Photovoltaics

How electricity is created from sunlight

Sunlight is the most common and freely available source of energy on our planet. Per hour, more energy is transported to the Earth in the form of light than the entire human population consumes in a year. More than 60 years ago, technology was developed that allowed light to be directly converted into electricity.

Raw material: sand

In order to convert sunlight into electricity, we need the aid of a very common material: sand. The sand needs to be converted into 99.999% pure silicon before it can be used to produce solar cells. To achieve this, the sand undergoes a complex purification process which involves heating it up to as much as 2000 degrees Celsius. This converts the raw silicon into a gaseous silicon compound form, which is mixed with hydrogen to create highly purified polycrystalline silicon. These silicon ingots are then cut into very thin silicon wafers, the heart of the photovoltaic cell.

Electrons and holes in a semiconductor

The pure silicon wafer is a so-called semiconductor. When at rest, the electrons in the crystal are responsible for creating the bonds between atoms. There are no free electrons available to create an electric current. However, when light is shone on the crystal, enough energy is transferred to the electrons to release individual electrons from their bonds, and they are free to move within the crystal lattice. The place in the crystal where the electron is freed from its bond is called a hole, and like the electron, the hole is free to move within the crystal. This movement of electrons and holes is however initially random and no current is produced through the load as a result.

Extra atoms make electricity possible

In order to guide the movement of the electrons in a particular direction when light hits the semiconductor, the silicon wafers at the top and the bottom of the photovoltaic cell are injected with additional elements. Phosphorous atoms, which have more electrons available for bonds than silicon, are add to the top. This results in free electrons at the top, also referred to as negatively doped or N-type doping, as a result of the excess of negatively charged electrons. Boron atoms, which have less bonding electrons than silicon, are added to the bottom, creating holes at the bottom. This is called positive or P-type doping. If light hitting the wafer frees up electrons and holes in the crystal, these move in a particular direction due to the doping at the top and bottom of the cell, and this creates an electric current.

Fingers and busbars transport the current

To transport the electricity generated by the cell requires electrical contacts made of metal. To allow as much light as possible to hit the cell, very thin metal contacts are printed onto the cells – so-called fingers. The electricity flowing in the large number of smaller fingers are routed from the cell using thicker busbars. The underside of the cell generally has a continuous metallic layer that also conducts the electricity.