Sensors based on organic thin film transistors

Abstract

Organic thin-film transistor (OTFT) sensors have proven to exhibit very promising properties when used as sensors and detectors. In this thesis, we are going to report on two types of sensors based on OTFTs, i.e. phototransistors and biosensors. Phototransistors are a type of optical transducers in which light detection and signal amplification are combined in a single device. Organic phototransistors (OPTs) are considered to be one of the feasible applications of OTFT because of their large adsorption properties in ultraviolet (UV) and visible light and the excellent photo current generation efficiency of organic semiconductors. More importantly, OPTs can be fabricated by a solution process, such as printing or spin coating, at room temperature and therefore can be easily integrated in smart clothings, packages, and biological systems as light sensors or biosensors. A light sensitive phototransistor based on the composite of poly(3-hexylthiophene) (P3HT) and titanium dioxide (TiO2) nanoparticles has been fabricated by a solution process. The device shows a quick change of channel current under light exposure, which can be attributed to a positive shift of the threshold voltage, while no change in the field effect mobility and off current can be observed. The shift of the threshold voltage is induced by the accumulated electrons trapped by the TiO2 nanoparticles in the channel. The photosensitivity of the device has been found to be dependent on the concentration of TiO2 nanoparticles, the incident wavelength and the voltage between the source and drain. OTFT are excellent candidates for the application on disposable sensors due to their potentially low-cost fabrication process. A novel DNA sensor based on OTFTs with semiconducting polymer poly(3-hexylthiophene) has been fabricated by solution process. Both single and double strand DNA molecules are immobilized on the surface of the Au source/drain electrodes of different devices, producing a dramatic change in the performance of the OTFTs, which is attributed to the increase of the contact resistance at the source/drain electrodes. Single strand DNA and double strand DNA are differentiated successfully in the experiments indicating that this is a promising technique for sensing DNA hybridization without labeling.