Polymorphism of terthio-phene with surface confinement.

The origin of unknown polymorphic phases within thin films is still not well understood. This work reports on crystals of the molecule terthio-phene which were grown by thermal gradient crystallization using glass-plate substrates. The crystalline domains displayed a plate-like morphology with an extended lateral size of about 100 µm, but a thickness of only a few µm. Specular X-ray diffraction patterns confirmed the presence of a new polymorph of terthio-phene. Crystal structure solution from a single crystal peeled from the film revealed a structure with an extremely large unit-cell volume containing 42 independent molecules. In contrast to the previously determined crystal structure of terthio-phene, a herringbone packing motif was observed where the terminal ends of the molecules are arranged within one plane (i.e. the molecular packing conforms to the flat substrate surface). This type of molecular packing is obtained by 180° flipped molecules combined with partially random (disordered) occupation. A densely packed interface between terthio-phene crystallites and the substrate surface is obtained, this confirms that the new packing motif has adapted to the flat substrate surface

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

Self-assembled π-conjugated organic nanoplates : from hexagonal to triangular motifs

Organic semiconducting materials in confined geometries present interesting properties connected to change or coexistence of molecular organizations, depending on their characteristic scales. In this work the preparation and structural characterization of self-assembled π-conjugated organic nanoplates of 2,7-diperfluorohexyl-[1]benzothieno[3,2,b]benzothiophene are studied. These nanostructures were prepared by spin coating, implementing a fast precipitation method. We observed an evolution of the nanostructures from hexagonal towards triangular nanoplates, possibly driven by enhanced solvent/molecule interactions, and accompanied by the formation of a new polymorph

Thermoelectric nanospectroscopy for the imaging of molecular fingerprints

We present a nanospectroscopic device platform allowing simple and spatially resolved thermoelectric detection of molecular fingerprints of soft materials. Our technique makes use of a locally generated thermal gradient converted into a thermoelectric photocurrent that is read out in the underlying device. The thermal gradient is generated by an illuminated atomic force microscope tip that localizes power absorption onto the sample surface. The detection principle is illustrated using a concept device that contains a nanostructured strip of polymethyl methacrylate (PMMA) defined by electron beam lithography. The platform's capabilities are demonstrated through a comparison between the spectrum obtained by on-chip thermoelectric nanospectroscopy with a nano-FTIR spectrum recorded by scattering-type scanning near-field optical microscopy at the same position. The subwavelength spatial resolution is demonstrated by a spectral line scan across the edge of the PMMA layer

Chasing the ‘Killer’ Phonon Mode for the Rational Design of Low Disorder, High Mobility Molecular Semiconductors

Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron – phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high mobility molecular semiconductors, we have combined state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electron phonon coupling constants with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way we have been able to identify the long-axis sliding motion as a ‘killer’ phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, we propose a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high mobility molecular semiconductors.Royal Society German Research Foundation European Research Council Engineering and Physical Sciences Research Council ARCHER UK National Supercomputing Service Belgian National Fund for Scientific Research Leverhulme Trust Wiener-Anspach Foundation Belgian Walloon Region GENCI-CINES/IDRI

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

Funder: Consortium des Équipements de Calcul IntensifFunder: The Leverhulme TrustAbstract: 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

Unraveling unprecedented charge carrier mobility through structure property relationship of four isomers of didodecyl[1]benzothieno[3,2-b][1]benzothiophene

Since the dawn of organic electronics in the 1970’s, academic and industrial research efforts have led to dramatic improvements of the solubility, stability, and electronic properties of organic semiconductors (OSCs).[1, 2] The common benchmark to characterize the electrical performances of OSCs is their charge carrier mobility μ (cm2 V–1 s–1), defined as the drift velocity of the charge carrier (cm s–1) per unit of applied electric field (V cm–1). Reaching high mobilities in OSCs is highly desirable as it allows faster operation of transistors and energy savings by reduced calculation times.[2, 3] However, OSCs performances (conventional values usually range from 1 to 10 cm2 V–1 s–1, with highest values obtained with single-crystal devices mostly exempt of structural defects) are still not comparable to that of state-of-the-art inorganic semiconductors (e.g. metal oxides with µ = 20-50 cm2 V–1 s–1 and polycrystalline silicon with µ > 100 cm2 V–1 s–1) thereby hampering important potential technological applications such as flexible organic light-emitting diode (OLED) displays and wearable electronics.[3, 4

From synthesis to device fabrication: elucidating the structural and electronic properties of C7-BTBT-C7

We report the polymorph investigation, crystallographic study and fabrication of organic field-effect transistors (OFETs) in solution-processed thin films of a prototypical organic semiconductor, i.e., 2,7-diheptylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (C7-BTBT-C7). We found that this molecule self-assembles solely into one type of stable crystal form, regardless of the experimental conditions employed when using conventional and non-conventional methods of crystallization. The integration of blends of C7-BTBT-C7 with polystyrene as active materials in OFETs fabricated using a solution shearing technique led to a field-effect mobility of 1.42 ± 0.45 cm2 V−1 s−1 in the saturation regime when a coating speed of 10 mm s−1 was employed. The intrinsic structural properties control the overlap of the frontier orbitals, thereby affecting the device performance. The interplay between the crystal packing, thin film morphology and uniformity and its impact on the device performance are reported.We acknowledge the Paul Scherrer Institut, Villigen, Switzerland, for the provision of synchrotron radiation beam time at the beamline MS-X04SA of the SLS (ID proposal 20201790). P. P., L. F., N. M., N. T., J. C., P. S., M. M. T., Y. G., E. M., and L. M., contributors of the paper, are the members of the UHMob project. This work has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant no. 811284. G. S. is a FNRS Research Associate (Belgian National Fund for Scientific Research). G. S. acknowledges financial support from the Francqui Foundation (Francqui Start-Up Grant). G. S. thanks the FNRS for financial support through the research project COHERENCE2 (no. F.4536.23). M. M. T. and L. F. also acknowledge MCIN through the project GENESIS PID2019 and the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S), and the Generalitat de Catalunya (2017-SGR-918).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

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