Morphological Characteristics and Printing Mechanisms of Grid Lines by Laser-Induced Forward Transfer

Abstract

Laser-induced forward transfer (LIFT) is an innovative metallization technique used in the processing of grid lines of solar cells for the photovoltaics industry. A study on the morphology and transfer mechanisms of formed lines with high-viscosity silver paste and small gap was performed in this paper. There were three different transfer states under different laser fluences: non-transferred lines or transferred but no continuous lines, continuous transferred lines, and explosive transferred lines. There was a critical transfer threshold for the continuous line transfer under different processing speeds. Higher processing speed required a larger critical transfer threshold. The line width increased as the laser fluence increased. For all continuous formed lines, the cross-sectional morphologies with single and double peaks were shown at critical and above transfer threshold, respectively. Two symmetrical protrusions with steep edges were observed for the formed line with double peaks. By comparing the silver paste remaining on the donor and transferred to the acceptor under different laser fluences, it can be found the transferred silver paste exhibited a retracting characteristic under the critical and above transfer threshold. While a stretching characteristic was obvious when the laser fluence was much higher than the transfer threshold. Morphological characteristics with single or double peaks were determined by the distance between the rupture position of the bridge and the bottom of the bubble, under the action of the axial combined forces. This work can provide insights for improving fine-line metallization and understanding transfer mechanisms in the photovoltaic application and flexible electronics devices.

Characterization of Grid Lines Formed by Laser-Induced Forward Transfer and Effect of Laser Fluence on the Silver Paste Transformation

“…The transfer process of silver paste and the morphology, as well as the quality of the formed grid line, will be strongly affected by the specific rheology of the silver paste used [16], making the transfer mechanism different from that previously observed in conductive inks or nano pastes [17]. Continuous lines with high aspect ratios have been printed at high speeds [18]; however, in this research, the influence of the interaction on adjacent pulses is not considered.…”