Papers by Author: Tonio Buonassisi

Abstract: In multicrystalline silicon for photovoltaic applications, high concentrations of iron are usually found, which deteriorate material performance. Due to the limited solubility of iron in silicon, only a small fraction of the total iron concentration is present as interstitial solute atoms while the vast majority is present as iron silicide precipates. The concentration of iron interstitials can be effectively reduced during phosphorus diffusion gettering (PDG), but this strongly depends on the size and density of iron precipitates, which partly dissolve during high-temperature processing. The distribution of precipitated iron varies along the height of a mc-Si ingot and is not significantly reduced during standard PDG steps. However, the removal of both iron interstitials and precipitates can be enhanced by controlling their kinetics through carefully engineered time-temperature profiles, guided by simulations.

Abstract: Light microscopy, electron backscatter diffraction and transmission electron microscopy is employed to investigate dislocation structure and impurity precipitation in commonly occurring dislocation clusters as observed on defect-etched directionally solidified multicrystalline silicon wafers. The investigation shows that poligonised structures consist of parallel mostly similar, straight, well-ordered dislocations, with minimal contact-interaction and no evidence of precipitate decoration. On the other hand, disordered structures consist of various dislocation types, with interactions being common. Decoration of dislocations by second phase particles is observed in some cases. Enhanced recombination activity of dislocations may therefore be a result of dislocation interaction forming tangles, microscopic kinks and jogs, which can serve as heterogeneous nucleation sites that enhance metallic decoration.

Abstract: The evolution of Fe-related defects is simulated for di erent P di usion gettering (PDG) processes which are applied during silicon solar cell processing. It is shown that the introduction of an extended PDG is bene cial for some as-grown Si materials but not essential for all of them. For mc-Si wafers with an as-grown Fe concentration 14 cm³, a good reduction of the Fe_i concentration and increase of the electron lifetime is achieved during standard PDG. For mc-Si wafers with a higher as-grown Fe concentration the introduction of defect engineering tools into the solar cell process seems to be advantageous. From comparison of standard PDG with extended PDG it is concluded that the latter leads to a stronger reduction of highly recombination active Fe_i atoms due to an enhanced segregation gettering e ect. For an as-grown Fe concentration between 10¹⁴ cm³ and 10¹⁵ cm³, this enhanced Fe_i reduction results in an appreciable increase in the electron lifetime. However, for an as-grown Fe concentration >10¹⁵ cm³, the PDG process needs to be optimized in order to reduce the total Fe concentration within the wafer as the electron lifetime after extended PDG keeps being limited by recombination at precipitated Fe.

Abstract: Dislocations are known to be among the most deleterious performance-limiting defects in multicrystalline silicon (mc-Si) based solar cells. In this work, we propose a method to remove dislocations based on a high temperature treatment. Dislocation density reductions of >95% are achieved in commercial ribbon silicon with a double-sided silicon nitride coating via high temperature annealing under ambient conditions. The dislocation density reduction follows temperature-dependent and time-dependent models developed by Kuhlmann et al. for the annealing of dislocations in face-centered cubic metals. It is believed that higher annealing temperatures (>1170°C) allow dislocation movement unconstrained by crystallographic glide planes, leading to pairwise dislocation annihilation within minutes.

Abstract: We present a comprehensive description of synchrotron-based analytical microprobe techniques used to locally measure the diffusion length and chemical character of metal clusters in multicrystalline silicon (mc-Si) solar cell material. The techniques discussed are (a) X-ray fluorescence microscopy, capable of determining the spatial distribution, elemental nature, size, morphology, and depth of metal-rich particles as small as 30 nm in diameter; (b) X-ray absorption microspectroscopy, capable of determining the chemical states of these metal-rich precipitates, (c) X-ray beam induced current (XBIC), which maps the minority carrier recombination activity, and (d) Spectrally-resolved XBIC, which maps the minority carrier diffusion length. Sensitivity limits, optimal synchrotron characteristics, and experimental flowcharts are discussed. These techniques have elucidated the nature and effects of metal-rich particles in mc-Si and the physical mechanisms limiting metal gettering from mc-Si, and have opened several promising new research directions.