The impact of the Ag particle (metal powder in the screen printed paste) size on the quality of Ag thick-film ohmic contacts to high-sheet-resistance emitters of Si solar cells is investigated. Spherical particle size was varied in the range of 0.10-10 mu m (ultrafine to large). Even though ultrathin glass regions are achieved for the large particle paste, giving low specific contact resistance (rho(c)), secondary ion mass spectroscopy measurements showed a higher Ag concentration (> 10(15) cm(-3)) at the p-n junction that increased the junction leakage current (J(o2)) and decreased the V-oc by similar to 7 mV and fill factor (FF) by similar to 0.02. Pastes with ultrafine Ag particles generally produced a thick glass layer at the Ag-Si contact interface, which increased rho(c) and series resistance (R-s) (>= 1 Omega cm(2)), and lowered the FF by similar to 0.03. Small to medium size Ag particles in the paste produced thin glass regions and many regularly distributed Ag crystallites at the contact interface. This resulted in low R-s (< 1 Omega cm(2)), high shunt resistance (60,558 Omega cm(2)), low J(o2) (similar to 20 nA/cm(2)), and high FF (0.781). Cell efficiencies of similar to 17.4% were achieved on untextured float zone Si with 100 Omega/square emitter by rapid firing of screen printed contacts in a lamp-heated belt furnace. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2126579] All rights reserved.