Theoretical Study On The Charge Carrier Transport Properties Of Anthracene And Its Derivatives

Organic optoelectronic functional materials have received more and more attentions from both academic and industrial application in the past two decades due to their tremendous advantages such as flexibility, low cost, easy tenability and so on.At present, they are widely used to fabricate various organic electronic devices, such as organic solar cells(OSCs), organic light-emitting diodes(OLEDs) and organic field-effect transistors(OFETs). The charge carrier mobility in organic optoelectronic functional materials is one of the essential parameters that determine electronic device performance.Design and synthesize new materials with demanding high mobility has been a formidable task in the past two decades and now plenty of materialsexhibit room temperature mobilities between 1 and 10 cm2/V·sin thin film forms and even larger mobilities in single-crystal forms.The charge carrier transport properties in organic materials has been a subject of theoretical interests that could trace back to 1959, when Holstein proposed the small polaron model, which depicted the electron motion in organic solids. It is difficult to judge the applicability of the approximations often applied at different levels to solve the Holstein model Hamiltonian. On the other hand, B?ssler and coworkers developed phenomenological disorder model to describe organic devices. Nevertheless, from the microscopic view of material design, there are two main transport mechanisms: the band model for delocalized electrons and the hopping model for localized electrons. Among these, significant progress has been made by Brédas and coworkers, who evaluated the charge transport property through investigating the intermolecular coupling and the molecular reorganization energy. Accordingly, this dissertation based on above theory, investigated the classical organic molecular crystals of Anthracene and its derivatives of 9,10-bis((E)-2-(pyrid-2-yl)vinyl) anthracene. We predict the molecular geometry, molecular frontier orbitals, reorganization energies, intermolecular coupling, charge carrier mobility and so ontheoretically.We demonstrate here that the optimal theoretical and computational method, the relationship between charge carrier transport property and molecular geometry, as well as intermolecular interactions. Main research contents and conclusions as the following:Part Ⅰ:We choose the classical organic small molecule Anthracene single crystal as the research object, study the molecular structure, organization energy based on the density functional theory(DFT) method, using B3LYP/6-31(d, p) basis sets. We calculate the intermolecular charge transfer integral with PW91PW91/6-31(d, p). Finally, we get the carrier mobility though Einstein relationship, the hole mobility is 0.662 cm2/V·swhile the electron mobility is 0.590 cm2/V·s. The results show that the carrier mobilitiesare in good agreement with the experimental results, which shows that the theoretical method is feasible. Compared with other theoretical results, the appropriate theoretical methodand accurate charge transfer ways could guarantee the credible results.Part Ⅱ:The electronic and charge transport properties of BP2 VA, BP3 VA and BP4 VA, which could be differentiated by the linkage(ortho-, meta-and para-) between 9, 10-divinyl anthracene unit and pyridine, have been systemically investigated based on the DFT theory. For the three molecules, theoretical charge diffusion mobilities for holes are dozens times higher than those of electrons. The linkage(ortho-, meta- and para-) between 9, 10-divinyl anthracene unit and pyridine significantly influence not only the intra-molecular conformation(as the reorganization energies), but also the intermolecular interaction(as transfer integrals), and finally the charge mobility. The charge mobilities show the trend as BP4VA>BP2VA>BP3VA, indicating that linkage of para- between 9, 10-divinyl anthracene and pyridine exceedingly can reduce the steric effects of surrounding molecules in the crystalline state. For BP4 VA, we obtain the hole mobility as high as ~1 cm2/V·s.The result is of great importance for the design of new organic semiconducting materials with demanding charge carrier mobility. Particularly in the design of organic materials with an army of functional groups, involving the linkage of ortho-, meta-and para-. In one word, linkage of para- between different functional groups in designing process might be a better choice in order to obtain a demanding material with high charge carrier mobility.Part Ⅲ:We investigated the electronic structure and charge transport properties of BP2 VA molecule with three different crystalline polymorphs based on Marcus theory. The BP2 VA molecule in all the crystalline polymorphs favours hole transport, and the hole mobility of β-BP2 VA reached as high as ~1 cm2V-1S-1. The shifted-cofacialπ-stacking with the appropriate center-to-center distance, the good overlap of orbital distribution with the same phase, and the weak interactions could result in the strong electronic coupling and high transfer integral, and high charge mobility furthermore. The comparison of reorganization energy between individual molecule in gas-phase and embedded molecule when considering the steric effect of surrounding molecules suggests that the reorganization energy is dependent on intermolecular interaction for BP2 VA. This could shed some light on the better understanding of the relationship between the charge transfer properties and different crystalline polymorphs of organic semiconductors. Additionally, when design new optoelectronic material with high charge mobility, one should become more focused on the effect of intermolecular packing.