Continuous Conductive Path Research Articles

Formation of Organic Crystalline Nanopillar Arrays and Their Application to Organic Photovoltaic Cells

To enhance the performance of organic photovoltaic (OPV) cells, preparation of organic nanometer-sized pillar arrays is fascinating because a significantly large area of a donor/acceptor heterointerface having continuous conduction path to both anode and cathode electrodes can be realized. In this study, we grew cupper phthalocyanine (CuPc) crystalline nanopillar arrays by conventional thermal gradient sublimation technique using a few-nanometer-sized trigger seeds composed of a CuPc and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) stacked layer. We optimized the pillar density by tuning crystal growth condition in order to apply it to OPV cells.

Block copolymer/ionic liquid films: The effect of ionic liquid composition on morphology and ion conduction

The effect of morphology on ion transport in ionic liquid-based solid-state films was investigated. In this study, mixtures of a block copolymer, poly(styrene-b-methyl methacrylate) (SbMMA), and an ionic liquid (IL), 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide (EMIm-TFSI), were prepared as clear solid-state films at various IL compositions (0–50 wt%) by solution casting from a volatile co-solvent. The IL was preferentially miscible with the MMA block as evidenced by visual inspection and differential scanning calorimetry. Both equilibrium and non-equilibrium morphologies were identified with X-ray scattering and transmission electron microscopy and the morphology varied with MMA/IL volume fraction. The morphology and microdomain orientation had a significant impact on ionic conductivity. Higher through-plane conductivities were observed in morphologies with a three-dimensionally continuous conducting path (e.g., non-conducting S cylinders) compared to morphologies with a non-continuous conducting path (e.g., lamellae). When the lamellae were oriented in the plane, the through-plane conductivity was significantly lower than the in-plane conductivity, while the conductivity was direction-independent when the morphologies have a continuous conductive path. Also, a significant increase in conductivity was observed with increasing IL content at the glass transition of the conductive (MMA/IL) microdomain. Finally, significantly higher ionic conductivities can be achieved in a block copolymer/IL solid-state film compared to a homopolymer/IL film at the same IL content (wt%), because the non-conductive microdomain excludes IL, which produces a higher local IL concentration in the conductive phase.

Morphological and electrical characterization of Si xCr yC zB v thin films

Si x Cr y C z B v thin films with several compositions have been studied for integration of high precision resistors in 0.8 μm BICMOS technology. These resistors, integrated in the back-end of line, have the advantage to provide high level of integration and attractive electrical behavior in temperature, for analog devices. The film morphology and the structure have been investigated through transmission electron microscopy analysis and have been then related to the electrical properties on the base of the percolation theory. According to this theory, and in agreement with experimental results, negative thermal coefficient of resistance (TCR) has been obtained for samples with low Cr content, corresponding to a crystalline volume fraction below the percolation threshold. Samples with higher Cr content exhibit, instead, a variation of the TCR as a function of film thickness: negative TCR values are obtained for thickness lower than 5 nm, corresponding to a crystalline volume fraction below the percolation threshold; positive TCR are obtained for larger thickness, indicating the establishment of a continuous conductive path between the Cr rich grains. This property seems to be determinant in order to assure the possibility to obtain thin film resistors almost independent on the temperature.

Fabrication and Thermal Analysis of Submicron Silver Tubes Prepared from Electrospun Fiber Templates

Submicron silver tubes have been synthesized by a polymer-based template approach. Two different approaches to metallization, electroless deposition and exchange plating, were evaluated within the template approach. Silver films with average thickness approximately 50-100 nm were deposited on polycarbonate fibers approximately 400 nm in diameter by each technique, resulting in tubes with a diameter between 450 and 500 nm after thermal degradation of core fibers. These nanomaterials were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and scanning thermal microscopy. The thermal conductivity of the silver submicron tubes was found to differ depending on the method of preparation, with tubes from electroless plating possessing relative thermal conductivity values that were 1 order of magnitude higher than that from exchange plating, 3000 W/m x K and 660 W/m x K, respectively. Interestingly, these results indicate that silver submicron tubes possess higher thermal conductivity than the bulk metal. This observation is discussed in the context of the continuous conduction path of the tubes and their high surface area-to-volume ratio.

On the improved properties of injection-molded, carbon nanotube-filled PET/PVDF blends

The mechanisms for improved mechanical and electrical properties of an injection molded, carbon nanotubes (CNTs) filled, polyethylene terephthalate (PET)/polyvinylidene fluoride (PVDF) blend have been investigated. It is found that the improved properties are due to the formation of a triple-continuous structure in the CNT-filled polymer blend; CNT segregates in the continuous PET phase, forming a continuous conductive path to provide the composite an electrical short circuit. The continuous PVDF phase free from CNT, on the other hand, offers crack bridging and the interface between the PET and PVDF phases provides crack deflection for the composite. As a result of such a combination, the CNT-filled PET/PVDF has better electrical conductivity, strength and elongation than the CNT-filled PET with the same CNT loading. The segregation of CNT in the PET phase of the CNT-filled PET/PVDF blend is due to the thermodynamic driving force that favors the segregation of CNT in the PET.

Porous silicon as a near-ideal disordered semiconductor

From the temperature dependence of the dark conductivity in porous silicon materials we find activation energies which are larger than half of the optical bandgap. The phototransport properties indicate a continuous conduction path through the tetrahedrally distorted shell of the links which form the porous silicon coral-like network. The results are interpreted in terms of a mobility gap which is due to disorder in the tetrahedral covalent network.

Remote monitoring of thermal conductivity of a powder by a flash radiometry technique

We study the dependence of the thermal conductivity of a carbon-loaded epoxy powder on its porosity using a noncontact flash radiometry technique. The thermal conductivity is found to be very small when the porosity of the sample is higher than a critical value. Below the critical porosity Pcrit, thermal conductivity increases linearly with packing density. This phenomenon can be explained by a two-phase model; the powder exhibits continuous thermal conduction paths only when the porosity is less than Pcrit.

Anisotropic electrical conductivity of drawn elastomeric ionene–TCNQ salts

AbstractElastomeric ionene–TCNQ salts with favorable electrical, mechanical, and processing characteristics were drawn mechanically. The electrical conductivity parallel and perpendicular to the drawing axis was investigated. Correlation between anisotropic conductivity and the change in microstructure was discussed. The resistivity ρ at 25°C of the simple salt (EI‐TCNQ0) and the complex salt (EI‐TCNQ0.5) were on the order of 105 and 102 Ω cm, respectively. In the drawn TCNQ salts, the ρ parallel to the drawing axis increased greatly; on the other hand, the ρ perpendicular to this axis increased slightly or was similar to the ρ of the undrawn TCNQ salts. The anisotropy in the ρ of EI‐TCNQ0.5 between the two directions reached 40 times. The activation energy also increased in the direction parallel to the drawing axis. In the undrawn TCNQ salts, the continuous conduction paths exist isotropically. With drawing, the continuous conduction paths, particularly in the direction parallel to the drawing axis, break or make a structural change. The anisotropic conductivity disappeared with time in EI‐TCNQ0; however, it was present in EI‐TCNQ0.5 even after 200 days under ambient conditions.