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, 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. 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 cm V s, supported by field-effect transistor studies of slightly doped samples. The thermal conductivities were in the range of 0.2-0.5 W m K, typical for conductive polymers. The results point to further enhancements that could be obtained by increasing doped polymer mobilities.
We report the synthesis and characterization
of two solution-processable
pyromellitic diimide (PyDI)-acetylene-based conjugated homopolymers.
Adjacent PyDI cores were connected with triple bond linkages by reacting
3,6-dibromo-N,N′-dialkyl
pyromellitic diimides with bis(tributylstannyl)acetylene under Stille
coupling conditions. Cyclic voltammetry revealed that these polymers
have sufficient electron affinity to accept electrons. Absorption
spectra revealed that one polymer, with a simple octyl chain, has
greater intermolecular interaction or conjugation after forming a
thin film, and that film exhibited electron transport in top-gate
bottom-contact mode organic field-effect transistor (OFET) devices.
X-ray diffraction (XRD) and atomic force microscopy (AFM) results
show that the octyl polymer is amorphous on the bulk scale. The polymer
exhibited electron mobility of about 2 × 10–4 cm2 V–1 s–1 with
on/off ratio of 103 and is the simplest n-channel polymer
yet reported. A 4-trifluoromethylphenethyl side chain did not result
in measurable electron mobility. The octyl polymer exhibited negative
Seebeck coefficient on the order of −40 μV/K in thermoelectric
devices, further substantiating its n-channel activity. The demonstration
of electron transport from such a simple polymer has opened a new
path for obtaining n-channel semiconducting activity from polymer
films.
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.
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