Sacrificial high-temperature phosphorus diffusion gettering process for lifetime improvement of multicrystalline silicon wafers

Iron is among the most deleterious lifetime-limiting impurities in crystalline silicon solar cells. In as-grown material, iron is present in precipitates and as point defects. To achieve solar cell conversion efficiencies above 20%, bulk minority-carrier lifetimes in excess of 300 μs (p-type) and 900 μs (n-type) are required [1]. For cost-effective multi-crystalline silicon wafers, achieving this lifetime requires gettering. Gettering at higher temperatures for longer times is often necessary to fully dissolve and remove precipitated impurities. However, such time-temperature profiles can result in unacceptably deep emitters, affecting the blue response of the finished device. Here, we explore a “sacrificial” gettering step in which gettering and emitter-formation are decoupled and optimized independently. The optimization is guided by the Impurity-to-Efficiency simulation tool [2] and explores high-temperature regimes. While models predict that increasing the gettering temperature decreases total iron concentration resulting in an increased lifetime, experimental results show that for the highest temperatures tested, the minority carrier lifetime is reduced.