Small-molecule organic thin film transistors

This thesis reports state-of-the-art organic thin film transistors (OTFTs) fabricated using a variety of organic active layer materials on rigid and flexible substrates with record device performance. The processing and characterization of materials for high performance OTFTs was initially developed using OTFT test structures on oxidized single-crystal silicon wafers; providing a convenient substrate, gate contact, and gate insulator. Technology was then transferred to glass and flexible polymeric substrates to demonstrate patterned gate OTFTs and simple OTFT circuits with performance suitable for several low-cost, large-area electronic applications.; Using pentacene as the active material, OTFTs were fabricated on rigid low-cost glass or thermally oxidized silicon substrates with saturation field-effect mobility as large as 2.5 cm2/V-s, on/off current ratio larger than 108, and subthreshold slope as low as 0.4 V/decade. All are record values for organic thin film devices (mobility and current on/off ratio are approximately one order of magnitude better than the best previous results) and are comparable to or better than results obtained using hydrogenated amorphous silicon.; High performance OTFTs were fabricated from a variety of thermally evaporated small-molecule organic semiconductor materials. In addition to pentacene OTFTs with saturation field-effect mobility as large as 2.5 cm2/V-s, OTFTs were fabricated using alpha-sexithienyl, naphthacene, or sexiphenyl as the active layer material with field-effect mobility as large as or greater than 0.1 cm2/V-s. OTFTs with Cu-phthalocyanine, quaterphenyl, or quinquephenyl active layers were fabricated with field-effect greater than 0.01 cm2/V-s. These mobility are in the range of interest for several current and future low-cost, large-area electronic applications.; Pentacene OTFTs were fabricated using chemically-modified source and drain contacts with improved contact and linear region characteristics. OTFTs had saturation field-effect mobility greater than 0.8 cm2/V-s, on/off current ratio larger than 107, and subthreshold slope less near 0.7 V/decade. The maximum linear carrier field-effect mobility was greater than 0.55 cm2/V-s at a drain-source voltage of −0.1 V; to our best knowledge this is the highest linear region mobility reported for an OTFT. The process developed for improving the linear region performance of pentacene OTFTs is currently being be extended to flexible plastic substrates and may also prove useful for improving charge injection in organic electroluminescent devices.; In addition to improvements in OTFT performance using simple test structures, pentacene OTFTs with photolithographically patterned gate electrode, gate dielectric, and source and drain contacts were fabricated on flexible polyethylene napthalate (PEN) film (a high temperature polyester film) with field-effect mobility greater than 1 cm2/V-s and large on/off current ratio. These results greatly increase the likelihood that organic thin film transistors will find use in current and future low-cost, large area electronic applications.