The Three Types of Solar Cells
There are three basic types of solar cell. Monocrystalline cells are cut from a silicon ingot grown from a single large crystal of silicon whilst polycrystalline cells are cut from an ingot made up of many smaller crystals. The third type is the amorphous or thin-film solar cell.
Amorphous Solar Cells
Amorphous technology is most often seen in small solar panels, such as those in calculators or garden lamps, although amorphous panels are increasingly used in larger applications. They are made by depositing a thin film of silicon onto a sheet of another material such as steel. The panel is formed as one piece and the individual cells are not as visible as in other types of solar panels.
The efficiency of amorphous solar panels is not as high as those made from crystalline solar cells. Due to their lower power density; amorphous cells require up to three times the number panels for a standard installation to achieve the same power output; therefore taking up more usable space. Amorphous solar cells have a much shorter lifespan, therefore returning a much smaller investment for your investment.
Crystalline Solar Cells
The Great Debate - MonoCrystalline or PolyCrystalline
A lot of people say ‘What is the difference between mono and poly? I heard mono is better than poly.’ The answers to these questions are ‘not much’ and ‘no’.
How are the wafers made?
Mono cells are made by growing a cylindrical ingot of crystal silicon from a small seed crystal. The ingot is one crystal, hence the name ‘mono-crystalline’ or single-crystal. This ingot is then trimmed into a square and sliced into wafers. The ‘mono-crystalline’ cell is most effective when placed at 0° (North in the southern hemisphere and South in the Northern hemisphere).
While poly cells are made by pouring molten silicon into a square mold and allowing it to set. Since the silicon cools at different rates (the outside sets faster than the inside) and there is no seed crystal to ‘grow’ the new material from the resulting block contains many crystals. This gives the name ‘poly- or multi-crystalline’ and gives the resulting cells their multifaceted sparkly appearance. The ‘poly-crystalline’ cell due to its multifaceted surface is a high performer in weak light and on east and western roof surfaces. It’s other advantage is that per square meter of space it has a much larger wattage of power production.
So the question again - which is better? How about a question for a question - how do we define better?
Is better actually more efficient? The answer here is yes!
Poly cells have internal losses associated with the boundary lines where the different crystal facets meet. Mono cells, being only one crystal, do not have these internal losses. Mono cells have another advantage as well. The specific orientation of the mono crystal allows the creation of surface pyramids. These pyramids help to absorb more light into the module.
Different manufacturers also use many other processing techniques to improve their efficiency. Always check your module data to see what else is done to optimize the cell efficiency.
Suntech cell efficiency chart (v0809)
- Cell efficiency 180W-Ad 200W-Ub
- Mono 17.5 % 16.1 % Poly 16.0 % 15.2 %
Does more efficient mean more power? The answer here is yes, for the same area.
A mono module rated at 180W with an area of 1.28m2 may be 14.1% efficient while a poly 200W module with an area of 1.47m2 may be 13.6% efficient. So efficiency is only a measure of module output based on module area – the higher the efficiency the more power will be produced from the same size module. Or to look at it the other way, the same output with less modules. This could be very important where available roof space is an issue.
Does more efficient mean more energy in the real world? The answer here is no. So the next question is why not and how do we know which gives more energy in the real world?
Energy is Wh (or kWh), not just W (or kW) so we also need to consider weather factors such as the strength of the sunlight, the amount of cloud cover and the ambient temperature. To show this more easily many module manufacturers will give either a ‘temperature characteristics’ rating or a NOCT rating (Nominal Operating Cell Temperature). These ratings give an indication of how the modules will perform ‘on the roof’ rather than ‘in the lab’.
SUNTECH POWER NOCT chart (v0809)
- Model 180W-Ad 200W-Ub
- Pmax temp coefficient -0.48%/°C -0.47%/°C
- Pmax @ NOCT 131W 146W
Since the temperature characteristics of the mono and poly modules are almost identical they will both perform in the same manner in the ‘real world’.So the first question (‘what is the difference between mono and poly?’) has been answered. And the second question (’I heard mono is better than poly’) is really just an urban legend!