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Designed Organophosphonate Self-assembled Monolayers Enhance Device Performance Of Pentacene-based Organic Thin-film Transistors.
Published 2010 · Medicine, Materials Science
Organic thin-fi lm transistors (OTFTs) [ 1 ] are compatible with large area fabrication techniques, and they may yield new paradigms for manufacturing that would reduce device costs for large area circuits. [ 2 ] Pentacene-based OTFTs are well studied; yet they can be compromised with regard to critical device parameters: on/off ratios, carrier mobilities, sub-threshold performance, and threshold voltages. Surface treatments for the SiO 2 gate dielectric [ 2–8 ] can be effective; typically, however, improvement is realized in only one of these critical areas, often to the detriment of the others. [ 5 , 6 , 9–11 ] Here we show that systematic structural modifi cation of self-assembled monolayers of phosphonates (SAMPs) fabricated on SiO 2 gate dielectrics can yield pentacene-based devices in which all four critical parameters are enhanced simultaneously. We hypothesize that phosphonate structural motifs enable two-dimensional SAMP surface coverage to translate to a three-dimensional one through controlled lateral spacing between vertically oriented acene units, which can affect crystallization of vapor-deposited pentacene. Our approach to surface modifi cation of the SiO 2 gate dielectric on a Si device is based on a simple procedure in which a dilute solution of a phosphonic acid is drawn down across the face of the Si substrate. This process, nicknamed the T-BAG, [ 12 , 13 ] enables formation of high quality SAMPs on these substrates under mild conditions. In previous work we showed that a surface chemistry design approach to transistor function optimization based on T-BAGged SAMPs, progressing from simple enhancement of surface wetting to inducing more specifi c SAMP-pentacene interactions, enabled systematic improvement in OTFT behavior. [ 14 , 15 ] Our best-performing OTFT was based on a SAMP of 9-phosphonanthacene ( 1 ). Anthracenebased phosphonates are electronically acceptable for SiO 2 gate dielectric modifi cation: Their SAMP constituents should have a band gap larger than that of pentacene itself (1.86eV by optical