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Theory of Charge Transport in Carbon Electronic Materials

verfasst von: Zhigang Shuai, Linjun Wang, Chenchen Song

Verlag: Springer Berlin Heidelberg

Buchreihe : SpringerBriefs in Molecular Science

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik" , Springer Professional "Wirtschaft"

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Über dieses Buch

Mechanism of charge transport in organic solids has been an issue of intensive interests and debates for over 50 years, not only because of the applications in printing electronics, but also because of the great challenges in understanding the electronic processes in complex systems. With the fast developments of both electronic structure theory and the computational technology, the dream of predicting the charge mobility is now gradually becoming a reality.

This volume describes recent progresses in Prof. Shuai’s group in developing computational tools to assess the intrinsic carrier mobility for organic and carbon materials at the first-principles level. According to the electron-phonon coupling strength, the charge transport mechanism is classified into three different categories, namely, the localized hopping model, the extended band model, and the polaron model. For each of them, a corresponding theoretical approach is developed and implemented into typical examples.

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Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Abstract

The charge mobility, μ, which characterizes the ability of a charge to move in a bulk semiconductor, is the essential parameter in determining the overall performance of electronic devices reported by Coropceanu et al. (Chem Rev 107:926, 2007). By definition, it is the charge drift velocity, v, acquired per driving electric field, F, i.e., μ = v/F, usually expressed in unit of cm²/Vs. In the absence of scattering, the field-induced momentum gain for an electron, Δq = −eFt, should increase linearly with the time period t. However, according to the classical Boltzmann transport picture, due to the scattering with impurities, defects, and lattice vibrations, the electron momentum is restored to its original value after the mean scattering time, τ, i.e., the average time between two consecutive scattering events.

Zhigang Shuai, Linjun Wang, Chenchen Song

Chapter 2. Hopping Mechanism

Abstract

In the limit of strong electron–phonon coupling and weak intermolecular electronic coupling, a charged molecule undergoes a large geometry relaxation, which eventually traps the charge. In this case, the charge transport can be viewed as an intermolecular hopping process. With the known electron transfer rates between neighboring molecules, the charge carrier mobility can be evaluated through the Einstein relation from random walk simulations. In general, the classical Marcus electron transfer theory, which works well in the high-temperature limit. For a better understanding, we incorporate the nuclear tunneling effect arising from the intramolecular high-frequency vibrations to characterize the transport behavior at room temperature. Dynamic disorder effect arising from the intermolecular low-frequency vibrations is found to be very much materials structure or space-dimension dependent, which may give rise to the phonon-assisted current.

Zhigang Shuai, Linjun Wang, Chenchen Song

Chapter 3. Polaron Mechanism

Abstract

The intrinsic charge transport in organic semiconductors is an electron–phonon interacting process. Due to the “soft” nature of organic materials, the existence of an electron can cause significant deformation of local nuclear vibrations, which moves together with the electron itself, and thus the effective diffusing quasiparticle is composed of the electron and its accompanying phonons. This is the basic idea of the polaron mechanism. In principle, it is a more general description for charge transport since it does not presume that the charge is localized within one molecule as in the hopping mechanism described in Chap. 2. In this chapter, we adopt the general Holstein-Peierls Hamiltonian coupled with first-principles calculations to investigate the fundamental aspects concerning charge transport. All kinds of electron–phonon couplings, including both local and nonlocal parts for inter- and intra-molecular vibrations, have been taken into considerations. Detailed studies are performed to study their contributions to the total electron–phonon coupling strength and the temperature dependence of mobility, especially the band-hopping crossover feature. We also investigate the pressure- and temperature-dependent crystal structure effects on the charge transport properties.

Zhigang Shuai, Linjun Wang, Chenchen Song

Chapter 4. Deformation Potential Theory

Abstract

When the electron–phonon coupling is weak compared with the intermolecular electronic couplings, charge transport can be described by the band mechanism. Namely, the charge moves coherently in a wavelike manner and is scattered by phonon. In this chapter, we introduce the deformation potential theory, which is actually a band model including only the lattice scatterings by the acoustic deformation potential. It is based on the Boltzmann transport equation and sometimes, can be simplified using the effective mass approximation. Contrary to Chap. 3, where only optical phonons are considered, the acoustic phonons are the focus of this chapter. This approach is applied to a typical molecular crystal, naphthalene, and covalently bonded functional materials, graphene and graphdiyne sheets and nanoribbons.

Zhigang Shuai, Linjun Wang, Chenchen Song

Chapter 5. Outlook

Abstract

Modeling the charge transport in organic materials is a formidable task due to the complexity in dealing with various scattering mechanisms, which is intrinsically a many-body problem [1]. In this book, we present three approaches, namely, the localized hopping model, the extended band model, and the polaron model, to compute the mobility for organic and carbon materials at the first-principles level. We show that we indeed achieved some successes in, for instance, predicting the intrinsic mobility values from given materials structures, or in rationalizing the structure–property relationship.

Zhigang Shuai, Linjun Wang, Chenchen Song

Titel: Theory of Charge Transport in Carbon Electronic Materials
verfasst von: Zhigang Shuai
Linjun Wang
Chenchen Song
Copyright-Jahr: 2012
Verlag: Springer Berlin Heidelberg
Electronic ISBN: 978-3-642-25076-7
Print ISBN: 978-3-642-25075-0
DOI: https://doi.org/10.1007/978-3-642-25076-7

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