Low-voltage organic thin film transistors based on high-k gate insulators

xx, 175 leaves : col. ill. ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577P AP 2012 LiOrganic thin film transistor (OTFT) as a basic unit of organic integrated circuits has attracted much attention recently. In the past ten years, great progress has been made in this field. Many novel organic semiconductors were developed with the performance that can rival the amorphous silicon, which pave the way to practical applications of flexible OTFTs. However, OTFTs are encountering great challenges in high operating voltages due to the lack of suitable gate dielectrics. OTFTs with high operating voltage will not only result in high energy consumption but also be incompatible with many emerging applications, including wearable electronics, disposable sensors, and radio frequency identification (RFID) tags. In principle, low-voltage OTFTs can be realized by decreasing the thickness of the gate insulator layer or using high-k gate dielectrics. It will be more convenient to fabricate low-voltage OTFTs with high-k gate dielectrics because only a gate insulator with enough thickness (> 100 nm) is compatible with solution process. Therefore we systematically studied the application of high-k gate dielectrics in OTFTs in the thesis. Relaxor ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) ( P(VDF-TrFE-CFE) ) 56/36.5/7.5 mol% exhibits high relative dielectric constant (~ 60) that is much bigger than those of the organic insulators used in OTFTs before. We successfully used the P(VDF-TrFE-CFE) high-k polymer in low-voltage and solution processible OTFTs as the gate insulator for the first time. Both n-channel and p-channel OTFTs based on conjugated polymers were fabricated and showed the carrier mobilities equal to or higher than 0.1 cm²/Vs at the operating voltage of 3 V, suggesting that P(VDF-TrFE-CFE) is an excellent high-k gate dielectric material for OTFTs. This work paves a way for developing various high-k gate dielectrics based on relaxor ferroelectric polymers for low-voltage transistors.OTFTs based on a high mobility n-type semiconductor poly{[n,n9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) and different gate dielectrics were fabricated. The average carrier mobility decreases from 0.76 to 0.08 cm²/Vs with the increase of the gate dielectric constant from 2.6 to 7.8. We find that the P(NDI2OD-T2) polymer film shows unconventional face-on molecular packing, which results in short distance between carriers and gate dielectric and pronounced interactions between them. Therefore the decrease of the electron mobility with the increasing dielectric constant is attributed to the Frohlich polaron effect for the interaction between electrons in the channel and ionic polarization cloud in the gate dielectric. Flexible low-voltage OTFTs based on the important small-molecule organic semiconductor pentacene were fabricated by using the high-k relaxor ferroelectric polymer (P(VDF-TrFE-CFE)) as gate dielectrics. P(VDF-TrFE-CFE) films were modified with different thin polymer films (few nanometers) by solution process, which can optimize the dielectric/semiconductor interface and the performance of pentacene OTFTs. The low-voltage OTFTs fabricated on plastic substrates show the carrier mobility up to 0.6 cm²/Vs at the operating voltage of 4 V and a good stability after 1000 times bending tests. It is expected that the high-k terpolymer gate dielectric is suitable for low-voltage OTFTs based on various small-molecule organic semiconductors since the dielectrics surface can be easily modified with different thin polymer films by solution process. We have successfully fabricated low-voltage nonvolatile floating gate memories based on OTFTs with the n-typed semiconducting polymer P(NDI2OD-T2) and high-k gate dielectric P(VDF-TrFE-CFE) on flexible substrates by solution process. A thin layer of Au nano-particles was embedded in the gate insulator layer, which can keep charge and result in a memory effect. The floating gate memory demonstrated low programming/erasing voltages of ±6 V, little degradation after 10⁵ programming/erasing cycles and good retention after 10⁴ s, which suggested great promise in the application of the memory devices in flexible electronics.Department of Applied PhysicsPh.D., Dept. of Applied Physics, The Hong Kong Polytechnic University, 201