Study On The Design, Preparation And Performance Of Polymer Dielectrics For Organic Thin Film Transistors

Organic thin film transistors (OTFTs) play a critical role in flexible electronicdevices, wearable smart cards and sensors, flexible radio-frequency identification tags,etc., due to the possibility of large-area coverage, structural flexibility, low-costdevice fabrication, and compatibility with flexible substrates. At the beginning ofacademic research, in order to realize high-performance OTFTs, semiconductorsespecially their mobility were the center of research and great efforts had been exertedto describe the chemistry and processing of active organic semiconductors andunderstand the properties of the interface between the semiconductor and contacts.However, with further research, researchers found that the properties of the gatedielectric and its interface with organic semiconductors are also quite important forthe performance of OTFTs. In recent years, OTFTs had progressed remarkably toenable the realization for future flexible image devices in industry. A key shortcomingof the OTFTs developed to date is their high operation voltages which stems fromlow-k (k: dielectric constant) of polymer gate insulators. Thus far, reduction of theOTFTs driving voltages has been linked to the use of high-k inorganic metal-oxidefilms which deposited by sputtering that will prohibitively expensive, or the use ofself-assembled monolayer (SAM) which is difficult to be progressed large-area onOTFTs. Recently, there has been increasing interested in obtaining better gate insulators for the ultimate goal-large-area and flexible electronics and cheap liquiddeposition and/or patterning techniques such as spin coating, stamping, printing, etc..In this dissertation, we have reviewed the history of development of OTFTsbriefly, showed various geometries of OTFTs, presented different kinds of materialsof semiconductors and insulators for OTFTs, illustrated the theories of OTFTs andsummarized the application of OTFTs in chapter1.In chapter2, a research on the design, synthesis, and characterization of novelcross-linked polymer organic-inorganic hybrid materials as gate insulators for organicthin-film transistors (OTFTs) with vanadyl-phthalocyanine as the organicsemiconductor is presented. The hybrid films (0.5-1.2μm thick) can be easilyprepared by sol-gel technology and fabricated by spin-coating a mixture of zirconiumn-butoxide sol with a side-chain triethoxysilane-capped polyurethane solution inambient conditions, followed by curing at low temperatures (～120°C) andcross-linking under UV light. OTFTs with this film as gate insulator were achievedwith good processability, high charge-carrier mobility of0.56cm2/Vs, surfaceroughness of around0.49-0.59nm, ultralow threshold of-6V, and ultralow leakage of0.24mA. Hybrid films with various compositions were investigated, and the resultsshowed that the field-effect mobility of the OTFTs was dominated by the highdielectric constant component ZrO2. The result indicated that these hybrid materialsare promising candidates for the exploration of OTFTs devices.In chapter3, a research on the design and synthesis of a novel, thermalcrosslinkable and high-k (k=5, Ci=7nF/cm2) copolymer as monolayer gate insulatorfor organic thin-film transistors (OTFTs) with vanadyl-phthalocyanine (VOPc) andp-6P as the organic semiconductor is presented. The film (680nm thick) can be easilyfabricated by spin-coating in ambient conditions, followed by curing at160°C for2h.OTFTs with this film as monolayer gate insulator was achieved with goodprocessability, high charge-carrier mobility of0.49cm2/Vs, ultralow threshold of-3.22V, on/off current ratio>105and ultralow leakage. The result indicated that thismaterial is promising candidate for the exploration of OTFTs devices. In chapter4, a research on the design and synthesis of novel polyurethanecontaining terphenyl groups as gate insulator to induce crystallization of p-6P fororganic thin-film transistors (OTFTs) is presented. Different sizes and shapes of p-6Pgrains were measured by AFM, and found that large size of p-6P grain can improvethe performance of OTFTs. The film (about900nm thick) can be easily fabricated byspin-coating in ambient conditions, followed by curing at UV irradiation for10min.OTFT with this film as gate insulator was achieved with good processability, highcharge-carrier mobility of1.1cm~2/Vs, threshold voltage of-25V, on/off current ratio>10~5.