Laser induced forward transfer (LIFT) and laser sintering of metal nanoparticle inks constitute a two-step digital fabrication technique which has been proven a key enabling technology for the fabrication of flexible microelectronic devices. In this work we will present the investigation of the laser printing and sintering process of Ag nanoparticle inks for the production of a conductive grid comprised of parallel lines as replacement for the bottom Indium Tin Oxide (ITO) electrode in organic photovoltaics (OPVs). We study the effect of a range of laser parameters and their impact on the morphological characteristics and the electrical performance of the laser printed conductive grid. The electrical conductivity of the laser printed lines is calculated by means of electrical measurements in a 4-point probe IV station while their morphological characteristics are assessed with profilometry measurements. As a result, flexible ITO-free OPVs incorporating laser-printed Ag grids as a bottom electrode on PET substrates will be presented. The results confirm that the laser printing and sintering combination is an advantageous technique, which can offer a distinguishing solution for applications in highly efficient ITO-free OPVs.

The laser induced forward transfer and sintering of metal nanoparticle inks has been proven a key enabling technology for flexible electronics. Nevertheless, many challenges concerning the conformal processing of non-planar substrates incorporating thermally sensitive layers are yet to be addressed. In this work, we study the behaviour of conformal laser printing of silver nanoparticle inks on patterned samples comprising sensitive underlying structures, by correlating the laser sintering powers employed to the undesired effects on the adjacent interfaces. The latter include demanding surface topographies with periodic patterns and micro-components exhibiting aspect ratio in the nano to 100-micron scale. We investigate the contribution of crucial processing parameters, such as the per pulse energy, repetition rate and the pulse to pulse spatial and temporal overlap to the overall result. The demonstrated results validate the versatility of laser processing which can offer application specific solutions on different use cases involving multilayered and multimaterial electronics.

Laser-induced forward transfer (LIFT) printing has emerged as a valid digital printing technique capable of transferring and printing a wide range of electronic materials. In this paper, we present for the first time LIFT printing as a method to fabricate silver (Ag) nanoparticle (np) grids for the development of indium tin oxide (ITO)-free inverted PM6:Y6 nonfullerene acceptor organic photovoltaics (OPVs). Limitations of the direct use of LIFT-printed Ag np grids in inverted ITO-free OPVs are addressed through a Ag grid embedding process. The embedded laser-printed Ag grid lines have high electrical conductivity, while the Ag metal grid transparency is varied by altering the number of Ag grid lines within the inverted OPVs' ITO-free bottom electrode. Following the presented Ag-grid embedding (EMP) process, metal-grid design optimizations, and device engineering methods incorporating an EMB-nine-line Ag np grid/ PH500/AI4083/ZnO bottom electrode, we have demonstrated inverted ITO-free OPVs incorporating laser-printed Ag grids with 11.0% power conversion efficiency.

Current trends in flexible and large area electronics require low temperature, solvent- free and mask-less processing of patterns with versatile form factors and aspect ratio. In this work, we explore the boundaries of conformal laser printing and laser processing of Ag nanoparticle inks applied on particularly sensitive substrates and structures. The latter involve challenging surface topographies, such as patterns and micro-components with periodicity and aspect ratio in the nano to 100-µm scale. The demonstrated results validate the versatility and flexibility of laser sintering, which can offer a specific solution to particularly challenging use cases and applications in flexible multimaterial multilayered electronics.

The recent developments in the field of large area, flexible and printed electronics have fueled substantial advancements in Laser Printing and Laser Sintering, which have been attracting interest over the past decade. Resulting applications, ranging from flexible displays and sensors, to biometric devices and healthcare, have already showcased transformational advantages in terms of form factor, weight and durability. In HiperLAM project, Laser-Induced Forward Transfer (LIFT), combined with high speed laser micro-sintering are employed, as digital microfabrication tools for the demonstration of fully functional RFID antennas and fingerprint sensors based on highly viscous Ag and Cu nanoparticle inks. Having previously successfully demonstrated complex structures, this work's focus is on increasing the process throughput and yield by increasing the laser repetition rate (up to 40 kHz) and scanning speed (up to 2 m/s), without compromising reliability and resolution. In order to gain insight into the effects of the incremented repetition rate on the printing procedure, the latter was monitored in real time via a high-speed camera, able to acquire up to 540.000 fps, coupled to the setup. Examples of resulting structures comprise well-defined interdigitated and spiral micro-electrodes with post-sintering electrical resistivity lower than 5 x bulk Ag and 3 x bulk Cu. The aforementioned results validate the compatibility of laser based processing with the field of flexible RFID tags and OTFT based fingerprint sensors and foster the wider adoption of LIFT and laser micro-sintering technology for laboratory and industrial use.

Recent advances in flexible electronics have highlighted the importance of high throughput, digital additive microfabrication techniques. In this work, we demonstrate the combination of laser printing and laser sintering of a novel copper nanoparticle ink onto flexible substrates in order to produce oxide free conductive copper patterns in ambient atmospheric conditions. The printed patterns exhibit high reproducibility, very low resistivity (about 2x bulk), and negligible oxidation according to Raman spectroscopy. The process has been employed for the fabrication of an on-chip antenna on a flexible substrate for use in combination with a flexible circuit, in applications where a small form factor and simplicity of integration are required alongside ultra-low cost, e.g. consumable tagging. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Current technological trends in the field of microelectronics highlight the requirement to use cost-effective techniques for precise deposition of highly resolved features. Laser-induced forward transfer (LIFT) meets these requirements and is already applied for direct printing of electronic components. However, to improve the process' reproducibility and printing resolution, further research has to be conducted, regarding the rheological characteristics of the printable fluids and their jetting dynamics. Herein, a high-speed imaging setup is used to investigate the liquid jet's propagation during the printing process. Different Ag nanoparticle inks are studied and compared, over a wide range of viscosities and two different values of surface tension. The main focus of this investigation is the influence of the ink's rheological properties, both on the jet propagation and on the spatial and temporal evolution of the printed droplet during the wetting phase on three different receiver substrates (glass, SU-8, and gate dielectric). The results indicate that both the surface tension and the wetting properties of the receiver determine the shape of the printed droplet, whereas the inks' viscosity and laser fluence determine the printed volume.

The increasing development of flexible and printed electronics has fueled substantial advancements in selective laser sintering, which has been attracting interest over the past decade. Laser sintering of metal nanoparticle dispersions in particular (from low viscous inks to high viscous pastes) offers significant advantages with respect to more conventional thermal sintering or curing techniques. Apart from the obvious lateral selectivity, the use of short-pulsed and high repetition rate lasers minimizes the heat affected zone and offers unparalleled control over a digital process, enabling the processing of stacked and pre-structured layers on very sensitive polymeric substrates. In this work, the authors have conducted a systematic investigation of the laser sintering of micro-patterns comprising Ag nanoparticle high viscous inks: The effect of laser pulse width within the range of 20–200 nanoseconds (ns), a regime which many commercially available, high repetition rate lasers operate in, has been thoroughly investigated experimentally in order to define the optimal processing parameters for the fabrication of highly conductive Ag patterns on polymeric substrates. The in-depth temperature profiles resulting from the effect of laser pulses of varying pulse widths have been calculated using a numerical model relying on the finite element method, which has been fed with physical parameters extracted from optical and structural characterization. Electrical characterization of the resulting sintered micro-patterns has been benchmarked against the calculated temperature profiles, so that the resistivity can be associated with the maximal temperature value. This quantitative correlation offers the possibility to predict the optimal process window in future laser sintering experiments. The reported computational and experimental findings will foster the wider adoption of laser micro-sintering technology for laboratory and industrial use.

Current technological trends in the field of microelectronics have highlighted the requirement to use cost-effective techniques for the precise deposition of highly resolved features. Laser-induced forward transfer (LIFT) meets these requirements and has already been applied for the direct printing of devices and components. However, in order to improve the process' reproducibility and printing resolution, further research has to be conducted, regarding the rheological characteristics of the printable fluids and their jetting dynamics. In this work, we employ both pump-probe and high-speed imaging in order to investigate the formation and expansion of the liquid bubble, as well as the liquid jet's propagation. Newtonian as well as non-Newtonian fluids are studied and compared, over a wide range of viscosities. Furthermore, a computational model is utilized in order to gain more insight on the transfer mechanisms of the process. The simulation predictions are validated against experimental results, and found to be in good agreement, even in the case of non-Newtonian fluids. The results indicate that such accurate modelling can be developed as a new cost- and time-effective tool for the technique's optimization.

The large-scale synthesis of polyhedral Ag nanoparticles (NPs) described here follows a mediated-particle mechanism and the formulation of NPs in high volume fraction ink easily forms highly electrically conductive lines for printed electronics.