| Organic thin film transistors (OTFTs) have been developed rapidly in the past decate since they can be fabricated on flexible, lightweight and low-cost substrates by simple techniques. OTFTs can be used in the microelectronics and semiconductor industries as the alternative of conventional silicon counterparts. However, there is much work to be completed for OTFTs compared with conventional silicon thin film transistors. For example, the lower carrier mobility, high drive voltage and the relatively poor stability. Particularly the high operating voltage will increase the energy consumption, thus limit the large-scale application of OTFTs. Therefore, excellent gate dielectrics applied in OTFTs become a hotspot in recent years. Various gate dielectrics, including oxides, polymers, organic/inorganic composite have been widely studied. Although much progress has been obtained, the large-scale preparation of high-performance dielectrics and the corresponding OTFTs is still a challenge for researchers. In this thesis, a simple solution method was applied to prepare a variety of gate dielectric layers and the corresponding OTFTs performance was studied in details.The relationship between the film-forming technology of organic dielectrics and the device performance of OTFTs with organic/inorganic bilayer dielectrics was first explored. We selected polymethyl methacrylate (PMMA) and polystyrene (PS) as the organic dielectric materials. The results show that the electrical properties of OTFTs are related with the thickness of organic dielectric layer and the chemical structure of the organic dielectric material. When the PMMA thickness decreases from200nm to40nm, the device mobility increases from0.06to0.1cm2/Vs, and the on/off current ratio increases from103to104. When the PS thickness decreases from60nm to10nm, the device mobility increases from0.26to0.56cm2/Vs, and the on/off current ratio increases from105to106. The difference between the PS/SiO2devices and PMMA/SiO2devices are attributed to the different polarity of organic dielectric materials. The non-polar PS layer is more conducive to the transmission of the carriers. The organic/SiO2devices with two layers organic dielectrics were also fabricated, we found that the second organic dielectric layer can modify the dielectric/semiconductor interface and therefore improve the device performance. For example, the mobility and on off current ratio reached0.71cm2/Vs and8.14×106for the OTFTs with double PS layer。Then, a highly effective solution method for significant performance improvement of OTFTs was demonstrated. The PS/SiO2dielectric layer in top-contact pentacene-based OTFTs was modified with hexamethyldisilazane (HMDS) self-assembled monolayers (SAMs). The study systematically investigated the relationship between the interface modification and device performance. It is found that the performance of OTFTs is greatly enhanced by this HMDS modification. For example, for the HMDS/(10nm PS)/SiO2devices, the charge carrier mobility reached1.05cm2/Vs, while (10nm PS)/SiO2devices was just0.56cm2/Vs and the on/off current ratio increased from105-106to106-107, respectively.We also introduce a facile and universal solution fabrication of high-k amorphous oxide dielectrics for high-performance OTFTs. ZrTiOx as a typical example, a k value and capacitance as high as53and467nF/cm2could be achieved by controlling the composition and annealing temperature. The polystyrene (PS) modification of ZrTiOx dielectric films results in a leakage current as low as4×10-8A/cm2. Based on their implementation as a gate insulator, the solution-processed high-k ZrTiOx dielectric films realize high and stable performances performance OTFTs during operation at a low voltage. A carrier mobility of0.58cm2/Vs, an on/off current ratio of104, and a low operating voltage of6V were achieved. Our results show the possibility of the solution-processed high-k amorphous oxide dielectric layer as a gate insulator for OTFTs. |
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