Sponge LID - A new degradation mechanism?

High performance multi wafers are a new wafer class introduced some years ago. These wafers are characterized by creating rather small grains at the start of crystallization leading to a reduced density of structural defects and an increase of up to 0.5 % in solar cell efficiency. During the evaluation of high performance multi wafers, we observed for some wafer types very high light-induced degradation of up to 10 % relative in cell efficiency. More detailed investigations revealed that the affected wafers stem from the “bottom” part of the ingots with small grain size and the degraded cells show a “sponge-like” structure in electroluminescence images. The investigation of regeneration characteristic revealed that this degradation is not caused by formation of boron oxygen complexes or splitting of iron boron pairs. Although we could reduce this degradation mechanism by adapting the solar cell manufacturing process, avoiding this effect on wafer / crystallization level will be needed for future cell efficiency improvements.