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.

Downscaling of Organic Field‐Effect Transistors with a Polyelectrolyte Gate Insulator

“…However, ions from the electrolyte can potentially penetrate the semiconductor and cause electrochemical doping, which then could lead to deteriorated transistor behavior and slow switching. [24][25][26] For polyanionic electrolytes, the negative ions are covalently attached to polymer chains, which makes the anions effectively immobile; thus preventing undesired electrochemical doping of the semiconductor bulk. [19,20] In this work, we have used the polyanionic electrolyte poly(vinyl phosphonic acid-co-acrylic acid) (P(VPA-AA)), inset Fig.…”

Vapor‐Phase Deposited Ultrathin Polymer Gate Dielectrics for High‐Performance Organic Thin Film Transistors

“…[ 6 ] Use of polymer electrolyte materials such as ion-gel [ 7 ] had also been attempted as dielectric layers to increase k , thus C i . Although capacitance exceeding 10 µF cm −2 was demonstrated with the ion-gels via suffi cient mobile ions in the electrolyte, low polarization speed, and high gate leakage current ( I G ) hindered the broad use of ion-gel-based gate dielectrics.As far as the gate dielectrics are concerned, polymeric thin fi lms have been considered as ideal candidates, which exhibit a wide range of dielectric constants, [ 8 ] low-temperature processability, mechanical robustness, and cost competitiveness. [ 9 ] However, with only few exceptions, [ 10,11 ] the insulating property of the polymer fi lms degrades severely as the fi lms become thinner.…”

Polyelectrolyte‐Gated Organic Complementary Circuits Operating at Low Power and Voltage

“…[ 20 ] Also, under certain circumstances, ions from the electrolyte may penetrate into the semiconductor and cause electrochemical doping of the semiconductor bulk, which can result in considerably slower switching. [21][22][23][24] We and others have recently demonstrated unipolar electrolyte-gated transistor circuits based on polythiophene semiconductors, which operate below 1 V and show propagation delays down to 0.3 ms. [ 25 , 26 ] However, unipolar circuits suffer from large power consumption, low gain and narrow noise margins, which restrict the range of possible applications. Complementary circuits, on the other hand, which combine p -and n -channel transistors, typically exhibit considerably better characteristics.…”