Self-Adhesive Organic Thin Film Transistors on Elastomeric Nanofilms

Devices that can be intimately contacted with our skin are increasingly gaining attention because of their huge potential in collecting long-term vital data for future healthcare, nursing, or sports activities. One of the key components is flexible or stretchable transistors, since they are necessary for signal amplification close to the body [1], or sensor matrixes to collect spatial data [2]. Flexible transistors in ultrathin form, especially below few-100 nm-thick, can conform to the complex shape of the skin surface [3], whereas thicker elastomeric transistors can also attach to the skin by their elastomeric property. Although these devices can contact well to the human skin, stability of the device-skin contact and robustness needed improvement in order to achieve long-term signal collection. Nanofilms, which are below few-100 nm thick and are made of elastomeric materials, have shown sufficient stability of the device-skin contact and robustness to be worn on skin for over a week [4]. These elastomeric nanofilms has become a candidate for wearable devices’ interface, but only electrodes have been fabricated on them. Fabrication of transistors on them has remained a challenge, due to difficulty in handling and physical damages that occur during the fabrication.<br/><br/>In this work, we report novel device structure and process of organic thin film transistors (OTFT) on elastomeric nanofilms. Total device thickness is only 0.5 µm, enabling the devices to have stable contact on any parts of the body. The substrate film is ultrathin with average thickness of ~200 nm, enabling the film to be self-adhesive requiring ~130 µJ/cm² for detachment from artificial skin. This film was fabricated by dip-coating electrospun polyurethane nanofibers into polydimethylsiloxane solution, according to ref.3. The fiber-reinforced structure enhances the robustness of the film. The film was then placed onto a flexible supporting substrate with release agent. Bottom gate was formed by thermally evaporating gold (50 nm) through a shadow mask, followed by chemical vapor deposition of parylene (200 nm) as the dielectric layer. Subsequently, organic semiconducting layer of dinaphthothienothiophene (DNTT) (30 nm) and gold contact electrodes (50 nm) were formed by thermal evaporation. Finally, OTFTs on nanofilms were delaminated manually from the supporting substrate to become free-standing state.<br/><br/>The fabricated transistors exhibited electrical characteristics comparable to those fabricated on conventional substrates. They showed excellent yield of 75/77, with mobility of 0.19 ± 0.02 cm²/Vs and maximum on/off ratio of above 10⁵. To demonstrate stability, devices were laminated and then delaminated from a glass substrate, which showed negligible effect on the electrical characteristics.<br/><br/>[1] Sugiyama, M., <i>et al</i>. <i>Nat. Electron. </i>2, 351–360 (2019).<br/>[2] Wang, S., <i>et al</i>. <i>Nature</i> 555, 83–88 (2018).<br/>[3] Viola, F.A., <i>et al. Nat. Commun.</i> 12, 5842 (2021).<br/>[4] Wang, Y., et al. <i>Proc. Natl. Acad. Sci</i>. 118, e2111904118 (2021).