Summary: Single junction solar cells have just one p-n junction, as against multi-junction solar cells, which have more than one p-n junction

Solar cells generate electricity owing to the presence of p-n junctions. What is a p-n junction, what are single junction solar cells, and how are they different from multi-junction solar cells?

The conventional solar cells being used in solar power plants are of the single-junction type, and generate electricity using the single p-n junction.

One side of a solar cell is “doped” with boron, the p type, and the other side with phosphorus, the n-type, creating a single p-n junction.

What indeed is a p-n junction?

A p–n junction refers to the boundary between two types of semiconductor material. One type is called the p-type and the other is called the n-type. In a typical solar cell, one side of the cell is “doped” with boron (the p type), and the other side is doped with phosphorus, the n-type. Note that it is this doping that creates the p and n junctions.

What is the function of this complex-sounding p-n junction?

Let us make this simple for you: Visualise a solar cell having two regions, one called n-region that has a high concentration of electrons, and another called the p-region that has a low electron concentration. This combination of n- and p- regions, with a boundary between them, is called a p-n junction.

When the photons in the sunlight hit the solar cell and removes an electron from the n-region (with high electron concentration), the dislodged electron escapes from this region, and through a circuit, reaches the p-region (low electron concentration). This flow of electrons constitutes the electricity. This is a simplistic explanation, but to a large extent gives a picture of what really happens inside a solar cell that results in a flow of electric current.

Where there is only one combination of p and n regions, it is called a single junction or single p-n junction. The conventional solar cells use single junction technology, meaning it uses just one p-n junction.

If we are talking about single-junction, it means that more than one junction is possible, correct?

Yes, it is possible to have more than one junction in a solar cell. These are called multi-junction solar cells.

Questions from the curious cat

What is the highest efficiency possible (theoretical) with single junction solar cells?

The highest theoretical efficiency possible with a single junction solar cells is about 34%. Given that some single junction solar cells have about 22%, there is still considerable room for improvement in single junction cell efficiencies.

What are the advantages of multi-junction solar cells over single junction solar cells?

Simply put, single junction solar cells have upper limits on their efficiency.

Current studies indicate that single junction solar cells have a theoretical maximum efficiency of about 35%. That is, do what you may, these cells are not going to give efficiencies beyond 35%.

In order to derive lot more power from solar panels in future, we might want efficiency beyond 35%. Single junction cells might not be able to carry us all the way.

What the heck is doping?

We know what you are thinking. No, this doping has nothing to do with grass and LSD.

In the context of solar power and semiconductor, doping refers to the practice of intentionally introducing impurities into a pure semiconductor material such as silicon. Doping is done to change the electrical properties of the semiconductor. In semiconductor lingo, you need two types of such impurity introduction – the n-type doping, and the p-type doping.


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