Reduced fill factors in multicrystalline silicon solar cells due to injection-level dependent bulk recombination lifetimes

Recombination lifetimes of multicrystalline silicon solar cell precursors have been measured experimentally as a function of injection-level, and modeled using Shockley-Read-Hall statistics. The expressions for the variable lifetimes are then used to predict the final cell open-circuit voltages and fill factors using a simple analytic method. When accurate recombination lifetimes measurements are possible, the predicted parameters match well with the measured values on finished cells. The cells are shown to be limited by the presence of bulk recombination, which not only limits the open-circuit voltage through lower lifetimes, but also reduces the fill factor due to a strong injection-level dependence around one-sun maximum-power conditions. It is shown that such non-ideal behaviour cannot be adequately explained by junction recombination. The specific effect of interstitial iron, an important impurity in silicon, on voltages and fill factors is modeled numerically and discussed.