Combining the one-dimensional tight-binding Su−Schrieffer−Heeger (SSH) model and the extended Hubbard model (EHM), we analyze the scattering and combination in conjugated polymers of two polarons w...

Formation and evolution dynamics of bipolarons in conjugated polymers.

Combining the one-dimensional tight-binding Su–Schrieffer–Heeger (SSH) model and the extended Hubbard model (EHM), the scattering and combination of oppositely charged bipolarons in conjugated polymers are investigated using a nonadiabatic evolution method. On the basis of this physical picture, bipolarons can scatter into a singlet biexcitonic state, in which two electrons and two holes are trapped together by a lattice distortion. This biexcitonic state can emit one photon to an exciton state, which can subsequently decay to the ground state. The results indicate that the scattering and combination of oppositely charged bipolarons may provide guidance for improving the internal quantum efficiency for electroluminescence to as high as 75% in polymer light-emitting diodes.

Electroluminescence Enhancement in Polymer Light-Emitting Diodes through Inelastic Scattering of Oppositely Charged Bipolarons

Conducting polymers have become standard engineering materials used in many electronic devices. Despite this, there is a lack of understanding of the microscopic origin of the conducting properties, especially at realistic device field strengths. We present simulations of doped poly(p-phenylene) (PPP) using a Su–Schrieffer–Heeger (SSH) tight-binding model, with the electric field included in the Hamiltonian through a time-dependent vector potential via Peierls substitution of the phase factor. We find that polarons typically break down within less than a picosecond after the field has been switched on, already for electric fields as low as around 1.6 mV/A. This is a field strength common in many flexible organic electronic devices. Our results challenge the relevance of the polaron as charge carrier in conducting polymers for a wide range of applications.

Breakdown of Polarons in Conducting Polymers at Device Field Strengths

The collision processes between two acoustic polarons, two acoustic bipolarons, and a polaron and a bipolaron in a one-dimensional electron-lattice system are analyzed by numerical simulations based on Su, Schrieffer and Heeger's model extended to include short-ranged Coulombic electron-electron interactions. The collisions between two polarons with the same spin and between two bipolarons are essentially elastic because of a rather strong repulsion due to the Pauli principle. On the other hand, those between two polarons with opposite spins and between a polaron and a bipolaron show some inelastic behaviors depending on the relative velocity of the two objects before the collision. This knowledge will serve to understand the dynamics of the system when many polarons and/or bipolarons are created in the system e.g. by photoexcitation.

Collisions among Acoustic Polarons and Acoustic Bipolarons in One-Dimensional Electron-Lattice System

In order to estimate the mobility of an acoustic bipolaron in a typical non-degenerated polymer such as polydiacetylene (PDA), which can be regarded as a one-dimensional electron-lattice system, the dynamical behavior of an acoustic bipolaron in an external electric field is numerically investigated in the presence of a phenomenological damping of the lattice motion. In these simulations, we use the Su, Schrieffer and Heeger (SSH) Hamiltonian extended to include on-site and nearest-neighbor Coulomb repulsion terms, and the damping is introduced by adding a phenomenological friction term in the lattice equation of motion. The repulsive interactions are treated within the unrestricted time-dependent Hartree–Fock approximation. It is confirmed that the mobility of a bipolaron increases with the increase of the repulsion strength and that the relevant quantity determining the mobility of a bipolaron is its width as far as the phenomenological friction is kept fixed. The mobility of a bipolaron is compared wit...

Electric Conduction Due to an Acoustic Bipolaron in One-Dimensional Electron-Lattice System

Dynamics of an Acoustic Bipolaron in One-Dimensional Electron-Lattice Systems : Condensed Matter: Electronic Properties, etc.

Quantum Tunneling of an Acoustic Polaron in One-Dimensional Electron-Lattice System

Collision between two acoustic polarons, which are complexes of an electron near the band bottom and lattice distortions around it, is numerically studied by using Su, Schrieffer and Heeger's model extended to include short-ranged electronic interactions. When the two polarons have opposite spins, their collision looks quite repulsive in the absence of electronic repulsive interactions. Contrary to intuition, this effective repulsion is reduced by relatively weak electronic interactions. Detailed analyses indicate that a kind of “Coulomb blockade effect” might explain this reduction.

Toshiyuki Ozawa

Papers

Dynamics of an Acoustic Bipolaron in One-Dimensional Electron-Lattice Systems : Condensed Matter: Electronic Properties, etc.

Collisions among Acoustic Polarons and Acoustic Bipolarons in One-Dimensional Electron-Lattice System

Electric Conduction Due to an Acoustic Bipolaron in One-Dimensional Electron-Lattice System

Quantum Tunneling of an Acoustic Polaron in One-Dimensional Electron-Lattice System

Coulomb blockade effect in collision of two acoustic polarons