Dear Ali Bahrami:

Since every sub-cell in a tandem (multi-junction) solar cell is electrically connected in series, the total current of the tandem cell (Jsc,total) is determined and governed by the sub-cell providing the lowest Jsc. One of necessary tasks in designing the tandem cell is to obtain the nearly same Jsc for each sub-cell. This is called current-matching.

Let us consider two single-junction cells (called A and B). If the cells A and B are individually characterized under the same condition (i.e., they have to be made and measured separately), they will exhibit their own characteristics (e.g., Jsc,A and Jsc,B) which are derived from the exposure of entire solar spectrum at a particular irradiation intensity. That is, they are able to absorb high- and low-energy photons as much as they can, depending on the materials. However, when the cells A and B are combined into a single tandem cell (not electrically wired between two cells), the rear sub-cell (typically made from low-bandgap material) of the tandem cell can absorb only low-energy photons because high-energy photons are already harvested by the front sub-cell (typically made from high-bandgap material) of the tandem cell. Consequently, Jsc,total of the tandem cell will never exceed the lowest current of single-junction cell (either Jsc,A or Jsc,B).

As the carrier recombination reduces the current, the carrier lifetime is important in addition to the above issue because the carriers have to survive a number of layers in a tandem cell.

Hope it helps.

Dear Ali,

In order to answer this question one needs to analyse the circuit of at least two cells connected in tandem.  Let u assume that one of the cells has a photo current Iph1 greater than the photo current of the other cell Iph2. Let us assume that the two cells has negligible shunt resistances. That is Rsh1, Rsh2=infinity.

Now let u short circuit  every cell alone, then the the shortcircuit currents will be equal to the corresponding photocurrent; Ish1= Iph1 and Ihs2=Iph2 and the cell diodes shunting the current sources will be zero biased as cell voltage will be zero.

Now let us short the tandem connection of the two cells, then by simple intuitive circuit analysis: the cell with the higher photo current will push its excess current into its diode making it forward biased. Since the two cells now are connected back to back, the cell with smaller photcurrent will be revere biased and its current flows through the other cell to the outer short. In this case then, Ish of the tandem  cell= Ish2 the smaller one.

Let us now add a shunt resistance to the smaller current cell, The photo current Iph2 will partly flow through this resistance, leaving only smaller current than Iph2 flowing outside the cell.

Then in case of a presence of a shunt resistance in the smaller photocurrent cell,The global short circuit  of the tandem will be smaller than its photocurrent and its own short circuit current. That is Ish global < Ish2.

This is full explanation of the effect.

wish you success

Both Maetee and Abdelhalim have given excellent explanations of the effect. I would like to add some more practical aspects of this for real world cells. As you increase the number of cells in tandem, the more accurately you have to match the cutoff wavelengths to the exact spectrum of incident light. By the time you get to 4 layers, you will find you need to define the cutoff wavelengths to 1 nanometer for any given spectrum of incident light. You will find that a cell optimised to work when the sun is overhead is useless when it's near the horizon, and cells designed for normal incident sunlight won't work very well under xenon arcs, even though such an arc is a good match for the solar spectrum.

While I wish researchers working on tandem cells well, it should be remembered that it is easy to overspecify such cells and find they do not work well in the real world. Their ideal application is of course spacecraft, which are exposed to full sun except when shadowed, and must have high (>40%) efficiency because of weight constraints.

The tandem cell is a complex structure in which the subcell is not illuminated like in real sun. There is certainly a loss of current due the spectral mismatch. In additionThe optical  (antireflection coating) is certainly modified and the surface passivation of the subcell is not so efficient like in an individual cell. Endly, the illumination level is low and in some matrials the lifetime of minority carriers decreases with this level (except for n-type silicon). As a consequence the photocurrent density is also lower than in an individual cell. Obviously, sun light concentration is a partial solution to the preceding problems. Endly, the considerations of AbdelHaim Zekri concerning the equivalent electrical circuit of a tandem cell must be taken into account.

Farhana, consider what would happen if both cells were identical and both designed to work as well as possible with the incident solar spectrum. Now change one cell - by definition, you have made things worse because  both the first cells were optimum and now  one isn't. But this doesn't solve the same problem as the tandem cell is designed to solve. The tandem cell is designed to extract as much power as possible from unit incident irradiation. This is exactly the right approach when making two cells in tandem results in a lower mass structure, essential for a spacecraft. On the earth's surface it is rather different. Highly efficient conversion of solar energy is desirable when the energy supplied has a cost. But sunlight doesn't have a cost - it's free, so designing a cell to extract more power may not (in practice, won't) be a better economic solution than just making your array a bit bigger.

The cost of doubling the size of a ground based array is probably  slightly less than 2. The cost of using tandem cells instead of silicon might increase the cost 50 or 100 times. It isn't worth it!

why the current in the top layer is lower than the bottom layer?