Low‐Dimensionality Effects in Organic Field Effect Transistors

Abstract

The organic field effect transistor (OFET) is a device where a thin film of an organic semiconductor (OS) bridges a channel between source and gate electrodes. The gate electrode, separated by a dielectric thin film from the organic semiconductor, controls the charge carrier density in the organic semiconductor by capacitive coupling. OFET responds with a current between source and drain to a voltage bias applied to the gate and drain electrodes, the source being grounded. Response of OFET is measured by a set of parameters that are extracted from the current voltage characteristics as a function of the gate voltage (transfer curves) or the drain voltage (output curves). The OFET has been studied for more than two decades because of its potential applications in flexible circuits, RFID tags, wearable electronics, and back-panel active matrix displays. Although organic electronics is the main technology driver, OFET plays a central role in the fundamental studies aimed to elucidate charge transport in organic semiconductors. OFET is widely used as an experimental gauge for probing charge mobility, carrier density, and doping levels in organic thin films and nanostructures. Despite the apparent simplicity of the architecture, the device physics is more subtle and elusive. OFET response cannot be simply elicited from molecular design and crystal packing, as it is dominated by the interactions of the OS with the device interfaces. At the OS/metal electrodes, charge injection/extraction occurs; at the OS/gate dielectric interface, charge carriers are capacitively accumulated, depleted, trapped, and transported; charge carriers cross organic semiconductor domain boundaries and are scattered/trapped by morphological/structural defects; the outer OS surface is exposed to the environment. The OFET response is extremely sensitive to any change occurring at these interfaces, happening either spontaneously (like in the case of charge trapping and bias stress), accidentally (a parasitic dopant), or by design (specific interaction of an analyte with species adsorbed or grafted at the interfaces). The effect of these interfaces is intertwined in the OFET response, and it is difficult to experimentally disentangle it. On one hand, the OFET inherent instability is detrimental to electronics applications; on the other hand, it makes the device interesting for exploring new paradigms of sensing and transduction. In the past five years, an increasing trend of publications with OFETs used as (bio-)sensors is observed [1]. Advantages of OFETs with respect to more robust and established devices as CMOS are: the ease of interface tailoring toward analytes, living cells, and tissues; the use of low-cost scalable fabrication (important for single shot sensing); technology transferrable on flexible substrates with tunable mechanical compliance; a library of biocompatible and biodegradable materials, the latter yet in nuce (both crucial features for implantable devices); fabrication of devices with minimal amounts of materials (semiconductors, conductors, dielectrics, recognition groups); upscaling and integration are simpler than for resistive or amperometric sensors. This chapter looks into the OFET as a low-dimensional device and how this distinctive feature can be exploited for