Bias Stress in Organic Thin-Film Transistors Towards Low-Cost Flexible Gas Sensors

Abstract

This work is focused on the bias stress (BS) effects in Organic Thin-Film Transistors (OTFTs) from poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14) on both highly-doped Si and glass substrates. While the former had a thermally-grown SiO2 dielectric, the latter demanded an alternative dielectric that should be capable to withstand bottom contact lithography, as well as semiconducting thin-film deposition. In addition, it should represent one more step towards flexible electronics. In order to do that, poly(4-vinylphenol) (PVP) was blended to poly(melamine-co-formaldehyde) methylated (PMF). OTFTs on glass with a cross-linked polymer dielectric had a charge carrier mobility (μ) of 4.0x10-4 cm2/Vs, threshold voltage (VT) of 18 V, current modulation (ION/OFF) higher than 1x102, and subthreshold slope (SS) of -7.7 V/dec. A negative BS shifted VT towards negative values and produced an increase in ION/OFF. A positive BS, on the other hand, produced the opposite effect only for OTFTs on Si. This is believed to be due to a higher trapping at the PVP:PMF interface with PBTTT-C14. Modeling the device current along time by a stretched exponential provided shorter time constants of ca. 105 s and higher exponents of 0.7–0.9 for devices on glass. Due to the presence of increased BS effects, the application of organic TFTs based on PVP:PMF as flexible sensors will require compensating circuits, lower voltages or less measurements in time. Alternatively, BS effects could be reduced by a dielectric surface treatment.

Detection of Water Contaminants by Organic Transistors as Gas Sensors in a Bottom-Gate/Bottom-Contact Cross-Linked Structure

“…A careful bias stress investigation was carried out to assess the effect of electrical polarization on OTFT performance over time. These results can be found in detail in [ 37 ]. In short, there was a noticeable shift in the threshold voltage at high bias voltage (∼5 V).…”

“…Notably, the degree of physisorption depends on the chemical affinity between the active layer and the analyte, occurring both in the bulk of the semiconducting film and at the interface with the dielectric [ 57 ]. In summary, it has been shown that PBTTT-C14 is not only suitable for moisture and ammonia detection [ 23 , 37 , 98 ], but also has enormous potential for alcohol detection and water quality assessment.…”

“…The acquired measurements were (i) versus transfer curve for from 10 to V with a V step and from 0 to V, with a V step; and (ii) versus output curve for from 0 to V with a V step and from 4 to V, with a V step. In order to avoid bias-stressing the device [ 36 , 37 ], once in the gas chamber, just the versus transfer curve was acquired, being limited to a scan from 1 to V with a V step at V.…”

“…For the same reason, was determined to be the average current in that same scan. More details on parameter extraction from OTFT data can be found in [ 10 , 36 , 37 ].…”