Bipolaron

About: Bipolaron is a research topic. Over the lifetime, 1335 publications have been published within this topic receiving 29154 citations. The topic is also known as: bipolarons.

Analyses of Spectroelectrochemical Behavior of Polypyrrole Films Using the Nernst Equation “Monomer Unit Model” and Polaron/Bipolaron Model

Abstract: A series of the absorption spectrum changes in polypyrrole films were analyzed using the Nernst equation by two models. One is the "monomer unit model" and the other is the "polaron/bipolaron model." In the first model, formal electrode potentials and the n-values were obtained and usec~ to fit calculated hAbs vs. E curves to the experimental plots at three wavelengths. In the second model, the "apparent" excess chemical potentials were introduced to correct for the large deviations between the Nernst equation and the Nernst plots obtained from the absorption spectra. The calculated hAbs vs. E curves were then fitted to the experimental plots. The "apparent" excess chemical potentials were assigned as corrections for species concentrations. Advantages of the monomer unit model over the polaron/bipolaron model are pointed out in the precise analyses of the spectroelectrochemical behavior of the films.

Optical and electrical characterization of a conducting polypyrrole composite prepared by in situ electropolymerization

Abstract: A study of the optical and electrical properties of a conducting polypyrrole–polyoxyphenylene composite, PPy–POP, prepared by insitu electropolymerization is presented. Electropolymerization was performed potentiostatically in a solution of pH 9 which contained the monomers pyrrole, allylphenol and sodium 4-hydroxybenzenesulfonate (4HBS), by applying a potential of 1.25 V vs. SCE. The films obtained were characterized optically by UV/VIS and IR spectroscopy and electrically by measurements of the temperature dependence of the ac and dc conductivity. FTIR measurements indicated that the polymer blend obtained consists of PPy and the insulating polymer poly-2-allyloxyphenylene (POP), whereas the third monomer, 4HBS, is incorporated into the PPy–POP film as dopant for the conducting PPy. Furthermore, optical characterizations show a light degree of overoxidation of PPy in the PPy–POP composite. In the UV/VIS spectra, the formation of both polaron and bipolaron electronic states of the band structure of PPy can be seen, but the IR spectra demonstrate the transition of the PPy structure from a conducting quinoid to benzoid type with increasing polymerization potential. This is accompanied by the introduction of a carbonyl group into the PPy backbone and a reduction of the conjugation length of the polymer chain, which has a strong influence on the conductivity of the polymer composite. Despite this overoxidation process, the PPy–POP film retains a conductive character which allows the growth of thick films. The temperature dependence of the ac and dc conductivity of PPy–POP was investigated. The total ac conductivity, σtot(ω), in the frequency range 102–105 Hz, changes by approximately four orders of magnitude in the range from 77 to 300 K, showing a sub-linear dispersive behavior. The temperature dependence of the dc conductivity of such a polymer composite can be described by Mott's variable range hopping (VRH) model according to σ=σ0 exp[-(T0/T)γ], with γ=1/2

Insulator-to-metal transition in polythiophene.

Abstract: In the present work, the electronic structure of polythiophene at several doping levels is investigated by the use of the H\"uckel Hamiltonian with \ensuremath{\sigma}-bond compressibility. Excess charges are assumed to be stored in conformational defects of the bipolaron type. The Hamiltonian matrix elements representative of a bipolaron are obtained from a previous thiophene oligomer calculation, and then transferred to very long chains. Negative factor counting and inverse iteration techniques have been used to evaluate densities of states and wave functions, respectively. Several types of defect distributions were analyzed. Our results are consistent with the following: (i) the bipolaron lattice does not present a finite density of states at the Fermi energy at any doping level; (ii) bipolaron clusters show an insulator-to-metal transition at 8 mol % doping level; (iii) segregation disorder shows an insulator-to-metal transition for doping levels in the range 20--30 mol %.

Polaron states in polydiacetylenes

Abstract: The ground state and lowest-lying excited states of polydiacetylene chains are investigated using a tight-binding Hamiltonian which includes electron-phonon interactions but does not explicitly include electron-electron repulsion. The parameters of the model are obtained by comparison with experiment and the character and properties of the low-lying charge states are determined. This calculation shows that localised polaron states will dominate charge conduction in these systems. In contrast to the situation for polyacetylene, bipolaron states are stable in the polydiacetylenes. Moreover, they are so strongly bound that they may survive the inclusion of electron-electron repulsion.