Recombination Of A Biexciton In Polyaniline

The present work studied an oxidized form of polyaniline, a conducting polymer. Mechanism of conducting polymers is different from metals and semiconductors. Unconstrained electrons are voltaic carriers of metals, and electrons or holes are carriers of inorganic semiconductors, while conducting polymers' carriers are kinks, solitons, bisolitons or excitons etc. Polymers can be used for energy source (recharge batteries, solar batteries and solid batteries), light emitting diodes (LED), sensors, technology of shelter and life sciences etc., for the particular structures and excellent properties of physics and chemistry. It is important that the theoretical study works of electricity, magnetics and optics for conducting polymer materials. In present work, we studied some properties of biexciton in polyaniline.Theoretical foundation of the present work is theories of electronic structure in one-dimension systems, which is established on the Peierls unstable properties of ID systems. The theories showed us concepts of kinks, solitons, excitons of conducting polymers, and successful explained the conducting phenomenons of these "particles" as carriers. W.P. Su, Schrieffer and Heeger founded the famous SSH model and explained the electronic structures of polyacetylene and the Peierls phase-transform of ID systems. Many scholars generalized the SSH model to polyaniline, such as Ginder, Baraniwski etc. But electron-electron interactions were neglected in those theoretical works, especially influence of the long-range Coulomb force. It is a flaw to study recombination of excitons. So we figured that the influence of the electron-electron interactions must be considered in our work.Added an electron-electron interaction (up to the fourth neighbor) term to the Baranowski-Buttner-Voit (BBV) model for polyaniline, we carried out a self-consistent-variation calculation for an biexciton in the pernigranlilne-base polymer, in order to confirm the model Hamiltonian. The results showed that two peaks (near l.OeV and 1.5eV) appeared in the calculated optical absorption, besides the reproduced electronic structure, bond order wave (BOW) and ring-torsion angles.This result is consistent with the experimentally observed features of short-life. That shows our model is reasonable. The parameters of the extended BBV model are as follows: the near neighbor interaction intension F=0.75eV and the coefficient of the Hubbard term U=1.5eV.Based on above model, molecular dynamics simulation was used to study recombination process of an exciton in the polymer. The results showed that the excitons were completely recombined after 900 fsec and the geometry structure of the polymer such as the BOW and the ring-torsion angles returned to the ground states, which were the dimerization states. Comparison with the experimentally observed life time -100 msec, the recombination time 900 fsec is quite short. Thus we suggested a stable period of dozens msec for the PNB excitons.Our work shows that the extended BBV model can reproduce most observed data of the polymer. More works can be done on the other oxide forms of polyaniline. And the method can be generalized to other quasi-one dimensional systems.