Doping and oxygen dependence of efficiency of EFG silicon solar cells

The doping dependence of polycrystalline EFG silicon cell efficiency differs markedly from single-crystal float zone (FZ) cells. The optimum resistivity for EFG cells is decisively higher than 0.2-0.3 Omega -cm, the absolute efficiency is lower than the counterpart single-crystal cells, and a broad maximum in efficiency is observed in the range of 1 to 5 Omega -cm, as opposed to a sharp maximum at 0.2-0.3 Omega -cm for single-crystal FZ cells. It is shown by model calculations that all three effects can be explained on the basis of lifetime-limiting discrete trap levels whose activity increases with doping concentration. The addition of a small amount of oxygen (>