The Vapour Sensing Capabilities of Organic Field-Effect Transistors

The work in this doctoral thesis is mainly concerned with the detection of volatile organic vapours (analytes) using organic field-effect transistors (OFETs) as transducers, in some cases using a ‘sensitiser layer’ on top of the devices to improve their response to certain analytes; some work has also been carried out using a gold nano-particle chemi-resistor to detect amine vapour and the development of an aqueous sensing system is also discussed. It was found that the porphyrins PtOEP (platinum (II) octaethyl porphyrin) and PtEP-I (Etioporphyrin-I) could be used as organic semiconductors and that PtOEP was sensitive to isopropanol (IPA) and acetone vapours; PtOEP was also used to successfully sensitise a pentacene OFET to ethylene vapour at low ppm concentrations. Pentacene OFETs were found to be sensitive to octylamine (an amine), ethylethanoate (an ester), formamide (an amide) and ethylene (an alkene); through the use of a 2:1 molar ratio blend of the calixarene calix[8]arene (calixarene 2) and the porphyrin 5,10,15,20-Tetrakis (3,4-bis (2-ethylhexyloxy) phenyl)-21H,23H-porphyrinato cobalt (II) (Co-EHO) as a sensitiser layer, it was possible to introduce sensitivity to both octanal (an aldehyde) and octan-2-one (a ketone) into a pentacene OFET; the calixarene: 5,17-(34-nitrobenzylideneamino)-11,23-di-tert-butyl-25,27-diethoxycarbonyl-methyleneoxy-26,28dihydroxycalix[4]arene (calixarene 1) was also be used to improve OFET recovery after exposure to ethylethanoate and formamide, but some sensor response was lost in the process. The n-type organic semiconductor PDI8-CN2 (N,N’-bis (n-octyl)- dicyanoperylene-3,4:9,10-bis(dicarboximide)) was found to be sensitive to octylamine vapour, but the nature of its response seems to indicate some kind of amine base-doping mechanism is at work within the device, analogous to the acid doping possible with p-type semiconductors. Gold nano-particles were found to be sensitive to octylamine vapour as the amine group has an affinity for gold and coats the nano-particles, increasing the resistance of the nano-particle film. Creating a water gated P3HT (poly(3-hexylthiophene-2,5-diyl)) OFET without the electro-chemical doping normally experienced by such devices was found to be possible through the use of a calixarene 1 barrier layer, paving the way for the development of an aqueous sensing system.