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.

Electrochemical, Magnetic, and Electrical Properties of α,ω-Capped Sexithiophene Films. 2. Conduction in Sexithiophenes with α,ω-Aryl-Extended Conjugation

Abstract: Thin films of tetrahexylsexithiophenes, fluorenonyl- and fluorenyl-protected at the terminal α,ω-positions, were investigated by cyclic voltammetry, electrochemical quartz crystal microbalance analysis, in situ ESR, and in situ conductivity. Reversible oxidation is composed of three separate steps, two one-electron processes and a further multielectron process. ESR indicates strong magnetic dimerization for the one-electron-oxidized species. Conductivity is redox type at the cation−dication (polaron−bipolaron) state and metal-like at doping levels higher than the bipolaron with a 20-fold increase at full oxidation. Hexyl-substituted α,ω-capped octathiophene, decathiophene, and dodecathiophene films have been similarly investigated to compare the effects of fluorenonyl and fluorenyl extension of sexithiophene with those given by additional thiophene rings. The conductivity, which increases progressively (by 3 orders of magnitude) with the oligothiophene chain length, is metal-like. In dodecathiophene the d...

Understanding the evolution of the Raman spectra of molecularly p-doped poly(3-hexylthiophene-2,5-diyl): signatures of polarons and bipolarons.

Abstract: Molecular doping is a key process to increase the density of charge carriers in organic semiconductors. Doping-induced charges in polymer semiconductors result in the formation of polarons and/or bipolarons due to the strong electron-vibron coupling in conjugated organic materials. Identifying the nature of charge carriers in doped polymers is essential to optimize the doping process for applications. In this work, we use Raman spectroscopy to investigate the formation of charge carriers in molecularly doped poly(3-hexylthiophene-2,5-diyl) (P3HT) for increasing dopant concentration, with the organic salt dimesityl borinium tetrakis(penta-fluorophenyl)borate (Mes2B+ [B(C6F5)4]-) and the Lewis acid tris(pentafluorophenyl)borane [B(C6F5)3]. While the Raman signatures of neutral P3HT and singly charged P3HT segments (polarons) are known, the Raman spectra of doubly charged P3HT segments (bipolarons) are not yet sufficiently understood. Combining Raman spectroscopy measurements on doped P3HT thin films with first-principles calculations on oligomer models, we explain the evolution of the Raman spectra from neutral P3HT to increasingly doped P3HT featuring polarons and eventually bipolarons at high doping levels. We identify and explain the origin of the spectral features related to bipolarons by tracing the Raman signature of the symmetric collective vibrations along the polymer backbone, which - compared to neutral P3HT - redshifts for polarons and blueshifts for bipolarons. This is explained by a planarization of the singly charged P3HT segments with polarons and rather high order in thin films, while the doubly charged segments with bipolarons are located in comparably disordered regions of the P3HT film due to the high dopant concentration. Furthermore, we identify additional Raman peaks associated with vibrations in the quinoid doubly charged segments of the polymer. Our results offer the opportunity for readily identifying the nature of charge carriers in molecularly doped P3HT while taking advantage of the simplicity, versatility, and non-destructive nature of Raman spectroscopy.

Binding Energies, Effective Masses and Screenings Effects of Fröhlich Bipolarons

Abstract: The bipolaron ground state binding energy and the effective masses are calculated self-consistently in a scheme where the electron-phonon interaction is described by the Fr?hlich interaction. We explicitly use the total linear momentum conservation and both two-and three-dimensional systems are considered. We review results for binding energies and show that the bipolaron effective mass increases with the electron-phonon coupling constant ? more rapidly than two free polaron masses. As expected, the increase is greater in two than in three dimensions. We estimate the screening effects due to an electronic or hole density n in a range of values such that nR2b 1 (here Rb is the bipolaron radius). We find that the bipolaron binding energy decreases with n and eventually becomes positive indicating the existence of a metastable bipolaron state. Finally we discuss the possible connections between our results and high Tc superconductivity.