Nathaniel D. Robinson

The dynamic organic p-n junction

Static p-n junctions in inorganic semiconductors are exploited in a wide range of todays electronic appliances. Here, we demonstrate the in situ formation of a dynamic p-n junction structure within an organic semiconductor through electrochemistry. Specifically, we use scanning kelvin probe microscopy and optical probing on planar light-emitting electrochemical cells (LECs) with a mixture of a conjugated polymer and an electrolyte connecting two electrodes separated by 120 microm. We find that a significant portion of the potential drop between the electrodes coincides with the location of a thin and distinct light-emission zone positioned >30 microm away from the negative electrode. These results are relevant in the context of a long-standing scientific debate, as they prove that electrochemical doping can take place in LECs. Moreover, a study on the doping formation and dissipation kinetics provides interesting detail regarding the electronic structure and stability of the dynamic organic p-n junction, which may be useful in future dynamic p-n junction-based devices.

Flexible and Metal-Free Light-Emitting Electrochemical Cells Based on Graphene and PEDOT-PSS as the Electrode Materials

We report flexible and metal-free light-emitting electrochemical cells (LECs) using exclusively solution-processed organic materials and illustrate interesting design opportunities offered by such conformable devices with transparent electrodes. Flexible LEC devices based on chemically derived graphene (CDG) as the cathode and poly(3,4-ethylenedioxythiophene) mixed with poly(styrenesulfonate) as the anode exhibit a low turn-on voltage for yellow light emission (V = 2.8 V) and a good efficiency 2.4 (4.0) cd/A at a brightness of 100 (50) cd/m(2). We also find that CDG is electrochemically inert over a wide potential range (+1.2 to -2.8 V vs ferrocene/ferrocenium) and exploit this property to demonstrate planar LEC devices with CDG as both the anode and the cathode.

Polymer field-effect transistor gated via a poly(styrenesulfonic acid) thin film

A polyanionic proton conductor, named poly(styrenesulfonic acid) (PSSH), is used to gate an organic field-effect transistor (OFET) based on poly(3-hexylthiophene) (P3HT). Upon applying a gate bias, ...

On the Current Saturation Observed in Electrochemical Polymer Transistors

Electrochemical transistors based on conjugated polymers are proposed as a path to printed electronics on paper. The electrochemical doping/dedoping of conjugated polymers clearly plays a role in the current vs potential (I-V) characteristics of these devices, however, the mechanism of current saturation (often referred to as pinch-off) is not clearly understood, and the relationship between electrochemical devices and field-effect transistors is unclear. This paper offers a semiempirical model of the steady-state behavior of electrochemical transistors and compares this model with experimental observations of potential and electrochromic measurements within a device to illustrate the science behind the functionality observed. ©2006 The Electrochemical Society

Electrolysis-reducing electrodes for electrokinetic devices.

Direct current electrokinetic systems generally require Faradaic reactions to occur at a pair of electrodes to maintain an electric field in an electrolyte connecting them. The vast majority of such systems, e.g. electrophoretic separations (capillary electrophoresis) or electroosmotic pumps (EOPs), employ electrolysis of the solvent in these reactions. In many cases, the electrolytic products, such as H+ and OH− in the case of water, can negatively influence the chemical or biological species being transported or separated, and gaseous products such as O2 and H2 can break the electrochemical circuit in microfluidic devices. This article presents an EOP that employs the oxidation/reduction of the conjugated polymer poly(3,4‐ethylenedioxythiophene), rather than electrolysis of a solvent, to drive flow in a capillary. Devices made with poly(3,4‐ethylenedioxythiophene) electrodes are compared with devices made with Pt electrodes in terms of flow and local pH change at the electrodes. Furthermore, we demonstrate that flow is driven for applied potentials under 2 V, and the electrodes are stable for potentials of at least 100 V. Electrochemically active electrodes like those presented here minimize the disadvantage of integrated EOP in, e.g. lab‐on‐a‐chip applications, and may open new possibilities, especially for battery‐powered disposable point‐of‐care devices.

Graphene electrodes for organic metal-free light-emitting devices

In addition to its fascinating electrical and mechanical properties, graphene is also an electrochemically stable and transparent electrode material. We demonstrate its applicability as both anode and cathode in a light-emitting electrochemical cell (LEC), an electrochemical analogue to a polymer organic light-emitting diode. Specifically, we summarize recent progress in carbon-based metal-free light-emitting devices enabled by chemically derived graphene cathodes on quartz and plastic substrates, and explain the advantages of using LECs in manufacturing large-area devices.

Visualizing the electric field in electrolytes using electrochromism from a conjugated polymer

Electrochromic polymer films, employed as display elements, smart windows, and the base material for electrochemical electronic devices, can be addressed solely through ionic transport via an elect ...

The electrochemical transistor and circuit design considerations

The electrochemical transistor is presented from a functional point-of-view. It is shown that this transistor has characteristics that are similar to p-channel depletion-mode MOSFET devices. Electrical design rules for proper operation are given. Based on these rules, we show how logical circuits such as inverters and gates can be constructed.

Conducting Polymer Electrodes for Gel Electrophoresis

In nearly all cases, electrophoresis in gels is driven via the electrolysis of water at the electrodes, where the process consumes water and produces electrochemical by-products. We have previously demonstrated that π-conjugated polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) can be placed between traditional metal electrodes and an electrolyte to mitigate electrolysis in liquid (capillary electroosmosis/electrophoresis) systems. In this report, we extend our previous result to gel electrophoresis, and show that electrodes containing PEDOT can be used with a commercial polyacrylamide gel electrophoresis system with minimal impact to the resulting gel image or the ionic transport measured during a separation.

Liquid bridge stabilization: theory guides a codimension-two experiment

Abstract Subject only to surface tension, a cylindrical liquid bridge is unstable at lengths longer than its circumference, the Plateau-Rayleigh limit. Perturbed by gravity and an axial flow, the liquid bridge becomes near-cylindrical with a modified stability. An unfolding captures the interactions between gravity and flow-induced pressure near the Plateau-Rayleigh limit. The stress balance of the free interface is solved using a lubrication-flow approximation and the Lyapunov-Schmidt method. Stabilization is predicted when the two perturbations counter-balance one another. Where accurate measurements are possible, experiment gives good comparison with predictions. The attempt to use theory to guide experiment to the most delicate predictions is only partially successful, however. ‘Why?’ is addressed.