title: Effect of self-assembled monolayers on charge injection and transport in poly(3-hexylthiophene)-based field-effect transistors at different channel length scales.
abstract: Charge injection and transport in bottom-contact regioregular-poly(3-hexylthiophene) (rr-P3HT) based field-effect transistors (FETs), wherein the Au source and drain contacts are modified by self-assembled monolayers (SAMs), is reported at different channel length scales. Ultraviolet photoelectron spectroscopy is used to measure the change in metal work function upon treatment with four SAMs consisting of thiol-adsorbates of different chemical composition. Treatment of FETs with electron-poor (electron-rich) SAMs resulted in an increase (decrease) in contact metal work function because of the electron-withdrawing (-donating) tendency of the polar molecules. The change in metal work function affects charge injection and is reflected in the form of the modulation of the contact resistance, R(C). For example, R(C) decreased to 0.18 MΩ in the case of the (electron-poor) 3,5-bis-trifluoromethylbenzenethiol treated contacts from the value of 0.61 MΩ measured in the case of clean Au-contacts, whereas it increased to 0.97 MΩ in the case of the (electron-rich) 3-thiomethylthiophene treated contacts. Field-effect mobility values are observed to be affected in short-channel devices (<20 μm) but not in long-channel devices. This channel-length-dependent behavior of mobility is attributed to grain-boundary limited charge transport at longer channel lengths in these devices.
authors.affiliation: Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States.
authors.forename: K A
authors.initials: KA
authors.lastname: Singh
authors.forename: T L
authors.initials: TL
authors.lastname: Nelson
authors.forename: J A
authors.initials: JA
authors.lastname: Belot
authors.forename: T M
authors.initials: TM
authors.lastname: Young
authors.forename: N R
authors.initials: NR
authors.lastname: Dhumal
authors.forename: T
authors.initials: T
authors.lastname: Kowalewski
authors.forename: R D
authors.initials: RD
authors.lastname: McCullough
authors.forename: P
authors.initials: P
authors.lastname: Nachimuthu
authors.forename: S
authors.initials: S
authors.lastname: Thevuthasan
authors.forename: L M
authors.initials: LM
authors.lastname: Porter
mesh_terms: D004583:Electrons; D006046:Gold / Q000737:chemistry; D013876:Thiophenes / Q000737:chemistry*; D014173:Transistors, Electronic
chemical_list: D013876:Thiophenes; C507295:poly(3-hexylthiophene); D006046:Gold