Sigma files provisional patent for metal 3D printing

Abstract

Conventional silver paste for crystalline silicon solar cells exhibits low cell efficiency when deposited by non-contact electrohydrodynamic jet printing because of either high volumetric shrinkage or low packing density when there is no printing pressure applied to the silver paste. Therefore, we develop bimodally dispersed silver paste for electrohydrodynamic jet printing to resolve both volumetric shrinkage and packing density of the conventional silver paste, and its electrical and rheological traits are investigated. The bimodally dispersed silver paste exhibits lower unit-line resistance, which is inversely proportional to the weight ratio of small-to-large silver particles due to the increased packing density, and higher contact resistivity above a certain weight ratio for small-to-large silver particles. This behaviour results from the obstructed melt flow of glass frit by the early coarsened and densified small silver particles at an elevated temperature. The increased weight ratio of the small particles also raises the viscosity of the bimodally dispersed silver paste above the point where electrohydrodynamic jet printing is possible. By employing a binary solvent mixture and metallorganic silver as a viscosity reducing agent, the bimodally dispersed silver paste is tuned for electrohydrodynamic jet printing, and the front-side metallization of a polycrystalline silicon solar cell with the emitter sheet resistance of 60 Ω/sq is constructed. With the abnormally high aspect ratio of silver electrodes at 0.86, a cell efficiency of 16.72% is achieved, which is higher than that of screen-printed cells with the similar emitter sheet resistance by +0.22–0.52%p.