| Organic conjugated polymers have attracted much theoretical and experimental attention since the discovery in 1977 that trans-polyacetylene can be doped to exhibit a very high conductance and become into the good conductor. As a new kind of functional material, organic conjugated polymers have the electronic properties of metals and semiconductors, as well as the processing and performance advantages of polymers for easy-processing, flexibility and low-cost, and have become into the focus of a lot of research works. Thanks to the rapid development in the past 20-plus years, numerous high-performance photoelectric devices based on organic conjugated polymers have been fabricated successfully, such as organic light-emitting diodes (OLED) and photovoltaic cells. The work principle of these devices is mainly based on physical processes involving the charge injection, the charge transport and the combination of electron-hole pairs, etc. Due to the strong electron-phonon interactions in the organic materials, the electron state and the lattice state influence each other. The additional charge (electrons or holes) or photoexcitation in organic conjugated polymers will induce lattice deformation, so the charge carriers in organic conjugated polymers are not the traditional electrons or holes but the self-trapping excitations, such as solitons (only in trans-polyacetylene), polarons, bipolarons, excitons or biexcitons.Since 1970's, Su-Schrieffer-Heeger (SSH) Hamiltonian, the semi-empirical tight-binding calculation method that was found by Su, Schrieffer and Heeger, has been demonstrated successfully for studying the electronic structure and optical properties of organic conjugated polymer-polyacetylene. In the later years, Bishop, Sun, Conwell and Xie et al. extended the SSH Hamiltonian to study the excitations and their dynamic processes. Using a nonadiabatic dynamic method, Wu et al. studied the dynamic process of polaron formation in a metal/polymer/metal sandwich structure. Yan et al. simulated the dynamic process of bipolaron formation in a metal/polymer/metal sandwich structure. Fu et al. studied the injection and transport of charge in a metal/polymer structure. The further development in this field not only favors our understanding on the microcosmic physical world of the polymers but also has the important application value, and can drive the study and exploiture of the new organic molecule devices.Within the tight-binding Su-Schrieffer-Heeger (SSH) model and a nonadiabatic dynamic evolution method, we simulate the injection and transport of charge in metal/impurity polymer/metal sandwich structure and in metal/coupled polymer chains/metal sandwich structure at the same time. Finally, we discuss the injection and transport of electrons and holes in a metal/polymer/metal sandwich structure. The detailed study and main results are given below:1. Charge injection and transport in metal/impurity polymer/metal sandwich structureIt is well known that there are impurities in the practical samples, these impurities may come from the cross coupling, complicated morphologic impurities or external impurities. And the injection and ejection of charge need to be considered in the practical application processes, so it is important and significative to study the dynamic processes of injection and transport of the charge in metal/impurity polymer/metal sandwich structure.It is found that the impurity ion acts as a control gate on the charge carriers in the polymer. The state of the control gate is determined by the strength of the electric field and the magnitude of the voltage bias. Keeping the voltage bias unchanged, when the external electric field is weak, the gate is closed, and the injected charges can not pass the impurity ion; when the external electric field is strong, the gate is open, and the injected charges nearly can all pass the impurity ion, and the most passed charges can over the interface potential to enter the right metal electrode. Keeping the electric field unchanged, when the voltage bias is big, the gate is half-closed, a part of the injected charges has big energy and can pass the impurity ion to continue moving along the polymer chain. 2. Charge injection and transport in metal/coupled polymer chains/metal sandwich structureIn actual organic materials, the length of molecular chains is limited, so it is important to study the interchain transport of charge carriers. And the interchain coupling plays a very important role in the conductance and other electrical properties of the polymers. At present, many experiments show that it is necessary to consider the interchain coupling of the polymer chains. Based on the cause, we study the injection and transport of charge in metal/coupled polymer chains/metal sandwich structure.It is found that the interchain transport of charge is determined by the driving electric field and the voltage bias. Strong electric field and big voltage bias are both favor of the interchain transport of charge. To enhance the efficiency of the charge transport in metal/ coupled polymer chains /metal sandwich system, we need to increase the strength of the driving electric field and the magnitude of the voltage bias. Furthermore, the coupled area of two polymer chains acts as a cutter on the charge carriers, and it can divide a wave packet into two parts. Thereout, we illustrate that the charge transports in the form of probability wave.3. Injection and transport of electrons and holes in metal/polymer/metal sandwich structureFormer investigations just focused on the dynamic processes of the electron injection and transport. However, in an OLED, electrons and holes are injected into the polymer from the opposite electrodes. These charge carriers move in the polymer under an external driving electric field. The injected electrons and holes may meet and annihilate to give light emitting. Therefore, it is helpful to study the injection and transport of electrons and holes to understand the microcosmic mechanism of an OLED performance. So we study the dynamic processes of the injection and transport of electrons and holes in a metal/polymer/metal sandwich structure.Due to the strong electron-lattice interactions, the injected electrons and holes form wave packets and move face to face along the polymer chain under the driving of the electric field. The system will form a neutral exciton state. Due to the competition between the electrical force and the electron-phonon couplings, the formed exciton presents an oscillation between positive and negative polarization. The system reaches a dynamic balance finally. |
