All-inkjet-printed flexible electronics
fabrication on a polymer substrate


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Abstract

All-printed electronics is the key technology to ultra-low-cost, large-area
electronics. As a critical step in this direction, we demonstrate that laser
sintering of inkjet-printed metal nanoparticles enables low-temperature metal
deposition as well as high-resolution patterning to overcome the resolution
limitation of the current inkjet direct writing processes. To demonstrate this
process combined with the implementation of air-stable
carboxylate-functionalized polythiophenes, high-resolution organic
transistors were fabricated in ambient pressure and room temperature without
utilizing any photolithographic steps or requiring a vacuum deposition
process. Local thermal control of the laser sintering process could minimize
the heat-affected zone and the thermal damage to the substrate and further
enhance the resolution of the process.

Introduction

The development of electric circuit fabrication on polymer
substrates has attracted significant interest as a pathway to
low-cost or large-area electronics [1, 2]. The conventional
vacuum deposition and photolithographic patterning methods
are well developed for inorganic microelectronics. However,
flexible polymer substrates are chemically incompatible with
resists, etchants and developers used in conventional integrated
circuit (IC) processing. In practice, conventional IC fabrication
4 Author to whom any correspondence should be addressed.
processes are subject to limitations in that they are multistep,
involve high processing temperatures and toxic waste
and are therefore expensive. Furthermore, the increasing size
of electronic devices such as displays poses great difficulty
in adapting standard microfabrication processes, including
lithographic patterning.

Fabrication and experimental details

Three different techniques were combined to achieve effective
deposition of high-resolution metal patterns on a heatsensitive,
low-cost lightweight plastic substrate at low
temperature and under ambient atmospheric pressure without
using an expensive, toxic and time-consuming conventional
lithographic process to realize low-cost and large-area flexible
electronics. Firstly, the reduced processing temperature
exploits the melting temperature depression of nanoparticles
due to the thermodynamic size effect. Nanomaterials exhibit
several interesting mechanical, chemical, optical and electrical
properties that cannot be observed in their bulk counterparts
due to the large surface to volume ratio, large surface energy
and spatial confinement. Bulk gold starts to melt above
1063 ◦C. However, the melting temperature of the material
drops to around 150 ◦C when the size of the nanoparticle
shrinks under 2 nm due to the thermodynamic size effect [21].
The metal nanoparticles allow handling and treatment of
metal components at a plastic-compatible low processing
temperature in an inkjet based solution process without using
any vacuum deposition method. Secondly, a focused laser
beam was irradiated to deposit the energy and induce highly
localized and efficient nanoparticle melting.

Summary

All-printed electronics is the key technology to ultra-low-cost
electronics such as radiofrequency identification devices and
large-area displays. As a critical step in this direction, airstable
OFETs were fabricated using inkjet printing and lowtemperature
selective laser sintering. OFET electrodes having
great resolution and highly electrically conductive gold lines
were fabricated in ambient pressure and at room temperature
without using any lithographic process.