Phonon-phonon interactions in the polarizarion dependence of Raman scattering

We have found that the polarization dependence of the Raman signal in organic crystals can only be described by a fourth-rank formalism. The generalization from the traditional second-rank Raman tensor $\mathcal{R}$ is physically motivated by consideration of the light scattering mechanism of anharmonic crystals at finite temperatures, and explained in terms of off-diagonal components of the crystal self-energy. We thus establish a novel manifestation of anharmonicity in inelastic light scattering, markedly separate from the better known phonon lifetime.Comment: 31 pages, 17 figure

Dinaphthotetrathienoacenes : synthesis, characterization, and applications in organic field-effect transistors

The charge transport properties of organic semiconductors are limited by dynamic disorder that tends to localize charges in organic crystals. It is the main hurdle to overcome in order to significantly increase charge carrier mobility. We propose an innovative design that combines a chemical structure based on sulfur-rich thienoacene with a solid-state herringbone packing and present the synthesis, physicochemical characterization and charge transport properties of two new thienoacenes bearing a central tetrathienyl core fused with two external naphthyl rings: DN4T and isoDN4T. Both compounds crystallize with a herringbone pattern structure and present transfer integrals ranging from 33 to 99 meV (for the former) within the herringbone plane of charge transport. Molecular dynamics simulations point towards an efficient resilience of the transfer integrals to the intermolecular sliding motion commonly responsible for strong variations of the electronic coupling in the crystal. Best device performances were reached with DN4T with hole mobility up to μ = 2.1 cm² V-1 s-1 in polycrystalline OFETs, showing the effectiveness of the electronic coupling enabled by the new aromatic core. These promising results pave the way to the design of high-performing materials based on this new thienoacene, notably through the introduction of alkyl side-chains

Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

International audienceWe study by scanning thermal microscopy the nanoscale thermal conductance of films (40-400 nm thick) of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8). We demonstrate that the out-of-plane thermal conductivity is significant along the interlayer direction, larger for BTBT (0.63 ± 0.12 W m-1 K-1) compared to C8-BTBT-C8 (0.25 ± 0.13 W m-1 K-1). These results are supported by molecular dynamics calculations (Approach to Equilibrium Molecular Dynamics method) performed on the corresponding molecular crystals. The calculations point to significant thermal conductivity (3D-like) values along the 3 crystalline directions, with anisotropy factors between the crystalline directions below 1.8 for BTBT and below 2.8 for C8-BTBT-C8, in deep contrast with the charge transport properties featuring a two-dimensional character for these materials. In agreement with the experiments, the calculations yield larger values in BTBT compared to C8-BTBT-C8 (0.6-1.3 W m-1 K-1 versus 0.3-0.7 W m-1 K-1, respectively). The weak thickness dependence of the nanoscale thermal resistance is in agreement with a simple analytical model

Chemical modifications suppress anharmonic effects in the lattice dynamics of organic semiconductors

The lattice dynamics of organic semiconducting crystals has a significant role in determining their electronic and mechanical properties. A common technique to control these macroscopic properties is chemically modifying the molecular structure. These modifications are known to change the molecular packing, but their effect on the lattice dynamics is relatively unexplored though it can be equally important to their performance at finite temperatures. Therefore, we investigate how chemical modifications to a core [1]benzothieno[3,2-b]benzothiophene (BTBT) semiconducting crystal affect the evolution of the crystal structural dynamics with temperature. Our study combines temperature-dependent polarization-orientation (PO) low-frequency Raman measurements with first-principles calculations and single-crystal XRD measurements. We show that the chemical modifications can indeed suppress expressions of vibrational anharmonicity in the lattice dynamics. Specifically, we detect in BTBT a gradual change in the PO Raman response with temperature, indicating a unique expression of anharmonicity. This anharmonic expression is suppressed in all examined chemically modified crystals (ditBu-BTBT and diC8-BTBT, diPh-BTBT, and DNTT). Our findings indicate that π-conjugated chemical modifications suppress anharmonic effects more effectively than alkyl chains