Conjugate Dynamical Properties Of The Polaron In The Polymer

Conjugated polymers which are different from traditional semiconductor, have some unique properties. Most of them are quasi-one-dimensional systems with strong electron-phonon interaction. Their carriers are composite particles, which are different from simply electron and hole, characterized with the surrounding distortion of lattice configuration (polaron with spin 1/ 2 , bipolaron with spin 0 etc.). The transport of the carriers and the process that oppositely charged polarons combine to from exciton are believed to be of fundamental importance for electroluminescence properties. In this paper, based on the tight-binding Su-Schrieffer-Heeger (SSH) model and by using a nonadiabatic molecular dynamic method, we investigated some unclear problems of polymers in details, such as polaron dynamic properties in different systems, the inelastic scattering processes of oppositely charged polarons driven by an external electric field,and the influence of electron-electron interactions on the dynamics of a charge polaron in conjugated polymers. The conclusions of this paper include three parts.1. The dynamics of charge polaron in conjugated polymersIn the third chapter, based on the tight-binding SSH model which neglects electron-electron interactions and its extended model, and by using a nonadiabatic molecular dynamic method, dynamics of charge polaron in conjugated polymers are studied, the results show that:(1) The polaron driven by an external electric field has a stationary velocity. It is found that the stationary velocities of polaron depend on the field strength and mode of the electric field ( E₀ ), confinement parameter ( t_e), and the electron-phonon coupling constant (α). But all these changes of stationary velocities are owing to the changes of the localization and/or the localized charges of polaron. The weaker the localization is and the fewer the localized charges are, the larger the stationary velocities are.(2) The polaronic migration in the polymer/polymer interface is believed to be fundamental importance for the transportation and light-emitting properties of conjugated polymers in polymer based light-emitting diodes (LED's). Our calculational results shoe that the polaronic migration at the interface dependents sensitively on the potential barrier induced by the energy mismatch in interfaces (Δ_e), the coupling interaction ( t_h) which effectively reduces the potential barrier in interface, and the applied electric field strength ( E₀ ) which provides the polaron kinetic energy to overcome the potential barrier. The stronger the E₀ is and the smaller the t_h is, as well as the lower theΔ_e is, the easier the polaron hops over the coupled interface to nearby chain.2. The inelastic scattering dynamics of oppositely charged polarons in polymersIn the fourth chapter, we investigate the inelastic scattering dynamics of oppositely charged polarons in single polymer chain and the polymer/ polymer interface. We find that the scattering process of the charge and lattice defect is different under different strengths of electric field and the interchain coupling interactions.(1) The results of the dynamics of oppositely charged polarons in polymer chains show that:â‘ At a weak field (<0.1 mV/A), they finally scatter into an entity, which will be dissociated as a pair of neutral solitons after their first collision and separation.â‘¡At a mediate electric field (0.1 mV/A < E0≤1.2 mV/A), the oppositely charged polarons will scatter into a pair of independent particles and each of them is a mixture of a polaron and an exciton. The yield of exciton in this process depends strongly on the strength of applied electric field.â‘¢At a stronger electric field ( E₀ >1.2 mV/A), the two polarons will merge together and form an entity, and then the entity will dissociate into a pair of oppositely charged solitons, or they will break into a lot of small lattice vibrations, depending on different initial conditions.(2) The results of the dynamics of oppositely charged polarons in the coupled conjugated polymer interface show that : If it is smaller coupling interactions ( t_h) in interface, the oppositely charged polarons return with primitive charge after collision under the driven of an external electric field, as the t h increases, the overlap of oppositely charges waves increase, the charges of two polarons decrease gradually, polarons return with the residual charges and signs remain, they are a pair of independent particles and each of them is a mixture of a polaron and an exciton. In degenerate polymer, continue to increase the coupling interactions in interface (e.g. E₀ =0.1 mV/A, t_h≥1.88eV), they will form a self-trapping exciton which evolves into a charge soliton and neutral antisoliton after a long time on one chain, and other chain is dimerization. At a stronger field and bigger coupling interaction (e.g. E₀ =2.0 mV/A, t_h= 2.0eV), we can find that two polarons with opposite sign charges will dissociate after collision, the electron and/or hole is freely scattered in chains.3. Effects of e-e interactions on the dynamics of polaron in conjugated polymersIn the fifth chapter, based on the one-dimensional tight-binding SSH model and the EHM, by using a nonadiabatic molecular dynamic method, we investigate the effect of electron-electron interactions on dynamics of charged polaron in a conjugated polymer chain at the unrestricted Hartree-Fock (UHF) level. The polaron energy levels which are degenerate for spin without e-e interactions are split, and the localization of polaron is changed by e-e interactions.(1)In the simpler Hubbard model (HM), results show that the stationary velocities of polaron varies with the on-site Coulomb interaction (U ). One can find that both the Coulomb attraction ( U < 0) and repulsion ( U > 0) restrain the polaronic motion and the stationary velocity of the polaron reaches its local extremum value at U = 0, where the system undergoes a quantum phase transition between charge-density-wave (CDW) for U < 0 and spin-density-wave (SDW) for U > 0.(2) Next, we turn to considering both on-site repulsions (U ) and the nearest-neighbor interactions (V ), i.e., extended Hubbard model (EHM). Only positive U , V > 0 are considered, and the results show the stationary velocity of the polaron varies with e-e interactions. It is found that the maximum value of stationary velocities of polaron appears at U≈2Vwhere quantum phase transition between CDW and SDW undergoes in the extended Hubbard model.