title: Vibrational spectroscopy reveals electrostatic and electrochemical doping in organic thin film transistors gated with a polymer electrolyte dielectric.
abstract: We apply attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy to directly probe active layers in organic thin film transistors (OTFTs). The OTFT studied uses the n-type organic semiconductor N-N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) and a polymer electrolyte gate dielectric made from poly(ethylene oxide) and LiClO4. FTIR spectroscopy of the device shows signatures of anionic PTCDI-C8 species and broad polaron bands when the organic semiconductor layer is doped under positive gate bias (VG). There are two distinctive doping regions: a reversible and electrostatic doping region for VG <or= 2 V and an irreversible and electrochemical doping regime for VG>2 V. On the basis of intensity loss of vibrational peaks attributed to neutral PTCDI-C8, we obtain a charge carrier density of 2.9 x 10(14)/cm2 at VG=2 V; this charge injection density corresponds to the conversion of slightly more than one monolayer of PTCDI-C8 molecules into anions. At higher gate bias voltage, electrochemical doping involving the intercalation of Li+ into the organic semiconductor film can convert all PTCDI-C8 molecules in a 30-nm film into anionic species. For comparison, when a conventional gate dielectric (polystyrene) is used, the maximum charge carrier density achievable at VG=200 V is approximately 4.5 x 10(13)/cm2, which corresponds to the conversion of 18% of a monolayer of PTCDI-C8 molecules into anions.
authors.affiliation: Contribution from the Departments of Chemistry and Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: L G
authors.initials: LG
authors.lastname: Kaake
authors.forename: Y
authors.initials: Y
authors.lastname: Zou
authors.forename: M J
authors.initials: MJ
authors.lastname: Panzer
authors.forename: C D
authors.initials: CD
authors.lastname: Frisbie
authors.forename: X-Y
authors.initials: XY
authors.lastname: Zhu