Infrared Transmission Investigations of Rod - Like Defects in Multicrystalline Silicon

Abstract

Parts from a specially grown multicrystalline test ingot showed diffuse absorption regions in the IR transmission. The underlying defects were investigated with IR microscopy and FTIR spectroscopy. The typical features that can be observed in the IR microscope images of wafers from the diffuse absorption regions are long filament-like defects aligned in bundles and having a preferential orientation along the crystallization direction. In FTIR spectroscopy broad absorption bands in the wavenumber range between 750 and 1050 cm -1 can be found in specimens from the same diffuse absorption regions. EDS measurements with a scanning electron microscope yield an increased nitrogen concentration. It is thus suggested that the underlying defects are silicon nitride particles that can grow under certain growth conditions. The preferential orientation indicates that these rod-like defects grow at the liquid-solid interface from the melt during crystallization.

Investigation of Defects in the Edge Region of Multicrystalline Solar Silicon Ingots

“…For the NN profile it was found that in spite of the small segregation coefficient of 7x10 -4 the maximum of the concentration is measured near 2/3 of the height of the ingot. Si 3 N 4 precipitates were found here, in accordance with another report for a similar test crystal [4].…”

Growth of carbon and nitrogen containing precipitates in crystalline solar silicon and their influence on solar cells

“…In the mid 2000s (just during the shortage period of Si feedstock) many findings about precipitates in mc‐Si for solar cells have been made. Transmission infrared imaging and microscopy helped to identify particles within this material , which can be done at sawn wafers, but best image quality is obtained with polished wafers . These findings helped to identify the contaminated parts in ingots and avoid their use for solar cell production.…”

“…Furthermore, Si feedstock is oversupplied these times, hence solar cells manufacturer are not forced to use low quality material and also requirements to impurity contents improved with respect to higher solar cell and module efficiencies. After IR inspection of the bricks before sawing, only the parts of the bricks being free of precipitates are sawed to wafers . Nevertheless it is important to share the knowledge about these precipitates to hold the material quality as high as possible, which is even more important for the cell concepts emerging on the markets today such as mc‐Si PERC (passivated emitter and rear) and n‐type PERC cells.…”

Crystalline silicon for photovoltaics

“…Because of the much higher diffusion coefficient in the melt compared to the solid large precipitates can grow. In fact, large precipitates of Si 3 N 4 and SiC have been observed in directionally solidified multicrystalline silicon [12][13][14]. They mainly occur in form of long filaments, needles or tubes that run in growth direction over several mm, see Fig.…”