For crystalline silicon solar cells having an alloyed aluminum back contact, open-circuit voltage increases with aluminum thickness. The increase is ascribed to the formation of a p+/p "high-low" junction by liquid-phase epitaxy, resulting in increased efficiency for the cell. With thick aluminum layers, the characteristics of the p+ regrown region are in reasonable agreement with predictions based on the Al-Si phase diagram. With thin aluminum layers the junction is nonideal, and the improvement in V(oc) is much smaller. Pile-up of metallic impurities at the aluminum/p+ interface is attributed to rejection of these impurities during the silicon regrowth process.