Polaron Dynamics And Zener Tunneling In Conjugated Polymers

The discovery of high conductivity of doped trans-polyacetylene (tPA) broke through the conventional opinion that all organic polymers were insulators, and opened a new area of conducting polymers. The p_z-orbital of neighboring carbons overlap to form conjugated n bond, so conducting polymers can be called conjugated polymers. Conjugated polymers span a wide range from insulator to superconductor. The special structures and excellent physical and chemical properties have made it a focus of material science. As a new kind of irreplaceable fundamental functional materials, polymers has promising potential application in optoelectric device, sensing probe, molecular device, electromagnetic shielding, and metal preservation. A lot of significant progress has been made for conjugated polymers in many aspects, such as molecule design, material synthesis, doping method, solubility and processability, and conducting mechanism. Now, some optoelectric devices based on conjugated polymers have been turned into practical technique from pure experimental interest, including organic light-emitting diode (OLED), field-effect transistor (FET), photovoltaic cell, etc.Compared with the traditional organic semiconductor, organic material has its unique properties. First, most of the conjugated polymers have the quasi-one-dimensional structure due to the weak interaction between the organic molecules. Second, there are strong electron-lattice couplings in organic systems. Injected charges (electrons or holes) or photoexcitation in conjugated polymers will induce lattice deformation, and the lattice deformation in turn influences the energy band structure of the organic system. So, it is generally believed that these self-trapping excitations, including solitons (only in trans-polyacetylene), polarons and bipolarons, are charge carriers in conjugated polymers. These elementary excitations are of fundamental importance for transport and photoluminescence of conjugated polymer system. Therefore, the dynamical process of these excitations has always been the focus in both theoretical and experimental investigation, and extensive studies have been carried out on charge carrier dynamics and on the promising application of conjugated polymers in organic functional devices. In 1970's, a tight binding model-SSH model, developed by Su, Schrieffer and Heeger, was adopted to study the electronic structure and optical properties of the simplest conjugated polymer PA and showed considerable success. Since then Bishop, Sun and Xie et al have extended the SSH Hamiltonian to research both the static and dynamic process of excitations. At present, there mainly are four groups performing researches on charge transport in conjugated polymers based on the SSH model. The further research in this field not only can broaden our understanding of the microcosmic physical world but also have a substantial impact on the applications of organic devices.Within the tight-binding electron-lattice interacting model, by using a nonadiabatic dynamic method, we have simulated the dynamic formation process of a interchain delocalized polaron and its motion driven by the external field in a well-ordered system of coupled conjugated polymer chains. The dynamical behavior of the interchain delocalized polarons is compared with that of the intrachain localized ones. Moreover, we have also explored the effects of temperature on the polaron stability. At last, the behavior of conjugated polymer chain under high electric field is investigated. The detailed research and main results are given below:1. Dynamics of interchain delocalized polarons in conjugated polymersPolaron formation and motion in a single chain is now a relatively well studied subject, and a great deal of effort has been devoted to such studies. However, the dynamical behavior of polarons in a more complicated system of coupled conjugated polymer chains is not so clear. Several experiments have reported high mobilities in thin-film transistors separately, in which owing to self-assembly, organic molecules form a relatively well ordered film structure by interchain stacking. In Sirringhaus's experiment which aimed to probe the transport properties of the ordered microcrystalline domains, it revealed the 2D interchain character of the polaronic charge carriers (polarons), and suggested that radical cations on isolated 1D chains can not lead to high mobilities. Thus, it is of great interest to probe the polaron transport in well-ordered coupled conjugated polymers, such as self-assembly films.We construct a system consisting of as many as ten well-ordered coupled conjugated polymer chains to provide enough space for the polaron to travel. First, the dynamical process of polaron formation in such a system is investigated. It is found that once the electron is added to the system, it is shared by the several chains and induces lattice distortion in the chains within a short time. With enhancing the interchain coupling, it takes longer time for the electron to overcome the interchain coupling to localize in a single chain. Beyond a certain value of the interchain coupling, the electron will evolve into a 2D interchain delocalized polaron state extending over several chains.By applying an external electric field to the interchain delocalized polaron, we study the 2D motion of the polaron, including the motion along the polymer chains (in which case the field is parallel to the chain) and the motion perpendicular to the polymer chain (in which case the field is perpendicular to the chain). Compared with the motion of the intrachain localized one, the interchain delocalized polarons drift much faster than localized polarons under the same electric field, implying that high mobilities can be achieved by 2D charge transport. This is in agreement with earlier experiments.2. Effect of temperature on the stability of polaronsPolaron stability is another issue to which much attention has been paid. Up to now, the external electric field was demonstrated to have significant influence on the polaron stability. Moreover, it was found that, besides the electric field strength, the application mode of the field also plays an important role in polaron stability. Other effects, such as impurities, defects, and collision between charge carriers, have also been thoroughly studied. However, all these works were based on the lattice dimerization, i.e., lattice fluctuations were not taken into account.Dimerization has been well understood in the static limit and within the mean-field theory. As a matter of fact, however, lattice fluctuations always exist. Even in the limit of zero-temperature, lattice fluctuations are of the same order of magnitude of the lattice distortion. Furthermore, at temperature comparable to the transition temperature the thermal lattice motion can be several times larger than distortion. The lattice fluctuations should have an important effect on the one-dimensional materials. In this paper, we investigate the stability of the localized polaron in conjugated polymers in the presence of thermal lattice fluctuations.The temperature effect is simulated by introducing random forces to the equation of the lattice motion. It is found that the localized polaron state becomes delocalized even at low temperatures. The time of polaron keeping localized depends on the magnitude of temperatures. By taking into account the thermal effect, we find that the dissociation field is weaker compared with earlier works.3. Zener tunneling in conjugated polymersThe issue of high-field transport in solids is of great concern and has been investigated for many decades due to basic interest in the physical phenomena and many important applications. Early theoretical contributions were made by Bloch and Zener. Since Zener explained the electrical breakdown of solid dielectric in terms of interband tunneling, the phenomenon of Zener tunneling has attracted considerable interest, particularly in modern nanoscale devices. To the best of our knowledge, however, Zener tunneling in conjugated polymers (belong to the family of organic semiconductors) is rarely referred to.In chapter IV, we consider the dynamical evolution of electronic states in a conjugated polymer chain under a high electric field, and try to explore the possible Zener tunneling effect in organic semiconductors. It is shown that under a sufficiently high field, electrons can transit from the valence band to the conduction band, which is Zener tunneling in organic semiconductors. The result also indicates a field-induced insulator-metal (I-M) transition accompanied by the vanishing of the energy gap, that is, the Peierls phase is destroyed. Meanwhile, the lattice configuration undergoes significant change from dimerization to equidistant arrangement when I-M transition occurs.