Effect of Vertical Annealing on the Nitrogen Dioxide Response of Organic Thin Film Transistors.

Nitrogen dioxide (NO₂) sensors based on organic thin-film transistors (OTFTs) were fabricated by conventional annealing (horizontal) and vertical annealing processes of organic semiconductor (OSC) films. The NO₂ responsivity of OTFTs to 15 ppm of NO₂ is 1408% under conditions of vertical annealing and only 72% when conventional annealing is applied. Moreover, gas sensors obtained by vertical annealing achieve a high sensing performance of 589% already at 1 ppm of NO₂, while showing a preferential response to NO₂ compared with SO₂, NH₃, CO, and H₂S. To analyze the mechanism of performance improvement of OTFT gas sensors, the morphologies of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) films were characterized by atomic force microscopy (AFM) in tapping mode. The results show that, in well-aligned TIPS-pentacene films, a large number of effective grain boundaries inside the conducting channel contribute to the enhancement of NO₂ gas sensing performance.