Figure 9: Zoom into upper current range of 1-SunI -V curves with +/- 2.0% photogeneration inhomogeneity,
plotted for the four cases of different cell reverse bias
characteristics in Fig. 6. (Also shown for comparison: I -V curve with homogeneous I PH distribution, grey
dotted line.) Despite different R Sh.Slope values,
all I -V curves share the same MPP.
The reason for all curves sharing the same MPP is that the maximum power
current of the module is still notably lower than the lowest
photogeneration current in the inhomogeneity distribution. Consequently,
even the solar cell with the smallest I SC in the
module circuit does not block the current at MPP, and this cell with the
lowest I SC can still produce a significant
fraction of its maximum power at the global module maximum power currentI MP.
Fig. 9 shows that the black dashed line I -V curve [i.e.
the module of case 3 in Fig. 6] is indistinguishable from the blue
line I -V curve [case 2 in Fig. 6]. The reverse
characteristics of the “case 3 cells” is defined by extending their
forward shunt resistance R Sh =10 kΩcm² to the
reverse bias direction, while the “case 2 cells”, that form the blueI -V curve in Fig. 9, have additionally to a reverse shunt
resistance of 10 kΩcm² also a breakdown characteristic that increases
current conduction superlinearly in reverse voltage direction. However,
despite their differences for larger reverse bias voltages, both reverse
bias characteristics are nevertheless virtually identical for the first
4 volts, from zero to -4 V. Since the moderate current inhomogeneities
considered here do not cause cells in these two module types to exceed
reverse bias voltages of -4V, their I -V curves in Fig. 9
do not differ.
In order to highlight the ‘near-irrelevance’ of the apparent shuntR Sh.Slope for module’s power production, Fig. 10
plots the module efficiency as a function of illumination intensity for
the two most extremely different inhomogeneity cases of this study: The
‘IBC homojunction’ case at an inhomogeneity level of +/- 0.4%, and the
case with the highest reverse bias shunt resistanceR Sh.rev = 30 kΩcm². at an inhomogeneity level of
+/- 2.0%. We can see that the efficiency differences between these two
cases is marginal and reaches only a maximum deviation of
0.026%abs, i.e. 0.11%rel, at the at
highest illumination intensity.