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!
