Study On The Dynamics Of Polaron Transport In One-dimensional Conducting Copolymers

In recent years, with the improvement of experimental instruments and the development of theoretical methods, the research on organic conducting polymers has been developed into a systematic and multidisciplinary field. Increasing interest of researchers has been attracted to the study on electronic, magnetic, and optical properties of organic polymers. Some results in common from both experiments and theories have become the bases of the further researches. Some optoelectronic devices firstly developed in the lab, like organic light-emitting diodes (OLED's), have brought about industrialization. The rapid development of the molecular electronics based on small organic molecules makes it possible to realize the new revolution of the electronic and information fields.As one of the most basic issues on organic optoelectronic devices, the properties of carriers in organic conducting polymers is all the while the focus of research. Due to the strong electron-phonon interactions in organic materials, the 'softness' of them leads the carriers to be not the electrons and holes, like in the traditional inorganic semiconductors, but the nonlinear excitations, such as solitons, polarons and bipolarons. To give guidelines to the development of experiments and devices fabrication, a great deal of theoretical work has been performed on the dynamical transport properties of carriers in the point of view of microcosmic under external electric field, and some valuable results in common have been obtained.Increasing attention has been paid to the research on organic optoelectronic devices based on copolymers, which is one kind of the most important polymers. Recently, moreover, much interest has also been attracted to small molecule copolymers with fine properties of rectification in the field of molecular electronics. Great deals of studies on the electronic structures of copolymers have been performed both theoretically and experimentally, and some valuable results, e.g., the properties of organic quantum well and superlattice, the tunable band structures, etc., have been obtained, which is significant for the further studies and applications of copolymers. However, most of the studies were considered in the aspect of electronic structure based on static pictures, but they were not enough for the understanding of transport properties of carriers.For this reason, we systematically studied the dynamical transport properties of polarons driven by external electric fields in copolymers formed by PPP and PT by using a nonadiabatic dynamical method, which has been proved to be an effective method in the studies of carriers transport in homopolymers. The outline of the research and the main results are shown as follows:1. The static electronic structure properties of copolymerWe firstly studied the static electronic structure properties of copolymer before the consideration of the dynamical behavior of polarons. It was found that the band structure of copolymers can be tuned by the scales and proportion of the segments or the strength of the interfacial couplings.2. The effect of scale of segments on the polaron transport in copolymersBy using an one-dimensional (1d) SSH tight-binding model in an extended form and the nonadiabatic dynamical method, we studied the dynamical transport properties of polarons driven by external electric fields in 1d model conducting copolymers -(A_x-B_y)_z-A_x-. We focused on the effect of scale of segments on the transport of the polaron, and found that with the increase of the scale of the segments, the behavior of the polaron gradually varied from that in homopolymers to that in copolymers.3. The effect of interface on the polaron transport in copolymersCombining with the results above, we focused on the effects of both the form and the strength of the interfacial couplings on the polaron transport in diblock copolymer -A_x-B_y-, which is in a more general form. It was found that for the gradual-changing (GC) interface, which serves as a fine bridge between the two segments, it is favorable for the movement of polarons, but for the abrupt-changing (AC) interface the transport of polarons depends on the relative difference between the bindingenergy E_b~A of the polaron and the offset of the polaronic level △E_p~- at the interface. For a polaron, the interface of weak coupling with β<1 serves as a 'barrier' and the one with β<1 serves as a 'well'. The most favorable strength of interfacial coupling for a polaron is the one with β= 1 of an AC form.