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 Ω-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 Ω-cm, as opposed to a sharp maximum at 0.2-0.3 Ω-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 (⩽5×10¹⁷ cm^-3) is found to improve the EFG cell performance and change the doping dependence such that the optimum resistivity shifts to a slightly higher value and the efficiency becomes nearly flat in the 1-5 Ω-cm range. J-V- T measurements revealed that oxygen passivates a 0.35 eV trap and exposes a 0.54 eV trap