Conjugated polymers at the verge of strongly correlated systems and 1D semiconductors (original) (raw)
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Physical theory of excitons in conducting polymers
2010
In this tutorial review, we cover the solid state physics approach to electronic and optical properties of conducting polymers. We attempt to bring together languages and advantages of the solid state theory for polymers and of the quantum chemistry for monomers. We consider polymers as generic one-dimensional semiconductors with features of strongly correlated electronic systems. Our model combines the long range electron-hole Coulomb attraction with a specific effect of strong intra-monomer electronic correlations, which results in effective intramonomer electron-hole repulsion. Our approach allows to go beyond the single-chain picture and to compare excitons for polymers in solutions and in films. The approach helps connecting such different questions as shallow singlet and deep triplet excitons, stronger binding of interchain excitons in films, crossings of excitons' branches, 1/N energies shifts in oligomers. We describe a strong suppression of the luminescence from free charge carriers by long-range Coulomb interactions. Main attention is devoted to the most requested in applications phenyl based polymers. The specifics of the benzene ring monomer give rise to existence of three possible types of excitons: Wannier-Mott, Frenkel and intermediate ones. We discuss experimental manifestations of various excitons and of their transformations. We touch effects of the time-resolved self-trapping by libron modes leading to formation of torsion polarons.
Understanding excitons in optically active polymers
Polymer International, 2008
We review the solid-state physics approach to electronic and optical properties of conducting polymers, and bring together the languages of solid-state theory for polymers and the quantum chemistry of oligomers. We consider polymers as generic one-dimensional semiconductors with specific features of strongly correlated electronic systems. Our model combines the large distance electron-hole motion within an exciton, governed by long-range Coulomb attraction with strong intramonomer electronic correlations, which results in effective intramonomer electron-hole repulsion. We exploit the dielectric screening to go beyond the single chain picture and to compare excitons for polymers in solutions and in films. Our approach allows the connecting, explaining, exploiting and organizing of such different experimental and numerical findings as shallow singlet and deep triplet excitons in phenylenes, anomaly in singlet-triplet exciton formation ratio, A g-B u crossing in polyenes and common 1/N energy dependencies in oligomers.
Localization of Electronic Excitations in Conjugated Polymers Studied by DFT
The Journal of Physical Chemistry Letters, 2011
We present an extensive density functional theory (DFT) study on the neutral and charged electronic excitations in oligophenylene vinylenes including lowest singlet (S 1 ) and triplet (T 1 ) excitons and positive (P þ ) and negative (P -) polarons. We investigated the vibrational and electronic properties of molecules using five different DFT functionals from pure GGA to long-range-corrected hybrids and found an explicit correlation between the spatial extent of the state and the fraction of the orbital exchange. While solvent effects are found to be negligible for neutral (S 1 and T 1 ) excitons, they play an important role for charged (P þ and P -) species. S 1 states are observed to be spatially less localized compared to the polaronic wave functions (P þ and P -). This is in contrast to the T 1 states, which exhibit more spatial confinement in comparison to P þ and Pstates.
Exciton sizes of conducting polymers predicted by time-dependent density functional theory
Physical Review B, 2005
The electronic structure and size scaling of spectroscopic observables in conjugated polymers are investigated using time-dependent density functional theory. We show that local density approximations and gradientcorrected functionals do not have an effective attractive Coulomb interaction between photoexcited electronhole pairs to form bound states and therefore do not reproduce finite exciton sizes. Long-range nonlocal and nonadiabatic density functional corrections ͑such as hybrid mixing with an exact Hartree-Fock exchange͒ are necessary to capture correct delocalization of photoexcitations in one-dimensional polymeric chains.
Physical Review B, 2013
The electronic properties of macromolecular semiconductor thin films depend profoundly on their solid-state microstructure, which in turn is governed, among other things, by the processing conditions selected and the polymer's chemical nature and molecular weight. Specifically, lowmolecular-weight materials form crystalline domains of cofacially π-stacked molecules, while the usually entangled nature of higher molecular-weight polymers leads to microstructures comprised of molecularly ordered crystallites interconnected by amorphous regions. Here, we examine the interplay between extended exciton states delocalized along the polymer backbones and across polymer chains within the π-stack, depending on the structural development with molecular weight.
The Journal of Physical Chemistry B, 2014
Using a modified version of the Su-Schrieffer-Heeger (SSH) model combined with the Extended Hubbard Model (EHB) the recombination between a singlet exciton pair is investigated under influence of an external electric field, electron-electron interactions, and temperature effects in the scope of a nonadiabatic evolution method. The excitons are positioned very close to each other in a way to mimic a high-density region in monomolecular conjugated polymer systems. Results show that there are mainly three possible channels resulting from singlet-singlet exciton recombination: (1) forming an excited negative polaron and an excited positive bipolaron; (2) forming two free and excited oppositely charged polarons, and (3) forming a biexciton. These results suggest that the recombination processes critically depends on the condition imposed to the system. The description of this dependence, as carried out in the present work, may provide guidance to improve the generation of free charge carriers in organic optoelectronic devices.
Polaronexcitons and electronvibrational band shapes in conjugated polymers
The Journal of chemical physics, 2003
The neutral excitations in poly(p-phenylenevinylene) are studied in conjunction with the vibronic structure of the lowest optical transitions. Combining the configuration interaction of Wannier-localized electron-hole pairs with an empirical description of electron-phonon coupling, we obtain the potential energy surfaces of monoexcited states and the Condon electron-vibrational spectra in absorption and emission. The S 1 →S 0 luminescence band shape is found compatible with self-localization of S 1 within about 10 monomers, driven exclusively by electron-phonon coupling. The singlet and triplet polaron-excitons are exchange-split by about 1eV and differ substantially in terms of average electron-hole separation. ________________________ 1
Getting excited: challenges in quantum-classical studies of excitons in polymeric systems
Phys. Chem. Chem. Phys., 2016
A combination of classical molecular dynamics (MM/MD) and quantum chemical calculations based on the density functional theory (DFT) and many-body Green's functions theory (GW-BSE) was performed to describe the conformational and optical properties of diphenylethyne (DPE), methylated-DPE and poly para phenylene ethynylene (PPE).