Efficacy of Phosphorus Gettering and Hydrogenation in Multicrystalline Silicon

The emitter formation step (POCl3 diffusion) in p-type crystalline silicon solar cell processing includes many variables, e.g., peak temperature, gas flows, temperature ramps, which can be optimized in order to improve material quality. Diffusion parameters of an 80-Ω/L1 emitter are varied, and the resulting change in electronic quality of multicrystalline silicon is analyzed. A detailed gettering analysis of multicrystalline material, surface passivated with hydrogen-rich amorphous silicon, after POCl₃ diffusion, and an additional gettering step combined with hydrogenation from SiN_x:H is presented. The industrial-type diffusion leads to material of lower electronic quality than the extended reference diffusion. A major finding of this paper is the fact that results on different 5 × 5 cm² samples out of one 15.6 × 15.6 cm² wafer can vary significantly. Hence, conclusions about which diffusion is most efficient in gettering strongly depend on wafer position. An edge position close to crucible walls, for example, might improve less effectively than another position close to the crucible center. In fact, the opposite can also be shown, and samples originating from edge regions reach their highest lifetimes after gettering. This is explained by the different defect structure of the investigated samples. Structures exhibiting high gettering efficacy contain fewer recombination active grain boundaries and are predominantly free of extended defect clusters.