Marco R. Cavallari

Enhanced Sensitivity of Gas Sensor Based on Poly(3-hexylthiophene) Thin-Film Transistors for Disease Diagnosis and Environment Monitoring

Electronic devices based on organic thin-film transistors (OTFT) have the potential to supply the demand for portable and low-cost gadgets, mainly as sensors for in situ disease diagnosis and environment monitoring. For that reason, poly(3-hexylthiophene) (P3HT) as the active layer in the widely-used bottom-gate/bottom-contact OTFT structure was deposited over highly-doped silicon substrates covered with thermally-grown oxide to detect vapor-phase compounds. A ten-fold organochloride and ammonia sensitivity compared to bare sensors corroborated the application of this semiconducting polymer in sensors. Furthermore, P3HT TFTs presented approximately three-order higher normalized sensitivity than any chemical sensor addressed herein. The results demonstrate that while TFTs respond linearly at the lowest concentration values herein, chemical sensors present such an operating regime mostly above 2000 ppm. Simultaneous alteration of charge carrier mobility and threshold voltage is responsible for pushing the detection limit down to units of ppm of ammonia, as well as tens of ppm of alcohol or ketones. Nevertheless, P3HT transistors and chemical sensors could compose an electronic nose operated at room temperature for a wide range concentration evaluation (1–10,000 ppm) of gaseous analytes. Targeted analytes include not only biomarkers for diseases, such as uremia, cirrhosis, lung cancer and diabetes, but also gases for environment monitoring in food, cosmetic and microelectronics industries.

On the Performance Degradation of Poly(3-Hexylthiophene) Field-Effect Transistors

Polymeric transistor degradation was investigated on bottom and top gate structures. Shelf-lifetime studies in both kinds of devices demonstrate an accelerated increase in effective charge carrier mobility, threshold voltage, and off current in modulus when poly(3-hexylthiophene) (P3HT) is exposed to atmospheric gases. Although P3HT is underneath PMMA dielectric and gold gate electrode films, electrical parameters degradation can be only delayed by 100 h. Therefore, only glass encapsulation of the active area is capable of effectively preventing current modulation decrease after exposure to atmospheric gases. Differently from their bottom-gate counterparts, capped top-gate TFTs clearly present negative threshold voltage and positive hysteresis. An interface with reduced deep traps concentration and electrical characterization influence on shallow traps filling are believed to play a significant role in these top-gate P3HT/PMMA transistors under operating conditions. Alternating gate voltage stress along 3000 cycles provides evidence of electrical sweep as a cause of performance degradation. Similarly, dc gate bias stress monitored for 127 min can impair current modulation and shifts threshold voltage depending on its sign. Finally, reversibility of both kinds of stress points that shallow traps are the major problem in these capped devices.

Optical absorbance of P3HT thin films used to estimate simultaneously thin-film thickness and morphology for gas sensing

Regioregular poly(3-hexylthiophene) (rr-P3HT) is suitable for electronic noses and the detection of gaseous biomarkers of human diseases to clinical diagnosis. Nevertheless, thin-film properties such as crystallinity and thickness play a major role in overall device performance. Thin-films were obtained from spin coating of 2–20 mg/mL solutions in chloroform, toluene, chlorobenzene and dichlorobenzene to form a thickness from 20 to 160 nm as measured by atomic force microscopy (AFM). Absorbance spectrum fitted by the sum of three Gaussian curves defined the following parameters, which correlate with the films electronic structure/morphology: (i) the abscissa at the center of the Gaussian from the highest wavelength, which responds for the P3HT band gap, and (ii) the ratio between the area under the Gaussian centered at the lowest wavelength over the one at the highest wavelength, which corresponds to the amount of amorphous and crystalline phase, respectively. Isosbestic point was determined by thermal annealing temperature variation, while keeping the thickness constant. It was observed that absorbance spectrum shape and, consequently, thin-film morphology depend not only on the concentration of the solution, but also on solvent. Finally, the isosbestic point determined at (470 ± 3) nm provides a linear relationship between absorbance and thickness with y-axis intercept approaching zero. The absorbance spectrum and isosbestic point of P3HT provides a non-destructive, faster and reliable way to estimate thin-film properties as thickness and crystallinity without recurring to AFM and X-ray diffraction (XRD) measurements.