Effect of iron in silicon feedstock on p- and n-type multicrystalline silicon solar cells

Abstract

The effect of iron contamination in multicrystalline silicon ingots for solar cells has been investigated. Intentionally contaminated p- and n-type multicrystalline silicon ingots were grown by adding 53 ppm by weight of iron in the silicon feedstock. They are compared to reference ingots produced from nonintentionally contaminated silicon feedstock. p-type and n-type solar cell processes were applied to wafers sliced from these ingots. The as-grown minority carrier lifetime in the iron doped ingots is about 1–2 and 6–20 μs for p and n types, respectively. After phosphorus diffusion and hydrogenation this lifetime is improved up to 50 times in the p-type ingot, and about five times in the n-type ingot. After boron/phosphorus codiffusion and hydrogenation the improvement is about ten times for the p-type ingot and about four times for the n-type ingot. The as-grown interstitial iron concentration in the p-type iron doped ingot is on the order of 1013 cm−3, representing about 10% of the total iron concentration in the ingot, and is reduced to below 1011 cm−3 after phosphorus diffusion and subsequent hydrogenation. The concentration of interstitial iron after boron/phosphorus codiffusion and hydrogenation is about 1012 cm−3, pointing out the reduced gettering effectiveness of boron/phosphorus codiffusion. The effect of the iron contamination on solar cells level is a decrease in the diffusion length in the top half of the ingots with a trend in agreement with Scheil’s model for segregation. This is, however, not the only impact of the iron. An increased crystal defect concentration in the top and bottom of the Fe doped ingots, compared to the reference ingots, is observed, which contributes considerably to the degradation of the solar cell performance.