Theoretical Study On Charge Transport Properties Of ?-conjugated And Nitrogen Organic Semiconductor Materials

The theoretical investigation on charge transport and electronic properties of a serious of?-conjugated and doping complexes derivatives were carried out by Marcus-Hush theory and density functional theory?DFT?based on quantum mechanics?QM?calculations.The study consists of five parts:?1?The pyrene derivatives are remarkable candidates for organic semiconductor materials.Various molecular structure modifications of 12 different pypene derivatives are investigated to reveal structure-mobility relationships of the organic materials,such as changing the central core by linking different backbone structure and introducing different type substitution groups in different positions.The charge mobility is estimated based on molecule single-crystal structures and incoherent charge-hopping model at 300K.The calculation results are in good agreement with the experimental results.The hole mobility is predicted to reach 2.95 cm² V^-11 s^-11 for 2,7-diphenyl-substituted pyrene derivative and the largest electron mobility for 4,5-dithien pyrene derivative as high as 6.24 cm² V^-11 s^-1.In this study,the charge transport properties is bound to be impacted with the molecular structural adjustment with the substituent group of organic semiconducting molecules.?2?The orientational dependence of the charge transport mechanism in the crystal structure are systematically investigated.Organic semiconductor molecules with large transfer integrals and small reorganization energies originating from the?-stacking structures show better charge transport properties.The transfer integral along hopping pathway should be attributed to parameters of stacking structures.A semi-classical Marcus model for calculating anisotropic mobility is extended from one to three dimensions of crystalline molecule.As we predicted,the mobility calculated from 3D model is improved by considering the transfer integral contribution from every directions in space,rather than just one plane.?3?The influence of the dimer packing structures and intermolecular interaction on the transport properties were investigated.Insights into the consideration of intermolecular interaction and electronic coupling,there will be a synergistic relationship when the?-?packing way and high intermolecular interaction coexist in the hopping direction,which tends to cause enhancement of charge transport properties.It turns out that intermolecular interactions are responsible for the change of charge transport,while interactions come from?-stacking structure dominate donor-acceptor transport process.The intermolecular interaction analyses demonstrates that hydrogen bonds play an important role with strong electrostatic interactions.?4?The charge transport properties of 1,3,6,8-tetraazapyrene?TAPy?derivatives are theoretically investigated.The orientational dependence of calculated mobilities in the crystal structures are carries out to estimate the effect of the introduction of various electron-attracting substituents on charge transfer properties,which does not disorganize the planar skeleton.It is shown that core chlorination and bromination in molecules can decrease the lowest unoccupied molecular orbital?LUMO?energy levels as well as improve the?-system molecular stability.In addition,the calculated reorganization energies and transfer integral values show that the TAPy derivatives with electron-withdrawing groups may well be potential n-type organic materials with the high electron mobility.The simulation results of the angle-resolved anisotropic mobility from master equation approach shows that the maximum mobility is along the direction with the largest transfer integral value.?5?At the interface of the doping complexes,the degree of charge-transfer is influenced through various factors based on the charge analyze method,including dimer stacking structure,the strength of the intermolecular orbital interaction,the offset defined by the donor ionization potential and the acceptor electron affinity,and HOMO_D-LUMO_A gaps.To better understand the charge transport properties in the complex crystals,a computing model was performed to calculate the reasonable mobility values by combining semi-classical Marcus theory with molecular dynamics simulations,in which the mobility values were in agreement with experimental values.The average and largest mobility were 0.21 and 4.59cm² V^-1 S^-1 for TTF-TCNQ in room-temperature based on anisotropic transport property and random-walk simulation of charge diffusion coefficient.Charge-transfer complexes possess unique metallic conducting properties with the predicted resistance of 4.43 k?in the interface of TTF-TCNQ.Charge-transfer complexes exhibit larger mobility and higher conductivity compared to the constituent donor and acceptor molecules.This theoretical study researched the importance of tuning the molecular geometry for high-performance organic semiconductor materials.Incorporating the understanding of the molecular packing ways and intermolecular interactions in the design of organic semiconductors can assist in developing novel materials.Our calculation results could provide enlightening information on possible methods to increase the charge mobility and the design for organic electronic devices with high-mobility.