Intimate monitoring of physical and chemical patient data is changing the health industry. Real-time monitoring devices, such as implantable and wearable tools for diagnostics, need truly flexible transistors for advanced electronics and elegant sensors. Previous reports have realized flexibility through geometric patterning or intrinsically stretchable semiconductors. For gate dielectric insulators, researchers have utilized elastomeric matrices and polymer electrolyte systems to achieve flexibility. Ionic liquid-based gel electrolytes offer high capacitance, a large electrochemical stability window, and are nonvolatile. Typical demonstrations of these ion gels use block copolymers or chemically crosslinked in situ polymerization to add support, though some have demonstrated that colloidal particles dispersed in ionic liquids can result in gels.
In this work, we differ from previous reports of ionic liquid-based transistors (IL-OFETs) by using a colloidal dispersion of silica nanoparticles to support the ionic liquid. This scaffold is advantageous over existing triblock copolymers or chemically crosslinked gels because of its facile fabrication for conformal gel coating. Specifically, triblock copolymers have complex chemistry and in situ methods require additional crosslinking steps, such as UV treatment to form a gel, which could be difficult to apply to certain substrate types and geometries. Contrastingly, these silica dispersion gels are easily applied after gelation for a conformal coating.
We employ 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as the ionic liquid and poly(3-hexylthiophene) as the organic semiconductor, both well studied materials in regards to IL-OFETs. We achieve OFET performance assessed by output and transfer characteristics. Use of this ion gel results in an on-off ratio of 104, threshold voltage of -1.65V, an operating window of 0 to -2.7V, and allows for transistor development on softer, more flexible substrates than typical silicon or glass. For demonstration, we employ our transistor on a linen thread, which could offer a route to 3D monitoring of health in the form of smart sutures and transdermal implementations. This resulted in an on-off ratio of 103, threshold voltage of -2.8V, and an operating window of 0 to -4V. These preliminary results validate this proposed material selection and design as ideal for electronics and sensors on both 2D and 3D flexible substrates.
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