Molecular Dielectric Multilayers for Ultra-Low-Voltage Organic Thin Film Transistors
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MOLECULAR DIELECTRIC MULTILAYERS FOR ULTRA-LOW-VOLTAGE ORGANIC THIN FILM TRANSISTORS Myung-Han Yoon, Antonio Facchetti*, and Tobin J. Marks* Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA. ABSTRACT Very thin (2.3 – 5.5 nm) self-assembled organic dielectric multilayers have been integrated into organic thin-film transistor (OTFT) structures to achieve sub-1 V operating characteristics. These new dielectrics are fabricated via layer-by-layer solution phase deposition of molecular silicon precursors, resulting in smooth, nanostructurally well-defined, strongly-adherent, thermally stable, virtually pinhole-free, organosiloxane thin films having exceptionally large electrical capacitances (400-700 nFcm-2). These multilayers enable OTFT function at very low source-drain, gate, and threshold voltages, and are compatible with a broad variety of vapor- or solution-deposited p- and n-channel organic semiconductors. INTRODUCTION During the last few years, thin-film transistors based on organic conjugated materials (OTFT) have been extensively investigated for applications where current inorganic semiconductors cannot be employed.1 The initial arena where this new technology will likely succeed relates to the production of “cheap” electronic devices such as low-performance printed circuits for smart cards and ID tags, low-resolution large-area displays, and eventually flexible/bendable active-matrix LCD/LED screens. The key requirement for these applications is availability of organic semiconductors having, besides obvious stability under operation, requisite FET performance (carrier mobility > 0.01 cm2/Vs; current on:off ratio > 105) at sufficiently low operating gate voltage (VG) to minimize power consumption. Many p- and, to a lesser degree, n-type organic semiconductors exceed the above metrics, however typical VG’s required to achieve such performance with conventional dielectric materials (silicon oxide, polymers, etc.) are unreasonably high for practical use (50-100 V). Relatively few studies have addressed the issue of substantially reducing TFT operating voltage. One approach is to employ inorganic materials having higher dielectric constants than commonly used SiO2, such as Si3N4, BZT (barium zirconium titanate), and Ta2O5.2 Another approach consists of using self-assembled monolayers (SAMs) of monofunctionalized alkyl or phenoxyl-terminated alkyl silane as the OTFT dielectric layer.3 Therefore, the principal strategies can be summarized as either increasing the dielectric constant or reducing the thickness of the insulating layer while minimizing leakage currents. Each approach has both strengths and limitations. We report here a new approach--the design, fabrication, and characterization of solutionphase, layer-by-layer assembled nanoscopic organic dielectrics which afford low-bias OTFTs based on combining the following elements (Figure 1): (a) Self-assembled α,ω-trichlorosilylfunctionalized hydrocarbon chains (Cl3Si(
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