorption, intralayer diffusion (on a terrace), interlayer diffusion (across steps), nucleation, and growth of islands.
One approach to describe the various relevant interactions uses the concept of surface and interface energies, , similar to what is done for wetting phenomena. Typically, the surface energies (i.e., the relative contributions of the free substrate surface, , the film surface, , and the film–substrate interface, ) are then related to the different growth modes, that is, Frank van der Merwe (layer by layer), Stranski–Krastanov (layer plus islands after a certain critical thickness), and Vollmer–Weber (islands starting at the first monolayer).
We will not discuss issues related to the epitaxial relation in much detail. (For clarity, we should emphasize that under epitaxial relation we understand the crystallographic relation between film and substrate, which does not necessarily imply smooth film growth). However, we should point out that, generally, the surface energies depend on the strain field induced by the lattice mismatch at the film–substrate interface, and thus also on the number of layers of the film. Therefore, the epitaxial relation of film and substrate is important not only in a crystallographic sense but also for the growth behavior.
It should be emphasized that growth is actually a nonequilibrium phenomenon, and equilibrium or near-equilibrium energy considerations alone cannot properly account for all growth scenarios. Thus, a dynamic description is needed. This description has to take into account the flux of adsorbates toward the surface (corresponding to a certain supersaturation), the adsorption and redesorption probabilities, and the diffusion processes on the surface (interlayer and intralayer) and their respective barriers. In the past two decades, a theoretical framework has been established, which relates growth mechanisms to a set of scaling exponents describing the dependence of the surface roughness on film thickness and lateral length scale. Much effort has been spent to theoretically predict scaling exponents for certain growth models, as well as to determine them experimentally [20–25].
The scaling theory of growth-induced surface roughness is based on the behavior of the height difference correlation function (HDCF), the mean square height difference of pairs of points laterally separated by . The HDCF displays distinct behaviors for and , where denotes the correlation length. For one expects a power law increase as , where is the static roughness exponent and the prefactor is a measure of the typical surface slope. For the heights at distance become uncorrelated. Hence, saturates at the value , where is the standard deviation of the film height (or “rms roughness”). The three parameters , , and evolve with film thickness according to the power laws , , and , defining the growth exponent , the dynamic exponent , and the steepening exponent . Assuming that the regimes and are connected through a scaling form , it follows that the scaling exponents are related by . For (no steepening) one has . Scaling with is referred to as anomalous [22]. The HDCF can be determined experimentally by real space methods (such as atomic force microscopy) or diffuse scattering, each having their advantages [25].
1.2.2 Issues Specific to Organic Thin Film Growth
While the general considerations presented above apply to both inorganic and organic thin-film systems, there are a few issues specific to organics (Figure 1.2), which can lead to quantitatively and qualitatively different growth behavior.
Figure 1.2 Issues specific to organics in the context of thin film growth. (a) Orientational degrees of freedom, potentially leading to orientational domains (additional source of disorder). They can also give rise to orientational transitions during growth. (b) Molecules larger than the unit cells of (inorganic) substrates, thus leading to translational domains. Generally, this can also lead to a smearing-out of the corrugation of the substrate potential experienced by the adsorbate.
Generally, most of the above points directly or indirectly impact the interactions and thus also the barriers experienced during diffusion. Thus, not only the static structure but also the growth dynamics exhibit differences compared to inorganic systems.
1.2.3 Overview of Popular OMBD Systems
Organic chemistry provides obviously a vast number of dyes and semiconductors, which are potentially interesting for thin film studies, and there is the additional possibility of specifically modifying certain functionalities. A fairly large number of compounds has indeed been employed for thin film work, but not for all of these have detailed growth studies been performed. We will limit ourselves to only selected systems, largely based on examples from our own work (see Figure 1.3).
Figure 1.3 Some popular organic semiconductors discussed in this chapter.
1.2.3.1 PTCDA
The perylene-derivative PTCDA (3,4,9,10-perylene-tetracarboxylic dianhydride, C24H8O6, a red dye) has long been regarded as a model system for OMBD [4, 5, 26–33]. Its bulk structure (actually and phase) exhibits layered molecular planes, and it was expected that the regular stacking of these planes (along the [102] direction in phase notation) is favorable for well-behaved film growth, which turned out to be not necessarily correct. The optical properties [34–38] and the vibrational properties [38–41] have been thoroughly studied.
1.2.3.2 DIP
Diindeno(1,2,3,-cd,1′,2′,3′-lm)perylene (C32H16, DIP, a red dye) has the same perylene core as PTCDA. It has been shown to exhibit interesting out-of-plane ordering behavior [25, 42–44] and, associated with this, good charge carrier transport properties [45, 46]. Recently, its spectroscopic behavior was analyzed in detail [47, 48], and it was also demonstrated to be very promising in OPV devices [49].
1.2.3.3 Phthalocyanines
Phthalocyanines (Pc's) are rather popular [50–55], and some of the early work on OMBD has employed Pc's [50]. They exhibit a certain degree of “specific tunability,” both due to the possible central metal ion, which can be changed within a broad range, and due to the choice of the side group(s) [51, 52]. F16CuPc is particularly attractive, since it is considered a good candidate as an n-type conducting organic material [56]. As a blue dye [54], it is also interesting for optoelectronic applications [51, 55].
1.2.3.4 Oligoacenes (Anthracene, Tetracene, and Pentacene)
The oligoacenes and in particular pentacene have attracted considerable attention, since their charge transport properties were reported to be excellent [9, 45]. An important feature of pentacene seems to be that it can be grown in well-ordered thin films, although the “bulk structure” and a “thin film structure” appear to be competing. A recent development is the synthesis of perfluoropentacene (PFP) [57], which exhibits structural similarities to pentacene (PEN), but electronically of course different [58–61], and allows the preparation of mixed films (see Section 1.7).
There are, of course, many other popular systems. These include, for example, oligothiophenes, oligophenyls, and also “sheets of graphite.” Besides the crystalline systems, there are amorphous small-molecule organic semiconductors prepared by OMBD, such as Alq3 and TPD. In terms of the growth physics, amorphous systems exhibit obviously some differences (no strain due to epitaxy, different diffusion barriers, no crystallographic domains, etc.). They are worth studying in their own right, but we cannot discuss them here. Another interesting case is rubrene, which can form crystals, but for conventional thin-film deposition results in amorphous structures. For examples from various other systems, we refer to Refs [3–15].
1.3 Films on Oxidized Silicon
Silicon wafers are among the most common substrates for thin film growth. They are stable in air with their oxidized surface layer, the thickness of which can be “tuned” by thermal oxidation (from some 15 Å, native oxide, to several 1000 Å). Also, they are very flat and relatively easy to clean.
In the context of organic electronics, of course, they are very popular as substrates for thin-film transistors (TFTs), since the oxide can serve as the insulating layer between the silicon as the bottom contact (gate) and the active organic semiconductor on top.
We should also mention that oxidized silicon surfaces are suitable for surface modification using self-assembled monolayers (SAMs) [62, 63], which has been exploited, for example, for the growth of pentacene [64] and other systems [61].
1.3.1 PTCDA
It was expected that the regular stacking of PTCDA molecules in the [102] direction (in phase notation) of the bulk structure would give rise to well-behaved film growth. This regular stacking is indeed observed on silicon oxide and many other substrates, unless the growth temperature is too low and no well-defined structure evolves or a too strong interaction with a very “reactive” substrate leads to other orientations of the first PTCDA monolayer. However, it is important to realize that a regular stacking and well-defined orientation of the molecules within the films does not necessarily imply smooth surfaces.
In an early study, it was already found that PTCDA on oxidized silicon exhibits smooth surfaces only for growth at low temperatures ( for deposition rates around 1 Å/s), where the crystallinity was not very good [65]. For growth at higher temperatures, the films exhibited good crystallinity, but showed a tendency to island growth (“dewetting”).
These results demonstrate a not uncommon feature of growth on substrates with low surface energies. If the films tend to dewet from the substrate near equilibrium, then the above pattern (relatively flat, but low-crystallinity films for low , and dewetting, that is, rough, morphologies with good crystallinity for high ) is quite frequently found.
1.3.2 DIP
DIP has the same perylene core as PTCDA, but the indeno end groups instead of the anhydride end groups give rise to a completely different structural behavior compared to PTCDA. DIP has been studied in detail both structurally [19, 25, 42–44, 66, 67] and spectroscopically [47, 48, 68–70], and it was found to exhibit excellent out-of-plane order on silicon oxide surfaces.
Films with various film thicknesses () were prepared on oxidized (4000 Å) Si(100) substrates at a substrate temperature of and at a deposition rate of . The out-of-plane X-ray spectra exhibit well-defined Bragg reflections corresponding to a lattice spacing of (suggesting essentially upright-standing molecules) and associated Laue oscillations, the analysis of which shows that the films are coherently ordered across the entire thickness [42]. The rocking width, which is a measure of the distribution of the out-of-plane lattice planes, is 0.01° and lower [42, 44]. The lattice spacing is consistent with a model of molecules standing essentially upright with a tilt angle presumably around 15–20°. The large number of higher order Bragg reflections could be used to deconvolute the out-of-plane electron density distribution in a Fourier series (Figure 1.4)
(1.1)
where the Fourier amplitude, , is associated with the intensity of the nth Bragg reflection [42]. We can speculate that the shape of DIP with its slightly narrow head and tail may be favorable for an ordering mechanism with some degree of interdigitation of molecules from neighboring (i.e., top and bottom) lattice planes.
Figure 1.4 Specular X-ray scan of a 206 Å thick DIP film. Many higher order Bragg reflections are observed, which can be used for the reconstruction of the electron density profile using the various Fourier components (close-up shown in the inset). Adapted from Ref. [42].
On silicon oxide, the in-plane structure is, of course, a 2D powder. The packing appears to follow a herringbone motif. The structure will be discussed also in the context of growth on Au (Ref. [43] and Section 1.5).
The growth including the evolution of the HDCF and the associated growth exponents, , , and , were studied using AFM and X-ray scattering (specular and diffuse) [25]. Whereas the static roughness exponent (average of AFM and X-rays ) is similar to that observed in many other growth experiments [21], the values for and were found to be rather large (Figure 1.5). Specifically, the DIP films belong to the class of systems that display the phenomenon of rapid roughening, for which , that is, the roughness increases faster with thickness than the random deposition limit [22]. This effect appears hard to rationalize in the absence of a thermodynamic driving force (e.g., dewetting). A model that is consistent with the scaling exponents involves random spatial inhomogeneities in the local growth rate, which are fixed during the growth process [25, 71]. It is plausible that when certain regions of the surface persistently grow faster than others, the surface will roughen very rapidly. It was suggested that the spatial inhomogeneities might be related to the different tilt domains of the film and the inevitable grain boundaries in between these [25].
Figure 1.5 Root mean square roughness of DIP films as a function of thickness . The inset shows a typical X-ray reflectivity data set and a fit to the data. The solid line in the main plot is a linear fit to the data and the growth exponent is obtained as . The dotted line denotes the random deposition limit . Adapted from Ref. [25].
Recently, these issues were followed by X-rays and optical spectroscopy in real-time in situ during growth [19, 72, 69] (Figure 1.6), and also steps made visible optically using near-field microscopy with a resolution as good as 17 nm [70].
Figure 1.6 (a) The DIP molecule and a schematic of standing () and lying () structures. (b) Evolution of the real-time X-ray reflectivity as a function of time (i.e., film thickness) during growth on silicon oxide (at 130 °C). (c) Layer coverages Θ(t) as obtained from fits to the above real-time data. A change from layer-by-layer growth to rapid roughening is clearly discernible after about 10 ML (see arrow). Adapted from Ref. [19].
1.3.3 Phthalocyanines
Phthalocyanines also tend to grow in a standing-up configuration in thicker films on “inert” substrates. Films of F16CuPc between 120 and 450 Å were recently found to exhibit very good crystalline out-of-plane order with rocking widths around 0.01° and well-defined Kiessig interferences and Laue satellites around the out-of-plane Bragg reflection [73].
The in-plane structure is, of course, azimuthally disordered, since the substrate is isotropic. One of the complications for phthalocyanines is a strong anisotropy of the crystal structure and the associated growth properties, which can lead to needle-like features, both for F16CuPc [74] and for H16CuPc [75]. The structure and the optical properties were recently studied in real time [74, 75] revealing changes during growth. Anomalous scaling behavior and surface roughening for H2Pc deposition were studied in Ref. [76].
1.3.4 Pentacene
Pentacene on silicon oxide has been studied intensely due to its relevance for OFETs, and it is impossible to provide a complete list of references here [9]. Ruiz et al. studied the initial stages of the growth [77]. Their analysis of the island distribution in (sub)monolayer films by dynamic scaling showed that the smallest stable island consisted of four molecules. Meyer zu Heringdorf et al. showed that under appropriate growth conditions the single-crystal grain sizes can approach 0.1 mm [78].
For thicker films, pentacene thin films exhibit some complication in the sense that there is a “thin film structure” and a “bulk structure,” which can coexist, depending on the growth conditions. Also, the defect structure is of interest [79]. Some light was shed on these issues in real-time growth studies [80, 81].
An interesting idea is that of surface modification involving self-assembled monolayers [63]. Shtein et al. studied the effects of film morphology and gate dielectric surface preparation on the electrical characteristics of organic vapor-phase-deposited pentacene thin-film transistors including surface modification using SAMs [64]. Meyer zu Heringdorf et al. employed cyclohexene saturation of Si(001) to modify the growth dynamics [78]. Voigt et al. studied the growth of tetracene on oil-covered surfaces [82]. While they actually used ITO as solid substrates, the concept might equally well be applicable to silicon oxide surfaces.
1.4 Films on Aluminum Oxide
Interfaces of organics with insulators are of obvious relevance for organic electronics, and aluminum oxide is one of the most commonly used insulators. Unfortunately, sputtered aluminum oxide layers frequently exhibit a rather high roughness and not well-defined starting conditions for growth studies. Sapphire is aluminum oxide (Al2O3) in its purest and best ordered form. It is also a popular substrate for epitaxy of metals and inorganic semiconductors, and it can be obtained in very high crystalline quality. We will focus here on sapphire, since it is very suitable for model studies of the growth of organics on insulator surfaces (see Section 1.6 for other substrates).
Surfaces of ionic substrates, which are not charge balanced, tend to be unstable and/or exhibit strong relaxations/reconstructions. In the case of sapphire, the () surface (“A plane”) is charge balanced and rather inert, and it has been used for growth studies. An important feature to realize for surfaces of crystals is that they commonly exhibit a miscut, that is, a difference between the physical surface and the (low-index) crystallographic plane. This gives rise to a step pattern, which in the case of essentially perfect crystals like sapphire is the dominating feature of the surface morphology (Figure 1.7). Issues related to the surface preparation have been discussed in Ref. [83].
Figure 1.7 Topography of an A-plane sapphire substrate (top) and an F16CuPc film (120 Å) (bottom) on this substrate determined by noncontact AFM. The step pattern of the substrate serves to azimuthally align the film (see text). Adapted from Ref. [84].
1.4.1 PTCDA
PTCDA on sapphire has, to our knowledge, not been studied in detail. Test results, however, indicate that the overall behavior is similar to that for PTCDA on oxidized silicon, that is, for growth at high temperatures the films tend to (partially) dewet (Krause et al., unpublished).
The overall growth scenario is most likely not changed significantly by the presence of steps, but the in-plane order of PTCDA may be affected. However, even with alignment at the step edges, PTCDA would most likely still exhibit multiple domains (see also the discussion of PTCDA on metals).
1.4.2 DIP
Based on the results for DIP on silicon oxide it is expected that DIP would also exhibit good out-of-plane ordering on the similarly “inert” sapphire. Preliminary data indicate that this is, in fact, the case (Osso et al., unpublished). In addition, the stepped sapphire substrate can induce in-plane ordering, as first demonstrated for the growth of phthalocyanines [84] (see below), which was also found for DIP (Osso et al., unpublished).
1.4.3 Phthalocyanines
As described above, the regular surface steps associated with miscut sapphire can serve as templates for film growth with azimuthal alignment. While the concept of stepped substrates has been used frequently for monolayer adsorbates, its use for comparatively thick films (5–50 ML) of relatively large molecules was first demonstrated by Osso et al. for F16CuPc on A-plane sapphire [84]. The resulting azimuthal ordering has been shown by four methods sensitive to different aspects of order [84]. AFM was used to image the surface morphology of the bare substrate. After film growth, the characteristic step pattern of the bare substrate was shown to be basically replicated, suggesting an azimuthal coupling of the film structure to the substrate morphology (Figure 1.7). In-plane X-ray diffraction (GIXD) showed that the crystal structure of the film was indeed not a 2D powder, but was aligned. The distribution width (“mosaicity” of the in-plane lattice) was several degrees broad, which suggests a rather soft/weak driving force for the ordering. The in-plane order was also visible in the azimuthal intensity distribution of the vibrational modes detected by Raman scattering. Finally, the resulting anisotropy of the dielectric function was studied by spectroscopic ellipsometry, offering the chance to study the “intrinsic” behavior of these systems without a strongly reduced disorder-induced broadening of the optical transitions. We should note that the strong optical anisotropies of these systems are an interesting field of study in their own right, and give rise to nontrivial effects in the propagation of light [54].
The out-of-plane ordering was similarly good for F16CuPc or DIP on silicon oxide, that is, a well-defined Bragg reflection with Laue oscillations and mosaicities around 0.01°, although the tendency of phthalocyanines to grow in needles can cause some complications. We note that the tilt angle of the molecules as well as the out-of-plane lattice parameter was found to depend on the growth temperature (and are different from the bulk structure parameters), indicating that the structure may not be in full equilibrium.
1.4.4 Pentacene
Similar concepts and mechanisms as observed for DIP and F16CuPc in terms of azimuthal alignment should be applicable to pentacene on sapphire, but to our knowledge there are no published results yet.
1.5 Films on Metals
Interfaces with metals are of obvious relevance for contacting organic semiconductors. The choice of the metal is frequently determined more by the desired work function and thus electron or hole injection properties than by growth considerations. Nevertheless, there are a wide variety of metals in terms of behavior as substrates for organic thin film growth, and it is important to realize that this can have a profound impact on the growth and the resulting structural and functional properties. Besides issues related to the surface morphology, crystalline quality, potentially crystalline orientation, and size of the unit cell (epitaxy), it is very important how “reactive” or “inert” the metal is, since this determines the mobility of the molecules on the surface and thus the growth.
For strongly reactive substrates, the molecules tend to behave almost in a “hit-and-stick” fashion, that is, without significant mobility and thus no long-range order. Less reactive metals such as noble metals, to which we will limit ourselves here, turned out to be rather popular and suitable.
We will concentrate on metal single crystals. From a practical point of view, for growth studies it is important that their surfaces can be “recycled” by sputtering and annealing, that is, several growth experiments can be performed on the same substrate and on (essentially) the same surface. Less reactive metals are also easier to keep clean before growth. Obviously, with metal substrates the application of electron-based surface science methods is possible, since the signal does not suffer from charging effects. This has been used excessively by the surface science community in particular for molecular monolayers on surfaces of metal single crystals.
We should also mention that metal surfaces are suitable for surface modification using SAMs [62, 63], which has been employed in particular for Au(111). Examples include the growth of PTCDA on alkanethiol SAMs [85–88].
1.5.1 PTCDA
PTCDA on metal surfaces has been thoroughly studied, with the noble metals being particularly popular.
1.5.1.1 Structure and Epitaxy of PTCDA/Ag(111)
On Ag(111), well-defined epitaxial growth of PTCDA(102) has been observed [4, 26, 30, 32]. The 2D structure is characterized by a herringbone arrangement of the flat-lying molecules, which corresponds to a layer of the (102) plane of the bulk structure, with a small degree of distortion (strain). Possible mechanisms leading to the well-behaved 2D structure of PTCDA on Ag(111) were discussed in Ref. [89]. The vertical PTCDA-Ag(111) spacing was found to be Å based on X-ray diffraction [31], but it may differ for low-temperature deposition if the adsorption state differs. Subsequently, still more precise and even element-resolved vertical bonding geometries of PTCDA monolayers were determined using XSW [90, 91].
Overall, in particular monolayers of PTCDA on Ag(111) have been studied in detail using a broad range of techniques; for a collection of references see Refs [92, 93].
For growth extending beyond the monolayer, a more complex azimuthal distribution arises, and, depending on the growth temperature, also domains noncollinear with principal axes of the substrate can form to relieve strain [30]. Interestingly, the epitaxial relations could be rationalized similar to the Nishiyama–Wassermann versus Kurdjumov–Sachs relations for fcc(111)/bcc(110) growth, although the PTCDA structure is not bcc [30].
We note that PTCDA/Ag(111) has also been subject of a number of spectroscopic studies. One of the challenges is understanding the molecule–substrate interaction, which is “between pure van der Waals and clear covalent binding.” Details are beyond the scope of this chapter (see Ref. [93] and references therein).
1.5.1.2 Comparison with Other Substrates
The comparison with PTCDA/Au(111) yields a qualitatively similar picture [28, 33, 94], although details of the epitaxy appear to differ, which is not too surprising given that structure is a result of a rather delicate balance of different factors and given that the corrugation of the substrate potential experienced by PTCDA is different.
It is interesting to compare monolayers of PTCDA on different noble metal surfaces. The tendency is as expected from “a stronger chemical interaction” on Cu(111) via the intermediate case on Ag(111) to the weakest interaction on Au(111), which is seen both in the vertical bonding distances and in the valence band spectra [91, 95, 96].
On the more open Ag(110) surface, an entirely different structure already in the monolayer was found, characterized by a “brick-stone” arrangement [26]. Phase transitions of PTCDA/Ag(110) were studied in Ref. [97].
Growth on Cu(110) was studied by Möller's group [98–100]. The monolayer was found to differ from those known from other substrates. For thicker films, Stranski–Krastanov growth was found, similar to the case on Ag(111) (see below).
1.5.1.3 Dewetting and Thermal Properties
While the structure and epitaxy in the monolayer regime are well defined, the later stages of the growth (potentially after a certain threshold thickness) can, of course, exhibit islanding and a very rough resulting morphology. It was found that PTCDA on Ag(111), a well-behaved system in the monolayer regime, indeed exhibits Stranski–Krastanov growth. At growth temperatures T 350 K, relatively smooth epitaxial films have been found, whereas at T 350 K, well-separated crystallites with bulk crystalline structure on top of a 2 ML thick wetting layer have been observed [30, 31, 101, 102]. These results are qualitatively the same as those for PTCDA on Au(111) [28].
The thermally induced post-growth dewetting of “low-temperature” grown films was also studied, confirming that the films tend to dewet if given sufficient thermal energy [31]. In these experiments, the thermal expansion of PTCDA was also determined ( out of plane) [31]. For a comparison with other systems (Alq3 and TPD), see Ref. [103]. While islanding of the films is usually not desirable, it should be pointed out there might also be ways to exploit islanding or dewetting and the formation of small crystallites for “self-organized nanostructures” (similar to Si–Ge quantum dots).
1.5.1.4 Real-Time Growth
In order to shed light on the dynamics and the temperature dependence of the 2D–3D transition (layer by layer to islanding), a real-time X-ray diffraction study of the growth of PTCDA on Ag(111) was performed [102]. The idea is as follows (Figure 1.8). In kinematic theory the specular X-ray scattering intensity is the sum of the scattering contributions from the film and the substrate,
(1.2)
where and are the form factors of the film and the substrate, respectively, and are the corresponding lattice spacings, and Å is the distance between the substrate and the first layer of the film [30]. is the time-dependent fractional coverage of the nth layer within the organic film. At the anti-Bragg point of the PTCDA film (), the first term in Eq. (1.2) equals . Therefore, the coverage difference
(1.3)
can be deduced from the measured intensity . Specifically, it is possible to distinguish the coverage of the first and the second layer in the initial stage of the growth. In the case of layer-by-layer growth, characteristic intensity oscillations are observed.
Figure 1.8 Simulation of the specular rod scattering of a thin PTCDA film on Ag(111) as a function of the out-of-plane momentum transfer (top). The time axis in this figure (for a fixed ) indicates the intensity oscillations at the anti-Bragg point during growth (see text for explanation). The central figure shows the time dependence (in units of monolayer deposition times) of the normalized scattered intensity at the anti-Bragg point for various temperatures (233 K (red), 283 K (green), 303 K (blue), and 358 K (black)). The bottom figure shows the temperature dependence of the deviation from layer-by-layer growth expressed in terms of the intensity of the minimum (open symbols) and of the maximum (filled symbols) of the scattered intensity at 1 ML and 2 ML, respectively (see text for details). Adapted from Ref. [102].
Figure 1.8 shows typical time-dependent intensity data during growth in a dedicated chamber [104], measured at various substrate temperatures between 233 and 258 K [102]. is defined as the starting time of the deposition. The signal is normalized to the substrate scattering intensity, , and the time is normalized to the deposition time, , of one monolayer, which corresponds to the intensity minimum. A typical growth measurement exhibits distinct intensity oscillations for t, followed by a constant intensity during further deposition, similar to the observations for PTCDA/Au(111) [105]. The intensity oscillations correspond to layer-by-layer growth. The transition to a constant intensity indicates the breakdown of layer-by-layer growth and the onset of islanding characteristic of SK growth. As can be seen from the transition to a time-independent scattering signal (associated with an equal probability for a given molecule to be accommodated in even and odd layers), the islanding starts rapidly after completion of a 2 ML “wetting” layer.
Comparing the growth data for different temperatures (Figure 1.8), we find that for the oscillations are not visibly damped for . They are followed by a sharp transition to a time-independent intensity (islanding). For lower temperatures, the oscillations are progressively damped, and the 2D–3D transition is smeared out as the temperature is lowered.
The experimental data, that is, in particular the 2D–3D transition, could be modeled by kinetic Monte Carlo simulations using a relatively simple model for the energies/barriers, the most important feature of which is the dependence of the interlayer transport barrier, , on the layer number , namely, and [102].
The growth of PTCDA/Ag(111) could also be monitored in real time in real space using PEEM [106]. Moreover, for elevated temperatures strong postgrowth diffusion was observed [107].
1.5.2 DIP
In the monolayer regime, DIP, like many other organic semiconductors, was studied by STM. The molecules were found to be lying down flat on the substrate [108]. The interaction of DIP with Au was found to be physisorptive[43]. In the regime of thicker films, DIP was studied in detail on Au contacts [43] (and as substrate for Au contacts evaporated on DIP [43, 66–68]). Importantly, in contrast to growth on silicon oxide, due to the stronger interaction with the Au substrate, the lying-down configuration tends to prevail not only for monolayers but also for multilayers. Since the standing-up configuration (which again followed a herringbone-like motif) appears to have the more favorable surface energy (as seen on silicon oxide), there is obviously a competition between the two configurations (standing-up versus lying-down), and they are found to coexist [43]. From the point of view of growth kinetics, this competition is very interesting, but it is certainly a further complication and an additional source of disorder that is usually undesirable. These issues were recently studied using (electronic) spectromicroscopy [109]. Furthermore, mixed films of DIP and CuPc were studied by STM in the monolayer regime [110].
1.5.3 Phthalocyanines
Phthalocyanines were among the first “large” molecules that were studied by STM with (sub)molecular resolution [50]. In the monolayer regime, the molecules lie down flat on the surface, and the 2D structures have been thoroughly studied. Recently, the (vertical) bonding distance to the metal substrate was determined using XSW [2, 111, 112]. For thicker films, there is a competition between the lying-down configuration of the first layer and the tendency to stand up. The impact of roughness on the ordering behavior was studied in Ref. [113].
1.5.4 Pentacene
Acenes on metal substrates were studied by several groups. Early work on the orientation of various aromatic hydrocarbons including tetracene on metal surfaces using NEXAFS was done by Koch and coworkers [114].
More recent work focused on pentacene. Pentacene structures on Au(111) as a function of coverage (up to the equivalent of around 2 ML) were studied by Parkinson's group [115]. A very detailed study of pentacene on clean and SAM-modified gold surfaces was presented by Käfer et al. [116].
The structure and binding distance as well as associated electronic properties such as work function and interface dipole of PEN and also of PFP on Cu(111) were determined with high precision in Ref. [117].
In the monolayer regime of pentacene on Cu(110), Lukas et al. reported a novel mechanism giving rise to long-range order on Cu(110), based on the modulation of the adsorption energy due to charge-density waves related to a surface state [118]. While it is not too surprising that the molecules in the monolayer regime tend to be lying more or less flat on the surface, importantly, for the growth of thicker films on Cu(110) an orientational transition from a lying-down configuration to an essentially standing-up configuration was observed [119].
An interesting study of the “hyperthermal” growth of pentacene (exhibiting hyperthermal energies in a seeded supersonic molecular beam) on Ag(111) was presented by Casalis et al. [120]. They found that at low substrate temperatures (200 K), highly ordered films can be grown by hyperthermal deposition when thermal deposition leads only to disordered films. The results were interpreted as a result of “local annealing” due to the impact of the hyperthermal molecules. This technique appears to have the potential to tailor the growth of molecular systems in addition to what is possible by changing the impingement rate and the substrate temperature, and it may be further tested in the future (see Ref. [61] for recent work on PFP).
1.6 Films on Other Substrates
Many substrates other than the above ones have been employed, all of which we cannot review. We shall mention only some of the most important other substrates.
Quite popular for growth studies is graphite, since it is easy to prepare. In our general classification of substrates, graphite would be “weakly interacting.” For spectroscopic studies, this offers the opportunity to study the film without strong “interference” from the substrate; see, for example, Ref. [121] and references therein. Other examples from this group of layered substrates are MoS2, GeS, and Sb2S3 [6].
Also rather weakly interacting would be MgO, which falls essentially in the same category as sapphire and silicon oxide. Mica, which can be easily prepared by cleavage, may also be seen in the category of rather inert substrates. It can also be used well for real-time differential reflectance spectroscopy (DRS) [122].
Alkalihalogenides, such as NaCl and KCl, are quite popular as simple substrates for growth studies, since they are easy to prepare. For some studies, they have the additional benefit that they can be easily dissolved and the film can be studied by TEM.
Metals, as indicated above, span a broad range from the noble metals to very reactive substrates.
A very important class of substrates are certainly (inorganic) semiconductors such as Si and GaAs, since they may be used in the integration of organic–inorganic hybrid devices. Moreover, they are very well defined in terms of their surface and overall structural quality, which is favorable for growth studies. If the surface is clean, however, they can exhibit “dangling bonds” and be rather reactive. In these cases, the organic adsorbates then tend to “hit and stick,” that is, they usually do not diffuse over significant distances, hence they do not form long-range ordered structures. A strategy to avoid these problems, but still benefit from the above advantages, is the use of surface-passivated semiconductors, such as H–Si or Se–GaAs.
Polymeric substrates and possible routes for oriented growth of pentacene have been studied in Ref. [123].
1.7 More Complex Heterostructures and Technical Interfaces
Organic semiconductor devices frequently do not only consist of a film on a substrate, but involve additional layers such as metal contacts or insulating layers or also different organic components in a multilayer structure.
1.7.1 Inorganic–Organic Heterostructures
Metal contacts are one obvious requirement for many applications of organic semiconductors. It turns out that the controlled deposition of metals on organics (“top electrode”) is nontrivial. In order to reduce problems related to interdiffusion (and ultimately short-circuiting) and traps related to surface states, different strategies can be pursued.
