Study Of Polarization Of Exciton And Bi-exciton In Organic Polymers

Traditionally, organic materials based on carbon atom are insulator which have notthe properties of optics, electricity, and magnetism as well as conductor andsemiconductor have. In the late1970s, Alan J. Heeger et. al found polymers which areconductive. Then the organic electronics becomes an emerging subject area. Alan J.Heeger et. al won the Nobel Prize in chemistry in2000because of that found. In recentyears, organic materials have the properties of optics, electricity and magnetism arerealized gradually and their application in various electronic devices got the researchers’favor at the same time. In recent decade, electronic materials and devices based on hetraditional inorganic semiconductor developed and widely used and we know moreabout the physical properties of organic material. Organic conjugated polymer as a newtype of functional material which not only have the character of low cost, easyprocessing good flexibility but also have rich electrical, optical and magnetic propertiesare widely used in organic electronics, organic molecular electronics and organicspintronics etc. In early90s, intense research work has been undertaken in the study ofphysical properties of electroluminescence in conjugated polymer since light-emittingdevices based on conjugated polymers were demonstrated. Organic optoelectronicdevices such as organic light-emitting diodes (OLED), organic field-effect transistors(OFET) and organic photovoltaic cells (OPVC) based on organic conjugated polymershave a great commercial prospects and get more and more people’s attention.Polymer theory model constantly modify and improve in the last few decades. In1979, W. P. Su and others established SSH model promoted the development of thetheory of organic polymer in order to study of organic polymer.SSH model considers theelectron-lattice interaction and ignores the electron-electron interaction. The system willbecome a many-body system which difficult to deal with if considering theelectron-electron interaction. We can use Hartree-Fock approximation combined withSSH model and Hubbard model to deal with many-body system with electron-electroninteraction due to the energy gap of organic polymer materials is greater than the electronsrepulsion potential energy on the same site. In recent years, it was found that electroncorrelation has a great influence on the nature of the electronic states and the carrier inpolymers. Zhao deduced long-range correlation Hamiltonian and long-range correlationenergy formula which is of great significance for the study of electron correlation. In2000, Sun Xin academicians discovered that an electron on the top of valenceband leaps into the bottom of conduction band when a polymer molecule absorbs onephoton by light stimulation, this results in forming self-trapping exciton state wherepolarizability is positive because electron-hole will cause systemic lattice structure todistort. When the self-trapping exciton state absorbs a photon, it will lead to theformation of a self-trapping bi-exciton where electric dipole will be reversed. Thisphenomenon is called photoinduced polarization inversion (PIPI). This is a veryinteresting physical phenomenon which not only has great research value on basicphysics but also has potential application value in technology. We can develop newfunctional materials with negative polarizability and ultra-fast quantum switch device.Bi-exciton states is extremely unstable, however, it’s hard to control and observe inexperiment because of its life expectancy is10-10s. Therefore, it is make a lot of senseto find factors which can have effect on the polarizability of the exciton and bi-exciton.In this paper, the problems and conclusions are studied as follows:(1) The effects of the long-range electron correlation on the polarizability ofexciton in finite chain length polymer were studied in Sec.Ⅲ. The effect of thelong-range electron correlation on binding energy, charge density, linear polarizabilityand third order no-linear polarizability of exciton in finite chain lengthcis-polyacetylene were studied using the tight-binding model (SSH model) includingelectron-electron interaction and long-range electron correlation. It was found thatexciton is more likely to be dissociated into other particles duce to the binding energy ofexciton reduced by long-range electron correlation effect. Electron-electron interactionreduces linear polarizability (1)xxand third order no-linear polarizability (3)xxxxof excitonand long-range electron correlation reduces linear polarizability (1)xxand third order no-linear polarizability (3)xxxx of exciton again.(2) The effects of the long-range electron correlation on the polarizability ofbi-exciton in finite chain length polymer were studied in Sec.Ⅳ. It were found that inpolymer with external electric field long-range electron correlation effect increases thebinding energy, Local area of the lattice and the degree of reverse polarizationbi-exciton, so that bi-exciton is difficult to dissociation into the charged double polaronsdue to the stability of the bi-exciton increased. The degree of reverse polarization ofbi-exciton increases gradually, charge transfer increases linearly and electric dipolemoment decreases linearly and the linear polarizability increases firstly then decreasesby variation of V at the same external electronic field. The linear polarizability of bi-exciton is to be maximized when V=0.0950eV. In variation of external electric field,electron-electron interaction and long-range correlation effect increases linearpolarizability and electric dipole moment of bi-exciton before bi-exciton dissociation.Electronic interactions and long-range correlation effect reduces the linear polarizability,while increases the electric dipole moment when bi-exciton dissociation into positiveand negative bi-polaron. Both electronic interaction and long-range electron correlationcan increase third-order polarizability of bi-exciton.