Laser-Induced Forward Transfer of High Viscosity Graphene Inks

Laser-induced forward transfer (LIFT) is gaining significant attention as a non-contact printing technique for high-viscosity conductive inks in printed electronics. However, the high wet thickness of printed tracks is essential for achieving effective electrical pathways, a requirement that has not been thoroughly considered so far. The wet thickness is a function of ink viscosity, substrate wettability, and the laser processing parameters. In this study, the printing mechanism of conductive graphene inks with viscosities ranging from 1 to 15 Pa.s using LIFT was investigated. The effects of pulse energy (30 to 120 µJ) and gap distance (50 to 300 µm) in printing voxels with a green nanosecond laser were systematically examined, providing a phenomenological understanding of the material transfer mechanism. The findings highlight the significant role of the temporal pulse distance in enhancing the wet thickness achievable during LIFT of high-viscosity inks, attributed to capillary healing phenomena. Additionally, the acceptor substrates' hydrophobicity was found to increase the wet thickness and improve the resolution of the printed voxels/tracks. Especially, the aspect ratio of LIFT-printed tracks was increased by more than 175% with 10 printing passes when a hydrophobic accepter was used. So, the optimal LIFT processing conditions were identified to achieve high-quality, high-aspect-ratio tracks, by considering synergistically the effects of the temporal pulse distance and the substrate wettability. Moreover, the resistivity of the LIFT-printed graphene tracks decreased by more than 84% after a 100-minute sintering step at 120°C. This research advances understanding of LIFT printing high-viscosity conductive inks, particularly underpinning the development of high-resolution and high-aspect-ratio electrical circuits for printed electronics.