Quantum confinement effects on the ordering of the lowest-lying excited states in conjugated chains

The symmetrized density matrix renormalization group approach is applied within the extended Hubbard-Peierls model (with parameters U/t, V/t, and bond alternation \delta) to study the ordering of the lowest one-photon (1^{1}B^{-}_u) and two-photon (2^{1}A^{+}_g) states in one- dimensional conjugated systems with chain lengths, N, up to N=80 sites. Three different types of crossovers are studied, as a function of U/t, \delta, and N. The U-crossover emphasizes the larger ionic character of the 2A_g state compared to the lowest triplet excitation. The \delta crossover shows strong dependence on both N and U/t. The N-crossover illustrates the more localized nature of the 2A_g excitation relative to the 1B_u excitation at intermediate correlation strengths.Comment: Latex file; figures available upon request. Submitted to PR

Spin-dependent electron-hole capture kinetics in conjugated polymers

The recombination of electron-hole pairs injected in extended conjugated systems is modeled as a multi-pathway vibron-driven relaxation in monoexcited state-space. The computed triplet-to-singlet ratio of exciton formation times $r = \tau_T/\tau_S$ increases from 0.9 for a model dimer to 2.5 for a 32-unit chain, in excellent agreement with experiments. Therewith we rationalize recombination efficiency in terms of spin-dependent interstate vibronic coupling and spin- and conjugation-length-dependent exciton binding energies.Our model calculations for various length polymers indicate that the ratio of the singlet to triplet formation ratios, $r = \sigma_S/\sigma_T$, is inversely related to the ratio of the singlet and triplet binding energies, $\epsilon^b_S/\epsilon^b_T$

Low-Lying Electronic Excitations and Nonlinear Optic Properties of Polymers via Symmetrized Density Matrix Renormalization Group Method

A symmetrized Density Matrix Renormalization Group procedure together with the correction vector approach is shown to be highly accurate for obtaining dynamic linear and third order polarizabilities of one-dimensional Hubbard and $U-V$ models. The $U-V$ model is seen to show characteristically different third harmonic generation response in the CDW and SDW phases. This can be rationalized from the excitation spectrum of the systems.Comment: 4 pages Latex; 3 eps figures available upon request; Proceedings of ICSM '96, to appear in Synth. Metals, 199

Theoretical model of structure-dependent conductance crossover in disordered carbon

We analyze the effects of sp^2/sp^3 bond-aspect ratio on the transport properties of amorphous carbon quasi-1D structures where structural disorder varies in a very non-linear manner with the effective bandgap. Using a tight-binding approach the calculated electron transmission showed a high probability over a wide region around the Fermi-level for sp^2-rich carbon and also distinct peaks close to the band edges for sp^3-rich carbon structures. This model shows a sharp rise of the structure resistance with the increase of sp^3C % followed by saturation in the wide bandgap regime for carbon superlattice-like structures and suggests the tuneable characteristic time of carbon-based devices.Comment: 6 pages, 6 figure

A Novel Mitigation Mechanism for Photo-Induced Trapping in an Anthradithiophene Derivative Using Additives

© 2020 Wiley-VCH GmbH A novel trap mitigation mechanism using molecular additives, which relieves a characteristic early turn-on voltage in a high-mobility p-type acene-based small-molecule organic semiconductor, when processed from hydrous solvents, is reported. The early turn-on voltage is attributed to photo-induced trapping, and additive incorporation is found to be very effective in suppressing this effect. Remarkably, the molecular additive does not disturb the charge transport properties of the small-molecule semiconductor, but rather intercalates in the crystal structure. This novel technique allows for the solution-processing of small molecular semiconductors from hydrous solvents, greatly simplifying manufacturing processes for large-area electronics. Along with various electric and spectroscopic characterization techniques, simulations have given a deeper insight into the trap mitigation effect induced by the additive

Quantum inelastic conductance through molecular wires

We calculate non-perturbatively the inelastic effects on the conductance through a conjugated molecular wire-metal heterojunction, including realistic electron-phonon coupling. We show that at sub-band-gap energies the current is dominated by quantum coherent transport of virtual polarons through the molecule. In this regime, the tunneling current is strongly increased relative to the case of elastic scattering. It is essential to describe the full quantum coherence of the polaron formation and transport in order to obtain correct physics. Our results are generally applicable to one-dimensional atomic or molecular wires.Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letter

Strong Suppression of Thermal Conductivity in the Presence of Long Terminal Alkyl Chains in Low-Disorder Molecular Semiconductors

While the charge transport properties of organic semiconductors have been extensively studied over the recent years, the field of organics-based thermoelectrics is still limited by a lack of experimental data on thermal transport and of understanding of the associated structure–property relationships. To fill this gap, a comprehensive experimental and theoretical investigation of the lattice thermal conductivity in polycrystalline thin films of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (Cn-DNTT-Cn with n = 0, 8) semiconductors is reported. Strikingly, thermal conductivity appears to be much more isotropic than charge transport, which is confined to the 2D molecular layers. A direct comparison between experimental measurements (3ω–Völklein method) and theoretical estimations (approach-to-equilibrium molecular dynamics (AEMD) method) indicates that the in-plane thermal conductivity is strongly reduced in the presence of the long terminal alkyl chains. This evolution can be rationalized by the strong localization of the intermolecular vibrational modes in C8-DNTT-C8 in comparison to unsubstituted DNTT cores, as evidenced by a vibrational mode analysis. Combined with the enhanced charge transport properties of alkylated DNTT systems, this opens the possibility to decouple electron and phonon transport in these materials, which provides great potential for enhancing the thermoelectric figure of merit ZT

Unification of trap-limited electron transport in semiconducting polymers

Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking. Here we investigate electron transport in a wide range of semiconducting polymers by current-voltage measurements of single-carrier devices. We observe for this materials class that electron transport is limited by traps that exhibit a Gaussian energy distribution in the bandgap. Remarkably, the electron-trap distribution is identical for all polymers considered: the number of traps amounts to 3 × 1023 traps per m3 centred at an energy of ∼3.6 eV below the vacuum level, with a typical distribution width of ∼0.1 eV. This indicates that the electron traps have a common origin that, we suggest, is most likely related to hydrated oxygen complexes. A consequence of this finding is that the trap-limited electron current can be predicted for any polymer. © 2012 Macmillan Publishers Limited. All rights reserved