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The influence of gettering or defect passivation steps on recombination activity in the vertically grown, multicrystalline ribbon materials edge-defined film-fed growth and string ribbon silicon has been investigated with the help of photoconductance decay. In contrast to well-known results of integral measurements, spatially resolved lifetime mappings have been obtained by applying microwave detection technique.This aspect of spatial resolution has been found to be indispensable for investigating the impact of different processing steps on material quality in an accurate way. Apart from strong variations in as-grown lifetimes that have been found throughout vertically grown silicon wafers, this is due to areas of comparable starting lifetimes which have been revealed to react very differently to applied processing steps. After processing, some of them reach minority charge carrier lifetimes of more than 300 μsμs whereas others just show values of a few microseconds. As a consequence, the results of integral measurements strongly depend on the nature of areas incorporated in the specific sample. An impression of the corresponding uncertainties inherent to integral measurements has been obtained by statistical evaluation of spatially resolved lifetime data.

Tri-crystalline silicon material (Tri-Si) is a promising alternative to mono-crystalline CZ silicon because of a faster crystallization process and higher wafer stability. The orientation of the three crystals [110] is different to the [100] orientation in CZ material typically used in the photovoltaic industry. As a consequence the common alkaline texture technique using KOH or NaOH cannot be applied. For this reason we compared several alternative texturing methods like mechanical grooving and acidic texture etching, which work independently of crystal orientations. Solar cells were processed from these wafers, and an overview of their surface morphology as well as the resulting cell parameters is given. By applying an antireflection coating we reached the highest efficiencies reported on Tri-Si material.

Recently obtained results of a process monitoring based on spatially resolved lifetime measurements revealed that bulk lifetime values above 300 /spl mu/s can be reached within edge-defined film-fed growth (EFG) silicon ribbons with the help of gettering and hydrogen passivation steps. Therefore, recombination losses at the wafer backside have to be considered in this material with low as grown lifetimes. The solar cell processing sequence has been adapted to the needs of this material. Besides phosphorous gettering and remote hydrogen plasma passivation a screen printed back surface field has been implemented instead of an evaporated and subsequently alloyed thin Al BSF used for Al gettering. This allows to make use of low energy photons in regions with very high bulk lifetimes. In this way an independently confirmed solar cell efficiency of 16.7% has been obtained which is the highest value that has been reported so far. published

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