Impact of iron precipitates on carrier lifetime in as-grown and phosphorus-gettered multicrystalline silicon wafers in model and experiment

The impact of iron point defects on the measured injection-dependent minority carrier lifetime in silicon after different processing steps (described by the Shockley-Read-Hall equation) is well known. However, in some parts of multicrystalline silicon (mc-Si) (used for solar cells), a large share of the iron atoms is precipitated. In this study, we simulate realistic iron precipitate distributions in mc-Si after crystallization, as well as after phosphorus diffusion gettering within grains by employing the Fokker-Planck equations. Taking the Schottky contact between metallic precipitates and semiconductor into account, in a second step, we analyze the effect of recombination at iron precipitates on carrier lifetime by means of finite-element simulations. The results are compared with experimental injection-dependent lifetime measurements on a p-type mc-Si wafer before and after phosphorus diffusion. Our simulations show that in the low-lifetime edge region close to the crucible wall, a considerable share of the carrier recombination can be attributed to iron precipitates in both the as-grown and in the phosphorus-diffused state. In addition, the simulated injection dependences of iron precipitates and iron interstitials differ significantly, with the precipitates influencing the carrier lifetime especially in the mid- to high-injection range, which is supported by carrier lifetime measurements.