Improvement of dispersion of the Al 2 O 3 nanoparticles in the poly͑4-vinyl phenol͒ ͑PVP͒ matrix by coupling agent treatment resulted in a reduction of the leakage current density of the nanocomposite gate dielectric in organic thin-film transistor ͑OTFT͒ devices, which, in turn, improved the device performance compared to that of the device with the pure PVP gate dielectric. Under repetitive cyclic bending, the leakage current density of the nanocomposite layer was not changed, while that of the PVP layer was increased significantly. The nanocomposite gate dielectric layer provided the flexible OTFT device with improved mechanical and electrical stabilities.During the past few years, flexible organic thin-film transistors ͑OTFTs͒ have been extensively studied for various applications such as radio-frequency identification, 1,2 low-cost sensors, 3,4 and flexible displays. 5-7 Achieving the electrical reliability of flexible OTFTs under repetitive mechanical deformation requires good mechanical flexibility and stability of the electrical properties. Therefore, gate dielectric, electrode, and semiconductor layers possessing good mechanical flexibility, with no cracking or delamination, and whose electrical properties remain stable in the device during repetitive mechanical bending, need to be developed for flexible devices.One of the important materials determining the performance of OTFTs is the gate dielectric layer. 8 Inorganic gate dielectric layers, such as SiO , and lead zirconate titanate, etc., 8 can provide a higher capacitance with a smaller thickness due to their large dielectric constant ͑k͒ and low-leakage characteristics. However, cracking during repetitive mechanical deformation can cause an increase in the leakage current. In the case of low-k organic gate dielectrics such as poly͑4-vinyl phenol͒ ͑PVP͒, poly͑vinyl alcohol͒, polyimide ͑PI͒, and benzocyclobutane, 8,9 the required large thickness of the gate dielectric layer due to the high leakage current lowers the gate capacitance. Furthermore, although organic gate dielectric layers are known to be mechanically flexible, their electrical stability is unknown. One way of overcoming the problems of inorganic and organic gate dielectrics would be to develop mechanically flexible nanocomposite gate dielectrics. Nanocomposite gate dielectrics in OTFTs have been studied for the purpose of increasing the gate capacitance and thereby improving device performance. 10-13 However, their leakage characteristics of the nanocomposite gate dielectrics and their electrical stability under repetitive mechanical deformation has not been reported.In this work, Al 2 O 3 /PVP nanocomposite layers with good dispersion of the Al 2 O 3 nanoparticles were obtained by treating the surface of the nanoparticles with a coupling agent ͑CA͒. A mechanically flexible, low-leakage, and high-capacitance nanocomposite gate dielectric layer was obtained and successfully applied to OTFT devices. ExperimentalAl 2 O 3 particles with the average size of Х50 nm ͑Sigma Aldrich͒ were surface...