The bottom-gate top-contact OTFTs were fabricated on an indium tin oxide (ITO)/glass substrate that had been cleaned with detergent, acetone, and isopropyl alcohol and treated in a UV-ozone cleaner for 30 min. A solution of PNIPAM (15 wt%) in deionized water was spin-coated (2000 rpm) onto the substrate and then baked at 85 °C for 40 min. A layer of pentacene semiconductor (70 nm) was thermally evaporated under a pressure of 5 ? 10–6 torr at a deposition rate of 0.
3–0.5 Å s–1. Following the preparation of the active layer, 40-nm Au source and drain electrodes were deposited through a shadow mask having a channel width of 200 µm and a channel length of 2000 µm. For fabrication of the flexible device on the polyethylene terephthalate (PET) substrate, 15 wt% PNIPAM in water was spin-coated twice (1000 rpm each time), followed by deposition of pentacene and the Au source and drain electrodes, using the same procedures described above.
The field-effect mobility of the OTFT was calculated from the following equationI_D=(W/2L) C_i ?(V_G-V_T )^2 (for the saturated regime) (1)where ID is the drain current in the saturated regime, VG is the gate voltage, VT is the threshold voltages, W and L are the channel width and length, Ci is the capacitance per unit area of the gate dielectric layer, and µ is the field-effect mobility.For measurement of the capacitance and leakage current, metal–insulator–metal (MIM) capacitors were fabricated comprising PNIPAM sandwiched between ITO (bottom) and Au (top) electrodes. The 15 wt% PNIPAM solution was spin-coated at 1000, 2000, 3000, or 4000 rpm and then baked at 85 °C for 40 min. The active area of the capacitor was 0.1 cm2. The dielectric constant was calculated using the equationk=Cd/(?_0 A) (2)where C is the capacitance, ?0 is the permittivity of a vacuum, d is the thickness of the film, and A is the active area of the capacitor.
Film thickness was measured using a Bruker Dektak profilmeter. Tapping-mode atomic force microscopy (AFM) images were recorded using a Bruker Dimension Icon atomic force microscope. The capacitance of the dielectric film was measured using a Solartron SI 1260 impedance/gain phase analyzer coupled with a Solartron 1296 dielectric interface.
The electrical performance of the fabricated devices was measured using a Keithley 4200 semiconductor analyzer inside a N2-filled glove box. The OTFT sensors were tested by heating the devices on a hotplate and recording the electrical performances. Bending tests of flexible devices were performed at a bending radius of 1.5 cm; changes in the electrical performance were measured after 25, 50, 75, and 100 cycles.