Explanation of commonly observed shunt currents in c-Si solar cells by means of recombination statistics beyond the Shockley-Read-Hall approximation

The current-voltage (I-V) characteristics of industrially fabricated, crystalline silicon solar cells are often influenced by non-linear shunts that originate from localized, highly disturbed regions and cause ideality factors n >2. We show that recombination within such locations needs model descriptions that go beyond the Shockley-Read-Hall (SRH) approximation, because the density of defects is so high that recombination does not occur via isolated, but coupled defect states. We use a variant of coupled defect level (CDL) recombination, the donor-acceptor-pair (DAP) recombination, but via deep levels (as opposed to shallow levels). With this model, we quantitatively reproduce the I-V curves of solar cells that we subjected to various degrees of cleaving, laser scribing or diamond scratching to form shunt locations in a controlled manner. The suggested model explains the transition from ideality factors <n 2 to n> 2 when going from low to high defect densities. We also explain the non-saturating reverse I-V characteristics. We show that an additional source of currents with ngt;2 is SRH recombination in an inversion layer that extends from the front p-n junction to the rear contact along the cell's edge or along a micro-crack.