Modification of the Poly(bisdodecylquaterthiophene) Structure for High and Predominantly Nonionic Conductivity with Matched Dopants.

Four p-type polymers were synthesized by modifying poly(bisdodecylquaterthiophene) (PQT12) to increase oxidizability by p-dopants. A sulfur atom is inserted between the thiophene rings and dodecyl chains, and/or 3,4-ethylenedioxy groups are appended to thiophene rings of PQT12. Doped with NOBF4, PQTS12 (with sulfur in side chains) shows a conductivity of 350 S cm-1, the highest reported nonionic conductivity among films made from dopant-polymer solutions. Doped with tetrafluorotetracyanoquinodimethane (F4TCNQ), PDTDE12 (with 3,4-ethylenedioxy groups on thiophene rings) shows a conductivity of 140 S cm-1. The converse combinations of polymer and dopant and formulations using a polymer with both the sulfur and ethylenedioxy modifications showed lower conductivities. The conductivities are stable in air without extrinsic ion contributions associated with PEDOT:PSS that cannot support sustained current or thermoelectric voltage. Efficient charge transfer, tighter π-π stacking, and strong intermolecular coupling are responsible for the conductivity. Values of nontransient Seebeck coefficient and conductivity agree with empirical modeling for materials with these levels of pure hole conductivity; the power factor compares favorably with prior p-type polymers made by the alternative process of immersion of polymer films into dopant solutions. Models and conductivities point to significant mobility increases induced by dopants on the order of 1-5 cm2 V-1 s-1, supported by field-effect transistor studies of slightly doped samples. The thermal conductivities were in the range of 0.2-0.5 W m-1 K-1, typical for conductive polymers. The results point to further enhancements that could be obtained by increasing doped polymer mobilities.

ZT > 0.1 Electron-Carrying Polymer Thermoelectric Composites with In Situ SnCl2 Microstructure Growth.

An n-type pyromellitic diimide polymer composite with in situ microstructure growth of the common element compound SnCl2 reaches power factor of 50-100 µW m-1 K-2, the highest purely n-type polymer composite power factor yet reported. The composite has a gigantic Seebeck coefficient between -4000 and -5000 µV K-1, many times higher than other polymer composites.

Reducing leakage currents in n-channel organic field-effect transistors using molecular dipole monolayers on nanoscale oxides.

Leakage currents through the gate dielectric of thin film transistors remain a roadblock to the fabrication of organic field-effect transistors (OFETs) on ultrathin dielectrics. We report the first investigation of a self-assembled monolayer (SAM) dipole as an electrostatic barrier to reduce leakage currents in n-channel OFETs fabricated on a minimal, leaky ∼10 nm SiO2 dielectric on highly doped Si. The electric field associated with 1H,1H,2H,2H-perfluoro-octyltriethoxysilane (FOTS) and octyltriethoxysilane (OTS) dipolar chains affixed to the oxide surface of n-Si gave an order of magnitude decrease in gate leakage current and subthreshold leakage and a two order-of-magnitude increase in ON/OFF ratio for a naphthalenetetracarboxylic diimide (NTCDI) transistor. Identically fabricated devices on p-Si showed similarly reduced leakage and improved performance for oxides treated with the larger dipole FOTS monolayer, while OTS devices showed poorer transfer characteristics than those on bare oxide. Comparison of OFETs on both substrates revealed that relative device performance from OTS and FOTS treatments was dictated primarily by the organosilane chain and not the underlying siloxane-substrate bond. This conclusion is supported by the similar threshold voltages (VT) extrapolated for SAM-treated devices, which display positive relative VT shifts for FOTS on either substrate but opposite VT shifts for OTS treatment on n-Si and p-Si. Our results highlight the potential of dipolar SAMs as performance-enhancing layers for marginal quality dielectrics, broadening the material spectrum for low power, ultrathin organic electronics.

Effects of pulsing and interfacial potentials on tellurium-organic heterostructured films.

Polycrystalline thin films of tellurium and organic semiconductor molecules are paired in heterostructured field-effect transistors built on Si/SiO2 substrates. While charge carrier mobilities can exceed 1 cm(2)/(V s), there is only a limited gate voltage range over which the current is modulated. We employ continuous and pulsed measurements on transistors to explore the influence of charge equilibration time on device behavior, finding that pulsed gating improves output characteristics. We also use surface potential measurements to investigate the interfacial vacuum level offset between materials, and we modify the interlayer potential profile by interposing statically charged dielectric layers on the silicon dioxide. We show that interfacial fields determine the gate voltage range over which Te shows a field effect in heterostructures with organic semiconductors and that modification of these fields can extend this range.

Tellurium thin films in hybrid organic electronics: morphology and mobility.

Elemental Te displays a wide variety of nanoscale morphologies of vapor-deposited films, depending on the substrate surface and temperature. These morphologies are correlated to field-effect mobilities in transistors made with Te as the lone semiconductor or from Te-organic multilayer semiconductors. Two examples of morphologies and transistor output characteristics, i.e., on a room temperature oxide and heated organic, are shown in the figure.