Polythiophene thin-film transistor array for gas sensing

A small array of various polythiophene derivative OTFT gas sensors with robust operation in air is demonstrated. Air stability is shown not to preclude gas sensor response, and good sensitivity to a range of gas analytes is observed down to 10 ppm. The active materials are all deposited from solution, making these sensors compatible with low-cost gas sensor arrays. Finally, the effect of varying the functional groups and size of a gas analyte on the response it induces in the sensor array is investigated. Organic thin film transistors (OTFTs) are an attractive option for low-cost gas sensor arrays. The synthetic richness of organic chemistry allows the development of arrays of materials with systematically varied ranges of sensitivities. Also, organic sensing materials tend to be soluble, and arrays of sensors can thus be cheaply integrated onto a single substrate through inkjet deposition. However, stability of OTFTs is a concern, especially for operation in ambient conditions. Encapsulation is not an option for gas sensors, so the development of air-stable materials is important, but the trade-off between air-stability and odor sensitivity is unclear and requires further study. A schematic cross section of the OTFT gas sensors is shown in Figure 1. For convenience, a standard substrate-gated architecture was used. 95 nm of wet oxide is grown on heavily doped Si substrates, and 50 nm thick Au pads with a 2.5 nm Cr adhesion layer are thermally evaporated to form source/drain contacts. The active materials, including poly-3-hexylthiophene (P3HT) and two air-stabilized polythiophene derivatives (M2 and P1), are deposited by spin-coating. Sensors are operated at room temperature in the dark, in an isolated chamber with controlled gas flow, and probed remotely with an HP 4145 parameter analyzer. The sensors show appreciable response to air, exhibiting large, repeatable shifts in on-current, mobility, and threshold voltage when purged with nitrogen or reexposed to air. A difference in sensor response is also observed when operated in air instead of nitrogen (Figure 2). This sensitivity to air is non-destructive, however, and the sensor array continues to exhibit robust operation when stored and operated in air for over two weeks. In order to investigate the mechanisms responsible for OTFT gas sensor response, the sensor array was exposed to a number of different odors (Figure 3), including hexane, hexanol, hexanethiol, hexylamine, and hydrochloric acid. While hexane had no affect on the sensor array up to a concentration of 5000 ppm, as little as 10 ppm of hexanol produced a detectable response, including a positive V _T shift and a degradation of extracted charge mobility. This response was quickly reversed upon removal of the hexanol, indicating that the interaction between hexanol and the polythiophenes is fairly weak, and that hexanol desorbs easily. Hexanethiol, on the other hand, produced a prolonged, non-reversible degradation of both threshold voltage and extracted charge mobility, suggesting that the thiol functional group interacts strongly with the polythiophenes, forming a permanent bond. The opposite directions of VT shift induced by the two gases indicate that they both diffuse to the dielectric interface, where they introduce opposite interface charges. Hexylamine, on the other hand, is electron-donating, and would be expected to de-dope polythiophene. Accordingly, all three active materials immediately became non-conductive upon exposure to 10 ppm of hexylamine. For comparison, hydrochloric acid should be proton-donating, and increase doping in the material. PI does, in fact, show a strong increase in on-current upon exposure to small concentrations of HCl, but all three materials quickly become non-conductive at higher concentrations. The sensor response to hexanol is particularly interesting because of its repeatability and reversibility. Notably, many inorganic sensors tend to be sensitive to alcohol poisoning, so this may be an advantage of organic gas sensors. To further understand the hexanol response, the sensor array was exposed to a series of alcohols with varying alkane chain lengths (Figure 4). Larger alcohols caused more on-current degradation for all three materials, implying that the sensor response is mechanically based, perhaps through film swelling. However, the fact that alkanes such as hexane and pentane produce no sensor response means that the alcohol group is necessary for sensor interaction with the analyte to occur. As understanding of the OTFT gas sensor response improves, the air stability, sensitivity, and solution-processability of the polythiophene gas sensor array demonstrated here suggests that OTFTs may, indeed be a practical option for low-cost gas sensor arrays. The demonstration of air-stable operation of an arrayed sensor herein is an important step in this direction.