Effective iron gettering in lightly-doped emitters

The goals of phosphorus diffusion in a multicrystalline silicon solar cell process are often contradictory. While high concentrations of phosphorus near the front surface are known to decrease blue response, a heavier diffusion generally leads to improved gettering of lifetime-killing iron impurities. To investigate the tradeoffs involved in selection of time-temperature profiles for lightly-diffused emitters, like those in a selective emitter formation, we use a coupled diffusion-segregation kinetics simulator to model the behavior of iron and phosphorus during phosphorus diffusion gettering. We propose novel approaches for mitigating the impact of high iron concentrations using shallow emitters. Firstly, our simulations indicate that lifetimes can be higher if higher-temperature processes are employed, since the rates of iron precipitate dissolution and iron point-defect diffusion are faster. (An additional benefit of higher-temperature processing, is a shorter annealing cycle time, i.e., higher throughput.) We assess the possible trade-offs of higher-temperature processing, including decreased emitter sheet resistance. However, we also show that for a fixed total process time and peak temperature, the sheet resistance of the emitter is a poor indicator of final lifetime. Instead, the final lifetime is a function of the fraction of time spent at high temperature (versus fraction spent cooling) and the shape of the cooling profile. Lastly, we show that different iron contamination levels demand different processes to maximize the processed lifetime.