Study of innovative organic transistors for applications in structural electronics and biosensors

Résumé

Organic Thin Film Transistors (OTFT) represent an interesting transductors class for next generation sensors and biosensors. Organic materials composing OTFTs exhibit interesting properties: biocompatibility, low environmental impact and low-cost processing techniques. In addition, they can be manufactured on flexible and stretchable large area substrates, which are well fitted for on-vivo applications and Lab-On-Chip devices.OTFTs also exhibit good sensing performances: very low detection limits and high selectivity have been achieved through direct physical interactions (oxidoreduction at the organic semiconductor (OSC) interface, high electrostatic field thanks to electrolyte gated devices or channel doping). The proximity between the target and the OSC increases the interactions effects, and so leads to better sensitivities. However, it exposes the OSC to the biological analyte, usually of aqueous nature. Consequently, it limits the use of water sensitive-materials.In this study, we designed a double-gate architecture that protect the OSC from direct interaction with the analyte. The transistor structure is based on a reliable single-gate OTFT technology fabricated using a versatile process (including spin coating, photolithography and dry etching steps) that was validated with a statistical analysis. The structure was modified to add one dielectric and one metal layer below the full stack, leading to a double-gate transistor architecture. Layouts for the double-gate architecture were designed, including characterisation patterns for further statistical data analysis on the process reproducibility and efficiency.Structural characterisation has been performed on the device to assess the morphology and geometry (layer thicknesses). Scanning electron microscopy of organic stack was successfully achieved, and profilometry measurements came to validate the observed thicknesses of organic layers. To validate the physical analysis, materials properties were investigated (dielectric permittivity) with capacitance measurements to evaluate the thicknesses with a complementary method.Electrical measurements in double-gate operation demonstrated the interdependence of the threshold voltages to the double-channel structure and allows at identifying the maximum transconductance leading to the most optimised readout voltage. With a large number of devices, a new set of statistical analysis was conducted to evaluate the capability of the process in producing similar devices for industrial purposes. As complementary results, the impact of functionalisation process over transistors integrity has been evaluated to guarantee the sensor performances. Preliminary experiments of sensing with saline solution have presented encouraging results with detection of potential variation on the sensing gate effectively tuned by the second gate voltage, proposing the presented technology as a well suited candidate for sensing and biosensing.

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