The Effects of Moisture in Low‐Voltage Organic Field‐Effect Transistors Gated with a Hydrous Solid Electrolyte

Abstract

The fi eld-effect transistor possesses an important function as a current and voltage switcher in various electronic applications. Ever since the fi rst reports on organic fi eld-effect transistors (OFETs) were published, [ 1 ] extensive research has been carried out in order to improve the device performance and, hence, lower the power consumption. The OFET is a three terminal device consisting of the drain, source and gate electrodes. The electronic current path between the drain and the source (i.e., the active channel) is situated at the semiconductor/insulator interface and the current is modulated by applying a gate voltage. The insulator is typically an inorganic or an organic dielectric. A high capacitance over the dielectric is desirable for low-voltage operation. [ 2 ] There are two standard methods to increase the capacitance: 1) by using an insulator with a high dielectric constant, which is, for example, accomplished with inorganic oxides, [ 3 ] and 2) by reducing the thickness of the dielectric. This has been achieved by utilizing ultrathin cross-linked polymeric insulators. [ 4 ] Indeed, low-voltage and high-performing OFETs have been demonstrated with these dielectrics. The downside is that they are often also thin and brittle to use on, e.g., bendable surfaces. The electrolyte gated OFET constitutes a specifi c class of OFETs. [ 5 ] The device structure is identical to the OFET. However, the dielectric is replaced with an electrolyte as an ion conducting insulator. In such a structure, cations and anions assemble at opposite interfaces when applying the gate voltage. The resulting capacitance over the electrolyte can be several orders of magnitude higher than in ordinary dielectrics. [6][7][8] Thus, a high charge density is generated at the semiconductor/electrolyte-interface already at low voltages, and the electrolyte gated OFET can, therefore, be operated within 3 V, occasionally also with high fi eld-effect mobilities. [ 7 ] Recently we demonstrated the possibility of using ion conducting membranes in order to manufacture electrolyte gated OFETs (MemFETs) operating at 1 V. [ 9 ] The major advantages with membranes are their thickness, robustness and chemical resistance. The thickness of used membranes varied between 50-150 μ m and especially fully and partially fl uorinated membranes resist common organic solvents, such as chloroform. Hence, the membrane can easily be sandwiched within a complex geometry, such as the transistor. The concept of using ion conducting membranes (50-150 μ m thick) for gating low-voltage (1 V) organic fi eld-effect transistors (OFETs) is attractive due to its low-cost and large-area manufacturing capabilities. Furthermore, the membranes can be tailor-made to be ion conducting in any desired way or pattern. For the electrolyte gated OFETs in general, the key to low-voltage operation is the electrolyte "insulator" (the membrane) that provides a high effective capacitance due to ionic polarization within the insulator. Hydrous ion conducting membranes are easy to pro...

Self-standing chitosan films as dielectrics in organic thin-film transistors

“…It was mentioned above that CS is a hygroscopic material and that its proton conductivity has indeed been explored in previous studies. The effect of the water content was not addressed here but, based on the studies carried out by Kaihovirta et al [16], for instance, in TFTs with hygroscopic dielectrics, the water content may in fact play an important role on the performance of transistors. Additional studies are in progress to better evaluate the role of the water content and the possible electric-double-layer effects on the behavior of the organic transistors.…”

Ultrahigh Mobility in Solution‐Processed Solid‐State Electrolyte‐Gated Transistors

“…Flexible P3HT/SEGI (98:2) FETs on a poly(ethylene naphthalate) (PEN) substrate (Figure 4j, top) were bent repeatedly at a bending radius (R c ) of 6.0 mm (Figure 4j, bottom) in ambient air (humidity level = 45.2-62.3%). For these devices, I D and off-current slightly increase due to the sensitivity of P3HT to oxygen-doping effect ( Figure S18, Supporting Information), [46,47] however they continue to function and they do not suffer from mechanical stress. For these devices, I D and off-current slightly increase due to the sensitivity of P3HT to oxygen-doping effect ( Figure S18, Supporting Information), [46,47] however they continue to function and they do not suffer from mechanical stress.…”

Controlling the dimensionality of charge transport in organic thin-film transistors

“…The latter mechanism, on the other hand, is typically characterized by mass transport of ions across the electric double layer and involves electrochemical doping of the semiconductor bulk. Even if identification of the mechanism can be ambiguous, absorption (7,10), Fourier transform (8), and impedance spectroscopy (11) as well as signatures in the electrical characteristics (12) have been used as evidence to judge the mode of operation. Electrochemical doping is most often (but not always) undesirable due to its deteriorating effect on the transistor performance such as considerably slower switching speed and large hysteresis in the electrical characteristics (13).…”