ION MODULATED ORGANIC TRANSISTORS - Doria

Nikolai Kaihovirta1. INTRODUCTIONelectrochemical sensors, micro-fluidic systems and drug delivery devices[6]. In other words, organic bioelectronics is an area of research thatdisplays the potential for polymeric materials.A high-performing transistor should provide a high on-current at lowvoltages. For a field-effect transistor (MOSFET and OFET) this sets arequirement to the capacitance (C) of the dielectric insulator:C κε= = , (1.1)A dC i0where C i is the effective capacitance (capacitance per unit area), κ is thedielectric constant of the insulator, ε 0 is the vacuum permittivity and d isthe insulator thickness. According to Equation (1.1), there are basicallytwo ways of increasing C i: (1) By decreasing the insulator thickness, or (2)by increasing the dielectric constant. High performing, low-voltage (< 5V) OFETs have been demonstrated with both alternatives. On the otherhand, it is challenging to reach low-voltage operation with OFETs andstill maintain robustness, stability and flexibility of the structure. Anotherroute has therefore been to replace the ordinary (dielectric) insulator withan ion-conducting insulator (e.g., an electrolyte) to construct eitherelectrolyte gated OFETs or organic electrochemical transistors (ECTs). Inthis thesis, both transistor types are defined as ion modulated organictransistors. The electrolyte gated OFET, which is the central type in thisthesis, uses the same transistor structures and follows the same currentvoltage(I-V) analysis as the conventional OFET.1.2 The OFET and the Ion Modulated OrganicTransistors1.2.1 Transistor StructuresThe OFET constitutes of thin films fabricated as subsequent layers. Thepolymeric materials are often soluble (or dispersible) in common organicsolvents, and are, therefore, easily processed by drop-casting, spincoatingand/or by various large-scale coating and printing techniques.The thickness range of each layer may vary from a few tens ofnanometers up to several micrometers. The OFET is a three-terminaldevice with a source, drain and a gate electrode. The electronic currentpath (the active channel) is formed between the source and drain at the2