Interchain coupling effects on dynamics of photoexcitations in conjugated polymers

A review on nonlinear DNA physics

The study and the investigation of structural and dynamical properties of complex systems have attracted considerable interest among scientists in general and physicists and biologists in particular. The present review paper represents a broad overview of the research performed over the nonlinear dynamics of DNA, devoted to some different aspects of DNA physics and including analytical, quantum and computational tools to understand nonlinear DNA physics. We review in detail the semi-discrete approximation within helicoidal Peyrard-Bishop model and show that localized modulated solitary waves, usually called breathers, can emerge and move along the DNA. Since living processes occur at submolecular level, we then discuss a quantum treatment to address the problem of how charge and energy are transported on DNA and how they may play an important role for the functioning of living cells. While this problem has attracted the attention of researchers for a long time, it is still poorly understood how charge and energy transport can occur at distances comparable to the size of macromolecules. Here, we review a theory based on the mechanism of 'self-trapping' of electrons due to their interaction with mechanical (thermal) oscillation of the DNA structure. We also describe recent computational models that have been developed to capture nonlinear mechanics of DNA in vitro and in vivo, possibly under topological constraints. Finally, we provide some conjectures on potential future directions for this field.

Charge Carrier Scattering in Polymers: A New Neutral Coupled Soliton Channel

The dynamical scattering of two oppositely charged bipolarons in non-degenerate organic semiconducting lattices is numerically investigated in the framework of a one-dimensional tight-biding-Hubbard model that includes lattice relaxation. Our findings show that it is possible for the bipolaron pair to merge into a state composed of a confined soliton-antisoliton pair, which is characterized by the appearance of states within less than 0.1 eV from the Fermi level. This compound is in a narrow analogy to a meson confining a quark-antiquark pair. Interestingly, solitons are quasi-particles theoretically predicted to arise only in polymer lattices with degenerate ground state: in the general case of non-degenerate ground state polymers, isolated solitons are not allowed.

Dynamic Formation of Bipolaron-Exciton Complexes in Conducting Polymers

The recombination dynamics of two oppositely charged bipolarons within a single polymer chain is numerically studied in the scope of a one-dimensional tight-binding model that considers electron-electron and electron-phonon (e-ph) interactions. By scanning among values of e-ph coupling and electric field, novel channels for the bipolaron recombination were yielded based on the interplay between these two parameters. The findings point to the formation of a compound species formed from the coupling between a bipolaron and an exciton. Depending on the electric field and e-ph coupling strengths, the recombination mechanism may yield two distinct products: a trapped (and almost neutral) or a moving (and partially charged) bipolaron-exciton. These results might enlighten the understanding of the electroluminescence processes in organic light-emitting devices.

Bloch oscillations in organic and inorganic polymers

The transport of polarons above the mobility threshold in organic and inorganic polymers is theoretically investigated in the framework of a one-dimensional tight-binding model that includes lattice relaxation. The computational approach is based on parameters for which the model Hamiltonian suitably describes different polymer lattices in the presence of external electric fields. Our findings show that, above critical field strengths, a dissociated polaron moves through the polymer lattice as a free electron performing Bloch oscillations. These critical electric fields are considerably smaller for inorganic lattices in comparison to organic polymers. Interestingly, for inorganic lattices, the free electron propagates preserving charge and spin densities' localization which is a characteristic of a static polaron. Moreover, in the turning points of the spatial Bloch oscillations, transient polaron levels are formed inside the band gap, thus generating a fully characterized polaron structure. For the organic case, on the other hand, no polaron signature is observed: neither in the shape of the distortion-those polaron profile signatures are absent-nor in the energy levels-as no such polaron levels are formed during the simulation. These results solve controversial aspects concerning Bloch oscillations recently reported in the literature and may enlighten the understanding about the charge transport mechanism in polymers above their mobility edge.

Nonadiabatic dynamics of injected holes in conjugated polymers

The dynamics of injected holes in short transient times that precede polaron formation is numerically studied in the framework of a tight-binding electron-phonon interacting approach aimed at describing organic one-dimensional lattices. In particular, the direct impact of internal and external factors on the conversion of injected holes into polarons is carefully investigated. The results show that a hole injected at levels lower than the highest occupied molecular orbital forms self-trapped bound structures that can merge spontaneously to form a polaron after, at least, one picosecond. On the other hand, the life-time of such structures substantially decreases (up to a few hundreds of femtoseconds) when the influence of external electric fields, temperature effects and impurities is considered. Importantly, the critical values of the aforementioned factors in promoting the quenching of the self-trapped structures are obtained. These findings may enlighten the understanding of the mechanism of charge carrier generation in Polymer Light Emitting Diodes when several kinds of excitations are present.

Concentration effects on intrachain polaron recombination in conjugated polymers

The influence of different charge carrier concentrations on the recombination dynamics between oppositely charged polarons is numerically investigated using a modified version of the Su–Schrieffer–Heeger (SSH) model that includes an external electric field and electron–electron interactions.

Critical temperature and products of intrachain polaron recombination in conjugated polymers

The intrachain recombination dynamics between oppositely charged polarons is theoretically investigated through the use of a version of the Su–Schrieffer–Heeger (SSH) model modified to include an external electric field, an extended Hubbard model, Coulomb interactions, and temperature effects in the framework of a nonadiabatic evolution method.

Effects of disordered interchain interactions on polaron dynamics in semiconducting polymers

Polaron dynamics in a system of two randomly coupled polymer chains is simulated using a nonadiabatic evolution method. The simulations are performed within the framework of the Su-Schrieffer-Heeger model modified to include disordered interchain interactions and an external electric field. By analysing the polaron velocity statistically, we find that the polaron motion is determined by the competition between the electric field and the disordered interchain interactions. Polaron dynamics are classified into two types, weak-coupling dynamics and strong-coupling dynamics. It is found that the strength of interchain interactions is the dominant factor controlling charge propagation in weak-coupling dynamics, whereas the effects of disorder are dominant in strong-coupling dynamics. The charge carriers tend to have higher mobility for stronger interchain coupling, and interchain coupling disorder can be favorable for charge transport depending on the coupling strength and the electric field.

Molecular dynamics investigation of charge carrier density influence over mobility in conjugated polymers

Charge carrier mobility is known to be one of the most important efficiency delimiting factors in conducting polymer-based electronic devices. As the transport mechanism in this class of material is nonconventional, many works have tried to elucidate the charge carrier's interaction with temperature, external electric field, and the collective effects they present. Even though the multiple trap-and-release model is often used to describe these effects, its applicability is known to be restricted to electronic properties. In this work we make use of a modified version of the Su-Schrieffer-Heeger model, the most used method to describe the important properties of conducting polymer in general, to investigate the influence of temperature and carrier densities over the transport mechanisms. We obtained different regimes of temperature and carriers density influence over the systems mobility, consistent with most of the experimental data available.

Field effect on polaron dynamics and charge transport in conducting polymers

We investigate the formation and motion dynamics of polarons in one-dimensional conjugated polymers within the extended Su-Schrieffer-Heeger model combined with an adiabatic dynamics method. In the presence of external electric fields, the initial location of the polaron is stressed and concerned with the charge transport property of organic semiconductors. Three regimes for the electric fields are categorized in terms of the forming place of the polaron. In the low field strength regime, the polaron is formed around the center of the chain and thus the charge undergoes a long-time travel before being extracted into the electrode because of the strong electron and phonon (e-p) interaction. In the intermediate strength regime, the polaron is formed near the chain end. Due to the chain-end scattering, the electron mobility increases in linear relation with the field strength. In the high strength regime, the polaron is formed at the chain end. This results in a nonlinear enhancement in the electronic mobility, in agreement with the experimental observations on the field dependent transient photocurrent in poly(phenylene vinylene). The electron-electron correlation, as well as the field mode effect on polaron dynamics, is also discussed.