High mobility flexible polymer thin-film transistors with an octadecyl-phosphonic acid treated electrochemically oxidized alumina gate insulator

Abstract

Flexible solution-processed polymer thin-film transistors (PTFTs) with a low band-gap (LBG) donor–acceptor (D–A) conjugated polymer as the active layer and electrochemically oxidized alumina (AlO_x:Nd) as the gate insulator are fabricated on polyethylene naphthalate (PEN) substrates. The AlO_x:Nd insulator exhibits excellent insulating properties with low leakage current, a high dielectric constant and a high breakdown field. To improve the interface coupling between the polymer active layer and the AlO_x:Nd insulator, the AlO_x:Nd insulator is treated with octadecyl-phosphonic acid (ODPA), forming self-assembled monolayers (SAMs) on the surface, and great improvement in TFT performance with the highest mobility of 2.88 cm² V⁻¹ s⁻¹ is attained. The performance improvement is attributed to the smoother surface and lower surface energy of the ODPA-treated AlO_x:Nd compared to those of bare AlO_x:Nd. In addition, the flexible PTFT exhibits only small shifts in the transfer curves at bending curvatures (R) at 30 mm, but the device shows larger threshold voltage and higher off current (I_off) after bent at R = 5–20 mm, which may be attributed to the damage in the insulator-semiconductor interface.