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.

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

“…Extensive data on the macroscopic distribution of iron and its effect on performance in these samples have already been published. 12,42,43 The samples were selected from a relative ingot height of 29%, 61%, 88% and 30%, 68%, and 85% for the 53 and 200 ppmw Fe ingots, respectively. The wafers from these heights will be subsequently referred to as "Bottom," "Middle," and "Top."…”

“…Interpolating the neutron activation analysis (NAA) data from the CrystalClear project, 42,43 the iron concentrations for the samples are estimated to be: 3:3 Â 10 13 ; 3 Â 10 13 ; 6 Â 10 14 for the 53 ppmw ingot and Three sister wafers were pulled out of the processing line at each of the three heights in both ingots: one as-cut, one after gettering, and one following SiN x deposition and firing. 12 The phosphorus diffusion process was a standard industrial process conducted in a POCl 3 tube furnace. For the hydrogenation and firing step, the same temperature profile was applied as for the metallization firing, but no metals were present so as to avoid contamination effects.…”

“…3,4 Iron, in particular, limits the bulk minority carrier lifetime of most as-grown p-type silicon wafers, 5,6 in part because of its large electron capture cross section, 7,8 but also because of its inevitable presence in feedstocks, 9 crystal growth crucibles and their linings, 10 and throughout the industrial growth environment. 11 Recently, several authors have investigated the macroscopic device-level effects of iron contamination, [12][13][14] updating foundational studies of metal contamination in silicon solar cells (e.g., the Westinghouse study of Davis, Jr. et al…”

Building intuition of iron evolution during solar cell processing through analysis of different process models

“…In «-type Si devices, phosphorus can be used to getter impurities [36,38,[76][77][78] and to form a front (or rear) surface field layer [79,80]. …”

Synchrotron-based investigation of transition-metal getterability in n-type multicrystalline silicon

“…One advantage is that some common metal point defects, notably interstitial iron, are less recombination active in n-type than in p-type Si. [1][2][3] It has been shown that phosphorus diffusion gettering (PDG) can increase the lifetime of n-type mc-Si, including in low-lifetime ingot border regions, [4][5][6][7] and that industrially relevant efficiencies are achievable. 8 For p-type mc-Si, simulation of the redistribution of metal impurities during PDG and the resulting lifetime impact has enabled development of PDG processes that improve yield and extract higher performance, especially in border regions and the top of the ingot.…”