Organic vehicle engineering for high-aspect-ratio front-side fine-line silver paste in TOPCon solar cells

Fine-line screen printing of front-side Ag paste is essential for reducing shading loss and silver consumption in TOPCon silicon solar cells, but the process remains strongly constrained by the organic vehicle. Poorly matched solvent volatilization, binder-induced viscosity, and thixotropic recovery can cause particle agglomeration, line broadening, intermittent finger breakage, and rough sintered electrodes. In this work, a process-oriented organic vehicle design strategy is proposed for high-aspect-ratio fine-line Ag paste. A mixed-solvent system was constructed by considering volatility, binder solubility, and wetting compatibility; ethyl cellulose (EC) was used to tune the viscosity window; and polyamide wax was introduced to regulate post-printing shape retention. The rheological response, printed line morphology, sintered cross-section, series resistance, fill factor, and photoelectric conversion efficiency (PCE) were systematically correlated. At 6 wt% EC, the paste showed a balanced flow-recovery behavior, enabling continuous fine lines close to the 13 μm stencil opening and increasing the PCE to 26.41%. Further optimization with 6 wt% polyamide wax improved shape retention and produced a favorable height-to-width ratio without severe surface roughening, giving the highest PCE of 26.58%. The results indicate that the key to TOPCon fine-line metallization is not simply increasing viscosity, but matching shear-induced flow during printing with rapid but controllable recovery after printing. This study provides a practical organic vehicle design window for reducing Ag consumption and improving the metallization quality of high-efficiency TOPCon cells.