Ideality factor behavior between the maximum power point and open circuit

The local ideality factor analysis of dark and light I-V curves has been used in the past to study various performance degradation effects in solar cells. Trapping, edge recombination and injection-level-dependent recombination are expressed as “lumps, humps and bumps” in the plots of the local ideality factor over cell voltage (m-V plots). Earlier applications of this differential technique did not correct the plots for the series resistance effect. Thus, the bumps at the higher voltages introduced by some mechanisms were more difficult to quantify. A possible solution is to analyze I
_SC
-V
_OC
curves, but their measurement is not always possible. We present a formula for calculation of the R
_S
-corrected local ideality factor. The m-I plot can be more informative than the m-V plot which varies strongly with the cell temperature. We present results from several crystalline-silicon (c-Si) PV technologies showing the m-I plot behavior between the MPP and open circuit. For some of the tested modules, the ideality factor is much higher at the MPP.