Prajwal Kumar

Solvent-induced changes in PEDOT:PSS films for organic electrochemical transistors

Organic electrochemical transistors based on the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) are of interest for several bioelectronic applications. In this letter, we investigate the changes induced by immersion of PEDOT:PSS films, processed by spin coating from different mixtures, in water and other solvents of different polarities. We found that the film thickness decreases upon immersion in polar solvents, while the electrical conductivity remains unchanged. The decrease in film thickness is minimized via the addition of a cross-linking agent to the mixture used for the spin coating of the films.

Water stability and orthogonal patterning of flexible micro-electrochemical transistors on plastic

Water-stable, flexible and micro-scale organic electrochemical transistors (OECTs) based on poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) were fabricated on a plastic substrate using a new process based on a fluorinated photoresist. The PEDOT:PSS films, mixed solely with a biocompatible conductivity enhancer, show robust adhesion on plastic substrates, and exhibit unchanged electrical properties under extreme bending. This work simplifies the fabrication of high-performance OECTs and places them in a highly competitive position for flexible electronics and healthcare applications.

Conducting Polymer Transistors Making Use of Activated Carbon Gate Electrodes

The characteristics of the gate electrode have significant effects on the behavior of organic electrochemical transistors (OECTs), which are intensively investigated for applications in the booming field of organic bioelectronics. In this work, high specific surface area activated carbon (AC) was used as gate electrode material in OECTs based on the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS). We found that the high specific capacitance of the AC gate electrodes leads to high drain-source current modulation in OECTs, while their intrinsic quasi-reference characteristics make unnecessary the presence of an additional reference electrode to monitor the OECT channel potential.

Melanin-based flexible supercapacitors

Biocompatible and biodegradable materials that store electrochemical energy are attractive candidates for applications in bioelectronics and electronics for everywhere. Eumelanin is a ubiquitous biopigment in flora and fauna. It exhibits strong broad-band UV-visible absorption, metal chelation as well as good thermal and photo-stability. Eumelanin is based on 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole carboxylic acid (DHICA) building blocks, present in different redox forms (hydroxyquinone, semiquinone and quinone). The synergy between the redox activity of the building blocks and the capability of several of their functionalities to reversibly bind cations constitutes the foundation for the use of melanin in pseudocapacitive energy storage systems. In this work, we report on the energy storage properties of eumelanin in supercapacitor configuration. Initially, a gravimetric specific capacitance as high as 167 F g−1 (specific capacity of 24 mA h g−1) was observed for eumelanin on carbon paper electrodes, in aqueous electrolytes. A maximum power density of up to 20 mW cm−2 was deduced for the corresponding melanin supercapacitors. Capitalizing on these results, we used an unconventional patterning approach to fabricate binder-free flexible micro-supercapacitors on plastic substrates. Our results demonstrate that melanin is a valid candidate for future supercapacitor electrodes. The biocompatibility and biodegradability featured by eumelanin, combined with its easy availability and room temperature processing, make it an extremely attractive material for environmentally and human friendly energy storage solutions.

Effect of channel thickness, electrolyte ions, and dissolved oxygen on the performance of organic electrochemical transistors

We investigated the device characteristics of organic electrochemical transistors based on thin films of poly(3,4-ethylenedioxythiophene) doped with poly(styrene-sulfonate). We employed various channel thicknesses and two different electrolytes: the micelle forming surfactant cetyltrimethyl ammonium bromide (CTAB) and NaCl. The highest ON/OFF ratios were achieved at low film thicknesses using CTAB as the electrolyte. Cyclic voltammetry suggests that a redox reaction between oxygen dissolved in the electrolytes and PEDOT:PSS leads to low ON/OFF ratios when NaCl is used as the electrolyte. Electrochemical impedance spectroscopy reveals that doping/dedoping of the channel becomes slower at high film thickness and in the presence of bulky ions.

Ionic liquid–water mixtures and ion gels as electrolytes for organic electrochemical transistors

Organic Electrochemical Transistors (OECTs) are widely investigated for applications in bioelectronics. Ionic liquids (ILs) are, in principle, interesting candidates as gating media in OECTs. Nevertheless, ILs can exhibit excessively high viscosity that prevents their straightforward application in OECTs. Here we report two processing approaches to apply the highly viscous ionic liquid triisobutyl(methyl)phosphonium tosylate (Cyphos® IL 106) in OECTs based on poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS), namely IL–H2O binary mixtures and ion gels. The use of Cyphos® IL 106–H2O binary mixtures and ion gels as gating media determines an increase of the OECT modulation, with respect to the pure ionic liquid. This increase cannot be explained simply by the change of the viscosity and ionic conductivity of the ionic liquid–H2O mixtures with the increase of the H2O content. Using high surface area activated carbon gates, ON/OFF ratios as high as 5000 are achieved with Cyphos® IL 106–H2O mixtures at 5 and 10% H2O v/v.

Highly stretchable electrospun conducting polymer nanofibers

Biomedical electronics research targets both wearable and biocompatible electronic devices easily adaptable to specific functions. To achieve such goals, stretchable organic electronic materials are some of the most intriguing candidates. Herein, we develop highly stretchable poly-(3,4-ethylenedioxythiphene) (PEDOT) doped with tosylate (PEDOT:Tos) nanofibers. A two-step process involving electrospinning of a carrier polymer (with oxidant) and vapor phase polymerization was used to produce fibers on a polydimethylsiloxane substrate. The fibers can be stretched up to 140% of the initial length maintaining high conductivity.

Photolithographically Patterned TiO2 Films for Electrolyte-Gated Transistors.

Metal oxides constitute a class of materials whose properties cover the entire range from insulators to semiconductors to metals. Most metal oxides are abundant and accessible at moderate cost. Metal oxides are widely investigated as channel materials in transistors, including electrolyte-gated transistors, where the charge carrier density can be modulated by orders of magnitude upon application of relatively low electrical bias (2 V). Electrolyte gating offers the opportunity to envisage new applications in flexible and printed electronics as well as to improve our current understanding of fundamental processes in electronic materials, e.g. insulator/metal transitions. In this work, we employ photolithographically patterned TiO2 films as channels for electrolyte-gated transistors. TiO2 stands out for its biocompatibility and wide use in sensing, electrochromics, photovoltaics and photocatalysis. We fabricated TiO2 electrolyte-gated transistors using an original unconventional parylene-based patterning technique. By using a combination of electrochemical and charge carrier transport measurements we demonstrated that patterning improves the performance of electrolyte-gated TiO2 transistors with respect to their unpatterned counterparts. Patterned electrolyte-gated (EG) TiO2 transistors show threshold voltages of about 0.9 V, ON/OFF ratios as high as 1 × 10(5), and electron mobility above 1 cm(2)/(V s).