You probably know that plants get their energy from the sun through their leaves in a process
called photosynthesis.
We humans don’t have our own leaves, but maybe we can get energy from the sun in a
different way--electrical energy, that is!
Could there be some kind of electrical “leaf”?
A solar cell gathers light from the sun and generates electricity--just as a leaf generates
food for a plant--let’s find out how!
The sun emits energy in the form of waves.
These waves can range in length, from short, ultraviolet waves, through the rainbow of
the visible spectrum, to long infrared waves.
When the sun is shining, these waves move towards the earth and hit the surface of solar
cells.
Let’s take a closer look!
The active part of a solar cell is a wafer made of a semiconductive material, typically
silicon.
A semiconductor is a type of material that normally doesn’t conduct electricity well,
but it can be made more conductive under certain conditions.
The semiconductor part of the solar cell has three layers.
The thin top layer contains silicon and a very tiny amount of an element, such as phosphorus,
that has more electrons than silicon.
This gives the top layer an excess of electrons that are free to move and make the material
more conductive.
The top layer is also called negative-type, or n-type, as it favors the collection and
transport of electrons.
The thin bottom layer contains both silicon and an element, such as boron, that has fewer
electrons than silicon.
This gives the bottom layer fewer electrons that are free to move, therefore making the
material less conductive for electrons.
A missing electron can be described as an effective positive charge.
Therefore, the bottom layer is called positive-type, or p-type, as it preferentially favors the
collection and transport of these positive charges, also dubbed ‘holes.’
The thicker middle layer has only slightly fewer electrons, making it marginally p-type.
Thin metal lines, typically made of silver, are printed on the top n-type layer, and the
bottom p-type layer is in contact with an aluminum plate.
When light waves hit the top surface of the silicon solar cell, only light with wavelengths
from a specific window of the solar spectrum (350-1140 nm) are absorbed into the middle
layer of the solar cell.
This range of wavelengths includes the visible spectrum: ultraviolet wavelengths are so short
they stop at the surface, and infrared wavelengths are so long they cannot be absorbed and pass
right through the cell or are reflected back.
The light wave knocks an electron off a silicon atom, setting the electron loose and leaving
an area of positive charge (a ‘hole’) where the electron used to be.
The loose electron then moves towards the top and reaches the top n-type layer, which
readily accepts electrons.
Similarly, the loose hole moves towards the bottom and reaches the bottom p-type layer,
which readily accepts holes.
This continues as long as sunlight shines on the solar cell.
Now that the electrons and the holes have been separated, connecting a wire between
the top and the bottom metal electrodes provides a pathway for the electrons to move towards
the holes.
Flow of electrons is electrical current!
One solar cell produces several Watts of power.
This may be sufficient for running a calculator or a phone charger, but it is not sufficient
for running a toaster, for example, which uses one thousand Watts.
So, several solar cells, typically 32, are wired together to make a solar panel.
Several solar panels are needed to generate enough electricity to power
a household.
Like leaves, solar cells are a viable way to convert the sun’s rays directly into
electricity that we can use.
Future challenges include improving the efficiency by which a solar cell converts sunlight to
electricity and making the solar energy cheaper.
Of course, solar cells produce electricity only during the day, so, storing the electricity
efficiently for use during nighttime is another important challenge.
Advancements in solar technologies present the potential to power our lives, if we just
leave it to sun!