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.

Theoretical study on the emeraldine salt - impact of the computational protocol

Abstract: We report a theoretical study on the geometry characteristics and the electronic properties of bipolaron defects introduced in oligomers (tetramer to hexadecamer) of emeraldine salt. The main goal of the present investigation is to establish a quantum chemical method suitable for description of the target system and for determination of the oligomer segment perturbed by protonation. For the purpose, a series of first principles methods (HF and DFT) combined with different basis sets are tested and the influence of the medium (water) dielectric constant is estimated using PCM. It is shown that the optimized molecular geometry is critically dependent on the theoretical formalism applied. HF yields geometry of the tetramer substantially distorted from planarity and bipolaron, which is highly localized in the quinoid ring. The bipolaron produced by DFT tends to delocalize, thus enhancing the conjugation along the oligomer chain. The reliability of the separate methods is commented on the basis of a list of criteria such as reproducibility of available experimental geometry, spin contamination of wavefunctions and quantification of correlation effects. The BLYP/6-31G*/PCM method is outlined as the most appropriate. It is found that irrespective of the oligomer chain length and the degree of protonation (partial or full) a bipolaron spreads on three phenyl rings and the nitrogen atoms bound to them. © 2010 Elsevier B.V.

Density Functional Theory Study of Selenium-Substituted Low-Bandgap Donor–Acceptor–Donor Polymer

Abstract: Theoretical study of an optically transparent, near-infrared-absorbing low energy gap conjugated polymer, donor–acceptor–donor (D-A-D), 2,1,3-benzosele-nadiazole (A) as acceptor and 3,4-ethylenedioxyselenophene (D) as donor fragments, with promising attributes for photovoltaic application is reported herein. The D and A moiety on the polymeric backbone has been found to be responsible for tuning the band gap, optical gap, open circuit (VOC), and short-circuit current density (JSC) in the polymers solar cells. D-A-D has a key role in charge separation and molecular architecture which ultimately influences the charge transport. Reduction in the band gap, high charge transformation, and enhanced visible light absorption in the D-A-D system is because of strong overlapping of molecular orbitals of D and A. The polaron and bipolaron effects are also investigated which has a direct relation with visible light photocurrent generation. In addition, the enhanced planarity and weak steric hindrance between adjacent...

Effect of mixing transition ions in glasses. II. The P2O5-Fe2O3-MnO system

Abstract: The first article of this series reported electrical properties of phosphate glasses containing V and Fe ions, both of which contributed to conductivity. A mixed transition-ion effect was detected in such glasses which had several characteristics similar to the mixed alkali effect, observed in glasses containing two alkali ions. This article reports electrical and optical properties of phosphate glasses containing Fe and Mn ions, one of which (Mn) does not contribute directly to conductivity. DC resistivity by the Van der Pauw four-probe method and optical absorption (UV–VIS–IR) measurements have been performed on x P 2 O 5 − (100 − x ) (Fe 2 O 3 + MnO) (PFM) glasses. The transport mechanism has been identified to be hopping of small polarons between Fe 2+ and Fe 3+ sites. For 0.3 n Fe n Fe , the atomic fraction of Fe ions is given by (Fe/(P + Fe + Mn)), a marginal linear increase in resistivity has been observed with decreasing n Fe . However, when n Fe is reduced below ∼0.3, the resistivity increases exponentially with decreasing n Fe , which is similar to ‘metal–insulator transition’ (MIT), observed in many crystalline systems. The resistivity transition in these glasses has been explained by a (small) polaron to (small) bipolaron transition (PBT) in the composition range 0 ⩽ n Fe ⩽ 0.3. The occurrence of PBT is confirmed by the shifting of the polaronic optical absorption band to higher energies with increasing small bipolaron concentration.