The chapter reports the use of organic electrochemical transistors in sensor applications. These transistors are excellent ion-to-electron transducers and can serve as very sensitive transducers in amperometric sensor applications. To further improve their sensitivity, we outline various amplification circuits all realized in organic electrochemical transistors.

The field of organic electronics promises exciting new technologies based on inexpensive and mechanically flexible electronic devices. It has progressed over the past three decades to the point of commercial viability and is projected to grow to a 30 billion dollar market by the year 2015. Exploring new applications and device architectures, this book sets the tone for that exploration, gathering a community of experts in this area who are focused on the use of ionic functions to define the principle of operation in polymer devices. The contributors detail relevant technologies based on organic electronics, including polymer electrochromic devices and light-emitting electrochemical cells.

Polymers are very attractive materials that can be tailored for specific needs and functionality. They can for instance be made electrically insulating or (semi)conducting, with specific mechanical properties. Polymers are often processable from a solution, which enables the use of low-cost manufacturing techniques to fabricate polymer devices. Polymer-based electronic and electrochemical devices and sensors have been developed.

This thesis is related to the polarization characteristics in polyelectrolyte thin film capacitor structures. The polarization characteristics have been analyzed at various humidity levels for polyelectrolyte capacitors alone and when incorporated as the gate-insulating material in polyelectrolyte-gated organic field-effect transistors. Both limitations and possibilities of this class of transistors have been identified. Also, a concept for wireless readout of a passively operated humidity sensor circuit is demonstrated. The sensing mechanism of this sensor is related to the polarization in a polyelectrolyte thin film capacitor. This sensor circuit can be manufactured entirely with common printing technologies of today and can be integrated into a low-cost passive sensor label.

Polymers are very attractive materials that can be tailored for specific needs and functionalities. Based on their chemical structure, they can for instance be made electrically insulating or semiconducting with specific mechanical properties. Polymers are often processable from a solution, which enables the use of conventional low-cost and high-volume manufacturing techniques to print electronic devices onto flexible substrates. A multitude of polymer-based electronic and electrochemical devices and sensors have been developed, of which some already has reached the consumer market.

This thesis focuses on polarization characteristics in polyelectrolyte-based capacitor structures and their role in sensors, transistors and supercapacitors. The fate of the ions in these capacitor structures, within the polyelectrolyte and at the interfaces between the polyelectrolyte and various electronic conductors (a metal, a semiconducting polymer or a network of carbon nanotubes), is of outermost importance for the device function. The humidity-dependent polarization characteristics in a polyelectrolyte capacitor are used as the sensing probe for wireless readout of a passively operated humidity sensor circuit. This sensor circuit can be integrated into a printable low-cost passive sensor label. By varying the humidity level, limitations and possibilities are identified for polyelectrolyte-gated organic field-effect transistors. Further, the effect of the ionic conductivity is investigated for polyelectrolyte-based supercapacitors. Finally, by using an ordinary electrolyte instead of a polyelectrolyte and a high-surface area (supercapacitor) gate electrode, the device mechanisms proposed for electrolyte-gated organic transistors are unified.

The combination of electrolytes and organic semiconductors has opened up new opportunities in photonics1, electronics2 and in energy storage3. In most of these devices, the key mechanisms involve the transport of charge carriers (electrons or ions) across the organic semiconductor-electrolyte interface. The formation of an electric double layer (EDL) at this polarized interface is fuzzier than at a metal-electrolyte interface since weak intermolecular interactions in the organic solid favour the penetration of ions4. An EDL established at the organic semiconductor-electrolyte interface, defined by a sheet of electronic charge carriers and a sheet of ions, has been proposed recently as the basic mechanism for electrolyte-gated organic field-effect transistors (EGOFETs)5, 6. Here, organic thin film transistors are used as a probe to investigate the organic semiconductor-electrolyte interface. We demonstrate that the capacitance value of the gate counter electrode dictates the degree of advancement7 of the electrochemical halfreaction (the extent of the reaction) at this interface. This finding unifies the mechanisms proposed for EGOFETs and organic electrochemical transistors (OECTs); and sets the ground description for an electrochemical half-reaction induced entirely by capacitive coupling.

The dimensionality of charge transport in an organic electrochemical transistor depends on the degree of advancement of the electrochemical half-reaction at the organic semiconductor/electrolyte interface. A carbon nanotube (CNT) nanoporous gate electrode leads to bulk transport in the semiconductor, while a flat Au gate electrode allows for localizing of the electrochemical oxidation of the semiconducting polymer at the organic semiconductor/electrolyte interface.

Electrolyte-gated organic field-effect transistors (OFETs) hold promise for robust printed electronics operating at low voltages. The polarization mechanism of thin solid electrolyte films, the gate insulator in such OFETs, is still unclear and appears to limit the transient current characteristics of the transistors. Here, the polarization response of a thin proton membrane, a poly(styrenesulfonic acid) film, is controlled by varying the relative humidity. The formation of the conducting transistor channel follows the polarization of the polyelectrolyte, such that the drain transient current characteristics versus the time are rationalized by three different polarization mechanisms: the dipolar relaxation at high frequencies, the ionic relaxation (migration) at intermediate frequencies, and the electric double-layer formation at the polyelectrolyte interfaces at low frequencies. The electric double layers of polyelectrolyte capacitors are formed in 1 µs at humid conditions and an effective capacitance per area of 10 µF cm^-2 is obtained at 1 MHz, thus suggesting that this class of OFETs might operate at up to 1 MHz at 1 V.

Low-cost passive wireless electronic sensor labels glued onto packages are highly desirable since they enable monitoring of the status of the packages for instance along the logistic chain or while stored at a shelf. Such additional sensing feature would be of great value for many producers and vendors, active in e.g. the food or construction industries. Here, we explore a novel concept for wireless sensing and readout, in which the humidity sensitive ionic motion in a polyelectrolyte membrane is directly translated into a shift of the resonance frequency of a resonance circuit. Thanks to its simplicity, the wireless sensor device itself can be manufactured entirely using common printing techniques and can be integrated into a low-cost passive electronic sensor label.

Polyelectrolytes are promising materials as gate dielectrics in organic field-effect transistors (OFETs). Upon gate bias, their polarization induces an ionic charging current, which generates a large double layer capacitor (10-500 µF cm^-2) at the semiconductor/electrolyte interface. The resulting transistor operates at low voltages (<1 V) and its conducting channel is formed in 50 µs. The effect of ionic currents on the performance of the OFETs is investigated by varying the relative humidity of the device ambience. Within defined humidity levels and potential values, the water electrolysis is negligible and the OFETs performances are optimum.

Printed electronics promise various kinds of sensor circuit labels, for applications in distributed sensing and monitoring, which can be manufactured using traditional printing tools at very low cost. Elevated humidity levels or water leakages cause tremendous costs in our society, such as in construction industries and in transportations. Distributed monitoring and remote sensing of the humidity level inside walls of buildings and packages is therefore desired and urgently needed. Here, we report a wireless humidity sensor label that is manufactured using screen-printing and dry-phase patterning. The sensor label includes a planar antenna, a tuning capacitor and a printed sensor-capacitor head. Through electromagnetic coupling between a reader and the printed sensor label, changes in humidity level were remotely detected and read-out as a shift of the resonant frequency. The manufacturing process of the humidity sensor label is fully compatible with inexpensive, reel-to-reel processing technologies, thus enabling low cost production.

In the emerging technology field of printed electronics, circuits are envisioned to be powered with printed energy sources, such as printed batteries and printed supercapacitors (SCs). For manufacturing and reliability issues, solid electrolytes are preferred instead of liquid electrolytes. Here, a solid-state, polyanionic proton conducting electrolyte, poly(styrenesulfonic acid) (PSS:H), is demonstrated for the first time as an effective ion conducting electrolyte medium in SCs with electrodes based on carbon nanotube (CNT) networks. The effect of the ionic conductivity in the PSS:H film of those SCs is studied at different levels of relative humidity (RH) with impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge techniques. High capacitance values (85 F g(-1) at 80% RH) are obtained for these SCs due to the extremely high effective electrode area of the CNTs and the enhanced ionic conductivity of the PSS: H film at increasing RH level. The charging dynamics are primarily limited by the ionic conductivity of the electrolyte rather than a poor contact between the electrolyte and the CNT electrodes. The use of polyelectrolytes in SCs provides high mechanical strength and flexibility, while maintaining a high capacitance value, enabling a new generation of printable solid-state charge storage devices.