Transient Transport Properties Of Polythiophene

With the development of experimental methods,such as scanning microscope and molecular self-assembly,more and more attention has been paid to the research of molecular electronics.The study of molecular electronics is mainly based on aromatic organic polymers with benzene ring as the basic unit.These organic compounds can be regarded as quasi one dimensional systems due to their conjugated ? electronic structures.The interaction between electrons and lattices can support the nonlinear elementary excitation of multiple structures,such as solitons,polarons and excitons.These elementary excitations can make the organic polymers play unique physical properties.Polythiophene and its derivatives are good candidates for organic molecular devices due to their unique optical and electrical properties.Su-Schrieffer-Heeger(SSH)model is used to study the static and dynamic processes of isolated electron lattice coupling system,but for the molecular and electrode coupling,that is the dynamics of the excited states in open quantum systems,lack of understanding,especially because of the various physical phenomena caused by the electro acoustic interaction.In this paper,the extended SSH model is used to study the transient transport properties of polythiophene based on the coupled motion equations and the nonadiabatic molecular dynamics.Firstly,we study the initial properties of polythiophene molecular dynamics by self consistent iteration method,we found that the thiophene molecule has four periodic structures and due to the existence of the heterogeneous sulfur atoms,the energy band is asymmetric.Then,the transient transport properties of organic molecular devices composed of four thiophene rings are calculated by using the method of the hierarchical equations of motion and non-adiabatic molecular dynamics.It is found that the system reaches the steady state after a period of time,about 68 fs,when it is subjected to external excitations.When the bias voltage is small,the current is very small and the oscillation amplitude is small due to the absence of conductive channel.When the bias voltage reaches a certain value,there will be a conductive channel in the bias window and system current increase.When the bias voltage islarge,the amplitude of the system current is large,and the main reason is that the oscillatory behavior of some electric conduction levels enters the bias window instantaneously.Molecular length is also an important factor affecting the transport properties of the system,we also discuss the effect of chain length on the transient transport of thiophene.It is found that the steady of the current decreases with the increase of the chain length within a certain bias.This is due to the resonance level of the longer chain closer to the bias window,the effect of electron phonon coupling increases the scattering effect of lattice atoms on electrons,so the current is smaller.With the increase of bias voltage,the current increases greatly,and then appears the current platform,the longer the chain,the earlier the platform,and the height of the platform will be reduced.This is due to the lowest unoccupied orbit and the highest occupied orbit of the system into the bias window,while the other levels do not enter the bias window.At the same time,when reached the resonance energy level,the effect of the electron phonon coupling is the largest,and the scattering effect of the lattice is the strongest,so the current platform drops.Finally,the influence of electron transition(t_?~S)between the carbon atoms and carbon atoms on the two sides of the hetero sulfur atom on the transient transport properties of polythiophene is discussed,it is found that when the transition integral is 0eV,the phenomenon of current surge appears relatively late,about 1.8eVat,this point the lattice exhibits a regular dimerization state,and there is no transmission peak into the bias window,so the current is smaller,with the increase of the bias voltage,the presence of the sulfur atoms in a certain range of bias will suppress the steady flow,this is due to the increase of the transition integral,the transition energy increases,the value of the steady flow is smaller.