Theoretical calculations reveal that the quality of an aluminum -back-surface field (BSF) in a silicon solar cell can be improved by either increasing the thickness of the deposited aluminum (Al), peak alloying temperature, or both. However, this study shows that there is a critical temperature for a given screen-printed Al thickness, above which the BSF quality begins to degrade because of nonuniformity triggered by the agglomeration of Al-Si melt in combination with the bandgap narrowing resulting from the high doping effect in the agglomerated regions. It is found that this critical temperature decreases with the increase in the thickness of the deposited Al layer and, therefore, limits the quality and thickness of the Al-BSF that can be achieved before degradation sets in. This nonuniformity of Al-BSF is observed in the form of scattered Al bumps with thick and thin BSF regions. A combination of experimental results and model calculations is used to provide improved understanding and guidelines for choosing the optimal combination of Al thickness and alloying temperature. (c) 2005 The Electrochemical Society. [DOI: 10.1149/ 1.2129106] All rights reserved.