Masafumi Kunii1 Hiroaki Iino1 Jun-Ichi Hanna1

1, Tokyo Institute of Technology, Yokohama, , Japan

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-[1]benzothieno[3,2-b][1]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 [1].
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 [2].
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.

[1] M. Kunii, H. Iino, and J. Hanna, IEEE Elec. Dev. Lett., 37, 486, (2016).
[2] M. Kunii, H. Iino, and J. Hanna, Appl. Phys. Lett., 110, 243301 (2017).