Polaron bipolaron

Figure 7-2H. (a) ll-l ADMR and (b) A-PADMR spectra of u-6T film. The peaks of polarons, bipolarons, and triplet cxei-(ons are labeled. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

We investigated the ultrafast dynamics in a Na-NaBr melt at 1073 K by fs pump probe absorption spectroscopy. A simple model was used to simulate the dynamics of polaron-, bipolaron- and Drude-type electrons. The relaxation times for polarons and bipolarons are 210 fs and 3 ps, respectively. The existence of an isosbestic point at 1.35 eV indicates an inter-conversion between bipolarons and Drude-type electrons. 

Mobile defects are Frenkel78 excitons, Mott-Wannier excitons, polarons, bipolarons, polaritons, and solitons. 

Bipolaron — Bipolarons are double-charged, spinless quasiparticles introduced in solid state physics [i]. A bipolaron is formed from two -> polarons (charged defects in the solid). For chemists the double-charged states mean dications or dianions, however, bipolarons are not localized sites, they alter and move together with their environment. By the help of the polaron-bipolaron model the high conductivity of -> conducting polymers can be explained. 

Charge transport — When charged species move within a phase, this is called charge transport [i-viii]. Electron transport occurs in metals and -> semiconductors. (In the latter case the - charge carriers - holes, polarons, - bipolarons (- electronic defects) are also considered as moving charged species, while the superconductivity occurring at very low temperatures is explained by... 

Soliton — Solitons (solitary waves) are neutral or charged quasiparticles which were introduced in solid state physics in order to describe the electron-phonon coupling. In one-dimensional chainlike structures there is a strong coupling of the electronic states to conformational excitations (solitons), therefore, the concept of soliton (-> polaron, - bipolaron) became an essential tool to explain the behavior of - conducting polymers. While in traditional three-dimensional -> semiconductors due to their rigid structure the conventional concept of - electrons and -> holes as dominant excitations is considered, in the case of polymers the dominant electronic excitations are inherently coupled to chain distortions [i]. 

Neutral Polymer Polaron Bipolaron Bipolaron Bands... 

We should mention that another possible explanation of the Pauli-like behavior has recently been proposed in terms of contributions of the triplet-excited bipolaronic states to the spin susceptibility [94]. As shown by Bussac and Zuppiroli, the polaron-bipolaron energy difference U is essentially determined by the interdopant distance Ld. The authors also consider the bipolaron triplet state, which reduces to two separate polarons for large Ld. Due to disorder, the Ld are distributed, giving rise to a distribution for the energy of the magnetic states. Summing over the distribution yields a Curie-like contribution, plus a smoothy temperature-dependent term, which resembles a Pauli contribution. 

Poly(phenylene vinylene), PPV, and its soluble derivatives have emerged as the prototypical luminescent semiconducting polymers. Since PPV has a nondegenerate ground state, structural relaxation in the excited state leads to the formation of polarons, bipolarons, and neutral excitons. However, prior to treating the structural relaxation in the excited state, one needs to develop a satisfactory description of the electronic excited states. 

Jahn-Teller Polarons, Bipolarons and Inhomogeneities. A Possible Scenario for Superconductivity in Cuprates... 

A.S. Alexandrov, N. Mott, Polarons, Bipolarons, Edit. World Scientific (1995)... 

Bredas JL, Street GB (1985) Polarons, Bipolarons, and Solitons in Conducting Polymers. Accounts of Chemical Research 18( 10) 309—315... 

A revisal of the equilibrium between polarons and doubly charged states of the polymer chains shows that the previous assumptions on the extension of polaron, bipolarons and polaron pairs were not justified. A more general formulation is presented for the equilibrium concentrations and the kinetics. But the differences are almost negligible up to the maximum charge concentrations that can be achieved in accumulation layers. The resulting rate constants for formation and dissociation of (immobile) bipolarons can be estimated using a rate constant for the bipolaron formation determined recently by Salleo and Street, and indicate that these processes can cause the hysteresis on the time scale of the measurements. 

However, when exceeds said critical valne a significant interface dipole can be formed. Positive charges are transferred from the metal to the semiconductor and the position of the Fermi level at the interface becomes pinned at an energy level interpreted as the hole polaron/bipolaron energy level in the polymer semiconductor. This simple picture suggests that, at least in the case of solntion-deposited polymers on common hole-injecting contacts, chemical interactions between the metal and... 

Oxidative doping of conjugated polymers usually gives rise to new infrared active modes whose appearance is associated with the formation of solitonic or polaronic (bipolaronic) structures in the polymer chain [62,67-70]. The postulate that doping induced IR-active modes are inherently associated with the oxidized chain of the polymer is based on two experimental observations ... 

Neutral soliton Charged soliton Polaron Bipolaron... 

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