Study Of Re-excitation And Interchain Coupling In Organic Conjugated Polymers

As a new sort of functional material with a broad prospect in application, organic conjugated polymers have been an indispensable position in the field of semiconductor materials because of their unique plasticity, light-weight, low cost, easy processing and good flexibility since they were found being conducting through doping in the eighties of last century. And their optical, electrical, magnetic properties are so rich that have attracted much attention to materials physics researchers. In thirty years, the theoretical study of organic conjugated polymers materials has made great progress, which is guiding the production and application of organic conducting materials effectively. Now the organic conjugated polymers materials have already been successfully applied to the manufacturing of semiconductor device and solar cell. And their future is full of infinite possibilities.The most essential difference between organic conductive polymers and inorganic materials is that the carrier of inorganic materials is electron or hole; however, the carriers of organic conducting polymers are soliton, polaron and so on. W. P. Su, Schrieffer and Heeger established the SSH model by using the "tight binding approximation" and ignoring the quantum effect of carbon atoms for the first time in the study of the properties of polyacetylene, which made a great contribution to the research on organic conjugated polymers. And then they put forward some new concepts in the field of organic conjugated polymers, for example, polaron and exciton. In the theoretical study of exciton and bi-exciton of conducting polymers, Sun Xin academician discovered that the polarization of exciton is normal in the external electric field, but the polarization of bi-exciton is reverse. When conducting polymer absorbs a photon, the exciton state will be re-excited and turn into a new state, bi-exciton, in which cases, polarization is reverse. Reverse polarization is a very peculiar and important physical phenomenon in semiconductor materials which not only has great research value on basic electronics but also has potential value in production application.The contents and conclusions of this paper mainly include the parts as follows:(1) In the third chapter, we study further the properties of single electron re-excitation state of polaron in the trans-polyacetylene by using the SSH model with the natural boundary condition. Single electron re-excitation state of polaron is found appearing reverse polarization in the external electric field. And the electric field strength determines the degree of reverse polarization. In the strong electric field, reverse polarization will disappear gradually, normal polarization occurs at the same time. So we can produce the quantum switch, by controlling the intensity of electric field to change the polarization when system is a single electron re-excitation state of polaron. Our investigation results suggest that te is not conducive to the generation of reverse polarization of single electron re-excitation state of polaron.(2) The effects of interchain coupling on the polaron, exciton, bi-exciton for twochain system(a number of chains) of finite chain length cis-polyacetylene were studied in Sec.Ⅳusing the SSH model including interchain interactions with the natural boundary condition.First of all, we discussed the effects of interchain coupling on each chain for twochain system. Then our study is focus on the interchain delocalization of polaron for a number of chains system. Charge transfer is also found in a number of chains system with considering interchain coupling. When the interchain coupling is strong enough for system, charge transfers in the adjacent chain and polaron spreads in all chains hierarchically.Secondly, we researched the effects of interchain coupling on the degree of electronic delocalization of exciton state and the total energy of the system. There exists a critical chain coupling, which makes exciton be distributed in the two chains evenly and makes the energy of the system reach the maximum value and remain unchanged. And then we discussed the dissociation of exciton in two-chain system. In an electric field, exciton on one chain in two-chain system will be dissociated into two oppositely charged polaron, which is the same to that of the single chain. Interchain coupling reduces the binding energy of exciton, and makes it more easily to be dissociated into other carriers with setting the value of the electric field. Keeping a weak value of coupling, we find that the strong electric field is also conducive to the formation of new carriers.At last, we analyzed the effects of coupling interaction on bi-exciton for cis-polyacetylene two-chain system. It is found that interchain coupling makes charges transfer in the chains, forming a lattice distortion on the ground state chain; and makes the inversed polarization of biexciton in the first chain be enhanced remarkably, because the coupling interaction can reduce the localized energy difference of the system. As the coupling increasing, there is a bi-exciton appearing on the ground state chain, whose polarization is inverted in the electrical field. There is a critical value of coupling interaction making the reverse polarization be reduced on two chains. However, the bi-exciton state is primarily localized in one chain even the interchain coupling is included. And there is no the situation that the two chains have the same lattice distortion like polaron and exciton in the two-chain system. Charge may transfer in the chains within a local interchain coupling. However, the polarizations in chains of system with the coupling of different styles are different. Some local coupled model will make charge transfer in the adjacent chain, but cannot form bi-exciton on the ground state chain in the system.In conclusion,there is a reverse polarization phenomenon in re-excitation of polyacetylene, which is noticeable problem in the study. Interchain coupling stops charges from being localized in a chain, and makes them transfer in the chains, forming a lattice distortion on other chains; Interchain coupling reduces the binding energy of carriers in polyacetylene and is conducive to the formation of new carriers. The reverse polarization phenomenon will also appear in bi-exciton of two-chain system. However, there is no phenomenon in some local coupled model.