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A semiclassical model for studying charge transport in a two-dimensional molecular lattice is presented and applied to both a well ordered system and a system with disorder. The model includes both intra- and inter-molecular electron-lattice interactions and the focus of the studies is to describe the dynamics of a charge carrier in the system. In particular, we study the dynamics of the system in which the polaron solution is dynamically stable. It is found that the parameter space for which the polaron is moving through the system is quite restricted and that the polaron is immobile for large electron-phonon coupling and weak intermolecular electron interactions and dynamically unstable and disassociates into a delocalized electronic state decoupled from the lattice for small electron-phonon coupling and strong intermolecular electron interactions. Disorder further limits the parameter space in which the polaron is mobile. [ABSTRACT FROM AUTHOR]

We report results from simulations of charge and energy dynamics in poly(para-phenylenevinylene) (PPV) and PPV interacting with C60. The simulations were performed by solving the time-dependent Schrödinger equation and the lattice equation of motion simultaneously and nonadiabatically. The electronic system and the coupling of the electrons to the lattice were described by an extended three-dimensional version of the Su-Schrieffer-Heeger model, which also included an external electric field. Electron and lattice dynamics following electronic excitations at different energies have been simulated. The effect of additional lattice energy was also included in the simulations. Our results show that both exciton diffusion and transitions from high to lower lying excitations are stimulated by increasing the lattice energy. Also field induced charge separation occurs faster if the lattice energy is increased. This separation process is highly nonadiabatic and involves a significant rearrangement of the electron distribution. In the case of PPV coupled to C60, we observe a spontaneous charge separation. The separation time is in this case limited by the local concentration of C60 molecules close to the PPV chain. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The complete electronic (occupied and unoccupied states) and vibrational structure of thin films of bithiophene in undoped form and after alkali doping have been investigated experimentally and theoretically. The ultraviolet (UV) photoemission and inverse photoemission spectra demonstrate clearly the filling of the LUMO of bithiophene in the doping process due to charge transfer from the alkali atoms, and density functional theory (DFT) calculations confirm the formation of dianion (bipolaronlike) states in the HOMO-LUMO band gap at high doping levels, respectively. High-resolution electron energy loss spectra (HREELS) have been used to probe the vibrational structure of the bithiophene films, and the formation of a quinoidic structure on Cs doping is well reflected in HREELS by the advent of an intense vibrational structure at 206 meV, which is due to the stiffened C-C interring bond as also predicted by theory. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations are performed to theoretically study the interaction between low work function metal atoms and trans-tetradecaheptaene, an extended model molecule for π-conjugated polymers. We present characterizations of metal/polyene complexes obtained by the local spin density method. The results show that charge transfer reactions occur for Na and Ca and the formation of a covalent bond for Al. Mg atoms, however, are found not to react significantly with π-conjugated systems. Differences in the bonding mechanism between the metals are discussed and the theoretical density of valence states are compared to experimental photoelectron spectroscopy (UPS) data. The results demonstrate that the local spin density method can be successfully used to interpret valence UPS spectra from early stages of metal/π-conjugated interface formation. [ABSTRACT FROM AUTHOR]

Changes in the geometric and electronic structure of polyenes upon sodium doping have been studied in detail. A short polyene containing three double bonds, hexatriene, forms a charge transfer complex with a maximum of one sodium ion. The interaction results in the formation of a polaron (radical anion) on the polyene chain. This type of charge storage state is shown to be unstable relative to the formation of a hexatriene dimer when interaction between two polyenes is allowed for; the two polyenes are then bound to each other via a carbon–carbon single bond. The charge storage states in this case correspond to two singly charged, spinless defects, one on each polyene segment of the dimer molecule. Decapentaene, which contains five carbon–carbon double bonds, is shown to be able to accept two electrons from a pair of sodium atoms. Similar to the case of doped trans-polyacetylene, the charges are stored as a soliton–antisoliton pair. However, when there is an excess of the number of polyene molecules relative to the number of sodium atoms, i.e., below saturation doping, this type of charge storage state is also unstable towards the formation of a decapentaene dimer of the same type as in hexatriene. [ABSTRACT FROM AUTHOR]

Interactions at the interface between aluminum and polythiophene are studied theoretically using ab initio Hartree–Fock and Mo\ller–Plesset perturbation theories as well as semiempirical techniques. Aluminum is found to interact strongly with the polythiophene chain. The fundamental interaction unit corresponds to an aluminum dimer (Al2) bound to a single thiophene ring. This type of interaction strongly modifies the geometrical structure of the polymer chain and induces localization of the π-electron system. The calculated charge transfer between aluminum and the thiophene system is found to be in good agreement with the charge transfer deduced from experimental core level spectra of the aluminum/polythiophene interface. A detailed analysis of the electronic structure, including the correlation effects, of a model system (Al2 interacting with α-trithienylene) is performed. Higher concentrations of aluminum atoms on a polythiophene chain results in the formation of additional Al2–thiophene complexes. Within the first monolayer, clustering of aluminum on polythiophene is shown to be energetically unfavorable. [ABSTRACT FROM AUTHOR]

We investigate the electronic excitation in aniline tetramers at various levels of oxidation and protonation. We show that for certain geometrical conformations there is excellent agreement between our results and recently published experimental data. The neutral forms of the tetramer are shown to have excitation spectra almost identical to polymer equivalents. For the doubly protonated emeraldine tetramer we propose a bipolaron type of defect state, unlike in its polymer equivalent for which a polaronic metal model has been proposed. The electronic transitions associated with the bipolaron are shown to involve only one bipolaron level in contrast to the common picture of two bipolaron levels. [ABSTRACT FROM AUTHOR]

The interactions between different low work function metals aluminium, calcium and sodium, and α,ω-diphenyltetradecaheptaene, a model molecule for certain conjugated polymers, have been investigated using both x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum chemical calculations performed within the local spin density (LSD) approximation methodology. The metals are found to interact with the conjugated system in very different ways. Aluminium forms a covalent bond, which strongly modifies the π-electronic structure of the conjugated molecule, while both the sodium and the calcium atoms act as doping agents, inducing new states in the otherwise forbidden bandgap. These new gap states can be viewed as a soliton–antisoliton pair for the Na/DP7 and a bipolaronic-like defect for Ca/DP7. [ABSTRACT FROM AUTHOR]

The undoped and ferric chloride p-doped ‘‘head-to-head’’ ‘‘tail-to-tail’’ analog of poly(3-decylthiophene) obtained from substituted bithiophenes, i.e., poly(4,4’didecyl-2,2’bithiophene)-PDDBT has been investigated. The samples were studied by means of ultraviolet photoelectron spectroscopy, optical absorption measurements, and polymer bands structure calculations. Experiments were carried out at different temperatures between 100 °C and -180 °C. The results indicate that PDDBT is nonplanar in the undoped state. No temperature effects have been observed in the π-electron properties. In the doped state, however, lowering of the temperature results in an increase of the system planarity that modifies the electronic band structure. This effect has been shown to be fully reversible with temperature. [ABSTRACT FROM AUTHOR]

We have investigated the chemical nature and the electronic structure of the interface between a low work function metal, aluminum, and a conjugated polymer semiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core-level x-ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymer electronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two polythiophene systems (i) the alkyl-substituted poly-3-octylthiophene and (ii) the α-sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythiophene with Al atoms is found to modify dramatically the structure of the conjugated backbone, as strong carbon–aluminum bonds are formed in the α positions of the thiophene rings. A large charge transfer takes place from the Al atoms to the polymer chain, and the upper π levels of the polymer are strongly affected. The metallization is contrasted to the doping of conjugated polymers with alkali metals. [ABSTRACT FROM AUTHOR]

The shake-up features of the N1s and C1s core-level x-ray photoelectron spectra of model molecules of polyaniline are studied theoretically using the intermediate neglect of differential overlap method in combination with configuration interaction. The calculations are performed on N,N’-diphenyl-1,4-phenylenediamine and N,N’-diphenyl-1,4-benzo-quinondiimine, which are model molecules for fully reduced and fully oxidized polyaniline, respectively. The core-level spectra of these two molecules are also studied experimentally by means of x-ray photoelectron spectroscopy. Experimental and theoretical results are found to be in excellent agreement, which allows for a detailed interpretation of the observed shake-up spectra. Core ionization of imine nitrogens is shown to be accompanied by large shake-up intensity. Our studies of model molecules are compared with recently published core-level spectra of polyaniline at various oxidation levels. The increase in imine nitrogen content upon oxidation of the polymer give rise to an increase in the N1s shake-up intensity, a result which is in agreement with what we find for the model molecules. [ABSTRACT FROM AUTHOR]

The electronic structure of poly-3-hexylthiophene (P3HT) is studied in the solid state with photoelectron spectroscopy, as the polymer is gradually doped from NOPF6. Solubility and processability of P3HT allow for the preparation of very clean and very thin films, which are then doped without air-exposure. The evolution of the core level binding energies is related to the modification of the electron density on the conjugated backbone, due to the creation of polaron and bipolaron defects. Upon doping, valence spectra show a shift in the Fermi level of the system, and at saturation doping a finite density of states at the Fermi level is observed unambiguously for the first time in electrically conducting polymers with photoelectron spectroscopy. The experimental data on core and valence levels can be interpreted in terms of the formation of a polaron lattice at high doping. [ABSTRACT FROM AUTHOR]