Liquid-crystalline semiconductors exhibit the high carrier mobility comparable to those of molecular crystals as well as solubility in common organic solvents.  We have already reported the fabrication of field-effect transistors (FET’s) using liquid-crystalline phenylterthiophene derivatives with the spin-coating method [1,2].  The FET devices fabricated on Si/SiO₂ substrates exhibited p-type operation with a field-effect mobility of 0.042 cm²/Vs and an on/off ratio of 10⁷.  This good performance of the FET devices should be attributed to the close molecular packing within the smectic layers and the low defect density in the smectic phase.  

In this study, the FET devices are fabricated on flexible polymer sheets aimed for the application to electronic papers.  The liquid crystalline semiconductor 5-propyl-5’’-(4-pentylphenyl)-2,2’:5’,2’-terthiophene was synthesized.  On a polyimide sheet with the thickness of 0.3 mm, a gold gate electrode was deposited under vacuum. Then a dielectric layer made from PVA was fabricated by the spin-coating of the PVA/H₂O solution and baked in vacuum. The chlorobenzene solution of this compound was spun on the PVA layer to form the liquid crystalline layer, finally gold source and drain electrodes were vacuum-deposited on it to produce defined top-contact FETs. 

The FET devices exhibit a p-type operation with the mobility of 0.01 cm²/Vs and the on/off ratio of 10⁴.  These characteristics keep almost constant even when the strain (bending the substrate) reached 2.7 %.  It is superior to conventional OFETs using the polycrystalline thin films, in which the performances degrade remarkably when the strain exceeds 1 %.  The soft and flexible structure of the liquid crystalline semiconductor results in this good mechanical flexibility of the FET devices.         

[1]     M. Funahashi, F. Zhang, and N. Tamaoki, Adv. Mater., 19, 353 (2007).

   [2]     F. Zhang, M. Funahashi, and N. Tamaoki, Appl. Phys. Lett., 91, 063515 (2007).

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