Organic field effect transistors (OFETs) have attracted high attention in the past decades for its potential in industrial applications such as flexible displays, organic bio-medical sensors, and flexible radio frequency identification (RFID) tags. In fact, among various OFET materials so far proposed, some of which exhibited high OFET mobility over a few cm2/Vs. However, FET performance have hardly been investigated in detail where practical device performance are concerned such as OFET characteristics under low voltage driving and high bias-stress stability.
In this study, we evaluated OFET performance fabricated with solution-processed polycrystalline thin films of liquid crystalline organic semiconductor of 2-phenyl-7-decyl-benzothieno[3,2-b]benzothiophene (Ph-BTBT-10) under the low voltage and high bias-stress conditions described above.
Ph-BTBT-10 OFETs (bottom-gate and bottom-contact FET with W/L of 350 μm/70μm) were fabricated with low-κ polystyrene/SiO2 hybrid gate dielectric using highly ordered smectic E (SmE) liquid crystalline phase, which helps organic molecules form a well-aligned ccrystalline thin film during spin-coating process and gives it high thermal stability. The transfer characteristics were measured at drain voltage (Vds = −0.1 V for linear regime and at Vds = −2 V for saturation regime) under ambient air at room temperature. Despite the polycrystalline nature of Ph-BTBT-10 and low-κ gate dielectric, the solution-processed, bottom-gate, bottom-contact Ph-BTBT-10 OFET exhibits saturation mobility up to 4.9 cm2/Vs with the subthreshold swing of 79 mV/decade, which is very close to the theoretical limit of 60 mV/decade at room temperature .
On the other hand, we evaluated OFET bias-stress stability that depends on gate voltages as a function of operation time up to 104 s. The negative gate bias-stress caused negative threshold voltage shift (△Vth) for Ph-BTBT-10 FET in ambient air, but △Vth rapidly decreases as the gate bias decreases and approaches to near zero when the gate bias went down to 9V in amplitude. In contrast, positive gate bias-stress causes negligible △Vth even with a relatively high bias voltage .
All these results conclude that Ph-BTBT-10 FET has excellent FET performance at a low voltage operation as well as excellent bias-stress stability in ambient air in the range of low to moderate operating voltages.
 M. Kunii, H. Iino, and J. Hanna, IEEE Elec. Dev. Lett., 37, 486, (2016).
 M. Kunii, H. Iino, and J. Hanna, Appl. Phys. Lett., 110, 243301 (2017).