Spectral Analysis On Charged Carrier Dynamics In Conjugated Polymers

The organic conductive polymer is a new type of functional material,which has been widely used in organic electronic devices,sensors,electromagnetic shielding and metal anti-corrosion technology,especially in organic light-emitting devices?OLED?.Due to the strong electron-phonon interactions,the electrons or holes are injected from the metal electrode into the polymer chains,and deform the lattice distortion to induce self-localized excitations,such as polarons,bipolarons,which are used as charge carriers and transport under the action of an applied electric field.Therefore,the investigation of the migration process of such carries under an external electric field is of great importance.In view of this,the dynamic evolution of the polaron/bipolaron in a polyacetylene chain is simulated in terms of nonadiabatic dynamics method,by using the Su-Schrieffer-Heeger?SSH?tight-binding approximation model that includes an external electric field within periodic boundary conditions.In order to better understand the dynamic properties of these carriers,especially the oscillatory behavior existed in dynamic process,the frequency-domain analysis method--the power-spectrum density?PSD?and amplitude-spectrum methods are adopted.The specific research contents and main results of this work are as follows:1.Spectral analysis of polaron dynamic in conjugated polymersIn this section,we described the relationship between bond-current density and polaron velocity firstly,then performed the spectral analysis of polaron dynamics taking the bond-current density as the core.The results show that the zero-frequency peak of the bond-current density corresponds to the average velocity of the polaron.Further,the transition of the characteristics frequency of bond-current density is corresponding to the transition of the polaron velocity.Importantly,the fundamental frequencies of bond-current density are proportional to the average velocities of the polaron.We calculated and analyzed the power spectra of the charge density and the lattice vibration of acoustic mode as well as optic mode.It has been found that the characteristic frequencies of the charge density,acoustic and optic modes are identical when the field is lower than the critical field,the polaron moves at a sonic velocity;Once the field is higher than the critical field,the characteristic frequencies of the charge density and the optic mode still stay the same,and they also keep the same as that of bond-current density,about 3 times that of the characteristic frequencies of the acoustic mode,the polaron moves at a supersonic velocity.The characteristic peak corresponding to breather has been identified mainly in the spectrum of optic mode at about 0.18fs^-1.The relationship between the weight of the breather peak and the velocity of the polaron has been studied by using the PSD method.It has been found that,when the electric field is lower than the critical field,the polaron moves at a sonic velocity and the breather cannot be excited.Once the field is higher than the critical field,the polaron moves at a supersonic velocity,the breather can be excited.Finally,the effects of electron-electron interactions on the breather are briefly discussed,and the lifetime of breathers are estimated by using the Lorentz fitting method.These results provide guidance for a better understanding of the polaron dynamics in conjugated polymers.2.Spectral analysis of Bloch-like oscillations in conjugated polymersThe polaron will be dissociated under the action of a strong electric field.In this section,the dynamic properties of a dissociated polaron under a strong electric field in a conjugated polymer chain are investigated based on Su-Schrieffer-Heeger model,by using nonadiabatic molecular dynamics and frequency-domain analysis methods.We mainly discussed the effects of electric field,electron-phonon coupling,Brazovskii-Kirova symmetry-breaking term and thermal fluctuation of lattice.The results show that,the phonon scattering increases with the increase of electron-phonon coupling and Brazovskii-Kirova symmetry-breaking term,which not only make the drift motion of the charge and the DC component of the current density increase,but also lead to the half-width of the fundamental oscillation peak increase.As a result,the deviation between the Bloch-like oscillations in organic polymer and the perfect Bloch oscillations increases.In addition,the thermal fluctuation of lattice is not favorable to the Bloch-like oscillations in organic polymer.3.Spectral analysis of bipolaron dynamic in nondegenerate conjugated polymersIn this section,the spectral characteristics of the bipolaron dynamics has been investigated by using the PSD method.It has been found that,the fundamental frequencies of bond-current density are proportional to the average velocities of the bipolaron,which is the same as that for polaron.The characteristic peak corresponding to breather has been identified mainly in the spectrum of optic mode.The frequency of the breather is the same as the polaron case in nondegenerate conjugated polymers.However,the weight of peak corresponding to breather in the case of the bipolaron is lower than that of polaron.The fundamental frequency of the bond-current density and the weight of characteristic peak corresponding to breather are inhibited by electron-electron coulomb interactions.The dependence of the fundamental frequencies of bond-current density on temperature has been carried out.It is clearly presents two critical field regions.The critical field is about 4.0mV/?.For the case of the critical field higher than 4.0mV/?,the fundamental frequency of the bond-current density is enhanced by thermal effects.When the field is lower than 4.0mV/?,the fundamental frequency of the bond-current density is inhibited.This reflects the competition between electric field and temperature.