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

Design, synthesis, chemical stability, packing, cyclic voltammetry, ionisation potential, and charge transport of [1]benzothieno[3,2-b][1]benzothiophene derivatives

Five new molecular semiconductors that differ from dioctylbenzothienobenzothiophene, by the introduction of ether or thioether side chains, have been synthesized and obtained in good yields. Their availability in sufficient quantities has allowed investigation of their electrochemical behaviour in solution and their electronic properties in solid state. Both ether and thioether compounds oxidise rather easily in solution, but nevertheless, they exhibit rather high ionisation potentials. This is a consequence of their crystal structure. Dioctylthioetherbenzothienobenzothiophene is rather sensitive to oxidation and degrades easily in close to ambient conditions. Dioctylletherbenzothienobenzothiophene is more stable. Its charge carrier mobility remains however rather moderate, on the order of 0.5 cm2/V.s, whereas that of dioctylbenzothienobenzothiophene reached 4 cm2/V.s, in the same conditions. The difference is explained by intrinsic factors as shown by a theoretical modelling

High‐Efficiency Ion‐Exchange Doping of Conducting Polymers

Abstract: Molecular doping—the use of redox‐active small molecules as dopants for organic semiconductors—has seen a surge in research interest driven by emerging applications in sensing, bioelectronics, and thermoelectrics. However, molecular doping carries with it several intrinsic problems stemming directly from the redox‐active character of these materials. A recent breakthrough was a doping technique based on ion‐exchange, which separates the redox and charge compensation steps of the doping process. Here, the equilibrium and kinetics of ion exchange doping in a model system, poly(2,5‐bis(3‐alkylthiophen‐2‐yl)thieno(3,2‐b)thiophene) (PBTTT) doped with FeCl3 and an ionic liquid, is studied, reaching conductivities in excess of 1000 S cm−1 and ion exchange efficiencies above 99%. Several factors that enable such high performance, including the choice of acetonitrile as the doping solvent, which largely eliminates electrolyte association effects and dramatically increases the doping strength of FeCl3, are demonstrated. In this high ion exchange efficiency regime, a simple connection between electrochemical doping and ion exchange is illustrated, and it is shown that the performance and stability of highly doped PBTTT is ultimately limited by intrinsically poor stability at high redox potential

Fingerprints for Structural Defects in Poly(thienylene vinylene) (PTV): A Joint Theoretical–Experimental NMR Study on Model Molecules

In the field of plastic electronics, low band gap conjugated polymers like poly(thienylene vinylene) (PTV) and its derivatives are a promising class of materials that can be obtained with high molecular weight via the so-called dithiocarbamate precursor route. We have performed a joint experimental- theoretical study of the full NMR chemical shift assignment in a series of thiophene-based model compounds, which aims at (i) benchmarking the quantum-chemical calculations against experiments, (ii) identifying the signature of possible structural defects that can appear during the polymerization of PTV's, namely head-to-head and tail-to-tail defects, and (iii) defining a criterion regarding regioregularity

F Center Mediated Growth of Patterned Organic Semiconductor Films on Alkali Halides

Organic semiconductors combine flexible tailoring of their optoelectronic properties by synthetic means with strong light matter coupling, which is advantageous for organic electronic device applications. Although spatially selective deposition has been demonstrated, lateral patterning of organic films with simultaneous control of molecular and crystalline orientation is lacking as traditional lithography is not applicable. Here, a new patterning approach based on surface localized F centers halide vacancies generated by electron irradiation of alkali halides is presented, which allows structural control of molecular adlayers. Combining optical and atomic force microscopy, X ray diffraction, and density functional theory DFT calculations, it is shown that dinaphthothienothiophene DNTT molecules adopt an upright orientation on pristine KCl surfaces, while the F centers stabilize a recumbent orientation, and that these orientations are maintained in thicker films. This specific nucleation results also in different crystallographic morphologies, namely, densely packed islands and jagged fibers, each epitaxially aligned on the KCl surface. Spatially selective surface irradiation can also be used to create patterns of F centers and thus laterally patterned DNTT films, which can be further transferred to any including elastomer substrate due to the water solubility of the alkali halide growth template

Palladium(0) NHC complexes : a new avenue to highly efficient phosphorescence

We report the first examples of highly luminescent di-coordinated Pd(0) complexes. Five complexes of the form [Pd(L)(L’)] were synthesized, where L = IPr, SIPr or IPr* NHC ligands and L’ = PCy3, or IPr and SIPr NHC ligands. The photophysical properties of these complexes were determined in toluene solution and in solid state and contrasted to the poorly luminescent reference complex [Pd(IPr)(PPh3)]. Organic light-emitting diodes were successfully fabricated but attained external quantum efficiencies of between 0.3 and 0.7%

The self-assembly of amphiphilic oligothiophenes: hydrogen bonding and poly(glutamate) complexation

The self-organization behavior of an amphiphilic sexithiophene bearing amide functionalities is studied and compared to that of a deriv. bearing an ester group at the same position. The introduction of hydrogen-bond interactions in assemblies of these pi-conjugated oligomers is found to affect the mol. organization both in protic media and in thin deposits on mica. The amphiphilic 2,2';5',2'';5'',2''';5''',2''''; 5'''',2'''''-sexithiophene-5,5'''''-dicarboxylic acid bis[(4,7,10,13,16-pentaoxaheptadecyl)amide] forms assemblies in n-butanol and water and partially aggregates in toluene. Spectroscopy reveals that the presence of a hydrogen-bonding moiety increases the thermal stability of the assemblies in n-butanol and even more in water soln. On mica surfaces, the formation of rod-like one-dimensional nano-structures is obsd. after deposition from toluene solns. In addn., transmission electron microscopy in combination with selected area electron diffraction shows that in water plate-like structures are formed built from parallel oriented stacks, with a pi-pi distance of 3.5 .ANG.. Comparison of these data to mol. modeling and quantum chem. calcns. is used to better understand the influence of the amide group on the stacking of these compds. The introduction of these H-bonding interactions leads to denser and more stable stacks. Furthermore, we show that a deriv. of the amide compd., bearing terminal ammonium groups, forms a complex with chiral polyanions in aq. media such that the sexithiophene segments are stacked in a meta-stable helical fashion with preferred handedness. We obsd. that poly(glutamate) and DNA generate a chiral sexithiophene assembly. In time the induced chirality disappears, which is explained by the meta-stability of the kinetically formed adduct. This constitutes one step forward towards the controlled formation of functional multi-component systems in aq. soln. [on SciFinder (R)