Figure 6: Reverse-characteristics of cell I -V curves used in the simulations in this paper for modules withI SC-mismatched cells.
characteristics of the case 2 blue curve is approximately identical to a
3-fold increase of the reverse break-through reported by Clementet al. [8] for PERC solar cells under 1-Sun illumination
intensity.
The yellow curve in Fig. 6, i.e. case 4, uses in reverse direction a
three-times higher shunt resistance than in forward direction. Neither
cases 3 nor 4 feature a reverse breakthrough and have only a linear
reverse-bias characteristics. While a real solar cell does have a
break-through at some reverse-bias voltage, these purely linear reverse
bias cases can be regarded as cases representing cells with a very high
break-through voltage.
3 RESULTS
3.1 I SC-matched cells
For a scenario of I SC matched cells we have to
consider only the parameters I 0, n ,R Sh and R S, becauseI PH is virtually identical toI SC for all practical scenarios that we need to
consider here. Variations of the series resistanceR S amongst the cells do not cause effects that
would be misinterpreted as an intensity dependent shunt. This can be
easily understood by noting that the individual series connected
elements of the string circuits in Fig. 3 can be regrouped, such that
all series resistances of the individual cells are placed on one end of
the re-grouped string circuit, and all diode-containing elements on the
other end. In other words, all series resistances can be lumped together
into one big resistance, and its effect does not depend on the
distribution of individual cell R S values that
the lumped big series resistance originated from.
It is therefore of bigger interest to examine what effect have
distributions of I 0 values, ideality factor
values n , and distributions of shunt valuesR Sh.
In a voltage range where the exponential part of equation 1 dominates
the current for all cells, we can approximate and re-write equation 1
for all cells as: