Solar Cell Emitters Fabricated by Flash Lamp Millisecond Annealing

Abstract

Phosphorus ion implantation was used for the emitter formation in monoand multicrystalline silicon solar cells. After ion implantation the silicon is strongly disordered or amorphous within the ion range. Therefore subsequent annealing is required to remove the implantation damage and activate the doping element. Flash-lamp annealing offers here an alternative route for the emitter formation at overall low thermal budget. During flash-lamp annealing, only the wafer surface is heated homogeneously to very high temperatures at ms time scales, resulting in annealing of the implantation damage and electrical activation of phosphorus. However, variation of the pulse time also allows to modify the degree of annealing of the bulk region to some extent as well, which can have an influence on the gettering behaviour of metallic bulk impurities. The μ-Raman spectroscopy showed that the silicon surface is amorphous after ion implantation. It could be demonstrated that flash-lamp annealing at 800 ◦C for 20 ms even without preheating is sufficient to recrystallize implanted silicon. The highest carrier concentration and efficiency as well as the lowest resistivity were obtained after annealing at 1200 ◦C for 20 ms both for monoand multicrystalline silicon wafers. Photoluminescence results point towards P-cluster formation at high annealing temperatures which affects metal impurity gettering within the emitter.