Summary: Some solar cells have more than one p-n junction. Multi-junction cells with more than one p-n junction have much higher efficiency than single-junction cells.

Conventional solar cells are single p-n junction solar cells. But there are solar cells with more than one such junction. What are these multi-junction solar cells about, and what advantages do they have over single-junction solar cells?

Conventional solar PV cells comprise just one layer of semi-conductor cells, each with one p-n junction.

Multi-junction solar cells are solar cells with more than one p–n junction. Each of these junctions is made of different semiconductor materials.

Multi-junction solar cells have higher efficiencies than conventional solar cells that have only one p-n junction. While single junction solar cells have a theoretical maximum efficiency of about 35%, multi-junction solar cells have a theoretical maximum of over 80%.

Multi-junction solar cells have more than one p-n junction, and as a result can have higher efficiency than single junction cells.

In real world, single junction solar cells have hit a maximum efficiency of about 22% as of mid 2014. For the same timeline, multi-junction solar cells have hit an efficiency of about 35% in real world conditions.

Questions from the curious cat

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.

What is the maximum number of junctions possible for a multi-junction solar cell in theory? What is the maximum that has been achieved so far?

In theory, we can have an infinite number of junctions, but that’s just theory. In practice, companies have achieved up to triple junction solar cells at close to commercial levels (2014)

In real world, what is the largest number of junctions being used? What is the efficiency of this solar power plant?

In research labs, the highest number of junctions achieved so far is upto six (sextuple). While there is considerable uncertainty on highest efficiency reached owing to authenticity of claims, it appears some researchers have been achieve efficiencies just exceeding 50% in lab conditions (2014 data).

Do multi-junction solar cells need concentrating devices?

Yes, multi-junction solar cells will not be able to produce such high efficiencies, and as a consequence they will not be economical, without concentrating the sunlight.

Do multi-junction solar cells need trackers?

Yes, as multi-junction solar cells require really high concentration of sunlight at any point in time, they will need tracking devices in order to provide satisfactory output

What are the disadvantages of a multi-junction solar cell?

The most significant disadvantage of multi-junction solar cells is their high upfront cost.

How much do multi-junction solar cells cost?

Cost of multi-junction solar cells are difficult to estimate, owing to the variance in the number of cells considered, and also owing to the nascent nature of this sector. However, it can be safely said that as of 2014, multi-junction solar cells cost at least 2-2.5 times as much as single junction solar cells of equivalent wattage.

Some nice videos for you

p-n junction solar cells


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