How a solar cell works

Photovoltaics (PV) are semiconductor devices that convert light into electricity. The solar cell is the core element of the PV system. When light hits the solar cell, it produces the photovoltaic effect, freeing electrons inside the panel that can be extracted from the module as electricity.

If the photons have enough energy, the electrons can overcome the electric field at the junction and are free to move through the silicon atoms in the cell and into an external circuit as energy.

As the photons flow through the external circuit they give up their energy as useful work (e.g. turning motors, lighting lamps) and return to the solar cell.

Solar cell technologies that have varying conversion rates include:

• Amorphous silicon (Tandem) 6% - 7%
• Cadmium telluride (CdTe) thin film 9% - 12%
• Multicrystalline (or polycrystalline) silicon 15% - 17%
• Monocrystalline silicon (SiN) 17% - 19%.

BP Solar currently uses  only multicrystalline and monocrystalline technologies, which we believe offer the best balance of overall performance, reliability and value. Our research scientists continue to actively seek alternative methods to improve efficiency.

One square metre of solar cells that are exposed to the sun at noon will receive approximately 1 kilowatt (kW) of energy from the sun. BP Solar’s multicrystalline cells convert roughly 16.5% of this into electricity, generating 165 Watts in full sunshine.

How solar cells are manufactured

Solar cells are made from a variety of materials and technologies available in the industry today. Silicon is the material most often used to make solar cells and the process to manufacture crystalline silicon cells consists of the following steps:

Casting

Silicon, the raw material used for making solar cells, needs to be purified by melting it at very high temperatures.  This process gives silicon the properties and quality needed to make a solar cell.

Once the raw material has the grade of purity and quality needed for the solar cells, it is solidified in crucible to form ingots or bricks.

Depending on the process used to solidify the silicon, different types of cells (monocrystalline or polycrystalline) can be created.

Ingots or bricks are sawed into thin slices called wafers.

The wafers are then cleaned and very thin layers of other materials are added to make the photovoltaic cell able to produce electricity when exposed to light.

Etching and texturing

Wafers are cleaned with chemicals. Monocrystalline wafers are further textured to form square-based pyramids to reduce the reflection of sunlight.

Diffusion and edge isolation

Wafers are pre-loaded with boron during the casting process, giving them a positive (p-type) characteristic. The wafers are given a negative (n-type) characteristic by diffusing them with a phosphorous source at high temperature, which in turn creates a negative/positive (n-p) junction.

Anti-reflection coating

To further reduce reflection and allow the capture of more sunlight, BP Solar wafers are coated with an anti-reflection coating.

Metallization

The cells can now generate electricity. Contacts (usually in the form of metal strips made of silver) need to be added to the front and  rear surfaces to collect the electricity.

Because it can be soldered, silver is the most widely used metal for contact formation. Silver (in the form of a paste) is screen printed onto the front and the rear surfaces. In addition, aluminium paste is applied to the rear to achieve back surface field (BSF), which improves the performance of the solar cell.

These metal pastes are subsequently heated above the alloying temperature to form a good ohmic contact material. They become electronically conductive when they are supplied with light or heat: they operate as insulators at low temperatures.