Mechanism of charge transport in organic semiconductors and carbon nanomaterials
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Mechanism of charge transport in organic semiconductors and carbon nanomaterials
Yuqian Jiang, Jinyang Xi, and Zhigang Shuai MOE Key Laboratory of Organic OptoElectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China [email protected]
ABSTRACT We develop theoretical descriptions for charge transport in organic semiconductors and carbon nanomaterials. For the localized charges, we found the quantum nuclear tunneling effect is essential which could manifest isotope effect for mobility as well as exotic optical feature. Because the nuclear tunneling tends to favor electron transfer while heavier nuclei decrease the quantum effect, isotopic substitution should reduce carrier mobility. Moreover, the isotopic effect only occurs when the substituted nuclei contribute actively to vibrations with appreciable charge reorganization energy and coupling with carrier motion. For the band-like transport, we propose a Wannier extrapolation scheme for computing the electron-phonon interaction matrix for the Boltzmann equation. Our calculation indicates that the intrinsic electron-phonon scatterings in two-dimensional carbon materials are dominated by low-energy longitudinal-acoustic phonon scatterings over a wide range of temperatures, while by high-frequency optical phonons at high temperature. The electron mobilities of α- and γ-graphynes are predicted to be ca.104 cm2V-1s-1 at room temperature.
INTRODUCTION The optoelectronic properties in organic semiconductors and inorganic nanomaterials depend strongly on the charge carrier mobility, which have been extensively investigated theoretically and experimentally [1-8]. However, our understanding on the charge transport mechanism is still very limited and under controversy. Traditional wisdom says that when mobility increases with the rising temperature, thermal activated hopping transport was considered to be dominant, where the charge reorganization energy (λ) is much larger than the intermolecular electron coupling (V), and the charge is considered to be fully localized on a single molecule. Semiclassical (SC) Marcus theory has been widely used to study such hopping behavior [9, 10]. On the other hand, when mobility decreases with temperature, a band-like picture was proposed instead and Boltzmann transport theory combined with deformation potential (DP) theory with longitudinal-acoustic phonon scattering is commonly used [3, 6]. However, beyond such thermal activated hopping or acoustic-phonon-scattered band model, intramolecular electron-phonon (e-ph) coupling was found to play an important role in both hopping and band transport by theoretical and experimental studies.
For hopping transport, several approaches with considering intramolecular e-ph interaction have been proposed to interpret experimental phenomena [11-16]. For example, Asadi et al. derived a macroscopic current expression based on the quantum nuclear tunneling picture under external field, which was found to be universal for at least
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