Novel Dielectric Materials for High Performance and Low-Voltage Thin-Film Transistors: Organic-Inorganic Hybrid Blends and Multilayers

Organic thin-film transistors (OTFTs) have been the focus of intense research over the past few decades as alternatives to silicone-based conventional transistors. In these devices, dielectric characteristics play a critical role in influencing OTFT operation and performance. My thesis research focuses on two novel classes of dielectric materials based on organic-inorganic hybrid blends and hybrid multilayers. At first, the design, synthesis, processing, and dielectric properties of novel crosslinked inorganic/organic hybrid blend (CHB) dielectric films for low-voltage organic thin film transistor (OTFT) operation are introduced. CHB thin films are readily fabricated by spin-coating a zirconium chloride precursor + an alpha,o-disilylalkane crosslinker solution in ambient, followed by curing at low temperatures (∼150°C). The very smooth CHB dielectrics exhibit excellent insulating properties (leakage current densities ∼ 10-7 A/cm2), tunable capacitance (95 -- 365 nF/cm2), and high dielectric constants (5.0 -- 10.2). OTFTs fabricated with pentacene and PDIIF-CN2 as the organic semiconductor function well at low voltages (< -4.0 V). The morphologies and microstructures of representative pentacene films grown on CHB dielectrics prepared with incrementally varied compositions and processing conditions are investigated and shown to correlate closely with OTFT response. Furthermore, pentacene OTFTs fabricated on plastic substrates with these new hybrid dielectrics are shown to operate at low voltages (< -4.0 V), and to offer high on-off current ratios (>105) as well as substantial hole mobilities (0.2-1.5 cm2V-1s-1).;Secondly, the rational synthesis, processing, and dielectric properties of novel layer-by-layer organic/inorganic hybrid multilayer dielectric films are introduced. These new zirconia-based self-assembled nanodielectric (Zr-SAND) films (5 ∼12 nm thick) are readily fabricated via solution processes under ambient atmosphere using polarizable pi-electron phosphonic acid building blocks and ultra-thin ZrO2 layers. Attractive Zr-SAND properties include amenability to accurate control of film thickness, large-area uniformity, well-defined nanostructure, exceptionally large electrical capacitance (up to 750 nF˙cm-2), excellent insulating properties (leakage current densities as low as 10-7 A˙cm-2), and excellent thermal stability. Thin-film transistors (TFTs) fabricated with pentacene and PDIF-CN2 as representative organic semiconductors and zinc-tin-oxide (Zn-Sn-O) as a representative inorganic semiconductor, function well at low voltages (< +/- 4.0 V). Furthermore, the TFT performance parameters of representative organic semiconductors deposited on Zr-SAND films, functionalized on the surface with various alkylphosphonic acid self-assembled monolayers, are investigated and shown to correlate closely with the alkylphosphonic acid chain dimensions.