Studies on solution-processable gate dielectrics and semiconductors for high performance organic field-effect transistors

Abstract

Organic semiconductor and dielectric materials that can be obtained using simple solution-based processes have attracted the attention of many research groups in recent years because of the widely increased demand for low-cost and flexible electronic products. The potential applications of these organic materials have led to intense research in the field of organic electronics, especially in the field of organic field-effect transistors (OFETs). OFETs are expected to be used as driving circuits for large scale and flexible electronic devices, such as radiofrequency identification tags, large area sensors, and bendable/foldable/rollable displays. However, poor performance and stability (air-stability or gate-bias hysteresis), and high operating voltage are still serious obstacles to their commercial use. The work presented in this thesis is to study on the organic semiconductor and gate dielectric layers including their interface, and to demonstrate solution-processed, high performance OFETs with good stability and low operating voltages. Various polymers and organic-inorganic hybrid materials are examined as solution processed gate dielectrics for p-type and n-type OFETs. Also, crystallization and field-effect transistor behavior of various organic semiconductors (vacuum-evaporated or solution-processed) are investigated. Basic background of the organic semiconductor and dielectric materials, and OFETs are introduced in Chapter 1. Hysteresis-free OFETs and inverters using photo-crosslinkable poly(vinyl cinnamate) (PVCN) as a gate dielectric are demonstrated and discussed in Chapter 2. The photo-crosslinked PVCN dielectric film has superior insulating properties and does not require thermal curing. The high water-resistance of the dielectric, which arises because PVCN is hydroxyl group-free, means that the devices were found to be hysteresis-free in all operations. The OFETs with the PVCN dielectric were found to exhibit a carrier mobility of 0.51 cm2/Vs, an on/off ratio of 106, and a subthreshold swing of 0.913 V/dec. An organic inverter consisting of two OFETs exhibited a high inverter gain of 17.9.Photopatternable ultrathin gate dielectric for the fabrication of low-voltage-operating OFETs and inverters are dealt with in Chapter 3. The gate dielectric material is composed of a photo-crosslinkable polymer, poly(vinyl cinnamate), and a thermally crosslinkable silane crosslinking reagent, 1, 6-bis(trichlorosilyl)hexane. The spin-coated dielectric is photocured with ultraviolet light, which enables fine film patterning via regular photolithography. After thermal curing (at 110oC), the dielectric showed excellent insulating properties (a leakage current density of 10-7 A/cm2 at 2.0 MV/cm) for an ultrathin film thickness of 70 nm, thus reducing the operating voltage of the OFETs and inverters to -5 V.In Chapter 4, I reports a novel application of ethylene-norbornene cyclic olefin copolymers (COC) as gate dielectric layers in OFETs that require thermal annealing as a strategy for improving the OFET performance and stability. The thermally-treated N,N’-ditridecyl perylene diimide (PTCDI-C13)-based n-type FETs using a COC/SiO2 gate dielectric show remarkably enhanced atmospheric performance and stability. The COC gate dielectric layer displays a hydrophobic surface (water contact angle = 95±1°) and high thermal stability (glass transition temperature = 181°C) without producing cross-linking. After thermal annealing, the crystallinity improves and the grain size of PTCDI-C13 domains grown on the COC/SiO2 gate dielectric increases significantly. The resulting n-type FETs exhibit high atmospheric field-effect mobilities, up to 0.90 cm2V–1s–1 in the 20 V saturation regime and long-term stability with respect to H2O/O2 degradation, hysteresis, or sweep-stress over 110 days. By integrating the n-type FETs with p-type pentacene-based FETs in a single device, high performance organic complementary inverters that exhibit high gain (exceeding 45 in ambient air) are realized.Solution-processed ultrathin cyclic olefin copolymer (COC)/Al2O3 bilayer gate dielectrics for low-voltage and flexible N,N’-ditridecyl perylene diimide (PTCDI-C13)-based n-type OFETs and their complementary circuits are introduced in Chapter 5. The PTCDI-C13 thin films grown on the COC/Al2O3 bilayer gate dielectrics formed large and flat grains with thermal treatment, resulting in high OFET performance, and stability in an ambient air atmosphere. Despite the high glass transition temperature of the COC, the COC thin films showed good mechanical flexibility with the application of bending strain, and OFETs with bilayer gate dielectrics showed stable operation up to a strain of 1.0%. Complementary inverters based on the PTCDI-C13 and pentacene OFETs with bilayer dielectrics functioned at a low voltage of 5 V, and exhibited a high gain of 63.The use of photocurable poly(vinyl cinnamate) (PVCN) as a gate dielectric in high-performance cylindrical OFETs with high bending stability is described in Chapter 6. A smooth-surface metallic fiber (Al wire) was employed as a cylindrical substrate, and polymer dielectrics (PVCN and poly(4-vinyl phenol) (PVP)) were formed via dip-coating. The PVCN and PVP dielectrics deposited on the Al wire and respectively cross-linked via UV irradiation and thermal heating were found to be very smooth and uniform over the entire coated area. Pentacene-based cylindrical OFETs with the polymer dielectrics exhibited high-performance hysteresis-free operation. Devices made with the PVCN dielectric showed superior bending stability than devices made with PVP dielectrics or previously reported cylindrical OFETs due to the good flexibility of the PVCN dielectric. The devices maintained their excellent performance under bending at a bending radius comparable to the lowest value reported for planar OFETs. The preparation of uniform large-area highly crystalline organic semiconductor thin films that show outstanding carrier mobilities remains a challenge in the field of organic electronics, including organic field-effect transistors. Quantitative control over the drying speed during dip-coating permits optimization of the organic semiconductor film formation, although the kinetics of crystallization at the air–solution–substrate contact line are still not well understood. In Chapter 7, I report the facile one-step growth of self-aligning, highly crystalline soluble acene crystal arrays that exhibit excellent field-effect mobilities (up to 1.5 cm2V–1s–1) via an optimized dip-coating process. I discover that optimized acene crystals grew at a particular substrate lifting-rate in the presence of low boiling point solvents, such as dichloromethane (b.p. of 40.0°C) or chloroform (b.p. of 60.4°C). Variable-temperature dip-coating experiments using various solvents and lift rates are performed to elucidate the crystallization behavior. This bottom-up study of soluble acene crystal growth during dip-coating provides conditions under which one may obtain uniform organic semiconductor crystal arrays with high crystallinity and mobilities over large substrate areas, regardless of the substrate geometry (wafer substrates or cylinder-shaped substrates).