Effects Of Electron-phonon Interaction On The Transient Transport Through A Polymer Chain

In order to obtain non-equilibrium state,open quantum system closely related to quantum dissipation has been the focus of attention of researchers.The influence of electron-phonon interactions,in an open quantum system,on the carrier transport properties in the system is always one of the hot spots.In this paper,the method of combining hierarchical equations of motion(HEOM),based on Green's function,with non-adiabatic molecular dynamics is used to discuss the meta-excitation kinetics and transient transport process in the electron-phonon interaction system represented by conjugated polymers.The intermediate part of the electronic part of the Hamiltonian is described with Su-Schrieffer-Heeger(SSH)model and the source and the drain are described by non-interaction Fermi reservoirs.We can get the differential equations describing the density matrix of the electronic part of the system and the first order adjoint density matrix by means of the HEOM method based on the Green function.The classical part of the lattice is used to satisfy the Newtonian equation of motion.And then through the numerical calculation method to carry out time simulation.The effects of different electron-phonon coupling on the carrier transport properties of the open quantum system was investigated by HEOM method.It is shown that the electron-phonon coupling hinders the carrier transport in the system when the bias voltage is small,while as the bias voltage increases to a certain extent,the electron-phonon coupling promotes the carrier transport.This is due to the increase of the bias so that the carrier in the electrode can be injected into the system,and then induced lattice distortion which affect the carrier carrier transport.Moreover,the greater the electron-phonon interaction,the stronger the lattice relaxation capacity.At the same time we also found that:the number of mid-lattice points were increased,in our consideration of the bias range,the electron-phonon coupling first impede the carrier transport,in promoting the phenomenon of carrier transport will be repeated.When reducing the coupling between the system and the electrode,the greater the electron-phonon interaction,I-V curve appears in the current platform more obvious.Because the system and the electrode coupling decreases which makes the energy level of the system widen and the greater the electron-phonon coupling lattice relaxation capacity,the more obvious the emergence of the ladder-like platform.At last We compare the results of the non-equilibrium Green's function method and the HEOM method to verify the accuracy of the HEOM method for handling the carrier transport properties of the open quantum system.We also simulated the dynamics of the system under the Sweep bias.It is found that when the bias voltage is added and increased to the maximum value,then decreases.In the I-V curve,the current will be hysteresis.Because of this strong electron-phonon interaction in organic polymers,moving electrons and holes induce distortion of the lattice which produces a local potential field where electrons and holes are bound and form an exciton state.Because of this "self-trapping" effect,when an incremental bias and a decreasing bias were added by changing the electrode Fermi surface,the current is hysteresis in the volt-ampere characteristic curve.This is due to the addition of an incremental bias,the intermediate system will form an exciton state;by adding a decreasing bias,the exciton state of the intermediate system will dissociate.And the bias of the exciton state and the bias of the dissociation exciton state are different,resulting in a difference in the two bias voltages that cause the current to appear to hysteresis.It will make the current hysteresis phenomenon more obvious when the electro-acoustic coupling strength is reduced.At last,it is found that when the chain length increased,the hysteresis phenomenon of the current is repeated in the same range of the bias voltage as we consider.And when the electro-acoustic coupling increased or the coupling of the system and the electrode decreased,the effect of the current hysteresis becomes more obvious.