Web Release Date: June 1,
Vibrational Spectroscopy Reveals Electrostatic and Electrochemical Doping in Organic Thin Film Transistors Gated with a Polymer Electrolyte Dielectric
Contribution from the Departments of Chemistry and Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
Received January 27, 2007
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 
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 × 1014/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 ~4.5 ×
1013/cm2, which corresponds to the conversion of 18% of a monolayer of PTCDI-C8 molecules into anions.
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