Theoretical Studies On The Charge Transport Properties Of Thienoacenes-based Optoelectronic Materials

Organic optoelectronic materials have been widely used in organic light-emitting diodes(OLEDs), organic field-effect transistors(OFETs) and organic photovoltaic cells(OPCs) due to their advantages such as abundant sources, low cost, easy fabrication,good flexibility and large-area production. However, the mobilities of organic materials need to be improved in comparison to those of inorganic materials. Over the past two decades, researchers are focusing on designing new materials with high carrier mobility and optimizing the device structures experimentally, so far the device performance has been significantly improved. Correspondingly, with the rapid development of computer technology and the updating of quantum chemical methods, theoretical calculation is becoming a powerful tool to study the charge transport properties of organic materials and predict their carrier mobilities.Oligothiophene-based organic optoelectronic materials have been extensively reported and applied in organic semiconductors, but low conjugation because of the torsion of single bonds limit their application in practical devices. Fused oligothiophenes(thienoacenes) possess the advantages of rigid planarity and extended conjugation, thus leading to improved charge transporting properties and chemical stability. The sulfur atoms in thienoacenes have high polarizability and thus facilitate electron-donating properties. Multiple short intermolecular S...S contacts originating from the sulfur atoms at the molecular periphery can make the molecules densely packed in the solid state, resulting in the enhanced charge transport properties. Herein, we study the charge transport properties based on thienoacenes materias.In this dissertation, based on quantum chemistry theory and method, we explore the molecular reorganization energy, frontier orbitals, ionization potential and electron affinity, aromaticity, transfer integral and orbital interaction, and carrier mobility, aiming to explain why a structural change leads to significant effects on the charge transport properties at a microscopic level and establish the structure-property relationship. We hope our work could shed light on designing new organic optoelectronic materials and improve the performance of electronic devices experimentally. Here, we choose two typical systems based on thienoacenes and investigate their charge transport properties. Our work mainly includes two parts as follows:Part 1: The effect of structural symmetry and linking mode on the electronic and charge transport properties of dimers of dithienothiophenes were investigated by using density functional theory(DFT). To gain a better understanding of the effects of the structural symmetry and linking mode on the dimers, the geometrical structures, molecular reorganization energies, molecular ionization potentials(IPs) and electron affinities(EAs), molecular aromaticities, frontier molecular orbitals, as well as charge mobilities are analyzed for the investigated dimers of dithienothiophenes. The calculated results show that the vinylene-linked dimers have advantages over the directly single-bond linked dimers because of the large extent of π conjugation and thus enhanced π–π stacking interactions in their crystal structures. The molecular symmetry could affect the electron density distributions in the molecules, and further determine the molecular orientations and intermolecular arrangements. High molecular symmetry could facilitate the molecular packing in order, thus enhancing the charge transport. Therefore,the highly symmetrical vinylene-bridged dimers could be promising candidates for transistor applications. Our calculated results shed light on the molecular design of high performance organic optoelectronic materials.Part 2: The effects of sulfur oxidation on the electronic and charge transport properties of fused oligothiophene derivatives were studied in this part. Taking aim at the effects of oxidized location and extent on the modulation of electronic structures and charge transport properties of oligothiophenes, the geometrical structures, molecular reorganization energies, molecular ionization potentials(IPs) and electron affinities(EAs), molecular aromaticities, frontier molecular orbitals, as well as charge mobilities of oligothiophenes and corresponding S, S-dioxides are investigated through comparative analysis. The calculated results show that increasing the extent of oxidation will decrease the reorganization energy, which is favorable for charge transport; the introduction of sulfone group makes the EAs of S,S-dioxides more negative, thus lower the energy barrier for electron injection and improve the stability of their anions; The oxidation decrease the molecular aromaticities and facilitate the π-π packing. The calculated results would hint us that chemical oxidation of the sulfur atoms to S,S-dioxides is an effective strategy to modulate the electronic and charge transport properties of fused oligothiophene. It is also demonstrated that such oxidation of thiophene could be a potential approach to transform p-type thiophene-based semiconductor to ambipolar or n-type OFET materials.