O Ok Park

Enhancement of Donor–Acceptor Polymer Bulk Heterojunction Solar Cell Power Conversion Efficiencies by Addition of Au Nanoparticles

This research was supported by Future-based Technology Development Program (Nano Fields, 2010-0029321) and the WCU (World Class University) program (R32-2008-000-10142-0) through the NRF of Korea funded by the MEST. J. H. Park acknowledges the support from NRF of Korea funded by the MEST (NRF-2009- C1AAA001-2009-0094157, 2011-0006268). Research at UCSB was supported by the US Army General Technical Services (LLC/GTS-S- 09-1-196) and by the Department of Energy (BES-DOE- ER46535).

Rheology and dynamics of immiscible polymer blends

The fundamental problems in two immiscible polymer blends, such as deformation, break‐up, and coalescence of the dispersed phase, were considered. Instead of formulating a single droplet problem, it was assumed that there is a kind of structure of the interfaces, in which the interfacial area (Q) and its anisotropy (qij) are equilibrated due to the competition between flow and interfacial tension. Relaxation mechanisms of the interfaces in heterogeneous systems were phenomenologically considered so that a more general constitutive equation was proposed, which can be used not only for arbitrary volume fractions but also for arbitrary flow fields. Also, the effect of simple shear flow on the morphology of polystyrene (PS)/linear low‐density polyethylene (LLDPE) blends was experimentally investigated. Whereas most works along these lines have been done visually in a flow cell, our samples were quenched after steady shear and their resulting structures were analyzed by scanning electron microscopy. In order to achieve a better understanding of morphological effects on polymer blending processes, the semiphenomenological expressions describing the interface contributions of immiscible polymer blends were formulated and compared with dynamic shear experiments of PS/LLDPE blends.The fundamental problems in two immiscible polymer blends, such as deformation, break‐up, and coalescence of the dispersed phase, were considered. Instead of formulating a single droplet problem, it was assumed that there is a kind of structure of the interfaces, in which the interfacial area (Q) and its anisotropy (qij) are equilibrated due to the competition between flow and interfacial tension. Relaxation mechanisms of the interfaces in heterogeneous systems were phenomenologically considered so that a more general constitutive equation was proposed, which can be used not only for arbitrary volume fractions but also for arbitrary flow fields. Also, the effect of simple shear flow on the morphology of polystyrene (PS)/linear low‐density polyethylene (LLDPE) blends was experimentally investigated. Whereas most works along these lines have been done visually in a flow cell, our samples were quenched after steady shear and their resulting structures were analyzed by scanning electron microscopy. In order t...

Highly Stretchable and Wearable Graphene Strain Sensors with Controllable Sensitivity for Human Motion Monitoring

Because of their outstanding electrical and mechanical properties, graphene strain sensors have attracted extensive attention for electronic applications in virtual reality, robotics, medical diagnostics, and healthcare. Although several strain sensors based on graphene have been reported, the stretchability and sensitivity of these sensors remain limited, and also there is a pressing need to develop a practical fabrication process. This paper reports the fabrication and characterization of new types of graphene strain sensors based on stretchable yarns. Highly stretchable, sensitive, and wearable sensors are realized by a layer-by-layer assembly method that is simple, low-cost, scalable, and solution-processable. Because of the yarn structures, these sensors exhibit high stretchability (up to 150%) and versatility, and can detect both large- and small-scale human motions. For this study, wearable electronics are fabricated with implanted sensors that can monitor diverse human motions, including joint movement, phonation, swallowing, and breathing.

An Electrochemical Capacitor Based on a Ni ( OH ) 2/Activated Carbon Composite Electrode

In order to enhance energy density, a hybrid type pseudocapacitor/electric double layer capacitor (EDLC) was considered and its electrochemical properties were investigated. At various current densities, stable charge/discharge behaviors were observed with much higher specific capacitance values of 530 F/g compared with that of EDLC (230 F/g), by introducing as a cathode material. By using the modified cathode of a /activated carbon composite electrode, the specific capacitance was less sensitive to charge/discharge current density exhibiting stable power characteristics.

Carbon Nanotube/RuO2 nanocomposite electrodes for supercapacitors

Electrochemical characteristics of electrodes for supercapacitors built from RuO 2 /maltiwalled carbon nanotube (CNT) nanocomposites have been investigated. Capacitances have been estimated by cyclic voltammetry at different scan rates from 5-50 mV/s. Electrostatic charge storage as well as pseudofaradaic reactions of RuO 2 nanoparticles have been affected by the surface functionality of CNTs due to the increased hydrophilicity. Such hydrophilicity enables easy access of the solvated ions to the electrode/electrolyte interface, which increases faradaic reaction site number of RuO 2 nanoparticles. The specific capacitance of RuO 2 /pristine CNT nanocomposites based on the combined mass was about 70 F/g (RuO 2 : 13 wt % loading). and the specific capacitance based on the mass of RuO 2 was 500 F/g. However, the specific capacitance of RuO 2 /hydrophilic CNT nanocomposites based on the combined mass was about 120 F/g (RuO 2 : 13 wt % loading), and the specific capacitance based on the mass of RuO 2 was about 900 F/g.

Dye-sensitized solar cells with Pt- and TCO-free counter electrodes.

Here for the first time, we demonstrate novel DSSCs with a Pt- and TCO-free counter electrode, which have a highly conductive polymer exhibiting high catalytic ability and charge transport, and the photovoltaic performance increases as we increase the PEDOT film conductivity.

The fabrication of syndiotactic polystyrene/organophilic clay nanocomposites and their properties

The fabrication of nanocomposite of syndiotactic polystyrene (sPS)/organophilic clay was conducted by melt intercalation. To avoid the decrease of interlayer spacing due to desorption of organic materials at high temperature, various amorphous styrenic polymers were introduced during the melt mixing process. The nanocomposites were fabricated via two different methods, one is the stepwise mixing method, which is the melt intercalation of amorphous styrenic polymers into organophilic clay followed by blending with sPS, and the other is the simultaneous mixing method, in which all components are melt mixed together. The microstructures of nanocomposites were investigated by X-ray diffraction (XRD) and transmission electron microscopy. The mechanical properties of the nanocomposites such as tensile strength, flexural modulus and izod impact strength were measured and discussed in relation to their microstructures. Both fabrication methods yielded the nanocomposites with different microstructures ranging from intercalated structure to exfoliated structure depending on the kind of amorphous styrenic polymers, which was revealed by the increase in interlayer spacing on X-ray spectrum. Amorphous polymers intercalated into the clay gallery previously is considered to play an important role in maintaining the intercalated or exfoliated structure without any contraction of interlayer spacing even at sPS melting temperature. The fabrication method also influenced the microstructure and mechanical properties, especially tensile strength. In the case of nanocomposites having intercalation structure, the stepwise mixing method yielded more obvious intercalation structure than the simultaneous mixing method so that the former method resulted in higher tensile strength. On the other hand, the nanocomposite having exfoliated structure showed similar mechanical properties between the two fabrication methods.

Extreme bendability of single-walled carbon nanotube networks transferred from high-temperature growth substrates to plastic and their use in thin-film transistors

In this paper we describe printing methods for transferring single-walled carbon nanotubes (SWNTs) from high-temperature growth substrates to flexible, low-cost plastic supports. Thin-film transistors (TFTs) built with networks of transferred SWNTs grown by chemical vapor deposition show good performance—mobilities and on∕off current ratios similar to those of devices fabricated on the growth substrates for a wide range of channel lengths. Bending tests on these TFTs show that their output current varies only in a narrow (±5%) range, even for bend radii that induce surface strains larger than 1%. Similar structures evaluated under sharp folding, with strains larger than 20%, show that the SWNT networks are operational even under extreme bending conditions. This level of mechanical robustness, the good electrical performance, and optical transparency make transferred SWNT networks an attractive type of electronic material for applications in macroelectronics, sensors, and other systems that require wide ar...

The property and formation mechanism of unsaturated polyester–layered silicate nanocomposite depending on the fabrication methods

Abstract The properties of unsaturated polyester (UP)/montmorillonite (MMT) nanocomposite greatly depend on the preparation procedure because of the chemical reactions and physical interactions involved. To investigate the properties and formation mechanism of UP/MMT nanocomposite, samples were prepared by two different mixing methods. The first method, simultaneous mixing, is similar to the method used for preparing the conventional unsaturated polyester and filler composite. The second method is the sequential mixing, a new approach for preparing unsaturated polyester–layered silicate nanocomposite. In the first step, pre-intercalates of the unsaturated polyester and MMT nanocomposites were prepared. In other words, mixture of the UP and organophillic-treated MMT are prepared in the first step; a styrene monomer was then added to the pre-intercalates of UP/MMT with varying mixing time. The structures of UP/MMT nanocomposite were investigated by X-ray diffraction and transmission electron microscopy. To investigate the formation mechanism of UP/MMT nanocomposite, dynamic mechanical thermal analysis, solution-rheometry and melt-rheometry were performed. The properties and formation processes depending on two methods are compared. These tests enable us to understand the mechanism of UP–silicate nanocomposite formation. Based on this mechanism, we have been able to increase the crosslinking density and the degree of dispersion in UP/MMT nanocomposite.

Printed thin-film transistors and complementary logic gates that use polymer-coated single-walled carbon nanotube networks

This paper reports on the electrical properties of thin-film transistors (TFTs) that use polymer-coated networks of single-walled carbon nanotubes (SWNTs) as the semiconductor with source and drain electrodes formed by high-resolution printing techniques. P-channel, n-channel, and ambipolar TFTs are demonstrated with bare SWNT networks, networks coated with polyethylene imine and with polyethylene oxide, respectively. Studies of the scaling of properties with channel length and tube density reveal important information about the operation of these devices. Complementary inverters made with n- and p-channel devices show gain larger than one and illustrate the potential use of these types of TFTs for complex logic circuits.