The Molecule–Metal Interface · 3.5.2 Magnetic Molecule/Magnetic Metal Interfaces 79 3.6 ConcludingRemarks 81 References 81 Part Two Atomic Structure 91 4 STM Studies of Molecule–Metal

The Molecule–Metal Interface

Edited by

Norbert Koch, Nobuo Ueno, andAndrew T.S. Wee

Related Titles

Bruetting, W., Adachi, C., Holmes, R. J.(eds.)

Physics of OrganicSemiconductors2012

ISBN: 978-3-527-41053-8

Klauk, H. (ed.)

Organic ElectronicsMaterials, Manufacturing andApplications

2012

ISBN: 978-3-527-32647-2

Kampen, T. U.

Low Molecular Weight OrganicSemiconductors2010ISBN: 978-3-527-40653-1

Brillson, L. J.

Surfaces and Interfaces ofElectronic Materials2010ISBN: 978-3-527-40915-0

Butt, H.-J., Kappl, M.

Surface and Interfacial Forces2010ISBN: 978-3-527-40849-8

Stallinga, P.

Electrical Characterization ofOrganic Electronic Materialsand Devices2009

ISBN: 978-0-470-75009-4

Förch, R., Schönherr, H., Jenkins, A. T. A.(eds.)

Surface Design: Applicationsin Bioscience andNanotechnology

2009ISBN: 978-3-527-40789-7

Schwoerer, M., Wolf, H. C.

Organic Molecular Solids

2007ISBN: 978-3-527-40540-4

The Molecule–Metal Interface

Edited by

Norbert Koch, Nobuo Ueno, andAndrew T.S. Wee

WILEY-VCH Verlag GmbH & Co. KGaA

The Editors

Prof. Norbert KochHumboldt Universität zu BerlinInstitut für PhysikBerlin, [email protected]

Prof. Nobuo UenoChiba UniversityGraduate School of Advanced IntegrationScienceChiba, Japan

Prof. Andrew T.S. WeeNational Univ. of SingaporeDepartment of PhysicsSingapore

Cover PictureLowest unoccupied molecular orbital of5,7,12,14-pentacenetetrone on the Au(111)surface as calculated by density-functionaltheory. The illustration was created with VMD.VMD was developed by the Theoretical andComputational Biophysics Group in theBeckman Institute for Advanced Science andTechnology at the University of Ilinois atUrbana-Champaign. Courtesy of G. Heimel.

All books published by Wiley-VCH are carefullyproduced. Nevertheless, authors, editors, andpublisher do not warrant the informationcontained in these books, including this book, tobe free of errors. Readers are advised to keep inmind that statements, data, illustrations,procedural details or other items mayinadvertently be inaccurate.

Library of Congress Card No.:applied for

British Library Cataloguing-in-Publication Data:A catalogue record for this book is availablefrom the British Library.

Bibliographic information published by theDeutsche NationalbibliothekThe Deutsche Nationalbibliothek lists thispublication in the Deutsche Nationalbibliografie;detailed bibliographic data are available on theInternet at http://dnb.d-nb.de.

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA,Boschstr. 12, 69469 Weinheim, Germany

All rights reserved (including those of translationinto other languages). No part of this book maybe reproduced in any form – by photoprinting,microfilm, or any other means – nor transmittedor translated into a machine language withoutwritten permission from the publishers.Registered names, trademarks, etc. used in thisbook, even when not specifically marked assuch, are not to be considered unprotected bylaw.

Composition le-tex publishing services GmbH,LeipzigPrinting and Binding Markono Print MediaPte Ltd, SingaporeCover Design Grafik-Design Schulz,Fußgönheim

Print ISBN 978-3-527-41060-6ePDF ISBN 978-3-527-65320-1ePub ISBN 978-3-527-65319-5mobi ISBN 978-3-527-65318-8oBook ISBN 978-3-527-65317-1

Printed in Singapore

Printed on acid-free paper

V

Contents

Preface XI

List of Contributors XIII

1 Introduction to the Molecule–Metal Interface 1Nobuo Ueno, Norbert Koch, and Andrew T.S. Wee

1.1 From Organic Semiconductors to Organic Electronic Devices 11.2 Role and Function of Interfaces in Organic Electronic Devices 31.3 What Will We Learn about the Interfaces? 41.4 The Fermi Level and Related Fundamentals 61.4.1 Definition of the Fermi Level in this Book 61.4.2 Measuring the Fermi Level of Organic Semiconductors 81.4.3 The Work Function and the Vacuum Level of a Solid with Finite Size 9

References 13

Part One Theory 15

2 Basic Theory of the Molecule–Metal Interface 17Fernando Flores and José Ortega

2.1 Introduction 172.2 The Molecule Energy Gap Problem: Image Potential Effects 202.2.1 Molecule Self-Interaction Energy 202.2.2 Image Potential Effects 252.3 The Unified IDIS Model: Charge Transfer, Pauli Exclusion Principle

(“Pillow”) Effect and Molecular Dipoles 282.3.1 The IDIS Model 282.3.2 Pauli Repulsion (“Pillow”) Effect and the Unified IDIS-Model 312.3.3 Molecular Dipole Corrections and the Unified IDIS Model 342.4 DFT Calculations for a Single Molecule on a Surface 352.4.1 C60 on Au(111) 352.4.2 TCNQ/Au(111) 382.4.3 TTF on Au(111) 392.5 From a Single Molecule to a Monolayer 422.5.1 The Unified IDIS Model for an Organic Ad-layer on a Metal 422.5.2 C60/Au(111) 43

VI Contents

2.5.3 TTF/Au(111) 452.5.4 More on the Unified IDIS Model 46

References 48

3 Understanding the Metal–Molecule Interface from First Principles 51Leeor Kronik and Yoshitada Morikawa

3.1 Introduction 513.2 A Brief Overview of Density Functional Theory 533.3 Electronic Structure of Metal–Molecule Interfaces from Density

Functional Theory: Challenges and Progress 593.4 Understanding Metal–Molecule Interface Dipoles from First

Principles 643.4.1 n-Alkane/Metal Interfaces 673.4.2 Benzene/Metal Interfaces 683.4.3 Pentacene/Metal Interfaces 723.4.4 PTCDA/Metal Interfaces 753.5 Two Examples of Collective Effects at Metal–Molecule Interfaces 773.5.1 Quantum-Confined Stark Effect in Monolayers of Molecules Consisting

of Polar Repeating Units 773.5.2 Magnetic Molecule/Magnetic Metal Interfaces 793.6 Concluding Remarks 81

References 81

Part Two Atomic Structure 91

4 STM Studies of Molecule–Metal Interfaces 93Swee Liang Wong, Han Huang, Andrew T.S. Wee, and Wei Chen

4.1 Introduction to Scanning Tunneling Microscopy 944.1.1 Basic STM Operation 944.1.2 Theory of STM 964.1.3 Scanning Tunneling Spectroscopy 984.2 Factors Affecting Molecular Packing on Perfect Surfaces 1004.2.1 Molecule–Substrate vs. Intermolecular Interactions 1004.2.2 Commensurability with Substrate 1034.2.3 Molecular Density Dependent Phase Transitions 1044.3 Influence of Inhomogeneity at Metal Surfaces 1064.3.1 Physical Inhomogeneity at Crystalline Interfaces 1064.3.2 Surface Electronic States 1084.3.3 Molecule-Induced Modification of Surface Topography 1094.4 Manipulation of Molecules Using STM 1124.5 Summary 116

References 116

5 NEXAFS Studies of Molecular Orientations at Molecule–SubstrateInterfaces 119Dong-Chen Qi, Wei Chen, and Andrew T.S. Wee

5.1 Principles of NEXAFS 120

Contents VII

5.1.1 The X-Ray Absorption Process 1205.1.2 Molecular Orbitals and Characteristic Resonances in K-shell NEXAFS

Spectra 1235.1.3 Molecular Orientation and Polarization Dependence of the Resonance

Intensities 1255.1.4 Techniques and Instrumentation of NEXAFS 1275.1.5 Radiation Damage of NEXAFS 1295.2 Molecular Orientations at Interfaces: the Effect of Molecule–Substrate

Interactions 1295.2.1 Organic/Metal Interfaces 1305.2.2 Organic/Semiconductor Interfaces 1325.2.3 Organic/Organic Heterojunction Interfaces 1365.2.4 CuPc on Other Technologically Important Substrates 1395.3 Molecular Orientations at Interfaces: the Effect of Strong Intermolecular

Interactions 1405.4 Molecular Orientations of Self-Assembled Monolayers 1435.5 Summary and Outlook 147

References 148

6 X-Ray Standing Waves and Surfaces X-Ray Scattering Studies of Molecule–MetalInterfaces 153Alexander Gerlach, Christoph Bürker, Takuya Hosokai, and Frank Schreiber

6.1 Introduction 1536.2 X-Ray Standing Wave Theory 1546.2.1 General Considerations on Wave Fields in Crystals 1546.2.2 The Two-Beam Approximation 1556.2.3 The Darwin Curve 1566.2.4 X-Ray Absorption and Dipole Approximation 1596.2.5 The Coherent Position and the Coherent Fraction 1616.3 X-Ray Standing Wave Experiments 1626.3.1 Beamline Setup at ID32 1626.3.2 Experimental Details 1636.4 Examples: Organic Monolayers on Metals 164

References 170

Part Three Electronic Structure 173

7 Fundamental Electronic Structure of Organic Solids and Their Interfaces byPhotoemission Spectroscopy and Related Methods 175Nobuo Ueno, Satoshi Kera, and Kaname Kanai

7.1 Introduction 1757.2 General View of Electronic Structure of Organic Solids 1767.2.1 From Single Molecule to Molecular Solid 1767.2.2 Contribution of Polaron 1797.2.3 Requirement from Thermodynamic Equilibrium 1797.3 Electronic Structure in Relation to Charge Transport 180

VIII Contents

7.3.1 Ultraviolet Photoemission Spectroscopy 1807.3.1.1 Energy and Momentum Conservation 1807.3.1.2 Energy Band Dispersion and Estimation of Band Transport

Mobility 1837.3.1.3 Density of States Effects in Polycrystalline Films 1847.3.2 Electron Spectroscopy Using Metastable Atom Beam: Characterization

of the Molecular Orientation via Wavefunction Spread 1867.3.2.1 Principle and Characteristics 1887.3.2.2 Characterization of the Molecular Orientation 1897.3.2.3 Spatial Wavefunction Distribution of Band Gap States 1907.3.3 Inverse Photoemission Spectroscopy (IPES) 1917.3.3.1 Characteristics of IPES 1917.3.3.2 Comparison between UPS-IPES and Tunneling Spectroscopy 1957.3.3.3 Comparison with Near-Edge X-Ray Absorption Fine Structure

Spectroscopy (NEXAFS) 1987.3.4 Probing Electron–Phonon Coupling, Hopping Mobility and Polaron by

UPS 2007.3.4.1 Basic Background 2007.3.4.2 Experimental Reorganization Energy and Polaron Binding Energy 2027.4 Electronic Structure at Weakly Interacting Interfaces 2067.4.1 Effects of Inhomogeneity of the Substrate Surface on the Energy Level

Alignment 2067.4.2 Strange Band Bending 2077.4.3 Radiation Effects on the Energy Level Alignment 2087.4.4 Mysterious Phenomena: Fermi Level Alignment Issue 2097.4.4.1 Impacts of Interface Dipole Layer on the Energy Level Alignment 2097.4.4.2 Impacts of Disorder on the Energy Level Alignment and Band

Bending 2117.5 Summary 213

References 214

8 Energy Levels at Molecule–Metal Interfaces 219Antoine Kahn and Norbert Koch

8.1 Introduction 2198.2 The Organic–Electrode Interface 2218.3 Gap States 2238.4 Metal Electrodes 2288.5 Tuning of Charge Injection Barriers 2328.5.1 Strong Electron Acceptor and Donor Molecules 2348.5.2 Self-Assembled Monolayers with Dipoles 2368.6 Conductive Polymer Electrodes 237

References 238

9 Vibrational Spectroscopies for Future Studies of Molecule–Metal Interface 243Wei-Yang Chou

9.1 Introduction 243

Contents IX

9.2 Selection Rules for Infrared and Raman Spectra 2449.3 Raman/IR Application in Organic Films 245

References 249

10 General Outlook 251Norbert Koch, Nobuo Ueno, and Andrew T.S. Wee

Index 253

XI

Preface

Organic electronics is a branch of electronics that utilizes carbon-based entitiessuch as semiconducting polymers and molecules as its basic building blocks. Thisis in contrast to traditional electronics that use inorganic semiconductors, for exam-ple, silicon, to fabricate the basic microelectronic components such as the transis-tor. Significant progress has been made in the field of organic electronics over thepast few decades, driven largely by lower cost manufacturing methods and the useof flexible substrates. Organic devices are already in use today as photoconductorsin copiers and laser printers, and newer applications such as organic light emittingdiodes (OLED), organic solar cells (OSC) and organic field effect transistors (OFET)have already reached the market.

Nobel laureate Herbert Kroemer coined the phrase the interface is the device whenhe referred to heterogeneous inorganic semiconductor structures. As is the casewith inorganic semiconductors, the most important components of the organic de-vice are its interfaces. In particular, the interactions between the organic semicon-ductor and electrodes critically determine the properties of the organic device, andhence the molecular-metal interface is chosen as the theme of this book. For exam-ple, in an OLED, the injection of electrons and holes from the electrodes is a crucialprocess, and formation of a molecular exciton via the formation of a bound stateof an electron and hole gives rise to light emission. In an OSC, where the functionis basically the reverse of the OLED, electrons and holes, resulting from excitondissociation at relevant organic–organic interfaces, are separated in the interfacialband bending regions that also depend on the organic–metal electrode interfaces.Hence, a fundamental understanding of the molecule–metal interface and its asso-ciated electronic structure forms the basis for improving the device performance.In the subfield of molecular electronics, which involves the use of single moleculesas building blocks for the fabrication of electronic components, an understandingof the molecule–metal interface is even more critical. Molecular electronics pro-vides a device miniaturization pathway to extend Moore’s Law beyond the limitsof silicon-integrated circuits, since a single molecule device is inherently in thenanometer scale.

This book is intended to serve as a textbook for a graduate level course, or as ref-erence material for researchers in organic electronics, molecular electronics, andnanoscience. It does not duplicate the many excellent books already written on or-

XII Preface

ganic electronics, but focuses on the science at the molecule–metal interface. Thechapters are written by leading experts in the various subfields, and are organizedinto three parts. In Part A, the basic theory and first principles theoretical studies ofthe molecule–metal interface is presented. In Part B, state-of the-art experimentaltechniques that elucidate the atomic structure of the molecule–metal interface aredescribed, namely scanning tunneling microscopy (STM), near-edge X-ray adsorp-tion fine structure spectroscopy (NEXAFS), X-ray standing wave, grazing incidenceX-ray diffraction (GIXRD) and X-ray reflectivity (XRR) studies. In Part C, the funda-mental electronic structure of organic solids and their interfaces elucidated by pho-toemission spectroscopy and related methods, as well as energy levels at molecule–electrode interfaces are discussed. Finally, infrared and Raman spectroscopies, aswell as the future outlook are presented.

This book attempts to give a concise coverage of important topics in the scienceof the molecule–metal interface, but as a disclaimer, we emphasize that the cover-age is by no means exhaustive. The selection of the materials is limited for spacereasons, and unavoidably reflects the research interests of the chapter authors. Nev-ertheless, the key concepts are presented so that the reader is given an overview ofthe recent progress in the field, important results in a few molecule–metal systems,and future directions of research.

This book came about as a result of the co-editors’ collaborative links betweenour respective institutions in Germany, Japan and Singapore. We are indebted toall our contributing authors for their efforts in writing chapters that are informativeand enjoyable reading for researchers in organic and molecular electronics, surfacescience, nanoscience and related fields. We truly hope that we have achieved ourgoal of writing a book that would become a useful reference for researchers andgraduate students interested in the fascinating science at the molecule–metal in-terface.

Humboldt-Universität zu Berlin Norbert KochChiba University Nobuo UenoNational University of Singapore Andrew T.S. WeeFebruary, 2013

XIII

List of Contributors

Christoph BürkerUniversität TübingenFakultät für PhysikAuf der Morgenstelle 1072076 TübingenGermany

Wei ChenNational University of SingaporeDepartment of Chemistry3 Science Drive 3Singapore 117543Singapore

Wei-Yang ChouNational Cheng Kung UniversityInstitute of Electro-optical Science andEngineeringTainan 701Taiwan

Fernando FloresDepartamento de Física Teórica de laMateria CondensadaFacultad de CienciasModulo C-VUniversidad Autónoma de Madrid28049 MadridSpain

Alexander GerlachInstitut für Angewandte PhysikUniversität TübingenAuf der Morgenstelle 1072076 TübingenGermany

Takuya HosokaiDepartment of Material Science andEngineeringIwate UniversityUeda 4-3-5Morioka 020-8551Japan

Han HuangDepartment of PhysicsNational University of Singapore2 Science Drive 3Singapore 117542Singapore

Antoine KahnDepartment of Electrical EngineeringPrinceton UniversityPrinceton, NJ 08544USA

Kaname KanaiDepartment of PhysicsFaculty of Science and TechnologyTokyo University of ScienceYamazaki 2641Noda 278-8510Japan

Satoshi KeraGraduated School ofAdvanced Integration ScienceChiba UniversityInage-kuChiba 263-8522Japan

XIV List of Contributors

Norbert KochHumboldt-Universität zu BerlinInstitut für PhysikNewtonstr. 1512489 BerlinGermany

Leeor KronikWeizmann Institute of ScienceDept. of Materials and InterfacesRehovoth 76100Israel

Yoshitada MorikawaDepartment of Precision Science andTechnologyGraduated School of EngineeringOsaka UniversityJapan

José OrtegaDepartamento de Física Teórica de laMateria CondensadaFacultad de CienciasModulo C-VUniversidad Autónoma de Madrid28049 MadridSpain

Dong-Chen QiDepartment of PhysicsNational University of Singapore2 Science Drive 3Singapore 117542Singapore

Frank SchreiberUniversität TübingenFakultät für PhysikAuf der Morgenstelle 1072076 TübingenGermany

Nobuo UenoChiba UniversityGraduate School ofAdvanced Integration Science1-33 Yayoi-choInage-kuChiba 263-8522Japan

Andrew T.S. WeeDepartment of PhysicsNational University of Singapore2 Science Drive 3Singapore 117542Singapore

Swee Liang WongDepartment of PhysicsNational University of Singapore2 Science Drive 3Singapore 117542Singapore

1

1Introduction to the Molecule–Metal InterfaceNobuo Ueno, Norbert Koch, and Andrew T.S. Wee

1.1From Organic Semiconductors to Organic Electronic Devices

In 1954, Inokuchi coined the term organic semiconductors for polycyclic aromaticcompounds with a molecular structure similar to fragments of graphene. He con-firmed the notion that such organic materials are electrically conductive [1] througha number of careful experiments by himself and other pioneers [2]1)2). As such,it is generally accepted that organic semiconductors were discovered in the mid-twentieth century. Following their pioneering work in this period, much of the re-search concentrated on revealing the nature of the electrical conduction in molec-ular single crystals, which exhibited charge carrier mobility values of a few cm2/Vsat room temperature, and much higher values at low temperature, as shown inthe work of Karl et al. [5]. So far, the highest mobility (40 cm2/Vs) was reportedfor rubrene single crystals in organic field effect transistors [6]. For practical deviceapplications, however, organic semiconductor thin films, comprised of evaporatedsmall-molecule compounds or polymers processed from solution, are more viable.

Organic semiconductors are already widely used today as photoconductors incopiers and laser printers, and have recently gained much attention because oftheir potential applications in electronic and optoelectronic devices, such as organiclight emitting diodes (OLEDs) [6–11], organic solar cells (OSCs) [12, 13] and organicfield effect transistors (OFETs) [10, 11, 14–17]. OLEDs are already used in displaysof mobile phones and are just entering the commercial lighting market. As a re-sult of the continuous drive to fabricate organic electronic devices on lightweight,large-area plastic substrates by low-cost processing techniques, organic electronicsis fast-tracked for applications that help overcome general energy problems andglobal warming. Following this trend, OSCs and OTFTs have developed rapidly

1) For history before 1988 see [3], where review articles by (I) D.D. Eley (p. 1), (II) H. Inokuchi (p. 23),(III) N. Karl (p. 31), (IV) L.E. Lyons (p. 53), (V) C. Pecile et al. (p. 69), (VI) M. Pope (p. 89), (VII)Z.D. Popovic (p. 103), (VIII) R. Qian (p. 117), (VX) E.A. Silinsh (p. 135), and (X) J. Sworakowskiand S. Nešpurec (p. 145) are available.

2) See also [4].

The Molecule–Metal Interface, First Edition. Edited by N. Koch, N. Ueno, and A.T.S. Wee.© 2013 WILEY-VCH Verlag GmbH & Co. KGaA. Published 2013 by WILEY-VCH Verlag GmbH & Co. KGaA.

2 1 Introduction to the Molecule–Metal Interface

Figure 1.1 Evolution of reported charge carrier mobility of OFETs employing the organic semi-conductor pentacene (adapted from [23, 24]).

over the past decade. Many potential applications of OFETs have been demon-strated, ranging from flexible displays [18] and sensor systems [17, 19] to radiofrequency identification tags [20], and some of these systems are now close to com-mercialization.

Organic semiconductors have other unique physical properties that offer numer-ous advantages compared to their inorganic counterparts: (i) The extremely highabsorption coefficient of many organic molecules in the visible wavelength rangeoffer the possibility of very thin, and therefore resource-efficient, photodetectorsand solar cells. (ii) Many fluorescent molecules emit light efficiently. However,charge transport in organic semiconductors is often limited by low intrinsic car-rier density and mobility. Therefore, controlled and stable doping, in full analogyto doping of inorganic semiconductors for increasing carrier density, is desirablefor reaching higher efficiency of many organic-based devices [11, 21, 22]. In addi-tion, if one succeeds in shifting the Fermi level toward the transport states upondoping, this could reduce Ohmic losses at contacts, improve carrier injection fromelectrodes, and increase the built-in potential of Schottky or p–n junctions.

To understand the recent progress and expansion of organic electronics we showthe temporal evolution of reported charge carrier mobility of OFETs employing theorganic semiconductor pentacene in Figure 1.1 [23, 24], and the progress of OSCefficiency compared with different solar cell technologies in Figure 1.2 [25]. Therapid improvement of device performance is undeniably related to the progress inthe science of interfaces and the availability of techniques to control interfaces.

1.2 Role and Function of Interfaces in Organic Electronic Devices 3

Figure 1.2 Efficiency progress of different solar cell technologies (Source: L.L. Kazmerski, Na-tional Renewable Energy Laboratory (NREL), Golden, CO).

1.2Role and Function of Interfaces in Organic Electronic Devices

As organic semiconductors generally have a wider band gap and narrower band-width than their inorganic counterparts, the density of thermally excited chargecarriers in organic films is not sufficient to sustain high current density. We thusneed injection of carriers into organic films from electrodes to achieve sufficientlyhigh current in organic devices. This requirement is directly linked to the energet-ics at electrode–organic and organic–organic interfaces that exist in devices, thuscontrol of the interfacial energy level alignment is the key technology required forthe fabrication of organic electronic devices with high efficiency [21, 22, 26–29].

Typical examples for the electronic structure at interfaces in an OLED, an OSC,and an OFET are schematically shown in Figure 1.3. In the OLED, electrons areinjected into the electron transporting layer (ETL) from the right electrode andholes are injected to the hole transport layer (HTL) from the left electrode. Theinjected charges are transported to the ETL/HTL interface when a potential is ap-plied between the two electrodes. In modern OLED devices, more than just twoorganic layers are used to realize highly efficient light emission. The injection ofelectrons and holes from the electrodes is more important than high carrier mo-bility in OLEDs, and the molecular exciton, produced via the formation of a boundstate of an electron and hole at the ETL/HTL interface, yields light emission uponradiative decay. In the OSC, the physical process is essentially the reverse of the

4 1 Introduction to the Molecule–Metal Interface

Figure 1.3 Schematic electronic structureof OLED (a), OSC (b) and OFET (c). As thebands derived from the highest occupiedmolecular orbital (HOMO) and the lowest un-occupied molecular orbital (LUMO) have verynarrow bandwidth, they are represented bylines. HTL and ETL in the OLED stand for holetransport layer and electron transport layer,respectively. In the OFET, as the hole injectionbarrier (Δh) is smaller than the electron injec-

tion barrier (Δe), holes are injected and trans-ported near the surface of the gate insulator tothe source electrode (p-type OFET). The Fermilevel (EF) is not defined across the structures,because the electron systems are not in ther-modynamic equilibrium but in a steady statewith current flowing. Therefore EF in the fig-ures indicates the Fermi level position in themetals before device operation.

OLED. In the OSC it is necessary that photogenerated bound electron–hole pairs(molecular excitons) in the organic layer are dissociated to generate mobile elec-trons and holes, which are further separated in the band-bending region, whichdepends on the electronic structure of the organic/electrode interface. As the elec-tric current that can be used in an external circuit necessitates good charge trans-port to the electrodes, one needs high carrier mobility in the organic layer in OSCs.The elucidation of the interfacial electronic structure therefore forms the basis forunderstanding and improving the performance of these devices. In particular, theorganic/metal interfaces have attracted much interest in relation to the rapid de-velopment of organic electronic and optoelectronic devices, since organic/metalinterfaces are crucial for realizing efficient charge exchange between the organiclayer and the external circuit.

1.3What Will We Learn about the Interfaces?

Fundamental interface phenomena, which are the subject of study in interface sci-ence, may be understood by investigating the difference between Figure 1.4a,b:Figure 1.4a the metal and the organic semiconductor are in direct contact, and Fig-ure 1.4b the metal and the organic are just wired. For Figure 1.4b we assume thatthe wire acts only for exchanging electrons between the two solids. Both systemsthereby allow exchange of electrons between the two solids to achieve thermo-dynamic equilibrium. Figure 1.5 compares the electronic states of the two cases.Since the electrons fill the energy levels strictly following Fermi–Dirac statistics, it

1.3 What Will We Learn about the Interfaces? 5

Figure 1.4 What do we need to consider forthe interface? Two cases of interacting solids:(a) metal and organic semiconductor are di-rectly contacted to form an interface, wherethe electronic coupling is not zero even for

the interface with an extremely weak interac-tion; (b) metal and organic semiconductor arewired only to allow for the exchange of elec-trons, and the two surfaces face each otherwithout direct contact.

Figure 1.5 Hypothetical energy level align-ment at thermal equilibrium of the electronsystem for the two cases from Figure 1.4a,b,direct contact (a) and contact by wiring (b),both of which allow for charge exchange be-tween organic and metal. VL, CPD and Vbistand for the vacuum level, contact potential

difference, and built-in potential, respectively.A continuum media model is assumed andan n-type organic semiconductor with a donorlevel is used. The interface dipole exists at thedirect contact interface. There must be differ-ence between (a) and (b) due to the electroniccoupling at the interface in (a), see text.

is required that an interface system consisting of two different materials must have asingle Fermi level (EF) throughout the system when the electrons are in thermodynamicequilibrium. This concept is always valid for electron systems in thermodynamicequilibrium, and is thus also valid for the cases in Figure 1.5a,b. It can be convinc-ingly shown, however, that there are some differences between Figure 1.5a andFigure 1.5b, which are related to what we will discuss in this book.

When we discuss interfaces with organic semiconductors, we divide them intotwo groups, (i) strongly interacting interfaces and (ii) weakly interacting ones. Inthe former case (i), new electronic states appear at or near the interface due to

6 1 Introduction to the Molecule–Metal Interface

strong electronic coupling or chemical reactions between the two materials, whichresult in changes of the energy level alignment. The latter case (ii), where by andlarge no new states at the interface occur, may also be adapted to organic/metalinterfaces that are not atomically clean. This is highly relevant for practical devicefabrication that proceeds in moderate vacuum conditions or involving solvents.For instance, exposing metal surfaces, which in general are chemically reactive,to air during device fabrication may passivate the surfaces. To comprehensivelyunderstand the energy level alignment at organic/metal interfaces, one needs thehelp of theoretical studies in addition to experimental work. This book covers the-oretical methods that unravel the interface electronic states in Chapters 2 and 3.However, experimental results have demonstrated that EF moves in the organicband gap upon contact of the two materials though the new electronic state den-sity may be considered negligible. In some cases, theory is incapable of explain-ing experimental results, when, for example, the results are related to structuraland/or chemical imperfections (Chapter 7). Cumulated experimental evidence hastriggered the introduction of an intuitive idea, that there must be yet unidentifiedimpurity molecules/atoms, which control the position of EF in the HOMO-LUMOgap of the organic semiconductor, in analogy to the physics of inorganic semicon-ductors. A very low concentration (� ppm) of impurities is sufficient for inorganicsemiconductors to control EF and thus to realize n- or p-type properties effectively,while a much higher dopant concentration (� %) is required for organic semi-conductors [10, 11, 21, 22]. Why do we need such high doping concentrations?The answer may be related to the nature of molecules, molecular solids, molecule–molecule heterocontacts (intermolecular interaction between different molecules),and to yet unknown electronic states in the gap. This book tries to offer a clearerinsight into these issues.

1.4The Fermi Level and Related Fundamentals

1.4.1Definition of the Fermi Level in this Book

As mentioned above, it is important to adhere to the concept of the electronic statesof interacting solids and the resulting energy level alignment. The occupation prob-ability of electronic states by electrons in solids is given by the Fermi–Dirac distri-bution function, f (E ),

f (E ) D 1

eE�μ(T )

kB T C 1, μ(T ) � μ0 � π2

6

hd D(E)

d E

iμ0

D(μ0)(kBT 2)(1.1)

where kB is the Boltzmann constant, T is the absolute temperature, E is the elec-tron energy, μ is the T-dependent chemical potential of the electrons, μ0 is μ(0),and D(E ) is the density of states (DOS). Often we observe that there is some con-

1.4 The Fermi Level and Related Fundamentals 7

Figure 1.6 Relation of the Fermi–Dirac distribution function [ f (E)], density of states [D(E)] andelectron density [n(E)] in a semiconductor without impurities (dopants) at T > 0.

fusion in using the terms “chemical potential,” “Fermi level” or “Fermi energy”for the parameter μ(T ). Therefore, we go back to the theoretical work on metals bySommerfeld to understand the difference between “chemical potential” and “Fermilevel,” which have different reference energies [30]. We begin here by assuming thesimilarity of “chemical potential” and “Fermi level,” and use “Fermi level [EF(T )]”instead of using μ(T ). For example, therefore, f (E ) D 0.5 at E D EF(T ) for T > 0,independent of considering metals, semiconductors and insulators. On the otherhand, “Fermi energy” is defined as the energy of the highest occupied energy levelof electronic states in a metal at T D 0, thus it is different from the Fermi level andcorresponds to EF(0)[μ(0)] for metals, see for example [31].

Adhering to this, we will use the Fermi level (EF) for μ(T ) in this book, namelyEF D EF(T ) D μ(T ). The number density of electrons [n(E )] between energy Eand E C dE is given by n(E )dE D f (E ) � D(E ) � dE , since D(E ) describes allavailable states and can be occupied by electrons only in accordance with f (E ) (seeFigure 1.6).

If a semiconductor is free of impurities or dopants, and the DOS of the conduc-tion band [Dc(E )] and the valence band [Dv(E )] are approximated by the model offree-electron-like DOS near the band edges, one obtains [32]

Dc(E ) D 12π2

�2m�

e

„2

�3/2

(E � Ec)1/2 and

Dv (E ) D 12π2

�2m�

h

„2

�3/2

(Ev � E )1/2 (1.2)

where me* and mh* are the effective mass of electron and hole, respectively, Ec isthe energy of the bottom of the conduction band and Ev is the energy of the top ofthe valence band. Using Eqs. (1.1) and (1.2), the charge neutrality condition, wherethe density of thermally excited electrons (ne) is the same to that of holes (nh), isgiven as

ne D 2�

m�e kB T

2π„2

�3/2

e(EF�Ec)

kB T D nh D 2�

m�h kBT

2π„2

�3/2

e(Ev�EF)

kB T (1.3)

8 1 Introduction to the Molecule–Metal Interface

EF is obtained from Eq. (1.3) as

EF(T ) D 12

Eg C 34

kBT ln�

m�h

m�e

�(1.4)

where Eg is the band gap energy. If m�e D m�

h , EF is located exactly in the middleof the energy gap independent of T. As often m�

e � m�h , EF indeed is found near

the middle of the gap.Impurity doping yields occupied (electron donor) or unoccupied (electron accep-

tor) levels in the gap, thus EF moves in the gap depending on the density of theselevels, their energy position, and temperature [33]. As the mechanism of doping inorganic semiconductors may be different from their inorganic counterparts, andstill is under investigation, one should be aware of possible shortcomings in apply-ing this model for organic systems.

1.4.2Measuring the Fermi Level of Organic Semiconductors

An important issue in studying organic/metal interfaces is to consider thermo-dynamic equilibrium of electrons throughout the system [34]. If thermodynamicequilibrium is achieved after contact, the Fermi level in the organic layer alignswith that of the metal by electron exchange, which results in shifts of the energylevels and band bending in the organic layer. A measurement of the EF positionin the HOMO-LUMO gap is critically important, since the EF position dominatesvarious electronic properties of the interface system. However, the experimentaldifficulty is that ultraviolet photoemission spectroscopy (UPS) cannot directly mea-sure the EF position for materials with a band gap, such as organic semiconduc-tors, because there are no electrons at EF. In this case, we assume thermodynamicequilibrium for the organic/metal systems and use EF measured for the metal sub-strate also as that in the band gap. In practice, this is achieved by measuring filmthickness-dependent UPS spectra of an organic semiconductor on a metal sub-strate as shown in Figure 1.7. Note that possible (positive) surface charging of theorganic film upon photoionization has to be prevented. Charging decreases theelectron kinetic energy in the measurement, thus increasing the binding energy inUPS. It can be identified by looking carefully at time-dependent changes in spec-tral position and/or spectral shape especially, near the vacuum level. The latter isbecause the energy distribution of electrons with small kinetic energies is sensi-tively influenced by the nonuniform surface potential due to the charging. Notethat charging-induced changes can occur on time scales smaller than the time re-quired for accumulating a spectrum. Therefore, initial studies with much reducedphoton flux are very useful. If photoelectrons emission is observed above EF inUPS of an organic film, one must consider that the organic/metal system is not inthermodynamic equilibrium, or may suffer from photovoltage effects during UPSmeasurements [35].

1.4 The Fermi Level and Related Fundamentals 9

Figure 1.7 Measurement of the Fermi level(EF) from the organic film thickness depen-dence of UPS spectra; here results are shownfor copper phthalocyanine (CuPc) on polycrys-talline Ag. In CuPc (thickness D 200 Å) onAg, EF is determined from the measurement

done on Ag. The step-like feature at EF in theAg spectrum, which is called the Fermi edge,is the Fermi–Dirac distribution function at themeasurement temperature, convoluted withthe energy resolution of the electron spec-trometer.

1.4.3The Work Function and the Vacuum Level of a Solid with Finite Size

The work function is defined as the energy difference between the Fermi level andthe vacuum level. The vacuum level refers to the energy of a zero-velocity elec-tron that is placed in vacuum. Here we consider an “empty” vacuum to be wherethe electrostatic potential is constant. However, we should be aware of electrostaticpotential variations in vacuum in reality due to the omnipresence of charge dis-tributions if the material exists in vacuum. We first consider a biased semiinfinitemetal in this vacuum and discuss the potential at a position away from the infinitesurface (case I). In this case we know from classical electrostatics that there is aconstant potential independent of the position in the half space. When we put asimilarly biased small metal in this vacuum (case II), it is also clear that there is anelectrostatic potential, which is similar to that of case I near the surface, and the po-tential becomes 0 at a position infinitely far away from the surface. As the vacuumlevel is a property of the electron and free space, the vacuum level is defined as thelevel, where the kinetic energy of an electron is 0, to avoid problems between thecases I and II. The vacuum level can be used as a common reference level for theenergy levels of two different materials before contact. It is particularly importantto understand the meaning of the vacuum level, and therefore the work function,for the design of organic device components, such as cathodes and anodes. In thefollowing, we discuss a bit further the vacuum level/work function defined for asmall metal or solid that is actually used in experiments.

Consider a hydrogen atom in vacuum, the simplest atom that consists of a proton(the nucleus) and an electron with a positive and a negative elementary charge(˙e), respectively. Assuming point charges for both, the potential energy U(r) of

10 1 Introduction to the Molecule–Metal Interface

Figure 1.8 The vacuum level defined for asimple atom, hydrogen. The electrostatic po-tential [V(r)] of an electron in the field of thenucleus and the electron potential energy

[U(r)] are 0 for r ! 1, where the electro-static potential in vacuum is chosen to be 0. Ifthe electron is at VL1, its kinetic energy (Ek)is 0.

the electron at distance r from the nucleus is given by

U(r) D �eV(r) D (�e)

0@�

rZ1

e4π�0r 02 dr 0

1A D � e2

4π�0r(1.5)

where V(r) is the electrostatic potential by the nucleus, and ε0 is the vacuum per-mittivity. U(r) becomes 0 for r ! 1, which means that an electrostatic potentialin vacuum at 1 is selected to be 0. This energy level, U(r ! 1), is defined asthe vacuum level (VL1). Therefore, if the kinetic energy (Ek) of the electron is 0 atr ! 1, the total energy of the electron is 0 at VL1 because we have chosen thepotential to be 0 at r ! 1 (see Figure 1.8).

A metal single crystal has various values for the vacuum level, and thus the workfunction, depending on the surface crystal plane. Some examples are collected inTable 1.1. This phenomenon occurs because electrons form a solid tail into vacuumat the surface, which yields an electric dipole layer at the surface, as the charge neu-trality condition does not hold near the surface. Figure 1.9 shows such a surfacedipole layer of a metal using the Jellium model [36]. Thus the vacuum level and thework function depend on the specific surface of a crystal due to the electrostaticpotential generated by the surface dipole layer (see Table 1.1). Another surface phe-

Table 1.1 Surface dependence of work function of metals.

Metal Lattice Surface Surface atom density Ref.(110) (100) (111) (assuming no reconstruction)

Cu fcc 4.48 4.59 4.94 (110) < (100) < (111) [41]Ag fcc 4.52 4.64 4.74 (110) < (100) < (111) [42]W bcc 5.24 4.63 4.47 (110) > (100) > (111) [43]Mo bcc 4.95 4.53 4.36 (110) > (100) > (111) [44]K bcc 2.55 2.40 2.51 (110) > (100) > (111) [45]

1.4 The Fermi Level and Related Fundamentals 11

Figure 1.9 Self-consistent solution for thecharge density at a metal surface for rs D 2and rs D 5 (uniform positive backgroundmodel (Jellium model)). The term rs (in

atomic units) is defined by 4π r3s /3 D 1/n,

where n is the electron density. The figure wasreproduced with permission from [36]. Copy-right (1970) by the American Physical Society.

nomenon related to the anisotropy of the work function of metals was introducedby Smoluchowski [37]. The effect, called after Smoluchowski, originates from aredistribution of the electron cloud on a metal surface with a strong corrugation.Figure 1.10 illustrates this smoothing effect of the electron cloud, where the surfacecharge redistribution is represented by the wavy curve. The electron density redis-tributes from the “mountains” into the “valleys” to result in a net positive chargeon the “mountain” and a negative charge in the “valley”. This charge distributiondepends strongly on the surface atomic structure. For a close-packed surface, like(111) surfaces of face-centered cubic [ fcc(111)] crystals, this mechanism does nothave a momentous effect, whereas the surface charge distribution of an open sur-face, like (110) surfaces of simple cubic [sc(110)] crystals (see Figure 1.10), or a

Figure 1.10 Schematic of the Smoluchowski effect using a simple cubic (sc) metal. Electronsnear a bumpy surface redistribute to result in surface dipole layer as illustrated here for the(011) surface.

12 1 Introduction to the Molecule–Metal Interface

stepped surface, like fcc(311), is strongly effected. Work function values of vari-ous metals are summarized in [38]. Note that these work function values are onlyrelevant for atomically clean surfaces; any adsorbates, like organic semiconductormolecules or contaminants from air, change the charge distribution at the surfaceand thus the work function.

When we discuss interacting electron systems, we need to use the Fermi level asan energy reference. This is because the Fermi level can then be drawn as a hori-zontal line on a vertical energy axis, and the work function changes are representedby vertical shifts of the vacuum level, depending on the surface dipole potentialchanges. Thus, it is convenient to redefine the vacuum level at close proximity tothe surface (the distance from the surface should be smaller than the lateral exten-sions of the surface, and larger than the range of the electron density tailing into

Figure 1.11 (a) Direction dependence of thevacuum level and the work function (φ) for ametal with the Fermi level as the energy ref-erence, and (b) direction dependence of thevacuum level and ionization energy (IE) fornonpolar perfluoropentacene (local dipole ef-fects). In (b) small local dipoles due to spatialspread of π electrons perpendicular to themolecular plane are neglected. One should

be careful with ionization energy that is mea-sured for gas phase and solid phase samples.For the gas phase the ionization energy refersgenerally the vacuum level at r ! 1 (VL1

in Figure 1.8) that does not involve the localdipole potential, while for the solid phase itrefers the vacuum level near the surface thatinvolves the local (surface) dipole potential.