Physical theory of excitons in conducting polymers (original) (raw)
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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.
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).
Exciton and Charge-Transfer Dynamics in Polymer Semiconductors
Springer Series in Chemical Physics, 2007
Organic semiconducting polymers are currently of broad interest as potential low-cost materials for photovoltaic and light-emitting display applications. I will give an overview of our work in developing a consistent quantum dynamical picture of the excited state dynamics underlying the photo-physics. We will also focus upon the quantum relaxation and reogranization dynamics that occur upon photoexcitation of a couple of type II donor-acceptor polymer heterojunction systems. Our results stress the significance of vibrational relaxation in the state-to-state relaxation and the impact of curve crossing between charge-transfer and excitonic states. Furthermore, while a tightly bound charge-transfer state (exciplex) remain the lowest excited state, we show that the regeneration of the optically active lowest excitonic state in TFB:F8BT is possible via the existence of a steadystate involving the bulk charge-transfer state. Finally, we will discuss ramifications of these results to recent experimental studied and the fabrication of efficient polymer LED and photovoltaics.
Conjugated polymers at the verge of strongly correlated systems and 1D semiconductors
Synthetic Metals, 2004
We review the solid state physics approach to electronic and optical properties of conducting polymers also with attempts to bring together languages of the solid state theory for polymers and the quantum chemistry of oligomers. 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 intra-monomer electron-hole repulsion. We explain, exploit and organize various experimental and numerical findings. For example we connect such different questions as shallow singlet and deep triplet excitons in phenylenes, A g-B u exciton levels crossing in polyenes, common 1/N energy dependencies in oligomers.
Excitons in organic semiconductors
Synthetic Metals, 2011
The exciton binding energy (E b) is one of the heavily debated issues in conjugated polymer literature. Most of the experimental studies suggest values around 0.3-0.4 eV for PPV. On the other hand, binding energies ranging from 0.06 eV to more than 1 eV have been reported as well. Time Dependent Density Functional Theory (TDDFT) is employed to calculate E b for PPV and polythiophenes and their derivatives in order to (i) to clarify the controversial debate of the magnitude of the exciton in PPV and (ii) to study the influence of the substituents on the exciton binding energy of conjugated polymers.
Theory of non-Condon emission from the interchain exciton in conjugated polymer aggregates
The Journal of chemical physics, 2007
The authors present here a simple analysis that explains the apparent strengthening of electron phonon interaction upon aggregation in conjugated polymer materials. The overall scheme is that of an intermolecular Herzberg-Teller effect whereby sidebands of a forbidden transition are activated by oppositely phased vibrations. The authors show that upon aggregation, the 0-0 emission becomes symmetry forbidden and the apparent redshift and remaining vibronic structure are due to sideband ͑0-1,0-2, etc.͒ emission. At higher temperatures, the 0-0 peak is due to thermal population in a higher lying even-parity vibronic state rather than direct emission from the odd-paritied lowest intermolecular vibronic state.
The Journal of Chemical Physics, 2000
We examine the dynamics of exciton self-trapping in conjugated polymer systems using mixed quantum-classical molecular dynamics. The model treats the exciton as a two-dimensional quantum mechanical wave function representing a particle/hole quasiparticle interacting with a classical vibrational lattice ͓M. N. Kobrak and E. R. Bittner, J. Chem. Phys. 112, 5399 ͑2000͔͒. We show that the dynamics are influenced strongly by thermal disorder in the lattice, and that there is a dramatic change in the self-trapping mechanism as temperature increases. At low temperatures, the rate of localization is limited by the time required for the vibrational lattice to respond to the creation of the particle-hole pair, while at higher temperatures thermal disorder permits localization on time scales limited primarily by electronic response. We simulate the time-resolved fluorescence spectrum for the model system, and compare the temperature dependence of the spectrum to recent time-resolved fluorescence upconversion studies on polydiacetylene derivatives.
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.