The origin of ideality factors n > 2 of shunts and surfaces in the dark I-V curves of Si solar cells
So far, a general model for the explanation of non-linear shunts and edge currents, often showing ideality factors n> 2, has been missing. Non-linear shunts like scratches and edge currents are the major source of the recombination current of industrial crystalline silicon solar cells. Moreover, the reverse current of such cells behaves always linear or even superlinear instead of saturation-type, as expected. We propose that the edge recombination current and other non-linear shunts can be described as regions that are highly disturbed, even amorphous, crossing the pn-junction. The recombination within such regions needs model descriptions that go beyond the SRH approximation, because the density of defects is so high that carriers recombine via more than one defect (so-called multi-level recombination). Two variants of coupled defect level recombination, both leading to idealiy factors larger than 2, are realistically modelled by Sentaurus simulations. Under reverse bias, the high local density of states in the defective material enables hopping conduction, which explains the linear reverse characteristics.