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Solar cells are large area p-n junctions. A n-type solar cell consists of a thin p-type silicon (doped with <br>boron) layer over a much thicker n-type silicon (doped with phosphorus) layer. Electrical contacts are <br>applied to both sides. The p-side is the front side facing the sun. It is given an antirefelective coating, <br>over which is pasted a clear adhesive which holds the front protective glass layer. <br>Note: Do not be alarmed. This dope is legally recommended, not by doctors, but by engineers. <br>Currently most of the crystalline solar cells are p-type. This is because of a lower cost of production of p-<br>type. The reasons are probably attributable to history of development of solar cells. But performance <br>wise, n-type solar cells can give much better efficiency compared to p-type solar cells. Two main factors <br>account for this. Firstly, p-type material has boron (trivalent) doping. In the presence of light and oxygen <br>boron undergoes some undesirable action, which reduces efficiency of conversion. This is called Light <br>Induced Degradation or LID. <br>To understand the second disadvantage, note that sunlight breaks loose an electron hole pair. If this <br>takes place in the n-type region, the holes are minority carriers in this (n-type) region. A hole while <br>drifting could be filled by absorbing one of the free electrons which are the majority carriers here. In this <br>case the solar energy absorbed will be lost as heat, and only heat up the cell. Our aim is to allow the <br>holes to reach the depletion region where it will be swept by the electric field to the p-type material, <br>and on to the anode. In other words, the diffusion length of minority carriers must be as long as possible <br>to allow better solar efficiency. In the n-type cell this diffusion length of minority carriers is more than in <br>the p-type cells, because the n-type is less affected by minority impurities. Since n-type cells have a <br>better diffusion length for their minority carriers, they have a better conversion efficiency. <br>For this reason the relevant industry is engaged a lot of research in this direction to permit more cost <br>effective production of n-type cells. <br>Resources: <br>http://www2.pv.unsw.edu.au/nsite-files/pdfs/UNSW_Understanding_the_p-n_Junction.pdf <br>http://pv.energytrend.com/features/20141216-7939.html <br>www.pv-tech.org/guest_blog/n_type_silicon_solar_cell_technology_ready_for_take_off <br>https://www.sintef.no/globalassets/project/foxy/publications/nrel-n-type-cells-060724.pdf
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