Ink-Jet Printing of Organic Field-Effect Transistors

Low-cost flexible electronics with organic transistors calls for an easy and cost-effective patterning process. While contact printing, e.g. using reels, might be useful for large-scale production, ink-jet printing ensures high flexibility.

P3OT has been extensively studied as soluble organic semiconductor, but mostly in chloroform solutions. We tested the printing of organic field-effect transistors (oFETs) using different inks:

Semiconductor:

  • Commercial poly-(3-octylthiophene-2,5-diyl) (P3OT), 1 – 3 mg/ml in different solvents

Solvents:

  • Chloroform, chlorobenzene, trichloroethylene, xylene

Test vehicles:

  • Back side-metallized doped Si as gateBildschirmfoto 2014-09-02 um 16.09.26
  • 140 nm thermal oxide as gate dielectrics
  • 150 nm Ti/Au lift-off-patterned S/D-electrodes yielding 20-50 µm x 1-3 mm transistor channels
  • Rinsing in chloroform immediately before printing to obtain hydrophobic surfaces

 

Ink-jet printer:

  • Nano-PlotterTM , XYZ robotic stage with microwell plate for inks and washing solvents
  • Piezoelectrically driven Nano-Tip fabricated by Si-micromechanics with tip-outlet to reduce wetting
  • Camera system for function control of jetting and adjustment to substrate
  • Normal atmosphere (extendable to housed processing under nitrogen o.a.)Micro-Dosage of P3OT

Micro-Dosage of P3OT

  • Printing only possible from solutions with higher boiling points (not chloroform)
  • Line printing by overlapping of single dots
  • Problems (flake-like overlap) with trichloroethylene-based solution because of fast evaporation
  • Good printing from chlorobenzene and xylene solutions
  • Spot diameter ~ 150 – 180 µm, depending on concentration and jetting parameters

Bildschirmfoto 2014-09-02 um 16.11.02

Summary

We prepared oFETs with common Si/SiO2-Au bottom contact design using ink-jet printed semiconductor lines. To reduce evaporation, high-boiling solvents were used. We could print P3OT in trichloroethylene, chlorobenzene, and xylene. The printed oFETs revealed Ion/Ioff-ratios up to 20,000 and hole mobilities of up to 0.002 cm²/Vs (xylene solution). Although this does not represent ideal data, the ease of patterning offers a route to low-cost electronics. It also inspires to study patterning of other semiconducting materials with drastically decreased gate leakage.   Equipment: Nano-Plotter NP2.0