The term photovoltaics means the direct conversion of light into electrical energy using solar cells.  Semi conductor materials such as silicon, gallium arsenide, cadmium telluride or copper indium diselenide are used in these solar cells.  The crystalline solar cell is the most commonly used variety.

There are four main types of cells available:

Monocrystalline Silicon Cells

The production of monocrystalline silicon involves a crystal of silicon grown from highly pure molten silicon.  The foundation of the monocrystalline solar photovoltaic cell is formed by thinly slicing 0.2mm and 0.3mm thick sections from a single crystal cylindrical ingot.  These are then modified into a hexagonal shape to enable the maximum amount to be fitted onto each module.

Monocrystalline silicon cells are said to work better in direct sunlight and have efficiencies of between 13-17%.  They are the most efficient type of the three types of silicon cell.  

Polycrystalline Silicon Cells

The production of the polycrystalline silicon begins with a highly pure molten silicon using a casting process.  The silicon is heated to a very high temperature and then cooled under controlled conditions using a mould.  By using this process the silicon sets as an irregular poly or multi crystal form.  The block is then cut into slices 0.3mm thick.

Polycrystalline silicon cells are said to work better in diffuse daylight.  These cells are easily recognised by their blue appearance.  This colouring is due to an anti-reflective layer which means that it reflects the least and absorb the most light.  The polycrystalline cells on the market have efficiencies of around 11-15%.

Hybrid Cells

The hybrid cell differs from the cells detailed above as it is comprised of two different types of cells.  The hybrid modules that we supply and install are manufactured by SANYO (HIT range) and comprise of a monocrystalline cell which is then covered by an ultra thin amorphous silicon layer.

The hybrid cell has a distinct advantage as they perform well both at high temperatures and also in low light conditions whilst still maintaining a higher efficiency of up to 19%.  These cells therefore out perform conventional silicon cells such as the monocrystalline and polycrystalline.  However, whilst they are a higher price the higher efficiency of the cells means that more energy is generated and the yield of the system is increased thus increasing financial benefits.  Hybrid modules are perfect if roof space is limited as the modules themselves are physically smaller than average but with a much higher output.

Amorphous Silicon Cells

Amorphous Silicon Cells are more widely recognised as 'thin film' technology.  The amorphous silicon is different as it is a non-crystalline silicon.  You will have some of these cells already in your home as they are used for calculators.  Considerably less raw material is used in their production in comparison with the crystalline silicon cells as the layer of semi conductive material is only 0.5 - 2.0 um thick (1um is equivalent to 0.001mm).  The film of amorphous silicon is deposited as a gas on a surface such as glass.

The efficiency of amorphous silicon cells is around 6-10%.  Multi-junction amorphous thin film cells with each layer sensitive to different wavelengths of light are available and these have slightly higher efficiencies.  Thin film cells are more suited to BIPV (Building Integrated Photovoltaics) systems where they are used in the building facade.  Alternatively they are particularly suited if large roof areas are available as orientation is not so important and it is less affected by shading.  As a result amorphous cells are better suited to larger commercial applications.