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Improved sensor selectivity for chemical vapors using organic thin-film transistors

  • Author(s): Royer, James Edward
  • et al.
Abstract

Organic thin-film transistors (OTFTs) offer unique methods for chemical vapor detection due to multiple device parameters which are influenced by reactive gases. The simplest conventional readout for OTFT sensors is the drain current; however, the drain current is dependent on changes in fundamental device characteristics such as mobility and/or threshold voltage. The chemical properties of the analyte determine whether the mobility or threshold voltage response is dominant for the OTFT. The OTFT mobility is dependent on molecular properties of the organic semiconductor, as well as bulk film properties such as morphology and intermolecular stacking. Conversely, the OTFT threshold voltage is dependent on fixed charge at the semiconductor/gate insulator interface as well as fixed charge in the bulk film. Understanding and differentiating the effects of mobility and threshold voltage changes in OTFT sensors is necessary to enhance sensitivity and/or selectivity for OTFT sens. This work investigates novel organic semiconductors and sensing modalities for OTFT chemical vapor sensors. OTFTs based on spin-coated films of metal-free tetrabenzoporphyrin (H₂TBP) and octa-butoxy naphthalocyanine (OBNc) are compared with vacuum evaporated metal-free phthalocyanine (H₂Pc). The sensor response of H₂TBP is similar to H₂Pc, despite large differences in film morphology. H₂TBP and H₂Pc have very similar molecular structures, which suggest that the analyte sensing mechanism is attributed to nearly identical hydrogen-bonding at the inner N₄H₂2 group in both H₂TBP and H₂Pc. OBNc OTFTs have the highest mobilities of all devices in this work, and the most distinctive sensor response due to analyte induced threshold voltage (Vth) shifts. OBNc OTFTs exhibit analyte induced Vth shifts which appear to correlate with the analyte's octanol-water partition coefficient. The results are consistent with better charge stabilization by polar molecules in OBNc films compared with H₂Pc films. Selective hydrogen peroxide and organic peroxide vapor sensing is demonstrated by monitoring Vth shifts in metal- phthalocyanine (MPc) and OBNc OTFTs. MPc and OBNc OTFTs dosed with peroxide exhibit reversible mobility response and irreversible, dosimetric, Vth shifts. The dual response characteristics are consistent with chemisorption of the peroxide and subsequent catalytic decomposition to form reactive products which increase fixed charge in the semiconductor film

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