What are multi-junction solar cells? Are they more efficient?

Multi-junction solar cells are solar cells with more than one p–n junction. Each of these junctions is made of different semiconductor materials. Conventional solar PV cells comprise just one layer of semiconductor cells, each with one p-n junction.

Yes, multi-junction cells are more efficient that conventional solar cells.

It is worthwhile to note that while single junction solar cells have a theoretical maximum efficiency of about 35%, multi-junction solar cells have a theoretical maximum of over 80%.

Just like many emerging solar PV cell technologies, what is limiting widespread adoption of this technology is cost. Multi-junction cells are expensive, mainly due to their high manufacturing costs.

Many companies have come up with interesting multi-junction cell products. One such company is Insolight which seems to have developed multi-junction cells having efficiencies of 42%.

In another effort, a team of researchers from MIT and Masdar Institute of Science and Technology claim to have found a way to fabricate multi-junction cells called ‘step cell’ due to the stepwise fashion in which the two layers are arranged. They have estimated practical efficiencies of 35%.

In a world that is looking beyond silicon modules, solutions based on multi-junction cells are likely to be increasingly adopted when they get cost competitive with conventional panels.

How do multi-junction solar cells achieve higher output for the same area?

As there are different semiconductor materials forming the p-n junctions in a multi-junction solar cell, each material’s p-n junction will produce electric current in response to different wavelengths of light. Recall that sunlight is not just one “type” of light but is a spectrum comprising different lights, each with different wavelengths. Thus, the use of multiple semiconducting materials allows a broader range of wavelengths to  be absorbed, increasing the efficiency of the cell.

What is the highest possible efficiency for multi-junction solar cells?

The theoretical maximum efficiency possible for conventional single-junction cells is about 35%. For multi-junction cells, the theoretical maximum is about 87%, under highly concentrated sunlight. Of course, such a high efficiency will be achieved under a very large number of junctions. In labs, with a reasonable number of junctions, multi-junction cells have demonstrated efficiencies of about 43%. Compare this with the best efficiencies achieved for single-junction in labs, around 25% Commercially two-junction solar cells are widely available at 30% under conventional sunlight concentration and improves to around 40% for CPV.

Take a look at some nice videos below

p-n junction solar cells

You may read more on multi-junction solar cells here, here and here.

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