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 gate
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

Schematic cross section of a single test device

Ink-jet printer:

Nano-Plotter^TM , 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


Ink-jet printed dot matrix (2.5 mg/l P3OT in chlorobenzene) and enlarged single dot (SEM)				oFET test devices with ink-jet printed line of dissolved P3OT. Left: 1 mg/ml in trichlorethylene, right: 1 mg/ml in chlorobenzene.

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

Courtesy of:

Dr. Matthias Plötner
Semiconductor and Microsystems Technology Laboratory
Dresden University of Technology

Reference:
Plötner, M., Wegener, T., Richter, S., Howitz, S., Fischer, W.-J. (2004) Investigation of ink-jet printing of poly-3-octylthiophene for organic field-effect transistors from different solutions. Synthetic Metals 147, 299-303

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