Metal Content of Multicrystalline Silicon for Solar Cells and its Impact on Minority Carrier Diffusion Length

Abstract

The metal content of three types of silicon material for cost-efficient solar cells, Astropower silicon-film sheet material, Baysix cast material, and EFG ribbon-grown multicrystalline silicon was determined using instrumental neutron activation analysis. Iron, nickel, and chromium were found in concentrations between 10 12 and 1.8´10 15 cm -3 , depending on the material. The concentration of cobalt, molybdenum, and copper was between 10 12 and 5´10 13 cm -3 . Since the minority carrier diffusion length in all three mc-Si materials was much higher than one would expect if all metals were dissolved in an interstitial or substitutional state, we concluded that the metals most likely formed clusters or precipitates with a lower recombination activity than the interstitial/substitutional metals. No significant difference was observed between the metal content of the high and low lifetime areas of each material.

Analysis of copper-rich precipitates in silicon: Chemical state, gettering, and impact on multicrystalline silicon solar cell material

“…19 Multicrystalline silicon (mc-Si), the cost-effective material out of which an increasing 50% of solar cell modules are currently produced 20 , typically contains high transition metal concentrations combined with a high density and variety of structural defects. Not surprisingly, copper-rich particles have been observed at structural defects in poorly-performing regions of mc-Si solar cell material [21][22][23][24][25] , complementing neutron activation analysis (NAA) data reporting Cu concentrations in mc-Si as high as 10 13 cm -3 . 26 While Cu-rich clusters are undoubtedly not the only type of defect responsible for reducing the efficiencies of mc-Si solar cells, their known recombination activity and repeated observation in poorly-performing regions indicate they most certainly can be a contributing factor.…”

“…Cu 3 Si clusters, or individual Cu atoms, may be stabilized either by the lattice strains of adjacent structural defects or other metal clusters. 21,70 Specifically, the chemical interactions between Cu and other metal species (of which mc-Si contains an abundance 24,26 ) are not well understood at the present time. The singular result of all these effects would likely be a somewhat lower effective segregation coefficient for mc-Si than that for singlecrystalline silicon.…”

“…This is consistent with numerous µ-XRF observations of Curich clusters at structural defects in Al-gettered or fully processed mc-Si. 21,[23][24][25] Irrespective of the precise value of k, it follows from Eq. 2 that thicker Al layers should result in the removal of a larger fraction of the total Cu from the bulk during gettering.…”

External and Internal Gettering of Interstitial Iron in Silicon for Solar Cells

“…It is especially important in multicrystalline silicon wafers, in which it occurs primarily due to contamination from the crucible during ingot growth [1,2]. Typical concentrations of Fe in directionally-solidified multicrystalline ingots are 10 13 -10 14 cm -3 in the central regions, and up to 10 15 cm -3 near the bottom and top of the ingots [3,4], due to impurity segregation and solid-state indiffusion of Fe from the crucible walls.…”

Effects of high temperature annealing on the dislocation density and electrical properties of upgraded metallurgical grade multicrystalline silicon

“…When metal impurities conglomerate to form large precipitates at grain boundaries and dislocations in Si wafers, the precipitated impurities usually have low recombination activity. In contrast, the recombination activity of the metal impurities would be high enough to reduce the minority carrier diffusion length to less than 1 μm if the impurities with high concentrations were in interstitial or substitutional sites [13][14][15]. It can be concluded that the properties of Si wafers and the resulting performance of solar cells not only depends on the total concentrations of impurities, but also on the structure, chemical state and spatial distribution of the impurities in the Si wafers.…”