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- By Jane E. Adcock, Yahya Aghakhani, A. Anand, Eva Andermann, Frederick Andermann, Alexis Arzimanoglou, Sandrine Aubert, Nadia Bahi-Buisson, Carman Barba, Agatino Battaglia, Geneviève Bernard, Nadir E. Bharucha, Laurence A. Bindoff, William Bingaman, Francesca Bisulli, Thomas P. Bleck, Stewart G. Boyd, Andreas Brunklaus, Harry Bulstrode, Jorge G. Burneo, Laura Canafoglia, Laura Cantonetti, Roberto H. Caraballo, Fernando Cendes, Kevin E. Chapman, Patrick Chauvel, Richard F. M. Chin, H. T. Chong, Fahmida A. Chowdhury, Catherine J. Chu-Shore, Rolando Cimaz, Andrew J. Cole, Bernard Dan, Geoffrey Dean, Alessio De Ciantis, Fernando De Paolis, Rolando F. Del Maestro, Irissa M. Devine, Carlo Di Bonaventura, Concezio Di Rocco, Henry B. Dinsdale, Maria Alice Donati, François Dubeau, Michael Duchowny, Olivier Dulac, Monika Eisermann, Brent Elliott, Bernt A. Engelsen, Kevin Farrell, Natalio Fejerman, Rosalie E. Ferner, Silvana Franceschetti, Robert Friedlander, Antonio Gambardella, Hector H. Garcia, Serena Gasperini, Lorenzo Genitori, Gioia Gioi, Flavio Giordano, Leif Gjerstad, Daniel G. Glaze, Howard P. Goodkin, Sidney M. Gospe, Andrea Grassi, William P. Gray, Renzo Guerrini, Marie-Christine Guiot, William Harkness, Andrew G. Herzog, Linda Huh, Margaret J. Jackson, Thomas S. Jacques, Anna C. Jansen, Sigmund Jenssen, Michael R. Johnson, Dorothy Jones-Davis, Reetta Kälviäinen, Peter W. Kaplan, John F. Kerrigan, Autumn Marie Klein, Matthias Koepp, Edwin H. Kolodny, Kandan Kulandaivel, Ruben I. Kuzniecky, Ahmed Lary, Yolanda Lau, Anna-Elina Lehesjoki, Maria K. Lehtinen, Holger Lerche, Michael P. T. Lunn, Snezana Maljevic, Mark R. Manford, Carla Marini, Bindu Menon, Giulia Milioli, Eli M. Mizrahi, Manish Modi, Márcia Elisabete Morita, Manuel Murie-Fernandez, Vivek Nambiar, Lina Nashef, Vincent Navarro, Aidan Neligan, Ruth E. Nemire, Charles R. J. C. Newton, John O'Donavan, Hirokazu Oguni, Teiichi Onuma, Andre Palmini, Eleni Panagiotakaki, Pasquale Parisi, Elena Parrini, Liborio Parrino, Ignacio Pascual-Castroviejo, M. Scott Perry, Perrine Plouin, Charles E. Polkey, Suresh S. Pujar, Karthik Rajasekaran, R. Eugene Ramsey, Rahul Rathakrishnan, Roberta H. Raven, Guy M. Rémillard, David Rosenblatt, M. Elizabeth Ross, Abdulrahman Sabbagh, P. Satishchandra, Swati Sathe, Ingrid E. Scheffer, Philip A. Schwartzkroin, Rod C. Scott, Frédéric Sedel, Michelle J. Shapiro, Elliott H. Sherr, Michael Shevell, Simon D. Shorvon, Adrian M. Siegel, Gagandeep Singh, S. Sinha, Barbara Spacca, Waney Squier, Carl E. Stafstrom, Bernhard J. Steinhoff, Andrea Taddio, Gianpiero Tamburrini, C. T. Tan, Raymond Y. L. Tan, Erik Taubøll, Robert W. Teasell, Mario Giovanni Terzano, Federica Teutonico, Suzanne A. Tharin, Elizabeth A. Thiele, Pierre Thomas, Paolo Tinuper, Dorothée Kasteleijn-Nolst Trenité, Sumeet Vadera, Pierangelo Veggiotti, Jean-Pierre Vignal, J. M. Walshe, Elizabeth J. Waterhouse, David Watkins, Ruth E. Williams, Yue-Hua Zhang, Benjamin Zifkin, Sameer M. Zuberi
- Edited by Simon D. Shorvon, Frederick Andermann, Renzo Guerrini
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- The Causes of Epilepsy
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- 05 March 2012
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- 14 April 2011, pp ix-xvi
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7 - Examples
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp 84-139
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Summary
Background
In this chapter, the chemical and electronic structure of the interfaces between low work function metals and prototypical conjugated oligomers and polymers, appropriate for polymer LEDs, which have been studied over the past several years, are discussed. One of the underlying themes in these discussions is aimed at illustrating the combined experimental–theoretical approach, modelling the initial stages of the interface formation corresponding to the deposition of the metal onto the polymer surface, which has proven to be a very fruitful practice, yielding more than the simple sum of the individual parts. Following the description of the pertinent experimental sample preparation procedures, the examples are arranged in an order convenient for developing certain themes in the logic of the descriptions of the physical phenomena. More detail is supplied in the first examples, whereas less is supplied in subsequent examples, essentially pointing out differences. Also, an order has been chosen for presentation which minimizes the necessity of repetition of certain aspects.
From the experimental standpoint, the chemical and electronic structures of the surfaces of thin films of the conjugated materials are first studied in their pristine state, in order to generate surface electronic band structure parameters for input into device performance models. Then these surfaces are studied as metal atoms are slowly deposited, as the early stages for formation of the metallic overlayer develops. Two surface-sensitive techniques are used, each of which directly represents somewhat different but related aspects of the electronic structure of the polymer.
Index
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp 155-157
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4 - Materials
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp 50-63
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Summary
Molecular solids
Molecular solids are composed of discrete units which retain their identities as molecules in the solid phase, for example, when condensed from the gas phase into thin films. Molecular solids may be single crystals, polycrystalline or amorphous in structure. Bonding within the molecules is usually covalent, and is localized to within the molecular units. There are usually no inter-molecular covalent bonds, only weak Van der Waals forces. On the other hand, there are systems which are molecular yet more firmly bound in the solid, e.g., the amino acids and other hydrogen-bonded molecular solids. Hydrogen bonding in the zwitterionic state of condensed molecular solids has been studied by photoelectron spectroscopy, the primary experimental tool used in the results presented in this book.
In chapter 7, all works discussed on model molecular systems for conjugated polymers refer to condensed molecular solid ultra-thin films, generally prepared by condensation of molecules from the effusion of a Knudsen-type cell, in UHV, on to clean metallic substrates held at low temperatures. Clean is defined as atomically clean as determined by core-electron level XPS, such that there is intimate contact between the molecules at the substrate–film interface, without the influence of, for example, a metallic oxide, hydrocarbon or other contamination. In general, non-cross-linked polymers, and linear conjugated polymers in particular, comprise a special case of molecular solids. Exceptions are pointed out, as necessary.
π-Conjugated polymers
In linear polymer molecules which are not conjugated, the electronic structure of the chain of atoms or chemical groups which comprises the backbone of the macromolecule consists of only σ-bands (possibly with localized π-electronic levels).
3 - Experimental methods
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp 33-49
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Summary
Survey of measurement methods
Here we survey a series of possible surface-sensitive measurements which in principle can be used to study the surfaces of conjugated polymers and the early stages of metal interface formation. We then motivate the use of photoelectron spectroscopy.
NEXAFS, or near edge X-ray absorption fine-structure spectroscopy, is one method of particular interest, which provides both chemical and electronic information. NEXAFS, as photoelectron spectroscopy, is a photon-in-electron-out spectroscopy, and, therefore, surface sensitive. The basis for NEXAFS is carried out using polarized, monochromatized synchrotron radiation, which leads to sensitivity to molecular orientation, which can be of particular use in studying ordered polymer systems. Also, two electron probe methods should be mentioned. The first is high resolution low energy electron energy loss spectroscopy, HREELS. Generally somewhat destructive to organic surfaces, and fraught with serious electronic charging problems when used on electrically insulating organic surfaces, some success has been achieved with HREELS in special cases. Both electronic structure and vibrational information have been obtained by Pireaux and co-workers. Polarization effects are inherent, which can be important in studying ordered systems. The second method is that of high energy electron energy loss spectroscopy, which measures the momentum-dependent energy loss function. Information on the electronic transitions, as in optical spectroscopy, is obtained as a function of the momentum transfer within the electron scattering event. Free standing films of about 1000 Å thickness (or less) are required to minimize multiple scattering in transmission measurements. In addition, the equipment is very specialized and not available commercially.
6 - Optical absorption and emission in conjugated oligomers and polymers
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp 72-83
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Summary
In this section, the basic features of light absorption and emission (luminescence) processes in conjugated systems are reviewed. The discussion will focus on poly (p-phenylenevinylene), PPV, compounds, which provide typical examples of the physical phenomena to be highlighted in the context of polymer-based light emitting devices.
Optical absorption
In most cases, conjugated polymers present broad, inhomogeneous optical absorption bands that become more resolved when the material can be prepared in a more ordered way. This broadness originates in a number of features such as distribution of chain lengths, presence of chain bends, chain twists, or other defects, which contribute to the determination of an effective conjugation length. In the best-ordered PPV samples, the extent of effective conjugation length is estimated to be on the order of 15–20 phenylene vinylene units, by making comparison among a series of well-defined phenylene vinylene oligomers and PPV-based polymers.
Another most notable aspect relates to the vibronic coupling, that is the coupling between electronic excitations and vibrational modes. As has been stressed many times in the literature, much of the rich and fascinating physics of conjugated polymers is based on the strong connection between the electronic structure and the geometric structure: any electronic process (be it photoexcitation or charge transfer upon reduction, oxidation, or protonation i.e., doping, of the polymer chains) results in significant geometry relaxations that in turn modify the electronic properties.
Preface
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp x-xii
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Summary
The recent flourish in activities, both in academia and industry, in research on polymer-based light emitting devices has prompted a series of studies of the surfaces of conjugated polymers, and the early stages of interface formation when metals are vapour-deposited on these conjugated polymer surfaces. The summary of works presented here is, to a great extent, the result of a long and comprehensive co-operation between both the Laboratory of Surface Physics and Chemistry, and the Molecular Theory group within the Laboratory for Theoretical Physics, at the Department of Physics (IFM) at Linköping University, Sweden, and the Laboratory for Chemistry of Novel Materials and the Center for Research on Molecular Electronics and Photonics, at the University of Mons-Hainaut, Belgium. The results brought forth within this collaboration represent the product of a comprehensive combined experimental–theoretical approach to the study of the electronic and chemical structure of conjugated polymer surfaces and interfaces. The scope of the output of such a combined approach is greater than the sum of the (theory and experimental) parts. Following about 10 years' work, we were asked to compile a summary of the highlights of our studies in this area. This monograph represents a response to that request.
For generation of this book, we acknowledge specific support from the Commission of the European Union within the Network of Excellence on Organic Materials for Electronics NEOME). For participation in these activities, we acknowledge Commission support through the SCIENCE program (Project 0661 POLYSURF), the Brite/EuRam program (Project 7762 Poly LED), and the ESPRIT program (Project 8013 LEDFOS).
Conjugated Polymer Surfaces and Interfaces
- Electronic and Chemical Structure of Interfaces for Polymer Light Emitting Devices
- W. R. Salaneck, S. Stafstrom, J. L. Brédas
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- 12 January 2010
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- 10 October 1996
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The authors illustrate the basic physics and materials science of conjugated polymers and their interfaces, particularly, but not exclusively, as they are applied to polymer-based light emitting diodes. The approach is to describe the basic physical and associated chemical principles that apply to these materials, which in many instances are different from those that apply to their inorganic counterparts. The main aim of the authors is to highlight specific issues and properties of polymer surfaces and interfaces that are relevant in the context of the emerging field of polymer-based electronics in general, and polymer-based light emitting diodes in particular. Both theoretical and experimental methods used in the study of these systems are discussed. This book will be of interest to graduate students and research workers in departments of physics, chemistry, electrical engineering and materials sciences studying polymer surfaces and interfaces and their application in polymer-based electronics.
8 - The nature of organic and molecular solid surfaces and interfaces with metals
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp 140-154
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Summary
It is perhaps obvious that the nature of the interface between a molecular solid (polymer) and a (clean) metal surface is not necessarily equivalent to the interface formed when a metal is vapor-deposited (essentially ‘atom-by-atom’) on to the (clean) surface of the polymer or molecular solid. Atoms of all metals are active ‘in the form of individual atoms’, even gold atoms. In the context of the new polymer LEDs, some of the works discussed in chapter 7 involve the study of the early stages of formation of the interface in the latter configuration (metal-on-polymer interfaces). Very little has been reported on conjugated polymer-on-metal interfaces, however, primarily because of the difficulties in preparing ‘monolayers’ of polymer materials on well defined metal substrates appropriate for study (via PES or any other surface sensitive spectroscopy). The issues discussed below are based upon information accumulated over two decades of involvement with the surfaces of condensed molecular solids and conjugated polymers in ultra-thin form, represented by the examples in the previous chapter.
Polymer surfaces
It is most straightforward to begin with a brief description of the very nature of the free polymer surface, following which the polymer-on-metal interface is described, before returning to additional facts about the free polymer surface. Some fundamental differences between ideal polymer surfaces and those of conventional inorganic semiconductors are outlined below.
1 - Introduction
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp 1-7
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Summary
Although electroluminesence from organic materials has been known for a long time, research on light emitting diodes based upon conjugated polymers, with quantum efficiencies attractive for consideration in real devices, is quite new and currently growing into a topic commanding the attention of a wide variety of scientists and engineers, in both industry and academia, the world over. A great deal of the physics, and especially the chemistry, which governs the behavior of polymer-LEDs, occurs at the polymer surface, or the near surface region. The details are greatly determined by the metalic contact. Information obtained from detailed studies of the chemical and electronic structure of conjugated polymer surfaces and interfaces with metals, is becoming a basic ingredient in understanding device behaviour and optimizing device performance.
In this book, we attempt to bring together in one place the results of a relatively large number of basic studies of conjugated polymer surfaces, as well as the ‘early stages of metal–polymer interface formation’, in an attempt to produce a simple and coherent picture of some of the unique features of these surfaces and interfaces; features which are important in understanding and controling the performance of polymer-based LEDs. Instead of presenting a series of detailed chronological accounts of individual studies, we have tried to take a more global approach, at least in part, where the nature of the information allows, in order to make the book more comprehensible to readers from a range of different backgrounds.
Frontmatter
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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Contents
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 10 October 1996, pp vii-ix
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2 - Theory
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 10 October 1996, pp 8-32
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Summary
Introduction
The purpose of performing calculations of physical properties parallel to experimental studies is twofold. First, since calculations by necessity involve approximations, the results have to be compared with experimental data in order to test the validity of these approximations. If the comparison turns out to be favourable, the second step in the evaluation of the theoretical data is to make predictions of physical properties that are inaccessible to experimental investigations. This second step can result in new understanding of material properties and make it possible to tune these properties for specific purposes. In the context of this book, theoretical calculations are aimed at understanding of the basic interfacial chemistry of metal-conjugated polymer interfaces. This understanding should be related to structural properties such as stability of the interface and adhesion of the metallic overlayer to the polymer surface. Problems related to the electronic properties of the interface are also addressed. Such properties include, for instance, the formation of localized interfacial states, charge transfer between the metal and the polymer, and electron mobility across the interface.
In this chapter we discuss theoretical modelling, approximation schemes, and calculation methods. The description of the methods is on the level where we focus on the main steps in the theoretical development and not on details related to the solution of the resulting equations. For details of the derivation and evaluation of these equations we refer to some of the excellent books or reviews that are available.
5 - Device motivation for interface studies
- W. R. Salaneck, Linköpings Universitet, Sweden, S. Stafstrom, Linköpings Universitet, Sweden, J. L. Brédas, Université de Mons-Hainaut, Belgium
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- Conjugated Polymer Surfaces and Interfaces
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- 12 January 2010
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- 10 October 1996, pp 64-71
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Summary
Introduction
The application we have in mind for the metal–polymer interfaces discussed in this book is primarily that where the polymer serves as the electroactive material (semiconductor) in an electronic device and the metal is the electric contact to the device. Metal–semiconductor interfaces, in general, have been the subject of intensive studies since the pioneering work of Schottky, Strömer and Waibel, who were the first to explain the mechanisms behind the rectifying behaviour in this type of asymmetric electric contact. Today, there still occur developments in the understanding of the basic physics of the barrier formation at the interface, and a complete understanding of all the factors that determine the height of the (Schottky) barrier is still ahead of us.
The aim of this chapter is to present a simple but general band structure picture of the metal–semiconductor interface and compare that with the characteristics of the metal–conjugated polymer interface. The discussion is focused on the polymer light emitting diode (LED) for which the metal–polymer contacts play a central role in the performance of the device. The metal–polymer interface also applies to other polymer electronic devices that have been fabricated, e.g., the thin-film field-effect transistor, but the role of the metal–polymer interface is much less cruical in this case and therefore less relevant for detailed studies of the type discussed here.
The materials (metals and conjugated polymers) that are used in LED applications were introduced in the previous chapter.