Study On Mechanical Flexibility And Thermal Stability Of The Flexible Organic Field-effect Transistors Of High Performance Based On Double Gate Dielectric Layers

There is conspicuously increasing interest in flexible organic polymeric electronics during these past decades owing to their envisioned applications in radio- frequency identification tags, flexible displays, smart card, low-cost sensors, and electronic skins, etc. Flexible organic field-effect transistors(OFETs) are particularly interesting due to the advantages of low-temperature solution processing, large area processing low-cost production routes, mechanical flexibility, and compatibility with flexible plastic substrates. While the materials science community has given extensive consideration to the research of the semiconductor materials in regard of improving the charge transport, the gate dielec tric materials have drawn only modest attention. However, the realization of high-performance flexible OFETs via low-temperature solution processing relies greatly on the gate dielectric material. Furthermore, the development of the gate dielectric materia ls will be propitious to the popularization of commercial electronics based on organic transistors. Some research groups have demonstrated high-performance OFETs by applying metal–oxide high-k gate dielectric materials. However, metal–oxide gate dielectrics are not so well compatibility with flexible plastic substrates in comparison with polymer gate dielectrics, besides, metal–oxide gate dielectrics usually can’t provide good interface quality with organic semiconductor materials. Polymer gate dielectrics can provide a good interface with organic semiconductor materials as polymer gate dielectrics have nonpolar hydrophobic surface, thereby contributing to high carrier mobility. It is strongly desirable that the device can be fabricated by low-temperature solution processing in order to realize the advantages of flexible OFETs. In this regard, the polymer dielectric layers can be prepared using low-temperature solution processing method via spin coating.In this work, the thesis fabricates high mobility flexible OFETs on a flexible polyethylene terephthalate(PET) substrate based on PMMA and cross-linked PVP-HDA as double gate dielectric layers. Pentacene is chosen as the organic semiconductor layer owing to its relatively high mobility and extensive application for OFETs. The bottom gate pentacene transistor on flexible PET substrate exhibites high performance characteristics with an average saturation carrier mobility of about μ=1.5cm2V-1s-1, a threshold voltage about Vth =-20 V, and the on/off current ratio of 105. In addition the estimated saturation carrier mobility of the device has already achieves as high as μ=2 cm2V-1s-1. These above properties are comparable to and even superior to the rigid OFETs. The device is without any encapsulation. And then the thesis focused on the effects of various bending tests and the thermal stability of the device at different temperatures. The thesis discusses the mechanical bending tests in severl aspects,including the number of the mechanical bending cycles,the bending radius of 5mm and 3.5mm,the bending directions oriented parallel as well as perpendicular to the channel. In the study of the thermal stability of the device, the thesis increases the temperatures from 40 to 140 °C in steps of 20℃ in the oven and keeps each certain temperature for 2 hours. The results show that the device fabricated in the work has a relatively good thermal stability. Moreover, to systematically analyse the device properties, the thesis relates the properties to the morphological and structural characteristics of the organic semiconductor layer, which are partly determined by atomic force microscope(AFM),specular X-ray diffraction(XRD) etc.