Polycrystalline solar cells
Polycrystalline solar cells vary from Monocrystalline solar cells because they are made from various crystallites which are melted together. If you look closely you can see this.
I like to compare polycrystalline solar cells to square slices of ham. If you look closely, you can see how the ham is actually bits of ham, squashed together and cut into slices. It is the same principle with the polycrystalline solar cells. Where monocrystalline cells are cut from a single ingot, a crystal of up to 200 kg, polycrystalline cells are melted/formed in a square mould. This is also the reason that polycrystalline cells are square and monocrystalline cells have rounded corners (the monocrystalline ingots are round).
Due to the fact that polycrystalline does not have a single crystal structure, polycrystalline cells are in general a few percent less effective then monocrystalline cells. Their performance is low lighting is also up to 25% less the monocrystal cells.
Polycrystalline solar cells are sliced/sawn from the polycrystalline lump. Performance of solar cells is much attributed to the purity of the silicon, the crystal and its ability to conduct electricity, this is one of the reasons that polycrystalline performs less well then the mono cells.
Processes like sawing and polishing the cells is highly specialised and super sterile environments are maintained, tolerances are very fine.
Once the silicon wafer is complete it has a typical thickness of 0.2-0.3mm, depending on the process. Manufactures try and achieve the thinnest cells possible as pure silicon is very expensive due to the extensive refining.
The cells are then coated with an active substance like Boron which electrons which are readily pushed to higher orbits by photons in the correct wavelength (visible to infra-red light). The decay of this orbit is basically what you and me can measure as volts and current. This is collected by the silver lines on the front of the cells, this is actually silver. On the back of the cells you find an aluminium foil, it is very thin and this acts to collect the current on the back of the wafer.
All solar cells are individually tested to ensure power matching of cells. Power matching is important as all cells characteristics vary slightly and in a series of solar cells, the weakest cell will dictate the current and voltage of the rest of the string. A solar cell is negative on the front and positive on the back.
For more information:
http://en.wikipedia.org/wiki/Polycrystal
http://en.wikipedia.org/wiki/Solar_cell
