Influence of device geometry on sensor characteristics of planar organic electrochemical transistors.

2010 WILEY-VCH Verlag Gmb Organic electronic devices, such as light-emitting diodes, transistors, and solar cells, are attracting enormous interest for low-cost, large-area, mechanically flexible electronics. Recently, a great deal of attention is being paid to the ability of organic semiconductors to conduct ionic in addition to electronic carriers, and this has been exploited to demonstrate novel devices. Among these, organic electrochemical transistors (OECTs) (also known as conducting polymer transistors) are particularly promising for applications in chemical and biological sensing and are expected to play a primary role in the emerging field of organic bioelectronics. OECTs, which employ a conducting polymer film in contact with a gate electrode via an electrolyte (Fig. 1), exploit the principle of electrochemical gating. The application of a gate voltage induces a reversible redistribution of ions within the electrolyte. Some of these ions enter the polymer film, and the resulting electrochemical doping/de-doping leads to a modulation in the current that flows between the source and drain electrodes. The working mechanism of OECTs requires the conducting polymer to be electrochemically active and ion permeable. The device structure of OECTs resembles that of organic electrical double layer transistors, which also exploit gating through an electrolyte. However, in electrical double layer transistors, the ions do not enter the polymer layer, and the operation of these devices relies on a field effect rather than on electrochemical doping/de-doping. First reported by Wrighton in 1984, OECTs can be operated in aqueous electrolytes as ion-to-electron converters, thus providing an interface between the worlds of biology and electronics. This exceptional property, together with a simple device architecture, simple signal readout, inherent signal amplification, and straightforward miniaturization, makes OECTs ideal devices for applications in chemical and biological sensing. OECTs based on several conducting polymers have indeed been used for the detection of a wide range of chemical and biological analytes, and have also been integrated with microfluidic channels for lab-on-a-chip applications. Despite the growing interest in OECTs, their device physics is not well understood. This particularly impacts their application in sensors, where one would like to know how parameters, such as sensitivity and lowest detectable analyte concentration, depend on device geometry. As a result, the optimization of OECT-based sensors and the prediction of their ultimate limits of performance are severely hindered. In this Communication, we investigate the effect of the gate area (Ag) and channel area (Ach) on the ability of OECTs to detect hydrogen peroxide (H2O2) via an electrochemical reaction at the Pt gate electrode. H2O2 is a model analyte, as its electrochemical oxidation at a Pt electrode forms the basis of many commercial enzymatic sensors, including glucose sensors, which represent about 85% of the whole biosensor market. A unique feature of OECTs associated with electrochemical gating is that the gate electrode does not need to be positioned at a