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EBIC Investigation of a 3-Dimensional Network of Inversion Channels in Solar Cells on Silicon Ribbons

Ribbon growth on substrate (RGS) is a novel technique to produce thin sheets of silicon for solar cells with high throughput directly from the melt without a slicing step. By this kind of process the amount of impurities incorporated and the temperature gradients during the solidification are remarkable higher then e.g. in a block casting process. Solar cells produced on RGS material can reach sufficient efficiencies above 12 %, but show unusual properties. The short circuit current may reach exceptionally high values beyond 30 mA/cm2, which are usually only observed in monocrystalline silicon solar cells. The fill factor and the open circuit voltage, however, are lower than expected. The dark current-voltage (I-V) characteristic of such solar cells shows ideality factors (giving the steepness of the forward IV-characteristic) well above two, in disagreement with the theory for a simple emitter-base diode. These unusual properties are caused by the existence of a 3-dimensional network of inversion channels formed by densely packed precipitates around dislocations. These channels are able to collect minority carriers in the bulk of the solar cells and lead them to the pn junction. Thus, in spite of the low diffusion length, the collecting volume is drastically enhanced. It is possible to visualize and investigate the inversion channels by EBIC (electron beam induced current) measurements using special sample geometries. Under forward bias these channels may act as preferred injection sites. A physical model for the inversion channels is proposed explaining for the first time ideality factors above two over a large bias range.