Organic Field-Effect Transistors

Zhenan Bao, Jason Locklin, Zhenan Bao, Jason Locklin

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780849380808
• eBook ISBN: 9781351837576
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

The remarkable development of organic thin film transistors (OTFTs) has led to their emerging use in active matrix flat-panel displays, radio frequency identification cards, and sensors. Exploring one class of OTFTs, Organic Field-Effect Transistors provides a comprehensive, multidisciplinary survey of the present theory, charge transport studies, synthetic methodology, materials characterization, and current applications of organic field-effect transistors (OFETs). Covering various aspects of OFETs, the book begins with a theoretical description of charge transport in organic semiconductors at the molecular level. It then discusses the current understanding of charge transport in single-crystal devices, small molecules and oligomers, conjugated polymer devices, and charge injection issues in organic transistors. After describing the design rationales and synthetic methodologies used for organic semiconductors and dielectric materials, the book provides an overview of a variety of characterization techniques used to probe interfacial ordering, microstructure, molecular packing, and orientation crucial to device performance. It also describes the different processing techniques for molecules deposited by vacuum and solution, followed by current technological examples that employ OTFTs in their operation. Featuring respected contributors from around the world, this thorough, up-to-date volume presents both the theory behind OFETs and the latest applications of this promising technology.

1.1	Theoretical Aspects of Charge Transport in Organic Semiconductors: A Molecular Perspective

Demetrio A. da Silva Filho, Yoann Olivier, Veaceslav Coropceanu, Jean-Luc Brédas, and Jérôme Cornil

CONTENTS

1.1.1 Introduction

1.1.2 A Primer on Electron-Transfer Theory

1.1.3 Electron-Vibration Coupling and Reorganization Energy

1.1.3.1 Intramolecular Reorganization Energy

1.1.3.2 Intramolecular Reorganization Energy of Oligoacenes

1.1.4 Electronic Coupling

1.1.4.1 Influence of Intermolecular Separation

1.1.4.2 Influence of Long- or Short-Axis Displacements

1.1.5 From Molecular Parameters to Carrier Mobilities

1.1.5.1 Influence of the Electric Field

1.1.5.2 Influence of the Reorganization Energy

1.1.5.3 Influence of Intermolecular Distance

1.1.5.4 Influence of Molecular Translations

1.1.5.5 Introduction of a Gaussian Disorder

1.1.6 Concluding Remarks

References

1.1.1 INTRODUCTION

The development of the field of organic electronics has benefited from the unique set of characteristics offered by π-conjugated oligomers and polymers. These materials combine the electrical properties of semiconductors with the properties typical of plastics: low cost, versatility of chemical synthesis, ease of processing, and flexibility.

In organic field-effect transistors, the key steps of operation involve charge injection and formation of a conducting channel within the organic semiconductor due to application of a gate voltage; upon application of a drain voltage, the charges migrate across the organic layer and are collected at the drain electrode. Charge injection and collection processes and, in most instances, charge transport actually correspond to redox (electron-transfer) reactions. Much success in gaining a better understanding of charge-transport phenomena in organic materials has come recently from extending the theory of electron-transfer reactions, originally formulated by Marcus for the description of redox reactions in solution, to organic semiconductors [1–5].

The charge-transport properties in conjugated materials critically depend on the packing of the chains and degree of order in the solid state [6] as well as on the density of impurities and structural defects [7]. As a result, the measured mobility values can largely vary as a function of sample quality [8]. Overall, the transport mechanism results from a balance between the energy gained by electron delocalization in an electronic band and the energy gained by geometric relaxation and polarization around a charge on an oligomer or polymer segment to form a polaron [9].

In highly purified molecular single crystals, transport at low temperature can be described within a band picture, as shown by Karl and coworkers [10]. As a general rule of thumb, (effective) bandwidths of at least 0.1 eV are needed to stabilize a band regime [9]. In that case, the positive or negative charge carriers are fully delocalized and their mobilities are a function of the width of the valence or con-duction band, respectively (i.e., of the extent of electronic coupling between oligomer or polymer chains). When temperature increases, the mobilities progressively decrease as a result of scattering processes due mainly to lattice phonons, as is the case in metallic conductors. Transport can then be described on the basis of effective bandwidths that are smaller than the bandwidths obtained for a rigid lattice. At elevated temperatures, localization steps in and transport operates via a thermally assisted polaron hopping regime where charge carriers jump between adjacent molecules or chains, as described, for instance, by Conwell and coworkers [11].

The hopping regime generally applies in the presence of significant static disorder, dynamic fluctuations [12], and/or impurities; this transport mechanism is thus expected to be operative in most organic field-effect transistors. At the microscopic level, polaron hopping can be viewed as a self-exchange electron-transfer reaction where a charge hops from an ionized site to an adjacent neutral site. In that context, the carrier mobilities are a direct function of the self-exchange electron-transfer reaction rates.

In this chapter, we focus on the hopping regime and start with a primer on electron-transfer theory in Section 1.1.2. This section will underline the three major parameters that enter the expression of the electron-transfer rate: reorganization energy, electronic coupling, and driving force. We then discuss some examples of the impact of chemical structure and packing mode on these parameters. Section 1.1.3 deals with reorganization energy, and Section 1.1.4 is devoted to electronic coupling. In Section 1.1.5, the role of the driving force (due to the application of an external electric field) is incorporated. This section provides an illustration of how the information on electron-transfer rates gathered at the intramolecular and intermolecular levels can translate into charge carrier mobilities at the macroscopic level.

1.1.2 A PRIMER ON ELECTRON-TRANSFER THEORY

Electron-transfer processes, as well as energy-transfer processes, can be viewed as special cases of the nonradiative decay of an electronic state. In the framework of perturbation theory [1,2], the probability for a transition from a discrete initial state ψ_i (corresponding to the reactants) to a discrete final state ψ_f (corresponding to the products of the reaction) writes under application of a perturbation V to first order:

$P_{if}=\frac{1}{{\hslash }^2}\left|<{\psi }_i\right|V{\left|{\psi }_f>\right|}^2{\left[\frac{\sin \left({\omega }_{fi}t/2\right)}{{\omega }_{fi}/2}\right]}^2$

(1.1.1)

where t denotes time, ω_fi the transition energy between the electronic states i and f, and <ψ_i|V|ψ_f> is the corresponding electronic coupling matrix element.

To account for a continuous distribution of final (vibrationally coupled) electronic states, Equation 1.1.1 can be recast by introducing the density of final states ρ(E_f) and summing over all probability densities. Assuming that the function |<ψ_i|V|ψ_f>|² ρ(E_f) varies slowly with energy, the transition probability per unit time (or transition rate) adopts, in the long-time limit, the simple and widely exploited Fermi’s golden rule form:

$k_{if}=\frac{2\pi }{\hslash }\left|<{\psi }_i\right|V{\left|{\psi }_f>\right|}^2\mathrm{\rho }\left(E_f\right)$

(1.1.2)

The transition mechanism involves vibr...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Section 1.1 Theoretical Aspects of Charge Transport in Organic Semiconductors: A Molecular Perspective
• Section 2.1 Charge Carrier Transport in Single-Crystal Organic Field-Effect Transistors
• Section 2.2 Charge Transport in Oligomers
• Section 2.3 Charge Transport Physics of Solution-Processed Organic Field-Effect Transistors
• Section 2.4 Contact Effects in Organic Field-Effect Transistors
• Section 3.1 Design, Synthesis, and Transistor Performance of Organic Semiconductors
• Section 3.2 Dielectric Materials: Selection and Design
• Section 4.1 Grazing Incidence X-Ray Diffraction (GIXD)
• Section 4.2 Near-Edge X-Ray Absorption Fine Structure (NEXAFS) Spectroscopy
• Section 4.3 Scanning Probe Techniques
• Section 5.1 Vacuum Evaporated Thin Films
• Section 5.2 Solution Deposition of Polymers
• Section 5.3 Solution Deposition of Oligomers
• Section 5.4 Inkjet Printed Organic Thin Film Transistors
• Section 5.5 Soft Lithography for Fabricating Organic Thin-Film Transistors
• Section 6.1 Radio Frequency Identification Tags
• Section 6.2 Organic Transistor Chemical Sensors
• Section 6.3 Flexible, Large-Area e-Skins
• Section 6.4 Organic Thin-Film Transistors for Flat-Panel Displays
• Index