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Silicon Solar Cells
The silicon wafer has an electron rich (n) layer and an electron poor (p or hole) layer. A photon of light energy is absorbed by the silicon semiconductor and if it has sufficient energy it forms an electron/hole pair or exciton. Under the right conditions the charges are separated and the free electron flows through the circuit creating an electrical potential. Meanwhile another electron flows from the circuit back into the hole layer where it recombines with the original hole that was created.
Crystalline silicon solar cells were the first generation of solar cells to be developed. The silicon used in solar cells comes from highly refined wafers that are only slightly less pure than those used to make computer chips. Monocrystalline silicon consists of a single, highly organized and compact crystal. Solar cells made from the material can achieve very high efficiencies of around 15-19% commercially, and up to 25% in the laboratory. Mono-Si solar cells currently are the second-largest solar panel technology by market share, making up around 35% of new production. However, monocrystalline cells are fairly expensive to produce because of the material purity required. In addition, these cells are much more sensitive to temperature than other technologies, meaning that in hotter conditions their efficiency advantage decreases.
Polycrystalline silicon solar cells are made from highly refined silicon wafers with material purities only slightly less than those used to make computer chips. The poly-Si material is cheaper than monocrystalline silicon, although cells made from have somewhat lower efficiencies of around 13-16%. Another advantage is that poly-Si solar cells can be made square, unlike their mono-Si counterparts, which are made from cylindrical ingots and therefore cannot be as densely packed. For these reasons poly-Si PVs currently occupy the largest market share of any solar panel technology, composing around 55% of new production last year.
Amorphous silicon solar cells are made from a layer of silicon deposited as thin film on a plastic backing. The silicon atoms in a-Si are randomly structured, unlike in crystalline cells, and have a comparatively low efficiency of up to 5-8%. They tend to degrade with long sun exposure but they are flexible and inexpensive to produce. In addition, a-Si cells have a much lower temperature sensitivity than other silicon-based cells, meaning that they lose less of their conversion efficiency at higher ambient temperatures.