SunPower manufactures high-efficiency rear-contact solar cells. To offer these cells at a competitive price, SunPower requires a source of low-cost wafers with the necessary lifetime, thickness and resistivity to attain a cell efficiency of at least 20%. With simulation and experiment, this paper investigates how each of these parameters affects the efficiency of SunPower's low-cost, rear-contact solar cell. It concludes that the cell efficiency approaches an optimum when the wafers have (i) a lifetime in excess o 1 ms, (ii) a thickness of 160-280 /spl mu/m, and (iii) a resistivity of 2-10 /spl Omega/cm (n-type). The requirement of the high lifetime restricts the wafer choice to FZ, PV-FZ or n-type Cz; but the wide tolerance on wafer thickness and resistivity help lower the wafer cost by maximizing the yield from a silicon ingot.