Hi, welcome back to new Physicist. There are two types of crystalline silicon solar cells. Monocrystalline and polycrystalline. What’s the difference between them? How they are manufactured? Why one is much more efficient than others? Why polycrystalline solar cell surface looks like cracked glass?
Once I learned about What’s happening inside a solar cell factory all my doubts vanished. Do you know, How crystalline solar cells are manufactured?
It all starts with the raw material, which in this case is sand or quartzite, more precisely silicon dioxide, from which we’ve to extract precious silicon. Silicon is abundantly available, making it the second most available element on Earth.
Together with coal or other sources of carbon, the silicon sand is melted down at over 2000 degrees Celsius in an electro arc furnace. The result is metallurgical silicon with a purity of 99%.
The silicon is collected, usually in the form of solid rocks. These rocks are being melted together at very high temperatures in a polycrystalline factory and then to the distillation plant, and finally to Siemens factory to get high purity silicon crystals. This process is known as chemical vapor deposition. In Siemens factory, a hot seed rod of silicon is used to get 99.9999 % pure polysilicon.
Siemens process is very expensive but other alternatives like fluid bath reactors or blowing gas methods cannot generate this much pure silicon out of metallurgical silicon.
The next process is making Ingots of Silicon. We can either proceed with this pure polysilicon to make polycrystalline ingots or we follow czochralski process or float zone process to make monocrystalline silicon ingots.
In the process of ingot making, attention is given so that all atoms are perfectly aligned in the desired structure and orientation by controlling temperature and other atmospheric conditions. This process is very expensive that’s why mono crystalline-solar panels are also expensive.
Whereas polycrystalline ingots are manufactured by directly casting melted polysilicon crystals in a dedicated crucible. So silicon crystals have different structures and orientations.
Monocrystalline silicon ingot gives us monocrystalline solar cells whereas polycrystalline ingot gives polycrystalline solar cells. Or in other words, Monocrystalline cells are made out of a single crystal of silicon whereas polycrystalline solar cells from several crystals of silicon melted together. You can recognize them by the shattered glass look given by the different silicon crystals.
The higher efficiency of monocrystalline solar cells can be attributed to the uniform structure of silicon atoms inside monocrystalline silicon. During ingot making Boron is added, which will give the silicon positive electrical polarity. This can be termed as p-type doping.
Polycrystalline ingots are usually in square shape whereas monocrystalline ingots are cylindrical because of their production method. After the ingot has cooled down, grinding and polishing are being performed, leaving the ingot with flat sides.
The next step is Wafer manufacturing. The silicon ingot is sliced into thin disks or we call it wafers. A wire saw is used for precision cutting. The thinness of the wafer is similar to that of a piece of paper.
The following processes will convert a wafer into a solar cell capable of converting solar power into electricity.
In an oven-like chamber, phosphorous is being diffused in a thin layer over the surface of the wafers. This will charge the surface with a negative electrical orientation.
The combination of boron and phosphorous will give the PN junction, which is critical for the proper function of the PV cell.
Each of the wafers is being treated and metal conductors are added on each surface. The conductors give the wafer a grid-like matrix on the surface.
Because pure silicon is shiny, it can reflect the sunlight. To reduce the amount of sunlight lost, an anti-reflective blue color coating is put on the silicon wafer.
The blue color is given on purpose to ensure the conversion of solar energy into electricity. The coating will facilitate the absorption of sunlight, rather than reflecting it. This is how monocrystalline or polycrystalline solar cells are made.