Modeling the effect of trap levels on the optimum resistivity of silicon solar cells

Numerical studies were performed to determine the effect of different trap levels on the optimum resistivity required to produce high-efficiency solar cells from materials of varying initial quality. Lifetime decreases with the doping concentration due to Auger, band-to-band, and SRH (Shockley-Read-Hall) recombinations and shallow traps, make the SRH lifetime more sensitive to the doping density for the same initial lifetime in the undoped material. It was found that the optimum resistivity for high-efficiency cells depends on several factors including the initial lifetime, the trap level, the relationship between the trap and doping density and the injection level in the cell. For low initial lifetimes, around 10 mu s, the optimum resistivity is approximately 0.2 Omega -cm, regardless of the deep level location. In high-lifetime materials (10 ms), high injection level drives the optimum choice to high resistivity, 200 Omega -cm, regardless of the deep level location. For intermediate starting lifetimes (1000 mu s, 1 ms), location of the trap level can switch the optimum choice from low resistivity to high resistivity. It is shown that low resistivity is preferred for midgap deep levels, but high resistivity becomes desirable for shallow traps.