Theoretical Study On Intrinsic Electron And Hole Mobility Of Acenes And TTF Molecular Crystals

Organic conjugated molecules and their derivatives due to their delocalization π－electrons,which usually show a certain charge transport capacity,are called organic semiconductor materials.According to the polarity(electron or hole)of charge carriers,organic semiconductors can be divided into n-type and p-type transporting materials.These two different types of conducting semiconductors play a key role in making up the organic field effect transistors(OFET).Molecular conductivity significantly affects the performance of OFET devices.Marcus-Hush electron transfer theory shows that the charge transfer rate is dependant on the electron coupling between molecules and the intramolecular and intermolecular structural relaxation during electron transfer reactions.The degree of structural relaxation is usually measured by the magnitude of the reorganization energy,including the internal reorganization energy that directly related with doner and acceptor molecules,and the external reorganization energy(λext)of DA peripheral molecules.A majority of theoretical studies measure reorganization energy using only the internal reorganization energy(λint)approximation.At present,there are two kinds of methods,implicit and explicit,to estimate λext.Among which,the explicit method can clearly consider environmental molecules,better reflect the characteristics of conjugated organic solid discrete media and anisotropic dielectric tensor.In this thesis,I uses the AMOEBA polarizable force field to describe the intermolecular interactions among the conjugated molecules,the state-specific polarizable force field(SS-PFF)is used to describe the relaxation DA molecular and DA peripheral molecular structures during charge transport in organic molecular crystals,and to investigate the influence of peripheral molecules on charge transport using a fully explicit method.The organic semiconductors usually behave huge difference in electron and hole transport.If improved engineering skills can eliminate the charge-carrier traps,the contrary polarity carrier also can well transport in traditional monopole preferred materials no matter n-type or p-type materials.Even similar electron and hole mobility can be obtained in them.In order to study whether the intrinsic mobility of electron and hole shows a big different,we choose two kinds of systems including oligoacene crystals and its derivatives and tetrathiafulvalene,to investigate the effect of external reorganization energy on the different polar charge transport using the SS-PFF explicit method.The Master thesis is mainly divided into following two parties:1.Taking the oligoacene crystals and its derivatives as research object.Two-point model is used to investigate λext for electron and hole carriers.I would like to know more about whether there is an asymmetric feature in the λext of electrons and holes.The results show that λint of holes and electrons are close to the results of QM,and λext behave notable asymmetric feature,which is inconsistent with the conclusion of implicit evaluation of non-equilibrium polarizble contiuum model(PCM).For acenes crystals,the λext of the electron is 7 to 10 times that of the hole carrier,but it is the opposite in the perfluoroacenes crystals.According to Marcus theory,small λext will lead to better charge transfer performance.The asymmetric λext can well explain why the hole transport ability of oligoacene system is stronger than that of electrons,which is usually used as p-type transport materials,while perfluorine substitution can change the conductive polarity of oligoacen.In addition,perfluorination not only increases the electron affinities of acenes and ambient stability,but also decreases the λext of electron and enlarges electron intrinsic mobility.2.In part two,Choose the tetrathiafulvalene of sulfur-containing atoms as the research object,QTAIM is applied to obtain the state specific atom polarizability in combination with the finite field method,and the state-specifc polarizable force field(SS-PFF)are parameterized Hessian matrix localized vibration analysis based on tetrathiafulvalene QM calculations.SS-PFF was used to investigate the intrinsic electron and hole charge mobility of tetrathiafulvalene.The results show that the λext of the hole carrier of tetrathiafulvalene is much smaller than that of the electron carrier,which makes the hole carrier more readily to migrate in tetrathiafulvalene materials.In addition,the electron coulplings of velocity dominant channels shows that the coupling of hole is larger than that of electron.Taken two factors into account,the intrinsic mobility of tetrathiafulvalene for hole is larger than that of electron,thus it is easier to show p-type transport properties.