How Do Solar Cells Work? How Semiconductors Convert Sunlight into Usable Energy
A solar cell is a type of semiconductor that can convert the
sun’s rays (also known as photons) directly into electricity. Each
solar cell produces a minuscule amount of electricity from one photon
of light. If enough solar cells are
gathered together and the sun is shining brightly, an electric
current will be generated. But how do
solar cells work?
Solar cells (also known as photovoltaic cells or PV cells)
are made of special materials called semiconductors. In the 1940’s, Bell
Labs discovered that when light strikes this material, an electron would
get knocked lose, allowing it to flow freely in the material. An electric
field within the material makes all of the electrons flow in one direction,
thus creating an electric current. If metal wires are placed on either end
of the PV cell, the current will be externally routed through a load. With
enough PV cells and enough photons of light, you can power a calculator,
an LED light or even household appliances.
When two silicon atoms are placed next to one another in a crystalline
structure, they share 4 electrons amongst themselves. In a pure silicon
crystal, this electron sharing is quite stable and the bonds are quite
strong. Pure silicon is a non-conductive material since electrons cannot
easily be removed from these bonds. It is electrically neutral. However,
‘good’ impurities can be mixed into the structure to make it behave
Phosphorous is an element that can share up to 5 electrons with its
neighbors. Since silicon only needs to share 4 electrons with its neighboring
atoms, when phosphorous is added to the crystalline structure, there is one
extra electron that will not strongly bond with its neighbors and will sort
of ‘float around’. The extra electron makes this doped material electrically
negative and is called N-type (negative-type). That’s half of the
Boron is an element that can share only 3 electrons with its neighbors.
Since silicon needs to share 4 electrons with its neighboring atoms, when
boron is added to the crystalline structure, there is a ‘hole’ created.
One electron is needed to fill this hole but none are available in this
silicon-boron solid. The material is electrically positive and is called
P-type (positive-type). That’s the other half of the explanation.
If we sandwich the two materials together, we’ll have one side wanting
an extra electron (P-type) while the other side can donate one extra
electron (N-type). It’s a match made in heaven, but only if we add some
energy, such as sunlight. When photons strike the silicon-phosphorous
solid, it has enough energy to ‘kick’ the extra electron out of the
N-type material. If we put metal wires on either side of the sandwich,
then these ‘floating’ electrons will start moving through the wire,
through an external circuit (i.e. a light bulb or battery) and then
to the P-type side of the sandwich. The moving electrons form an
electric current also know as electricity.
You may ask why the electrons will travel though the external
wires and not just cross the gap where the two sides of the sandwich
meet. At this location, some of the extra electrons from the N-type
side do hop over to fill the holes in the P-type side but not entirely.
As more and more electrons make the hop, an electric field is created
(a voltage). As this field strengthens, it makes it harder for the
extra electrons to hop over the barrier. They find it easier to travel
around the external circuit than try to overcome this strong electric
field at the junction. As the electrons travel through the wire, it
creates an electric current and we can now power our appliances & toys.
You might also wonder why a semiconductor is called a semiconductor.
This N-type and P-type material is only conductive when energy (in this
case, sunlight) is applied. Take away the energy -- for instance, after
the sun has set for the day -- and it does not conduct electrons anymore.
The material is only semi-conductive a semiconductor.