Interpretation of the Commonly Observed I-V Characteristics of C-SI Cells Having Ideality Factor Larger Than Two

The dark I-V characteristics of crystalline silicon solar cells usually deviate from that expected by classical diode theory by an unusually high ideality factor and magnitude at biases smaller than about 0.5 V. It had been shown that the recombination current is flowing preferentially in certain local extended defect positions like the edge or local shunts. There, the local density of recombination centers in the pn-junction is higher than in the bulk by orders of magnitude. In this work, we go beyond the SRH theory to explain the recombination effects occurring in such heavily damaged pn-junction regions. Firstly, we apply the coupled defect recombination via two shallow (or one shallow and one deep) level, which explains the observed high ideality factors due to trap-assisted tunneling. Secondly, we apply the coupling of two defects to recombination via deep donor-acceptor pairs, which cause the high ideality factors due to saturation of the recombination rate between the two defects. Thirdly, the local extension of the recombination region across the edge of the cell (due to electrostatic charging) is an other explanation of very high recombination currents