Pen-Cheng Wang

Highly Conductive PEDOT:PSS Treated with Formic Acid for ITO-Free Polymer Solar Cells

We proposed a facile film treatment with formic acid to enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by 4 orders of magnitude. The effect of formic acid concentration on conductivity was investigated; conductivity increased fast with increasing concentration up to 10 M and then increased slightly, the highest conductivity being 2050 S cm(-1) using 26 M concentration. Formic acid treated PEDOT:PSS films also exhibited very high transmittances. The mechanism of conductivity enhancement was explored through SEM, AFM, and XPS. Formic acid with its high dielectric constant screens the charge between PEDOT and PSS bringing about phase separation between them. Increased carrier concentration, removal of PSS from the film, morphology, and conformation change with elongated and better connected PEDOT chains are the main mechanisms of conductivity enhancement. ITO-free polymer solar cells were also fabricated using PEDOT:PSS electrodes treated with different concentrations of formic acid and showed equal performance to that of ITO electrodes. The concentrated acid treatment did not impair the desirable film properties as well as stability and performance of the solar cells.

Effect of molecular weight of additives on the conductivity of PEDOT:PSS and efficiency for ITO-free organic solar cells

We systematically investigated the effect of the molecular weight of additives on the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by using different concentrations and molecular weights of polyethylene glycol (PEG) and ethylene glycol (EG). The conductivity enhancement depends on both the molecular weight and concentration of PEG used. The conductivity of PEDOT:PSS was enhanced from 0.3 S cm−1 to 805 S cm−1 with 2% PEG but to only 640 S cm−1 with 6% EG. PEGs with molecular weight higher than 400 have too low mobility to impart the required screening effect, and hence, the conductivity enhancement is less. Through FTIR, XPS and AFM investigations, the mechanism for the conductivity enhancement is found to be charge screening between PEDOT and PSS followed by phase separation and reorientation of PEDOT chains leading to bigger and better connected particles. The molecular weight and concentration of PEG also affect solar cell performances even though the conductivities are the same. Due to their high conductivity and high transmittance, ITO-free organic solar cell devices fabricated using PEDOT:PSS treated with 2% PEG anodes exhibited performance almost equal to that of the ITO counterparts.

Integration of polymer-dispersed liquid crystal composites with conducting polymer thin films toward the fabrication of flexible display devices

Abstract A short review on the current development in all-organic PDLC (Polymer Dispersed Liquid Crystal) “light valves” using conducting polymer thin films as the driving electrodes is presented in this article. Due to conducting polymers’ better mechanical and interfacial compatibility with plastic substrates, integration of driving electrodes based on conducting polymer thin films in display devices can have some advantages over ITO (Indium Tin Oxide) for the improvement of display devices’ mechanical flexibility. As rigid alignment structures on the substrates sandwiching the liquid crystal components are not required, PDLC-type devices intrinsically can better tolerate mechanical bending than other types of LCD devices. With the integration of conducting polymer thin films as the driving electrodes, it can be expected that the flexibility of PDLC devices could be further enhanced.

Selective deposition of films of polypyrrole, polyaniline and nickel on hydrophobic/hydrophilic patterned surfaces and applications

Abstract Computer-generated patterned hydrophobic films are produced on hydrophilic glass substrates by a microcontact “stamp” printing technique. The hydrophobic pattern is produced when the silicone elastomeric stamp (“inked” with C18H37SiCl3) contacts the hydrophilic surface. Thin films of polypyrrole and polyaniline are deposited preferentially on the hydrophobic patterned surfaces from dilute aqueous solutions of the polymerizing monomer. Conversely, strongly adhering films of metals such as nickel are deposited selectively by an electroless process on the hydrophilic substrate surfaces. The properties of the polymers deposited on the hydrophilic and hydrophobic surfaces will be discussed as well as the application of the patterned polymer in liquid crystal displays.

Integration of polymeric membranes with microfluidic networks for bioanalytical applications

The concept of microfluidics has significantly influenced the design and the implementation of modern bioanalytical systems due to the fact that these miniaturized devices can handle and manipulate samples in a much more efficient way than conventional instruments. In an analogy to the development of microelectronics, increasingly sophisticated devices with greater functionalities have become one of the major goals being pursued in the area of micrototal analysis systems. The incorporation of polymeric membranes into microfluidic networks has therefore been employed in an effort to enhance the functionalities of these microfabricated devices. These commercially available membranes are porous, flexible, mechanically robust and compatible with plastic microfluidic networks. The large surface area‐to‐volume ratio of porous membrane media is particularly important for achieving rapid buffer exchange during microdialysis and obtaining ultrahigh concentration of adsorbed enzymes for various biochemical reactions. Furthermore, the membrane pore diameter in the sub‐νm range eliminates the constraints of diffusional mass‐transfer resistance for performing chiral separation using adsorbed protein as the chiral stationary phase. A review on the recent advancement in the integration of polymeric membranes with microfluidic networks is presented for their widespread applications in bioanalytical chemistry.

Transparent electrodes based on conducting polymers for display applications

The materials science and engineering related to the fabrication of conducting polymer thin films and the progress in the development of devices integrated with organic transparent electrodes based on conducting polymers for display applications are reviewed. Transparent electrodes are essential components for many display modules. With the evolution of display technologies, conducting polymers are recently emerging as important alternative materials for the fabrication of transparent electrodes. Conducting polymers offer some advantages, such as light weight, low cost, mechanical flexibility and excellent compatibility with plastic substrates for the development of next-generation display technologies and, in particular, are expected to play an important role in the development of flexible display technologies.

Preparation and characterization of electrostrictive polyurethane films with conductive polymer electrodes

All-polymer electrostrictive soft films were developed for the first time by depositing conductive polymer (polypyrrole) directly on both sides of solution-cast electrostrictive polyurethane elastomer films. The final composite films are flexible with strong adhesion between the polyurethane film and the conductive polymer electrode. The conductivity (sheet resistivity ∼1000 Ω/□), of the polymer electrode is appropriate for its intended use. The compatible interface between the polypyrrole electrode polymer and the electrostrictive polyurethane significantly improves the acoustic and optical transparency of these composite films, compared with using a metal electrode film. The all-polymer films also exhibit comparable dielectric properties to gold-electroded polyurethane films in the temperature range from −40°C to +80°C. The temperature range covers the soft segment glass transition temperature of the polyurethane elastomers, which is about −20°C. The films also show large electric field induced strain responses which are dependent on film thickness and measurement frequency. The electrostrictive characteristics in the all-polymer films show similarities to those of the films with gold electrodes under identical measurement conditions.

Field‐effect flow control in a polydimethylsiloxane‐based microfluidic system

The application of the field‐effect for direct control of electroosmosis in a polydimethylsiloxane (PDMS)‐based microfluidic system, constructed on a silicon wafer with a 2.0 νm electrically insulating layer of silicon dioxide, is demonstrated. This microfluidic system consists of a 2.0 cm open microchannel fabricated on a PDMS slab, which can reversibly adhere to the silicon wafer to form a hybrid microfluidic device. Aside from mechanically serving as a robust bottom substrate to seal the channel and support the microfluidic system, the silicon wafer is exploited to achieve field‐effect flow control by grounding the semiconductive silicon medium. When an electric field is applied through the channel, a radial electric potential gradient is created across the silicon dioxide layer that allows for direct control of the zeta potential and the resulting electroosmotic flow (EOF). By configuring this microfluidic system with two power supplies at both ends of the microchannel, the applied electric potentials can be varied for manipulating the polarity and the magnitude of the radial electric potential gradient across the silicon dioxide layer. At the same time, the longitudinal potential gradient through the microchannel, which is used to induce EOF, is held constant. The results of EOF control in this hybrid microfluidic system are presented for phosphate buffer at pH 3 and pH 5. It is also demonstrated that EOF control can be performed at higher solution pH of 6 and 7.4 by modifying the silicon wafer surface with cetyltrimethylammonium bromide (CTAB) prior to assembly of the hybrid microfluidic system. Results of EOF control from this study are compared with those reported in the literature involving the use of other microfluidic devices under comparable solution conditions.

High-resolution chiral separation using microfluidics-based membrane chromatography.

A plastic microfluidic system, containing porous poly(vinylidene fluoride) (PVDF) membranes adsorbed with bovine serum albumin (BSA), is demonstrated for high resolution chiral separation of racemic tryptophan and thiopental mixtures. Microfluidic networks on poly(dimethylsiloxane) (PDMS) substrates are fabricated by capillary molding technique. This miniaturized chiral separation system consists of two layers of PVDF membranes which are sandwiched between two PDMS slabs containing microchannels facing the membranes. On-line adsorption of BSA onto the membranes is employed for the preparation of chiral stationary phase and the evaluation of solution conditions in an effort to achieve maximum protein adsorption. Variations in the mobile phase conditions, including solution pH and ammonium sulfate concentration, are studied for their effects on chiral separation. Based on the large surface area to volume ratio of porous membrane media, adsorbed BSA onto the PVDF membranes enables high resolution separation of racemic mixtures with sample consumption of sub-nanogram or less in the integrated microfluidic networks. In addition, the membrane pore diameter in the submicron range eliminates the constraints of diffusional mass-transfer resistance during protein adsorption and chiral chromatographic processes.

Critical dependency of the conductivity of polypyrrole and polyaniline films on the hydrophobicity/hydrophilicity of the substrate surface

Abstract The conductivity, electronic properties and morphology of polypyrrole and polyaniline films deposited from aqueous solutions of the polymerizing monomer differ greatly according to whether they are deposited on a hydrophobic or hydrophilic substrate surface. For each polymer the conductivity of the film deposited on a hydrophobic substrate is ~10 4 greater than that of the film of approximately the same thickness deposited on a hydrophilic surface. Films of increasing thickness exhibit a “memory effect” induced by the nature of the original substrate surface.