Characterization and analysis of Organic Electro-Chemical Transistors printed with 3D-Stereolithography

The Master Thesis work presented here has been developed in order to study and characterise the performances of organic electrochemical transistors (OECT), fabricated by stereolithographic printing. OECTs are devices whose functional part is constituted by a semiconducting organic material. The general structure and the working principle are similar to traditional transistors, with some differences in the kind of charge which is transported in the channel. The physics of the device and a physical-mathematical model are thoroughly described in this work. An overview of the most common organic materials is presented. Nowadays the most commonly exploited material for this kind of devices is PEDOT, a conductive polymer which can be used as a filler inserted in an insulating polymeric matrix. In this work, in order to fabricate the functional part of the transistor, a polymeric mixture composed by PEGDA (an insulating resin which confers the structure and the mechanical properties to the final product) and PEDOT (which is conductive and confers the electrical properties to the device) is exploited. A review of the main printing techniques is presented, with a particular focus on stereolithography, a 3D printing technique. Stereolithography is widely exploited in micro and nanotechnology, but the novelty of this work consists in employing this technique to print OECTs. A stereolithographic process consists in reticulating a resin through the exposure to a laser source, which follows a particular path set by the user. The polymeric mixture is loaded into the stereolithographic printer, and through a particular software, many parameters can be customized, such as the layout and the design of the device, the laser power or the laser speed. Two different channel widths have been printed on the same substrate and tested. After the printing process, the final product undergoes a thermal treatment, after which is ready to be tested and analysed. In this work, a characterization of the devices has been performed, with the goal of analysing the performances from a qualitative and quantitative point of view and finding the best electrical parameters and conditions with which this kind of devices can be operated. In order to perform electrical measurements, an electrolyte with a known concentration is needed, and in this work a saline solution of NaCl has been chosen. A trans-characteristic curve (drain-source current as a function of gate-source voltage) and an output characteristic curve (drain-source current as a function of drain-source voltage) can be generated exploiting a sourcemeter. Also the transient behaviour of the devices has been analysed. The transistors showed good performances in terms of amount of current flowing in the channel and between the terminal, of threshold voltage and transconductance. Moreover they showed quite fast response times. OECTs showed to be advantageous since they allow to use a small amount of material during fabrication (reducing the costs), and in terms of mechanical and electrical properties; in the future such a device could be exploited as biosensor, due to its performances and to the fact that it requires a small amount of analyte in order to be operated.