Through the use of solution‐based materials, the field of printed electronics has not only made new devices accessible, but enabled the process of manufacture to move towards a high-throughput industrial scale. However, while the solution‐based active layer materials employed in these types of systems have been studied quite intensely, the conducting structures that feature in printed circuits, RFID applications, logic systems and electrodes in optoelectronic devices have not received as much attention.
Inkjet-printing in particular, as an additive, upscalable, direct write technique that requires no masks or lithographic pre-patterning of substrates, has been utilized to produce such structures in a wide variety of (opto)electronics, paving the way to fully solution-processed devices. However, for full compatibility with flexible, low cost substrates, the processing conditions of the deposited structures need to be controlled.
This contribution highlights our work on utilizing inkjet-printing to deposit copper nanoparticles (CuNPs) in order to form conducting structures within a range of electronic applications, specifically optoelectronic devices and printed circuits, and discusses methods to improve the conductive and interfacial properties. A reductive sintering approach is presented as an alternative to commonly used laser or flash lamp curing techniques.
The findings presented address the importance of continuing work in improving the effectiveness of printed conductive structures, including in their use in organic and hybrid (opto)electronic devices, in order to move towards fully solution-processed and flexible electronics.
Most current electronics manufacturing technologies utilise subtractive processing that is expensive, wasteful and energy intensive. Printed electronics is revolutionising the electronics industry by enabling additive processing that significantly reduces expense, waste and energy consumption.
The EU-funded PLASMAS project demonstrates the capability of printed electronics based on novel nanoparticle Cu inks with favourable cost to performance ratios, through development of large area printed circuit boards and printed logic as well as OLED and OPV elements with printed Cu nanoparticle electrodes.
However, a number of challenges need to be overcome when printing these metal nanoparticle inks – the typical feature height of printed structures of several 100 nm tend to exhibit a rough surface, which can lead to shorts in the device after subsequent overcoating of the organic active layer materials. Furthermore, the sintering temperature of the nanoparticle inks needs to be low (< 130 °C) in order to allow deposition and curing on transparent flexible substrates such as PET.
We therefore present the process development of solution-processed electrodes based on inkjet-printed Cu grids, by embedding the inkjet-printed metal grids to yield ITO-free optoelectronic devices. Secondly, we present roll-to-roll inkjet-printed RFID antennas based on Cu inks. Finally, we demonstrate a truly low-temperature sintering route for a Cu nanoparticle ink by using a reducing atmosphere of formic acid, yielding stable highly conducting layers.
The results of the project highlight overall parameters for solution processing and implementation of novel metal nanoparticle materials and architectures in printed electronics.