my first question is that what is the definition of current matching? does it mean matching between short circuit currents or currents at maximum power operation or there is another definition?
next question is that if we have two subcells in a tandem solar cell with current mismatch, how can it affect on the performance of tandem solar cell?
1. It should be current matching at the mpp of the tandem solar cell (which need not be at the mpp of either subcell). However, unless the cells differ strongly in fill factor matching short circuit currents brings you very close to matching currents at mpp.
2. Provided each cell provides good rectification, tandem cell current is limited by the cell that produces lower current. The cell that could, in principle, produce higher current, is limited to this low current, and operates at a voltage above its mpp. This means the voltage of the tandem cell increases slightly but this in no way compensates the large efficiency loss arising from the lower tandem cell current.
Interestingly, if one cell yields much lower current and much poorer rectification, the cell providing more current can "push" some of this current through the limiting cell, increasing tandem cell jsc to above the jsc of the limiting cell. However, the poor rectification in the limiting cell implies such strong recombination in that cell that this is purely of academic interest and certainly not a viable way of achieving improved tandem cells.
There is only one current in the sries connected subcells.
As mentioned already by Manuel, this is the small current.
Dear Manuel Schnabel, Thanks a lot for your complete answer. i have questions from your answer, why subcell with higher current should operates at a voltage above its mpp? Can current mismatch lead to open circuit reduction in the tandem cell?
Ali, ( or more likley anyone who sees this old thread), In open circuit no current flows and there can be no current mismatch.
In an in-series connected sub cells, (Current matched), the output current is restricted to the poorest supplying sub cell. The solution to this issue is a parallel connection of sub cells, which are equal in voltage supply. And the voltage simply depends on the materials utilized.
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