The Fabrication and Optimization of Thin-Film Transistors Based on Poly(3-Hexylthiophene) Films for Nitrogen Dioxide Detection

Poly(3-hexylthiophene) (P3HT) films were used as active layers in bottom contact organic thin-film transistor (OTFT) gas sensors to detect nitrogen dioxide (NO
₂
). The films with different thicknesses were deposited on the devices to measure current–voltage characteristics and to test the gas sensing properties of the OTFT devices. All prepared OTFTs exhibited p-type semiconductor behaviors. The effect of film thickness on the electronic characteristics of OTFTs was investigated, and the OTFT gas sensor with the thinnest film presented best electronic characteristics. The OTFT gas sensors with thinner P3HT film showed smaller baseline drift, higher response to NO
₂
gas of a certain concentration, and larger sensitivity enhancement. The sensitivity increased from 0.038 to 0.11 ppm
$^{{-1}}$
(up to 190%) with the decrease in film thickness from 600 to 200 nm. The absorption of NO
₂
changed the surface doping level, and thereby perturbed the charge transport properties of the devices, which made OTFT gas sensors with thinner film more sensitive. Compared with the response curves and repeatability curves of all devices, the response of the OTFT gas sensor with the thickest film was not precise owing to the baseline drift. The selectivity was also investigated. The results demonstrated that the response to NO
₂
was improved due to the film thickness optimization. Moreover, the sensing mechanism was analyzed as well.