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| N-type solar cell |
Solar cells are large area p-n junctions. An N-type solar cell consists of a thin p-type silicon (doped with boron) layer over a much thicker n-type silicon (doped with phosphorus) layer. Electrical contacts are applied to both sides. The p-side is the front side facing the sun. It is given an anti-reflective coating, over which is pasted a clear adhesive (for instance EVA) which holds the front protective glass layer.Note: Do not be alarmed. This dope is legally recommended, not by doctors, but by engineers!
Why N-type solar cells are less common?
Currently most of the crystalline solar cells are p-type. This is because of a lower cost of production of p-type. The reasons are probably attributable to history of development of solar cells. But performance wise, n-type solar cells can give much better efficiency compared to p-type solar cells. Two main factors account for this. Firstly, p-type material has boron (trivalent) doping. In the presence of light and oxygen boron undergoes some undesirable action, which reduces efficiency of conversion. This is called Light Induced Degradation or LID.
Disadvantages P-type solar cells.
To understand the second disadvantage, note that sunlight breaks loose an electron hole pair. If this takes place in the n-type region, the holes are minority carriers in this (n-type) region. A hole while drifting could be filled by absorbing one of the free electrons which are the majority carriers here. In this case the solar energy absorbed will be lost as heat, and only heat up the cell. Our aim is to allow the holes to reach the depletion region where it will be swept by the electric field to the p-type material, and on to the anode. In other words, the diffusion length of minority carriers must be as long as possible to allow better solar efficiency. In the n-type cell this diffusion length of minority carriers is more than in the p-type cells, because the n-type is less affected by minority impurities. Since n-type cells have a better diffusion length for their minority carriers, they have a better conversion efficiency.For this reason the relevant industry is engaged a lot of research in this direction to permit more cost effective production of n-type.
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