Date | Time
13/11/2015 | 11 h 00 min - 18 h 00 min
Salle du Conseil – IJL – Saurupt
Séminaire organisé par par le LabEx Damas et par l’équipe Solidification de l’IJL
Grain structure, iron precipitation and minority carrier lifetime in multicrystalline silicon
présenté par Mohammed M’Hamdi (Directeur de recherche, SINTEF Materials and Chemistry, Oslo et Professeur adjoint, NTNU, Trondheim, Norvège)
Multicrystalline silicon is, together with monocrystalline silicon, the most widely used material in the photovoltaic industry. It contains a high density of extended defects, i.e. grain boundaries and dislocations, and a wide range of impurities, affecting the output solar cell performance. Iron has been identified as one of the most detrimental impurities in multicrystalline silicon, and is found in relatively high concentration in ingots, originating from the crucible, its coating and the silicon feedstock. Iron is present in multicrystalline materials in interstitial state or in the form of metal silicide nano-precipitates, mainly identified as FeSi2. Previous studies have shown that a large majority of iron present in as-grown multicrystalline materials is precipitated. Iron precipitation has a positive impact on the as-grown wafer quality, as the recombination activity of a precipitate per iron atom is generally considered being lower than the recombination activity of an isolated interstitial iron atom. Iron precipitation at extended defects is, however, a strong limitation to the phosphorous gettering efficacy, as only the mobile dissolved iron atoms have the ability to segregate towards the emitter. Due to its relatively high solid diffusivity, iron precipitates mostly at extended defects. Those defects present favorable precipitation sites and act as internal gettering sites during the ingot cooling. Recent improvements in the silicon growth technology have led to the solidification of multicrystalline silicon ingots of higher quality, i.e. with lower densities of dislocation clusters. This newly developed material is commonly referred as high performance multicrystalline silicon (HPMC-Si), and presents a smaller grain size and a higher proportion of random angle grain boundaries compared to conventional multicrystalline silicon. The precipitation behavior of iron is affected by the structure evolution of multicrystalline silicon, and it is of major interest to evaluate and predict the influence of each type of extended defect. This study presents an examination of the spatial occurrence of iron precipitation during the cooling of an HPMC-Si ingot, and aims at investigating using numerical simulation the separate effects of grain-boundaries, sparse intra-granular dislocations, and dislocation clusters and their impact on the minority carrier lifetime.